Compatibility Testing Guidance for Bulking Operations in the Waste Treatment Industry

Compatibility Testing Guidance for
Bulking Operations in the Waste
Treatment Industry
Environment Agency / Health and Safety Executive Joint Guidance on Compatibility - November 2011
PUBLISHED BY:
Environment Agency
Rio House
Waterside Drive, Aztec West
Almondsbury, Bristol BS32 4UD
Tel: 0870 8506506
Email: [email protected]
www.environment-agency.gov.uk
© Environment Agency
All rights reserved. This document may be
reproduced with prior permission of the
Environment Agency.
Author(s):
1
1
Steve Rowe , Keith Middle
1
Chilworth Technology Ltd
Dissemination Status:
Publicly available
Keywords:
Compatibility, waste, bulking
Research Contractor:
Chilworth Technology Ltd, Beta House,
Southampton Science Park
Southampton, S16 7NS
Environment Agency’s Project Manager:
Jill Rooksby, Olton Court
Collaborator(s):
Chris Hall, Environment Agency
Janet Etchells, Health and Safety Executive
Ian Priestley, Syngenta
Product Code:
GEHO1111BVDU-E-E
Environment Agency / Health and Safety Executive Joint Guidance on Compatibility - November 2011
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CONTENTS
1.
INTRODUCTION
5
1.1
Objective of the Guidance
5
1.2
Scope of this Guidance
5
1.3
Scale-up Issues in Bulking Processes
6
1.4
Incidents in the Waste Industry
7
1.5
Purpose of Compatibility Assessment and Testing
7
1.6
Risk Assessment
8
1.7
Competence of Managerial, Laboratory and Supervisory Staff
9
2.
DESKTOP REACTIVITY HAZARDS SCREENING
10
2.1
Information on Received and Existing Wastes
10
2.2
Methods for Reactivity Prediction (Desktop Screening)
10
2.3
Procedure for Desktop Screening
13
2.4
Conclusions from Desktop Screening Studies
17
3.
LABORATORY TESTING OF POTENTIALLY ADVERSE REACTIONS
19
3.1
General
19
3.2
Dewar Calorimetry
21
3.3
Adiabatic Calorimetry
25
3.4
Acceptance Criteria
27
3.5
Conclusions from Laboratory Compatibility Studies
28
4.
CONCLUSIONS
30
APPENDICES
31
A.1
Appendix 1 – Heat Losses of Small and Large Scale Vessels
32
A.2
Appendix 2 – Details on Reactivity Prediction Methods
35
A.3
Appendix 3 – Determining Heat Loss & Phi Factor of Non-Adiabatic Dewar Vessels 42
A.4
Appendix 4 – Comparison of Laboratory Test Methods and Protocols
50
A.5
Appendix 5 – Determining Phi Factor & Heat Loss of Plant Vessels
56
Environment Agency / Health and Safety Executive Joint Guidance on Compatibility - November 2011
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A.6
Appendix 6 – Precautions for Using Electrical Apparatus
60
A.7
Appendix 7 – Adiabatic pressure dewar calorimetry equipment
62
A.8
Appendix 8 - Glossary
65
A.9
References and Bibliography
68
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1.
INTRODUCTION
This guidance has been produced jointly by the Environment Agency and the Health and
Safety Executive for operators of waste treatment plants who undertake bulking operations.
This guidance is not an authoritative interpretation of the law, but if you do follow this
guidance you will normally be doing enough to comply with the law. Other alternative
measures to those set out in this guidance may be used to comply with the law.
1.1
Objective of the Guidance
The mixing of waste streams can cause unexpected and undesirable chemical reactions
(referred to hereafter as “adverse” reactions) to occur in vessels that are not adequately
designed for removing heat and / or preventing gas release to atmosphere. Waste streams
that react, often unexpectedly, to cause heat generating or gas generating reactions are
termed “incompatible” waste streams. This has been the cause of a number of incidents in
recent years.
The guidance in this document is specifically aimed at bulking operations at permitted waste
management facilities only (that is, physical blending of wastes that are not intended to react
in any way), and does not relate to bulking operations at producers premises as other issues
would need to be addressed e.g. links to the waste hierarchy and keeping different waste
types separate to promote waste recovery. Although many of the concepts described can be
applied to treatment stages in waste processing, the guidance is not intended for treatment
processes. The bulking of wastes in which adverse reactions are expected, known or
identified at the pre-acceptance or acceptance stage would be in contravention of the site
permit, irrespective of whether the site is permitted for treatment.
The aim of this guidance is to provide operators with a structured and scientifically sound
procedure for assessing potential chemical reactions, foreseeing adverse reactions and then
enabling bulking processes to be carried out without risk of unexpected chemical reaction.
The methods include “desktop” analysis techniques for spotting potential adverse reactions
and experimental methods for screening for, or accurately quantifying the magnitude of,
adverse reactions. In particular, it provides a testing framework that will enable operators to
develop and evaluate laboratory derived data on the industrial scale.
By adopting this Guidance, potential adverse reactions can be identified and avoided before
large scale bulking, thus ensuring that waste bulking operations can be undertaken without
adequately controlled risk to personnel and the environment.
1.2
Scope of this Guidance
This guidance is limited to the identification of adverse reactions associated with bulking
liquid waste streams. Organic and aqueous (water-based) wastes are included. The
guidance is aimed at preventing adverse reactions occurring in general purpose tanks and
vessels which are not designed as reactors or are not designed for the specific
consequences of any reaction which could occur.
The guidance is not intended to cover intentional waste treatment reactions (such as
neutralisation) although many of the concepts and methods can be applied in these
situations. The guidance is not restricted in the scale of bulking operations that are
considered. It is equally applicable to small scale and large scale bulking operations – albeit,
the consequences of large scale bulking processes can be considerably more severe than
those of small scale operations.
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The guidance does not include coverage of the following important subject areas:
•
Toxicity of the waste streams and any associated by-products formed as a result of
reaction (except where toxic gas is formed by adverse reaction)
•
Ecotoxicity – impact on the environment of any unplanned release
•
The effect of mixing and storage of the waste on the vessel. This includes potential
corrosion, erosion and thermal (heat) effects on the tank
•
Flammability of the waste streams and any associated by-products formed as a
result of reaction (except where flammable gases are formed by adverse reaction)
1.3
Scale-up Issues in Bulking Processes
In the waste treatment industry, diverse ranges of industrial wastes are treated. The first step
of treatment, on receipt, normally involves “bulking up” operations to transfer the individual
received waste into a vessel containing similar streams. Reactions must not be intentionally
performed in bulking operations. Reactions can only be knowingly performed in treatment
processes, and these are not the focus of this guidance. It is, however, a possibility that
reactions may occur unintentionally due to the wide variety of potential components present
in the received waste or in the waste with which this will be bulked – the present guidance
aims to minimise such a possibility.
It is critical that potential adverse reactions are identified and avoided during bulking.
Although some adverse reactions can be readily anticipated and the mixing of those
materials prevented, other adverse reactions may not be foreseen. Reactions which may
pose a risk include exothermic (heat generating) chemical reactions and reactions which
generate a non-condensable gas as a by-product.
Most bulking vessels are not designed to have any ability to remove heat or withstand
pressure. Without any heat removal ability, any exothermic reaction that occurs in the vessel
will cause a temperature increase. Heat will then be lost to the surroundings, through the
walls of the vessel, at a rate that depends on the size of the vessel, its construction and the
external environment. For larger vessels, this heat loss normally occurs at a very low (often
imperceptible) rate. To provide valid data, laboratory simulation of plant scale processes
must be conducted under very low heat loss conditions to match the plant scale. This is why
sensitive calorimetric methods are required in the laboratory and simple glassware
experiments are inadequate. The heat losses of large and small vessels are considered
further in Appendix 1.
If the temperature increase is high enough, the mixture within the vessel may heat up
towards its boiling point. This vapour, or any gas generated by reaction, will escape through
the vent line into the atmosphere unless adequately treated. This can result in a flammability,
toxicity or ecotoxicity hazard or odour problem; or if the vessel vent is inadequately sized, in
a risk of over-pressurisation of the vessel.
The possibility of adverse reactions, or their consequences, can be reduced by, for example,
not bulking waste streams or using smaller vessels rather than large tanks for bulking
operations. In the case of particularly hazardous streams, or streams containing particularly
hazardous components, consideration should be given of discharge to treatment directly, in
order to mitigate the risks associated with bulking.
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1.4
Incidents in the Waste Industry
A significant number of incidents have occurred in the waste treatment industry, many during
bulking operations, and the Environment Agency has produced a review of a number of
these to demonstrate the primary causes and effects[1]. In addition, a paper presented by
HSE in 2008 reviews incidents associated with unplanned adverse reactions[2]. The review
examines the incidents and identifies departures in procedures for the facility from the
standards established in “Guidance for the recovery and disposal of hazardous and nonhazardous wastes (S5.06)”, also provided by the Environment Agency[3]. One incident
(March 2007, Heysham Works) from the review[1] can illustrate some of the issues involving
adverse reactions during waste bulking.
The incident occurred in 2007 when a tanker offload of 21 tonnes of hydrocarbon light
distillate (HLD) in a bulk tank led to exothermic reaction, pressurisation, venting and a loss of
over 4 tonnes of the vessel contents to atmosphere. This was a repeat load of HLD – with 16
previous loads having been satisfactorily accepted from the same customer.
The incident investigation revealed a number of failures in the operator’s procedures
including:
•
The pre-acceptance information collected by the operator was incomplete with
significant safety data omitted.
•
The HLD waste was highly variable from consignment to consignment and the use of
a generic HLD sample for compatibility testing was inappropriate.
•
The composition of the consignment load was only checked against a generic
specification.
•
There was inadequate instrumentation on the large scale bulking tanks to provide
any indication of internal reaction. With an earlier identification of the adverse
reaction, emergency measures taken promptly may have limited the extent of
escalation of the incident.
In cases where wastes are variable, compatibility screening and / or testing should be
performed on each variant load. The use of a generic test for a variable load is not reliable.
When a waste load varies significantly between pre-acceptance and acceptance,
compatibility testing should be triggered by the company’s procedures.
This incident occurred due to adverse reaction of the HLD waste consignment with a
component in the bulk storage tank. The exothermic reaction caused heating of the tank
contents, significant and prolonged vapour generation and venting of the tank direct to
atmosphere.
Reliable acceptance procedures, which would clearly highlight variability in the load, should
trigger the need for compatibility testing. This testing would almost certainly have identified
the potential adverse reaction and pressurisation risk, and this should have led to rejection of
the load or its isolation in a dedicated tank.
1.5
Purpose of Compatibility Assessment and Testing
Compatibility assessment and testing for bulking operations in the waste industry will occur
at different stages and will fulfil different functions. The two principal stages of concern in
this document are at pre-acceptance and at site acceptance to offload.
Pre-acceptance assessment is normally undertaken during initial discussions with a waste
supplier and is designed to:
Environment Agency / Health and Safety Executive Joint Guidance on Compatibility - November 2011
Page Number 7 of 69
•
•
•
•
•
Characterise the waste and its variability to understand its composition.
Determine if it may be handled within the terms of the site permit.
Determine if the site has the capability to treat the waste using the processes and
equipment available.
Determine the treatment route and the nature / quantities of chemicals required.
Assess the costs of treatment to allow the pricing for the contract to be calculated.
Whilst the characterisation of the waste may be carried out using a range of standard
chemical analyses, compatibility testing requirements are generally less defined at the preacceptance stage. Compatibility assessment will determine if there are any adverse
reactions from the new waste when bulked with the intended existing wastes. The number of
possible interactions should be limited by operational procedures established by the waste
disposal company. However, all reasonably foreseeable combinations should be assessed.
Thus, pre-acceptance testing is vitally important as it gives a baseline to work to and allows
limits to be defined for the various parameters which characterise the waste. Later
acceptance testing will then compare the incoming waste to the pre-acceptance sample as a
measure of acceptability to authorise off-loading.
Acceptance testing is fulfilling a different purpose. Ideally, it will:
•
•
Assess the waste characteristics to confirm the load is similar to the pre-acceptance
sample and is within the limits previously set and agreed.
Test the compatibility of the incoming load with site fluids present at the instant of offloading, for handling (off-loading, bulking and treatment) to the defined protocol.
This latter aspect is more difficult to assess and is often not a routine operation in current
practice. Its importance is to evaluate the nature of the waste received and identify if there
are any significant variations in the load composition arising from off-site activities, as well as
highlighting adverse consequences that could occur on site as transfers are carried out,
owing to possible variations in tank composition. As the number of possible combinations is
reduced, the testing becomes simpler, but it nevertheless is likely to require more time to
assess than is presently common in the industry.
1.6
Risk Assessment
Despite an earlier pre-acceptance assessment, there remains scope for a waste load to
undergo adverse reactions with undesirable consequences if compositions and conditions
are not as expected. The two principal categories of error are:
•
Wrong chemical composition in the incoming load. This could have occurred for a
number of reasons including incorrect loading at the supplier, the tanker being sent to
the wrong waste site, the waste stream being contaminated at the supplier or by the
previous contents of the tanker, or the waste specification varying over a period of time
from that examined at pre-acceptance.
•
Load discharged into the wrong destination at the waste treatment site (wrong receiving
tank, existing contents of receiving tank evolved from that examined at pre-acceptance,
subsequent treatment route incorrect, etc).
For many waste treatment companies, the first principal error is addressed by the existing
‘characterisation of waste’ tests, whilst the second is most commonly controlled by
Environment Agency / Health and Safety Executive Joint Guidance on Compatibility - November 2011
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management procedures. There is scope to address both with compatibility testing and this
aspect is considered later in this guidance.
There are other errors that may occur which should be considered in a robust risk
assessment. Procedures should be reviewed to ensure that representative samples are
taken from both the supplied load and offload tank. Reliable and robust identification,
labelling and storage of laboratory samples of wastes should also be assessed to ensure
that samples are not easily mixed up or incorrectly identified.
The possibility of certain errors and deviations occurring can be minimised by the production
of, and strict adherence to, Standard Operating Procedures (SOPs) for all stages of preacceptance and acceptance. This should include a specific SOP for compatibility testing.
Overall, the objective of these risk assessments is to reduce the likelihood of incidents to a
tolerably low level; as in all areas of life, complete elimination is most unlikely.
1.7
Competence of Managerial, Laboratory and Supervisory Staff
Laboratory managers responsible for the desk screening assessment, the selection of
calorimetric equipment, installation in the laboratory, provision of safety barriers, preparation
of test protocols, etc, should hold a minimum qualification of HNC chemistry, supplemented
by a recognised training course on calorimetry and the causes and consequences of
adverse reactions. The same qualifications and background knowledge should be held by
the responsible person reviewing the results from the laboratory tests and authorising the
mixing operation (for example, the discharge of a waste tanker into a storage vessel).
Laboratory staff conducting the tests according to the procedures should be fully trained on
the techniques involved, closely mentored to gain appropriate experience and have attended
the relevant supplementary training on calorimetry, adverse reactions and the hazards of
laboratory testing, before operating alone. These staff should meet the requirements
established in the Environment Agency sector guidance, S5.06[3].
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2.
DESKTOP REACTIVITY HAZARDS SCREENING
For any waste that will be bulked up (that is, added to existing wastes on site without
intentional reaction), it is necessary to conduct a risk assessment of the operation to identify
any possible adverse reactions that may occur. Once such reactions are identified, the
bulking should either not proceed or should be studied in suitable test apparatus to
determine if the operation will present an unacceptable risk. This assessment should form
part of the Waste Pre-Acceptance procedure, and must also be performed before
acceptance of the waste.
It should be stressed that the major advantage of desktop screening is to rapidly identify and
prevent the bulking of wastes which will undergo adverse reaction. Where potential reactions
are identified but the magnitude is uncertain or even minor, testing would always be required
for confirmation. The desktop assessment, alone, is not an acceptable way to accept a
waste for bulking. The other advantage of desktop screening is to provide a preliminary risk
assessment for laboratory compatibility testing.
The general procedure recommended for the desktop risk assessment for bulking wastes is
provided in Figure 1. Each stage of the assessment is discussed below with reference to the
box numbers in the flow chart.
2.1
Information on Received and Existing Wastes
Ideally, adequate information should be available on the waste that will arrive at site to
enable an assessment of potential adverse reactions. For reactivity prediction, the key
information on the received wastes (Figure 1; Box 1) should include:
•
Chemical composition and the possible variability between consignments, should be
known.
•
Source of the waste.
•
Quantity in the consignment.
•
pH, physical appearance.
•
Presence, strength and description of odour assessment (note COSHH
implications)
•
Other specific information on flammable, oxidising, explosive or water, air, acid or
base reactive properties or specific reactive chemicals that may be present.
This information should form part of the whole package of information in accordance with the
Environment Agency sector guidance S5.06[3].
This information should also be known for all existing streams already held on site (Figure 1;
Box 2). The most critical information for existing streams is composition, quantity and pH as
this data is important for identifying any possible adverse reactions on bulking up.
If the data is available (Figure 1; Box 4), it is possible to proceed to a theoretical assessment
of potential adverse reactions. If there is any data missing for the received waste or existing
streams, physical testing for compatibility is unavoidable as a desktop assessment of
potentially adverse reactions would be impossible. It may be possible to generate or request
the required outstanding data (Figure 1; Box 8).
2.2
Methods for Reactivity Prediction (Desktop Screening)
With adequate information, the possibility of a reaction between two or more of the
components in the respective streams can be examined. Binary interactions, that is
Environment Agency / Health and Safety Executive Joint Guidance on Compatibility - November 2011
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reactions between two components, can often be predicted using literature or binary
compatibility matrices. Reactions between more than two components (a ternary, or greater,
interaction) are possible but these are generally harder to foresee. Conducting such a review
for multi-component streams with many different constituents may be considered an onerous
task and a decision will normally be made to proceed directly to testing. However, even in
this case, some form of desktop screening would be necessary prior to allowing the
laboratory compatibility test, in order to protect the laboratory technician.
Methods for predicting binary interactions are discussed in Appendix 2. Adverse reactions of
particular concern are:
•
reactions that generate heat and cause an increase in the vapour pressure of the
mixture (potentially to boiling conditions), and / or
•
reactions that generate permanent gas or volatile products.
The binary reactivity prediction should seek to identify both types of potential reaction.
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Information on Received Waste
• Accurate composition
1
• Physical properties data
3
Binary
Interaction
Assessment
Information on Existing Stream
• Accurate composition
2
• Physical properties data
Existing data
adequate for
assessment?
Yes
4
No
Yes
Exothermic or
Gas Forming
6
Reaction
No reactions
predicted 5
Heat of Solution
Effects
7
Adverse
Reaction
Predicted?
Yes
Site Compatibility 10
Matrix
Very similar streams
previously mixed and
Data Available?
11
Yes
Generate or
request further
data?
8
No
9
No
Bulking Not
Allowed
Adverse
reaction 13
Yes
No
No adverse
reactions 12
Conduct
Testing
14
Testing Not
Essential 15
Figure 1: Procedure for Desktop Adverse Reaction Screening
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Page Number 12 of 69
Some reactions can be readily identified as being potentially adverse. Examples include:
•
Acid / base reactions or neutralisations. This can be predicted by significant
difference in the pH of the two streams and will result in exothermic reaction.
•
Chlorinated waste / aqueous base resulting in exothermic reaction.
•
Metals (particularly light metals such as aluminium, magnesium and sodium)
contacting with water or acid resulting in hydrogen gas generation.
In addition, some specific chemicals and groups of chemicals will present a high risk of
adverse reaction when present in appreciable concentration (some of these are referenced
in a HSE review of adverse reactions in waste handling operations[2]. Examples include, but
are not limited to, the following (with the nature of the potential adverse reaction in brackets):
•
Nitric acid solutions in combination with various solvents and acids (to produce
unexpected unstable nitrations and / or gas generation)
•
Other concentrated acids such as sulphuric acid or oleum solutions (exothermic
reaction and gas generation)
•
Peroxides - including aqueous hydrogen peroxide, organic peroxides and
hydroperoxides (exothermic reaction and gas generation)
•
Oxiranes – specifically epichlorohydrin, ethylene oxide and propylene oxide
(exothermic reaction)
•
Unsaturated monomers – for example, acrylic acid, styrene, methacrylic acid, methyl
methacrylate and any other acrylate or methacrylate monomers (exothermic reaction)
•
Hypohalites – for example, sodium hypochlorite with acids (exothermic reaction and
gas generation)
•
Hydrides – for example, sodium / potassium borohydride, sodium hydride, lithium
aluminium hydride (gas generation)
•
Acid chlorides – for example, thionyl chloride, acetyl chloride, phosphorus
oxychloride (exothermic reaction and gas generation)
•
Sulphides and cyanides (gas generation)
When these compounds and groups are present, in significant concentration, physical
testing should be immediately considered unless the received waste is to be mixed with
material of a very similar composition (for example, where a bulk tank is dedicated to a
specific stream from a specific customer and there is therefore, a high level of confidence
that the risks are insignificant).
2.3
Procedure for Desktop Screening
If a reaction is predicted from initial screening, it is necessary to predict the size of the
reaction and then decide if it presents a potentially unacceptable risk. In this assessment,
the rate of the reaction is not considered; we are simply looking to identify whether there is a
potential hazard.
2.3.1 Gas Forming Reactions (Figure 1; Box 6)
Where a gas generating reaction is foreseen, the potential amount and type of the gas must
be evaluated. The balanced chemical equation for the reaction must be determined to allow
this assessment to proceed. Based on the amount of the limiting reactant (that is, the
reactant present in the smallest amount according to the balanced equation and relative
quantities mixed), it should be possible to calculate the amount of gas that will be produced
(see Appendix 2).
If the quantity of gas generated is greater than 25 cm3(of gas).kg-1(of mixed waste), then the
proposed bulking operation should not proceed This is an intentionally low allowable limit of
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gas. Depending on the nature of the gas, a lower limit may be appropriate and this should be
considered as part of your risk assessment. For example, in the case of the gas being
hydrogen (extremely flammable), mercaptan (malodourous) or hydrogen cyanide (very
toxic).
The solubility of any foreseen gas in the liquid may be a significant factor in evaluating the
amount that will be liberated as free gas and hence the risk. However, any gas solubility in
the liquid must be ignored in desktop screening to ensure that results are conservative. The
potential for gas generation is generally considered to be greater in inorganic waste
treatment, than for organic.
In the case of reactions which involve the decomposition of a chemical, the nature of the
gaseous decomposition product may be more difficult to predict. In this case, testing will
invariably be required.
In any case where gas generation is noted to be theoretically possible, physical compatibility
testing should be performed if the predicted gas quantity is less than 25 cm3(of gas).kg-1(of mixed
waste). For higher quantities, the bulking operation should not proceed.
2.3.2 Exothermic Reactions (Figure 1; Box 6)
Where an exothermic reaction is foreseen, the potential temperature rise should be
calculated and compared to an allowable limit and to the boiling point of the resulting mixed
stream. Calculation of the potential temperature rise will require calculation, or estimation, of
the heat of reaction for the foreseen reaction. Methods for determining the heat of reaction
and converting this into a potential temperature rise are discussed in Appendix 2.
Once the potential temperature rise is known, for the concentrations and quantities to be
mixed, this can be compared to the likely boiling temperature of the mixture. The boiling
point of the mixture can be measured experimentally (accurately or crudely) or estimated
from the boiling points of the major volatile liquids present. For example, where the stream is
a 10% aqueous (water-based) solution of an involatile (high boiling) substance, it would be
reasonable to use the boiling point of water. Where mixtures contain significant
concentrations of liquids with a wide range of boiling points, it would be safer to assume that
the mixture has a boiling point equal to the lowest boiling point component. Alternatively, in
this case, experimental estimation of the initial boiling point may be prudent.
Small temperature increases, in the absence of gas generation, may be tolerable. However,
if the temperature of the mixture can rise by more than 10 K or to within 10 K of the boiling
point, then a risk exists of either creating a positive pressure in a sealed vessel or generating
a significant quantity of vapour. Either of these cases would be considered as unacceptable
for a bulking process and the proposed bulking operation should be disallowed. If the
temperature rise is smaller (<10 K) and will not reach within 10 K of the boiling point,
compatibility testing must be performed to fully characterise and confirm the
thermochemistry. Many vessels used as bulking tanks, particularly vessels manufactured of
plastic or composite materials, may not tolerate elevated temperatures. In this case, more
stringent allowable temperature rise limits may be appropriate.
Exothermic reactions resulting from mixing streams with different pH’s (not neutralisation)
may also occur and estimation of potential temperature rise from heat of solution effects can
be performed and compared with the allowable criteria (see Section A.2).
The mixing of liquid streams containing the same compounds at widely differing
concentrations can give rise to heat of solution effects (Figure 1, Box 7). This scenario also
requires evaluation to determine the potential temperature rise (see Section A.2).
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In addition to binary reactions of waste stream components, there may also be a possibility
that one (or more) of the materials may be thermally unstable at, or near, the proposed
storage temperature. The same literature used for reaction investigations highlighted in
Section A.2 may be used to predict any substance(s) which may be thermally unstable. If
any such components are present, do not bulk them together.
2.3.3
To Test or Not to Test?
A flow chart showing the procedure for exothermic reaction and gas generating reaction
assessment is shown in Figure 2. In this chart, any adverse reactions which generate
significant gas or yield a significant temperature rise, would be deemed unacceptable, and
mixing would not be allowed. In cases where small temperature rises or small quantities of
gas generation are foreseen, compatibility testing should be performed to characterise the
magnitude of reaction and determine if it is considered adverse.
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Binary
Interaction
Assessment
Gas
Generating
Reaction?
Yes
Amounts of
reactants in each
stream
Balanced
chemical equation*
Calculate quantity
of gas
No
Yes
Quantity > Limiting Criteria
No
Exothermic
Reaction?
Yes
Amounts of
reactants in each
stream
Balanced
chemical equation*
Calculate heat of reaction,
adiabatic temperature rise
and final temperature of
mixture
No
Thermal
Instability?
Yes
Temperature rise > 10 K or
Final temperature within 10 K
of boiling point
No
Yes
No
Site Compatibility
Matrix
Conduct
Testing
(Figure 1; Box 10)
Incorporate test data into
site compatibility matrix
Bulking Not
Allowed
Figure 2: Flow Chart for Binary Exothermic and Gas Generating Reaction Screening
* If a balanced chemical equation is not possible testing will normally be required.
Where an adverse reaction is predicted, the proposed bulking operation should not be
allowed. If a reaction is predicted, but it is not calculated to be large enough to be considered
an adverse reaction, it is necessary to proceed to experimental testing to more precisely
quantify the size of the hazard.
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The only exception where compatibility testing would not be necessary is where very similar
streams have been received and bulked, or tested, previously and no adverse reaction
observed. This information should, ideally, be stored in a “Site Compatibility Matrix” (Box 10,
Figure 1) where all previous stream mixing operations are available, along with information
on whether adverse reactions have been noted either on plant, or experimentally (Figure 1;
Boxes 10, 11, 12 and 13).
If the initial desktop evaluation does not indicate any possible interactions, there remains a
risk that some potentially adverse reactions have not been identified. For this reason, unless
there is experimental or plant evidence from previous studies on very similar streams,
experimental confirmation of compatibility should be sought. The company should also
document a procedure for, and consider the hazards associated with, treatment of any
residual material left in the tank after transfer.
The term “very similar streams” refers to previous mixing experience with streams of similar
composition and concentration. This may be the case where a consistent stream is provided
by a particular customer and is bulked in the same vessel each time a consignment is
received. If there is a significant deviation in the composition or concentration of a stream
(for example, if there is a change in the concentration of any hazardous component of
greater than 10%), then it would be prudent to conduct experimental confirmation.
2.4
Conclusions from Desktop Screening Studies
The conduct of thorough desktop studies on mixing of waste streams is invaluable in
highlighting potentially adverse reactions. Identifying potentially adverse reactions at an early
stage of pre-acceptance will enable the waste treatment operator to plan whether the waste
can be bulked, where the waste should go, and what testing will be required prior to, or at
the time of, waste acceptance. It will equally allow the operator to price the waste bulking,
and subsequent treatment, according to the work required.
In some circumstances, it may be possible, after a thorough desktop assessment, to receive
and bulk waste without the need for compatibility testing. However, to do this requires a
good level of material knowledge and absolute confidence in the absence of potentially
adverse reactions from previous experience in mixing very similar wastes. There are some
cases where experimental compatibility testing will be unavoidable. These include, but are
not limited to:
•
wastes with unknown composition,
•
wastes with highly variable concentrations,
•
where screening indicates potentially adverse reaction,
•
where components of the waste are known to be particularly reactive,
•
where components of the waste may potentially have catalytic properties towards
other wastes
•
where previous testing or experience indicates that there is a potentially adverse
reaction
•
wastes containing compounds for which balanced chemical equations cannot be
reliability written
It is critically important that the desktop assessment is conservative in the assumptions
made. In all cases where doubt exists regarding the compatibility of waste, physical testing
should be pursued. The desktop assessment process should intentionally focus on the size
of any potential events, and not their rate.
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In many cases, proceeding directly to compatibility testing will allow a simpler and quicker
assessment than performing the theoretical calculations discussed in Appendix 2. Even in
this case, some form of desktop screening should be performed to avoid the conduct of
hazardous reactions in laboratory compatibility testing.
The desktop study should be fully documented and made available to all persons who will be
associated with waste receipt. It should, after the consignment has been received, be
archived in a retrievable manner. Typically, records should be retained for a minimum period
of two years after receipt of the waste.
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3.
3.1
LABORATORY TESTING OF POTENTIALLY ADVERSE REACTIONS
General
When an adverse reaction cannot be reliably ruled out from preliminary screening, or where
there is sufficient uncertainty in the composition or concentration of species in the mixture to
reliably predict the absence of adverse reaction, it will be necessary to perform a meaningful
laboratory compatibility test.
A pre-acceptance sample of the waste stream should be made available prior to delivery of
the bulk waste. This sample should represent, as closely as possible, the composition and
concentration of the bulk waste. This pre-acceptance sample will be tested using standard
techniques to characterise the waste stream and to set the allowable range of variation for
the bulk waste receipt. The need for compatibility testing of this sample will have been
identified, from the desk screening exercise in Figure 1, and should proceed in all
circumstances where there is any doubt.
Laboratory compatibility testing may be undertaken either at the pre-acceptance stage or at
the site acceptance-to-offload stage, and in some cases, at both stages. Changing
composition in the waste tanks may prohibit or limit the applicability of testing at preacceptance. The level of investigation and the apparatus employed may be different, as will
the time available to complete the study.
Owing to the heat loss issues discussed in Section 1.3 and Appendix 1, appropriate
calorimeters must be used for testing to yield reliable results. Relatively cheap experimental
apparatus based around Dewar vessels (vacuum-jacketed flasks) is an economic approach
suitable for most companies. It is unacceptable to conduct compatibility testing where
possible exothermic and gas generating reactions are sought using beakers, bottles, flasks,
test tubes and other standard laboratory apparatus as the heat loss from these devices will
adversely affect the experiment outcome (as discussed further in Section 3.1.3). Commercial
calorimetric techniques can also be used for compatibility screening. Examples include
commercially available adiabatic calorimeters, reaction calorimeters or Thermal Activity
Monitors (TAM). This guidance focuses on low cost equipment that can be assembled from
readily available parts. The applicability of such commercial techniques should be reviewed
by competent individuals (see Section 1.7) based on the technique sensitivity and the way it
is operated (isothermal, adiabatic, or other).
3.1.1 Sampling of Waste Streams
Any material used for testing must be truly representative of the waste that will be received.
For homogeneous mixtures (for example, mixed liquid streams without layers or
suspensions) sampling is relatively easy. For heterogeneous mixtures (for example, liquid
streams with some deposited solid residues or two immiscible liquid phases), it is necessary
to obtain a representative sample for the entire mixture. This may involve multiple samplings
from different locations to obtain a mixture that represents the overall composition.
Sampling from tankers may be conducted via top access hatches using a gantry or via foot
valve discharge points, each of these locations having their own advantages and
shortcomings
• Top access hatches permit sampling of fluids at a variety of heights within the tanker,
thereby capturing potential for layering, sedimentation, immiscibility, etc. However, top
sampling can pose a risk to the operator of falling from height, exposure to fumes, etc.
and suitable precautions should be taken. Further guidance is given by HSE at
http://www.hse.gov.uk/foi/internalops/hid/spc/spctg04.htm.
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•
Foot valve sampling will always require flushing through of the sample point to ensure
freshness, but the sample will be restricted to fluid in close proximity to the valve.
Therefore, aspects such as layering and immiscibility may not be captured. Certain risks
to the operator may be reduced, but there will be a requirement to have a disposal point
for the flushing, there could be a risk of failure to close the sampling valve[1], and
appropriate PPE will be required to protect the operator from splashing, etc.
Selection of the best sampling location will depend upon an individual assessment of the
likely sample composition variation and the risks/protection measures to the operator.
Environment Agency sector guidance S5.06[3] advocates sampling from top hatches using a
gantry and may be consulted for further information in this area.
Sampling from other containers (for example IBCs, drums, bottles, etc) will offer fewer
options but attention will be needed to ensure the procedure can capture a fully
representative sample from all depths, and that the operator wears appropriate PPE.
Sampling from site tanks and vessels will require the same degree of consideration. Gaining
a representative sample from a large storage tank may involve sampling from a recirculation
loop following a suitable mixing time, sampling from an agitated tank, etc. Where a tank may
contain sludge residues, efforts should be made to incorporate a representative fraction of
this sludge into the sample to be tested.
Once the various samples are taken, it is imperative that they are kept separate, and only
mixed directly in the calorimeter when the test is started.
3.1.2 Safety in Laboratory Testing
Adverse reactions can present a serious hazard during small scale compatibility testing as
well as during large scale mixing. Many reportable laboratory incidents clearly demonstrate
the hazard that can exist.
Dewar vessels containing up to 1 litre of reactive liquid can give rise to highly dangerous
effects if a severe adverse reaction occurs. In addition to protecting the laboratory staff
against the health hazards (occupational safety) of the fluids being handled, protection
against shattering Dewar vessels must also be considered. Glass Dewars should preferably
be located in a fume cupboard (with the sash down and behind a secured internal shield).
Any gas collecting equipment should ideally also be placed in an adequately ventilated fume
cupboard, or exhausted via a scrubber; again a shield should be used. Flexible plastic mesh
tubes can be used to sheath a glass Dewar thereby minimising the scatter of fragments in
the case of rupture. Further, procedures should require the laboratory technician not to
stand directly in front of the Dewar during testing.
If the pre-testing assessment suggests that rapid gas formation is a possibility with the
chemistry being considered, a small sample should be mixed in an open beaker in a fume
cupboard before the Dewar flask is filled, in order to screen out laboratory hazards (see also
3.1.3).
Closed stainless steel Dewars capable of withstanding significant pressure should be sited
behind a blast resisting shield (or within an appropriately designed blast chamber), again
ventilated to scrubber or atmosphere, as appropriate.
As well as the overpressure hazard that can exist when studying adverse reactions, there is
the possibility of toxic or flammable gas generation. Any location where tests are conducted
should be adequately ventilated to prevent the possibility of accumulating a toxic or
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flammable gas cloud. The use of fume cupboards or extracted enclosures for testing is
normally essential. In the event of a possible flammable gas risk, any such location should
be free of potential ignition sources[19].
Generic procedures for conducting compatibility testing should be subject to a Standard
Operating Procedure (SOP). A risk assessment should be performed on the generic
procedure which considers the variety of possible adverse reactions that may occur. This
should include a consideration of flammability and toxicity hazards associated with the raw
materials and the potential by-product gases which may be generated. From this risk
assessment, a combination of safety measures (procedural and engineering control
measures) should be established. The SOP should incorporate the necessary safety
measures, including safety glasses and any other necessary Personal Protective Equipment
(PPE).
3.1.3 Invalid Test Methods
Later in this section, recommended valid test methods are discussed. However, there are
some tests, currently known to be used by operators which are not, alone, capable of
providing a valid compatibility assessment.
The use of standard laboratory glassware for simulating large scale mixing is wholly
inappropriate in isolation. Whilst indicative results may sometimes be obtained from simple
beaker mixing experiments, the high heat loss of such glassware will generally render the
simulation to be poor. There is a value in performing simple glassware experiments – if a
severe reaction is seen in laboratory glassware, then it is likely to be even worse in a large
scale environment. However, “no reaction” in simple glassware is not a definitive observation
due to the high heat losses of the glassware. It may be that a slow reaction is occurring but
that the heat losses of the glassware are balancing the heat output from the reaction –
causing the temperature to appear constant.
To produce a definitive test result, the heat losses of the test should ideally be equal to, or
less than, the heat losses of the large scale bulking vessel (see Appendix 1).
3.1.4 Important Considerations in the Test Method
When conducting a compatibility test, the following aspects should be considered in the
design and conduct of the test:
•
Inclusion of materials of construction. In the case of some reactions, a trace quantity
of a certain material can catalyse (speed up) a reaction. Laboratory scale simulations
should therefore always include a specimen of the material of construction of the
large scale equipment.
•
Agitation. The test should be agitated irrespective of the storage tank conditions as
this will provide a more conservative result.
•
Mixing Profile. The large scale mixing operation will be performed in a certain way –
normally dictated by the amount of material to be added and the methods available
for transfer. The test should seek to replicate the large scale process as closely as
possible. This may involve the addition of discreet portions, a continuous pumped
feed or the rapid introduction of the whole sample. Procedures will be needed to
ensure that full-scale operation does, indeed, proceed in this way.
3.2
Dewar Calorimetry
Non-adiabatic Dewar calorimetry is based around a vacuum jacketed flask used in standard
laboratory locations. These Dewar flasks may be glass or stainless steel and are freely
available from high street stores although industrial grade flasks are preferred. Care must,
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however, be taken in their use – a typical arrangement is described below. This test is
typically limited to the simulation of behaviour in smaller bulking tanks of up to 10 m3
capacity. However, this is not an absolute limit of applicability. Calibration of the vessel, as
described below, and estimation (or measurement) of the plant vessel heat loss should be
performed to evaluate the suitability and scalability of data generated by the technique.
3.2.1 Apparatus
The apparatus used for non-adiabatic Dewar calorimetry should:
• Consist of a glass or stainless steel Dewar flask with a loose-fitting cork, PTFE or rubber
bung (discard the screw top supplied with the flask). Glass vessels typically have lower
heat losses than stainless steel vessels but have no pressure capability.
• Be fitted with an adequate stirrer and a temperature probe connected to an electronic
monitor capable of recording temperature to ±0.3°C.
• Be located in a fume cupboard, but shielded from the air flow through the cupboard (air
currents will increase the heat loss); shielding is also required for protection against
possible explosion.
• Include an internal low power (20 – 30 W should suffice) immersion heater. It is
important that the heater has a controllable and readable DC power control and that the
“heated section” of the heater is fully immersed in the liquid (see Appendix 6 for a
consideration of precautions).
• Be connected at one outlet of the bung to a gas measurement device using a tube of at
least 6 mm internal diameter – this may be a gas burette, ‘over water’ using an inverted
cylinder (silicon oil, or similar, may be used instead of water, for water soluble gases), or
a ‘flowmeter’ type device
• Have the Dewar flask and head loosely wrapped in insulation (e.g. cork sheath, mineral
wool blanket, etc) to further reduce heat losses.
Typical equipment used in testing is shown in Figure 3. In this photograph the thermocouple
is shown in a glass sheath, and a simple ‘over water’ gas measuring device is shown. This
‘over water’ gas measurement method requires careful technician monitoring to yield rate
data and has limitations if significant quantities of gas are generated. A better instrument is
an automated gas burette (Figure 4) which is linked to the data acquisition computer and
yields both rate and total gas flows.
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Glass Dewar
vessel
Rubber
bung
Addition
line
Temperature
probe
(sheathed)
Agitator
Simple gas measuring equipment
Immersion
heater
Figure 3: Typical Equipment for Non-Adiabatic Dewar Testing (shown without shield
for illustration purposes)
Figure 4: Automated Burette for Gas Measurement
It is critical that all components of the apparatus are chemically compatible with the materials
to be tested (for example, hydrogen fluoride solutions would be incompatible with glass). If
not, the materials of construction of the flask may influence the test result.
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3.2.2
Calibration Procedure
Dewar Vessel Calibration
Before use, the Dewar apparatus must be calibrated to determine its phi factor (thermal
inertia) and heat loss. ALL Dewar vessels, even from the same supplier, will have varying
heat losses. It is therefore important that every Dewar vessel used is calibrated according to
this procedure.
The calibration is performed by running heating / cooling curve trials at an appropriate
starting temperature (typically around 90°C) using liquids representative of those used
normally on the site under test conditions (that is, around 70% vessel fill, agitation on, shield
in place). Liquids such as water or a light hydrocarbon such as toluene (boiling point 111°C)
may be considered to reasonably represent other aqueous or organic feeds. The calibration
would typically involve the following steps:
•
•
•
•
Assemble the apparatus, charge with a known quantity of liquid (around 70% vessel fill)
and insert the bung to the top of the flask. The specific heat capacity of the calibration
liquid should be known (this can be obtained from reliable internet literature sources
such as the NIST Chemistry Webbook[4]).
Connect to the monitoring equipment and switch on the stirrer.
Use the internal immersion heater to raise temperature in discreet steps to the start
value (typically across the intended measurement range, normally up to 90°C (but not
exceeding the liquid boiling point)). The heating steps should be approximately two or
three steps. Once the start temperature has been reached, turn off and electrically
isolate the heater and allow the contents to cool naturally.
Record the sample temperature, ambient temperature and heater power during heat up
and cool down.
Rapid cooling of a vessel may be indicative of a loss of vacuum in the flask – rendering it
useless for calorimetric purposes. “Failed” vessels can normally be rapidly identified by filling
them with hot water from a kettle and feeling if the outside wall of the flask gets warm within
1 or 2 minutes. Such vessels should be discarded. It is not realistic to calibrate each vessel
before each use. It is suggested that the “hot water touch test” is performed between each
use, but the vessel should be subject to full re-calibration after several experiments.
The calibration procedure is likely to take approaching 24 hours and should ideally be
started in the morning so that the cool down period can occur overnight (this phase does not
require constant supervision).
The data obtained in the test can be used to determine the approximate heat capacity and
heat loss value of the Dewar. The calculations to determine these parameters are described
in Appendix 3. Records should be maintained of all Dewar vessels used, their usage history,
calibration dates and calibration results.
Instrument Calibration
The Dewar calorimeter is a key item of safety apparatus with a role in the avoidance of
adverse reactions; as such it must be treated with respect and subjected to periodic
calibration. On a regular basis it must be visually examined for deterioration, heaters must
be tested and instruments calibrated against traceable standards. The frequency of this
work will depend upon the type of chemistry under examination. Typically, temperature and
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pressure sensors, gas flow meters and electrical calibration heaters should be calibrated
annually, or after any significant change or repair.
When any component of the system is replaced, or new equipment added, it should be
calibrated before first use.
3.2.3
Test Protocol
From a knowledge of the chemicals in the waste and in the receiving tanks (compositions
and concentrations), the relative quantities likely to give the most severe reaction should be
tested. If there is insufficient information, or knowledge, on potential reactions that may
occur, equal proportions of the two materials should be tested. The Dewar flask should be
filled to approximately the same fill as used for the calibration testing. An optimum fill level of
70% should be targeted as this maximises the sensitivity of the test.
With the calibrated Dewar open, charge it with the liquid representative of the storage tank
contents (or the least mobile of the two in the case of viscosity issues), install the bung with
all of its measurement devices, start agitation, and commence monitoring.
If the plant mixing operation is likely to start at a temperature above ambient, adjust the
temperature within the Dewar to this value using the internal immersion heater, and
equilibrate the liquid to be added to the same temperature – this will avoid temperature
changes during mixing owing to sensible heat effects, which would otherwise complicate the
interpretation of the test results.
With the bung in place and all monitoring equipment operational, add the second liquid via
the feed port in the bung and monitor behaviour for a minimum of 20 minutes.
•
•
•
If an exotherm is detected, allow it to proceed to completion (i.e. until there is no
further temperature rise)
If no exotherm is detected, increase temperature by 15 K and monitor for another 20
minutes
If no exotherm is detected, increase temperature by 15 K and monitor for another 20
minutes
These temperature steps are designed to accelerate any “slow-to-initiate” reactions, thereby
facilitating their identification. An illustration of the effectiveness of this approach is
presented in Appendix 1.
Monitor gas generation throughout test; if gas is detected despite absence of increasing
temperature, allow test to continue until completion (i.e. until there is no further gas
measured).
At test completion, allow the mixture to cool and visually examine the nature of the residual
liquid. If significant gas has been collected, this should be tested using standard laboratory
analytical techniques in order to assess its nature (if this is not evident from the chemical
nature of the species in the formulation).
3.3
Adiabatic Calorimetry
An improvement to the simple Dewar flask approach discussed in Section 3.2, is to locate a
glass Dewar or closed stainless steel Dewar within a fan assisted oven, controlled to follow
the temperature in the Dewar to within ±1 K. This approach significantly reduces the heat
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loss element of the basic configuration discussed above, although there remains the thermal
inertia of the apparatus (the ‘phi factor’). An adiabatic test, that is with practically zero heat
loss, can better simulate large tanks. The exact scale of applicability of an adiabatic test will
depend on its phi factor and heat loss. For a fully adiabatic test, the heat losses are zero, so
the limit of scale-up becomes linked only to the phi factors of the small scale and larger scale
equipment. The apparatus can be designed as pressure resisting (stainless steel flasks
only), able to be operated in a closed arrangement to maximise the adiabaticity, or as an
atmospheric pressure device operated as an ‘open cell’ set-up connected to the gas
measuring apparatus.
If using a stainless steel vessel, such as a Dewar, the pressure can be measured instead of
the gas flow. Open cell tests produce slightly inferior results as the venting of volatile vapour
components causes heat loss – this becomes very significant when approaching the boiling
point of the more volatile components. The UN H.2 “Adiabatic Storage Test” procedure
provides useful background information on the principles and execution of such studies[5]. If
using a closed cell adiabatic test, it should be recognised that any evolved gases may
dissolve in the liquid under higher pressure conditions. This can lead to under-estimation of
the evolved gas quantity and rate and would normally require supplementary data obtained
under lower pressure, or open cell, conditions.
The design details of pressure resisting adiabatic test facilities are lengthy and complex
owing to the significant pressure hazard which can result. Further discussion of design
concepts is beyond the scope of this guidance although the vessels, or their housing or
shield, must be adequately designed to withstand the maximum pressures which can be
developed in the vessel during closed cell operation. A number of adiabatic pressure
calorimeters are commercially available, including the ADC II (see Figure 5), the Vent Sizing
Package II (VSP), and Phi Tec II. The description below therefore concentrates on the
atmospheric pressure, open cell arrangement.
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Mechanical Agitator
Instruments (vent
systems, temperature
sensor, pressure
transducer, heater)
Stainless Steel Dewar
Adiabatic Enclosure (oven)
Data acquisition and control system
Figure 5: Example of Adiabatic Pressure Dewar Calorimetry Equipment
For details of the apparatus and how to use and interpret the results, refer to Appendix 7.
3.4
Acceptance Criteria
The acceptability of mixing a waste stream will be determined based on the thermal and gas
/ pressure effects observed in the test, in conjunction with an evaluation of the plant vessel
characteristics. A flow diagram depicting the assessment procedure is provided in
Figure 6. The process starts by evaluating whether the test is a valid simulation either
through adiabatic calorimetry testing (Box 1) or by using a Dewar test vessel with lower heat
losses than the plant vessel (Boxes 2, 3 and 4).
Bulking should be considered acceptable / allowable if:
1
• The temperature rise obtained in a valid simulation test is less than 10 K (Box 5) ,
and
• The maximum temperature observed in a valid simulation is not within 50 K of any
known decomposition onset temperature for any significant component in the wastes
to be mixed (Box 6). The heat steps in a Dewar test (adiabatic or non-adiabatic) will
provide valuable information on potential decomposition reactions for use in this
assessment, and
1
However, you should note that where any unexpected temperature rise is detected, further
investigation may be required.
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•
Any observed gas evolution does not exceed the established threshold criteria of 25
cm3.kg-1 or lower for specific, hazardous, gases (Box 7) (Section 2.3.1).
For an invalid simulation (where the plant heat losses are smaller than the test vessel heat
losses), non-adiabatic data is more difficult to interpret. Normally, in this case, any observed
temperature rise to completion of significant magnitude (for example greater than 2 K rise)
should disallow mixing (Box 8). In this case, a valid simulation should be sought through the
conduct of a further adiabatic mixing simulation (Box 10). The resulting data can then be
reassessed (starting at Box 5) using the criteria described above. If a valid simulation cannot
be done then mixing should not proceed (Box 11).
No
Adiabatic 1
Test?
Heat Loss of
Test Vessel 2
Heat Loss of
Plant Vessel 3
Yes
No
Temperature rise less
than 10 K
Yes
5
Yes
No
Peak temperature more than
50 K below any known
component decomposition
temperature?
6
Simulation
Valid?
4
No
Yes
No
Amount of gas evolved below
25 cm3.kg-1 threshold
7
No
Temperature rise significant
(> 2 K)?
8
Yes
Provisionally accepted
for careful and
monitored offloading 9
Unacceptable
Situation
11
Yes
No
Can an 10
Adiabatic Test
be performed?
Yes
Figure 6: Assessing the Acceptability of Mixing Based on Plant and Test Data
If any of the thermal and gas criteria are violated, this represents an unacceptable mixing
process (Box 11). In this case, the mixing strategy (for example, the vessel to which the
waste will be discharged, the masses involved, pre-treatments, etc) should be re-evaluated
and the exercise repeated.
3.5
Conclusions from Laboratory Compatibility Studies
Applying the Results to Plant
The purpose of the compatibility testing is to identify the risk of adverse reactions. However,
a test result involving an exothermic temperature rise or gas generation does not
automatically preclude the handling of a particular waste, but it does require a more rigorous
review of the procedures and mechanisms. For example (and where the site permit allows)
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strategies such as choice of a different receiving vessel, quarantining and direct treatment,
dilution of the incoming stream (itself possibly an exothermic event), slower or staged
offloading, etc, can be considered. It is important that this revised receiving procedure is
carefully considered by responsible and competent personnel, and undertaken by trained
staff fully aware of the dangers of incorrect adherence.
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4.
CONCLUSIONS
Any waste treatment site where exothermic or gas generating adverse reactions could
potentially occur owing to delivery or treatment upsets, should be considered to be a
chemical site and subjected to the same considerations. As a result, the procedures and
equipment used for all operations should be designed with both normal and abnormal
conditions in mind. In particular, risk assessment of the potential for adverse reactions
needs to be comprehensive and focused, both for general site operation and for each
individual waste.
A key defence for the avoidance of adverse reactions in the storage tanks used for ‘bulkingup’ is laboratory compatibility testing. This test involves mixing a representative sample of
the waste load with a representative sample from the intended tank, and checking for
possible temperature changes and gas evolution. In order that the test results may be
considered a valid simulation of plant behaviour, the heat losses from the laboratory test
apparatus should be similar to or less than those of the plant equipment. This means that
ideally, an adiabatic calorimeter should be used for the test, but at least a low heat loss
Dewar arrangement should be employed.
The need for compatibility testing, both at waste pre-acceptance and acceptance, is
addressed in this document. Avoidance of compatibility testing by the use of desk-top
calculation techniques and material conformity tests is possible where sufficient information
is available to have confidence in the results. Flowcharts present the decisions to be made.
However, where there is any shortage of data leading to doubt, or where standard chemical
tests at acceptance indicate a change in composition or concentration, compatibility testing
will be required. In many cases, it will be preferable to conduct compatibility testing in
parallel with other work in order to minimise the time delay in obtaining approval for waste
offloading.
By working through this guidance in the structured way described, operators should be in a
position to identify and thus avoid potential adverse reactions when bulking wastes together.
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APPENDICES
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A.1
APPENDIX 1 – HEAT LOSSES OF SMALL AND LARGE SCALE VESSELS
Compatibility testing is important for the discovery of unintended chemical reactions, and to
measure their consequences. Chemical reactions occurring in waste processing can evolve
(exothermic) or consume (endothermic) heat. They can also give rise to gas formation.
Exothermic processes are generally of greater concern because the evolved energy can
cause self-heating and at higher temperatures reactions occur faster, thereby leading to
exponential self-acceleration; sometimes known as a ‘runaway reaction’. Whilst the
unexpected self-heating alone may be undesirable, it is the knock-on consequences that are
of particular concern; possible boiling of the mixture and vapour generation, formation of gas
from the reaction, potential decomposition of species with further gas formation – and these
gas-phase emissions may be flammable, toxic or just able to create pressure in equipment
not designed for this.
Compatibility testing must be conducted carefully if the results are to reliably indicate the
behaviour of the mixed waste stream in plant equipment. The equipment used must be able
to measure temperature progression and the liberation of gas, but it must do this whilst
simulating the characteristics of the large scale plant equipment. The question of scale-up of
heat losses from test equipment is a key factor here.
The heat released from an exothermic chemical reaction will be distributed:
•
To heat-up the reaction fluid and cause self-acceleration
•
To heat-up the walls of the container in which the reaction is conducted (i.e. lost energy)
•
To heat up the local environment (i.e. lost energy)
Clearly, as more heat is lost from the system, the self-heating will be lower, and the
unwanted consequences will be less extreme. In judging the acceptability of mixing, it is
therefore important that any laboratory testing simulates plant equipment in order for the
observations to be valid. However a problem arises in that heat losses from small scale test
apparatus are often disproportionately greater than those from large scale plant equipment.
This is due to two main reasons; the surface area to volume effect, and the vaporisation
loss.
The surface area to volume effect may be illustrated by considering three different scales of
vessel as in Table 1 below. This shows the relationship between the surface area of the
vessel (that is, the surface area through which heat can be lost to the surroundings) and its
volume.
Vessel Volume (litres)
2
Vessel Surface Area (m )
2
3
Area:Volume Ratio (m /m )
0.05
50
50000
0.0075
0.75
75
150
15
1.5
Table 1: Surface Area to Volume Ratio Comparison for Small and Large Vessels
As the heat lost to the walls and to the environment is a function of the surface area,
whereas the heat generation is dependant upon the volume of reactants present, a smaller
surface area to volume ratio indicates lower heat losses and a closer approach to adiabatic
(that is, zero heat loss) conditions. Thus, observations made in a 50 cm3 flask will be
considerably milder than those that would be experienced in a 50 m3 vessel.
The surface area to volume effect is more graphically illustrated in Figure 7. This shows the
results of modelling a reaction between equal quantities of hydrochloric acid and sodium
Environment Agency / Health and Safety Executive Joint Guidance on Compatibility - November 2011
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hydroxide solution in well stirred vessels with no applied cooling (just natural heat losses). In
passing from 0.1 litre (lowest curve) to 100 m3 (highest curve) the peak temperature in the
vessel is seen to rise from 32°C to 119°C and the rate of temperature rise to seen to
increase markedly. Thus, this reaction in a laboratory glass beaker could be concluded to
have no significantly adverse consequences, whereas when conducted in a large plant
vessel, the liquid boiling point is exceeded. This artificial modelling exercise is only to
illustrate the phenomena; the data in these curves must not be extrapolated to any plant
studies.
100000 L
Modelled temperature rise for various scales.
Modelled using a stoichiometric aqueous acid /
base reaction in a well stirred vessel with
natural heat loss (no forced cooling)
10000 L
1000 L
100 L
1L
10 L
0.1 L
Figure 7: Example Peak Temperature in Different Size Vessels during Acid/Base
Reaction (Source: Ian Priestley, Syngenta)
The second aspect, heat loss due to vaporisation, can occur more readily in open test
equipment than in closed plant equipment. In this case, significant heat loss can accompany
vaporised fluids owing to the latent heat of vaporisation; whilst a reduced amount of heat
loss will accompany gas flows without liquid vaporisation, because gas phase heat capacity
is small in comparison. The difference between test apparatus and plant vessels is unlikely
to be as severe in this respect, certainly when compared to the effect of the surface area to
volume change. However, testing conditions should aim to minimise the losses from this
gas-phase effect.
This objective of minimising heat losses in test apparatus leads to a requirement for the use
of adiabatic (or quasi-adiabatic) calorimeters for assessing compatibility. This is reinforced
by considering the characteristics of typical laboratory apparatus used on an open bench, as
summarised in Table 2. In this example, the reaction temperature starts at 80°C, the
ambient temperature is 20°C, the vessel is 80% full and the fluid is well agitated. Again, this
data is for illustrative purposes only and must not be used to scale between techniques.
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Loss Time for 80°C to Equivalent Plant
79°C (min)
Vessel (m3)
Vessel
Volume (ml)
Heat
(W.kg-1)
Test Tube
10
381
0.18
Beaker
100
209
0.33
Flask
1000
34.9
2
<0.5
Glass Dewar
500
1.95
14.3
<4
Glass Dewar
1000
1.04
53.7
<10
Glass Dewar
in a Shield 1000
Oven
0.26
247
<25
Table 2: Heat Loss Comparison for a Range of Laboratory and Plant-Scale Vessels
The Dewar (or vacuum) flask is the first apparatus that can start to be called a calorimeter.
Dewar flasks can be made from glass or from stainless steel, can be closed or open
containers, and can be operated on a bench or in a shield oven. The use of a controlled
temperature shield oven to match the internal temperature of the Dewar improves the heat
retention over that achieved by the vacuum construction alone; this may then be called an
adiabatic calorimeter. Commercial Dewar test apparatus is available, or home-made
installations can be constructed, each with varying ability to simulate different scales of plant
equipment.
This scale-up comparison shows that even an adiabatic calorimeter such as a Dewar vessel
located within a shield oven is unable to fully simulate the heat loss characteristics of the
largest vessels found in the waste industry (that is, those greater than 25 m3). However, for
fast reactions, or those whose kinetics can be increased, the Dewar technique is able to
detect adverse exotherms, even with these shortcomings. It should also be said that the
variability in performance found in commercial Dewar flasks means that these quoted heat
loss equivalence figures are only approximate; a factor which is addressed in the
recommended test protocol section (Section A.3).
This comparison also restricts itself to well stirred vessels which will consequently have a
uniform temperature profile across the fluid. In a non-stirred large tank where reaction
occurs only around the incoming liquid jet, the surrounding quiescent fluid will act as
insulation and the heat loss will be considerably reduced, leading to more closely adiabatic
behaviour which is unable to be simulated by the best calorimeter. Equally, highly viscous
fluids and thick slurries will behave similarly and scale-up will be difficult to simulate from
even the best test apparatus. Finally, reactions with very long induction times (for example,
weeks) are also difficult to detect in laboratory apparatus and successfully scale-up. Highly
specialised test apparatus may offer some improvements to address these shortcomings,
but a perfect adiabatic calorimeter is impossible and with practical techniques, there will
always be some limit to the scale-up capability.
Accepting that there are limitations, practical recommendations for calorimetry apparatus
and test protocol for the waste industry is necessary. By using low priced apparatus, and
adopting a stepped test protocol and focused results interpretation, a practical test technique
is able to detect unexpected reaction behaviour and provide a reasonable estimate of the
magnitude of the consequences.
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A.2
APPENDIX 2 – DETAILS ON REACTIVITY PREDICTION METHODS
Predicting potential chemical reactions requires both good resources and a sound chemical
knowledge. Some interactions can be readily foreseen. Examples include reactions of
• acids with bases
• nitric acid with organic substances (compounds based on carbon and hydrogen)
• carbonates (such as potassium, calcium or sodium carbonate) with acids
• other specific chemical groups identified in Section 2.2
Reactivity information on substances can be obtained from a variety of sources including:
• Safety Data Sheets (SDS). Sections 7, 9 and 10 of an SDS are particularly relevant as
they listo Section 7 – Handling and Storage
ƒ Storage
conditions
–
below
ambient
temperature
storage
recommendations may indicate instability (decomposition risk) at ambient
temperature or just above
o Section 9 – Physical and Chemical Properties
ƒ pH – can be used for classify a substance as an acid or base (pH < 5 =
acid, pH > 9 = base, pH 5 – 9 is considered neutral).
o Section 10 – Stability and Reactivity
ƒ Stability (see comments for storage conditions) – beware many SDS are
deficient on detailed data, often simply noting “stable under normal
conditions of use”
ƒ Conditions to avoid (may include light, shock, temperature or pressure).
Temperature limits noted on an SDS may be limited to small scale and
may result from testing of pure, uncontaminated, samples. Such data
should be considered indicative, and should be investigated further for the
specific bulking scale and conditions by a competent person.
ƒ Materials to avoid – an explicit statement of potential reactivity
ƒ Hazardous polymerisation or hazardous decomposition
• Other (print) literature sources
o Brethericks’ Handbook of Reactive Chemical Hazards[6]
o CCPS Essential Practices for Managing Chemical Reactivity Hazards[7]
o Guidelines for Safe Storage and Handling of Reactive Materials[8]
o Other hazards substances’ texts[9,10,11,12]
• Chemical Reactivity Worksheet (CRW) – a free software download available from the
National Oceanic and Atmospheric Administrations (NOAA)[13]. This application will
predict any potential reactivity from a user selected combination of chemicals.
• General Internet searching
The most valuable and accessible of these resources to waste operators are SDSs,
Brethericks’ Handbook and the Chemical Reactivity Worksheet.
As wastes, by their nature, will contain a variety of chemicals, it is only feasible to focus on
those present at higher concentrations. A limit of >1% (weight basis) is considered
appropriate for reactivity screening for exothermic reaction hazards (or a lower threshold of
0.1% for very toxic or carcinogenic components). However, it should also be recognised that
gas generation can be appreciable, and hazardous, for materials at lower concentrations.
For components that can generate gas, a lower consideration threshold should be applied
(0.1% or less, depending upon the likely nature of the gas).
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A.2.1 Estimating Gas Generation Potential
If a component of a mixture is found, from the literature, to be capable of generating a
permanent gas, the quantity of gas can generally be calculated providing the balanced
chemical equation for the reaction can be written. The procedure to do this is outlined below,
using a theoretical example. The calculation below illustrates a stark example which would
be considered a treatment process but is shown for completeness to exemplify the
procedure. Such a case would not be appropriate for a bulking operation since
acknowledgement of a reaction of such magnitude would always be considered as a
treatment process. Strictly, such treatment operations are outside of the scope and
applicability of this guidance.
Example
A laboratory Winchester bottle containing 2 kg of phosphorous oxychloride (phosphoryl
chloride) is to be discharged into an aqueous waste IBC containing, primarily, water. The
volume of the tank is 1000 litres and the quantity of aqueous waste present is 500 kg.
During pre-acceptance, a review of data on phosphorous oxychloride in Brethericks’
Reactive Chemical Hazards Database[6] reveals that it can react with water producing an
“exothermic hydrolysis reaction, which may proceed with enough vigour to generate steam
and liberate hydrogen chloride gas”. Ordinarily at this stage, the identification of a potential
major adverse reaction would disallow bulking as this would be regarded as a treatment
process. However, the calculation is continued below to illustrate the procedure.
Stages of calculation
1.
Write a balanced chemical equation for the reaction between phosphorus
oxychloride and water
From the literature, we know that HCl gas is a product of the reaction. With chemical
knowledge, we can predict that the phosphorous oxychloride will be hydrolysed
(adding oxygen and hydrogen) and will lose its chlorine. Phosphoric acid is the logical
product of the hydrolysis. The equation is therefore:
Phosphorous oxychloride + water
→ hydrogen chloride + phosphoric acid
POCl3 + H2O → HCl + H3PO4
This equation needs to be balanced so that the same number of atoms are on both
sides of the equation. This can be achieved with three moles of water giving 3 moles
of HCl.
POCl3 + 3.H2O → 3.HCl + H3PO4
2.
Determine which of the two reactants is limiting in the mixing operation
proposed
The amounts of each reactant in the mixture are 2 kg of phosphorous oxychloride
(molecular weight = 153.3 g.mol-1, 13.05 mol) and 500 kg of water (molecular weight =
18 g.mol-1, 27778 mol). The water is seen to be in significant excess so phosphorus
oxychloride is the limiting reactant.
3.
Calculate the number of moles of gas that will be generated by the mixing
operation.
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We know that phosphorous oxychloride is the limiting reactant and from the balanced
equation, we can see that one mole of this will generate 3 moles of hydrogen chloride
gas. Therefore, 13.05 mol of phosphorus oxychloride will generate 39.15 moles of HCl
gas.
4.
Calculate the volume of gas that will be generated (assuming there is no
dissolving of the gas in the liquid)
One mole of any gas occupies 22.4 litres at standard temperature and pressure (STP
which is 0°C (273 K), 1 bara). This volume of gas increases with increasing
temperature as gas expands. Assuming, in the current case, that the reaction occurs
at 20°C, the volume of gas occupied by one mole can be recalculated:
22.4 litres.mol-1 x ((273+20) / (273) = 24.0 litres.mol-1
39.15 moles of HCl will therefore occupy 939.4 litres (0.94 m3) at ambient conditions.
5.
Make decisions on the acceptability of the gas release
In very simple terms, we can see that for 502 kg of mixed waste, we would have 939.4
litres of gas generation. This equates to 1.87 litres.kg-1 (or 1870 cm3.kg-1). This is
significantly above the 25 cm3.kg-1 limit for a bulking process. The IBC will have a free
space above the liquid of around 500 litres (since it has a total volume of 1000 litres
and it is filled with around 500 litres of liquid). The pressure in the vessel caused by the
release of gas can be crudely estimated to be:
1.0 bara (start pressure) x ((939.4 litresof HCl gas + 500 litresinitial void space gas) /
500 litresvoid space) = 2.88 bara
Most plastic IBC’s will have a very modest pressure capability and this quantity of gas
and the resulting pressure, if it was sealed, is likely to cause rupture or at least
splitting. On this criterion, this is unlikely to be acceptable.
In the case of the reaction assessed above, there are a number of complicating factors
which may influence the observed effects. The most significant is the solubility of HCl gas in
water. The above calculation assumes zero solubility and stoichiometric gas generation,
which is a highly conservative approach (in this case the HCl would be fully soluble in the
water generating 0.3% hydrochloric acid). A second key factor is the temperature rise that
will occur owing to the exothermic nature of the reaction (not calculated); this will adversely
affect the predicted pressure owing to the increased water vapour pressure, and will reduce
the conservatism due to reduced gas solubility. In this case, ignoring any solubility of the
gas in the liquid during the assessment would lead to a ‘worst case’ conclusion of the
achievable peak pressure.
For reactions that are “instantaneous” (that is, reactions which occur as soon as two
materials come into contact), the rate of addition can be used to calculate the rate of gas
generation. This may not be conservative for reactions which occur when the reaction is not
instantaneous.
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A.2.2 Predicting Heat of Reaction
Once the balanced equation for a potential chemical reaction has been established, it may
be possible, from calculation or literature, to establish the heat of reaction. Fundamentally,
simple bond energy calculations (available in open literature) can be performed to estimate
the heat of reaction (Hess’s method). This can be refined by using the heats of formation
(ΔHf) of the products and reactants.
Hess’s Method:
ΔHf Method:
ΔHr = ∑(Energy of bonds broken) - ∑(Energy of bonds made)
ΔHr = ∑(ΔHf Products) - ∑( ΔHf Reactants)
Given that heats of formation are largely unavailable for novel molecules, prediction methods
may be used for estimation such as the CHETAH[14] computer program developed by the
ASTM. The programme utilises Benson’s method of group contributions and facilitates heat
of reaction calculation from its predicted heats of formation for products and reactants.
Despite some limitations (regarding predictions for salts, the absence of some functional
groups and the use of only gas phase data) CHETAH provides a useful preliminary tool in
predicting the heat of reaction based solely on chemical structure.
The relative accuracy of the various estimation methods, compared with reaction calorimetry
measurement, for the esterification reaction between methanol and acetic anhydride can be
gauged from Table 3, although no general conclusion can be taken from this example where
the experimental result is milder than the predicted ones. In this case, the CHETAH data is
based on gas phase thermochemistry data whereas the experimental and heat of formation
predictions are based on liquid phase data.
O
H3C
H3C
O
O
+
2 H3C
2 H3C OH
OH2
CH3
O
Method
+
O
ΔHr (kJ mol-1)
Reference
Heat of Formation Data
-75.8
ΔHf data from NIST[4] for liquid
phase reactants and products
CHETAH Prediction
-83.0
[14]
Reaction Calorimetry
-67.0
Measured using Mettler Toledo
RC1e reaction calorimeter
Table 3: Heat of Reaction for Methanol / Acetic Anhydride Esterification
Typical exothermic heats of reaction of liquid systems can range up to -500 kJ mol-1 (for
example aromatic nitro-group reduction); although less relevant reactions can be higher (for
example, combustion).
Once the heat of reaction has been estimated, it can be readily converted into a theoretical
adiabatic temperature rise (ΔTad) – this is the temperature rise that will occur if the reaction is
performed without heat loss, assuming there are no secondary or side reactions initiated at
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elevated temperature. From this, the theoretical maximum reaction temperature is readily
computed.
ΔHr.N = m.Cp.ΔTad
Where:
ΔHr
N
m
Cp
ΔTad
=
=
=
=
=
Overall heat of reaction (kJ.mol-1 of limiting reactant)
Number of moles of the limiting reactant (mol)
Mass of the entire reaction mixture (kg)
Heat capacity of the reaction mixture (kJ.kg-1.K-1)
Adiabatic temperature rise (K)
Once the adiabatic temperature rise is known, it can be compared to the allowable
temperature rise criteria shown Figure 2, Section 2.3.3 and decisions on the acceptability of
bulking made accordingly.
In the waste bulking and treatment industry, concentration effects become particularly
important. The addition of a relatively small quantity of reactive material into a very large
volume of existing waste, whilst reacting very exothermically, may only generate a very small
temperature rise owing to the substantial dilution. However, this dilution factor will depend
heavily on whether the reaction occurs locally, or uniformly throughout a mixture. Agitation,
viscosity, miscibility and other factors will influence the extent to which the reaction zone
remains localised.
It is possible that more than two components may react together producing exothermic heat
or gas. This risk becomes increasingly likely in complex multi-component mixtures. In
addition, there may be unanticipated side reactions that are not readily identifiable. In any
case where doubt exists, compatibility testing should be performed.
A.2.3 Heats of Mixing
Whilst the previous section has considered chemical reaction between species, another
factor, particularly relevant in bulking operations, is the heat of mixing in the absence of
reaction. There are two aspects to this; firstly the sensible heat change that occurs when
two inert materials at different temperatures are mixed, and secondly the temperature
change resulting from heat of solution effects.
As an example of the first case, equal quantities of water at 10°C and 30°C respectively,
when adiabatically mixed, will have a final temperature of 20°C. This results from a heat and
mass balance calculation which can be simply expressed as:
Where:
m1, m2
Cp1, Cp2
t1, t2
tmix
m1.Cp1.(tmix – t1) = m2.Cp2.(t2 – tmix)
Mass of cold and hot liquid, respectively (kg)
Average specific heat capacity of cold and hot liquid (kJ.kg-1.K-1)
Initial temperature of cold and hot liquid, respectively (°C)
Final liquid temperature after mixing (°C)
Heat of solution, occurs when solutes are dissolved in solvents; heat of mixing then occurs
when solutions of differing concentration are mixed. Many materials have exothermic heats
of solution (for example, mineral acid and bases in water), whilst a few are endothermic (for
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example, sodium chloride in water, methanol in benzene). Unfortunately, heat of solution is
generally not linear with respect to concentration, therefore the calculation of temperature
changes caused by mixing solutions of different concentration becomes complicated.
Literature data can often be found for heat of solution, usually at ‘infinite’ dilution, whilst for
more common materials, heats of solution as a function of concentration may be available.
Alternatively, heats of formation of solutions of varying concentration, or enthalpy/
concentration charts may be available in the literature.
Some examples of heat of solution are given in Table 4, with significant variation being
observed between compounds. In the case of materials such as sodium chloride, the heat
of solution is sufficiently small as to be ignored in a practical situation (and indeed this
compound is endothermic on dissolution in water), whereas the acids and sodium hydroxide
can result in significant self-heating on mixing. The data for hydrochloric acid illustrates the
non-linear profile with concentration often observed; it is this aspect that complicates the
calculation process.
Sodium Chloride
Sodium Hydroxide
Nitric Acid
Sulphuric Acid
Hydrochloric Acid
Hydrochloric Acid
Hydrochloric Acid
Hydrochloric Acid
Hydrochloric Acid
Hydrochloric Acid
Dilution
(molwater/molsolute)
400
400
400
400
400
30.0
17.7
10.0
6.8
3.7
Heat of Solution
(kJ.mol-1solute)
+4.2
-42.6
-32.9
-76.7
-74.4
-72.5
-71.3
-69.4
-67.1
-60.2
Table 4: Example Heats of Solution in Water of Common Materials[27,28]
Using the heat of solution data as a function of concentration, the energy released on mixing
and the consequent temperature change, can be calculated. The following demonstrates
the procedure.
Example
50 kg of 35% w/w hydrochloric acid is to be mixed with 50 kg of 10% w/w hydrochloric acid,
both materials being at the same initial temperature.
The result of mixing these amounts is 100 kg of 22.5% w/w hydrochloric acid
Converting these concentrations to molar values (molecular weight of hydrogen chloride =
35.46 g.mol-1; water = 18 g.mol-1) gives the values in Table 5.
(% w/w)
35
10
22.5
Concentration
(% mol/mol)
21.5
5.3
12.8
molwater/molsolute
3.7
17.7
6.8
Heat of Solution
(kJ.mol-1solute)
-60.2
-71.3
-67.1
Table 5: Concentration Calculations for Hydrochloric Acid Mixing Example
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The amounts of materials involved can also be calculated:
50 kg 35% w/w hydrochloric acid is 494 mol hydrogen chloride and 1806 mol water
50 kg 10% w/w hydrochloric acid is 141 mol hydrogen chloride and 2500 mol water
100 kg 22.5% w/w hydrochloric acid is 635 mol hydrogen chloride and 4306 mol water
Using the heat of solution values above, the enthalpy change in this mixing operation is:
(-67.1 x 635) – ((-60.2 x 494) + (-71.3 x 141)) = -2835 kJ (exothermic)
Average specific heat capacity of 22.5% hydrochloric acid at 25°C is 3.89 kJ/kg-1.K-1
This leads to a calculated adiabatic temperature rise of 2835/(3.89*100) = 7.3 K
Thus, mixing equal proportions of 35% and 10% hydrochloric acid at the same initial
temperature would lead to a predicted temperature increase, from 20°C to around 27.3°C.
As the temperature increase is within the limit of 10 K, this does not qualify as an adverse
reaction within the present guidance (Figure 2).
Using an alternative approach for mixing 40% w/w sodium hydroxide solution with 10% w/w
sodium hydroxide to form a 25% w/w solution leads to a predicted temperature rise of
around 12.2 K; this mixing operation would be precluded by the present limits.
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A.3 APPENDIX 3 – DETERMINING HEAT LOSS & PHI FACTOR OF NON-ADIABATIC
DEWAR VESSELS
Using the procedure described in Section 3.2.2, it is possible to determine the heat capacity
of the Dewar vessel (from the heating steps) and the heat loss coefficient of the Dewar. A
schematic calibration profile is displayed in Figure 7.
Heater Power (W) / Temperature (°C)
Heating
Phase
Cooling
Phase
ΔTH3
dT/dtC1
ΔTH2
ΔTC1
ΔTH1
tH1
tH2
tH3
Time (seconds)
Sample Temperature (°C)
Heater Power (W)
Ambient Temperature (°C)
Figure 7: Calibration Profile for Non-Adiabatic Dewar Vessel
A.3.1 Determination of Vessel Heat Capacity
From the heating steps in the calibration procedure, the heat capacity of the vessel can be
determined. Each heater activation causes a temperature rise. The difference between the
energy applied from the heater and the energy absorbed by the liquid (determined from the
temperature rise) provides a direct measurement of the heat capacity of the vessel (HCvessel
in J.K-1). The calculation is displayed in the equation below.
HCvessel = (Heat injected from heater) – (heat absorbed by the liquid)
HCvessel = ((Q.t) – (m.CpAV.ΔT)) / ΔT
Where:
HCvessel
Q.t
m
the heat capacity of the vessel (the energy required to increase the
(empty) vessel temperature by 1°C) (J.K-1).
the heater power (voltage x current for DC power supply units) and
time (t in seconds) during activation of the heater. This is the overall
energy entering the system from the calibration heater (J)
the mass of liquid in the vessel (kg)
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CpAV
the average heat capacity of the liquid in the vessel at the temperature
range of the measurement (J.kg-1.K-1)
the temperature rise measured in the vessel during the heat step (K).
See ΔTH1, ΔTH2, etc in Figure 7.
ΔT
The heat capacity of most standard glass Dewar vessels in the 500 ml to 1000 ml scale is
typically 50 to 200 J.K-1. This value depends on the inserts present in the vessel. For
1000 ml stainless steel Dewar flasks, the value is somewhat higher (normally in the range of
200 – 300 J.K-1). The use of large inserts (for example, heater, stirrer, temperature probe) in
the vessel will compromise its heat capacity and hence the size of inserts should be
minimised wherever possible.
Example
A 500 ml glass Dewar was filled with 350 ml (276.5 g) of methanol at approximately 16.5°C.
The specific heat capacity (Cp) of methanol is 2.503, 2.565 and 2.705 kJ.kg-1.K-1, at 20°C,
30°C and 50°C respectively. A magnetic stirrer, a thermocouple and a calibrated electrical
immersion heater were used. The sample was heated sequentially to 20°C, then 40°C, then
60°C, and finally was left to cool. The full results of this trial are presented in Figure 8, whilst
the heating steps are in Figure 9.
500 ml Glass Dewar Calorimetry
Methanol Calibration Curve
70
50
45
60
40
35
30
40
25
30
Power (W)
Temperature (°C)
50
20
15
20
10
10
5
0
0
0
2
4
6
8
10
12
14
16
18
Time (hours)
Sample Temperature
Ambient Temperature
Heater Power
Figure 8: Full Calibration Curve – Methanol
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500 ml Glass Dewar Calorimetry
Methanol Calibration Curve
70
50
45
60
40
35
30
40
25
30
Power (W)
Temperature (°C)
50
20
15
20
10
10
5
0
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time (hours)
Sample Temperature
Ambient Temperature
Heater Power
Figure 9: Heat-Up Steps – Calibration Curve – Methanol
Heat step 1
The methanol was heated at an average power of 10.36 W until the temperature of the
sample reached 20°C from 16.5°C. The time for the temperature rise was 284 s. The rate of
temperature rise was found to be 0.0126 K.s-1 (45.5 K.hr-1).
HCvessel
= ((10.36 J.s-1 x 284 s) – (0.2765 kg x 2503 J.kg-1.K-1 x 3.5 K)) / 3.5 K
= 148.6 J.K-1
Heat step 2
The methanol was heated at an average power of 10.35 W until the temperature reached
40°C. The time for the temperature rise was 1774 s. At the end of each heat-up step, the
temperature is seen to fall owing to heat losses which can increasingly be due to
vaporisation of the methanol at more elevated temperatures. By extrapolating these falling
endpoint temperatures and measuring the temperature difference at the midpoint of the
heat-up step, these losses can be accounted (see Figure 7). In the present case, this
extrapolation suggests that without losses, the heat injected would have caused the system
temperature to reach 41.13°C from 19.9°C, a rise of 21.23 K. The average rate of
temperature rise was calculated to be 0.0120 K.s-1 (43.1 K.hr-1).
HCvessel
= ((10.35 J.s-1 x 1774 s) – (0.2765 kg x 2565 J.kg-1.K-1 x 21.23 K)) / 21.23 K
= 155.6 J.K-1
Heat step 3
The methanol was heated at an average power of 10.42 W until the temperature reached
60°C. The time for the temperature rise was 2138 s. Extrapolating the temperature falls at
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either end of the heating step, it was calculated that without loss of heat through vaporisation
of the methanol, the system would have reached 62.97°C from 38.86°C. The average rate
of temperature rise was calculated to be 0.0113 K.s-1 (40.52 K.hr-1)
= ((10.42 J.s-1 x 2138 s) – (0.2765 kg x 2705 J.kg-1.K-1 x 24.11 K) / 24.11 K
= 176.1 J.K-1
HCvessel
It is seen, as would be expected, that the heat capacity of the glass Dewar and its internals
increases as the temperature rises. It is important to select values for use in the ‘phi factor’
calculation which represent the temperature range experienced during the compatibility test.
A.3.2 Determination of Phi Factor
The “phi factor” of a test describes the extent to which the heat generated by a reaction is
maintained within the reaction mixture compared with that which is used to heat the vessel
up to the same temperature. The phi factor is calculated from the equation below:
Phi Factor (Φ) = ((mvessel. Cpvessel) + (mreactants.Cpreactants)) / (mreactants.Cpreactants)
= ((HCvessel) + (mreactants.Cpreactants)) / (mreactants.Cpreactants)
Where:
m
Cp
=
=
mass of reaction mixture (reactants) / vessel (kg)
heat capacity of reaction mixture (reactants) / vessel (J.kg-1.K-1)
All vessels will require energy to heat the vessel up. In larger vessels, the proportion of the
heat required to heat of the vessel is very small compared to that retained by the reaction
mixture. In this case, the phi factor will be close to 1.0 (1.0 represents an ideal adiabatic
situation, impossible in practice, whilst efficient adiabatic calorimeters will be capable of
achieving values upwards from around 1.05). For an adequate simulation of the plant scale
process in the laboratory environment, the phi factor of the test cell should be lower than, or
equal to, the plant vessel.
Example
Using the heat capacity values derived above, the ‘phi factor’ of the 500 ml glass Dewar
flask can be calculated.
Φ at 20°C
= ((148.56 J.K-1) + (0.2765 kg x 2503 J.kg-1.K-1)) / (0.2765 kg x 2503 J.kg-1.K-1)
= 1.215
Φ at 40°C
= ((153.68 J.K-1) + (0.2765 kg x 2565 J.kg-1.K-1)) / (0.2765 kg x 2565 J.kg-1.K-1)
= 1.216
Φ at 60°C
= ((172.62 J.K-1) + (0.2765 kg x 2705 J.kg-1.K-1)) / (0.2765 kg x 2705 J.kg-1.K-1)
= 1.231
A.3.3 Determination of Vessel Cooling Coefficient
From the cooling phase in the calibration procedure, the heat loss coefficient of the vessel
can be determined. The measurement can be performed at multiple temperatures during
cooling but typically two or three points during the cooling phase will suffice.
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At a specific temperature, the rate of cooling (dT/dt in K.s-1) should be calculated from the
cooling data. The temperature difference (ΔT in K) between the calculation temperature and
the ambient temperature should also be evaluated. The cooling coefficient (W.kg-1.K-1) can
then be calculated from the following equation:
Cooling Coefficient = (dT/dt.CpAV) / ΔT
Example
Using the data presented above (Figure 8), the cooling coefficients can be calculated. Three
points on the cooling curve (Figure 10) were taken: at 55°C, 45°C and 35°C.
At 55°C, the cooling rate was 0.0013 K.s-1, the Cp of methanol is 2744 J.kg-1.K-1 and the
temperature difference from ambient was 32 K
Cooling coefficient at 55°C = 0.0013 K.s-1 x 2744 J.kg-1.K-1 / 32 K
= 0.1115 W.kg-1.K-1
At 45°C, the cooling rate was 0.00086 K.s-1, the Cp of methanol is 2688 J.kg-1.K-1 and the
temperature difference from ambient was 22 K
Cooling coefficient at 45°C =0.00086 K.s-1 x 2688 J.kg-1.K-1 / 22 K
= 0.1067 W.kg-1.K-1
At 35°C, the cooling rate was 0.00044 K.s-1, the Cp of methanol is 2598 J.kg-1.K-1 and the
temperature difference from ambient was 12 K
Cooling coefficient at 35°C =0.00044 K.s-1 x 2598 J.kg-1.K-1 / 12 K
= 0.0996 W.kg-1.K-1
In this example, the 500 ml glass Dewar vessel was located in an open fume hood.
Shielding the Dewar from air currents and wrapping with insulation would have reduced the
heat losses, as illustrated in Figure 11. The effect of this insulation in this case is to reduce
the cooling coefficient to around 70% of the values calculated above; clearly this would be
beneficial. Equally, locating the Dewar in a temperature controlled shield oven (Section 3.3)
would reduce the environmental heat losses to close to zero.
The cooling coefficient calculated above can also be expressed in other ways. A common
alternative is the half-life time of cooling (an empirical measurement of the time it takes for
the temperature difference between the sample and environment to reach 50% of its starting
value).
For comparing the laboratory apparatus to the large scale vessel, simply comparing the
rates of cooling of the Dewar and plant vessels for a defined temperature difference
(between ambient and the contents) may provide an immediate indication of whether the test
is a reliable simulation of the large scale. However, as is discussed in Appendix 5,
establishing a reliable heat loss coefficient for a plant tank is somewhat difficult; where a
plant assessment is not possible, tabulated estimates may be used.
It has been noted elsewhere that even the best adiabatic calorimeter cannot simulate the
heat loss characteristics of very large tanks (limits of 10 m3 for a Dewar in a fume cupboard
and 25 m3 for a Dewar in an adiabatic oven, are often quoted). However, the curves in
Figure 7 suggest that for a relatively fast reaction the loss of peak condition diminishes
markedly for larger tanks (the peak temperature in this example rises from 116°C in a 1 m3
stirred vessel to 119°C in a 100 m3 stirred tank). The influence will be more pronounced in
slower reactions, however the step heating protocol proposed in Section 3.2.3 aims to
Environment Agency / Health and Safety Executive Joint Guidance on Compatibility - November 2011
Page Number 46 of 69
minimise this effect. As a result, the data from a well conceived and operated laboratory
Dewar calorimeter can be considered suitable for predicting the behaviour in large storage
tanks.
500 ml Glass Dewar Calorimetry
Methanol Calibration Curve
65
50
45
60
40
55
30
45
25
Power (W)
Temperature (°C)
35
50
20
40
15
35
10
30
5
25
1.9
3.9
5.9
7.9
9.9
11.9
13.9
0
17.9
15.9
Time (hours)
Sample Temperature
Ambient Temperature
Heater Power
Figure 10: Cooling Phase – Calibration Curve – Methanol
Cooling Curve Comparison
70
Temperature (°C)
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
16
Time (hours)
Uninsulated Dewar
Uninsulated Ambient
Insulated Dewar
Insulated Ambient
Figure 11: 500 ml Glass Dewar Heat Loss Comparison for Insulation
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Page Number 47 of 69
A.3.4 Adjusting Test Data for Calibrated Losses
The data obtained from a Dewar compatibility test can be adjusted to compensate for the
heat lost to the ‘phi factor’ and the environment. For a reaction that has reached completion
in a test, a compensated peak temperature may be calculated.
From the test data, extract the measured overall temperature rise, ΔTmeasured. For Dewar
vessel calibration that has been conducted on a fluid having a similar specific heat capacity
(Cp) and a similar vessel fill, the phi factor (Appendix 3) may be directly used:
Phi adjusted temperature rise, ΔTcorr phi = Φ x ΔTmeasured
The adjustment for environmental heat loss results from an averaging across the whole
exotherm (Figure 12):
Heat loss adjusted rise, ΔTcorr phi + loss = ((ΔTcorr phi x Cpreactants) + (HLC x t x ΔT’)) / Cpreactants
Where
ΔTmeasured
ΔT’
t
HLC
Cpreactants
=
=
=
=
measured overall temperature rise (K)
average temperature difference to ambient (K)
duration of exotherm (s)
cooling coefficient (Appendix 3) at average exotherm temperature
(W.kg-1.K-1)
= specific heat capacity of reactant fluid (J.kg-1.K-1)
Temperature
This adjustment using the arithmetic average temperature difference to ambient is an
approximation; a more accurate approach would entail integration across the exotherm.
This can be achieved using a spreadsheet to manipulate the test data.
Sample
Temperature
ΔTmeasured
ΔT’
Ambient
Temperature
t
Time
Figure 123: Data Adjustment for Heat Loss – Nomenclature
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Page Number 48 of 69
The calibration data can be used directly in the initial interpretation of compatibility test
results. Whilst strong and rapid reactions may be readily identified, mild exotherms which
may not lead to a positive temperature rise above a temperature step (Appendix 4) can be
detected by superimposition of relevant curves as in Figure 134.
Temperature
Mild Self-Heating – Within Oven
Mild Self-Heating – No Oven
Cooling Curve
Time
Figure 134: Exotherm Interpretation – Mild Self-Heating
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A.4 APPENDIX 4 – COMPARISON OF LABORATORY TEST METHODS AND
PROTOCOLS
As has been discussed several times throughout this guidance, the aspect of heat loss is
fundamental to obtaining useful, scaleable, data from laboratory compatibility tests. An
example is presented to demonstrate this effect, and the need for the protocol discussed in
Section 3.2.3.
Testing has been performed on a mixture of methanol and acetic anhydride. As has been
shown in Appendix 2, this is a known reactive combination, yielding methyl acetate and
water; this would consequently be considered ‘treatment’ and not ‘bulking’ but is used here
for illustrative purposes. Using a formulation of 55.7% w/w methanol and 44.3% w/w acetic
anhydride (that is, a 50% molar excess of methanol over stoichiometric proportions), mixed
cold (to simulate plant samples taken into a warm laboratory) directly in the test apparatus,
the following results have been obtained.
500 ml Glass Beaker
Samples of each reactant from cool storage were mixed directly in a 500 ml glass beaker, to
a total of 350 g mixture. The beaker and ambient temperature in the fume hood were
monitored for an extended period. Owing to the cool reactants, the initial temperature of
12.5°C was below ambient. The full temperature trace is shown in Figure 145, whilst the
first hour of the test is shown in Figure 15. These results suggest insignificant heating, in
that the peak temperature achieved is less than 26°C, which is not reached until 10 hours
into the test. As the reactants were cooler than the ambient laboratory temperature, the
initial period of the test involved slow warming, with the mixture taking 3.4 hours to reach
ambient. Whether any reaction was occurring during this period is unclear from this data.
Under laboratory time pressures, any test that was terminated after 1 hour, would be
inconclusive; Figure 15 shows a total temperature increase of 4 K, but this could easily be
attributed to the sample warming up without any reaction. And indeed, if the test was
allowed to run for 20 hours, a temperature peak of less than 3 K above ambient could be
judged insignificant.
Environment Agency / Health and Safety Executive Joint Guidance on Compatibility - November 2011
Page Number 50 of 69
500 ml Beaker Compatibility Test
Methanol + Acetic Anhydride
30
28
26
Temperature (°C)
24
22
20
18
16
14
12
0
2
4
6
8
10
12
14
16
18
20
0.9
1
Time (hours)
Sample Temperature
Ambient Temperature
Figure 145: 500 ml Beaker Test – 20 hours
500 ml Beaker Compatibility Test
Methanol + Acetic Anhydride
26
24
Temperature (°C)
22
20
18
16
14
12
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Time (hours)
Sample Temperature
Ambient Temperature
Figure 15: 500 ml Beaker Test – First Hour
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Page Number 51 of 69
500 ml Glass Dewar
Identical quantities of each reactant, again from cool storage, were mixed directly in a 500 ml
glass Dewar flask and the temperatures monitored for an extended period. In this test the
reactants were slightly warmer, at an initial temperature of 19°C, but still below ambient.
The full temperature trace is shown in Figure 167, whilst the first hour of the test is shown in
Figure 18. These results clearly show significant heat is generated from the reaction with
the mixture attaining boiling point (approximately 70°C) at around 4.5 hours into the test, and
continued reaction maintaining this boiling for a further 0.8 hours. Again, the cooler
reactants took some time (1.3 hours) to reach ambient laboratory temperature. This is faster
than the beaker test owing to the higher start temperature, and is faster than the beaker from
19°C (1.3 hours compared to 1.8 hours in the beaker). Whilst this comparison may point to
incipient reaction, the Dewar data alone remains unclear. During the first hour of this test,
the results are again inconclusive, with Figure 18 showing a total temperature increase of
2.7 K, possibly able to be attributed to sample warming.
Once ambient temperature is reached, the next hour yields a temperature rise of 4.5 K
(Figure 17); at this stage the reaction can be clearly identified but remains slow, taking a
further two hours to reach its peak. Under the constraints of a waste treatment laboratory,
this timescale could pose significant difficulty.
500 ml Dewar Compatibility Test
Methanol + Acetic Anhydride
80
70
Temperature (°C)
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
16
18
20
Time (hours)
Sample Temperature
Ambient Temperature
Figure 167: 500 ml Dewar Test – 20 hours
Environment Agency / Health and Safety Executive Joint Guidance on Compatibility - November 2011
Page Number 52 of 69
500 ml Dewar Compatibility Test
Methanol + Acetic Anhydride
26
25
24
Temperature (°C)
23
22
21
20
19
18
17
16
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
2.2
2.3
Time (hours)
Sample Temperature
Ambient Temperature
Figure 18: 500 ml Dewar Test – First Hour
500 ml Dewar Compatibility Test
Methanol + Acetic Anhydride
30
29
28
Temperature (°C)
27
26
25
24
23
22
21
20
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
Time (hours)
Sample Temperature
Ambient Temperature
Figure 17: 500 ml Dewar Test – First Hour After Ambient
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500 ml Glass Dewar – Temperature Step
In this test, the 500 ml glass Dewar was fitted with a heater and stirrer and the protocol
discussed in Section 3.2.3 has been applied. Identical quantities of each reactant, again
from cool storage, were mixed directly in the Dewar flask and the temperatures monitored.
Following mixing, the reactants were at 16°C for this test, again below ambient. The full
temperature trace is shown in Figure 18, whilst the first hour of the test is shown in Figure 19
and the first 2 hours in Figure 20. As with the previous glass Dewar test, the reaction
mixture attains boiling point, this time at around 2 hours into the test. As before, the cooler
reactants were heating slowly towards ambient, but after 20 minutes the temperature had
only risen 1 K, the trace was linear (Figure 191) and there was therefore no evidence of
reaction. The heater was then applied to raise the temperature by 15 K, and in the
subsequent 20 minutes following the heat-up, the temperature rose 3 K, but now the trace
was accelerating (Figure 20). A further 15 K step could have been applied at this stage but
in this test the exotherm was allowed to run unaided, taking a further 1.2 hours to reach
boiling, where it persisted for a further 1.5 hours before completion.
This test clearly shows the benefit of the heating steps in allowing identification of the
reaction within a reasonable timescale. The sequence of three tests demonstrates the
influence of heat loss on both the ability to detect an exotherm, and the assessment of its
magnitude. Whilst in this example both Dewar tests attain boiling, in the stepped heating
test boiling persists for around twice as long as in the unassisted one. Data from these tests
can then be adjusted with the heat loss and phi factor calibrations to allow comparison
against plant equipment.
This example has considered a reaction with slow kinetics at ambient temperature, but which
is nevertheless capable of running away and would cause serious consequences in quasiadiabatic plant equipment.
500 ml Temperature Step Dewar Compatibility Test
Methanol + Acetic Anhydride
70
Temperature (°C) / Power (W)
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
16
18
20
Time (hours)
Sample Temperature
Ambient Temperature
Heater Power
Figure 18: 500 ml Temperature Step Dewar Test – 20 hours
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Page Number 54 of 69
500 ml Temperature Step Dewar Compatibility Test
Methanol + Acetic Anhydride
40
35
Temperature (°C) / Power (W)
30
25
20
15
10
5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (hours)
Sample Temperature
Ambient Temperature
Heater Power
Figure 19: 500 ml Temperature Step Dewar Test – First Hour
500 ml Temperature Step Dewar Compatibility Test
Methanol + Acetic Anhydride
70
Temperature (°C) / Power (W)
60
50
40
30
20
10
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time (hours)
Sample Temperature
Ambient Temperature
Heater Power
Figure 202: 500 ml Temperature Step Dewar Test – First 2 Hours
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A.5
APPENDIX 5 – DETERMINING PHI FACTOR & HEAT LOSS OF PLANT VESSELS
Determining the phi factor and heat loss characteristics of plant vessels is a complex
subject. There are two principal approaches, measurement and calculation, although neither
is straightforward.
The measurement approach would follow a similar mechanism as that described for the
Dewar vessels in Section A.3; however this is impractical for the majority of plant storage
tanks. Difficulties in reliably heating a large volume of liquid with a known amount of energy,
and closely monitoring its cooling under stable ambient conditions, would preclude direct
measurement for all but the smallest plant storage tanks.
Some progress can be made with engineering calculations, but these are somewhat
complex, involve many imprecise parameters and hence can yield inaccurate results. An
examination of some of the principles has been presented[16]. Some of the main issues are
discussed below.
Calculation of Phi Factor
The calculation of phi factor for a large tank uses the same formulae as those presented in
Section A.3 for analysing a Dewar. However, the heat capacity of the tank must be obtained
by calculation. From knowledge of the tank dimensions and the density and specific heat
capacity of the material of construction, together with that of any internal fittings, the heat
capacity can be calculated:
HCvessel = (Mass x Cp)tank walls + (Mass x Cp)internal fittings (J.K-1)
The heat capacity of the fluid is calculated from its specific heat capacity and the fill quantity.
In a working tank, however, the fill level will be variable, and this will influence both the mass
of liquid used in the ‘reactant’ calculation, and the extent of the tank walls used in the ‘tank’
heat capacity calculation. This therefore can introduce significant uncertainty in the
accuracy of the resulting phi factor.
Calculation of Environmental Heat Losses
This calculation involves complex heat transfer calculations and is very sensitive to
parameters that may not be readily available. A detailed calculation procedure is beyond the
scope of this guidance; a explanation is provided in literature[16], and commercial computer
programs are available to perform the calculations. Key elements can, however, be
addressed. A ‘cooling coefficient’ (Section A.3) is, in fact, an overall heat transfer coefficient.
This results from a combination of:
•
•
•
Heat transmission from the bulk of the fluid to the zone near the wall – heat loss is
favoured by the presence of mechanical agitation (large effect), and to a lesser extent by
convection currents that can occur in large non-uniform tanks; the liquid physical
properties are also influential with aqueous liquids having greater transmission than
organic oils
Heat transfer across liquid film to the tank wall – heat loss again favoured by agitation
and influenced by the liquid properties
Conduction across the tank wall – metal tanks will conduct heat more freely than GRP,
concrete or other such materials; however, the wall thickness is also important, with heat
loss being faster for thin walled tanks
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Page Number 56 of 69
•
•
Heat transmission from the outside surface of the bare wall – heat loss is influenced
hugely by the wind (external tanks) or ventilation currents (within a building), with the
heat loss being a function of the ‘wind’ speed; heat loss also increases significantly in
rain (or during water spraying), particularly for tanks containing more volatile liquids
Insulation on the outside of a tank can provide a significant barrier against heat loss
under all external conditions, as this will normally have a much reduced thermal
conductivity; however, maintenance is important as, for example, wet mineral wool
insulation can have a heat conductivity of an order of magnitude greater than that of dry
insulation
As can be seen, calculations must be individual for each plant vessel but will nevertheless
be subject to unknown or variable factors. Although not a replacement for the detailed
analysis of a specific plant tank, the table below given some example values of overall heat
transfer coefficient for uninsulated tanks obtained from commercial calculation software.
These values are very approximate, because they have been derived following a number of
assumptions.
Wind Speed
(km.h-1)
(knot)
0
0
9
16
13
24
17
32
22
40
Overall Heat Transfer Coefficient, U (W.m2.K-1)
Aqueous Liquids
Light Organic Oils
10.2
23.3
26.7
29.5
32.4
8.2
18.6
21.4
23.6
25.9
Table 6: Approx. Heat Transfer Coefficients for Use in Estimating Plant Scale Loss
For a specific plant vessel, the procedure for evaluating the cooling coefficient is as follows:
Calculate the total surface area, SA, of the vessel from its dimensions (m2)
• If the vessel is on legs or saddles, use the total surface area
• If the vessel has a flat base and is resting on the ground, ignore this base area
(conduction into the ground is likely to be small compared to heat losses to the air)
Estimate the typical liquid quantity, M, in the vessel after filling with waste (kg)
Calculate the heat loss coefficient, HLC (W.kg-1.K-1) from:
HLC = U x SA / M
The heat loss coefficient can then be used in the same way as that calculated for the Dewar
apparatus in Appendix 3.
For insulated plant vessels, the heat loss is controlled by the type, thickness and condition of
the insulation, and is practically insensitive to the wind speed and the nature of the tank
contents. Typical heat transfer coefficients would be expected to be an order of magnitude
less than those quoted in Table 6; for example, 50 mm of mineral wool may result in an
overall heat transfer coefficient of around 0.7 W.m2.K-1.
Using the data in Table 6, heat loss coefficients have been calculated for a selection of
typical vessels (Table 7). For the vertical vessel mounted directly on the ground, allowance
has been made for different heat losses by conduction. In each case, the vessel is assumed
to be 85% full, with liquids having densities of 1000 kg.m-3 (aqueous) and 850 kg.m-3
(organic).
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Vessel Details
Wind Speed
(km.h-1)
HLC (W.kg-1.K-1)
Aqueous
HLC (W.kg-1.K-1)
Light Organic Oil
5 m3 Horizontal Vessel
1.5 m Diameter, 2.83 m Long
Dished Ends, Saddle Mounted
0
16
24
32
40
0.043
0.099
0.113
0.125
0.137
0.041
0.093
0.106
0.118
0.129
10 m3 Horizontal Vessel
2 m Diameter, 3.18 m Long
Dished Ends, Saddle Mounted
0
16
24
32
40
0.034
0.078
0.089
0.098
0.108
0.032
0.073
0.084
0.093
0.102
25 m3 Horizontal Vessel
3 m Diameter, 3.54 m Long
Dished Ends, Saddle Mounted
0
16
24
32
40
0.025
0.057
0.065
0.072
0.079
0.024
0.053
0.061
0.068
0.074
25 m3 Vertical Vessel
3 m Diameter, 3.54 m Long
Flat Ends, Ground Supported
0
16
24
32
40
0.020
0.045
0.052
0.057
0.063
0.019
0.043
0.049
0.054
0.059
50 m3 Vertical Vessel
4 m Diameter, 3.98 m Long
Flat Ends, Ground Supported
0
16
24
32
40
0.016
0.035
0.040
0.044
0.048
0.015
0.033
0.038
0.042
0.046
100 m3 Vertical Vessel
6 m Diameter, 3.54 m Long
Flat Ends, Ground Supported
0
16
24
32
40
0.012
0.027
0.030
0.034
0.037
0.011
0.025
0.029
0.032
0.035
250 m3 Vertical Vessel
8 m Diameter, 4.97 m Long
Flat Ends, Ground Supported
0
16
24
32
40
0.009
0.02
0.022
0.025
0.027
0.008
0.019
0.021
0.023
0.026
500 m3 Vertical Vessel
12 m Diameter, 4.42 m Long
Flat Ends, Ground Supported
0
16
24
32
40
0.007
0.016
0.018
0.02
0.022
0.007
0.015
0.017
0.019
0.021
Table 7: Typical Calculated Heat Loss Coefficients for a Simplified Range of Plant
Storage Vessels (Uninsulated)
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Comparing the data in Table 7 with the heat losses calculated for an example 500 ml
uninsulated, non-adiabatic glass Dewar (Appendix 3), indicates that the Dewar used in the
test calibration example can directly replicate uninsulated plant vessels of 5 m3 where the
wind speed is modest (16 – 24 km.h-1), and 10 m3 where the wind speed is high. However,
this Dewar could not be used to directly simulate any plant scale vessel of greater than
10 m3, without adjusting the Dewar data using the routine described in Appendix 3. The
specific heat losses of the Dewar would be lower for a 1000 ml flask, and could be further
reduced by wrapping the Dewar in insulation (Figure 11), thereby allowing direct simulation
of larger vessels. Thereafter, the use of adiabatic calorimetry techniques (Section 3.3)
would be required.
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A.6
APPENDIX 6 – PRECAUTIONS FOR USING ELECTRICAL APPARATUS
Experimental apparatus, such as a Dewar, used for calorimetry, is recommended to be
located within a fume cupboard. Such installations do not normally employ Ex rated
electrical apparatus. However in the present work flammable compounds, either waste
fluids or their decomposition products, may be involved in the testing. The flammability risks
can be exacerbated by the use of electrical immersion heaters, as recommended in Sections
3.2 and 3.3. A number of precautions should therefore be taken in any installation.
The main hazard arising from the use of an immersion heater comes from the potential for
one small part of the heater to attain a much higher temperature than the average. This may
be due to a fault (an electrical short) within the heater that causes a hot spot, or a point on
the heater where heat is not being removed at the rate it is being generated, such as where
there is no medium for removing heat. This could be a part of the heater not immersed in
liquid, or covered in insulating 'crud' that prevents heat transfer. Another hazard arises from
having electrical connections in close proximity to flammable gases or vapours. A short
circuit between connections, or a loose connection, could generate a spark powerful enough
to ignite any gas or vapour within its flammable range.
The consequences of an ignition may be limited by precautions, such as:
• Restricting the volume that a flammable vapour can occupy (the suggested Dewar fill
of 70% is beneficial here).
• Fitting the Dewar with a loose fitting cork or bung.
• Providing the fume cupboard with a spillage retention sill.
• Placing the Dewar in an operating fume cupboard.
• Use of shields.
• Not standing in front of the fume cupboard whilst the equipment is in operation (so far
as is reasonably practicable)
Concerning the electrical equipment, and particularly the immersion heater, the following
precautions should be observed:
• The immersion heater should not be used if there are signs of excessive heating on
the outside surface, such as carbonisation or discolouration.
• The immersion heater should be checked for signs of physical deterioration such as
damaged insulation, physical damage or corrosion, prior to use.
• The immersion heater should be clearly marked so that the extent of the heated
portion can be readily seen.
• The immersion heater should have continuous insulating leads that extend outside
the fume cupboard, and which have adequate physical protection against chafing,
pinching and being forced into a small bend radius.
• Any power supply for the immersion heater should be placed outside the fume
cupboard so there is no risk of ignition of any flammable atmosphere that may arise
in the fume cupboard.
• The immersion heater should operate at as low a voltage as possible, to reduce the
possibility of sparking.
• The source of power to the immersion heater should have a means of preventing
excessive currents from arising. This could be an electronic current limiter and/or
fuse or electro-mechanical device that isolates the supply in the event of excess
current being detected.
• The risks arising from a fault that leads to overheating should be assessed. It may be
necessary to limit immersion heater operation to attended operation only where the
consequences are particularly undesirable.
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•
•
•
Other electrical equipment should be placed outside the fume cupboard wherever
possible. Where other electrical equipment such as a stirrer has to be within the fume
cupboard, this should be suitable for the use to which it is being put and the
environment in which it is being operated.
Electrical equipment should be adequately earthed.
There should be a means of prevent overheating of the contents of the Dewar. Any
devices used to detect high temperature and remove electrical supply to the heater
should be suitable, bearing in mind the consequences arising from overheating.
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A.7
APPENDIX 7 – ADIABATIC PRESSURE DEWAR CALORIMETRY EQUIPMENT
The specific description below is restricted to the Dewar vessel technique, but the general
concepts, procedure and interpretation apply equally to other adiabatic techniques
A7.1 Apparatus
The apparatus for ambient pressure adiabatic Dewar calorimetry is identical to that for nonadiabatic Dewar calorimetry (Section 3.2.1) but with the addition of an adiabatic enclosure
(oven) around the test vessel. This will require a relatively high powered, fan assisted, oven
which is controlled by an automated control system to track the sample temperature within
the Dewar closely throughout the test (within ±1 K). Providing the control system is capable
of closely matching the oven temperature with the sample temperature, the heat losses of
the Dewar vessels will not be as critical as in non-adiabatic studies. The resulting data is
likely to be scalable to much larger volumes.
Where pressure resisting vessels are employed and operated closed, further instrumentation
and controls are required. This will include, but is not limited to:
• Pressure transducer capable of reading to an accuracy of at least 0.1 bar
• Safety devices and instrumentation to provide pressure relief (and relief containment)
if dangerous pressures are established.
It is again critical that the stainless steel Dewar flask is chemically compatible with the
materials to be tested (note that commercial units are usually a low grade of stainless steel).
If not, the materials of construction of the flask may influence the test results. In this case, it
may be necessary to purchase specific flasks manufactured of suitable material, or to coat
the inside of the flask (and any associated inserts) with an adherent, compatible, lining such
as PFA or Halar).
A7.2 Calibration Procedure
Dewar vessels used under adiabatic conditions should be subject to the same calibration
procedures as for non-adiabatic Dewars (see Section 3.2.2). There is less reliance on the
performance of the vacuum jacket in adiabatic testing – since the heat losses are minimised
by the use of the adiabatic tracking enclosure. However, vessels with “failed” jackets will
suffer from increased temperature drift and should not be used. Calibrations should therefore
still be performed periodically to confirm that the vacuum jacket remains efficient as a buffer
to heat loss.
The procedure for heat capacity (and hence phi factor) measurement and heat loss
determination are identical to those previously defined for non-adiabatic glass Dewars.
Equipment calibration and procedures are also the same as for non-adiabatic glass Dewar
testing.
A7.3 Test Protocol
For atmospheric pressure adiabatic calorimetry, the test protocol remains the same as
outlined in Section 3.2.3. Where pressure calorimetry is performed, the test cell configuration
should be checked for leaks immediately prior to the test. This typically involves pressurising
the flask with a compressed gas (air or nitrogen) and using a detergent solution applied to all
fittings to detect leaks (observed as bubbles). Other leak testing procedures are available
and can be substituted. Pressure calorimetry may be required, for example, where a volatile
substance is being bulked at near to its boiling point (although in reality this should not be
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happening at waste treatment facilities) – in atmospheric calorimetry, evaporative heat
losses can mask exothermic heat release and hence sealing of the calorimeter prevents
vapour loss and enables reliable results to be obtained.
The addition of materials to the flask during a test may, for pressure calorimetry, require the
use of a pump which can deliver against a positive pressure. Laboratory HPLC, or similar,
pumps can be utilised for this.
For systems which exhibit no, or low, reactivity at the initial test temperature, it is
recommended to perform one or two heating steps to force initiation of reactions near to
ambient temperature.
At the end of an adiabatic test using a sealed reactor, the reactor should be allowed to cool
to near ambient temperature before attempting to open it. Depressurisation of the flask after
cooling should be performed in a controlled way, and with the operator in a remote location
(to minimise the risk of personnel exposure to the released gases, vapours or even ejected
liquids).
A7.4 Interpretation of Results
A self-accelerating temperature trend is indicative of exothermic behaviour, with the rapidity
and magnitude indicating the severity of the reaction.
In the event of no temperature increase, or indeed a temperature fall, compare the
temperature trend against the relevant calibration cooling curve; any positive deviation is
again indicative of exothermic behaviour, albeit mild or slow.
A pocket of displaced air will be collected in the gas measurement device during the addition
of the second reactant and during a temperature step; however at all other times, gas
collection is indicative of an adverse reaction, with the rate and quantity indicating the
severity of the reaction.
Compare the heat loss and phi factor values of the test apparatus with those of the plant
tank (Appendices A.3 & A.5) to establish the validity of the test result. Assuming the
laboratory test is a valid simulation decisions can be made using the data. In any case
where the reaction goes to completion, multiply the maximum temperature rise observed by
the calibration ‘phi’ factor, and add this to the plant mixing temperature. Add to this an
estimate of the temperature rise due to heat loss (see Appendix 3). If, for a valid simulation,
the calculated temperature rise is more than 10 K, then mixing should not be allowed.
Compare the calculated peak temperature to any known threshold of instability
(decomposition onset temperature) of any component chemical – any closer than a margin
of 50 K should be considered unacceptable without a revised offloading procedure.
In any case where the reaction has not achieved completion by the end of the normal test
period, the test duration should be prolonged. It is potentially dangerous to make decisions
on incomplete reactions because secondary events (other chemical reactions or
decomposition reactions) can possibly occur at elevated temperatures. If complex modelling
techniques are available (their detail is outside the scope of this guidance), the temperature
and the rate of temperature rise at the end of the test can be used in calculations to balance
thermochemical dynamics against plant scale heat losses, thereby demonstrating
acceptability. Only if this analysis is performed, the test temperature is distant from the
criteria in the previous paragraph, and there are known to be no secondary reactions
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possible, can the uncompleted test results be considered acceptable without this modelling.
All such decisions must be undertaken by the competent person (see Section 1.7) and
documented.
In any case where gas is collected, its generation rate and total quantity should be evaluated
and compared to the proposed limiting criteria. The nature of the collected gas should also
be assessed to determine the acceptability of its direct discharge to atmosphere or the
suitability of the site scrubber to remove it from a ventilation stream.
NOTE: It is critically important, for open tests, that any test data obtained when the material
approaches the boiling point of any of the components is disregarded. In an open test,
evaporative heat loss will occur when nearing the boiling point thereby invalidating data from
that point onward.
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A.8
APPENDIX 8 - GLOSSARY
Word or Term
Adiabatic
Adiabatic
temperature rise
(ΔTad)
Adverse reaction
Aqueous
Calorimeter
Dewar vessel
DIERS
Diurnal
Endothermic reaction
Exothermic reaction
Fluid
Inorganic
Isothermal
Heat capacity
Heat of reaction
(ΔHr)
Meaning
Without transfer of heat. A true adiabatic calorimeter would not absorb
or loose any heat from a reacting mass, thereby allowing the reaction
mixture to heat up (or cool down) to the full extent dictated by the heat
of reaction or heat of solution. In reality, a true adiabatic calorimeter
cannot exist, although the term is commonly applied to commercial
apparatus having very good heat retention performance
The temperature rise caused by an exothermic reaction if all of the
chemical energy is used to heat up the reaction mixture. This value
ignores any heat losses and ignores any heat absorbed by the reactor
(units are normally K or °C)
Generally, a reaction that can generate sufficient heat and / or gas to
result in a harmful release either to people or to the environment is
called an adverse reaction. Within the context of the present guidance,
this is defined by limits for temperature rise and gas generation
quantity.
Water based liquids
Scientific apparatus of various types designed to measure enthalpy
changes. In this report this generally involves heat release or
absorption during chemical reaction, or liquid dissolution during mixing
Vacuum jacketed test apparatus designed to minimise heat loss. The
most well known commercial example would be a ‘Thermos’ flask
Design Institute for Emergency Relief Systems. This is a collaboration
of partners from industry, including academics and consultants, who
seek to understand the relief processes from reactors in which
uncontrolled exothermic or decomposition reactions occur, and to
provide reliable calculation methods for the sizing of emergency relief
systems to provide protection in such cases. The calculation
techniques, often referred to as DIERS Methodology, developed and
refined by this group, are considered best practice in relief system
design
Variation throughout the day; for example day to night temperature
changes
A chemical reaction in which heat is absorbed, resulting in a decrease
in temperature (conventionally, ΔHr is positive)
A chemical reaction in which heat is released, resulting in an increase
in temperature (conventionally, ΔHr is negative)
A description encompassing any of, liquid, gas and vapour phase
materials
Chemical elements and compounds that are not organic including
carbon. Inorganic compounds are generally salts, consisting of cations
and anions joined by ionic bonding
At constant temperature. An isothermal calorimeter removes and
measures the heat involved in a reaction to maintain a constant
reaction temperature
The product of the specific heat capacity and the mass of material
being considered (units are normally kJ.K-1)
The overall amount of energy absorbed or released by a chemical
reaction (units are normally kJ.mol-1 of the limiting reactant; that is, the
reactant that is present at the smallest stoichiometric amount)
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HPLC
Kelvin (K)
High Pressure Liquid Chromatography – a chemical analysis technique
An absolute scale of temperature with the same interval as degrees
Celsius: °C = K + 273.15. Convention in this report expresses
temperature in degrees Celsius (°C), and temperature difference in
Kelvin (K)
Organic
Carbon based chemical compounds with the following exceptions:
carbon, carbides, carbonates, bicarbonates, metal cyanides and metal
ions
Permanent gas
A fluid in the gas state at a temperature at which no liquid or solid can
(interchangeable with form (that is, it is above its thermodynamic critical temperature). In the
non-condensible gas) present context, this is usually taken as not being able to condense
during cooling to ambient temperature
Phi Factor (Φ)
A ratio expressing thermal inertia caused by heat losses to a
containing vessel. The phi factor is obtained as the ratio of (heat
capacity of the liquid mixture) + (heat capacity of the vessel) / (heat
capacity of the liquid mixture). Values of phi close to 1.0 indicate
adiabatic conditions are being approached, whereas higher values
indicate large heat loss to the containing vessel. Phi factor does not
involve a measure of any heat loss to the environment
Quasi-adiabatic
Near to, or appearing to be, adiabatic. This describes a more practical
situation than “adiabatic”. It is often applied to apparatus having poorer
performance than the commercial adiabatic calorimeters discussed
above
Quarantining waste
The action of separating a waste load into a dedicated storage facility
away from the normal process route (that is, no mixing with any other
stream is permitted). This would generally be done if a compatibility
test were to fail the acceptance criteria, but the site was still permitted
to handle the waste; in this case a revised treatment protocol would
needed to be developed
Quiescent
Calm or inactive. For example, an unstirred storage tank
Runaway reaction
An exothermic reaction occurring in a quasi-adiabatic environment. In
this case, the heat released by the reaction increases the reactant
temperature, which in turn accelerates the reaction. This can cause an
exponential temperature increase, and in the case of volatile fluids, an
exponential pressure rise
Sensible heat
Related to the heat capacity of a fluid, in the absence of phase change
Sensible heat effects Temperature changes associated with the mixing of two fluids at
different temperatures in the absence of any reaction or other physical
or chemical change
Shield oven
A containment oven (usually fan assisted) whose temperature is
regulated to equal the liquid temperature within an adiabatic Dewar
calorimeter. In the case of close temperature control, heat losses to
the environment are virtually eliminated during a test
Specific heat
The amount of energy required to increase the temperature of a certain
capacity (Cp)
mass of substance (units are normally kJ.kg-1.K-1)
Stoichiometric
A mixture containing quantities of each reacting compound that exactly
satisfy the balanced chemical equation (on a molar basis)
Thermal Activity
A type of isothermal calorimeter
Monitor
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Two-phase flow (in
venting)
Vapour
A vent relief stream consisting of a mixture of liquid and gas or vapour.
In the case of a runaway reaction, a two-phase mixture containing a
high proportion of liquid is frequently discharged through the relief vent.
Two-phase flow is important because its presence will cause a high
back-pressure in the vessel (could result in over-pressurisation if the
vent size is inadequate), and special consideration is required for the
safe treatment of the two phases discharged. Calculations relating to
two-phase flow are covered within DIERS methodology
A fluid in the gas state below its thermodynamic critical temperature.
In the present context, such a fluid can be condensed to liquid during
cooling to ambient temperature
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A.9
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