4.Thermal Screening

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HarsNetTHEMATIC NETWORK ONHAZARD ASSESSMENT OF HIGHLY REACTIVE SYSTEMS

4. Thermal Screening

This HarsBook text has been prepared by a HarsNet working

group. The text has been prepared in good faith but the authors

accept no responsibility for the consequences of application of the

information contained herein. The HarsNet Thematic network is

European Community Project BET2-0572 funded in part by

contract number BRRT-CT98-5066.

Keywords: thermal, screening, stability, exotherm, onset, decomposition, kinetics,

enthalpy

4.1. Introduction

The general objective of thermal screening of chemicals and mixtures is to identify

whether the sample can undergo an exotherm process and the temperature range of its

occurrence, which provides a preliminary indication of potential chemical reaction

hazards. The focus lies here on secondary reactions. Depending on the calorimeter used

additional information on the amount and rate of released heat, kinetics (normal or

autocatalytic) and pressure build up can be obtained. Thermal screening is the first

experimental stage in a hazard assessment.

4.2. Principle

Calorimeters for thermal screening are available from various producers in different

specifications, including DSC and various forms of DTA (single and twin set-up). All

screening calorimeters are characterised by the small sample size they require (mg- to g-

scale) and by the speed at which measurements can be performed. They are particularly

useful for:

Screening of a large number of samples.

Screening of highly unstable substances.

Screening of samples, which are only available in laboratory quantities, such asreactants, isolated intermediates, products and reaction mixtures.

Unintended mixtures, e.g. contamination with rust

In a typical screening apparatus a small amount of sample is placed in a pressure

resistant metal or glass container and is heated at a constant rate (0.1 to 10K/min) in the

temperature range of-20 to 500C. Besides this scanning mode, isothermal experiments

are also used for certain applications. Sensors measure the temperature of the sample or

sample container relative to a reference. In most of the available apparatuses this

temperature difference can be calibrated in units of rate of heat generation. Samples

which absorb a known amount of heat at a specific temperature, for instance the melting

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INDUSTRIAL AND MATERIALS TECHNOLOGIES PROGRAMME OF THE EUROPEAN COMMISSION. PROJECT: BET2-0572COORDINATION: Prof. Dr. R. Nomen Tel. +34-93-267 20 00 Fax. +34-93-205 62 66 http://www.iqs.url.es/harsnet


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of a known quantity of indium, are used for calibration. There is either no mixing or

only very limited mixing in the test cells.

A tabulated comparison of some screening calorimeters can be found in section 5.

Near-adiabatic gram-scale methods, such as RSST or ARC may also be used for

screening, but are not considered here. These methods are described in chapter 5 on

Adiabatic calorimetry.

4.3. The sample and sample container

For screening experiments closed high-pressure containers are typically used to

suppress/limit vaporisation and keep volatile decomposition products contained, which

may contribute to further decomposition. To ensure all compounds are kept in the

sample container, the mass of the filled sample container should be weighed before and

after a screening experiment.

The decomposition of a sample can depend strongly on its impurity profile. It is

therefore important that the screened chemicals or mixtures stem from the investigated

process. A sample taken from a heterogeneous mixture most likely does not represent

the mixture. This must be considered in the evaluation. The preparation of a

heterogeneous mixture in the sample container can generate more meaningful

information.

A disadvantage of using small sample sizes is that the sample has an unfavourable ratio

of surface to volume, which makes interactions with the container wall, the surrounding

air, etc. more likely. Catalytic as well as stabilising effects are possible, so that the

measured effect may not represent properties of the sample itself. For instance, many

compounds containing chlorine give artificial peaks when using steel or gold containers.

This kind of interaction can usually be eliminated by running screening experiments

with two containers of different material, e.g. steel and glass (Grewer, 1994).

Interactions with the surrounding air etc. can be eliminated by preparing the sample in a

glove-box with an inert atmosphere.

4.4. Evaluation

The previous section makes clear, that inherent features of thermal screeningmeasurements are considerable uncertainties and that an ideal evaluation should not be

based on one measurement only.

The interpretation and evaluation of screening experiments are often difficult because

the peak limits can not be clearly recognised and only rarely does an exothermic peak

appear independent from other peaks. However, using a different heating rate can

sometimes separate previously unseparated peaks. An illustration and interpretation of

different combinations of several peaks in screening experiments using some typical

examples of substances and mixtures can be found in /1, page 68/.

The following subsections describe the evaluation of a single peak.

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4.4.1. Qualitative Evaluation

The most obvious result of a screening test is the direction of the resulting peak

indicating an exothermic or endothermic process. The other information is the onset

temperature1, Tonset, the peak temperature and the peak shape, where the onset

temperature characterises the thermal stability, and the peak shape (size and sharpness)

gives an indication on the hazard potential of the sample (figure 4.1).

Area Q

TO

= Onset temperature

TP

= Peak temperature

Q = Heat of reaction

TP

TO

exo

heatgeneration

rate

temperature / time

Figure 4.1. Idealised screening curve of an exothermic reaction

In general, the larger and sharper the peak, the more hazardous the decomposition

reaction. However, low activation energy processes, which cause broad peaks, should beconsidered when extrapolating the data to adiabatic conditions.

The preliminary assessment of the thermal stability of chemicals or mixtures at

production scale is often based on a rule-of-thumb using Tonset of the secondary

reaction2. Such a rule-of-thumb states that if the maximum operating temperature of a

process isXKelvin lower than Tonset, the operation will not experience this secondary

reaction, and it is not necessary to obtain more detailed information by other means. For

instance, a safety margin of 100K is often used for DSC measurements. This kind of

rule-of-thumb can easily be misused, since the value ofTonset depends on the sensitivity

of the apparatus, the sample container, the heating rate and is sometimes difficult to

determine - especially if the signal is noisy and/or curvy. The DSC safety margin of100K is based on the assumption that the decomposition follows zeroth order kinetics

and has a defined temperature dependence. It has been shown that reactions with

activation energies below 80kJ/mol will violate the 100K rule (Hofelich, 1989). Hence,

the 100K rule was extended to ensure that low activation energy decompositions are

excluded. Beside the 100K safety margin, a shift of the peak temperature of less then

40K is demanded when changing the heating rate by a factor of ten (Steinbach, 1995).

1The observed Tonsetis the temperature at which the sample shows the first observable instrumental

response due to a reaction

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2See remark at the end of this subsection!


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Another rule-of-thumb for DSC measurements has been proposed. It assumes a

comparatively low activation energy of 50kJ/mol, zeroth order kinetics and a detection

limit of 20W/kg for the DSC (Keller, 1997). The maximum allowable process

temperature Tp,max is regarded as safe if the adiabatic time to maximum rate of the

secondary reaction is greater than or equal to 24 hours, which leads to the following

correlation:

[Eq. 4.1]..max, K50T650T onsetP +=

A more general correlation between Tp,maxand Tonset of a DSC measurement assuming

zeroth order kinetics (Dransfeld, 2001) is:

+

-=

2

onsetP

onsetA24

A

2

onsetonsetP

K50TRc

qEtmr

E

TRTT

)(lnmax, [Eq. 4.2]

whereR is the universal gas constant, EA the activation energy, tmr24the adiabatic time

to maximum rate of 24 hours, qonset the detection limit of the DSC and cp the heat

capacity.

The kind of kinetics (normal or autocatalytic) of the heat generation can be qualitatively

determined using the peak profile of an isothermal measurement.

Remark:

The definition of an upper safe temperature limit using a safety margin or an adiabatic

time to maximum rate is based on zeroth order kinetics. This simplifying assumption is

conservative for normal (nth order) kinetics but not necessarily for autocatalytic kinetics.This means, that the interpretation of screening traces to large scale manufacture

requires great care and experience in identifying reactions or decompositions that are not

proceeding via normal kinetics, i.e. screening tests are not like running a melting point

determination, where one just reads off the number from the trace!

4.4.2. Quantitative Evaluation

4.4.2.1. Heat Flow Profile

The measured heat flow contains the heat generation rate of the chemical reaction andother thermaleffects3 - the baseline. However, for evaluation only the heat generation

rate of the chemical reaction is of interest. Hence, the heat flow must be corrected prior

to an evaluation.

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3 E.g. many neat solvents reach their critical temperature (Acetone: 235C; Isopropanol: 235C; toluene:

319C; water: 374C) during screening measurements which causes an abrupt change in the baseline

profile. The same effect can be encountered when testing solutions.


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a) Baseline Correction

The baseline represents the theoretical profile, which would have been measured if the

reaction/decomposition would have taken place without any heat release. The difference

between the measured heat flow and the baseline is the heat production rate of the

reaction. For a baseline correction the peak limits have to be fixed, which can be quite

difficult - especially with a noisy and/or curvy heat flow profile. In the reaction interval

the unknown baseline must be interpolated, which is often an error prone task.

Commercial evaluation software often offers a large variety of interpolation methods,

e.g. linear, integral, spline, horizontal etc. The baseline type determines the obtained

results and should be chosen carefully. For normal scanning experiments an integral

baseline, which shows a smooth profile, is recommended (figure 4.2). For normal

isothermal measurements a baseline horizontal from the peak end is recommended. If

there is any doubt about the peak limits or the baseline profile, a second experiment on

the sample may help with the determination of the peak limits and baseline profile.

exo

heatgeneration

rate

temperature / time

Figure 2: Idealised screening curve with integral baseline

Peak limits and interpolated baseline should be plausible and chosen in a way to give

rather conservative results, i.e. lowest onset temperature and largest heat of reaction or

decomposition.

b) Deconvolution

Ideally, the difference between measured heat flow and baseline, although representing

the heat production rate of the chemical reaction, should not directly be used for akinetic or TMR evaluation. This is caused by the convoluting effect of the sample

container acting as a heat buffer, which leads to a broader peak. However, it has to be

pointed out that a TMR evaluation without deconvolution usually leads to conservative

results. The convolution is usually characterised by a time lag constant, tsignal, which can

be obtained by the evaluation of the rising part of a negative melting peak. The true

heat production rate of the chemical reaction, , can be recalculated (Hemminger,

1989) from the baseline-corrected heat flow using t

q&

bq& signalaccording to:

bsignalb qtqtq &&& t+= )()( [Eq 4.3]

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c) Heat of reaction

The integration of the baseline-corrected heat flow over time gives the heat of reaction,

which is a measure of the thermal hazard. Based on the amount of released heat

decisions are made how to proceed with further investigations.

The thermal hazard is usually related to the adiabatic temperature rise, which can be

calculated by dividing the heat of reaction with the heat capacity of the sample. An

adiabatic temperature rise of 50K, which equals a heat of reaction of about 100J/g for a

typical organic liquid, is often used as lower hazard limit. This means, if an exothermic

process generates less than 100J/g than this process is regarded as not hazardous

providing gaseous products or vapour are not generated in significant amounts. If the

sample undergoes several exothermic processes, the generated heat of each peak relative

to the temperature differences of their onset temperatures has to be considered.

The investigated sample may be subject to transport regulations. The UN committee on

transport of dangerous goods has defined a value of 300J/g as the energy limit for tests

on self-reactivity. If this limit is exceeded, further tests are needed as outlined by the

United Nations recommendations.

If the heat of decomposition exceeds 500J/g, especially in combination with a sharp

peak, the sample may be an explosive4. If the decomposition energy is higher than

500J/g further tests on explosibility are recommended.

d) Kinetics

In principle, screening calorimeters offer the capability to determine kinetic parameters

from isothermal and scanning experiments. Calculation methods for establishing

reaction kinetics from several scanning experiments are proposed in the literature e.g.

/8-10/. It was shown that reliable kinetic parameters can be obtained by quantitative

screening methods, such as DSC (Wagner, 1996). However, the determination of

reliable kinetic parameters is limited to simple homogenous reactions. Kinetic

parameters obtained from complex reactions give only an indication and can not be used

for quantitative interpretations or predictions.

From one scanning measurement an estimation of an overall activation energy can beobtained when plotting the natural logarithm of the corrected heat flow vs. the reciprocal

absolute temperature. The initial slope represents the negative activation energy divided

by the universal gas constant.From several scanning measurements at different heating

rates an estimated activation energy can be obtained using the Ozawa plot (Ozawa,

1970) - the natural logarithm of the heating rate vs. the reciprocal absolute peak

temperature. The slope of the resulting straight line represents the negative activation

energy divided by the universal gas constant.

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4

This energy limit is based on the experience that an explosive with a decomposition energy of less than500J/g is not known (Grewer. 1994).


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In isothermal mode the activation energy can be obtained by plotting the natural

logarithm of the maximum heat production rate versus the reciprocal absolute

temperature. The slope of the resulting straight line represents the negative activation

energy divided by the universal gas constant.

e) Time to Maximum Rate (TMR)

TMR can be used as an alternative to the use of a safety margin in order to define an

upper safe temperature limit of a process. The calculation of TMR is based on several

isothermal measurements at different temperatures. Using the maximum heat production

rate and the isothermal temperature, the TMR for a given set-temperature Ts can be

approximated by extrapolation, assuming a reaction of second (Townsend, 1980) or

zeroth order (Grewer. 1994):

AS

2

SP

ETq

TRc

TMR

=

)(& [Eq. 4.4]

where cp is the heat capacity, R the universal gas constant, (Ts) the heat production

rate at T

q&

s andEA the activation energy. Even though the application of TMR seems to be

straightforward, one has to consider that EA is typically determined from three

isothermal measurements, i.e. from three data points, and q (T& s) is obtained by

extrapolation far outside the regression interval. Taking the temperature where TMR

equals 24 hours and a correlation coefficient for three points of 0.999, the time span can

have an uncertainty of three hours. For a correlation coefficient of 0.99 the uncertainty

may be as big as eight hours!

As a rule-of-thumb: A secondary reaction does not generally pose a threat to the process

if TMR is longer than 24 hours at the highest attainable process temperature.

4.4.2.2. Pressure Profile

Some calorimeters enable the measurement of the pressure on-line indicating the

amount and rate of gas generation, which can afford valuable information about

hazardous gas production. The plot of the logarithm of the pressure vs. the reciprocal

absolute temperature gives an indication whether the investigated sample generatesgaseous products. An approximately straight line in such an Antoine plot indicates that

the pressure, in the sample container, rises due to vaporisation rather than the generation

of non-condensable products.

Considerable deviations of the slope from 10.5 times the absolute boiling temperature of

the liquid5 indicate possible generation of non-condensable products.

5Many organic (non-polar) liquids follow the Trouton rule reasonably well, i.e. the heat of evaporation

over the universal gas constants is approx. 10.5 times the absolute boiling temperature of the liquid.

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Since the pressure measurement is not convoluted, the time lag constant, tsignal, can be

checked or determined by measuring a simple gas producing reaction, e.g. an azo

decomposition. The profile of the heat production rate and pressure rate should be

identical if the correct tsignalwas used for deconvolution.

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4.5. Comparison of Thermal Screening Calorimeters

Calorimeter

Typica

l

sample

size

Tempe

rature

rangeinC

Modeof

operation

Stirring

Rawdataas

functio

nof

time

Sensitivity

Tabula

ted

inmW

/kg

Pricein

kEuro

DSC

Heat Flow 2-10 mg -50 to

700

Isothermal

T-ramp

no T, q 20 35

Power

Comp.

2-10 mg -50 to

700

Isothermal

T-ramp

no T, q 20 35

Twin DTA

TAM 0.5-2 g 20 to 80 Isothermal yes Q 10-4 70

C80 0.5 1.5

g

20 to

300

Isothermal

T-ramp

yes6 T, q, p 1 60

IET T-ramp T, DT

Single DTA

Carius tube 5-20 g 20 to

400

T-ramp no T, DT, p

DPT T-ramp T, p

Sikarex 1

Radex 1-3 g 20-400 T-ramp no T, DT

Sedex 2-100 g 0-400 Isothermal

T-ramp

Adiabatic

yes T, DT, p 80

TSu

1 7 g 20 to

400

Isothermal &

Several T

ramps

no T, DT, p 26

6 Mixing performance is limited. On-line pressure measurement not possible when

mixing.

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4.6. References

Grewer, T. Thermal Hazards of Chemical Reactions; Elsevier B.V., 1994

Hemminger, W.F.; Cammenga, H.K. Methoden der thermischen Analyse. Springer-

Verlag, 1989

Hofelich, T.C.; Thomas, R.C. Int. Symp. on Runaway Reactions; AIChE 1989; p.74

Hugo, P. et al;Z. Naturforsch. 50a (1995); p.549

Keller, A. et al;J. Loss Prev. Ind. 10 (1997); p. 31

Kissinger, H.; Anal. Chem. 29 (1957); p.1702

Ozawa, T.;J. Therm. Anal. 2 (1970); p. 301

Ozawa, T.;J. Therm. Anal. 9 (1976); p.369

Steinbach, J.; Chemische Sicherheitstechnik; VCH, 1995

Townsend, D.I..; Tou, J.C.; Thermochim. Acta 37 (1980); p. 1

United Nations; Recommendations on the transport of dangerous goods; Test and

criteria, 2nd revised Edition, 1995

Wagner, S.; Experimental and theoretical investigations of the use of Differential

Scanning Calorimetry as a thermokinetic measuring method (in German); PhD Thesis;

TU Berlin, 1996

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Documents

4.Thermal Screening