THE USE OF DEWAR CALORIMETRY IN THE ASSESSMENT OF CHEMICAL REACTION HAZARDS

THE USE OF DEWAR CALORIMETRY IN THE ASSESSMENT OF CHEMICAL REACTION HAZARDS R.L. ROGERS* Dewar Calorimetry is one of the simplest and most useful techniques used in the assessment of chemical reaction hazards. The thermal properties of Dewars i.e. their low heat loss rate and low phi factor are discussed and related to the conditions which occur during plant scale manufacture. The different experimental methods which can be used are described and the evaluation of the results to obtain the heat of reaction, minimum onset temperatures and runaway chemical reaction parameters is explained. (Key words: chemical reaction hazards, exotherms, Dewar calorimetry, heat of reaction, kinetics). INTRODUCTION The evaluation of chemical reaction hazards requires the identification of potential thermal instability in the reactants, reaction masses, and products; the measurement of the heat of reaction; and the detection of any gas evolution. The key aspect in any hazard evaluation is the relationship between the experimentally determined data and the conditions which actually occur on a plant scale. (1,2). Unfortunately no single experimental technique is able to produce all the data required to specify safe operating conditions. In most investigations some form of small scale thermo analytical method is generally used for the initial detection of exothermicity and or gas generation. However the use of such data is limited by the sensitivity of such techniques which range from.5 to 5 W/kg. In comparison a typical value for stirred 500 gallon reactor with a filled jacket is from.04 -.08 W/kg/ C. It can be seen therefore, that small scale tests can only be used to give an indication of the possibility of exothermic activity and an approximate value of the minimum onset temperature. They are however, useful for quickly screening large numbers of reactions, but in order to specify safe operating conditions either the results from such small scale tests have to be scaled or extrapolated, or more sensitive tests which more accurately simulate the plant scale heat loss conditions have to be used. The Dewar flask is one of the simplest test methods which has the sensitivity required to simulate conditions which occur on the plant * ICI Fine Chemicals Manufacturing Organisation, Blackley, Manchester 97

scale. Dewar calorimetry is one of the oldest test methods for measuring exothermic reactions and self heating which may lead to runaway reactions. The different types of Dewar Calorimeters that can be used have been reviewed by Grewer (3). This paper explains the principles of Dewar calorimetry, describes the different experimental techniques that can be used, and how the data obtained can be interpreted to assess the potential chemical reaction hazards. PRINCIPLES OF DEWAR CALORIMETRY The heat produced in a reacting mass is used in three ways: a) to heat the mass itself. The temperature rise observed depending on the specific heat of the material. b) to heat the container. The temperature rise is abated, by the phi factor where phi - heat capacity of sample + heat capacity of container heat capacity of sample and c) heat is lost to the surroundings. This is shown schematically in Figure 1. Of these only the heat used to heat the mass itself is not size dependent. Dewars minimise the heat that is lost from a reacting mass since the quantity of material used is relatively large, thus giving a low phi factor, and the vacuum jacketed construction minimises the heat loss to the surroundings. Sensitivity In order to be certain that the laboratory investigation identifies any thermal hazards that may arise on the plant it is important that Rate of Heat Loss <t_ Rate of Heat Loss in the Experiment in the Process The rate of heat loss depend on the heat loss characteristics and the temperature difference. Heat Loss from Process Vessels o The rate of heat loss from a number of large plant vessels up to 12m has previously been measured by filling them with hot water and allowing them to cool naturally (4). The vessels had insulation of various thickness and where the pan had a jacket, experiments were carried out with both the jacket full and empty. It was found that the rate of heat loss depended markedly on the size of the vessel and was in the range of 0,15 to 0.013 Wl" K for vessels ranging from 0.1 to 12 m. 98

Experimental Heat Loss/Adiabaticity Similar experiments have been carried out using 250ml and 500ml glass Dewar flasks, closed with corks, which were found to have cooling rates of 0.077 and 0.03 W.l" K~ respectively. These correspond to the rate of natural heat loss from 0.5m and 2.5 m plant vessels, therefore these plant conditions can be simulated by tests carried out in such Dewars. In order to simulate the conditions in larger plant vessels it is necessary to further minimise the rate of heat loss from the Dewar flask. This can be achieved by placing the Dewar in an oven whose temperature is controlled to follow the sample temperature thus approaching an adiabatic system i.e. one in which no heat is lost from the sample to the surroundings. However it should be recognised that experimentally it is not possible to achieve complete adiabaticity since a control system requires a finite temperature difference between the desired set point and the actual temperature in order to function. If the rate of heat loss from the process vessel is known i.e it has been measured or can be estimated, then the following simple formula can be used to determine the required oven temperature to ensure that the heat loss from the Dewar and oven system match that of the plant vessel:- T Q - T p(q p - Q v) + Q y x T A Where TQ is the required oven temperature, T and T. are the process and ambient temperatures respectively and Q D and Q v are the cooling rates for the Dewar and process Vessel expressed in W.l".K~ This equation can be applied to all vessels and experimental setups. For example: 3 1 1 The cooling rate of a typical 12 m vessel is 0.019 W.l K, if the process temperature is 50 C and the ambient temperature is 20 C then in order to experimentally simulate these conditions in the laboratory using a glass test tube having a volume of 10ml and a heat loss rate of 5.8 W.l" K~, then the shield oven temperature required would be % T - 150 (5.8-0.019) + (20 x 0.019) = 49.9 C 5.8 In practice such a continuous, precise following of the sample temperature by an oven i.e. better than 0.1K is difficult to maintain. By comparison if a 250ml Dewar which has a cooling rate of 0.077 W.l" K~ is used then for the same process conditions the required oven temperature is T Q - 50 (0.077-0.19 + (20 x 0.019) = 42.6 C 0.077

It should be recognised that the above calculations do not make any allowance for measurement errors and the actual experimental tests should be carried out with a smaller temperature difference to include a safety factor. Tests using Dewar are usually carried out under as near adiabatic conditions as possible usually with temperature difference of 1-2K which can be easily achieved using standard laboratory equipment. Thermal Inertia/Phi Factor The second important factor that affects the sensitivity of any experimental technique is the thermal inertia of the system which can be expressed as the Phi factor. Both the total exotherm rise and the rate of temperature rise actually measured will be abated by this factor. Thus if the heat capacity of the container equals that of the sample i.e. phi = 2 then a measured temperature rise of 0.5K is caused by an actual temperature increase of IK. Dewar calorimetry minimises this problem by using relatively large sample sizes and a phi factor of 1.05 to 1.5 can be obtained depending on the specific heat of the contents and the set up of the system. Safety Considerations EXPERIMENTAL SET UP The size of sample used in Dewar testing requires that, before conducting the test, the consequences of a runaway in the test equipment needs to be considered. The compositions being investigated must be pre-screened by small scale testing to seek out materials undergoing high rate decomposition with excessive heat/gas generation (2). In particular, before using a pressure Dewar, the basis of safe operation must be clearly specified. Failure of the Dewar to contain the exotherm/pressure rise must be allowed for by ensuring that the adiabatic shield oven is designed to contain the products which may be evolved and can withstand any vapour pressure generated. Venting needs to be provided to convey away any decomposition gases to a safe place. However, with the exception of pressure Dewar experiments which are carried out in a containment cell, the vast majority of Dewar tests can be performed in standard laboratory ovens fitted with magnetic door catches and situated in fume cupboards. Simple System As has been shown above simple experiments using small glass Dewars can provide information on the stability of reaction mixtures. A sample of the mixture, preheated to test temperature, is charged to a 250/500ml glass Dewar flask closed with a cork (to minimise heat loss from the top) and allowed to stand for prolonged periods with continuous monitoring of the mixture temperature Fig 2. Any exothermicity during the test period indicates a hazard if the reaction mixture is stored (at the test temperature) in plant vessels up to 0,5m /2.5m in size. Process Simulation/Adiabatic Systems Since the Dewar flask can be considered similar to any other reaction flask. The apparatus can be readily adapted to closely mimic the actual 100

plant configuration. Thus facilities for agitation, gas, liquid or solid addition and systems for heating/cooling can be readily provided. Descriptions of such systems are well documented in the literature (3,4,5). Adiabatic Pressure Dewar One limitation of using glass Dewars is that they cannot be used to investigate reactions carried out under reflux or under pressure. Several solutions have been used to overcome this problem. One option is to use a system in which the glass Dewar is contained within an Autoclave. A less successful alternative is to seal the sample in a glass ampoule and place this inside a standard glass Dewar in order to minimise the heat loss. These techniques have been reviewed by Grewer (1). A different approach is to use a stainless steel Dewar which is modified so that it an be fitted with a flange containing a stirrer, thermocouple, internal heater facility for liquid addition and a relief system. The whole assembly is mounted inside a fan assisted oven set to run adiabatically Fig 3, (2). Reaction in the Dewar can be launched by electrically heating the mixture in the Dewar to a start temperature where the reaction rate is low or by charging of one reactant to another. Calibration An understanding of the accuracy and sensitivity of the test apparatus used is one of the most important aspects in interpreting results from chemical hazard tests in order to specify safe operating conditions. As described above the two important parameters are heat loss and the thermal inertia. The main source of heat lost from a Dewar is via the neck and the nature of the closure can have a marked effect (e.g. the rate of cooling of a 500 ml Dewar is 1.6 K/hr with a cork and 2.0 K/hr when fitted with a rubber bung). Similarly agitation can put heat into system if the reaction mass is viscous. The rate of heat loss should therefore be determined for the system being studied. This can be simply achieved by switching off the shield oven at the end of the test and measuring the rate of temperature decrease. The heat capacity of the system can readily be determined either by filling the Dewar with a quantity of hot fluid of known specific heat and measuring the temperature loss or electrically, by measuring the temperature rise for a known power input. For long term induction time measurements with.the Dewar in an oven the stability of the system should be checked by performing a blank experiment with a similar non reactive material. Temperature drift rates of better than 0.1 K/hr or 2 K/day can be readily attained using Dewar systems. EVALUATION OF THE RESULTS Batch Reactions The temperature time curves which can be obtained from a batch or all in process using the different types of Dewar calorimeters are shown in Fig 4. 101

Self Heating Trace A shows the natural cooling curve that is obtained using a simple Dewar with no shield oven, when no exothermic reaction occurs. A constant temperature v time trace is obtained for the same system run with a shield oven set isothermally. Any deviation from the natural cooling curve or from the isothermal temperature indicates that self heating is occurring - curves B and D. If the sensitivity of the experimental set up has been chosen such that its rate of heat loss is less than that on the plant then a self heating hazard will only exist if it is observed in the test, similarly the measured induction time will reflect that which will occur in practice. Heat of Reaction Trace G shows an ideal or theoretical curve for a reaction where all the heat produced is used to raise the temperature of the reactant, i.e. completely adiabatic and phi = 1. Since in a practical situation some heat is always used to heat the container i.e. phi > 1 a more typical curve is shown as F. It can be seen that both the maximum temperature and the rate of temperature rise is abated because of this heat loss. The theoretical adiabatic temperature rise AT can be obtained by multiplying the measured temperature rise by the phi factor. The heat of reaction A H is simply obtained from AH - m CpAT where m and Cp are the mass and specific heat of the reactants. When the same reaction is carried out non adiabatically the temperature rise is further abated by the heat loss from the system - curve E and an additional correction needs to be made to obtain the actual heat of the reaction. A safe approximation is to correct the measured temperature rise by the rate of heat loss at the end of the reaction multiplied by the reaction time. Reaction Kinetics If satisfactory assumptions about the reaction mechanism can be made the temperature time traces obtained from Dewar calorimetry experiments can be analysed to yield thermodynamic and kinetic data. This is of course much easier when the experiment has been monitored by a computerised data logging system. This analysis assumes that the rate of reaction is proportional to the power output or rate of temperature rise and is based on dimensionless rates and concentrations. Thus the dimensionless rate (fraction reacting per minute) is given by the fraction of the total temperature rise that occurs per minute and the dimensionless concentration is given by the fraction of the temperature rise still to occur. These parameters can then be used in the dimensionless rate expression and the activation energy and pre exponential factor determined. This technique has previously been described in some detail. (4) 102

Semi-batch Reactions Since Dewar calorimeters work by preventing heat escaping from the reaction mass, the simulation and investigation of the course of the desired reaction during an isothermal semi-batch process (i.e. one where one reactant is added over a period of time to a second) where heat is being evolved involves studying the reaction by aliquot additions, i.e. the quantity of the added reactant is divided into a number of portions whose size is chosen such that the temperature rise obtained on their addition is measurable but not so excessive that the reaction mechanism would be changed or that side reactions might occur. The reaction mixture must be cooled back to the desired starting reaction temperature after each addition. A typical trace is shown diagrammatically in Fig 5. The heat of reaction can be obtained from the sum of the individual temperature rises and the mass and specific heats of reactants. Corrections for heat loss and phi have to be made as above if an accurate determination is required. Whilst this technique does not directly provide data on the course of a reaction, an indication can be obtained from the shape of the trace. For example, the time for the temperature rise to occur will seem to increase on subsequent additions if the reaction rate is falling off. In addition, this technique readily allows the evaluation of typical maloperations that might occur in practice. For example the effect of a double charge - does reactant accumulation occur; the effect of agitator failure - is there any build up of reactants or does the reaction continue. Runaway Reactions The Dewar Calorimetry can be used to simulate the fault condition of the loss of cooling during a semi batch reaction and therefore can be used to investigate potential undesired reactions. Such experiments can be carried out in normal Dewars to qualitatively identify the likelihood of a hazardous situation occurring. However if it is necessary to quantify the runaway then the experiment has to be repeated in an adiabatic pressure Dewar. The rates of temperature rise and pressure rise can be directly measured and used to design a release system or indeed to directly measure maximum temperatures and pressures to identify whether containment is an option (6). Other Factors Influencing Experimental Results It has to be remembered that the evaluation of chemical reaction hazards using a Dewar is a calorimetric technique. As such any experimental conditions which may occur that effect the heat production or removal will markedly affect the results. For example, if the temperature in an open system reaches the reflux temperature or if gas evolution occurs this will tend to remove heat from the system. Similarly, agitation particularly of viscous reaction masses may in fact cause the temperature to rise. 103

CONCLUSIONS Dewar calorimeters are one of the simplest tools available to the hazard investigator in the evaluation of the potential hazards associated with a chemical reaction. Because of the low heat loss which occurs experimentally in such a technique the data obtained can be directly applied to the plant situation. ACKNOWLEDGEMENT S The author would like to acknowledge the contribution that his co-workers in Hazards and Process Studies Group have made in developing and understanding the Dewar calorimeter and its use in the evaluation of chemical reaction hazards. REFERENCES 1. W. Regenass, I.Chem.E. Symp. Series No. 85. p.l (1984) 2. N. Gibson, R.L. Rogers & T.K. Wright, I.Chem.E. Symp. Series No. 102. p.61 (1987) 3. Th. Grewer, I.Chem.E. Symp. Series No.68. p.2/e:l (1981) 4. T.K. Wright & R.L. Rogers, I.Chem.E. Symp. Series No. 97. p.121 (1986) 5. J. Cronin, P.F. Nolan & J.A. Barton, I.Chem.E. Symp. Series No 102. p.113 (1987) 6. N. Maddison, This symposium 104

HEAT 2 j 3 Heat Produced is u9ed to 1. Heat itself 2. Heat container - PHI factor 3. Heat surroundings PHI factor Heat capacity Sample + Container Heat capacity Sample Figure 1. HEAT BALANCE DIAGRAM Thermocouples M "YCiS Measurement 8 Control Figure 2. SIMPLE DEWAR APPARATUS 105

Bursting Disc Heater Additions Ove Thermocouples Pressure Transducer o Agitator Dewar Figure 3. PRESSURE DEWAR APPARATUS Sample Temp 6 Ideal Adiabatic phi-1 F Adiabatic phi>i E Reaction isothermal Shield Oven D Self Heating C Isothermal B Self Heating No Shield Oven -A Natural Cooling Curve Time Figure 4. TEMPERATURE/TIME TRACES FDR DIFFERENT DEWAR SYSTEMS 106

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