I.CHEM.E. SYMPOSIUM SERIES NO. 85

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1 APPLICATIONS OF THE ACCELERATING RATE CALORIMETER IN THERMAL HAZARDS EVALUATION J. Keith Wilberforce* The Accelerating Rate Calorimeter (ARC) is used in the study of exothermic reactions. Older adiabatic conditions it produces temperature, pressure and time data, which is extremely useful in thermal hazard evaluation. After briefly describing the ARC and its operational principle, the paper discusses a wide range of specific applications. 1. INTRODUCTION The Accelerating Rate Calorimeter (ARC) is a well established tool for the evaluation of thermal hazards, being used by more than 200 establishments worldwide. In its short history, much has been written about the ARC, particularly concerning the instruments, design, construction, operation and its thermokinetic performance as characterised by its measurement of the decomposition parameters of pure di-t-butyl peroxide (1,2,3). To date, rather less has been published on the application of the ARC data to industrial thermal hazard problems. Interox Chemicals purchased an ARC in 198O and in the ensuing 4 years have gained substantial experience in operating the instrument and applying the resulting data to a wide range of problems associated with the research, production, storage, transport and application of unstable chemicals. The purpose of this paper is to review those experiences. 2. THE INSTRUMENT AND ITS OPERATION Ihe key factor in the design of the ARC is the maintenance of a near perfect adiabatic environment in which the reaction under study, takes place. The design philosophy of the complete ARC system is threefold. Firstly it is fully automated and hence requires minimal operator attention. Interox Chemicals Limited, Widnes 329

2 Secondly, incorporation of a standard design establishes data uniformity which enhances absolute data accuracy. Finally, the ARC system has preprogrammed data processing routines which facilitate the generation of the most useful types of data for kinetic and hazard evaluation. The calorimeter jacket and sample system, used to ensure adiabatic conditions, are shown in Figure 1. The sample holder which normally contains between 1 and 1O gm of solid or liquid is suspended inside the calorimeter jacket. This jacket is constructed from nickel-plated copper and contains three thermocouples and eight heaters. A fourth thermocouple is attached to the outer wall of the sample holder. The thermocouples are proprietary Type N. They are chosen to overcome thermal hysteresis of effects associated with other types of thermocouples. The sample holder is directly connected, by capillary tubing, to a diaphragm type pressure transducer which continuously mcnitors the reaction pressure. Operating ranges are O-500 C and O-2500 psia. Figure 2 shows a schematic of the Accelerating Rate Calorimeter. All ARC operations are controlled by the microprocessor system. Push button switches, a keyboard, status lights and a numeric display are provided. In addition there is a line printer for presentation of experimental parameters and data. The cycle of searching for and following an exothermic reaction is programmed to permit user selection of the run parameters and to provide online display of the most important variables such as time, temperature and pressure. A search for an exothermic reaction is achieved by elevating the sample temperature by a fixed increment and then checking to see if the sample self-heat rate exceeds a user selected threshold (usually 0.2 C min 1 ). Once an exotherm is detected, automatic collection of time, temperature and pressure data is carried out until the reaction has finished. This is shown in Figure 3. The processor continuously controls the temperature difference between the sample holder and the three separate zones of the calorimeter jacket. Our experience suggests a normal operating temperature difference between jacket and sample holder of ±0.007 C. Sample and jacket thermocouple inputs, together with span and zero reference voltages, are multiplexed through a voltage to frequency converter circuit to provide stable, low noise, operation. At low self heat rates, additional filtering of thermocouple inputs is achieved by time averaging of signals. An ice point unit references all thermocouples to 0 C for accurate temperature measurement. This unit contains an actual block of pure water which is held exactly at freezing point by a small refrigeration unit. The ice point is stable within 0.01 C. A typical ARC run initially requires about 1O minutes of operator time to enter the run parameters, weigh the sample and locate it in the calorimeter. Thereafter the experiment proceeds automatically without the requirement for any further operator attention. At the end of the run (typically less than 24 hours later), a further 15-2O minutes of operator time is required in order to plot all of the raw and processed data. The results from a single ARC experiment are shown in Figures It can be seen that a very wide range of information is made available, and given proper interpretation, this renders the ARC a very efficient generator of 330

3 thermal hazard data. Applications of the data are discussed later. Apart from efficiency, our experience has confirmed the ARC to be a very safe method of data acquisition. The sample system is surrounded by a steel clamp shell which is rated in excess of 1 ounce of TNT. Given the maximum sample size of c 1O g, the ARC is a safe open laboratory instrument for even the most powerfully explosive materials. Its introduction at Interox has enabled a massive reduction in the need for dewar vessel tests, which, since they involved several hundreds of grammes of samples, required specially protected test enclosures and often resulted in violent condensed phase or vapour phase explosions. In addition the smaller ARC sample size has largely removed the requirement to produce potentially dangerous development products on kg scales in advance of hazard evaluation. Another advantage we have experienced, is the virtual elimination of operator related deficiencies in the data collected. In this respect, the fully automated operation has yielded a greatly enhanced confidence in results. In thermal hazard testing it is vital that measurements are made under conditions which are representative of those under which the hazard is anticipated. In this respect, the ARC's automatically controlled adiabaticity is crucial and well known. However it is equally important to design the experiment correctly, particularly with respect to the choice between sealed and open sample containers. The choice should take into account several factors. Most notable are the effects of evaporative cooling, air oxidation and the retaining of materials in the sample mass which may catalyse or inhibit the exothermic reaction under study. A further choice which has to be made is that of the material of construction of the sample holder. This is particularly true in the peroxide chemistry and other fields where surface induced effects can be massive. With the above in mind it has been necessary to develop a wide range of sample holders (in addition to those supplied with the instrument) so that such effects could be eliminated. Holders made from glass, Aluminium and a variety of steels have been readily produced in a small workshop. A significant disadvantage of the ARC is in the loading of solid and viscous liquid samples into standard holders (4). In Interox the problem has been largely overcome by the use of a screw top holder, specifically developed for the purpose (Figure 14). Another disadvantage is the inability to stir samples continuously whilst undergoing test. It is understood that a stirred sample holder is available in the USA. Its introducticn in Europe is eagerly awaited. 3. DATA APPLICATION The thermal hazard evaluation of a chemical system involves many aspects. The ARC supplies data by means of which each aspect may be evaluated. 3.1 How to keep the chemical reaction under control? The answer to this question is required at all stages of the processing, handling, storage and transport. In every case the answer is in the same form ie. the rate of removal of heat from the system must be greater than or equal to the rate of heat generation by the reaction. 331

4 If we fail to do this, then the reaction rate will continue to accelerate until its maximum rate is reached. Classically, the latter is the point of thermal explosion. A normal ARC run provides the rate of self heating of the chemical system as a function of temperature, under adiabatic conditions. This is shown as Figure 5 in the earlier illustration. The use of such data is now considered in more detail by means of an example. Figure 15 shows similar data from a single ARC run plotted in a slightly different way. Whereas the ARC plots the data as log of self heat versus temperature (on a 1/T scale), for clarity Figure 15 uses purely linear scales. The curved line represents the.adiabatic rate of heat generation, as produced by the ARC. This specifies the minimum cooling rate required to maintain control of any operation of the chemical system, at any temperature. If we consider the operation of a reactor, the straight line ABC may represent its cooling system. This may be an existing cooling system or a design proposal. The average coolant temperature is shown at point A. Ihe slope of the line is given by the function "UA" of the cooler where U is the overall heat transfer coefficient and A is the heat transfer area. The reactor equipped with this cooling system will operate safely at point B. Any small increase in reactant temperature above B will be controlled since the cooling rate will then be higher than the generation rate. If the reactant temperature ever exceeds C, control will be lost and the reaction will runaway. So in this simple way we can specify all the chief parameters for safe normal operation. In addition to this we can use the same ARC data and same basic approach to evaluate the consequence of abnormal operating conditions. For example, if our coolant temperature increases, the straight cooling line will move to the right but remain parallel with its original position. Eventually a point will be reached where the reactor is only just controllable. This is represented by the line DE. Point D is the highest coolant temperature for which the reactor can be safely operated. Reverting to our normal operation as represented by ABC, the effects of another type of maloperation can also be investigated. If the coolant flow rate is reduced or the cooler becomes 'fouled', the slope of the cooling line, will become less steep. Such a case is shown by the line AFG. It is easily seen that, without any change in the coolant temperature, the stable operating point increases from B to F and the maximum recovery temperature falls from C to G. The example chosen above was that of a reactor with an applied cooling system. The same principles could be used for other unit process or operation as long as rate of heat generation and removal are within the measuring range of the ARC. The sensitivity of the ARC is such that it can measure rates of heat generation down to the range O:l to 1.O mw.g 1. This is sufficient sensitivity for most process decisions involving thermal hazards. In terms of storage the data can be used, with only a little extrapolation, for quantities up to 25 5O kg. Where there is significant knowledge 332

5 of the chemical reactions involved, the extrapolation may be extended so that safe storage of kg masses may be specified. This point has been illustrated by measuring, on the ARC, the Self Accelerating Decomposition Temperatures (SADT) of a range of organic peroxides for packaged storage in quantities between 25 kg and 20O kg(5). Results were compared with those obtained from full scale SADT tests and agreement found to be excellent. For larger masses the ARC is too insensitive (as are other techniques such as Dewar tests and Sikarex). For these larger masses, the rates of heat loss are so low (and hence so too are the critical rates of heat generation) that measurement can only be achieved directly by a microcalorimeter or indirectly by measurement of long term concentration changes. 3.2 If control is lost, how much time is available? So far we have concentrated on using ARC data to specify requirements to keep our chemical system under thermal control. Having done this, one is faced with answering another type of question. On certain occasions (hopefully they will be rare), control will be lost and so the question of corrective action will need to have been dealt with. The type of corrective action will depend upon, among other things, the amount of time available before the reaction reaches a dangerously fast runaway rate. For example if the time available is 1 week, then almost any type of action can be contemplated. If only 1 second is available, then nothing can be done and the reactor will need perhaps to be barricaded. If 1 hour is available, one could envisage a procedure which permitted ca 50 minutes to physically rectify the situation and this would still leave 1O minutes for evacuation if it became necessary. During an ARC run, the time between any temperature and the point of maximum, reaction rate, is directly measured and recorded. Of course, the reaction occurs under adiabatic conditions (worst case) and hence this "time to maximum rate" is directly applicable to the questions mentioned above. Figure 6 shows an example of time to maximum rate data from an ARC run. It is easy to see that an operating temperature of about 127 C would, in this example, provide one hour for corrective action and evacuation. It is important to note that the ARC is unique in its ability to directly measure time to maximum rate. An alternative method of obtaining this data is to predict appropriate values from a knowledge of kinetic parameters. However, great caution is required. Errors in activation energy have a devastating effect on the accuracy of the predicted time to maximum rate. Tnis is shown in Figure 16, where an error of even 10% in the prediction of activation energy causes an enormous error in time to maximum rate. This point has been considered in more detail elsewhere (6). In summary, if time to maximum rate is to be used as an operational criterion, it is desirable to measure it directly otherwise errors in estimation are likely to result in serious erosion of safety margins. Alternatively, the inclusion of a large safety margin may result in the unnecessary loss of progress efficiency through operating at too low a temperature. 333

6 In our experience it has been found that the ARC can directly measure times to maximum rate up to a limit which seems to be in the range 5-1O hours. Only very rarely have longer times been measured. For most of the process safety decisions, mentioned above, this is quite sufficient. Given knowledge of the chemistry involved, a little extrapolation can safely be carried out. 3.3 If control is lost can vessel bursting be prevented? During each ARC run, the pressure, temperature and time are automatically recorded throughout the adiabatic runaway. The pressure data is presented is several forms (Figures 7-1O). This information is intuitively useful in predicting the pressure-time behaviour of a vessel undergoing an exothermic runaway. Equally it has value in specifying pressure control measures. However, the phenomena involved are usually very complicated and resulting predictions of poor accuracy. In particular, during an ARC run, a relatively large (known) proportion of the heat of reaction is lost to the heating of the sample holder. The effect is to restrict the final temperature to a value significantly below that achievable on a practical (larger) scale where the thermal inertia of the vessel may be much smaller. Inevitably the need for extrapolation is thus incurred. The outcome has been the necessity to include very large safety margins in pressure calculations resulting in vents which are often grossly oversized. In practical terms this has come to mean a greater emphasis being placed on runaway prevention combined with emergency measures aimed at restricting the runaway to its early phase. Measures such as dumping, dousing and emergency cooling have all been employed. 3.4 Chemical aspects So far we have considered the application of ARC data to the engineering aspects of thermal hazard evaluation. Such aspects are important but always require to be underwritten by a sound understanding of the chemistry involved. Under the adiabatic conditions in the ARC, an equivalence exists between temperature and reactant concentration. Similarly rate of temperature rise and rate of reagent consumption are also equivalent. With this in mind, it is obvious that ARC data can be used to identify reaction mechanisms and determine kinetic parameters. Firstly by considering the start and end point of the exotherm, useful information can be obtained. Clearly the adiabatic temperature rise can be used to determine the overall heat of reaction (after allowance has been made for the heat capacity of the sample holder). Similarly, the pressure remaining in the holder when recooled after completion of exotherm, can be easily used to determine the total amount of gas liberated during the reaction. For a single Arrhenius reaction, the activation energy, preexpotential factor and reaction order can be readily calculated from the thermal data as shown elsewhere (1). A check of these calculations and the validity of the underlying assumption of a single reaction can then be checked by using software within the ARC to predict the experimental results from the calculated kinetic parameters. An example of this is shown in Figure 13. The dots are raw experimental data points and the continuous line is a prediction from calculated kinetic parameters. 334

7 If required further checks can be made by calculation. For instance, kinetic parameters can be estimated independently from the pressure data and compared with those which are temperature based. So far, we have ssen that ARC data can be readily used to carry out kinetic evaluations of single reactions. However, systems of interest are often more complex and it is of fundamental importance that such complexities of mechanism are recognised. The ARC has also proved to be useful in this area. The ARC's main value is in identifying non simple mechanisms. The shape of the self heat and pressure rate curves are particularly indicative (Figures 5 and 9). A single Arrhenius reaction will always provide a characteristic shape of this log rate vs 1/T scale. Initially, when concentration depeletion effects are small, the exponential rate increase with temperature is evident (a straight line on these plots). As the reaction proceeds and concentration depletion effects become more significant, the rate of increase is reduced until a maximum rate is reached where temperature increase and concentration depletion effects exactly balance. Thereafter a steady fall in rate occurs. Any variations in this shape are indicative of a more complex mechanism. A common complicating factor is that autocatalysis of which styrene polymerisation is an example. ARC work on this subject has been reported elsewhere (7). Figure 17 shows the results of one experiment. The steep initial slope of the heat rate curve is indicative of the autocatalytic mechanism (in this case the consumption of t.butyl catechol inhibitor). From the hazard point of view the recognition of such a mechanism is vitally important, since the occurrence of self heating will depend not only on temperature but also on the thermal history of the sample and its inhibitor concentration. The same paper (7) goes on to confirm that the self heating onset temperature decreases with sample ageing. Figure 18 shows the heat rate curve from an ARC run on a system of series reactions. Repeating the experiment at a higher thermal dilution (heavier sample holder or smaller weight of sample) results in the separation of the two reaction traces (Figure 19). It is then possible to analyse each reaction separately. When considering parallel reactions (Figure 2O), it is possible to suggest several (as yet unproven) possible solutions. Ageing of the sample at ca. 20 c below the onset may remove one of the reactions preferentially (in this case A-C). If so a further exotherm run could then be used to evaluate A-B on its own. Alternatively, if the two reactions produce different (known) amounts of gas, then the combination of pressure rate and temperature rate may be used without recourse to extra experimentation. 4. CONCLUSIONS The ARC has been used over the last 4 years as the main technique of thermal hazard evaluation at Interox. In that time it has proved to be of very valuable in identifying sources of thermal hazard and providing the data necessary for the specification of prevention and control measures. 335

8 Each ARC run can provide a wealth of high quality engineering, kinetic and mechanistic information with minimal operator intervention. In our experience it has proved to be a reliable, safe and most importantly an efficient generator of thermal hazard data. As with any technique, it has its measuring limits which must be borne in mind, All the common caveats about data extrapolation should be applied. REFERENCES 1. D.I.Townsend and J.C.Tou, Thermochimica Acta, 37 (1980) J.C.Tou and L.F.Whiting, Thermochimica Acta, 48 (1981) D.W.Smith et al, American Laboratory, June 198O. 4. C.F.Coates, Chemistry and Industry, 19th March 1984, T.D.Manly and J.K.Wilberforce, The use of the ARC to determine the SADT of organic peroxides. Unpublished Interox report. 6. J.K.Wilberforce, North American Thermal Analysis Society, Williamsburg, Va., September Columbia Scientific Industries Corp., Technical Information Bulletin 'ARCHIVES' No. 4. Figure 1. Calorimeter assembly and sample system. 336

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11 Figure 10. Plot of pressure rate vs temperature rate for the adiabatic decomposition of DTBP. Figure 11. Plot of pseudo zeroorder rate constant vs reciprocal temperature for the adiabatic decomposition of DTBP. Figure 12. Plot of experimental activation energy vs temperature for the adiabatic decomposition of DTBP. Figure 13. Plot of predicted self-heat rate vs reciprocal temperature for DTBP assuming an activation energy of 36.5 kcal/mol and an order of one. 339

12 Figure 14. Common ARC sample holders. From left to right: (a) Standard CSI spherical liquid holder. (b) Standard CSI wide-mouthed solids holder (c) Interox screw-top multipurpose holder 340

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