I. CHEM. E. SYMPOSIUM SERIES NO. 68

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1 ADIABATIC CALORIMETRY AND SIKAREX TECHNIQUE L. Hub* The suitability of adiabatic calorimetry for safety investigations, the specific requirements on the experimental set-up and the problems of correct interpretation of the results are discussed. The various aspects of these investigations such as detection of the starting temperature, evaluation of the time-to-explosion, accuracy limitation and influence of the drift and water equivalent, are considered. A brief description of the safety calorimeter Sikarex is given and the specific features of the adiabatic technique with this apparatus are described. Purpose ADIABATIC CALORIMETRY IN SAFETY INVESTIGATIONS Adiabatic calorimetric measurements are usually used for one or more of the four following purposes: 1. Safety investigations, expecially measurements for determining the course of an adiabatic self-heating process. Processes proceeding under adiabatic conditions (no exchange of heat between the compound or mixture and the surroundings), are generally accepted as representing the worst possible case, namely the total loss of cooling capacity. Such measurements often do not have the character of real calorimetry, because the result is a curve representing the temperature rise during the process. The usual calorimetric result, expressed as the heat of reaction, often cannot be evaluated due to deflagration of the compound toward the end of the experiment. Similarly the calorimetry itself may not be a matter of interest. 2. Search for the so-called "starting temperature". The lowest temperature by which an exothermicity of a reaction can be detected, is often called the "starting temperature". This "parameter" is not a defined physical or thermodynamic property of a material because it is dependent on the sensitivity of the instrument, the method used and other factors. However, the starting temperature, if properly defined, does provide us with information about the temperature at which the reaction is proved to be running. * SANDOZ Ltd., Basle, Switzerland 3/K:l

2 3. Derivation of kinetic data from the temperature or pressure changes obtained from a process running under adiabatic conditions (1). 4. Calorimetric measurements. From the adiabatic calorimetric measurements the heat of reaction can be obtained, a technique which is well known and used (2), but is of less importance in safety investigations. Measurement techniques Various methods can be used to approach the adiabatic process. Sample size. Generally, the larger the sample, the nearer the conditions are to adiabatic. However, on the laboratory scale the size of sample is mostly too small to achieve an acceptable approximation. Good insulation of the material is a very common method of achieving an approximate adiabatic condition. Simple Dewarvessels are frequently used for this purpose. With an ambient temperature different from the sample temperature, a true adiabatic process can never be obtained with this technique. There is an exchange of heat with the surroundings, which depends on the size and quality of the Dewar-vessel and also on other experimental variables. (Sometimes the insulation abilities of small Dewar-vessels are over-estimated). Other insulation techniques are also known but they are mostly used in combination with other devices. The combination of large sample size and good insulation (e.g. experiments in large Dewar-vessels) is a simple way to achieve small heat exchange, however, this technique has disadvantages as explained below. Control of the ambient temperature. This technique for achieving good adiabatic approximation is almost unavoidable, if small samples and/or high accuracy is required. This method is mostly used in combination with other above mentioned techniques such as measurements in Dewar-vessels installed in a heated oven (3). The Sikarex apparatus uses an air layer insulation combined with adiabatic temperature control. Problems of adiabatic measurements Accuracy of the measurement. A true adiabatic process is by definition a process with zero heat exchange. It is therefore only a theoretical phenomena, all real practical cases being approximations of this. Every instrument exhibits some heat flow from or to the sample. The corresponding error of the measurement depends on the construction of the instrument itself and often also on the conscientiousness of the operator. As a result, temperature drift can be observed also when inert compounds are tested. Correspondingly an existing small heat evolution can be suppressed by this non-adiabatic behaviour of the instrument or an exothermic reaction can be simulated where none exists. This inaccuracy of adiabatic control has a major influence in the early stages of selfheating process. Fig. 1 reflects the influence of a drift on the adiabatic measurement. The curve a represents a theoreti- 3/K:2

3 cal case of an adiabatic process. If heat losses (negative drift) are larger than the heat evolution, the reaction can be suppressed and curve b results. A reaction supported by a heat flow from the instrument into the sample simulates a faster reaction (curve c). Water equivalent of the equipment. By controlling the ambient temperature to be equal to that of the sample temperature, one part of the heat produced by the process will be consumed for the heating of the sample container. This effect, expressed by the water equivalent of the equipment, creates another type of error. This is demonstrated by the curve d in Fig. 1. Depending on the ratio of the heat capacity of the sample and the water equivalent, the temperature curve would differ from the theoretical adiabatic curve more and more in the direction of slower and less exothermic processes. Limitations of the temperature control at high speed. On the other hand,the limited maximum rate of heating-up of the instrument is mostly not critical for safety investigations. High speed of temperature rise normally occurs towards the end of the experiment. This phase lasts usually only for short periods of time, so that only a small proportion of the liberated heat is lost. Errors caused by incorrect temperature measurement. The following sources of error in the temperature measurements can effect the adiabatic process: - Non-linearity and mutual differences of sensors used for measurement of the sample and of the surrounding temperature. - Sample temperature is not realised inside the tested sample but somewhere on the sample holder. - The sensor itself creates heat (e.g. Pt-100 resistence thermometer) which is not compensated. Simulation of real processes. As mentioned above, a process running under adiabatic conditions is generally accepted as the standard measurement in investigating hazard of self-heating processes. Sometimes it is known that the real process in question deviates significantly from the adiabatic conditions. Examples are dissipation of the heat of stirring of highly viscous reaction mixtures, or known high heat losses at elevated temperatures. In such cases it would be ineffective to carry out measurements under adiabatic conditions and recalculate such influences later. The better way is to simulate, as far as possible, all circumstances of the production plant, in the laboratory test. There are a number of other effects which should be taken into the consideration such as the stirring of heterogeneous reaction mixtures, adding of other components during reaction etc. Consumption of test compound, safety. Methods based on measurements with large samples can lead to difficulties with the high consumption of expensive materials. Sometimes damage of the instrument due to the explosion of a large amount of chemicals 3/K:3

4 must be considered. Interpretation of the measurements The obvious interpretation of the adiabatic curve is to assume that the behaviour of the tested sample is representative of the behaviour of the same compound on a large scale. If the temperature rises only a few degrees, and there is no large gas evolution or foaming, the reaction can be considered as harmless from the safety point of view. On the other hand, exponential acceleration of temperature, ending with high rates of temperature increase and possibly with a deflegration, are definitely clear signs of a dangerous reaction. Many intermediate cases lying between these extremes can be easily interpreted. This "qualitative" result is the most important one and is quite reliable. Pitfalls for this interpretation have been already mentioned: negative drift of the instrument, suppression of the reaction in the early stages or the influence of water equivalent. The quantitative interpretation of adiabatic curves is much more difficult, and in some cases extremely tricky. The influence of temperature drift and/or water equivalent can significantly change the profile of temperature rise. Even more dangerous are evaluations based on assumed kinetic models leading to extrapolation outside of reliable measurement. Firstly many decomposition reactions do not have simple kinetics. The quite complicated decomposition reactions often run through a number of different chemical steps with quite different kinetics. Fig. 2 represents some examples of experimentally measured adiabatic decompositions showing reaction running in distinguishable different steps. Details concerning the pitfalls of safety testing and interpretations of adiabatic measurements have been published elsewhere (4). One example is reproduced in Fig. 3. Curve a. represents the typical evaluation of the so-called time-to-explosion, based on the extrapolation of measured data in the direction of longer times and lower temperatures. Curve b represents the measurement with the same compound, when the water equivalent is taken into consideration. Curves c to f represent the confidence limits of such measurements due to a limited sensitivity of the instrument. The diagram, shows that even if a relatively high sensitivity of the instrument is assumed, dangerously large errors could be made. As an example, with a starting temperature of 70 C and with an instrument sensitivity of 0,lW/kg a time-to-explosion of approximately fifteen hours would be found. It can be easily seen from the diagram that in reality the time to explosion under the same conditions could be as short as four hours. It is therefore recommended to make a conscientious interpretation of each measurement and avoid too much mechanized or routinised interpretation. CHARACTERISTIC FEATURES OF ADIABATIC MEASUREMENTS IN SAFETY CALORIMETER SIKAREX Concept of Sikarex measurements Adiabatic measurements are imbedded in the Sikarex safety 3/K:4

5 testing procedure. The safety calorimeter Sikarex itself is a modular system enabling screening, isoperibolic, adiabatic, isothermal and toxicologic measurements (5). For a better understanding of the adiabatic measurements the other operating modes will be briefly sketched her. A more detailed description of the whole system can be found for example in (5). The basic scheme of the apparatus is shown in Fig. 4. The testing tube is inserted in the double-wall jacket, with circulating air. The temperature of the sample is measured with help of Pt-100 resistance thermometers and the temperature of the jacket is measured and controlled correspondingly to the operating mode. Screening measurements can be made with help of a so-called screening attachment to the Sikarex and it is recommended that these are carried out with all unknown compounds. A large proportion of these turn out to be completely harmless (no exothermicity at all) and it would be too expensive and time consuming to test them with other operating modes. Isoperibolic measurements. In our experience, isoperibolic measurements proved to be the most suitable method for investigating the onset of exothermicity (starting temperature). The principle of isoperibolic measurements is shown schematically on Fig. 5 and 6. The ambient temperature is kept constant and the sample temperature is measured. If both temperatures equilibrate (Fig. 5) the tested compound doesn't create any heat. Higher sample temperatures (Fig. 6) depict an exothermic process. Compared with dynamic or adiabatic measurement techniques used to detect the first exothermicity the isoperibolic modes has the following advantages: - The accuracy is higher compared with adiabatic measurement in the same instrument, because only the measurement and recording of the temperatures is necessary, i.e. no adiabatic control is used. - The time of the experiment can be extended, so that autocatalytic reactions can also be detected. - The influence of superimposed physical processes such as evaporisation of solvent in the sample, melting etc. can be easily studied. - Isoperibolic measurements provide a solid base for carrying out reliable adiabatic measurements. The known starting temperature enables one to begin the adiabatic measurements at the proper temperature. The knowledge of the temperature difference between sample and jacket reduces the danger of errors due to the drift. This point will be discussed in more detail later. Isothermic calorimetry is suitable for reliable kinetic investigations and for some engineering purposes. Adiabatic measurements in Sikarex 3/K:5

6 Activities before the experiment. As mentioned above, in the Sikarex safety investigation procedure the starting temperature for an adiabatic experiment is already known from the previous measurements. Also another very important piece of information is already known: The exact temperature difference between sample and the jacket in the steady state, necessary for maintaining adiabatic conditions. Adiabatic measurements. During the adiabatic control mode, the temperature of the sample will be taken as the set point for the temperature control of the jacket. Generally, for the exact adiabatic control, these two temperature readings (sample and jacket) are not equal due to the differences in the temperature measurement with two sensors, (the signals given by these two measurement systems can differ slightly even if the temperatures are in reality equal). In such a situation, a control system keeping both measuring signals at the same value would cause actually an error in adiabatic control. Fortunately, in the Sikarex procedure the necessary difference in the measurement (corresponding actually to the same real temperatures) is already known from the isoperibolic measurement. It is easy then, to equilibrate in adiabatic mode using the same temp difference, as was found in the steady state condition of the previous isoperibolic experiment. This setting should garantee proper adiabatic control. It is still recommended to check for a certain period of time if none, or only acceptable drift is observed. If, in spite of the used procedure, the result is not satisfactory further correction can be made. In this way, the proper setting is checked before each measurement, which increases significantly the reliability of the whole procedure. Also the accuracy of the individual measurement can be influenced by the effort invested in minimising of the drift. Practical long term tests have shown that with moderate effort drift of about 10 C/week can be obtained. Simulation of the real plant condition. As mentioned above, the adiabatic conditions are not necessarily the best experimentell set-up for investigation of real processes. If, in the production plant, the heat exchange between the reacting compounds and the surroundings is known, it can be easily simulated in Sikarex by setting the temperature difference between sample and jacket in the instrument to the proper value. The instrument is calibrated and the temperature differences can be expressed in the terms of heat flow. Such a situation is represented in Fig. 7, where the constantly higher jacket temperature simulates a dissipation of heat of the stirring. Compensation of water equivalent. Even with extremly accurate adiabatic control, an error of measurement will be created due to the fact, that one part of the sample holder (test-tube) will be heated up at expense of the heat liberated in the sample. The Sikarex system provides an unit for automatic water equivalent compensation, called Sikadiff. This attachement is schematically shown in Fig. 8. The system calculates the first derivative with respect to time of the sample temperature and multiplies this by the set value of the water equivalent. The product of this calculation is equal to the necessary power required to compensate the heat consumed by the sample holder. This heat flow will be 3/K:6

7 supplied to the system by means of the auxilliary heater wound on the test-tube. Toxicological Investigations in Safety Calorimeter Sikarex Self-heating process and thermic explosions usually result in conditions, which can be defined as total loss of control. These chemical reactions are often very complicated and run through a number of unknown chemical steps. It is therefore not impossible, that some intermediates formed during these chemical processes can be highly toxic. (One example is the known accident in Seveso). A conscientious safety investigation procedure should also give information about possible toxic hazards resulting from uncontrolled run-away reactions. In our Sikarex safety testing procedure, this hazard is investigated for all adiabatic processes by which deflegration or flow of material out of the instrument is observed. In this case the adiabatic (or other simulating) experiment will be repeated with a simple attachement to collect all materials coming out of the instrument, (gases, condensable vapours, liquids and solids). From these, the condensable vapours, liquids and solids will be mixed together with material remaining in the instrument itself and this mixture subjected to toxicological investigation. (Of course other strategies in handling the resulting products are also possible, e.g. investigation of gases). Practical examples and user experience Since 1965 a number of Sikarex systems have been used routinely so that thousands of experiments under various conditions have been carried out. Experiences were exchanged during "Sikarex-User-Meetings" (5) and some examples are published in (6) and in the manufacturer documentation (7). REFERENCES 1. Townsend D.I., Chem. Eng. Progr. 73 (1977), Hemminger W., Höhne G: Grundlagen der Kalorimetrie, Verlag Chemie, Lütolf J.: Staub, Reinhalt. Luft 31 (1973) Hub L.: Kann eine thermische Gefahr unentdeckt bleiben?, to be published 5. Prospectus material of Systag Inc., Bahnhofstr. 76, 8803 Ruschlikon/ZH, Switzerland 6. Hub L.: Dissertation Nr. 5577, ETH Zurich, Configurations, Applications, Brochure published by Systag Inc., Bahnhofstr. 76, 8803 Rüschlikon/ZH, Switzerland 3/K:7

8 Figure 1 Inaccuracy of the measurement Figure 2 Adiabatic decomposition curves Figure 3 Limits of confidence of the time-to-explosion evaluation 3/K:8

9 Figure 4 Scheme of the Safety Calorimeter Sikarex 3/K:9

10 Figure 5 Isoperibolic measurement, no exothermicity Figure 6 Exothermicity found by isoperibolic measurement Figure 7 Simulation of a real process 3/K:10

11 Figure 8 Scheme of the water equivalent compensation (Sikadiff) 3/K:11