evaluation of chemical reaction hazards and loss... · evaluation of chemical reaction hazards ......

EVALUATION OF CHEMICAL REACTION HAZARDS

P. F. Nolan*

A review is presented of current laboratory methods used for detecting potential. thermal runaway behaviour of batch chemical processes. The methods are described in the context of the development of a process, including reference to the effect of scale of operation. Adiabatic calorimetry appears to offer the best way of following thermal runaway processes but isothermal calorimetry gives valuable thermodynamic, heat transfer data for reactions under relatively normal The data from laboratory methods needs to with other hazard assessment techniques, e.g data from the envisaged plant to ensure safe operation. Reaction hazard evaluation is also of use in defining the basis of process control and any necessary emergency relief sys terns .

Keywords: Laboratory methods, Runaway behaviour, Process development

and kinetic cond i tions . be combined , HAZOP, and

INTRODUCTION

In 1984, Barton and Nolan (1) presented the first of a series of papers (2, 3) on incidents arising from thermal runaway in batch reactor plant. Their analysis showed that many incidents occurred due to the lack of knowledge of the reaction chemistry.

It is axiomatic that in order to be able to properly define a specification for the design, operation and control of reactor plant so as to avoid conditions for runaway arising, it is necessary first to have a knowledge of the chemistry and thermochemistry (kinetic and thermodynamic data) of the desired reaction. It mav be possible to acquire some, at least, of this knowledge from standard reference works (4, 5, 6 ) . Ideally, it should be possible to derive at least the following data from laboratory studies:

a. heat of reaction b. accumulation of react ants/heat and the factors which

affect it c. specific heat of the reaction mass d. rates of heat evolution and the factors, which affect

it e. temperature range and nature of any decomposition

reactions f. gas evolution data g. effects of mischarging, impurities and errors h. heat transfer properties of reaction masses

*Department of Chemical Engineering, South Bank Polytechnic, Borough Road, London, SE1 OAA

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IChemE SYMPOSIUM SERIES No. 115

i. effects caused by materials of construction of plant

Process development usually occurs in four stages, primarily defined by the scale of operation. At each stage, the potential chemical reaction hazard requires definition. Such continuous definition and in some cases re-definition leads to a comprehensive knowledge of the reaction at the final, manufacturing stage. The stages are:

i. concept - usually the chemistry is set down on paper ii. laboratory preparation iii. pilot plant operation iv. full scale manufacture

While a continuous development of safety considerations is recommended, it has to be recognised that a substantia] amount of the experimental work on chemical reaction hazard evaluation occurs, in practice, between the laboratory preparation and the pilot plant operation stage.

FOUNDATION WORK

During the conceptual stage, the literature can be searched for data, potential hazards (including any toxicity problems) identified and some elementary thermochemical calculations can be performed, particularly since heats of formation of many substances are available (7) and calculation methods are given in the literature (8). Computer programs (9, 10) to evaluate heats of reaction are also readily available. The CHETAH (Chemical Thermodynamic and Energy Release Evaluation) program is popular in the UK fine and speciality chemical industry and it only requires knowledge of the basic chemical structures. Craven (11) has also reviewed a method for calculating the exothermicity of a chemical reaction. The method utilises a knowledge of the atomisation energies of reactants and products. The foundation work should provide sufficient evidence of the process viability in terms of scientific feasibility, economics and safety.

SCREENING PRIOR TO LABORATORY SCALE SYNTHESIS

Following the foundation work, it is necessary to characterise the materials in detail, in terms of their potential hazards. In particular, recourse should be made to the standard work by Bretherick (4). This provides a comprehensive list of chemical groups associated with explosibi1ity and prior to any practical work, irrespective of scale of operation, materials encountered in the process, e.g. reactants, intermediates and products, should be tested for detonation and deflagration properties. Cutler (12) has carried out a comprehensive review of such tests. They include the Trauzel lead block test for detonation potential in close confinement and various friction and impact tests, e.g. ballistic mortar, fall-hammer and Koenen tube. It is always advisable to seek expert advice in relation to such testing procedures .

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An approximate guide to the need for such explosibility screening is the calculation of the oxygen balance. For the molecule CxHyOz , the oxygen balance is

- 1600 x (2x + Y/2 - Z) molecular weight

Nearly all known explosive materials have values between - 100 and + 40. It is normal practice to consider any balance more positive than - 200 to indicate the need for explosibi1ity screening tests.

LABORATORY SCALE SYNTHESIS

This stage provides practical information on the reaction, its components, the methods for purification of the reaction products and any waste disposal. Purification, particularly distillation, can be very hazardous and hence the use of very small amounts of materials can limit potential problems within a fume cupboard.

Simultaneously with the synthesis experiments, screening tests can be carried out to determine the existence of any potential thermal hazards. This can be achieved using conventional, analytical techniques such as differentia] scanning calorimetry or by using custom-built, in-house larger scale differentia] thermal analysis ( 1 3 ) . Approximations to the onset of exotherm temperature, the magnitude of the exotherm and the induction time can be determined, however, the results are sensitive to sample size, material of construction, heating rate, thermal inertia and the presence of any endothermic effects (i.e. evaporation, gas evolution and phase changes can mask the onset of exotherms). A DSC employing a low heating rate of say, 1, 2, or 5°C min using sealed gold-lined pans appears the most satisfactory arrangement for potential hazard detection. The typical sensitivity of DSC instruments is 2 - 5 watt kg . Other milli-gram scale techniques (DSC uses samples of 10 rag) may also supply useful qualitative information. These techniques include hot stage microscopy and differential thermal analysis.

The laboratory scale synthesis will provide details of the norma] operating process and the associated use of very small samples, of typically a few milligrams, in thermal methods will provide indicative data on the thermal stability of substances at every stage of the process, i.e. reaction mixture, products at normal temperatures, any distillation residues and any recycled material. It is important to realise that very small sample sizes may not be representative of bu]k reaction mass behaviour and that some thermal responses may not be detected. An associated kinetic study during the laboratory synthesis stage will also provide data on:

a. catalytic effects caused by potential materials of construction of the envisaged plant

b. autocatalytic effects c. heat release rates

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d. rate of gas evolution as decomposition proceeds e. induction time effects f. secondary decompositions, i.e. the desired reaction

goes out of control to promote a subsequent decomposition reaction.

From small scale laboratory synthesis of the desired process carried out in a well-ventilated fume cupboard, it is necessary to consider a more precise definition of the thermochemistry and then the conditions which may stem from process deviations and lead to potential runaway and thermal explosion. The main investigative tools for thermochemistry are calorimeters. Adiabatic and isothermal measurements are usually employed, although further data can also be provided by semi-isothermal and reflux measurement techniques.

CALORIMETRIC TECHNIQUES

Regenass (14) noted that the most important feature for classifying thermal methods is in the treatment of the evolved heat. In accumulation methods (adiabatic), the sample temperature is well insulated from Its environment and its temperature change is used as a measure of the extent of conversion. In heat transfer or heat flow methods the evolved heat flows to the environment in a manner that can be measured, while the sample temperature remains near its set point. The equipment ranges from specialist, expensive commercial instruments to custom-built, in-house apparatus. General designs of the latter are particularly well described in the Association of the British Pharmaceutical Industry guide (15) and by Cronin and Nolan ( 1 3 ) . Cronin and Nolan (16) also give some indication of the comparative sensitivity of the instruments with regard to the temperature, at which an exotherm is first detected.

Consideration must, as always, be given to sample size, material of construction of container, sample heating rate, thermal inertia and instrument response time and detection sensitivity. In many instances, it must be appreciated that the experiment may need to be designed for the specific application, i.e. the experimental arrangement and procedure may need to be tailored in order to obtain the required data. Experience and/or advice is essential. It is not always possible to carry out calorimetric investigations to a set procedure, e.g. substantial rates of gas evolution and subsequent condensation may cause the in-operation of some commercial instruments.

The prime uses of purpose-built calorimetric equipment is the acquisition of data relating to:

i. desired process operation

the definition of - normal operating temperature range

- reactant concentration - pressure

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- addi t ion rate - minimum reaction temperature - acceptable hold time

ii. process with non-specific faults

agitator failure loss of cooling ingress of coolant process maloperation

- overcharging/undercharging

iii. process failure

contamination of batch by reactive chemical change in raw material quality involvement in incident initiated elsewhere, i.e. domino effect.

Many commercial instruments state that they can be operated in a number of heating modes, e.g. isothermal, adiabatic and isoperibolic ( 1 6 ) . The desired process can ideally be studied in isothermal calorimeters.

Isothermal Calorimeters

These are usually classified by the method of heat transfer control (17):

i. Passive system: the heat transfer is induced by temperature changes in the sample due to partial accumulation of the evolved heat.

ii. Active systems: a heat transfer controller causes heat transfer, induced by the slightest deviation of the sample temperature from its set point, i.e. the heat flow can be achieved by use of:

a. Peltier elements b. "back-off" or compensating electrical heating c. secondary fluid for thermostatic control of

reaction mass temperature.

a. Peltier elements: The Peltier effect describes the liberation or absorption of heat at a joint of dissimilar metals where the current passes from one material to another, whereby the joint becomes heated or cooled. Nilsson, Silvegren and Tornell (18, 19) have described an isothermal calorimeter based on the Peltier effect. In such calorimeters, it is normal to use the Peltier elements to maintain isothermal conditions by "pumping" any thermal energy produced to a metallic block, which acts as a thermal heat reservoir. The commercial equipment available tends to bear considerable design similarities to differential scanning calorimeters, but uses a much larger sample and control cell size. The major problem with Peltier calorimeters is that they cannot cope with a large power output

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from the reaction mass. This is because they "pump" rather than conduct heat. The maximum heat transfer rate is fixed by the maximum electrical current passing through the elements rather than by temperature differentials. From practical considerations this usually restricts the maximum rate of heat dissipation to relatively low levels in comparison, to say, heat flow calorimeters. A commercial instrument using the principle is the Setaram C80 heat flux calorimeter. The C80 has an operating temperature range of 293 - 573K and an absolute sensitivity of 10 uW. This corresponds to a relative sensitivity of 0.67 mW kg in an aqueous system with a maximum sample volume of 0.015 dm . Continuous mixing in the liquid phase is possible.

Another small scale calorimeter is the LKB Thermal Activity Monitor and this is intended for use in an isothermal mode. It permits the monitoring of the heat flow between a sample and an "infinite" heat sink. The 0.004 to 0.025 dm sized sample can be held in steel or glass containers. The container fits inside a measuring cell, which is linked via two thermopile arrays to a heat sink block. A reference measuring cell is similarly held within the unit, which is held within a thermostatically controlled water bath. It is primarily used for the isothermal study of low energy exchange systems in the 285 - 363K temperature range.

b. Compensating electrical heating: Power compensation heating has proved to be a popular and inexpensive method for obtaining isothermal calorimetric data. It can also, depending on the quality of the test apparatus, be an extremely sensitive technique. The advantage is that power output from a reaction exotherm can be measured directly. The main differences between the various designs are to be found in the nature and distribution of the electrical heating element and cooling interface .

Schildknecht (20) designed a novel method of compensation heating; in that the electrical heating element forms an Integral part of the inner reactor wall. Cooling coils pass around the outside of this inner wall, and are welded to it, thus providing the thermal "sink" for the compensation principle to function. The outer wall of the vessel is essentially an adiabatic container. The coil chamber is filled with fluid and maintained at the reaction mass temperature. The compensation heater is calibrated by means of an electrical heater immersed within the reaction fluid. By using a thermostatic control the circulating coolant can be maintained at a constant temperature below the reactor set point temperature. The reaction fluid is maintained at the set point temperature by continual pulsing of the compensation heater. Therefore, at equilibrium conditions a constant energy flux exists between the heater and circulating coolant. When a reaction exotherm Is generated, the power output from the electrical heater Is reduced by an amount equal to the power output from the reaction. The reduced heat flux from the heater is, therefore, supplemented by a heat flux from tine

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reaction mass to the cooling coil. The energy dissipated by the cooling coil remains constant at all times.

Wright and Butterworth (21) have described an isothermal calorimeter based on the compensation or "back-off" heating effect. The calorimeter consists of a twin jacketed vessel; the upper half of the vessel is vacuum jacketed whilst the lower half contains the circulating cooling fluid. A heating element is placed within the reaction vessel to dissipate thermal energy in a regulated manner by using pulse width modulation. Since the reaction mass surface area is always greater than the cooling jacket surface area, power dissipation by the jacket remains constant for a given temperature differential between jacket and reaction mass. Hence, dosing into the reactor will not affect the control algorithm. Wright and Butterworth (21) have found that the major problem with the design of the calorimeter occurs in the interpretation of data when there is a significant change in the heat transfer coefficient of the reaction mass during the progress of the exotherm.

Blitz (22) has designed and used an isothermal calorimeter based on power compensation principle to monitor fermentation reactions. The reactions do not include changes in the volume of the reaction mass and there is no significant change in the viscosity of the reaction mass throughout the fermentation. The calorimeter uses a standard jacketed glass vessel through which coolant is passed. Two electrical cartridge heaters are immersed in to the reaction mass, one is used for calibration, the other for power compensation. Blitz claims a resolution better than 0.2 w for a 1 dm reaction vessel; this figure can be significantly enhanced by regression techniques used in the evaluation program.

Stockton et al (23) have designed an isothermal calorimeter which is the basis of the commercial system marketed by Columbia Scientific Industries Inc. The reaction vessel is fabricated from one of several chemically resistant metals, such as Hastalloy. The high thermal conductivity of a metal vessel ensures rapid heat transfer and temperature stability of the reaction mass. The temperature of the reaction mass is regulated by a heat exchanger consisting of heating and cooling elements placed directly beneath the base of the reaction vessel. The vessel is heated by using a circular metal foil resistance heater, insulated by a polymer film, bonded to the top side of a metal disc. The underside of the metal disc is cooled by circulating silicone oil, which serves as the heat sink. Heat flow through the walls and cover of the vessel is eliminated by means of an adiabatic shield. The adiabatic shield consists of a single heated air jacket contained within the outer canister of the apparatus.

Harris (24) has designed a power compensation Isothermal calorimeter. The design uses a Dewar flask as the reaction vessel. Inserted into the Dewar flask is a cooling coil and electrical cartridge heater. The heater power is regulated using

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pulse width modulation techniques. In addition to monitoring the power input to the cartridge heater, Harris also logs a power balance on the cooling coil which is derived from measuring the coil temperature differential and coolant flow rate. Adiabaclty of the reaction vessel is further enhanced by installing the assembly in an oven. System control, data logging and evaluation are carried out using an IBM compatible PC with an instrumentation interface.

The thermokinetic reactor designed by Litz (25) also operates on the power compensation principle. The reaction vessel is surrounded by an intermediate thermostatic bath which in turn is encapsulated within a base thermostat. The reactor consists of a chamber provided with a stirrer, a controlled electric heater and a temperature sensor. The intermediate thermostat is designed in exactly the same manner. The temperature of the base thermostat is maintained at a constant set value via a control loop. The controlled heating of the intermediate thermostat maintains a set temperature difference between the intermediate thermostat and base thermostat, while controlled heating of the reaction chamber maintains a constant set temperature difference between the reactor and intermediate thermostat. Control of both heaters is by a PID control algorithm. The thermal reaction power is determined by the difference between the sum of the heating powers in the reactor and intermediate thermostat, and the sum of these values during the reaction.

c. Secondary fluid for thermostatic control of reaction mass temperature: Coates and Riddel] (26) developed a "heat

evolution" apparatus. This isothermal device uses a cooling coil placed within a vacuum jacketed glass reaction vessel, such that It Is immersed within the reaction fluid. By using a peristaltic pump, cooling water is extracted from a large Dewar flask reservoir, circulated around the coil, and returned to the reservoir. Thus during a reaction exotherm the pump is operated such that no increase in reaction mass temperature occurs. The resulting increase in the temperature of the cooling reservoir is a measure of the reaction enthalpy.

Horak and Silhanek (27) have used a similar approach. However, in their system the coolant is pulsed through the cooling coil from an overhead thermostatic controlled tank. An electromagnetic valve situated under the tank is linked to a computer and pulsed using a suitable control algorithm. By measuring the coolant reservoir temperature, coolant outlet temperature and duration of the pulse, a measure of the reaction enthalpy can be obtained.

The most widely used commercial isothermal calorimeters are those which are based on a thermostatically controlled heat transfer fluid circulating around a jacketed reaction vessel. The measured power output, however, may be based on one of two principles, popularly known as "heat balance" and "heat flow".

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Hub's initial design of calorimeter (28) was based upon a normal 2 litre reaction flask built into a specially constructed bath and equipped with a stirrer, feed vessel and reflux condenser. A thermostat ted fluid was passed around the bath at a slow flow rate. Hub found that low flow rates were needed to obtain a measurable temperature differential from the coolant inlet and outlet of the reactor bath. He experienced problems with the stability of the temperature differential which led to designing a rotating outer container (in effect a Dewar vessel). He found that the gap between the reactor wall and Dewar vessel wall had to be made as small as possible since this increases turbulence and so results In more efficient and uniform heat transfer. Hub's initial design principle was used by Contraves in a modified form to produce a commercial calorimeter known as "Contalab" (29). This calorimeter consists of a double jacketed reaction vessel. The outer vessel is vacuum jacketed whilst the heat transfer medium is passed through the inner jacket. The instrument permits both heat flow and heat balance operation. The temperature differential between the inlet and outlet flows of the jacket coolant is measured.

At about the same time, Regenass was developing a heat flow calorimeter. Regenass (14, 17) used the temperature differential between the reaction mass and jacket coolant. His calorimeter requires a uniform jacket temperature and in consequence a high flow rate of circulating fluid around the jacket. Technically the coolant should be referred to as a heat transfer medium since in many situations the jacket will be required to provide heat to the reaction mass, e.g. during a temperature ramp. The design principle of the Regenass calorimeter (14, 17) was adopted by Ciba-Geigy as the basis for an "in-house" calorimeter - the Bench Scale Calorimeter. This has subsequently been commercialised by Mettler AG (30).

The Mettler RC 1, in its standard configuration, consists of a jacketed two litre glass reaction vessel. Thermos tat ted silicone oil is circulated at a very high rate through the jacket of the vessel. The temperature of the silicone oil in relation to the reaction mass determines the rate of heat transfer.

Both the Hub and Regenass designs of calorimeters are able to achieve semi-isothermal measurement by controlling the jacket temperature to the set point rather than reaction mass temperature. This is particularly useful when it is required to investigate self-heating reactions whilst under a partial cooling load. It is also possible to operate the Mettler RC 1 as an adiabatic calorimeter by compensating for environmental heat loss effects such as evaporation.

Bonvin (31) has used the Mettler RC 1 in the kinetic reconstruction of chemical reactions. By combining theoretical considerations with bench scale measurements, Bonvin has Investigated systems involving competing reactions.

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Steele (32) has designed and used a computer-controlled reflux system in conjunction with the Mettler RC 1 and other calorimeters and accurate thermal analytical data can now be obtained for reaction mixtures .iust below, at and through the boi1ing point.

Adiaba tic Calorimeters

This type of calorimeter can also provide data for process development but its main use" is for Investigating "runaway" conditions. The data obtainable includes:

a. temperature of initiation of runaway reaction b. rate and quantity of heat release as decomposition

proceeds c. rate of gas evolution d. pressure generated on decomposition.

Grewer (33) regards the use of Dewar vessels as one of the oldest test methods for studying exothermic decomposition and runaway reactions. He has claimed that the method is not primarily a cal.or ime t ric method, although he agrees that the heat production rate can be estimated if the heat capacity of the sample and the Dewar flask are known. Grewer has found that calorimetric measurement is valid only if the substance under test does not reach its boiling point. He emphasised that the main purpose of a Dewar-based test is to monitor the time and temperature during self-heating and obtain the induction time for thermal runaway.

Test methods can fail to detect the onset of an exotherm due to:

i. heat loss from the sample to its surroundings ii. heat loss from the sample to the test cell.

The latter is particularly important when the heat capacity of the test cell is large compared with that of the sample. A parameter, 0 is used to characterise the effect and is termed the therma1 inertia

0 = 1 + M Cv v c c

M Cv s s

where M = mass, Cv = specific heat at constant volume. The subscripts s and c relate to the sample and container respectively. An analysis of the importance of the thermal inertia term is given by Townsend ( 3 4 ) . For the direct simulation of plant conditions, the thermal inertia term should have a numerical value of 1.0 to 1.5.

Wright and Rogers (35) measured the rate of heat loss from a number of large plant vessels by filling them with hot-water and allowing them to cool naturally. Vessels up to 12 m capacity were employed. Similar experiments were carried out with 250 ml and 500 ml glass Dewar flasks closed with corks. They found that


the heat loss rates from the 250 ml and 500 ml Dewac flasks corresponded with those from 0.5 ra and 2.5 m plant respective]y.

Wright and Rogers (35) went on further to develop an adiabatic calorimeter based on the use of a Dewar vessel, under the low heat loss conditions found in full-scale chemical plants. They claim that their calorimeter provides temperature - time data in the near absence of environmental heat loss; the rate of change of reaction mass temperature is indicative of the reaction power output. The data is used with plant natural cooling data to assess reactor stability or at high temperatures to size reactor vents. Both Grewer (33) and Wright and Rogers (35) use ovens to regulate the environmental temperature of the Dewar, so reducing convective heat loss. Cronin (36) has used a water bath and insulated reactor head to minimise environmental, loss. At higher temperatures Cronin compensates for heat loss by direct electrical heating of the reaction mass.

In adiabatic calorimetry, it is possible to determine kinetic constants from a single experiment (35, 36). Grewer has used a simple kinetic analysis to derive an adiabatic induction time. Noronha et al (37), Hugo and Shaper (38) and Frankvoort and Dammers (39) have carried out extensive kinetic studies. Such studies have indicated the importance of including terms for the energy input from agitation and energy dissipation due to convective heat transfer (i.e. environmental loss).

The use of Dewar vessels permits an excellent replication method for the conditions in industrial plant with loss of agitation and cooling. With the increasing use of stainless steel Dewars with pressure-tight tops many gas generating reactions can be studied. The stainless steel Dewar is capable of withstanding pressures of, about 40 bar. However, since adiabatic calorimetry can provide data on potential thermal runaway • conditions a number of commercial instruments, which permit near adiabatic environments, exist. These include the Accelerating Rate Calorimeter (ARC), Sikarex, Radex and the instruments, which have evolved from the AIChE Design Institute for Emergency Relief Systems programme.

The ARC has become virtually the standard instrument and it can be found in most laboratories, which seriously examine thermal stability problems. It was originally developed by the Dow Chemical Company (34, 40) to provide temperature - pressure -time data for chemical reactions occurring at near adiabatic conditions. It allows the calculation of kinetic data, including the time to maximum rate and the temperature of no return (34, 40). The 1 - 10 g sample is placed in a spherical container, which is suspended inside a calorimetric jacket, containing eight heaters and three thermocouples. A fourth thermocouple Is attached to the sample container. A line from the sample container to a pressure transducer allows monitoring of the reaction pressure. The heat - wait - search routine for detecting exotherms has been well described (34, 40). The ARC is now so well established that many workers have developed specific

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experimental techniques with considerable confidence to cater tor decomposition anomalies. A stirred sample cell is available but the energy input due to agitation must be considered in any heat transfer analysis related to chemical plant. The mathematical modelling of reactions in the ARC to obtain kinetic and thermodynamic data is now in a very advanced state. Rival commercial instrument manufacturers point to problems with regard to the loading of the sample containers and the possible high value of the thermal inertia term; however, such problems are not really difficult to overcome.

In the Sikarex (28, 4 1 ) , a sample is placed inside a test tube which fits into a cylindrical jacket, through which heated air is circulated by means of a blower. The jacket temperature is controlled by a heater and a second heating element is wound round the test tube. Sensors inside the sample and tube allow measurements and control of the temperatures. The progress of a self-heating reaction is followed by making the temperature the set point for the jacket temperature controller. The sample surroundings are, therefore, maintained at the same temperature as the sample itself, heat transmission by conduction is minimised and the worst case of inadequate cooling is simulated. The sample size is usually between 10 - 30 g and a sensitivity of 4 W g is claimed. It can be used for reactions in the temperature and pressure ranges of 273 - 573K and 0 - 100 bar respectively. It can also be operated in isothermal mode. Experience of the use of Sikarex in the UK is very limited at the present time.

The Radex system is a replacement for the Sedex instrument ( 4 2 ) . In the latter a 5 - 30 g sample was employed. The type of sample container is optional but it is fixed within a fan assisted oven and the temperatures of the sample and circulating air are monitored by platinum resistance probes fitted through the oven roof. The oven temperature may be controlled in scanning, adiabatic, isoperibolic and isothermal modes. The oven and sample temperatures are recorded as functions of time. The initiation of exothermic activity is indicated by a decrease in the difference between oven and sample temperatures. It is possible to estimate heats of reaction and kinetic data for decompositions by using an adiabatic mode and compensating for the thermal inertia of the container. Experience of the system in the UK i s smal] .

The AIChE initiative, entitled DIERS (Design Institute for Emergency Relief Systems), concentrated on the design of vents, i.e. vent area requirement. While vents are common forms of emergency relief systems in other parts of the chemical industry, they are not commonly used for that purpose in the fine and speciality chemicals sector. This does not infer that the provision of vents is not included in the reactor plant in that sector of the industry for normal operational procedures. However, the DIERS initiative saw the introduction of a pressure equalisation technique to minimise thermal inertia problems of sample containers, which could maintain their integrity and the

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provision of vented sample test cells to allow the measurement of cell emptying times as well as temperature - pressure - time data. The instrument developed under the DIERS contract was commercialised by Fike and became known as the Vent Sizing Package (VSP). A very similar instrument is the Phi-tec, which is also commercially available. The worldwide experience with pressure equalisation systems has been obtained with the Fike VSP; however, it is expected that considerably more knowledge on the use of pressure equalisation techniques will be available on the completion of present research (43).

In the Fike VSP, a 120 ml stirred metal test cell is heated by a main heater and an auxiliary heater wound on its outer surface. The latter reacts to the temperature of the sample and minimises heat loss. The pressure control system produces a pressure outside the test cell equal to that produced by the reaction inside the cell. This enables pressure reactions to be studied in thin-walled cells. Closed test cells can be used to obtain maximum pressures and temperatures attained under runaway conditions. The open or "vented" cells can be used to obtain data on the flow behaviour of the discharging mass - single or two phase, tempered or non-tempered reactions (44, 45, 46).

The very latest commercial instrument to appear on the market is the Reactive System Screening Tool and is relatively cheap in comparison to all other commercial systems. It was developed by Fauske and Associates Inc., the Company which devised the DIERS methodology and henc<=> the new instrument bears some similarity in concept and design co the Fike VSP, however, the sample can be contained in a glass flask.

PILOT PLANT OPERATION

The adiabatic and isothermal calorimetric techniques will have provided thermodynamic and kinetic data. In addition, heat flow calorimetry provides the opportunity to gain data on the inside film convective heat transfer coefficient for the jacketed vessel. If complete similarity has been possible between the pilot plant reactor and a heat flow calorimeter then scale up predictions from the latter to the former are applicable; however, Steele (32) has shown that complete similarity is difficult to attain. The current work by Steele (32) and Hirst (47) has indicated that similarity needs to be extended to process control algorithms and the fluid mechanics (level of turbulence, degree of fouling) inside the laboratory-scale and pilot plant scale needs to be completely comparable.

The pilot plant scale affords the ideal opportunity to obtain further data on the reaction system, while under constant human supervision. Effects, which have not been indicated during the calorimetry stage are often observed. Even a 2 litre scale operation does not always permit the detection of important subtle changes and hence true bulk effects are not always appreciated prior to pilot plant operation. Steele (32) has shown that the pilot plant stage permits a better evaluation of


the influence of the plant on hazard and on the definition of safety procedures than reliance on laboratory scale calorimetry studies. The collaborative programme between the Boots Company pic and South Bank Polytechnic has resulted in the development of a 675 litre heat flow calorimeter based on a computer-controlled jacketed Pfaudler reactor. This work has indicated that it is far easier to operate a large scale calorimeter in the heat flow rather than the heat balance mode of operation.

Workers at the EEC Joint Research Centre, Ispra are employing a 100 litre reactor to gain kinetic and thermodynamic data on desired reactions prior to a major study of runaway reactions, in large plant.

MANUFACTURING SCALE OPERATIONS

Following pilot plant work, a suitable plant must be found to allow manufacturing scale operations. This plant can be:

i. an existing reactor ii. a reactor modified to accommodate the new process iii. a new purpose-designed and built reactor.

The required cooling area for batch operation can be calculated by a number of methods. The method described by Hugo (48) for batch reactors requires data on the rate of reaction and the a d i a b_a,t ic temperature rise. Depending on the kettle size, 2 - 5m m can be instil led in reaction vessels with only jacket cooling and 4 - 8 m m if the vessel is also provided with an interna] cooling coil. The determination of the required cooling area may mean taking the decision about moving from batch to semi-batch operation. The latter, of course, allows the option of feed control to lessen potential runaway conditions.

The design of a suitable kettle should be accompanied by a series of hazard assessment exercises. The easiest technique for the non-specialist is the use of HAZOP (49). The Hazard and Operability Study can be carried out at various stages In the design and other decision-making processes about the plant and its operation. Numerous texts describe the technique in detail. More quantitative techniques, e.g. application of fault tree analysis and probabilistic risk assessment require some specialist knowledge.

The manufacturing scale also raises the possibility of designing for inherent safety; however, where this is not possible, then greater reliance must be placed on process control. The basis of process control is set by the experimental results from the calorimetric investigations. These include the definition of onset of exotherra temperature, the safety margins, amount of cooling required, the temperature limit to prevent accumulation of unreacted material and the effect of loss of agitation.

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The topic of process control may also be linked to containment, venting and reaction inhibition. In the fine and speciality chemicals industry the emphasis is correctly, on process control. However, the calorimetric techniques can provide useful data for the design of venting arrangements (e.g. use of DTERS methodologies) and the maximum decomposition pressures found in tests using the ARC or adiabatic pressure Dewar calorimetry can be used in the design of containment systems. A knowledge of any decomposition reactions is also necessary for inhibition. Inhibition may be achieved by inter alia removal of free radicals, quenching by the addition of inert diluents or dumping of the reactor contents into a larger vessel containing a quench liquid.

DISCUSSION

An attempt has been made to indicate that at each stage in the development of a process, from initial concept to full scale manufacture, the information requirement on reaction hazards increases. It is essential to combine kinetic and thermodynamic data with existing experience of chemical reactor design and opera t ion.

Numerous papers (26, 50, 51) have developed typical hazard assessment strategies for reactions, including thermal decompositions and all take a familiar route of:

a. literature searches, computer modelling and simplistic reactivity calculations

b. explosibility screening tests c. milligram scale scan heating tests d. isothermal or isoperibolic calorimetry e. adiabatic calorimetry

and then combining such results with information relating to the plant, e.g. possible effects caused by the proposed materials of construction and those due to the scale of operation. The above six steps only form part of the total hazard assessment, which should also incorporate plant details and the conclusions reached from HAZOP studies. Any hazard can be mitigated by design or control. Process control is vital in the fine and speciality chemical sector of the industry.

Many commercial calorimetric instruments are now available; however, Che vast majority tend to be expensive to purchase and operate. An approximately equal number of ad-hoc "in-house" techniques are described in the literature and on the whole provide data comparable with that achie-ved from commercial instruments. It must be remembered that experiments need to be tailored to the specific reaction under investigation. Certain events, e.g. the generation of large quantities of vapours, do not allow the adoption of standard testing procedures. It is also critically important to reraember that all instruments have finite response times to rapidly changing events and the sensitivity of an instrument is very much dependent on the type

59


of reaction system under test. The sensitivity quoted for an aqueous system will be significantly different to that for a highly viscous system.

With the in to establis by use of purchase or assessment calorimetry model devia studied in calorimeter instrument the evaluat

formation and techniques now available it Is poss h chemical reaction hazard testing facilities, ei simple cost-effective procedures (15, 36) or by

an isothermal calorimeter, such as a heat It is apparent from the above that no si

or technique can provide all Che necessary data ion of chemical reaction hazards.

ible ther the

za rd ewar t to y be f low ngl e for

CONCLUSIONS

Calorimetric techniques provide some of the evidence required for the evaluation of chemical reaction hazards. The combination of such data with formal hazard assessment studies, e.g. HAZOP, relating to the design and operation of reactor plant can provide the basis for the selection of process control and safe operation.

It is important that at each stage of process development details relating to any potential reaction hazards are considered, particularly in relation to scale of operation. The details relating to potential hazards need continuous expansion and redefinition. Reaction hazard evaluation should be seen as an integrated part of process development. Only with a thorough investigation can safety measures be chosen to mitigate the hazards.

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G. T. Austin; "Hazards of commercial, chemical operations" in "Safety and accident prevention in chemical operations" by H. H. Fawcett and W. S. Wood (ed) Interscience (1982).

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