Abstract of Special Research Report (SRR-88)

National Institute of Occupational Safety and Health, Japan

Study on Evaluation of Thermal Hazards in ManuFacturing Processes for Reactive Substances.



: Chemical industries in Japan have begun extensively to produce so-called fine-chemicals such as pharmaceuticals, functional resins, pesticides and so on. The production of these chemicals is usually carried out by a varied and small quantity type process to enhance value of the chemicals. As a result, chemicals with complicated structures have a tendency to be synthesized in a same reaction vessel, and then some different kinds of processes are often used as a multipurpose system for the production of these chemicals. Therefore, the potential hazards of the chemicals including raw materials, intermediates, and finished goods may be increased in connection with unsuitability of installations and difficulty of operation.
    In recent years, explosions or fires broke out at times in the batch processes for fine-chemicals in Japan, killing some workers and causing extensive damage to plants and buildings on the site. For the prevention of these accidents in batch processes, it is critical to evaluate the potential hazards of chemicals themselves, and to investigate safety measures on manufacturing facilities and operating system on the basis of safety knowledge of the chemicals.
    In this specific report, different kinds of experiments were carried out with relatively large amounts of sample to investigate the thermal hazards such as runaway reaction or thermal decomposition for the purpose of prevention of explosion or fire in batch processes of reactive substances. The contents of the research are briefly outlined below.
  1) Introduction
  2) Study on nitration with a twin-type calorimeter
    Reaction heat of nitration of benzene and heat of formation of nitro group were evaluated with a twin-type calorimeter using a few grams of samples, and also the effect of sulfuric acid on the nitration was investigated.
  3) Evaluation of reaction hazard by Accelerating Rate Calorimetry (ARC)
    Nitration of benzene and amination of p-chloronitrobenzene were carried out with ARC to clarify the danger by considering adiabatic condition in these reactions.
  4) Evaluation of runaway reaction with a 1 liter reaction vessel
    5-t-butyl metaxylene was nitrated in a 1 liter reaction vessel to see the process of runaway reaction, and the effect of concentration of nitric acid on the nitration was also examined.
  5) Evaluation of reaction hazard in a bench scale batch plant
    Nitration of benzene was carried out using a 20 liter reaction vessel to see the effects of feed rate of mixed acid and of concentration of sulfuric acid on the heat-rate or the temperature rise in the reaction system.
  6) Evaluation of thermal hazard in distillation process
    To evaluate the thermal hazard of distillation process, chlorinated aromatic amines were tested by a spontaneous ignition apparatus to investigate the effects of metal ions and of evoluted hydrogen chloride on the distillation.
  7) Analysis of case histories in batch processes
    The case histories of 297 accidents in batch processes were analyzed according to their accident types, reaction types and so on, and 12 case histories were cited as typical.

Measurement of Heat of Nitration by a Twin-Type Calorimeter


: In chemical industry in Japan, the production of fine chemicals has been becoming a mainstream in this field. These manufacturing processes involve some serious problems on safety as the production is sometimes carried out with less safety information such as runaway reaction and thermal decomposition. This may be due to the fact that various kinds of chemicals with small quantities are required to produce.
    In this chapter, measurements of reaction heat in nitration of benzene, which may be a very important factor for the prevention of runaway reaction, were conducted using a twin-type calorimeter. The nitration of benzene carried out in this chapter is shown below:
    C6H6 + Mixed Acid ( H2S04 + HN03 ) ⇒ C6H5NO2
    The experiments were carried out by a twin-type calorimeter (HTIC-200) made by Tokyo Riko Co. The total heat of reaction in benzene nitration process by mixed acid must involve both heat of nitration and of dilution by sulfonic acid. Then, the heat of nitration can be calculated by subtruction of heat of dilution from the above total heat of reaction. The heat of nitration thus determined was 34.2 kcal/mol. This value is rather bigger than the reported one, so the above value may involve other heats of reaction, such as heat of dilution by nitric acid.

Evaluation of Runaway Reaction and ThemalExplosion with Adiabatic Calorimetry

Takayuki ANDO and Shigeru MORISAKI

: The thermal runaway process is characterized particularly by a progressive increase in heat generation rate, temperature and pressure. It begins when heat generation from particular chemicals undergoing exothermic reaction or decomposition becomes greater than cooling capacity of and/or heat loss from facilities.
    Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) have been the most common techniques used to study the thermal behavior of chemicals and reaction hazards, as these methods require small sample sizes, typically, a few milligrams, and short analysis times of a few hours at most.
    However, it will be advisable to know the thermal hazards of chemicals under adiabatic condition, because, in actual cases, the chemicals may sometimes be under adiabatic condition when, for example, agitation failed or flow of cooling water was lost.
    In this report, we have shown the results of two examples of ARC (Accelerating Rate Calorimeter) experiments on aromatic nitration and amination under adiabatic condition.

Reproducting Experiment of a Runaway Incident of Nitration Reaction


: A reproducing experiment of a runaway incident of a nitration reaction was carried out, in which 5-tert-butyl-m-xylene (hereinafter referred to as BX) was nitrated to 2,4,6-trinitro- 5-tert-butyl-m-xylene (hereinafter referred to as TNBX) in the scale of 1/2,000 amount.
    Firstly mixed acid is prepared in the reaction vessel by mixing 195 ml of 98 % sulfuric acid and 250 ml of 98 % nitric acid. Then, in accordance with the progress of incident, 49 ml of BX is added with agitation onto mixed acid over 3.5 hours. After that, the agitator motor is switched off, but addition of BX is not interrupted, as the result, 28 ml of BX is added into the vessel without agitation over 2 hours before the restart of agitation (Fig.3).
    As soon as the agitation is restarted, the temperature of the reaction system rises at once by ca. 30 K to about 50 °C. It is confirmed by calculation that this temperature increment is due to the heat of the proper trinitration reaction of 24g of BX. After temperature levels of about 50 °C has been kept for 1 minute, the system begins a rapid heating phenomenon with a rate of temperature rise of ca. 220 K/min, evolving violently yellowish-brown smoke of NO2 . After the maximum temperature of 127 °C has been recorded, a cooling state lasts for about 5 minutes, next a slow heating phenomenon again begins (Fig.4)
    Then the heating behaviours of TNBX itself or (TNBX + mixed acid or others) systems are tested at 130 °C (Fig.5). (TNBX + mixed acid) system shows a heating rate of 1 K per 3 min at 130 °C. It is shown that concentrated nitric acid is a powerful oxidizing agent, compared to concentrated sulfuric acid.
    After that, the exothermic decomposition behaviour of TNBX itself is examined as a whole with DSC (Fig.6). Total calorific value of TNBX was found to be 1,043 cal/g. This value sufficiently corresponds to those of high explosives.
    Based on the above experimental facts, the progress of the incident has been revealed as follows: When a large amount of BX and mixed acid are suddenly mixed at room temperature in the vessel, the temperature of the reaction system rises to several tens degrees C, since the proper trinitration reaction immediately occurs. But, if excess nitric acid still exists, oxidation reaction of tert-butyl group in TNBX molecule begins at these temperature levels by nitric acid with the evolution of NO2 gas, and the temperature of the system rises to the levels of one hundred and several tens degrees C. After that, at such temperature levels, the more exothermic oxidation reaction of methyl group or benzene ring in the molecules of the above-stated partially oxidized nitration products continues by nitric acid, and ultimately some explosion phenomena may arise in the vessel.

Runaway Reactions in Benzene Nitration Process in a Bench Scale Batch Reactor

Toshihiro HAYASHI

: Runaway reactions in benzene-nitric acid-sulfuric acid system have been studied in a 20 liter batch reactor, in order to make some contributions on the safety of chemical processes dealing with exothermic reactions. The experimental facility, shown in Fig.1, was closely simulated to commercial batch plants. Benzene was agitated under constant temperature (50 °C) in the reactor, and then mixed acid was either fed continuously by a feed pump or thrown rapidly into the reactor by gravitational force. A 61 % nitric acid and a 95 % sulfuric acid were mixed to prepare mixed acids of various contents of both components. Reactants in the reactor were dumped by a relief valve into a blow-down tank filled with water if their temperature exceeded a pre-set value. Temperature-time curve was recorded for each test so as to determine a temperature rise rate, which was an average rate of temperature rise from 50 °C to relief valve activation temperature or the maximum temperature attained.
    Results of tests for continuous feed of mixed acid are shown in Figs.2 and 3, which show rather slow temperature rise rates and suggest less probability to cause reaction runaway. On the other hand, rapid mixing of mixed acid into benzene often brought to high temperature rise rates, especially for large contents of sulfuric acid in mixed acid (Figs.4 and 5). The content of sulfuric acid which caused higher temperature rise rates, i.e. more than 100 °C/min, depended largely on equivalent ratio of nitric acid to benzene. Such processes with temperature rise rate larger than 100 °C/min could be called as "runaway reaction", and any cooling might probably have no effect on controlling such processes, as shown in Fig.8. Effect of the total amount of reactants or the quantity of raw materials was also tested (Fig.7).
    Based on experimental evidences, a brief discussion was made on preventive and protective methods for reaction runaway, with reference to such items as composition and feed rate of raw materials, monitoring of temperature, cooling and agitation, together with confinement of a runaway reaction in a reactor and suppression of high temperature reactants by dumping.

Study on the Cause of Distillation Incidents of Chlorinated Aromatic amines


: Chlorinated aromatic amines are used as intermediates for synthesis of a variety of medical and pharmaceutical products or agricultural chemicals. Amines of this kind are purified by vacuum distillation after synthesis.
    Chlorine-containing organic compounds such as chlorinated aromatic amines have inherent property of partly decomposing, if heated for distillation, etc., to liberate hydrogen chloride (HCl) and give HCl salts of amine proper or dehydrochlorinated amines or tar (i.e. amine polymer, high molecular weight-addition products of various amine residues which arise by dehydrochlorination reaction of the amine proper; amine polymer exists partly also in the form of HCl salts). These decomposition products are hereinafter referred to as amine-HCl salts in the lump.
    If the normal temperature inside the vessel should have been slightly exceeded, in the process such as distillation, due to failure of temperature control, etc., the decomposition reactions are remarkably promoted. And if operators on duty are not aware of the abnormal situation occurring in the vessel, the inside temperature gradually gets higher, since the dehydrochlorination-polymerization-neutralization reaction proceeds slightly exothermally as the overall process.
    On the other hand, HCl thus formed, unless measures such as alkali addition to the crude product have been taken in advance, reacts with metallic structure of the vessel or with metallic catalysts, which have been carried through to the destination stage together with the crude product, to give metallic chlorides. Since these metallic chlorides, e.g. FeCl3, exerts in most cases powerful catalytic effect on the decomposition reactions of amines of this kind, the reactions are greatly accelerated as the chlorides are formed. In the final stages, the contents in the vessel changes largely into amine-HCl salts and the temperature inside the vessel rises also by a large margin at the same time.
    When a temperature of two hundred and several tens degrees centigrade is reached in the vessel, dissociation reaction of amine-HCl salts into free amines and gaseous HCl suddenly commences. In other words, explosive HCl-liberating reactions take place suddenly over a certain temperature level in the vessel, and thus, burst incident of distillation plant or environmental contamination incident due to blowing-off of chlorine-containing contents is brought about (Ref. 1).
    Two incidents occurred in Japan from 1973 on to 1975. In each case, the batch distillation vessel burst due to abnormal increase in temperature and pressure in the vessel accompanied by blowing-off of the contents, during vacuum distillation of a substance belonging to chlorinated aromatic amines. Those were 4-chloro-2-methy-aniline (m.p.30 °C, b.p. 240 °C, hereinafter
 referred to as CMA) in one case, and 3,4-dichloroaniline (m.p. 71.5 °C, b.p. 272 °C, hereinafter referred to as DCA) in the other case.
    The above-mentioned dangers which may happen in vacuum distillation of chlorinated aromatic amines are the ones inherent in amines of this kind. So it is thought to be reasonable to consider these phenomena in the lump. Thus in this paper, firstly the two incidents are outlined; next, phenomena observed in common in distillation incidents of amines of this kind are stated; then a series of experimental informations concerned are showed; finally incident-preventive measures based on those informations are mentioned.

Case Histories of Accidents in Batch Processes and Analyses

Yasuhiro FUJIMOTO and Shigeru MORISAKI

: In this chapter, the case histories of 297 accidents in batch process industories in Japan were classified into several patterns according to their accident types, reaction types, ignition sorces and so on. In particular, the accidents in the reaction or distillation process were examined in detail.
    The important characteristics of the accidents are summarized below.
  (1) The accidents in batch processes are mostly attributable to explosion and fire, showing about 90 % of all the accidents.
  (2) The accidents in the reaction processes break out most frequently in batch processes. This fact may be a remarkable feature for the batch process industries.
  (3) The accidents in polymerization process break out most frequently in batch processes, accounting about 20 % of all the accidents. Nitro- and nitroso-compounds and other compounds containing nitrogen cause a high ratio of the accidents.
  (4) Among various kinds of ignition sources, reaction heat is the most frequent ignition source of explosions or fires.

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