by J. C. Rapean and D. L. Pearson Shell Development Co.
I
H. Sello Shockley Semiconductor Laboratory
SAFETY A
W O R K B O O K
F E A T U R E
E C
A Test for Hazardous Chemical Decomposition Predicting the course of a chemical reaction is most desirable in the development of a process M
ANY POTENTIAL HAZARDS
exist
in a chemical plant. Those associated with the handling of flammable, toxic, or highly unstable materials are widely recognized, whereas those which arise from the unexpected explosion of process materials are not. To design and operate a plant with proper concern for safety, it is necessary to have a thorough knowledge of the stability characteristics of the various process streams—particularly when a new product is being made for the first time on plant scale or a new process is being scaled up. It is not enough that the process steps have been carried out on laboratory scale without incident, for plant conditions are different. For example, metals are used in place of glass for the construction of equipment; pressures and temperatures may be higher; residence times may be longer ; by-products may accumulate in recycle streams ; or impurities may be introduced by substituting commercial for reagent chemicals. The scale of operation may also be an important factor, particularly in batch operations. Long heating of large quantities of materials may impose hazards to both personnel and equipment as a result of violent decompositions. In some cases, the stability of a compound to heat or chemical agents can be predicted ; however, in many cases it must be determined experimentally under simulated plant conditions. With this purpose in mind, a laboratory screening test was devised to detect conditions which could cause explosions in chemical plants. Laboratory Stability Test
In this test, the material being studied is confined in a bomb and heated either alone or in the presence of possible contaminants or catalytic agents, and the pressure generated by
the material, due to the formation of gases or low boiling compounds, is measured. A schematic diagram of the apparatus used is shown in Figure 1. T h e bomb is made of stainless steel and has a capacity of about 250 cc. It has a working pressure of 2000 p.s.i.g. and is protected from overpressure by a conventional bursting disk. As a further precaution, the vessel is used within a barricade. The vessel is heated either in an oil bath or in an electrical furnace, which is provided with a coil for rapid cooling. Temperatures in the liquid and in the vessel wall are measured by thermocouples and recorded. The pressure is measured by a strain gage pressure transducer and also recorded. A gas sample is collected for analysis either during or at the end of a run. The normal test procedure is to fill the bomb about half full with the material to be tested, and, after being flushed with nitrogen, the vessel is closed and heated at a rate of 2° to 5° C. per minute. Heating is continued until a reaction is observed or a predetermined temperature is reached which should be well above any expected process temperature. After the heating program is completed, the vessel is cooled and liquid and gas samples are taken for analysis. In this test, a potentially hazardous decomposition is indicated by a sudden increase in the pressure of the system due to the formation of low boiling compounds or permanent gases such as hydrogen, carbon monoxide, carbon dioxide, or nitrogen. Decomposition reactions are usually accompanied by observable heat effects. An exothermic reaction is indicated bya changein the liquid heating curve or by liquid temperature exceeding vessel wall temperature. Examples
This test has been used to study a I/EC
large number of compounds and a few examples are discussed below. Allyl Alcohol. Studies with allyl alcohol show that the pure commercial grade material is relatively stable on heating. In stainless steel, less than 1 % decomposes per hour at 225 ° C. ; iron, iron salts, and copper salts had no significant effect on the decomposition rate. This compound is stable when heated with sodium carbonate. However, in the presence of sodium hydroxide, extensive decomposition of the alcohol takes place at a much lower temperature. Figure 2, A, shows a typical heating curve for allyl alcohol in stainless steel with 9 . 1 % by weight of sodium hydroxide added. The pressure increased slowly until a temperature of about 135° C. was reached. At this point, a change in slope of the heating curve showed the onset of an exothermic reaction. At about 175° C , the liquid temperature exceeded the wall temperature, and the reaction becomes explosive at about 190° C. During the last 30 seconds, pressure increased at a rate of about 20 p.s.i.g. per second. At temperatures below about 125° C , the decomposition was not self-sustaining in this test apparatus; however, a subsequent, more detailed study showed that in the presence of sodium hydroxide, decomposition takes place at temperatures as low as 100 ° C. The decomposition was not stoichiometric. One mole of sodium hydroxide destroyed from 4 to 8 moles of allyl alcohol as the original sodium hydroxide concentration was decreased from 9.1 to 1.0% by weight. Gaseous decomposition products were almost entirely hydrogen with only a trace of propylene. Liquid products identified by GLG and mass spectrometric analysis were mainly methyl, ethyl, and propyl alcohol, and the nonvolatile materials
W
ORKBOOK FEATURES
77 A
I/EC
SAFETY
•
A Workbook
Feature
HEATER COOLING COIL
F i g u r e 1. Sche matic diagram of the thermal stability test a p p a r a t u s , showing the stain less steel bomb and safety measures used against ex plosion
TR
PR
AL BLOCK Further studies showed that the hy drogen was formed by a corrosive at tack on steel by the hydrogen chlo ride released in the first stage of the decomposition. Allyl glycerol monochlorohydrin ether is relatively stable in the absence of iron or iron salts and can be heated in glass to about 250° G. with only a relatively slow evolution of gas and no evidence for an exothermic reaction. Several related compounds were studied and reported below.
formed were the sodium salts of weak organic acids, mainly formic. Allyl Glycerol Monochlorohydrin Ether. Allyl glycerol monochlorohydrin ether, an intermediate in the synthesis of allyl glycidyl ether, has also been studied in some detail. Figure 2, B, shows a typical heating curve for this compound in mild steel. At a temperature of about 190° G , there was an indication of the start of an exothermic reaction, and at 210° C. the liquid temperature ex ceeded the wall temperature. After a short induction period, the pressure increased very suddenly. I n these experiments, the rate of pressure in crease reached a maximum value of 50 to 75 p.s.i.g. per second and the ultimate pressure exceeded 1000 p.s.i.g. Gaseous decomposition products were mainly hydrogen and pro pylene. Some polymerization to high boiling liquids also occurred.
i
Ι
WALL T E M P LIQUID T E M P
PRESS
Decomposition
Temperatures
Steel
Compound Allyl glycerol monochlorohydrin ether Isopropyl glycerol monochlorohydrin ether Allyl glycidyl ether Allyl glycerol ether
Il
in
Mild
Decom position Temper ature, °C. 20S-10
Exo ther mic Yes
230
Yes
245 250-60
Yes No
I
PHESS
PRESS
ID
w 2 H W w
WALL T E M P LIQUID T E M P
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Β.
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ω" Ci ^
Η < s:
PRESS
T I M E . MINUTES
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
When results indicate the existence of a hazard, further experimental work is usually necessary to deter mine safe operating conditions. Much care must be exercised in extrapolating results from these small-scale tests to large-scale plant operations. T h e laboratory results are strongly affected by the rate of heating, the heat capacity of the bomb, and the sample size relative to the vapor space. In a large vessel effectively insulated from its sur roundings, much lower decomposi tion temperatures may be observed. However, in spite of these limitations, this laboratory test has served as a useful tool in our effort to achieve safe operation in the plant.
WALL T E M P LIQUID T E M P
PRESS
:
T I M E . MINUTES
Figure 2. These curves show rate and temperature of decomposition of various process materials which were given a screen test devised to aid in detecting con ditions which could cause explosions in chemical plants 78 A
Although the allylamines are rela tively stable when heated alone or in the presence of iron, copper, or the salts of these metals, diallylamine mixed with 25 to 50 mole % of its hydrochloride explodes on heating to about 225 ° G Figure 2, D, shows a typical heating curve for this mix ture. The decomposition was exo thermic and shows a pressure rise greater than 10 p.s.i.g. per second. Extensive breakdown of carbon-car bon bonds is indicated. Discussion
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LIQUID T E M P
Allylamines. The a l l y l a m i n e s have been commercialized recently and tests indicate that they are rela tively stable to heat. Monoallylamine decomposes very slowly and after heating 4 hours at 225 ° G , only traces of gaseous decomposition prod ucts were formed. Diallylamine is somewhat less stable. It decomposed slowly starting at about 230° G to form propylene and ammonia with no evidence for evolution of heat. Figure 2, C, shows a typical heating curve for diallylamine in stainless steel. When triallylamine is heated to temperatures in the range of 240° to 250° G , extensive polymerization of the liquid occurs. There was no evidence for a strong exothermic re action and only very small amounts of gaseous products were formed.
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