SAFETY IS EVERYBODY’S BUSINESS A trio of articles outlining the basic factors in safely studying hazardous reactions, and telling how to cope with hazards in handling alkali metals and using compressed gases
Safetv Problems in Studv of Hazardous Reactions d
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Industrial research in the past 10 years has been extended into fields which were once considered much too dangerous for routine investigation. Many studies involve not only high pressure reactions but also the use of reactants which are combustible, explosive, and toxic. Such reactions can be studied in complete safety in properly designed equipment operated by personnel trained to understand and respect the hazards involved.
R. S. BRODKEY, R. G. NEWBERG, AND JOSEPH STEWART Esso Research and Engineering Co., Linden, N . J .
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ONSIDERATIOK of the safety problems inherent in the study of hazardous reactions has led to the formulation of procedures and equipment designs which can be combined t o approach the “ideal” for a hazardous reaction laboratory. While, to the authors’ knowledge, no such laboratory exists, each of the components of the suggested design has been used for some time by many laboratories. Some of the particularly hazardous reactions are: Reaction Polymerization Acrylic synthesis Hydroformylation Direct acid Hydrogenation
Gases
co, c=c, CZCd co, c=c, czcz CO/H,, olefin CO, olefin H2
Catalyst Peroxides Ni( CO), [CO(CO)412 Nl(C014 Xi, [CO(C0),lz
These reactions are generally run a t moderate temperatures, but reaction pressures can be extremely high, because in many cases uncontrolled pressure surges and detonations may occur. The reactants and products are not only flammable but readily form explosive mixtures with air. Not only are such reactants as carbon monoxide poisonous but some of the catalysts noted are extremely toxic and have maximum allowable concentrations of the order of 0.1 to 1.0 p.p.m. The hydroformylation reaction, for example, is highly exothermic as shown in Figure 1. Fortunately, as the heat of reaction is liberated and the temperature rises, the gases are consumed resulting in a rapid decrease in pressure. In this reaction, an additional safety factor is afforded by the destruction of the catalyst at high temperature. These factors prevent the development of dangerously high pressures, which might occur if the reaction goes out of control. I n other reactions such inherent self-quenching characteristics do not exist and suitable provision must be made t o prevent destruction of the reaction vessel. The problems involved can be considered in terms of the major components of the equipment a8 shown in Figure 2. All of these components are common to reaction of this type. February 1956
COMPRESSION SYSTEM
As indicated in Figure 3, three gas compression systems are used: trunk compressors, gas compressors, and hydraulic boosters. One safety problem common t o all compressors is air leakage into the gas feed lines. Compressors create a vacuum at the suction or intake. If a leak exists in the intake line, air is drawn into the compressor and mixed with the gas being compressed. Explosions may then occur in the compressor or in the reaction vessel. Suction lines should be maintained under a slightly positive pressure of 2 or 3 pounds. A combination vacuumpressure gage should be used to indicate the intake pressure. A trunk-type compressor is similar in design t o the familiar internal combustion engine. It uses a common drive and a common crankcase, which receives any leakage or blowby of gas around the cylinder rings. The blowby into the crankcase presents several serious safety problems, Gases can build up excessive pressure in the crankcase and cause failure by rupture. The gas flow removes the lubricant between the piston rings and the cylinder wall, resulting in inadequate lubrication and seizing of the piston. Explosive mixtures can form by leakage of air into the crankcase. These difficulties can be avoided by venting the crankcase and flushing it before and during operation with nitrogen. Trunk compressors are not recommended for hydrogen or hydrogen mixture service since hydrogen is more likely t o blowby and leak than such gases as nitrogen or carbon monoxide. A common drive shaft is used in the gas compressor, and leaks from the higher pressure stages either flow t o the previous stage or to a dead space connected t o the suction or intake. Since this is also a reciprocal compressor, pressure fluctuations occur a t the intake. T o avoid these fluctuations, a large feed line or surge tank should be used. Otherwise a high average inlet pressure must be maintained t o prevent air intake during the suction stroke. The high pressure hinders the compressor operation by
INDUSTRIAL AND ENGINEERING CHEMISTRY
223
High pressure reactions using cornbustible, explosive, and highly toxic reactants and products can be investigated safely, if *
. .reaction i s stu
. . .equipment is designed adequately . . .personnel i s Brained properly 1
I
causing the first and secoiid stages t o operate at excessive pressure. This type of compressor is preferred for study of pressure reactions where a gas is uscd as onr of the reagents. A third and very coninion system employed for coniprcpsion of gases is the hj-draulic Loostei method. Here cylinder gas is displaced from a pressure tank by oil or water t o the reactor or t o a, second preesure storage tank. Khereas the capacity of s u ~ h:L system is low, the attainable pressure is very high. T l i ~most efficient technique. employ a gas compressor for pressui'es in thc range of GO00 to 15,000 pounds per square inch a11d a hydr.aulic. system for pressures to 60,000 pounds per square inch. If the reaction system has a lox- gas requirement, one can use the hydraulic displaceincnt method or a compressor in conjunction with a storage drum. For higher capacit3pression directly into the reactor is convenient.
missile of the weight of the reactor head moving a t the speed of sound. Explosive mixtures formed by leaks from the reactor may be prevented by providing adequate stall ventilation. To provide for rapid leaks into the stall or ventilation failure, the stall should have a blowout back, blowout roof) or no back a t all. In the study of those reactions using combustible gases as reagents, each stall should be equipped wit,h a combustible gnq alarm. h continuous meter is considered best for the detection of carbon monoyide, though canaries have been used si~ccesslully. Spot testing of carbon monoxide concentrations may be carried out with a chemical detector, Both measurement and control of the process variables is of course desirable. Temperature measurement is simple, using thermocouples as the sensing devices. Temperature control is usually on-off rather than proportional; explosion-proof instruments are available. TKOtechniques are available for measuring and recording reaction pressures. From a snfrtv &ndpoint these differ in the means of signal transniitt:ince. The strain gage technique transmits its sensing electrically whereas, in general, the Bourdon tube sensing devices transmit their iinpluses pneuniatically
THE STALL
The second major item of equipment is the stall. Depending upon the application, stalls are constructed of boiler plate, structural steel, or reinforced concrete.
Figure 2 .
TIME
Figure 1.
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Hydroformylation reaction
The development study of a well known reaction is often carried out without a stall. Structural steel stalls are used conimonly for the study of potentially high pressure reactions. A reinforced concrete stall is recommended for study of an esploratory nature, Thicknesses of 6 to 36 inches have been used. The stall should be designed considering tn-o major sources of danger: detonation of the reactor with formation of missiles and explosion outside of the reactor due to ignition of leakiilg gases. Missiles can be generated by an explosion inside of the reactor, and the stall should be designed t o withstand the impact of a
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BPajor components of equipment
The major advantage inherent iri the straiii gage in its elimina,tioti of the need of an air supply. Vihereas reliable pneuniatjc systems are available and have been widely used, the all electrical system eliminates the danger of coinpressed air supply failure. Thip type of failure may be aeiious if the pressure measurement is used for control of the reaction. Portholes or other holes in the stall wall are not recommended, because they will weaken the structure and provide a possible point for fracture of thc wall, as ell as a path for pressure release. Gages outside of stall i d l s should be eliminated if at all possible. Visual observat,ion may be obtained by the use of a television system or the judicious placement of mirrors. Such precautions are most desirahle where particub rly dangerous reactions are studied. In general, valves are operated mlinualiT- by an extended stein protruding through the stall wall. I t is desirahle, where possible, t o use a right angle gear drive for manual valve control. Holes through the wall and the possible danger of flying steams may be eliminated by the use of remotely controlled valves. Such automatic valves either of the on-off or throttling type are available commercially.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48,No. 2
SAFETY PRACTICES
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REACTORS
All reactor systems should have integral safety relief devices. Yreferably these should be rupture disks rather than springloaded valves. The patch should be sized in accordance with reaction pressure rather than vessel strength. For a known reaction the safety relief device need not be integral with the reactor and may be placed in a feed or exit line. This should be satisfactory since the development of high pressures is caused by compressing too much gas into the reactor or by overheating. Both of these pressure rises are slow, and surges are not usually found.
GAS TYPE
COMPRESSORS
-
GAS TANK
^,,
f
'L\
UIL
!
/j
COMPRE'SSioN CYLlN DER 7 \
HIGH PRESSURE
JL
RESERVOIR
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cooled rapidly in a n emergency, and in addition] these jackets are not truly explosion-proof. One disadvantage of the liquid jacket is the limited temperature range of any one heating medium as steam or Dowtherm. Reactors should be inspected routinely by pressure testing. Testing schedules based on length and severity of use should be followed. More frequent inspection is needed, of course, for those reactors exposed t o high temperatures and pressures or corrosive reagents. VENT SYSTEM
Reaction gases must be removed from the reactor without permitting combustible or toxic concentrations to accumulate in the stall or working area. I n some cases relatively large amounts of such gases as carbon monoxide, hydrogen, and smaller amounts of the extremely toxic catalysts must be removed. Some of these catalysts have maximum allowable concentration of less than one part per million. In a n ideal system, each reactor has a separate vent line which should be a straight run of piping with no bends or corners, When more than two or three reactors are involved] a common vent system is often used. The main advantage of the individual vent system is that the safety patch for each reactor can be sized for the expected pressure of each reaction. I n the common system, all patches must be sized at the value as determined by the lowest pressure reaction. This procedure in the common vent system is necessary t o prevent back failure of safety patches, Figure 5 shows the arrangement of the vent system piping for a batch reaction of known characteristics. For exploratory reactions a n additional safety disk is mounted at the reactor and connected to the common vent. A continuous unit using the same common vent system may have six safety patches. The exit product lines usually go t o a separator for product recovery and the off gas is fed through a back pressure regulator t o the common vent.
TO VENT
HYDRAULIC BOOSTER SYSTEM Figure 3.
Gas compression systems
For exploratory studies, where there exists any doubt as to the pressure effect, a safety patch should be located close t o the bomb. One such design is shown in Figure 4. For optimum safety the rupture disk is a part of the reactor head. Note that the exit from the safety should always lead to a vent and not into the stall. In systems where the safety patch may be in contact with the charge or where corrosion may occur, precious metal disks are recommended. Platinum has been found t o be satisfactory for general use. An additional safety factor is provided by the cooling coil in the bomb. The cooling water can be turned on by a solenoid valve or a manual bypass. Stationary reactors may be heated electrically, by the use of liquid metal baths, or preferably by fluidized sand baths ( I ) . Rocking bombs are normally heated electrically, but a more suitable design would entail the use of a heated jacket. By an arrangement of solenoid valves, cold liquid could be introduced into the jacket for rapid cooling. Electrical jackets cannot be February 1956
Figure 4.
Reaction vessel
One inherent disadvantage of the common vent system is the possibility of back failure of a safety disk or of operator neglect in leaving the bypass vent valve open. The problem is a major one when the reactor is not in operation and has been disconnected from the system lines. The danger consists in the failure of another reactor and the possibility of toxic gases flowing back into the stall rather than out the common vent line. Rating all safety patches at the same value will minimize back failure.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Good, careful operators are necessary to keep the proper valves closed a t the proper time. It may be possible to eliminate the hazard in this operation by the use of quick connective couplings. The lower right field of the figure s h o w the arrangement used, The coupling on the left', when disconnected, closes a valve in the system line giving a positive shutoff preventing toxic gases which might flow from the common vent throuph an open valve or blown patch. The unit on the right closes OK the line lvhich is attached directly to the common vent.
some dissolved in the reaction products. \There possible, toxic materials should be destroyed in situ. This can be done-for example, in the case of the metal carbonyls-by heating in the presence of hydrogen. Hydrogen is introduced into the reactor and heated up to the catalyst decomposition temperature. The procedure can be repeated if necessary to remove the last traces. Often t,he reaction products cannot be treated I n this manner, since changes in product yield or selectivity will occur. In this case a number of repeated flushings with nitrogen are made to remove as much of the catalyst as possible. The reactor is removed to a hood area and t.he products are gas stripped until safe for normal work-up. Care should always be taken to avoid contact of nickel hydrogenation catalysts with gases containing carbon monoxide or liquids containing the dissolved gas. If the hydrogenation is to be run a t elevated temperatures above the deconiposition point of nickel carbonyl, the presence of small amounts of carbon monoxide will not lead t o nickel carbonyl formation. PERSONKEL
SAFETY
3kk
U Figure 5.
Vent system piping using batch reaction of known characteristics
Operationi should be carried out only by those personnel who have been trained to operate high pressure equipment. Compressors should have safety locks so that unauthorized personnel cannot at.tempt t o operate the units. Personnel should be trained on the unit before permitting them to use the pressure facilities and must be periodically retrained.
TO STACK The ultimate disposal of reactoi gases must be considered In the laboratory design. Usually a design t o handle emergency venting also will be more than adequate for general venting operations. Gases like carbon monoxlde and hydrogen can be vented through a stark t o the atmosphere without danger. The more toxic gases-for exampie, the carbonyls-should be desti oyed and not vented to the atniospheie. Provision should be made for an extreme emergency, when these toxic components have been released. These provisions should include signaling for evacuation of the work area. The area should, of course, be checked for the presence of toxic agents before allowing workmen in the vicinity. Figure 6 shows a proposed installatlon for safe operation using any extremely toxic catalyst. The common vent line is constantly s ~ e p with t a nitrogen purge and check valves prevent backup into the nitrogen source. The common vent feeds into a large surge tank. The reducing valve slowly bleeds the gases either to a flare or scrubber. If the safety disk of a reactor ruptures, the entire contents are vented rapidly. The high pressure drives the reactants out of the bomb into the common vent and surge drum. Here the gases expand and the pressure drops to a controllable level. The gases are then vented slowly through the reducing valve or orifice to the flare or scrubber. A surge of material is avoided. This permits venting over a long period of time, a procedure essential in the case of the highly toxic metal carbonyls. PRODUCT WORK-UP
After completion of the reaction, toxic concentrations of catalyst are still present in the reactor, some in the gas phase and
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m
FLOW
CHECKS
+ON4 N2
SWEEP
COMMON VENT
SURGE TANK
I Figure 6 .
TO SCRUBBER, OR VENT
DQFLARE, I
DRAIN
Vent system using toxic catalyst
The training should emphasize respect for pressure research and knowledge of the highly toxic nature of the reactants used. 411 personnel using toxic materials should be checked regularly by the medical department. It is advisable to have the working area checked by an industrial hygiene group Tvhenever a change in equipment has been made or is requested. Ventilation is included as an important part of the industrial hygiene survey. LITERATURE CITED
(1) Adams, C.E., Gernanol, hl. O., and Kimberlin, C. N . , Jr., IXD. ENO.CHEM. 46, 2458 (1954). RECEIVED for review April 23, 1955.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTED November 28, l(155.
Vol. 48, No. 2
