8 Liquid-Phase Oxychlorination of Ethylene
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To Produce Vinyl Chloride L E O F R I E N D , L E O N A R D W E N D E R , and JOSEPH C. YARZE The M . W. Kellogg Co., Research and Development Division, Piscataway, N . J.
A new technology for producing 1,2-dichloroethane from ethylene, wet or dry hydrogen chloride, and oxygen uses a promoted aqueous solution of copper salts as the catalyst. The process features a selectivity better than 96%, flexibility in the use of HCl or Cl in any proportion, moderate temperature, direct heat removal by vaporization of water and the use of recognized materials of construction. Extensive pilot-plant studies were carried out to determine the mass transfer, chemical kinetics, heat removal and scale-up characteristics of the process and to determine selectivity and the stability of the system as well as appropriate materials of construction. These results, together with extensive engineering design studies provide the basis of the new process for producing dichloroethane. Economics of the process for a commercial size plant are described. 2
^ p h e past 10 years have seen a dramatic change in the technology of -* poly (vinyl chloride) as the price has dropped from 33 to 14 cents/lb. and as U . S. demand has increased nearly five-fold to an estimated two billion lbs./year in 1966. This has created an unprecedented demand for the monomer, has added to the imbalance in chlorine/caustic de mand, and has created large surplus quantities of HC1—a by-product of the ethylene route to vinyl chloride. In producing vinyl chloride from ethylene, a two-step process is used (6) in which ethylene is first chlorinated to 1,2-dichloroethane (DCE). C H 2
4
+ C l -> C , H C L2 2
4
L
(1)
D C E is then thermally cracked to produce vinyl chloride and by-product HC1. 168 Luberoff; Homogeneous Catalysis Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
8.
FRIEND ET AL.
Oxychlorination of Ethylene
169
C H C 1 -> C H ,C1 + H C l
(2)
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2
4
2
2
;
Some producers achieve a balanced operation by a combination of processes in which the by-product H C l reacts with acetylene to yield additional vinyl chloride. This is not generally the most attractive alter native because of the relatively high cost of acetylene compared with ethylene. Acetylene and ethylene can be produced in balanced yield ratios (1, 2), but these processes involve separation problems. Conse quently, a very considerable amount of research effort has gone into other techniques to use H C l directly for chlorination purposes. One obvious method is the historic Deacon process for oxidizing H C l back to chlorine. 2HC1 + i0
-> C l + H 0
2
2
(3)
2
Several modern versions of this process ( 7 ) have been proposed. Another technique, which has been used with varying success, is the direct oxychlorination of ethylene to D C E (4) via Reaction 4. C H 2
+ 2HC1 + O > -> C H C 1 + H . O
4
2
l
2
4
2
(4)
Both of the above processes have normally been carried out over supported metal halide catalysts at elevated temperature and pressure. One of the most difficult problems has been removing the large quantity of heat generated at the surface of the catalyst by the reaction. If tem perature is not adequately controlled in oxychlorination, a serious loss in selectivity will result, and catalyst volatilization will occur. In some cases fluidized solids or moving-bed techniques have been used, but these have generally met with difficulties owing to the volatile nature of the more active metal halide catalysts, such as copper chloride and the corrosive nature of the system. During a series of basic experiments in homogeneous catalysis at the M . W . Kellogg Laboratory, it was discovered that ethylene could be made to react with an aqueous solution of certain metal halides to give 1,2-dichloroethane at moderate temperatures and pressures (e.g., 300°F., 150 p.s.i.g. ). The reaction appears to occur between complexed ethylene and the metal halide. For example using copper chloride, ethylene com plexed with cuprous chloride is believed to react with cupric chloride to produce dichloroethane. The over-all reaction can be written simply as: C H 2
4
+ 2CuCl -> C H C 1 + 2CuCl 2
2
4
2
(5)
Since the oxidation of aqueous cuprous-cupric solutions is well-known (8): 2CuCl + 2HC1 + J 0
2
-> 2CuCl + H 0 2
2
(6)
it appeared that this system might constitute a novel way (3, 5) to carry
Luberoff; Homogeneous Catalysis Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
170
HOMOGENEOUS CATALYSIS
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out the over-all oxychlorination (Reaction 4) in which precise tempera ture control could be achieved by vaporizing water from the catalyst. Homogeneous catalysis of chemical reactions is a fast-growing new technology, which has now achieved a considerable degree of commercial acceptance. Because of its novelty, however, extensive pilot-plant studies were necessary to determine mass transfer, chemical kinetics, heat re moval, and scale-up characteristics of the process and to determine selectivity and the stability of the system as well as appropriate materials of construction. Based on these results, together with extensive engi neering design studies, a new process for producing D C E has been developed which offers unusual flexibility and selectivity. Process Description A flowsheet of the process is shown in Figure 1. It can operate with either H C l / 0 , chlorine, or H C l / 0 and chlorine in any proportion as régénérants. It offers outstanding flexibility in that it w i l l also accept wet HC1 or contaminated by-product HC1 from the cracking of D C E in a vinyl chloride plant. These régénérants, along with ethylene, are fed to a prestressed brick-lined reactor containing the promoted, aqueous C u C l / C u C l catalyst. Ethylene chlorination and catalyst regeneration occur si multaneously at steady state in the reactor, and D C E is produced at a selectivity of better than 96%. The reactor operates in the ranges 3 4 0 ° 360°F. and 250-275 p.s.i.g. Other reactor conditions, particularly catalyst composition and partial pressures of D C E , HC1, and water vapor have been set to assure high selectivity operation as delineated by pilot-plant studies. Reaction heat is removed simply and smoothly by vaporizing water. No cooling surface is required in the reactor; thus, there are no temperature gradients, and close temperature control is attained. 2
2
2
Reactor effluent gases are quenched with water in a prestressed brick-lined, packed tower. The liquid leaving the tower is cooled further and separated into aqueous and D C E phases. The aqueous phase is split, part being recycled to the tower as quench liquid and the remainder recycled to the reactor, except for a purge equal to the water produced in the oxychlorination reaction. The water recycled to the reactor is first used to absorb part of the HC1 feed and enters the reactor as an aqueous HC1 solution. D C E product is cooled further and flashed to separate out more water (purged) and dissolved ethylene (recycled). Uncondensed gases from the quench tower are recycled to the re actor, except for a purge stream to remove inerts. The purge stream goes through an ethylene recovery system to keep the over-all utilization of ethylene high—i.e., better than 95%. Careful attention has been given to the oxygen level in the reactor effluent and recycle gases. Experimental determinations were made, under severe ignition conditions, of flammability limits for these ethyleneand oxygen-bearing gases at actual process temperature, pressure, and
Luberoff; Homogeneous Catalysis Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
8.
Oxychforination of Ethylene
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171
ETHYLENE RECYCLE
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PURGE (INERTS)
QUENCH WATER LIQUID-LIQUID f SEPARATOR WATER PURGE HCI 1ABSORBER Figure 1.
FLASH
