Trichloroethane pyrolysis

Chlorinated by-products of ethylene oxychlorination typically include 1,1,2-trichloroethane chloral [75-87-6] (trichloroacetaldehyde) trichloroethylene [7901-6]-, 1,1-dichloroethane cis- and /n j -l,2-dichloroethylenes [156-59-2 and 156-60-5]-, 1,1-dichloroethylene [75-35-4] (vinyhdene chloride) 2-chloroethanol [107-07-3]-, ethyl chloride vinyl chloride mono-, di-, tri-, and tetrachloromethanes (methyl chloride [74-87-3], methylene chloride [75-09-2], chloroform, and carbon tetrachloride [56-23-5])-, and higher boiling compounds. The production of these compounds should be minimized to lower raw material costs, lessen the task of EDC purification, prevent fouling in the pyrolysis reactor, and minimize by-product handling and disposal. Of particular concern is chloral, because it polymerizes in the presence of strong acids. Chloral must be removed to prevent the formation of soflds which can foul and clog operating lines and controls (78). 

By-products from EDC pyrolysis typically include acetjiene, ethylene, methyl chloride, ethyl chloride, 1,3-butadiene, vinylacetylene, benzene, chloroprene, vinyUdene chloride, 1,1-dichloroethane, chloroform, carbon tetrachloride, 1,1,1-trichloroethane [71-55-6] and other chlorinated hydrocarbons (78). Most of these impurities remain with the unconverted EDC, and are subsequendy removed in EDC purification as light and heavy ends. The lightest compounds, ethylene and acetylene, are taken off with the HCl and end up in the oxychlorination reactor feed. The acetylene can be selectively hydrogenated to ethylene. The compounds that have boiling points near that of vinyl chloride, ie, methyl chloride and 1,3-butadiene, will codistiU with the vinyl chloride product. Chlorine or carbon tetrachloride addition to the pyrolysis reactor feed has been used to suppress methyl chloride formation, whereas 1,3-butadiene, which interferes with PVC polymerization, can be removed by treatment with chlorine or HCl, or by selective hydrogenation. 

Pyrolysis. The pyrolysis of 1,1,1-trichloroethane at 325—425°C proceeds by a simultaneous unknolecular and radical-chain mechanism to yield... 

The monomer is produced from trichloroethane by dehydrochlorination Figure 17.2). This may be effected by pyrolysis at 400°C, by heating with lime or treatment with caustic soda. The trichlorethane itself may be obtained from ethylene, vinyl chloride or acetylene. 

Vinylidene chloride is produced by the pyrolysis of 1,1,2-trichloroethane at 400°C in the presence of lime or base. Since both vinylidene chloride and vinyl chloride are carcinogenic, their concentrations must be kept low. 

Simulate the vinyl chloride process (Problem 5.4) using Aspen Plus. Take the feed at room temperature and 20 psia. Operate the direct chlorination reactor at 65°C and 560 kPa. A distillation column removes the trichloroethane and the rest of the stream is sent to the furnace. Heat the stream to 1500 F so pyrolysis takes place. Cool the effluent from the furnace, and recycle the vapor (mostly HCl). Send the hquid (vinyl chloride and ethylenedichloride) to a distillation column for separation. 

Ketcnc, prepared by pyrolysis of acetone vapor, is bubbled through a solution of 83 mg (0.25 mmol) of quinidine in 50 mL of toluene at — 50 CC while 1.47 g (10 mmol) of anhyd 2,2.2-trichloroethanal in 20 mL of toluene is added dropwise during 1 h. After the reaction is complete, the mixture is warmed to r.t. and extracted repeatedly with 4 N hydrochloric acid to remove the catalyst. The toluene layer is washed with sat. brine, dried over MgS04 and the toluene is removed under reduced pressure. Evaporative distillation at 120"C/0.5 Torr affords 1 yield 1.67 g (89%) [a]D —15.3 98% ee. 

Most of the condensate is mixed with the effluent from the recycle cooler to be processed in the pyrolysis loop. However, a portion is refluxed to the rectifying section of the column, which has several trays, to recover any of the less volatile species (e.g., trichloroethane) that may have vaporized. These heavies accumulate at the bottom of the liquid pool and are removed periodically as impurities. 

Vinylidene fluoride, CH2=CF2, is obtained by the pyrolysis of 1,1-difluoro- 1-chloroethane, which in turn is produced from acetylene, vinylidene chloride, or 1,1,1-trichloroethane by reaction with hydrogen fluoride. Because of its low boiling temperature, —84°C, vinylidene fluoride is suspension or emulsion polymerized under pressure. Considerable head-head linkage quantities are produced in these polymerizations. 

The EDC produced from the direct chlorination, oxychlorination and the recovered from the cracking step is required to be treated to reach more than 99.5% purity before entering the pyrolysis tmit. The by-products are removed in a sequence of two distillation columns. The first column removes the light wastes while the heavy wastes, mainly C2H3CI3 (1,1,1-trichloroethane), are removed in the second column. 

In the first stage, vinylidene chloride undergoes addition with hydrogen chloride at about 30°C and atmospheric pressure in the presence of a Friedel-Crafts type catalyst. The resulting trichloroethane is then treated with hydrogen fluoride at about 180°C and 30 atmospheres in the presence of antimony pentachloride to give chlorodifluorethane. Pyrolysis of this product yields vinylidene fluoride Vinylidene fluoride is a gas, b.p. —84°C. 

In the first alternative, trichloroethane is prepared by the liquid phase chlorination of vinyl chloride at 30-50°C under pressure. In the second alternative, trichloroethane is obtained by liquid phase chlorination of ethylene dichloride at about 60°C in the presence of aluminium chloride as catalyst. The trichloroethane is then dehydrochlorinated by agitating with an aqueous suspension of calcium hydroxide at about 50°C. Crude vinylidene chloride distills off as it is formed and is then purified by distillation under pressure. The dehydrochlorination of trichloroethane may also be accomplished by pyrolysis at 400°C. Vinylidene chloride is a colourless liquid (b.p. 32°C). It is rather difficult material to handle since it readily polymerizes on standing. Polymerization occurs rapidly on exposure to air, water or light but even storage under an inert atmosphere does not completely prevent polymer formation. The monomer is therefore commonly inhibited with a phenol, such as p-methoxyphenol, which is removed by distillation or alkaliwashing before polymerization. 

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