The manufacture of vinyl acetate, to be used for preparing polyvinyl acetate, is preferably carried out as a vapor-phase reaction between acetylene and acetic acid. Zinc acetate is used commonly as the catalyst for these vapor phase processes. A suitable catalyst consist of 3-5 mm granules of a mixture of 42 parts of zinc acetate to 100 parts of activated charcoal. The catalyzers are steel boxes in which the catalyst is held between vertical plates spaced 1 in. apart. Cooling of the exothermic reaction is provided by horizontal steel tubes in the catalyst bed through which water is passed.

By : Stephanus Sulaeman@upv.pertamina

VINYL AND VINYL POLYMERS

VINYL ACETATE

The manufacture of vinyl acetate, to be used for preparing polyvinyl acetate, is preferably carried out as a vapor-phase reaction between acetylene and acetic acid. Zinc acetate is used commonly as the catalyst for these vapor phase processes. A suitable catalyst consist of 3-5 mm granules of a mixture of 42 parts of zinc acetate to 100 parts of activated charcoal. The catalyzers are steel boxes in which the catalyst is held between vertical plates spaced 1 in. apart. Cooling of the exothermic reaction is provided by horizontal steel tubes in the catalyst bed through which water is passed.

Purified acetylene, from which all traces of hydrogen sulfide and phosphine have been removed by scrubbing with sulfuric acid and passing over potassium dichromate and kieselguhr, is bubbled through acetic acid heated to 60oC in the vaporizer. The rate of flow is adjusted so that the gas leaving the vaporizer carries with it 23% by weight of acetic acid. The gas, which is heated to 170oC by passing through a series of heat exchangers and heaters, flows into the catalyzer, in which the gas temperature is held at 170oC.

The conversion to vinyl acetate in the catalyzer may be regulated either by adjusting the rate of gas flow through the catalyzer or by varying the reaction temperature. The best economic balance is obtained at a conversion of 60%. A charge of catalyst has a life of about two months when vinyl acetate is produced at a rate of 400-500 metric tons per month.

The crude vinyl acetate vapor emerging from the catalyzer passes through the heat exchanger and then to a separator where carbon dust is removed. The vapor is then passed through a series of three condensers. Condensate from each of these units, plus the liquid from the mist remover, is collected in a common line which flows to the still. The condensate averages 60% of pure vinyl acetate and 40% of acetic acid. Resin formation during distillation is prevented by the addition of thiophenylamine to the crude vinyl acetate in the still. The yield of vinyl acetate varies from 92-95% based on acetylene and from 97-99% based on acetic acid.

Since no highly corrosive catalysts are used in the process, the materials of construction need not be special grades of highly corrosion resistant materials. The vaporizer, heat exchanger, separator, preheater, heater, and catalyzer may be made of steel; condenser, mist remover, and still are often of stainless steel.

Long chain vinyl esters, such as vinyl stearate, can be prepared by the liquid phase reaction of acetylene and a long chain acid in the presence of zinc stearate catalyst at 200 psi and 165oC in a stainless steel autoclave. Vinylation is complete in 8.5-9 hr; 3-5% of the acid is left unreacted.

VINYL CHLORIDE

The insatiable demand for vinyl chloride resins has resulted in a number of different processes being commercialized, depending on the economics of basic raw materials. Cost and availability of ethylene, acetylene, hydrogen chloride, natural gas and caustic soda have all been carefully weighed at each specific location, and often combinations of processes have been used.

ETHYLENE DICHLORIDE ROUTE

Ethylene and chlorine gases are reacted in liquid phase to form ethylene dichloride.

C2H4 + Cl2 → CH2ClCH2Cl

The addition of chlorine to ethylene may be effected in the liquid phase in the presence of a cosolvent or in the vapor phase in the presence of metal contact agents.

The use of solvents ordinarily leads to considerable substitution as well as addition, even at relatively low temperatures. From 10%-20% polychloroethanes are obtained when operating at 0-40 oC. the cosolvent may be some of the desired product, viz., dichloroethane or a polychlorohydrocarbon, which can be recovered and reused.

According to Hammond, the addition of relatively small amounts (0.05% – 0.25%, based on the weight of dichloroethane used as a cosolvent) of anhydrous ferric chloride will serve to depress substitution reactions. The reaction can be carried out in the apparatus of water cooled exothermic reactor. Chlorine and ethylene, under moderate pressure, are introduced into a reactor containing a body of dichloroethane and provided with a plurality of tubes through which cooling water is circulated. Ferric chloride may be introduced as such but, because of its hygroscopicity, may be first dissolved in anhydrous ethanol or delivered to the reactor with the replacement dichloroethane.

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