Pyrolysis vinyl chloride monomer process

Ethylene dichloride (EDC) is used to manufacture vinyl chloride monomer (VCM), which is one of the largest commodity chemicals produced in the world. EDC may be produced by the direct chlorination of ethylene or oxychlorination of ethylene in the presence of oxygen and hydrogen chloride. Pyrolysis of EDC produces VCM and an equal amount of hydrogen chloride as a co-product. This hydrogen chloride produced in the pyrolysis reactor is utilized by the oxychlorination process as one of the reactants. Therefore, the component processes of direct chlorination, EDC pyrolysis and oxychlorination are combined to develop a balanced process for the production of VCM with no net consumption or production of hydrogen chloride ... 

Thermal cracking of ethane, propane, butane, naphthas, gas oils, and/or vacuum gas oils is the main process employed for the production of ethylene and propylene butadiene and benzene, toluene, and xylenes (BTX) are also produced. Thermal cracking of these hydrocarbons is also called pyrolysis of hydrocarbons. Ethylene is the organic chemical produced worldwide in the largest amoimts and has been called keystone to the petrochemical industry. This technology is well documented in the literature. Somewhat similar thermal cracking processes are used to produce vinyl chloride monomer (VCM) from ethylene dichloride (EDQ, styrene from ethylbenzene, and allyl chloride from propylene dichloride (PDC). Production of charcoal and coke from wood and coal is actually a pyrolysis process, but it is not discussed here. 

Vinyl chloride monomer (VCM) is produced by the pyrolysis of 1,2-dichloroethane (EDC) at around 483 °C and 26.5 atm with a conversion of 55%. The figure below shows the process flow diagram. 

Ethylene Dichlonde and Vinyl Chloride. In the United States, all ethylene dichloride [107-60-2] (EDC) is produced from ethylene, either by chlorination or oxychlorination (oxyhydrochlorination). The oxychlorination process is particularly attractive to manufacturers having a supply of by-product HCl, such as from pyrolysis of EDC to vinyl chloride [75-01-4] monomer (VCM), because this by-product HCl can be fed back to the oxychlorination reactor. EDC consumption follows demand for VCM which consumed about 87% of EDC production in 1989. VCM is, in turn, used in the manufacture of PVC resins. Essentially all HCl generated during VCM production is recycled to produce precursor EDC (see Chlorocarbons and Cm OROHYDROCARBONS ViNYLPOLYAffiRS). 

By comparing the results for chlorinated polypropylene with those for polypropylene, it can be concluded that the two materials undergo very different pyrolytic reactions. Typical for polypropylene is the formation of fragments of the polymeric backbone with formation of monomer, dimer, etc., or with cleavage of the backbone in random places and formation of compounds with 3n, 3n-1, and 3n+1 carbon atoms (see Section 6.1). Pyrolysis of the chlorinated compound leads to a significant amount of HCI and also char. Very few chlorinated compounds are identified in the pyrolysate, since the elimination of HCI leaves very few chlorine atoms bound to carbons. Some aromatic hydrocarbons are formed by a mechanism similar to that of poly(vinyl chloride) pyrolysis. The elimination of HCI leads to the formation of double bonds, and the breaking of the carbon backbone leads to cyclization and formation of aromatic compounds. The reactions involved in this process are shown below for the case of formation of 1,3-dimethylbenzene ... 

Plastics waste can also serve as a source of chemical raw materials. The potential possibilities are considerable, here, since about 25%-30% of plastics consumed are thrown away as waste each year. The following process has proved to be useful hydrolyzable plastics are first hydrolyzed to their monomers below about 200° C the monomers are fractionally distilled off. Then, the poly(vinyl chloride) in the mixture is dehalogenated to poly(olefins) at about 350° C. The residues are then pyrolyzed at about 600-800° C in a sand-fluidized bed. The product fractions are very dependent on the composition of the pyrolyzed material. Generally, however, up to 40% fractions of the economically desirable aromatics are obtained by this high-temperature pyrolysis, and, indeed, when additional steam is blown into the system to reduce carbon char formation. Alternatively, what is known as a low-temperature pyrolysis can be carried out at about 400° C in poly(ethylene) wax as reaction medium. In this case, readily volatile oils of high olefin content are obtained together with waxes and carbon black. 

As the reaction diagram in Fig. 2 shows, hydrochloric acid and oxygen are used to convert ethylene into ethylene dichloride and then the PVC monomer - vinyl chloride - is produced by means of pyrolysis. At present, this procedure is used to use up waste hydrogen chloride from various chemical processes. An important precondition is that the HCl must be as clean as possible. 

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