Producing sales quality PVC from raw materials involves several steps. Initially, ethylene dichloride (EDC) must be produced from the feed stocks of ethylene and chlorine. The EDC is then thermally cracked to produce vinyl chloride (VCM), which is the chemical precursor to the PVC polymer. The VCM must be purified and dosed with a polymerization inhibitor to prevent spontaneous polymerization. Below, we will examine each stage of the process and the analysis requirements.
The direct chlorination process involves a liquid phase reaction of ethylene and chlorine using an FeCl3 catalyst. This exothermic reaction is controlled by mass transfer and can be operated either at low temperature (for low byproduct generation) or high temperature (in order to recycle heat).
Direct Chlorination Reaction
CH 2CH2 + Cl2 → ClCH2CH2Cl
A properly optimized direct chlorination reactor produces an effluent stream of 99% EDC purity with trace amounts of trichloroethane, HCl, ethylene, and chlorine. Some operations inject oxygen into the reactor to subdue free radicals and increase selectivity to EDC product. Inhibiting the formation of trichloroethane is especially important because it is very difficult to remove through distillation.
Downstream from the reactor, a caustic scrubber using NaOH removes FeCl3 contamination from the catalyst. The EDC then goes through a water wash and 2 distillation columns to achieve >99.5% purity. This EDC is sent to a storage tank for later processing.
Direct chlorination produces waste gas saturated with HCl which is repurposed in the oxychlorination process, which also produces raw EDC.
The oxychlorination process is used to obtain additional EDC using the HCl-rich waste gas from the direct chlorination reaction. The ethylene feed stock is reacted with HCl and oxygen to produce EDC and water in an exothermic reaction (using a CuCl2 catalyst):
CH 2CH2 + 2HCl + 1/2O2 → ClCH2CH2Cl + H2O
Oxychlorination is a less efficient process than direct chlorination due to the significant formation of water and carbon oxides; the typical reactor effluent stream contains 47 mol% EDC, 47 mol% water, 5 mol% CO2, and trace amounts of oxygen, HCl, ethylene, and various chlorinated hydrocarbons.
Following the oxychlorination reactor, the produced EDC is sent through a caustic scrubber (removing HCl) and a flash (removing impurities) before entering the purification stage.
The purified EDC is sent to a thermal cracker (pyrolyzer) which outputs VCM and HCl. The EDC cracker runs an endothermic reaction at 500 °C and 15-30 atm with VCM selectivity of >90%, yet only has a conversion rate of ~50%.
The pyrolysis of EDC involves a complex set of reactions that can be summarized as below:
ClCH 2CH2Cl → CH2CHCl + HCl
The effluent must be immediately quenched to avoid forming coke or tar. The VCM product is then sent through a flash drum to prepare it for purification. There are many undesirable products from EDC cracking that can foul the reactor and reduce the end product quality, while other impurities (e.g. chloromethanes) may actually be desirable for their promotion of VCM selectivity and are sometimes added intentionally.
The VCM purification unit comprises two distillation columns which are tasked with producing 99.9 %wt VCM. The first column removes HCl as the overhead product, sending the HCl gas back to the oxychlorination unit as raw material. The second column removes EDC and other impurities as the bottoms, sending this stream back to the EDC purification unit. VCM is produced as the overhead of the second column.
Following the distillation columns, catalytic oxidation and hydrogenation steps are usually performed on the distillation byproducts to recover some of the unwanted chlorinated hydrocarbons formed, converting them to CO2, HCl, and water.
The purified VCM is dosed with a polymerization inhibitor (e.g. benzoquinone diimide) at under 500 ppm to prevent spontaneous polymerization of the monomer in pipes and vessels.