Sodium Hydroxide l€i
Caustic Light-Ends Heavy-Ends
FIG. 3.8.6 The direct chlorination step of the vinyl chloride process using a liquid-phase reactor. (Reprinted, with permission, from Smith and Patela, 1992.)
In early designs, the reaction heat was typically removed by means of heat transfer to cooling water. Crude dichlo-roethane was withdrawn from the reactor as a liquid, acid-washed to remove ferric chloride, then neutralized with a dilute caustic, and purified by distillation. The material used for separating the ferric chloride could be recycled to a point, but a purge had to be taken. This process created waste streams contaminated with chlorinated hydrocarbons which had to be treated before disposal.
The problem with the process shown in Figure 3.8.6 is that the ferric chloride is carried from the reactor with the product and must be separated by washing. A reactor design that prevents the ferric chloride from leaving the reactor would avoid the effluent problems created by the washing and neutralization. Because the ferric chloride is nonvolatile, one way to prevent ferric chloride from leaving the reactor is to allow the heat of the reaction to rise to the boiling point and remove the product as a vapor, leaving the ferric chloride in the reactor. Unfortunately, if the reaction is allowed to boil, two problems result:
Ethylene and chlorine are stripped from the liquid phase, giving a low conversion. Excessive by-product formation occurs.
This problem is solved in the reactor shown in Figure 3.8.7. Ethylene and chlorine are introduced into circulating liquid dichloroethane. They dissolve and react to form more dichloroethane. No boiling takes place in the zone where the reactants are introduced or in the zone of reaction. As shown in Figure 3.8.7, the reactor has a U-leg in which dichloroethane circulates as a result of the gas lift and thermosiphon effects. Ethylene and chlorine are introduced at the bottom of the up-leg, which is under sufficient hydrostatic head to prevent boiling.
The reactants dissolve and immediately begin to react to form further dichloroethane. The reaction is essentially complete at a point two-thirds of the way up the rising leg. As the liquid continues to rise, boiling begins, and finally the vapor-liquid mixture enters the disengagement
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