Nearly all commercially made VCM is produced by thermal dehydrochlorination or cracking of EDC.
C2H4Cl2 → C2H3Cl + HCl
Cracking is an endothermic process (ΔHrxn = 71 kJ/mol EDC consumed) that occurs as a homogeneous, vapor-phase, first-order, free-radical chain reaction. Commercial EDC crackers operate at gauge pressures of 1.4-3.0 MPa (200-435 psig) and at temperatures of 475-525°C. EDC conversion per pass is normally maintained at 53-63%, with a residence time of 2-30 sec. Cracking reaction selectivity to vinyl chloride of >99% can be achieved at these conditions. Increasing conversion beyond this level gives progressively lower selectivity to VCM and higher coking rates. High-pressure cracking will also lead to lower selectivity to VCM, higher byproduct formation, and higher coke formation.
Cracking furnace effluent must be quenched, or cooled rapidly, to keep coking at a minimum. Therefore, the hot effluent gases are typically quenched and partially condensed by direct contact with cold EDC in a quench tower. Alternatively, the hot effluent can first be cooled by heat exchange with cold liquid EDC furnace feed or by vaporizing boiler feed water (BFW) to produce high-pressure steam in a transfer line exchanger (TLX) prior to entering the quench tower. This arrangement saves energy by decreasing the amount of fuel needed to fire the cracking furnace and/or steam needed to vaporize the feed.
EDC Cracking and VCM Purification Process