direct chlorination

Direct chlorination of ethylene is generally conducted in liquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the liquid phase and combine in a homogeneous catalytic reaction to form EDC.

Ferric chloride (FeCl₃) is an efficient and highly selective catalyst for this reaction, and is widely used commercially.

The direct chlorination process may be run with a slight excess of either reactant, depending on how effluent gases are treated. For instance, the vent stream could be sent to an oxychlorination process if ethylene is in excess. Conversion of the limiting reactant is essentially 100%, and selectivity to EDC is >99%. The principal byproduct is 1,1,2-trichloroethane, which most likely forms through free-radical reactions. Consequently, oxygen is often added to a level of about 0.5% of the chlorine feed. This increases selectivity to EDC by inhibiting formation of the free-radicals that lead to 1,1,2-trichloroethane.

The direct chlorination reaction is very exothermic (”H_rxn = -180 kJ/mol EDC made) and requires heat removal for temperature control. Operation at moderate temperatures (50-65°C) gives lower by-product formation, but limits the choice of heat transfer method to air or water cooling. Heat recovery requires higher reactor temperatures. One possibility is to operate the reactor at the boiling point of EDC, allowing the pure product to vaporize, and then either recovering heat from the condensing vapor, or replacing one or more EDC Purification column reboiler(s) with the reactor itself. Another option is to run the reactor at higher pressure to raise the boiling point. In this case, the reactor operates without boiling, but at higher temperatures (75-200°C) to allow heat transfer to some other part of the process. If the EDC product is not withdrawn as a vapor, it must be treated to remove ferric chloride, which would otherwise foul the EDC cracking furnace. This is usually achieved through washing with water or adsorption onto a solid.

Direct Chlorination Technology