The use of oxygen instead of air permits
operation at lower temperatures and results in improved operating
efficiency and product yield. Ethylene is generally fed in somewhat
larger excess over stoichiometric requirements than in the air-based process.
The reactor effluent is cooled, purified from traces of unconverted
HCl, separated from EDC and water by condensation, recompressed
to the reactor inlet pressure, reheated, and ultimately recycled.
This permits lower ethylene conversion per pass through the reactor
(giving higher selectivity to EDC) with minimal loss in overall
ethylene yield.
Another important advantage of oxygen-based oxychlorination over
air-based operation is the drastic reduction in volume
of the vent stream. Only a small fraction of the reactor off-gas,
typically 2-5%, is continuously purged to prevent accumulation
of impurities such as carbon oxides, nitrogen, argon, and unreacted
light hydrocarbons, which either form in the reactor or enter the
process with the feed streams. In the air-based process, a large vent flow rate is required
because of the nitrogen which enters with the air feed stream.
(Nitrogen is actually the single component with the highest molar
feed rate in the air-based process.)
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