While EDC
cracking is highly selective to VCM, a small fraction is
cracked all the way to acetylene. Since acetylene is close to
HCl in volatility, it is separated from the cracking product
along with HCl in the VCM purification process, and ends up in
the feed to the oxychlorination process.
Typical concentrations of acetylene in this HCl recycle stream
are 1,000-2,000 ppm by volume. If allowed to enter the oxychlorination
reactor, the acetylene would be readily converted to perchloroethylene
and other heavily chlorinated by-products, resulting in a significant
HCl efficiency loss. Consequently, the HCl recycle stream is
usually passed through a hydrogenation reactor to convert selectively
the acetylene to ethylene, which makes more EDC downstream.
Hydrogenation is generally carried out in a fixed bed
reactor packed with catalyst made from a precious metal on
an inert support. Hydrogen is added to the feed in stoichiometric
excess to ensure conversion of acetylene to ethylene. The reaction
is temperature dependant, with lower temperatures being preferable
to maximize conversion to ethylene. If the temperature
is too high, a fraction of the acetylene may, be further hydrogenated
to ethane. Although ethane is relatively inert to oxychlorination
and passes through, exiting with the vent stream, this represents
a loss in terms of hydrogen usage and lost ethylene value.
OxyVinyls Super H2 catalyst is especially efficient at converting
acetylene to ethylene at lower temperatures than other catalysts. Conversion
of acetylene is routinely above 99 percent with a selectivity
to ethylene of more than 70 percent. |
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