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 trichloroethylene, 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 selectively convert 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 dependent, 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 H-2® 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, with a typical catalyst bed run life in excess of 15 years.