During the production of polyethylene (PE), a portion of the ethylene and other hydrocarbon feedstock is lost. The value of the lost feedstock is substantial, ranging from $1 million to $3 million per year for a typical PE plant. Losses occur primarily at 3 points in the production process: distillation column overhead vents in the ethylene recovery and purification step, reactor purge vents, and resin degassing vents.
For resin degassing applications, the vent stream is compressed and cooled to
condense hydrocarbons. The gas leaving the condenser still contains a significant
amount of hydrocarbon. This gas is fed to the membrane section, which separates
the stream into a hydrocarbon-enriched permeate stream and a purified nitrogen
residue stream. The permeate is recycled to the inlet of the compressor and then
to the condenser where the hydrocarbon is recovered. The purified nitrogen
stream is recycled to the degassing bin.
For distillation column overhead and reactor purge applications, the VaporSep unit is very simple, consisting of membrane modules only, with no moving parts. The stream leaving the column or reactor is typically contaminated with light gases such as N2 and H2. The VaporSep unit splits this stream into a hydrocarbon-enriched stream and a light-gas-enriched stream. The hydrocarbon-enriched stream is returned to the distillation column or reactor where the hydrocarbon is recovered, and the light-gas-enriched stream is vented or flared. VaporSep units are currently used by major PE producers including ExxonMobil, Formosa Plastics, SABIC, and Sinopec.