During the production of polypropylene (PP), a portion of the propylene feedstock is lost. The value of the lost feedstock is substantial, ranging from $1 million to $3 million per year for a typical polypropylene plant. Propylene losses occur primarily in resin degassing vents.
For resin degassing applications, the vent stream is compressed and
then cooled to condense the propylene. The gas leaving the condenser
still contains a significant amount of propylene. This gas is fed to
the membrane unit, which separates the stream into a propylene-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 propylene is recovered. The purified nitrogen stream is recycled
to the degassing bin.
For C3 splitter overhead applications, the VaporSep unit is very simple, consisting of membrane modules only, with no moving parts. The stream leaving the column overhead is primarily propylene, mixed with light gases such as N2 or H2. The VaporSep unit splits this stream into a propylene-enriched stream and a light-gas-enriched stream. The propylene-enriched stream is returned to the distillation column, where the propylene is recovered, and the light-gas-enriched stream is vented or flared.
VaporSep units are currently used by major polypropylene producers including Formosa Plastics, Ineos, SABIC, Sasol, and Sinopec.