Propylene recovery in polypropylene production
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During the production of polypropylene (PP), lots of propylene is lost in monomer purification, polymerization reaction and resin degassing process. In the process of bulk polymerization, to reduce the monomer consumption, the compression and condensation technology is normally applied to recover the propylene monomer from the purge gas. However, subject to the cooling temperature and pressure, the noncondensable gas still contains a significant amount of propylene monomer up to 50~80 V%. Although the unit consumption decreases from 1.12~1.15 to 1.07~1.09 after using compression-condensation method to recover a portion of propylene, the propylene monomer loss is still as high as 70~90 kg for per ton PP. In BP-Amoco process for PP production, a typical plant with a capacity of 200 thousand ton per year loses more than 2700 ton propylene during polymerization reaction and resin degassing process. . We have developed a membrane system to recover the propylene from the purge gas. The gas is fed to the membrane system, which separates the stream into a propylene enriched permeate and a purified nitrogen residue. The permeate is recycled to the inlet of the compressor and subsequently to the condenser where the propylene is recovered.

　　System performance：
　　•  Propylene recovery up to 95%
　　•  For small bulk propylene polymerization process, unit consumption
         decreased by 30~40kg /PPt
　　•  Nitrogen purified to 97~99%, which can be reused.

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Ethylene recovery in EO/EG plant
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EO is produced by ethylene selective, catalytic oxidation in the presence of a silver catalyst. Because the oxygen purity is less than 100% with a small part of argon, some purging is necessary to avoid the accumulation of argon. Therefore, ethylene and methane are lost in purge stream. For example, 300ton ethylene will be lost per year in a 100kt EO/year plant.

The purge gas from EO plant is fed to the membrane unit. The permeate stream enriched ethylene is recycled to the inlet of existing compressor, subsequently is sent to reactor. The residue gas is sent to purging pipeline.
System performance Ethylene recovery up to 85% methane recovery up to 40~50% , in the case of using methane as stabilizing gas

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Ethylene and butenes recovery in polyethylene plant
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During the production of polyethylene, a significant amount of ethylene and butenes are lost in the purge gas of polymerization reaction and resin degassing process, which causes great economic loss.


　　－BP Innovene Technology：
In the production HDPE and LLDPE, ethylene conversion rate is less than 100%. So the unreacted ethylene and other hydrocarbons are recycled to reactor for further reaction by recycle compressor. To avoid the accumulation of inert gas such as nitrogen, a portion recycle gas is purged, containing 40% ethylene and 20% other hydrocarbons.
　　System performance：
 
The purge gas fist enters the condenser to recover a portion of hydrocarbons. Then the noncondensable gas is separated two streams by membrane unit. The permeate stream enriched ethylene and other hydrocarbons is sent to the inlet of existing compressor. The residue stream is sent to flare. Membrane process can recover more than 85% ethylene and 99% other hydrocarbons.
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－Unipol Technology:
In the production of PE by Unipol technology, the vent gas from resin degassing contains a significant amount of ethylene, butane, isopentane in addition to nitrogen. The compression-condensation technology is applied to recover hydrocarbons. But the noncondensable gas still contains a certain amount of hydrocarbon which cause great economic loss. Taking 130tPE/a as example, more than 2200 ton hydrocarbons and 300 million cubic meter nitrogen are lost in noncondensable gas.
System performance：

The noncondensable gas is fed to the membrane unit. The permeate stream enriched hydrocarbon is recycled to the inlet of compressor. The nitrogen-enriched residue stream is recycled to the gas bin. The recovery of butene and isopentane is up to 85%. Meanwhile, a fraction of ethylene can be recovered. Nitrogen can be purified to +98% for reuse.
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