Novel Membrane Systems for Olefin/Paraffin Separation

Ethylene and propylene are the primary feedstocks for the manufacture of polyethylene and polypropylene, respectively. These olefins represent a major component of the polymer manufacturing cost. Consequently, there is significant economic benefit in minimizing losses of unreacted olefin from the process. Some olefin loss is a result of the need to remove paraffin from the polymerization reactor. Low concentrations of paraffin are present in the feedstock (ethane in the case of ethylene feed and propane in the case of propylene feed). The paraffin builds up in the reactor as the olefin is consumed by reaction and reactor effluent is recycled. This makes it necessary to vent some of the reaction mixture in order to limit the buildup of paraffin. The vent stream carries both paraffin and the valuable olefin. A membrane that can efficiently separate the olefin from the paraffin would provide substantial economic benefit to these polymerization processes. Membrane processes have been previously evaluated for separating ethylene/ethane or propylene/propane. While somewhat encouraging results have been demonstrated, stability problems have led to systems that have been unable to maintain performance. In many cases silver (Ag+) salts were used to preferentially transport the ethylene or propylene. It is proposed to combine the stability features and high gas transport of Compact Membrane Systems proprietary membranes with transition metal complexes to create a facilitated transport membrane. These polymers have demonstrated superb chemical and thermal stability with the highest gas transport of any known stable polymer membrane. CMS has identified routes to adding transition metal complexes to the polymer. A robust chelate with a late-transition metal center incorporated into the membrane polymer will enable both high permeance and high selectivity for ethylene/ethane and propylene/propane separations. Detailed engineering and economic evaluations will be done with basic data from Phase I. Commercial Applications or other Benefits: In 2005 U.S. manufacturers produced about 16.3 million metric tons of polyethylene and 8.1 million metric tons of polypropylene. Annual reactor vent stream losses of the olefin feedstocks are estimated to have a value of $620 million and represent an equivalent energy loss of 29 trillion Btu. The proposed membrane can drastically reduce these costs by recovering olefins from vent streams for reuse in the reactor