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 inert 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 more valuable olefin. A process intensification membrane that can efficiently separate and recover 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. In many cases, silver salt facilitating agents have been incorporated into membranes to preferentially transport ethylene or propylene. Historically with earlier membranes, while good separations have been initially demonstrated in the laboratory, membrane stability problems have prevented development of commercial systems.
Using perfluoro based membranes, Compact Membrane Systems has developed laboratory membranes which have shown stable (180+ days) and very good propylene flux and propylene/propane selectivity. These results are far above the industry Robeson curve. In addition results were stable when exposed to high levels of conventional foulants (e.g., sulfur containing gases). Using basic data from these laboratory results payback times for propylene plant process intensification were less than 1 year. Payback times for propylene/propane splitters were less than 1-2 years. These and other results will be discussed more extensively at the presentation.