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. In many cases, silver salt facilitating agents have been incorporated into membranes to preferentially transport ethylene or propylene. While good separations have been demonstrated in the laboratory, membrane stability problems have prevented development of commercial systems. It is proposed to combine the stability features and high gas transport of Compact Membrane Systems proprietary membranes with stable transition metal facilitating agents to create a stable facilitated transport membrane. The facilitating agents proposed in this program are resistant to deactivation through reduction reactions. Routes to adding stable transition metal transport facilitators to the polymer have been identified in Phase I. When incorporated into the membrane polymer these facilitators will enable both high permeance and high selectivity for ethylene/ethane and propylene/propane separations. In phase I, a stable transition metal transport facilitator was successfully incorporated into the polymer membrane. The facilitator was shown to be in the desired chemical form in the membrane and the membrane was shown to exhibit high olefin permeance and high olefin/paraffin selectivity with a mixture of propylene and propane. The membrane was further demonstrated to provide stable separation performance over a period of 147 hours with no measurable decrease in permeance and selectivity. In the phase II program, an alternative approach to incorporating the transport facilitator into the membrane will be investigated and facilitated membrane composition will be optimized for olefin/paraffin separation performance. The process to produce the facilitated membrane material will be scaled up to larger quantities (10x greater). A prototype membrane will be fabricated and tested for stability in extended pilot tests. Commercial Applications and Other Benefits: The estimated combined production of polyethylene and polypropylene by U.S. manufacturers in 2011 is about 29 million metric tons. Annual reactor vent stream losses of the olefin feedstocks are estimated to have a value of $330 million and represent an equivalent energy loss of 16 trillion Btu. The proposed membrane can drastically reduce these costs by recovering olefins from vent streams for reuse in the reactor. The processes that produce the ethylene and propylene feedstocks can also benefit from the proposed membrane.