This Small Business Innovation Research Phase II project proposes to continue the development of carbon-ceramic membranes with excellent propylene/propane separation performance. The membranes serve as the enabling technology to be used in an environmentally benign and economically viable membrane process to separate propylene from propane for a variety of important petrochemical and refining processes. These composite membranes contain thin selective layers of a newly-developed microporous carbon material. The rigid structure of the material confers the membranes with exceptional resistance to plasticization. This allows the membranes to retain high mixed-gas selectivities at challenging industrial conditions. The mixed-gas propylene/propane selectivities and stability of the membranes achieved in Phase I work are far superior to those of previously examined polymer and facilitated transport membranes under industrially relevant conditions. In Phase II work, membranes developed in Phase I will be further optimized, and then used to produce prototype commercial-size modules for propylene/propane separations. In addition, this research is expected to increase general understanding of carbon-ceramic membranes and their potential for use in an array of other chemically and thermally challenging gas separations that are not possible with conventional polymeric membranes.
The broader impact/commercial potential of this project will be the use of the new carbon membranes for propylene recovery from polypropylene and propylene derivative reactor purge streams. This technology has important economic potential, considering the large volumes of propylene, polypropylene and other propylene derivatives produced annually in the petrochemical industry. With successful development and demonstration of the membrane-based processes, their potentially much larger applications include propylene/propane separations for monomer production at steam crackers and recovery of propylene from fluid catalytic cracker off-gases in refineries. The cost of making ceramic membranes is higher than that of polymeric membranes, but the savings from lower process energy requirements will easily outweigh the increased membrane costs. If successful, the new membranes will make membrane-based olefin/paraffin separations technically and economically attractive for use in conjunction with, or in place of, distillation.
The objective of this NSF Small Business Innovation Research Phase II project was to continue the development and scale-up of novel plasticization-resistant carbon-ceramic composite membranes for olefin/paraffin separations. Systems based on such membranes could provide serious competition to distillation, the workhorse separation technology of the petrochemical and refining industries. Development of effective membranes has the potential to cut the cost of olefin/paraffin separations in half, reducing the cost of ethylene and propylene production by two to six cents per pound. During Phase II, propylene permeance of small lab-scale, disc-like membranes were significantly improved without losing the propylene/propane selectivity. This was achieved by optimizing the thickness and pore structure of the selective layer in the membrane composite. In addition, the membrane stability under industrially relevant condition was systematically investigated; the study determined that the chemisorption of oxygen by carbon membranes reduces their pore size and porosity, and therefore, lowers the membrane performance over time. Our solution to achieve stable performance was to pre-treat the carbon membranes under optimized conditions, in order to disable the active sites for chemisorption of oxygen. Membrane scale-up was another focus of the project. In the initial scale-up steps, the surface pore size of tubular substrates was found to be the key to preparing high-quality composite membranes. Tubular membranes with performance comparable to the first disc-like membranes were successfully prepared. Our next step is to look for commercial partners that can produce low-cost substrates with desirable surface properties at an industrial production scale. The process economics for propylene recovery from a typical size polypropylene plant were updated using the performance properties attained in this project for single-tube membranes. At the current cost of the membrane substrates, a 10-month payback time can be expected when this technology is used for propylene recovery from a polypropylene plant purge stream. The payback time can be reduced to three months if lower cost substrates become available. The concept of using carbon membranes for gas separations was first proposed over 20 years ago, but to date, no commercial system has been sold. The key remaining issues include membrane scale-up and stability under practical conditions. This project helped clarify some of these issues, and provides guidance for future development work. One paper on the technical results obtained under this project has been published, with two more to be published in the near future.
