In-Process Recycling of Monomer From Purge Streams in Polyethylene and Polypropylene Plants

All polyethylene and polypropylene polymerization plants use a purge to remove paraffins entering the reactor with the olefin monomer. Loss of monomer with this purge represents 1-2% of the total monomer consumption of the plant and most of the plant¿s volatile organic compound emissions. The recovery value of the monomer is about $1 million/year for a typical plant. This research project proposes to use a membrane separation system to recover and recycle this monomer to the reactor. Industry-wide monomer recovery from these purge streams represents a large resource recovery opportunity. An estimated 100,000-200,000 thousand tons of recoverable monomer are flared at U.S. polyolefin plants each year. The EPA recognizes that in-process recycling¿reusing wastes within existing operations¿not only reduces discharges to the environment but also conserves raw materials, reducing the environmental impact of their production. In preliminary work, laboratory-scale composite membranes have been prepared with the permeation properties to make the process economically viable. In Phase I, candidate membrane materials for the application will be evaluated, and then bench-scale membrane modules with the required flux and selectivity will be prepared.