Hydrocarbon fractionation by adsorbent membranes
Reference Number: N 04-02
Inventors: Rao, Madhukar B.; Sircar, Shivaji; Abrardo, Joseph M.; Baade, William F.
The present invention is an improved method for effecting a ternary separation of a gas mixture containing at least one primary component and at least two secondary components, in which the secondary components are all more strongly adsorbed on an adsorbent material than the primary components, and in which the secondary components have relative adsorptive characteristics on the adsorbent material which range from most strongly adsorbed to least strongly adsorbed. The method comprises passing the mixture as a feed stream successively through the feed sides of two or more membrane zones or stages in series, in which each membrane zone has an adsorbent membrane containing the adsorbent material which divides the zone into a feed side and a permeate side. As the feed passes successively through the feed side of each membrane zone, it is depleted of the most strongly adsorbed secondary components in the first membrane zone and the least strongly adsorbed components in the last membrane zone. The permeate withdrawn from the permeate side of the first zone is enriched in the most strongly adsorbed secondary components and the permeate withdrawn from the permeate side of the last membrane zone is enriched in the least strongly adsorbed components. The non-permeate stream withdrawn from the feed side of the last membrane zone is depleted in secondary components and thus is enriched in the primary components. The method provides at least three products from a gas mixture containing three or more components, e.g., a product enriched in primary components, a product enriched in the most strongly adsorbed secondary components, and a product enriched in the least strongly adsorbed components.
The separation of the present invention is achieved through the mechanism of selective surface flow in which the secondary components are selectively adsorbed from the feed stream in the successive membrane zones by the adsorbent membrane, diffuse through the membranes as an adsorbed fluid phase, and are withdrawn from the permeate sides of the successive membrane zones to yield the secondary component permeate streams. The feed stream is successively depleted of the secondary components as it passes successively through the feed sides of the series of membrane zones, and the final non-permeate stream withdrawn from the last membrane zone is thus enriched in the primary components.
The method can be improved by further treatment of the primary component-rich non-permeate stream in a pressure swing adsorption (PSA) system which removes a substantial amount of the residual secondary components to yield a high-purity primary component product. In addition, adsorbent membranes can be integrated with cryogenic separation methods and PSA systems to yield improved energy efficiency compared with prior art PSA/cryogenic separation systems.