Hydrogen recovery by adsorbent membranes

Reference Number: N 04-02

Inventors: Rao, Madhukar B.; Sircar, Shivaji; Abrardo, Joseph M.; Baade, William F.

Owner: NISTAC

USPTO Link:5354547

Invention Summary

The present invention is a method for the recovery of one or more primary components from a gaseous feed mixture comprising these primary components and one or more secondary components, wherein the secondary components are more strongly adsorbed on an adsorbent material than the primary components. The method comprises passing the feed mixture into a separator containing a composite semipermeable membrane comprising microporous adsorbent material supported by a porous substrate and withdrawing from tile separator a nonpermeate intermediate product stream enriched in the primary components. The membrane has a feed side and a permeate side, and portions of the secondary components are selectively adsorbed by the microporous adsorbent material and diffuse from the feed side to the permeate side as an adsorbed fluid phase. A permeate comprising the secondary components is withdrawn from the permeate side of the membrane. The nonpermeate intermediate product stream is separated into a high-purity product stream comprising the primary components and a reject stream comprising additional portions of the secondary components. At least a portion of this reject stream is passed across the permeate side of the membrane as a sweep gas and a sweep gas/permeate effluent stream is withdrawn from the separator. The use of the reject stream, which is relatively lean in the primary components and contains a significant amount of the secondary components, as a membrane sweep gas increases the recovery of primary components and increases the permeation rate of the secondary components through the membrane. Preferably, the intermediate product stream is separated by passing the stream into a pressure swing adsorption (PSA) system in which the remaining secondary components are selectively adsorbed, and withdrawing from the PSA system a high purity product comprising the primary components and a reject stream comprising additional portions of the secondary components.

In a preferred embodiment of the invention, the feed mixture contains hydrogen as the primary component and one or more hydrocarbons as the secondary components in the gas mixture. The hydrocarbons are selected From methane, ethane, ethylene, propane, propylene, butane, isobutane, butylene, isobutylene, and mixtures thereof. In a related alternate embodiment, a methane-containing feed stream is reformed in a steam-methane reformer to produce a synthesis gas comprising hydrogen, carbon oxides, water, and unreacted methane, and the synthesis gas is combined with the intermediate hydrogen product stream from the adsorbent membrane system prior to separation in the PSA system. In an additional embodiment, at least a portion of the sweep gas/permeate effluent stream is fed to a dephlegmator, which separates this stream into a hydrocarbon product stream containing C.sub.3 and heavier hydrocarbons and an intermediate light hydrocarbon stream containing components lighter than C.sub.3. The intermediate light hydrocarbon stream is combined with the intermediate product stream From the membrane prior to separation in the PSA system.

In an alternate embodiment, the invention is a method for the production of hydrogen from a methane-containing feed gas and a gas mixture containing hydrogen and hydrocarbons. The method comprises passing the gas mixture containing hydrogen into a separator containing a composite semi permeable membrane comprising microporous adsorbent material supported by a porous substrate and withdrawing from the separator an intermediate product stream enriched in hydrogen. The membrane has a feed side and a permeate side, and portions of the secondary components are selectively adsorbed by the microporous adsorbent material and diffuse from the feed side to the permeate side of the membrane as an adsorbed fluid phase. A permeate stream comprising these portions of hydrocarbons is withdrawn from the permeate side of the membrane. A portion of the methane-containing feed gas is reformed in a steam-methane reformer to produce a synthesis gas comprising hydrogen, carbon oxides, water, and unreacted methane, and this synthesis gas is combined with the intermediate product stream. The resulting combined stream is fed to a PSA system wherein hydrocarbons and carbon oxides are selectively adsorbed; a high-purity hydrogen product and a reject stream comprising hydrocarbons and carbon oxides are withdrawn from the PSA system. The remaining portion of the methane-containing feed gas is passed across the permeate side of the membrane as a sweep gas to yield a sweep gas/permeate effluent stream, thereby increasing the recovery of hydrogen and increasing the permeation rate of hydrocarbons through said membrane. The sweep gas/permeate effluent stream can be used as feed or fuel in the steam-methane reformer.

The invention includes an integrated separation system for accomplishing the separation described above. The system comprises one or more separator modules each containing the composite semipermeable membrane comprising microporous adsorbent material supported by a porous substrate, each membrane having a feed side and a permeate side, wherein the adsorbent material is capable of selective adsorption and permeation of the secondary components. The integrated separation system also includes a pressure swing adsorption system having a plurality of adsorbent vessels containing granular adsorbent material capable of selectively adsorbing said secondary components, and further includes means for the following functions: (1) introducing the gaseous feed mixture into each of the separator modules on the feed side of the membrane; (2) removing a nonpermeate intermediate product stream from the feed side of the membrane from each of the separator modules; (3) introducing the intermediate product stream into the pressure swing adsorption system; (4) removing a high purity product stream comprising the primary components and a reject stream comprising the secondary components from the pressure swing adsorption system; (5) introducing at least a portion of the reject stream into each of the separator modules and passing the portions of the reject stream over the permeate side of each membrane as a sweep gas; and (6) withdrawing from the separator modules a mixed sweep gas/permeate stream.

The present invention is especially useful for the recovery of hydrogen at high purity from refinery waste gases comprising hydrogen and hydrocarbons. The combination of the adsorbent membrane and PSA systems for this separation is different and unique compared with the prior art combination of polymeric membrane and PSA systems, chiefly because the method of the present invention recovers hydrocarbon components at low pressure as membrane permeate and PSA reject while recovering hydrogen product at relatively high pressures. The adsorbent membrane system can be operated at a lower pressure than a polymeric membrane system thus requiring less feed compression; in addition, the hydrogen compression required between the membrane and PSA systems is lower for the present invention because the hydrogen-rich stream from the membrane is produced at near feed pressure.