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secondary components is withdrawn from the permeate side of the membrane.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side of the membrane as a sweep gas and a sweep gas/permeate effluent stream is withdrawn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.