Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-01-20
2002-04-30
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S103000, C095S130000, C095S139000, C095S140000, C095S143000
Reexamination Certificate
active
06379431
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
This invention relates to pressure swing adsorption (PSA) processes, and more particularly to such processes employing multiple adsorbent beds and multiple pressure equalization steps.
PSA processes are well-known for the separation of gas mixtures that contain components with different adsorbing characteristics. For example, hydrogen production via pressure swing adsorption (H
2
PSA) is a multi-million dollar industry supplying high purity hydrogen for chemical producing industries, metals refining and other related industries.
In a typical PSA system, a multicomponent gas is passed to at least one of multiple adsorption beds at an elevated pressure to adsorb at least one strongly sorbed component while at least one component passes through. In the case of H
2
PSA, H
2
is the most weakly adsorbed component which passes through the bed. At a defined time, the feed step is discontinued and the adsorption bed is depressurized with flow co-current to the direction of the feed in one or more steps which permits essentially pure H
2
product to exit the bed with a high recovery of the most weakly adsorbed component, H
2
. Then a countercurrent desorption step is carried out, followed by countercurrent purge and repressurization.
U.S. Pat. No. 3,986,849 to Fuderer et al. discloses PSA processes employing at least seven adsorbent beds, at least three steps of pressure equalization per bed. This patent teaches that an undesirable reversion of the desorbate profile from the inlet to the discharge end of the bed is substantially reduced when at least three pressure equalization stages are employed. Fuderer et al. does not disclose any embodiments comprising performing four pressure equalization steps in a ten-bed apparatus, or process cycles with on average two or more beds being purged at the same time.
The prior art in general teaches that increasing the number of beds typically facilitates increasing the number of equalizations, which minimizes the production costs of a PSA system. Unfortunately, increasing the number of beds typically increases the cost of a PSA system as well.
Accordingly, it would be very desirable to provide an improved PSA process which increases production and/or recovery per bed in a multiple bed system.
All references cited herein are incorporated herein by reference in their entireties.
BRIEF SUMMARY OF THE INVENTION
The invention provides a pressure swing adsorption process comprising providing a pressure swing adsorption apparatus having ten beds, and equalizing a pressure of each of said ten beds in four steps. At all times during the process, an average of at least two of said ten beds are being simultaneously regenerated by simultaneously providing off-gas from a product end of each of said two beds to an off-gas line. Each of the four pressure equalization steps comprises a depressurization phase spanning about {fraction (1/20)} of a total cycle time of said process and a repressurization phase spanning about {fraction (1/20)} of said total cycle time. The preferred product of the process is hydrogen.
The invention also provides a pressure swing adsorption process in which an average of at least two beds are being counter-currently purged throughout the process cycle.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
Not applicable.
DETAILED DESCRIPTION OF THE INVENTION
In a first embodiment of the invention, the improved PSA system employs ten adsorbent beds and four steps of equalization. Preferably, an average of at least two of said ten beds are simultaneously regenerated by simultaneously providing off-gas from a feed end of each of said two beds to an off-gas line throughout said process. Preferably, each of said four steps comprises a depressurizing provide equalization phase spanning about {fraction (1/20)} of a total cycle time of said process and a repressurization phase “R” spanning about {fraction (1/20)} of said total cycle time. The preferred product of the invention is hydrogen.
A second embodiment of the invention is based on the inventors' discovery that the average bed pressure during the purge step is very important in determining the recovery and productivity of the adsorption system. It is desirable to use a longer purge time to reduce the pressure drop in the bed, and therefore reduce the average bed pressure during the purge step. The slower the purge rates, the more effective a system is in removing adsorbed gases. Thus, the second embodiment of the invention requires that, at all times throughout the process, at least two beds are being counter-currently purged at the same time.
In a preferred embodiment, the process comprises:
(a) at least one adsorption step comprising feeding a feed mixture to a feed end of a first bed, adsorbing impurities onto an adsorbent in said first bed and permitting a product gas to exit a product end of said first bed;
(b) at least one depressurizing equalization step comprising reducing a pressure in said first bed by closing a feed valve and sequentially releasing gas from a product end of said first bed to other beds or to at least one other bed and a tank;
(c) at least one pressure reduction step comprising further reducing said pressure of said first bed co-currently and/or counter-currently;
(d) at least one counter-current purging step, comprising counter-currently purging said first bed with gas from another bed for a duration such that at least two of said beds of said apparatus are being purged simultaneously throughout said process;
(e) at least one repressurizing equalization step comprising increasing the pressure of said first bed with gas released from at least one other bed and/or tank undergoing said pressure reducing step; and
(f) at least one pressure augmentation step comprising further increasing said pressure of said first bed by feeding to said first bed at least one of a counter-current stream of product gas and a co-current stream of feed gas.
In certain embodiments, steps (a) to (f) are sequential.
The preferred product gas is hydrogen, but the invention is not limited thereto.
Preferably, the feed mixture comprises hydrogen and at least one member selected from the group consisting of methane, carbon dioxide, carbon monoxide, nitrogen and water vapor.
In embodiments, the feed gas is obtained by steam reforming of hydrocarbons.
In other embodiments, the feed gas is obtained by partial oxidation of hydrocarbons. In a number of these embodiments, the partial oxidation can occur in the presence of at least one catalyst.
In embodiments wherein the oxygen used for oxidation is provided by an ion transport membrane, it is preferred that the ion transport membrane be integrated with a reactor in which the partial oxidation occurs.
In embodiments, at least a part of said at least one repressurizing equalization step overlaps in time with said at least one pressure augmentation step.
In embodiments, at least a part of said at least one repressurizing equalization step overlaps in time with at least one of counter-current repressurization by the product gas and co-current repressurization by the feed gas.
In embodiments, gas from the product end of the bed in said at least one depressurizing equalization step is transferred directly to a bed in said at least one repressurizing equalization step.
In embodiments, gas from the product end of the bed in said at least one depressurizing equalization step is transferred to a tank before being transferred to a bed in said at least one repressurizing equalization step.
In embodiments, a bed being co-currently depressurized according to said at least one pressure reduction step provides purge gas to more than one other bed.
In embodiments, the depressurizing equalization step precedes said provide purge step, in other embodiments, the depressurizing equalization step overlaps in time with at least some portion of said provide purge step, and in still other embodiments, th
Occhialini James Michael
Pillarella Mark Robert
Rarig David L.
Weist, Jr. Edward L.
Xu Jianguo
Air Products and Chemicals Inc.
Chase Geoffrey L.
Spitzer Robert H.
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