Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-11-01
2002-07-30
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S105000, C095S130000
Reexamination Certificate
active
06425938
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
Pressure swing adsorption is a well-known method for the separation of bulk gas mixtures and for the purification of gas streams containing low concentrations of undesirable components. The method has been developed and adapted for a wide range of operating conditions, product purity, and product recovery. Many pressure swing adsorption systems utilize two or more adsorber beds operated in a cyclic sequence in order to maintain a constant product flow rate while selected beds undergo various steps including adsorption, depressurization, desorption, purge, pressure equalization, repressurization, and other related steps. Multiple adsorber beds using numerous process steps are required to achieve high purity and/or recovery of valuable gaseous products such as hydrogen, carbon oxides, synthesis gas, light hydrocarbons, and the like. The high cost of generating the feed gas mixtures containing these valuable components usually justifies the complexity and capital expense of multiple-bed pressure swing adsorption systems.
A number of single-bed pressure swing adsorption (PSA) processes have been developed and are known in the art. Many of these processes operate partially at pressures below atmospheric and are described as vacuum swing adsorption (VSA) or vacuum-pressure swing adsorption (VPSA) processes. In the present specification, pressure swing adsorption (PSA) is used as a generic term to describe all types of cyclic adsorption systems regardless of operating pressure levels.
Other gaseous products amenable to recovery by PSA do not require the high purity and/or recovery of the above-named products. In the recovery of oxygen and nitrogen from air by PSA, for example, a lower purity product containing 90 to 95 vol % oxygen is acceptable for many end uses, and simpler PSA systems can be used to provide such a product. These simpler PSA systems have significantly lower capital and operating costs than the multiple-bed systems earlier described. The simplest of these PSA systems for air separation utilize a single adsorber bed in conjunction with one or more gas storage vessels to allow constant product flow and provide gas for adsorber purge and pressurization during the regeneration portion of the PSA cycle.
PSA systems using a single adsorber and a single gas storage tank are well known in the art, being described in representative U.S. Pat. No. 4,561,865; 4,477,264; 4,892,566; 5,228,888; 5,415,683; 5,679,134; 5,876,485; 5,882,380; 6,102,985; and 6,096,115. Single adsorber/single tank systems also are described in Japanese Patent Application Kokai Nos. H9-77502 and H10-194708.
The use of two or more gas storage tanks can improve the overall performance of single adsorber PSA systems. Such systems are described in U. S. Pat. Nos. 3,788,036; 4,561,865; 5,370,728; 5, 658,371; 6,102,985; and 6,096,115; and in European Patent Publication EP 0 884 088 A1.
The invention described below and defined by the claims which follow is a single bed pressure swing adsorption process utilizing at least two gas storage tanks, an embodiment of which is useful for the recovery of oxygen from air with low capital and operating costs.
BRIEF SUMMARY OF THE INVENTION
The invention is a pressure swing adsorption process for the separation of a pressurized feed gas containing at least one more strongly adsorbable component and at least one less strongly adsorbable component which comprises:
(a) introducing the pressurized feed gas into a feed end of an adsorber vessel containing a solid adsorbent which preferentially adsorbs the more strongly adsorbable component, withdrawing from a product end of the adsorber vessel an adsorber effluent gas enriched in the less strongly adsorbable component, introducing adsorber effluent gas into a first gas storage tank, and withdrawing a final product gas from the first gas storage tank;
(b) terminating introduction of the pressurized feed gas into the adsorber vessel and depressurizing the adsorber vessel by withdrawing gas therefrom and introducing the withdrawn gas into a second gas storage tank;
(c) further depressurizing the adsorber vessel by withdrawing additional gas therefrom;
(d) purging the adsorber vessel by introducing gas from the second gas storage tank into the adsorber vessel while continuing to withdraw gas therefrom;
(e) terminating the introduction of gas from the second gas storage tank into the adsorber vessel and immediately thereafter repressurizing the adsorber vessel by introducing pressurized feed gas into the feed end thereof; and
(f) repeating (a) through (e) in a cyclic manner.
The feed gas can be air, the more strongly adsorbable component can be nitrogen, and the less strongly adsorbable component can be oxygen. A final product gas typically is withdrawn from the first gas storage tank during (b) through (e).
At least a portion of the purging of the adsorber vessel in (d) can occur while the pressure therein is decreasing. At least a portion of the purging of the adsorber vessel in (d) can occur while the pressure therein is at a minimum pressure. At least a portion of the purging of the adsorber vessel in (d) can occur while the pressure therein is increasing. The minimum pressure can be below atmospheric pressure.
The second gas storage tank can have a length to diameter ratio greater than about 5, wherein gas is introduced into the tank at one end in (b) and withdrawn from the tank at the same end in (d).
In an alternative embodiment of the invention, further depressurization of the adsorber vessel is effected by withdrawing additional gas therefrom and introducing the gas withdrawn therefrom into a third gas storage tank. This embodiment also can comprise, following (d), purging the adsorber vessel by introducing gas from the third gas storage tank into the adsorber vessel while continuing to withdraw gas from the adsorption vessel.
In another embodiment, following termination of the introduction of pressurized feed gas into the adsorber vessel and prior to depressurizing the adsorber vessel by withdrawing gas therefrom and introducing the gas withdrawn therefrom into a second gas storage tank, the adsorber vessel is depressurized by withdrawing gas therefrom and introducing the gas withdrawn therefrom into the first gas storage tank.
At least a portion of the further depressurizing in (c) can be effected by venting gas from the adsorber vessel to the atmosphere.
The feed gas can be air, the more strongly adsorbable component can be nitrogen, and the less strongly adsorbable component can be oxygen. In this case, a portion of the pressurized feed gas for repressurizing the adsorber vessel can be provided by atmospheric air which flows into the adsorber vessel while the vessel pressure is below atmospheric pressure.
The purging of the adsorber vessel in (d) can be effected by
(d1) introducing gas from the second gas storage tank into the adsorber vessel at a first flow rate; and
(d2) introducing the gas from the second gas storage tank into the adsorber vessel at a second flow rate which is greater than the first flow rate;
while continuing to withdraw gas from the adsorber vessel during (d1) and (d2).
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patent
Agrawal Rakesh
Dee Douglas Paul
Pillarella Mark Robert
Xu Jianguo
Air Products and Chemicals Inc.
Fernbacher John M.
Spitzer Robert H.
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