Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2002-12-05
2004-10-12
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S098000, C095S105000, C095S130000, C096S130000, C096S143000, C055S356000
Reexamination Certificate
active
06802889
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to pressure swing adsorption processes, systems and apparatus for the separation of a multi-component teed gas mixture by selectively adsorbing at least one more readily adsorbable component in a bed of adsorbent material.
Gas separations by pressure swing adsorption (PSA) are achieved by coordinated pressure cycling of a bed of adsorbent material which preferentially adsorbs at least one more readily adsorbable component present in a feed gas mixture relative to at least one less readily adsorbable component present in the feed gas mixture. That is, the bed of adsorbent material is contacted with a ready supply of a feed gas mixture. During intervals while the bed of adsorbent material is subjected to the ready supply of feed gas mixture and the bed is at or above a given feed pressure, a supply of gas depleted in the at least one more readily adsorbable component may be withdrawn from the bed. Eventually, the adsorbent material in the bed becomes saturated with the at least one more readily adsorbable component and must be regenerated. At which point, the bed is isolated from the ready supply of feed gas mixture and a gas enriched in the at least one more readily adsorbable component is withdrawn from the bed, regenerating the adsorbent material. In some instances, the bed may be subjected to a feed of depleted gas to facilitate the regeneration process. Once the adsorbent material is sufficiently regenerated, the bed is again subjected to the ready supply of feed gas mixture and depleted gas can once again be withdrawn from the bed once the pressure on the bed is at or above the given feed pressure. This cycle may be performed repeatedly as required. The period of time required to complete one such cycle is referred to as the “cycle time”.
The cyclic nature of the basic pressure swing adsorption process has resulted in the development of multibed systems which can provide a continuous stream of depleted gas. By way of example, one widely used system described in Wagner U.S. Pat. No. 3,430,418, herein incorporated by references as if set forth herein in its entirety, employs four adsorbent beds arranged in a parallel flow relationship. Each bed in the four bed system proceeds sequentially through a multistep cycle. Because a depleted gas stream cannot be withdrawn from a given bed continuously, the four beds are arranged so that a depleted gas stream may be withdrawn from at least one of the four beds at all times.
The efficiency of the separation of a gaseous mixture achieved using a given pressure swing adsorption system depends on various parameters, including the feed pressure, the regeneration pressure, the cycle time, the pressure gradient established across the bed, the type of adsorbent material as well as its size and shape, the dimensions of the adsorption beds, the amount of dead volume in the system, the composition of the gaseous mixture to be separated, the uniformity of flow distribution, the system temperature and the temperature gradient established within said bed. Variations in these parameters can influence the cost and productivity of a given system.
Every pressure swing adsorption system contains dead volume. For purposes of explanation of this dead volume, a conventional two bed pressure swing adsorption system is depicted in FIG.
1
. The conventional system comprises two identical beds
10
and
50
of adsorbent material
5
. Each of the identical beds
10
and
60
have: a feed gas inlet valve
15
and
55
, respectively; a depleted gas outlet valve
20
and
60
, respectively; and an enriched gas outlet valve
25
and
65
, respectively. The depleted gas outlet valves
20
and
60
are in fluid communication with bed outlet conduits
21
and
61
, respectively, and depleted gas conduit
70
. The conventional system further comprises a feed prime mover
30
and an exhaust prime mover
40
. The feed prime mover
30
intakes a feed gas mixture from the atmosphere or a storage container (not shown) and exhausts the feed gas mixture through feed gas inlet conduit
75
which is in fluid communication with the feed gas inlet valves
15
and
55
. The feed gas inlet valves
15
and
55
are also in fluid communication with bed inlet/outlet conduits
16
and
56
, respectively. The enriched gas outlet valves
25
and
65
are in fluid communication with the bed inlet/outlet conduits
16
and
56
, respectively; and an enriched gas conduit
80
. The enriched gas conduit
80
is also in fluid communication with the exhaust prime mover
40
.
It should be recognized that the dead volume of a pressure swing adsorption system includes (a) an “inlet void volume” which is the volume that is in fluid communication with the inlet end of the bed of adsorbent material and (b) an “outlet void volume” which is the volume that is in fluid communication with the outlet end of the bed of adsorbent material. It should be understood that, for the purposes of this disclosure, the sum of the “inlet void volume” and the “outlet void volume” for a given pressure swing adsorption system is the “total dead volume” for the given pressure swing adsorption system. It should also be recognized that the bed of adsorbent material will itself contain a “bed void volume” which includes the void spaces between and around the individual particles of adsorbent material or, in the case of structured adsorbents, the spaces not occupied by particles of adsorbent material. It should be understood that for the purposes of this disclosure, the “total dead volume” does not include the “bed void volume”.
For example, in
FIG. 1
, the “inlet void volume”
12
for the bed of adsorbent material
10
is indicated using a dashed line. Likewise, the “outlet void volume”
11
for the bed of adsorbent material
10
is indicated using a dashed line. That is, the “inlet void volume”
12
is the volume which is in communication with the inlet end of the bed of adsorbent material
10
. In the pressure swing adsorption system depicted in
FIG. 1
, it is therefore the sum of the volume of (a) the inlet/outlet conduit
16
between the bed
10
and the bed side of the feed gas inlet valve
15
and the enriched gas outlet valve
25
and (b) the free inlet space
13
, the free inlet space may contain a flow distribution system and/or confinement means for supporting the adsorbent material within the bed. Similarly, the “outlet void volume”
11
is the volume which is in communication with the outlet end of the bed of adsorbent material
10
. In the pressure swing adsorption system depicted in
FIG. 1
, it is therefore the sum of the volume of (a) the outlet conduit
21
between the bed
10
and the bed side of the depleted gas outlet valve
20
and (b) the free outlet space
14
, for example space required by a confinement means for retaining the adsorbent material within the bed and reducing the potential for fluidizing the adsorbent material.
Every pressure swing adsorption system contains some dead volume. Notwithstanding, the benefits of reducing the size of the dead volume are readily understood by those skilled in the art. Such benefits include improved recovery and productivity. Recognizing the benefits of reducing the total dead volume of a pressure swing adsorption system, however, is quite distinct from recognizing how to effect a reduction in the total dead volume.
To avoid an early breakthrough of an impurity through the bed, conventional adsorption systems incorporate a flow distributor in fluid communication with the inlet of the bed. The purpose of the flow distributor is to distribute the flow of feed gas uniformly across the entire bed cross-section to avoid inefficiencies caused by such early breakthroughs of impurities. Conventional flow distributors, however, introduce dead volume into the system. As noted above, such dead volume tends to negatively influence the efficiency of the gas separation.
Many conventional adsorption systems use a single pump or other type of conventional prime mover to transfer a feed gas mixture into the bed durin
Graham David Ross
Occhialini James Michael
Puri Pushpinder Singh
Air Products and Chemicals Inc.
Gourley Keith D.
Kiernan Anne B.
Spitzer Robert H.
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