Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1997-11-04
2001-02-06
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S103000, C095S130000
Reexamination Certificate
active
06183537
ABSTRACT:
FIELD OF THE INVENTION
This invention is directed to the field of gas separation by pressure swing adsorption (PSA), which when described herein shall also include vacuum pressure swing adsorption (VPSA) and vacuum swing adsorption (VSA). More particularly, the invention is directed towards reducing capital and operating costs in PSA systems through the use of particular rotary blowers.
BACKGROUND OF THE INVENTION
Two-bed PSA oxygen processes incorporate a vacuum step that is used to desorb nitrogen from the molecular sieve. During this vacuum step a Roots-type rotary blower is used. As the gas passes across the blower from a region of relatively low pressure to a region of relatively high pressure it is compressed, and consequently heated.
Table 1 illustrates typical heat of compression values (measured as &Dgr;T
rise
=blower discharge temperature−blower inlet temperature) associated with steady state operation of a standard rotary blower. By the term “standard rotary blower” we mean a blower that is not capable of being cooled via external means. An example of such a blower is the RAS-J Whispair, available from Dresser Industries, Connorsville, Ind.
The data in Table I below is measured at an ambient pressure of 14.5 psia and ambient temperature of 70° F. The table represents steady state conditions.
TABLE 1
Performance Data for a Rotary Positive
Blower having no external cooling
&Dgr;P = Inlet P −
Outlet P
(PSIA)
&Dgr;T
rise
0
7
0.5
13
1
19
1.5
26
2
33
2.5
42
3
51
3.5
61
4
72
4.5
84
5
98
5.5
113
6
131
6.5
151
7
175
7.5
202
8
234
8.5
272
9
318
9.5
375
10
447
As can be seen, as the difference between the pressure at the inlet of the blower and the outlet of the blower (&Dgr;P) increases, the heat of compression temperature at the outlet of the blower increases. This increase in temperature (&Dgr;T
rise
) leads to the heating of the compressor components which, if excessive, result in breakdown of the compressor. Thus if a Roots-type blower has a rated temperature tolerance (&Dgr;T
max
) of, for example, 230° F., this steady state &Dgr;T value is exceeded at a steady state &Dgr;P of about 8.
Unfortunately, typical PSA systems having one or more adsorbent beds may operate with &Dgr;P values that are greater than about 8. Such systems are described in U.S. Pat. No. 5,658,371 (single bed), U.S. Pat. No. 5,518,526 (multi-bed) and in co-pending application Ser. No. 08/611,942 (multi-bed), all of the above being incorporated herein by reference. Typical &Dgr;P values for the cycles disclosed in the above patents range from about 6 to 12 psia, preferably 7-11 psia, most preferably 8-10 psia.
Within the above ranges, the &Dgr;P values for a two-bed system are greater than those for a single-bed system.
The problem of excessive &Dgr;T has been addressed through the use of air or water cooled rotary blowers (e.g. blowers comprising additional means such that the compressor may be cooled via the use of external fluids) to lower the gas discharge temperature. Air cooled blowers are typically used with single bed systems, while water cooled blowers have been necessary for multi-bed systems. Examples of air cooled blowers may be found in U.S. Pat. Nos. 5,090,879 and 4,859,158. A commercially available blower of this type is the Roots DVJ Dry Vacuum Whispair Blower, available from Dresser Industries, Inc. in Connorsville, Ind. Other methods of cooling rotary blowers include the use of heat radiating fins, heat sinks and the like.
&Dgr;P and steady state &Dgr;T
rise
values for a typical air cooled rotary blower are illustrated in Table 2. The data in Table II below is measured at an ambient pressure of 15 psia and ambient temperature of 70° F. This table represents steady state conditions.
TABLE 2
Performance Data for a Cooled Rotary
Positive Blower
&Dgr;P = Inlet P −
Outlet P
(PSIA)
&Dgr;T
rise
0.5
5.2
0.98
10.6
1.46
17.8
1.95
23.2
2.43
30.4
2.91
39.4
3.4
48.4
3.88
57.4
4.36
66.4
4.85
77.2
5.33
89.8
5.82
100.6
6.3
113.2
6.78
125.8
7.75
152.8
8.23
167.2
8.72
181.6
9.2
196
9.68
210.4
10.17
226.6
As is apparent from these values, a cooled blower allows for operation at greater &Dgr;P than a non-cooled blower; and &Dgr;T
max
of 230° F. is not exceeded until &Dgr;P of greater than about 10 is achieved. Thus it is apparent from the available data that if one wished to operate a process having a &Dgr;P between about 8 to 10, then a cooled blower would be presumed to be required.
Unfortunately, such blowers have higher operating and capital costs. In addition, water cooled blowers require the availability of water to inject into the blower housing and in many cases water is not readily available. In addition, the cooling water causes scale to form on the blower end plates and rotors which cause mechanical problems for the blower.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to determine a VPSA operating process whereby a standard rotary blower may be used in place of a cooled rotary blower. This would allow for improved efficiency and lower capital cost.
SUMMARY OF THE INVENTION
This invention comprises a PSA process for the separation of a less adsorbable component from a mixture comprising said less adsorbable component and a more adsorbable component, said process comprising the cyclic performance of the following:
a) introducing a mixture into a feed end of a vessel which contains a material capable of selectively adsorbing said more adsorbable component, whereby the pressure in said vessel increases;
b) an adsorption step comprising adsorbing at least a portion of said more adsorbable component and recovering at least a portion of said less adsorbable component;
c) a desorption step comprising desorbing said more adsorbable component from said adsorbent material by evacuation of gas from said vessel, wherein said evacuation is at a selected pressure and occurs for a selected time such that a pressure reducing means utilized to evacuate said gas does not require extraneous cooling.
In a preferred embodiment the pressure reducing means comprise a rotary positive blower.
In another preferred embodiment a step time for said evacuation is less than 60 seconds.
In another preferred embodiment, the pressure reducing means has a particular temperature tolerance, and wherein said desorption step is operated under such pressure and time conditions so as to prevent said temperature tolerance from being exceeded.
In another preferred embodiment the total cycle time is between 10 and 180 seconds, the pressure ratio is between 2:1 and 6:1.
In another preferred embodiment, said desorption step lasts between 5 and 90 seconds.
The invention also comprises a PSA apparatus for the separation of at least one non-preferentially-adsorbable first gas from a gas mixture containing said first gas and one or more selectively-adsorbable second gas(es), comprising at least one adsorption bed unit containing a composition capable of selectively adsorbing said second gas(es) from said gas mixture at an elevated pressure, a receptor unit communicating with the downstream end of said bed unit through a control valve, to receive a pressurized supply of said first gas from the adsorption bed unit, a pressure-equalization unit for discharging void gas through a control valve into the downstream end of said adsorption bed unit to reduce the pressure therein and cause the desorption and depressurization of said second gas(es) therefrom, and compressor means for supplying a relatively-pressurized gas mixture through a control valve to the upstream end of the adsorption bed unit, means for causing the first gas to flow from said adsorption bed unit to said receptor unit when the pressure in the former exceeds the pressure in the latter, to collect the produced first gas, means for causing the depressurization and release of residual void gas present in said adsorbent bed unit through a control valve into said pressure-equalization unit after the supply of the pressurized gas mixture is discontinued to said adsorbent bed unit, and pressure reducing means communicating w
Fassbaugh John Harry
Rogan Michael Kenneth
Schaub Herbert Raymond
Smolarek James
Follett Robert J.
Praxair Technology Inc.
Spitzer Robert H.
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