Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-11-09
2001-07-03
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S652000, C062S654000
Reexamination Certificate
active
06253576
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH FOR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
The present invention pertains to the field of cryogenic air separation, and in particular to a process for the production and delivery of intermediate pressure oxygen from a cryogenic air separation plant.
There are typically two ways of delivering the oxygen produced from a cryogenic air separation plant. Historically, oxygen product was withdrawn as a vapor from the bottom of the lower pressure column of a double-column distillation system, warmed to ambient temperature, and either delivered to the user at very low pressure or compressed. This type of process is commonly referred to as a GOX-plant
The maximum oxygen pressure that can be realized when oxygen is withdrawn as a vapor from the lower pressure column is severely limited. This is due to the desire to operate the lower pressure column at a pressure as close to atmospheric pressure as possible to maintain efficient operation. The maximum oxygen delivery pressure also has been reduced further by the recent use of low pressure drop structured packing. In practice, the maximum, efficient, oxygen delivery pressure is only around 17 psia when oxygen is withdrawn as a vapor from a lower pressure column near atmospheric pressure. A supplemental product compressor may be justified for oxygen pressures greater than about 17 psia.
Many disclosures in the literature are directed at improving the efficiency of oxygen producing plants that produce oxygen as a vapor from a lower pressure column. U.S. Pat. No. 5,669,237 (Voit) is one example which is applicable to the production of low purity oxygen. A notable feature of this patent is the use of a portion of feed air to provide boilup to the bottom of the lower pressure column.
More recently, it has become commonplace to withdraw liquid oxygen from the lower pressure column, raise the pressure of the oxygen by using either static head or a pump, and vaporize the oxygen by condensing some suitably pressurized stream. This method of oxygen delivery is referred to as LOX-Boil or pumped-LOX. An example of LOX-Boil is taught in U.S. Pat. No. 4,560,398 (Beddome, et al.); an example of pumped-LOX is taught in U.S. Pat. No. 5,355,682 (Agrawal, et al.).
Oxygen delivery using LOX-Boil or pumped-LOX is commonly accomplished by condensing a portion of the incoming pressurized air. The source for the pressurized air is the discharge of a main air compressor. Since the discharge pressure of the main air compressor is set by the operating pressure of the higher pressure column, a lower bound on the condensing air pressure is established. As a result, the lowest pressure at which oxygen may be produced efficiently is approximately 23 psia. Of course, oxygen may be produced efficiently at pressures greater than 23 psia by using a booster compressor to raise the pressure of the condensing air stream. The absolute lowest efficient pressure may vary somewhat from 23 psia, depending on many factors such as: pressure of the lower pressure column, pressure drop in the distillation columns, heat exchanger temperature approaches, feed and product pressure drops, etc.
Many disclosures in the literature are directed at improving the efficiency of LOX-Boil and pumped-LOX plants. One example is U.S. Pat. No. 5,355,681 (Xu), which is applicable to the coproduction of liquid products. A key feature of one of the embodiments (as illustrated in FIG. 1 of U.S. Pat. No. 5,355,681) is the use of a portion of feed air to provide boilup to the bottom of the lower pressure column.
It is desired to provide an efficient process for producing oxygen from a cryogenic air separation plant at a pressure intermediate that which is achievable by either withdrawing vapor from the lower pressure column or by vaporizing liquid oxygen against a stream of air which is nominally at the pressure of the higher pressure column.
BRIEF SUMMARY OF THE INVENTION
The present invention is a process for the production and delivery of intermediate pressure oxygen from a cryogenic air separation plant.
The first embodiment of the invention is a process for separating air to produce oxygen at a intermediate pressure. The process uses a higher pressure column and a lower pressure column in thermal communication with the higher pressure column through a main reboiler-condenser. Each column has a top and a bottom, and the main reboiler-condenser provides at least a fraction of boilup at the bottom of the lower pressure column. The process includes multiple steps. The first step is to provide a first stream of compressed air. The second step is to divide the first stream of compressed air into a first portion of air and a second portion of air. The third step is to feed the first portion of air to the higher pressure column at a first pressure. The fourth step is to withdraw a stream of liquid oxygen from the lower pressure column. The fifth step is to heat exchange the stream of liquid of oxygen with the second portion of air, said second portion of air being at a second pressure lower than the first pressure, thereby at least partially condensing the second portion of air and at least partially vaporizing the stream of liquid oxygen.
In one variation of the first embodiment, the second pressure is lower than the first pressure by about 7 psia to about 8 psia.
A second embodiment of the invention includes the same multiple steps of the first embodiment, but includes three additional steps. The first additional step is to withdraw a third portion of air from the first portion of air or from the second portion of air. The second additional step is to expand the third portion of air. The third additional step is to feed the expanded third portion of air to the lower pressure column.
A third embodiment of the invention is similar to the first embodiment, but includes three additional steps. The first additional step is to withdraw an oxygen-enriched stream of liquid from the bottom of the higher pressure column. The second additional step is to feed at least a portion of the oxygen-enriched stream of liquid to the lower pressure column. The third additional step is to withdraw a nitrogen-enriched stream of vapor from the top of the lower pressure column.
A fourth embodiment of the invention is similar to the first embodiment, but includes four additional steps. The first additional step is to withdraw a nitrogen-enriched stream from the higher pressure column. The second additional step is to expand at least a portion of the nitrogen-enriched stream. The third additional step is to condense the at least a portion of the nitrogen-enriched stream. The fourth additional step is to feed at least a portion of the condensed at least a portion of the nitrogen-enriched stream to the lower pressure column.
A fifth embodiment of the invention is similar to the fourth embodiment, but includes three additional steps. The first additional step is to withdraw an oxygen-enriched stream from the bottom of the higher pressure column. The second additional step is to vaporize at least a portion of the oxygen-enriched stream by heat exchanging said at least portion of oxygen-enriched stream with the at least a portion of the nitrogen-enriched stream. The third additional step is to feed the vaporized at least a portion of the oxygen-enriched stream to the lower pressure column.
A sixth embodiment of the invention is similar to the first embodiment, but includes four additional steps. The first additional step is to withdraw a nitrogen-enriched stream from the top of the higher pressure column. The second additional step is to condense the nitrogen-enriched stream in a reboiler-condenser. The third additional step is to return a first portion of the condensed nitrogen-enriched stream to the higher pressure column. The fourth additional step is to feed a second portion of the condensed nitrogen-enriched stream to the lower pressure column.
A seventh embodiment of the invention is similar to the first embodiment
Brostow Adam Adrian
Herron Donn Michael
Air Products and Chemicals Inc.
Capossela Ronald
Jones II Willard
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