# US 5,701,764 A · Process to produce moderate purity oxygen using a double column plus an auxiliary low pressure column

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- Number: US 5,701,764 A
- Document ID: US5701764A
- Status: Granted
- Kind: A
- Application number: 08/692,990
- Filed: 1996-08-06
- Granted: 1997-12-30
- Earliest claimed: 1996-08-06
- Assignees: Air Products and Chemicals, Inc.
- Inventors: Rakesh Agrawal; Zbigniew Tadeusz Fidkowski; Donn Michael Herron
- Web page: https://patentgrep.com/patent/US5701764A

## CPC

- F25J 3/04303: {Lachmann expansion, i.e. expanded into oxygen producing or low pressure column}
- F25J 3/0409: {of oxygen}
- F25J 3/04418: {with thermally overlapping high and low pressure columns}
- F25J 3/0443: {A main column system not otherwise provided, e.g. a modified double column flowsheet}
- F25J 3/04872: {Vertical layout of cold equipments within in the cold box, e.g. columns, heat exchangers etc.}
- F25J 2200/32: using a side column fed by a stream from the high pressure column
- Y10S 62/939: Partial feed stream expansion, air
- F25J: LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS {OR LIQUEFIED GASEOUS} MIXTURES BY PRESSURE AND COLD TREATMENT {OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE (cryogenic pumps F04B37/08; gas storage vessels, gas holders F17; filing vessels with, or discharging from vessels, compressed, liquefied or solidified gases F17C; refrigeration machines, plants, or systems F25B)}
- Y10S: TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS

## Abstract

A process is set forth for the cryogenic distillation of an air feed to produce an oxygen product, particularly an oxygen product at moderate purity (80-99%, preferably 85-95%). The process uses an auxiliary low pressure column in addition to the conventional high pressure column and low pressure column. The auxiliary low pressure column, which is preferably operated at the same pressure as the main low pressure column and which is heat integrated with the top of the high pressure column by means of its bottom reboiler/condenser, pretreats the crude liquid oxygen from the bottom of the high pressure column. The resulting overhead vapor stream and bottom stream are subsequently fed to the main low pressure column. Preferably, the bottom stream is fed to the main low pressure column in a state which is at least partially vapor.

## Claims

### Claim 1

A process for the cryogenic distillation of an air feed to produce an oxygen product using a distillation column system comprising a high pressure column, a main low pressure column and an auxiliary low pressure column, said process comprising:
(a) feeding at least a portion of the air feed to the bottom of the high pressure column;
(b) removing a nitrogen-enriched overhead from the top of the high pressure column, condensing at least a first portion of it in a first reboiler/condenser located in the bottom of the auxiliary low pressure column and feeding at least a first part of the condensed first portion as reflux to an upper location in the high pressure column;
(c) removing a crude liquid oxygen stream from the bottom of the high pressure column, reducing the pressure of at least a first portion of it and feeding said portion as impure reflux to the top of the auxiliary low pressure column;
(d) removing a crude nitrogen overhead from the top of the auxiliary low pressure column and feeding it directly as a vapor to an intermediate location in the main low pressure column;
(e) removing an oxygen-enriched stream from a lower location in the auxiliary low pressure column as a vapor and/or liquid and feeding it to an intermediate location in the main low pressure column below the intermediate feed location of the crude nitrogen overhead in step (d);
(f) removing a nitrogen rich overhead from the top of the main low pressure column as waste nitrogen; and
(g) removing the oxygen product from a lower location in the main low pressure column as a vapor and/or liquid.

### Claim 2 (depends on 1)

The process of claim 1 wherein the entire amount of the nitrogen-enriched overhead which is removed from the top of the high pressure column is condensed against vaporizing oxygen-enriched liquid from the bottom of the auxiliary low pressure column, except for a second portion which may optionally be removed as a product stream.

### Claim 3 (depends on 1)

The process of claim 1 wherein the oxygen-enriched stream which is removed from the auxiliary low pressure column in step (e) is removed in a state which is at least partially vapor.

### Claim 4 (depends on 1)

The process of claim 1 wherein the auxiliary low pressure column is operated at the same pressure as the main low pressure column, plus the expected pressure drop between the auxiliary low pressure column and the main low pressure column.

### Claim 5 (depends on 1)

The process of claim 1 wherein the oxygen product is produced at moderate purity (85-95%).

### Claim 6 (depends on 1)

The process of claim 1 wherein the oxygen product which is removed from the bottom of the main low pressure column in step (g) is removed as a liquid and is subsequently vaporized and warmed in a heat exchanger.

### Claim 7 (depends on 6)

The process of claim 6 wherein the oxygen product is pumped to an elevated pressure prior to vaporization.

### Claim 8 (depends on 1)

The process of claim 1 wherein a second part of the condensed nitrogen-enriched overhead from the top of the high pressure column in step (b) is reduced in pressure and fed as reflux to an upper location in the main low pressure column.

### Claim 9 (depends on 1)

The process of claim 1 wherein prior to feeding the air feed to the bottom of the high pressure column in step (a), at least a portion of the air feed is at least partially condensed in a second reboiler/condenser located in the bottom of the main low pressure column.

### Claim 10 (depends on 9)

The process of claim 9 wherein prior to partially condensing the air feed in the second reboiler/condenser, the air feed is compressed, cleaned of impurities which will freeze out at cryogenic temperatures and cooled in a main heat exchanger to a temperature near its dew point.

### Claim 11 (depends on 10)

The process of claim 10 wherein prior to cooling the compressed and cleaned air feed in the main heat exchanger, the process further comprises removing an air reflux stream from the air feed, further compressing the air reflux stream, cooling and subsequently condensing the air reflux stream in an external heat exchanger, splitting the air reflux stream into a first portion and a second portion, reducing the pressure of the first portion across a third valve and feeding it as reflux to the high pressure column and reducing the pressure of the second portion across a fourth valve and feeding it as reflux to an upper intermediate location in the main low pressure column.

### Claim 12 (depends on 11)

The process of claim 11 where the external heat exchanger is the main heat exchanger.

### Claim 13 (depends on 12)

The process of claim 12 wherein during the cooling of the air reflux stream in the main heat exchanger, an air expansion stream is removed and expanded in an expander to produce an expanded air stream.

### Claim 14 (depends on 13)

The process of claim 13 wherein the expanded air stream is fed to an intermediate location in the main low pressure column which is between the intermediate feed locations of the crude nitrogen overhead in step (d) and the oxygen-enriched stream in step (e).

### Claim 15 (depends on 14)

The process of claim 14 wherein the waste nitrogen removed in step (f) is warmed in the main heat exchanger.

### Claim 16 (depends on 15)

The process of claim 15 wherein prior to warming the waste nitrogen in the main heat exchanger, said waste nitrogen is warmed in a subcooling heat exchanger against:
(i) the second part of the condensed nitrogen-enriched overhead from the high pressure column in step (b) prior to it being reduced in pressure fed as reflux to an upper location in the main low pressure column; and
(ii) the condensed air reflux stream prior to splitting said stream into said first portion and said second portion and feeding said portions as reflux to, respectively, the high and main low pressure columns.