Information
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Patent Grant
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6279344
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Patent Number
6,279,344
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Date Filed
Thursday, June 1, 200024 years ago
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Date Issued
Tuesday, August 28, 200123 years ago
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Inventors
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Original Assignees
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Examiners
Agents
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CPC
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US Classifications
Field of Search
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International Classifications
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Abstract
A cryogenic air separation system for producing oxygen employing a double column and a side column wherein side column liquid is vaporized against nitrogen heat pump fluid taken from the higher pressure column of the double column and then used to reflux the higher pressure and/or lower pressure columns of the double column.
Description
TECHNICAL FIELD
This invention relates generally to cryogenic air separation and, more particularly, to cryogenic air separation for producing oxygen, particularly at elevated pressure.
BACKGROUND ART
Oxygen is produced commercially in large quantities by the cryogenic separation of air, generally employing a double column arrangement having a higher pressure column in heat exchange relation with a lower pressure column. A recent significant advancement in the production of oxygen is the side column system which enables the production of oxygen with lower operating costs. Examples of side column systems may be found in U.S. Pat. No. 5,463,871—Cheung and U.S. Pat. No. 5,582,036—Drnevich et al.
When the production of elevated pressure oxygen is desired using the side column system, liquid oxygen from the side column is pumped and then vaporized against boosted feed air. The air pressure for the booster air compressor may fluctuate especially where the base load air compressor is also supplying air for another use such as the blast air for a blast furnace. Such fluctuations result in unstable operation.
Accordingly it is an object of this invention to provide a cryogenic air separation system using a side column arrangement which can produce oxygen with improved stability.
SUMMARY Of THE INVENTION
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for producing oxygen comprising:
(A) passing feed air into a higher pressure column and separating the feed air by cryogenic rectification within the higher pressure column into nitrogen-enriched vapor and oxygen-enriched fluid;
(B) passing oxygen-enriched fluid from the higher pressure column into a lower pressure column and producing oxygen-richer fluid within the lower pressure column;
(C) passing oxygen-richer fluid from the lower portion of the lower pressure column into the upper portion of a side column and producing oxygen-rich liquid within the side column;
(D) withdrawing nitrogen-enriched vapor from the higher pressure column, compressing the nitrogen-enriched vapor, and cooling the compressed nitrogen-enriched vapor by indirect heat exchange with oxygen-rich liquid to produce oxygen-rich vapor; and
(E) recovering vaporized oxygen-rich liquid as product oxygen.
Another aspect of the invention is:
Apparatus for producing oxygen comprising:
(A) a higher pressure column, a lower pressure column, means for passing feed air into the higher pressure column, and means for passing fluid from the higher pressure column to the lower pressure column;
(B) a side column and means for passing fluid from the lower portion of the lower pressure column to the upper portion of the side column;
(C) a compressor, a heat exchanger, means for passing fluid from the upper portion of the higher pressure column to the compressor and from the compressor to the heat exchanger;
(D) means for passing fluid from the lower portion of the side column to the heat exchanger; and
(E) means for recovering fluid from the heat exchanger as product oxygen.
As used herein the term “feed air” means a mixture comprising primarily nitrogen and oxygen, such as ambient air.
As used herein the term “column” means a distillation or fractionation column or zone, i.e., a contacting column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer's Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. The term, “double column” is used to mean a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure column. A further discussion of double columns appears in Ruheman “The Separation of Gases”, Oxford University Press, 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases can be adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein the term “indirect heat exchange” means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein the term “bottom reboiler” means a heat exchange device which generates column upflow vapor from column bottom liquid.
As used herein the terms “turboexpansion” and “turboexpander” mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration.
As used herein the terms “upper portion” and “lower portion” mean those sections of a column respectively above and below the mid point of the column.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a schematic representation of one preferred embodiment of the cryogenic oxygen production system of this invention.
FIG. 2
is a schematic representation of another preferred embodiment of the cryogenic oxygen production system of this invention wherein the invention is integrated with a blast furnace system.
DETAILED DESCRIPTION
In general the invention comprises the use of a nitrogen heat pump circuit operated using nitrogen-enriched fluid from the higher pressure column of a double column, to vaporize liquid oxygen within and/or taken from a side column to produce oxygen vapor. The nitrogen heat pump circuit relieves the feed air from some liquid oxygen vaporization duty, thus removing pressure fluctuations in the base load air compressor from disrupting the operation of the cryogenic air separation facility. Such pressure fluctuations are especially experienced when the base load air compressor is providing air to a facility, such as a blast furnace, in addition to the cryogenic air separation facility.
The invention will be described in detail with reference to the Drawings. Referring now to
FIG. 1
, feed air
100
is compressed in base load air compressor
200
to a pressure generally within the range of from 35 to 100 pounds per square inch absolute (psia). Compressed feed air
102
is then cooled of the heat of compression by passage through cooler
202
and then as stream
114
is passed to prepurifier
204
wherein it is cleaned of high boiling impurities such as carbon dioxide, water vapor and hydrocarbons. Cleaned compressed feed air
25
is divided into portion
115
and
116
. Portion
115
is increased in pressure by passage through booster compressor
252
. Boosted feed air portion
117
is cooled of the heat of compression in cooler
254
and then as stream
119
is passed to main heat exchanger
214
wherein it is cooled by indirect heat exchange with oxygen-rich liquid taken from the side column as will be more fully described below. Resulting cooled feed air portion
121
is passed through valve
248
and as stream
123
into higher pressure column
222
. Feed air portion
116
is passed into main heat exchanger
214
wherein it is cooled by indirect heat exchange with return streams. A portion
124
is withdrawn after partial traverse of main heat exchanger
214
and turboexpanded to generate refrigeration in turboexpander
216
. Resulting turboexpanded feed air portion
126
is passed into lower pressure column
226
. The remaining portion of stream
116
is passed from main heat exchanger
214
in stream
122
into higher pressure column
222
.
Higher pressure column
222
, which is part of a double column system which also includes lower pressure column
226
, is operating at a pressure generally within the range of from 30 to 95 psia. Within higher pressure column
222
the feed air is separated by cryogenic rectification into nitrogen-enriched vapor and oxygen-enriched fluid. Oxygen-enriched fluid is withdrawn from the lower portion of higher pressure column
222
in liquid stream
158
, subcooled by passage through heat exchanger
230
, and passed in stream
160
through valve
234
and as stream
161
into lower pressure column
226
. Nitrogen-enriched vapor is withdrawn from the upper portion of higher pressure column
222
in stream
130
and passed into main condenser
224
as shown by stream
131
. If desired, a portion of the nitrogen-enriched vapor may be recovered as product higher pressure nitrogen. Within main condenser
224
the nitrogen-enriched vapor is condensed by indirect heat exchange with boiling column
226
bottom liquid. Resulting condensed nitrogen-enriched liquid is withdrawn from main condenser
224
in stream
132
. One portion is passed into higher pressure column
222
as reflux in stream
133
and another portion
134
is combined with stream
181
(described below) to form stream
180
for passage into lower pressure column
226
as reflux.
In the practice of this invention nitrogen-enriched vapor from the higher pressure column is used to operate a heat pump circuit to boil oxygen-rich liquid typically in the main heat exchanger and/or the side column reboiler, although this could take place in a separate product boiler. In the embodiment of the invention illustrated in
FIG. 1
, this nitrogen-enriched vapor is taken as a portion of stream
130
. The nitrogen-enriched vapor for the heat pump circuit could be taken from the higher pressure column in a separate stream from stream
130
. If the nitrogen-enriched fluid for heat pumping is taken separately from stream
130
, then the nitrogen-enriched liquid of the heat pump circuit will be passed into the higher and/or lower pressure columns separately from the fluid in stream
132
. In the embodiment of the invention illustrated in
FIG. 1
, nitrogen-enriched vapor in stream
168
is warmed by passage through main heat exchanger
214
and resulting warmed nitrogen-enriched vapor stream
170
is compressed by passage through compressor
242
to a pressure generally within the range of from 50 to 1000 psia. Resulting compressed stream
172
is cooled of the heat of compression in cooler
244
and passed as stream
174
to main heat exchanger
214
wherein it is cooled by indirect heat exchange with return streams. Resulting nitrogen-enriched fluid
176
is passed into bottom reboiler
220
of side column
221
wherein it is cooled by indirect heat exchange with oxygen-rich liquid to generate oxygen-rich vapor for vapor upflow for the side column. Resulting nitrogen-enriched liquid is passed out of bottom reboiler
220
in stream
178
. A portion
179
is passed into higher pressure column
222
as reflux. Another portion
181
is combined with stream
134
to form stream
180
. Stream
180
is subcooled by passage through heat exchanger
228
to form subcooled stream
186
which is passed through valve
232
and as stream
188
into lower pressure column
226
as reflux.
Lower pressure column
226
is operating at a pressure less than that of higher pressure column
222
and generally within the range of from 16 to 25 psia. Within lower pressure column
226
the various feeds into that column are separated by cryogenic rectification into nitrogen-richer fluid and oxygen-richer fluid. Nitrogen-richer fluid is withdrawn from the upper portion of lower pressure column
226
in vapor stream
140
, warmed by passage through heat exchangers
228
,
230
and
214
and withdrawn from the system in stream
146
which may be recovered in whole or in part as product nitrogen having a nitrogen concentration within the range of from 95 to 99.999 mole percent. If desired some oxygen-richer fluid may be recovered from the lower portion of lower pressure column
226
as product oxygen having an oxygen concentration generally within the range of from 50 to 90 mole percent.
Oxygen-richer fluid is withdrawn from the lower portion of lower pressure column
226
as liquid stream
148
and passed into the upper portion of side column
221
which is operating at a pressure similar to that of lower pressure column
226
. The oxygen-richer liquid passes down through side column
221
against the upflowing vapor generated by the operation of bottom reboiler
220
and, in the process, lighter components such as nitrogen and argon are stripped out of the downflowing liquid into the upflowing vapor which is then passed in stream
150
from the upper portion of side column
221
to the lower portion of lower pressure column
226
. Typically stream
150
has an oxygen concentration within the range of from 20 to 65 mole percent and a nitrogen concentration within the range of from 30 to 80 mole percent. The stripping action within side column
221
serves to produce oxygen-rich liquid by cryogenic rectification in the lower portion of side column
221
. Oxygen-rich liquid, generally having an oxygen concentration within the range of from 70 to 98 mole percent, is withdrawn from the lower portion of side column
221
in stream
152
and pumped to a higher pressure by operation of liquid pump
240
. Resulting pressurized oxygen-rich liquid is passed in stream
153
to main heat exchanger
214
wherein it is vaporized by indirect heat exchange with the boosted feed air stream
119
as was previously described. The resulting oxygen-rich vapor is recovered as product oxygen in stream
154
.
FIG. 2
illustrates another embodiment of the invention and also illustrates a particularly advantageous application of the invention wherein the invention is integrated with a blast furnace system. In this application the base load air compressor also supplies air to the blast furnace, and product oxygen produced by the invention is supplied to the blast furnace. The numerals in
FIG. 2
are the same as those of
FIG. 1
for the common elements and these common elements will not be described again in detail.
Referring now to
FIG. 2
, compressed feed air
102
is divided into portion
106
, which may comprise from 25 to 90 percent of compressed feed air
102
, and into portion
110
which may comprise from 10 to 75 percent of compressed feed air
102
. Portion
110
is used as the feed air to the cryogenic air separation system. In the embodiment of the invention illustrated in
FIG. 2
, the cleaned feed air
25
is not divided upstream of main heat exchanger
214
but, rather, is passed thereto in its entirety. In addition, cooled feed air stream
122
is employed as the fluid driving side column reboiler
220
rather than the heat pump fluid used in the embodiment illustrated in FIG.
1
. After feed air stream
122
is used to reboil the bottom liquid of side column
221
it is passed as stream
128
into higher pressure column
222
. Oxygen-rich liquid
153
is vaporized by indirect heat exchange with compressed nitrogen-enriched heat pump fluid
174
in main heat exchanger
214
. Cooled, condensed nitrogen-enriched heat pump fluid
176
is not passed to bottom reboiler
220
but, rather, is passed through valve
246
and then as stream
178
is processed as was previously described. Product oxygen
154
is combined with blast air stream
106
to form oxygen-enriched blast air stream
190
. The oxygen-enriched blast air is then heated in stoves
280
and resulting heated oxygen-enriched blast air
192
is passed to blast furnace
290
.
Although the invention has been described in detail with reference to certain preferred embodiments, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims.
Claims
- 1. A method for producing oxygen comprising:(A) passing feed air into a higher pressure column and separating the feed air by cryogenic rectification within the higher pressure column into nitrogen-enriched vapor and oxygen-enriched fluid; (B) passing oxygen-enriched fluid from the higher pressure column into a lower pressure column and producing oxygen-richer fluid within the lower pressure column; (C) passing oxygen-richer fluid from the lower portion of the lower pressure column into the upper portion of a side column and producing oxygen-rich liquid within the side column; (D) withdrawing nitrogen-enriched vapor from the higher pressure column, compressing the nitrogen-enriched vapor, and cooling the compressed nitrogen-enriched vapor by indirect heat exchange with oxygen-rich liquid to produce oxygen-rich vapor; and (E) recovering vaporized oxygen-rich liquid as product oxygen.
- 2. The method of claim 1 further comprising passing nitrogen-enriched fluid, after the indirect heat exchange with the oxygen-rich liquid, into at least one of the higher pressure column and the lower pressure column.
- 3. The method of claim 1 wherein the product oxygen is the oxygen-rich liquid vaporized by indirect heat exchange with the compressed nitrogen-enriched vapor.
- 4. The method of claim 1 wherein the product oxygen is oxygen-rich liquid vaporized by indirect heat exchange with feed air.
- 5. The method of claim 1 further comprising passing recovered product oxygen to a blast furnace.
- 6. Apparatus for producing oxygen comprising:(A) a higher pressure column, a lower pressure column, means for passing feed air into the higher pressure column, and means for passing fluid from the higher pressure column to the lower pressure column; (B) a side column and means for passing fluid from the lower portion of the lower pressure column to the upper portion of the side column; (C) a compressor, a heat exchanger, means for passing fluid from the upper portion of the higher pressure column to the compressor and from the compressor to the heat exchanger; (D) means for passing fluid from the lower portion of the side column to the heat exchanger; and (E) means for recovering fluid from the heat exchanger as product oxygen.
- 7. The apparatus of claim 6 further comprising means for passing fluid from the heat exchanger into at least one of the higher pressure column and the lower pressure column.
- 8. The apparatus of claim 6 wherein the side column has a bottom reboiler, further comprising means for passing fluid from the heat exchanger to the bottom reboiler.
- 9. The apparatus of claim 6 wherein the side column has a bottom reboiler and wherein the means for passing feed air into the higher pressure column includes the bottom reboiler.
- 10. The apparatus of claim 6 further comprising means for passing product oxygen to a blast furnace.
US Referenced Citations (13)