Information
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Patent Grant
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6227005
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Patent Number
6,227,005
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Date Filed
Wednesday, March 1, 200024 years ago
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Date Issued
Tuesday, May 8, 200123 years ago
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Inventors
-
Original Assignees
-
Examiners
Agents
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CPC
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US Classifications
Field of Search
US
- 062 646
- 062 654
- 062 940
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International Classifications
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Abstract
A process for the production of oxygen and nitrogen is applicable when the oxygen product is withdrawn from a distillation column system as a liquid, pumped to an elevated pressure and warmed at least in part by cooling a suitably pressurized stream. At least a portion of the compressed, purified, and cooled air is introduced to a first of at least three distillation columns. The first distillation column contains at least a condenser at its top, produces at least an oxygen-lean stream from or near its top and a first oxygen-enriched liquid from its bottom. A second distillation column, which contains a reboiler in its bottom, has no condenser, receives at least a portion of nitrogen-enriched liquid as a feed to its top, and produces a first nitrogen-rich vapor stream from its top and a second oxygen-enriched liquid from its bottom. A third distillation column, which contains a reboiler in its bottom, has no condenser, receives at least a portion of nitrogen-enriched liquid as a feed to its top, receives at least said second oxygen-enriched liquid as a feed, and produces a second nitrogen-rich vapor from its top and a liquid oxygen-rich stream from its bottom. The liquid oxygen-rich stream from said third distillation column is elevated in pressure and warmed, at least in part, by indirect heat exchange with a pressurized stream having a nitrogen content greater than or equal to that in the feed air, said pressurized stream being cooled without being subjected to distillation. The second distillation column receives as a feed at least one of (a) a portion of said first oxygen-enriched stream from said first distillation column; or (b) a portion of said cooled pressurized stream. The third distillation column receives as a feed at least one of (a) a portion of said first oxygen-enriched stream from said first distillation column; or (b) a portion of said cooled pressurized stream. In the preferred mode of operation, the first distillation column is at the highest pressure, the third distillation column is at the lowest pressure, and the second distillation column is at an intermediate pressure.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH FOR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
The present invention relates generally to the production of oxygen and nitrogen from a cryogenic air separation plant, and more particularly to the production of pressurized oxygen using pumped-LOX (liquid oxygen) and the production of at least a portion of nitrogen as pressurized nitrogen.
The most well known cryogenic process for the production of both oxygen and nitrogen is the double-column cycle. This process uses a distillation column system comprising a higher pressure column, a lower pressure column and a reboiler-condenser which thermally links the two columns. Early versions of the double-column cycle produced both nitrogen and oxygen as vapors from the lower pressure column. Recently, it has become commonplace to withdraw the oxygen product from the distillation column system as a liquid, raise the pressure of the liquid oxygen by using either static head or a pump, and warm it in a main heat exchanger by cooling some suitably pressurized stream. This method of oxygen delivery is referred to as pumped-LOX. When large quantities of pressurized nitrogen are also required it is typical to elevate the pressure of the lower pressure column to recover nitrogen at some pressure greater than atmospheric.
Processes of this type are often called elevated pressure, or EP, cycles. Numerous examples of elevated pressure, double column, pumped-LOX cycles exist in the open literature. An example of one such prior art cycle is shown in FIG.
9
.
A commercial application for such a process is the production of low purity oxygen (less than 98 mole % oxygen) and nitrogen for Coal Gasification Combined Cycle (CGCC) power and chemical plants. Since an objective of such applications is to produce power, it is essential that the air separation process be energy efficient. The need for high efficiency has given rise to many modifications to the conventional elevated pressure, double-column, pumped-LOX cycle.
One solution for improving the efficiency of the double-column cycle is to utilize a third distillation column as in U.S. Pat. No. 5,682,764 (Agrawal, et al.). This patent teaches the use of a third column which operates at a pressure intermediate that of the higher and lower pressure columns. This third column receives a vapor air feed which is at a lower pressure than the main air feed to the higher pressure column. This intermediate pressure column has a condenser but no reboiler, and produces liquid nitrogen reflux for the lower pressure column. Power consumption is reduced by only having to compress a fraction of the feed air to the pressure of the higher pressure column.
Another patent which teaches the use of a third column to improve efficiency is U.S. Pat. No. 5,678,426 (Agrawal, et al.). This patent also teaches the use of a third column which operates at a pressure intermediate that of the higher and lower pressure columns. This third column receives oxygen-enriched liquid from the bottom of the higher pressure column as a feed. This intermediate pressure column contains both a reboiler and a condenser, and produces a nitrogen-rich stream from its top and a further-oxygen-enriched liquid from its bottom.
Another patent which teaches the use of a third column to improve efficiency is disclosed in U.S. Pat. No. 4,254,629 (Olszewski). Olszewski teaches the use of a third intermediate pressure column which functions much like that of U.S. Pat. No. 5,682,764. Olszewski also discloses a four-column version which has a pair of double columns in parallel. As taught by Olszewski, both lower pressure columns operate at essentially the same pressure. One higher pressure column operates at a lower pressure than the other. This is achieved by maintaining the composition in the bottom of one lower pressure column more oxygen-lean than the other - - the higher pressure column which is thermally linked to the lower pressure column having the more oxygen-depleted composition can thereby operate at lower pressure. Olszewski also teaches to pass oxygen-depleted vapor to the other lower pressure column.
None of the three patents discussed above teaches modes of operation using pumped-LOX.
U.S. Pat. No. 4,433,989 (Erickson) also teaches the use of a third column to improve efficiency. Erickson teaches the use of a third intermediate pressure column in conjunction with a double-column process. The steps taught by Erickson include: 1) passing all the air to the higher pressure column; 2) passing essentially all the oxygen-enriched liquid from the higher pressure column into the intermediate pressure column; 3) distilling in the intermediate pressure column to produce a nitrogen-rich vapor and a further oxygen enriched liquid; 4) passing the further oxygen-enriched liquid to the lower pressure column; 5) refluxing both intermediate pressure column and lower pressure column with nitrogen-enriched liquid from the higher pressure column; and 6) providing boilup to both the intermediate pressure column and the lower pressure column by indirect heat exchange with condensing vapor from the higher pressure column
Erickson also suggests an operating method using pumped-LOX. Erickson teaches that pressurized air is passed to the bottom of a fourth distillation column. This distillation column produces a nitrogen-rich liquid from its top and an oxygen-enriched liquid from its bottom—much like a typical higher pressure column would. The condenser for this fourth column is operated by vaporizing the oxygen product at elevated pressure.
It is desired to have an efficient process for separating air to produce oxygen and nitrogen, wherein the oxygen is produced as a pressurized product and at least a portion of the nitrogen is produced as a pressurized product.
It also is desired to have an efficient mode of utilizing pumped-LOX in a multi-column cycle comprising three or more distillation columns.
BRIEF SUMMARY OF THE INVENTION
The present invention is a process for separating air to produce oxygen and nitrogen using a distillation column system having at least three distillation columns. The invention also includes a cryogenic air separation unit using the process.
One embodiment of the invention is a process for separating air to produce oxygen and nitrogen using a distillation column system having at least three distillation columns. The system includes a first distillation column, a second distillation column, and a third distillation column, each distillation column having a top and a bottom. The process comprises multiple steps. The first step is to provide a stream of compressed air having a first nitrogen content. The second step is to feed at least a first portion of the stream of compressed air to the first distillation column. The third step is to withdraw a first oxygen-enriched stream from the bottom of the first distillation column and to feed at least a portion of the first oxygen-enriched liquid stream to the second distillation column and/or the third distillation column. The fourth step is to withdraw a first oxygen-lean vapor stream from or near the top of the first distillation column, to feed at least a first portion of the first oxygen-lean vapor stream to a first reboiler-condenser of the second distillation column or of the third distillation column, and to at least partially condense the at least a first portion of the first oxygen-lean vapor stream, thereby forming a first nitrogen-enriched liquid. The fifth step is to feed at least a first portion of the first nitrogen-enriched liquid to the top of the first distillation column. The sixth step is to feed a second nitrogen-enriched liquid and/or at least a second portion of the first nitrogen-enriched liquid to the top of the second distillation column. The seventh step is to withdraw a second oxygen-enriched liquid stream from the bottom of the second distillation column and to feed the second oxygen-enriched liquid stream to the third distillation column. The eighth step is to withdraw a first nitrogen-rich vapor stream from the top of the second distillation column. The ninth step is to withdraw a second nitrogen-rich vapor stream from the top of the third distillation column. The tenth step is to withdraw a liquid oxygen stream from the bottom of the third distillation column, wherein said liquid oxygen stream is elevated in pressure before being warmed at least in part by indirect heat exchange with a pressurized stream having a nitrogen content at least equal to the first nitrogen-content, said pressurized stream being cooled without being subjected to distillation. The eleventh step is to feed at least a portion of the cooled pressurized stream eventually to any or all of the first distillation column, the second distillation column, or the third distillation column.
There are variations of this embodiment. For example, in one variation, the pressurized stream is the first portion of the stream of compressed air. In another variation, the pressurized stream is another portion of the stream of compressed air. In a variant of that variation, the process includes an additional step. The additional step is to compress further the another portion of the stream of compressed air.
There are still other variations of this embodiment. For example, in one variation the pressurized stream is a compressed portion of an oxygen-lean vapor stream withdrawn from the distillation column system. In another variation, the first distillation column is at a first pressure, the second distillation column is at a second pressure lower than the first pressure, and the third distillation column is at a third pressure lower than the second pressure. In yet another variation, a boilup for the second distillation column is provided at least in part by indirect heat exchange with the first portion of the oxygen-lean vapor and a boilup for the third distillation column is provided at least in part by indirect heat exchange with another portion of the first oxygen-lean vapor.
Another embodiment of the invention has the same multiple steps as the embodiment discussed above, but includes five additional steps. The first additional step is to provide a fourth distillation column having a top and a bottom. The second additional step is to feed a second portion of the first oxygen-lean vapor stream from the first distillation column to the bottom of the fourth distillation column. The third additional step is to withdraw a third nitrogen-enriched liquid stream from the bottom of the fourth distillation column and to feed at least a portion of the third nitrogen-enriched liquid to the second distillation column and/or the third distillation column. The fourth additional step is to withdraw a second oxygen-lean vapor stream from or near the top of the fourth distillation column, to feed at least a first portion of the second oxygen-lean vapor stream to a second reboiler-condenser of the second distillation column or of the third distillation column, to at least partially condense the first portion of the second oxygen-lean vapor stream, thereby forming a fourth nitrogen-enriched liquid, and to feed at least a portion of the fourth nitrogen-enriched liquid to the top of the fourth distillation column. The fifth additional step is to withdraw a high purity nitrogen stream from the second oxygen-lean vapor stream or the fourth nitrogen-enriched liquid.
In a variation of this embodiment, a boilup for the second distillation column is provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapor stream, and a boilup for the third distillation column is provided at least in part by indirect heat exchange with the first portion of the second oxygen-lean vapor stream.
There is yet another embodiment of the present invention. This embodiment has the same multiple steps as the first embodiment, but includes five additional steps. The first additional step is to provide a fourth distillation column having a top and a bottom. The second additional step is to feed another portion of the stream of compressed air to the bottom of the fourth distillation column. The third additional step is to withdraw a third oxygen-enriched liquid stream from the bottom of the fourth distillation column, and to feed at least a portion of the fourth oxygen-enriched liquid stream to the second distillation column and/or the third distillation column. The fourth step is to withdraw a second oxygen-lean vapor stream from or near the top of the fourth distillation column, to feed at least a portion of the second oxygen-lean vapor stream to a second reboiler-condenser of the second distillation column or of the third distillation column, and to at least partially condense the second oxygen-lean vapor stream, thereby forming the second nitrogen-enriched liquid. The fifth step is to feed at least a portion of the second nitrogen-enriched liquid to the top of the fourth distillation column.
There are several variations of this embodiment. For example, in one variation, the fourth distillation column is at a fourth pressure greater than a first pressure of the first distillation column. In another variation, the fourth distillation column is at a fourth pressure less than a first pressure of the first distillation column. In yet another variation, a boilup for the third distillation column is provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapor stream, and a boilup for the second distillation column is provided at least in part by indirect heat exchange with the second oxygen-lean vapor stream.
There is still yet another embodiment of the present invention. This embodiment has the same multiple steps as the first embodiment, but includes three additional steps. The first additional step is to withdraw a vapor stream from the first distillation column at an intermediate location, to feed the vapor stream to a second reboiler-condenser of the second distillation column or of the third distillation column, and to at least partially condense the vapor stream, thereby forming an intermediate reflux stream. The second additional step is to feed the intermediate reflux stream to the first distillation column at or near the intermediate location. The third additional step is to withdraw the second nitrogen-enriched liquid from the first distillation column at or near the intermediate location for feeding at least a portion to the top of the second distillation column or the third distillation column.
There are several variations of this embodiment. In one variation, the boilup for the second distillation column is provided at least in part by indirect heat exchange with the vapor stream withdrawn at the intermediate location, and a boilup for the third distillation column is provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapor stream. In another variation, a boilup for the third distillation column is provided at least in part by indirect heat exchange with the vapor stream withdrawn at the intermediate location, and a boilup for the second distillation column is provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapor stream.
Another aspect of the present invention is a cryogenic air separation unit using a process as in any of the embodiments or variations thereof discussed above.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is best understood from the following detailed description when read in connection with the accompanying drawings, in which:
FIG. 1
is a schematic diagram of a first embodiment of the present invention;
FIG. 2
is a schematic diagram of a second embodiment of the present invention;
FIG. 3
is a schematic diagram of a third embodiment of the present invention;
FIG. 4
is a schematic diagram of a fourth embodiment of the present invention;
FIG. 5
is a schematic diagram of a fifth embodiment of the present invention;
FIG. 6
is a schematic diagram of a sixth embodiment of the present invention;
FIG. 7
is a schematic diagram of a seventh embodiment of the present invention;
FIG. 8
is a schematic diagram of an eighth embodiment of the present invention;
and
FIG. 9
is a schematic diagram of a conventional elevated pressure, double-column, pumped-LOX process.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a process for the production of oxygen and nitrogen using a distillation column system. The process is applicable when the oxygen product is withdrawn from the distillation column system as a liquid, pumped to an elevated pressure, and warmed at least in part by cooling a suitably pressurized stream. In the preferred mode of operation, nitrogen product is produced at a pressure greater than 20 psia and the purity of the oxygen product is less than 98 mole % (low purity oxygen). In the most preferred mode of operation, the nitrogen product is produced at a pressure greater than 30 psia and the ratio of nitrogen production to oxygen production is greater than 1.5 mole/mole.
The term “oxygen-rich” is understood to represent the oxygen product and corresponds to an oxygen content less than 99.9 mole %, preferably greater than 85 mole % and, preferably less than 98 mole %. It also is understood that the term “nitrogen-rich” represents nitrogen product and corresponds to a nitrogen content greater than 95 mole %, preferably greater than 98 mole %.
The term “oxygen-enriched” is understood to mean having an oxygen concentration greater than that of air. The term “nitrogen-enriched” is understood to mean having a nitrogen concentration greater than that of air. (The concentration of a “nitrogen-enriched” stream is typically similar to that of a “nitrogen-rich” stream.)
The term “oxygen-lean” means having an oxygen concentration less than that of air. An “oxygen-lean” stream could have a composition similar to a “nitrogen-enriched” stream, but it could contain much less oxygen than a nitrogen-enriched or nitrogen-rich stream (e.g., it could be a nitrogen product with an oxygen level of only a few parts per million (ppm)).
According to the present invention, at least a portion of the compressed, purified, and cooled air is introduced to a first of at least three distillation columns. The first distillation column, which contains at least a condenser at its top, produces at least an oxygen-lean stream from or near its top and a first oxygen-enriched liquid from its bottom. A second distillation column, which contains a reboiler in its bottom, has no condenser, receives at least a portion of nitrogen-enriched liquid as a feed to its top, and produces a first nitrogen-rich vapor stream from its top and a second oxygen-enriched liquid from its bottom. A third distillation column, which contains a reboiler in its bottom, has no condenser, receives at least a portion of nitrogen-enriched liquid as a feed to its top, receives at least said second oxygen-enriched liquid as a feed, and produces a second nitrogen-rich vapor from its top and a liquid oxygen-rich stream from its bottom. The liquid oxygen-rich stream from the third distillation column is elevated in pressure and warmed, at least in part, by indirect heat exchange with a pressurized stream having a nitrogen content greater than or equal to that in the feed air, and said pressurized stream is cooled without being subjected to distillation. The second distillation column receives as a feed at least one of (a) a portion of the first oxygen-enriched stream from the first distillation column; or (b) a portion of said cooled pressurized stream. The third distillation column receives as a feed at least one of (a) a portion of the first oxygen-enriched stream from the first distillation column; or (b) a portion of said cooled pressurized stream.
In the preferred mode of operation, the first distillation column is at the highest pressure, the third distillation column is at the lowest pressure, and the second distillation column is at an intermediate pressure between the highest and lowest pressures.
One embodiment of the invention is shown in FIG.
1
. This embodiment comprises a first distillation column
130
, a second distillation column
164
, and a third distillation column
166
. The oxygen product is removed from the distillation column system as an oxygen-rich liquid stream
172
. Two nitrogen-rich streams are produced from the distillation column system as a first nitrogen-rich vapor stream
194
, a vapor from the top of the second distillation column
164
, and a second nitrogen-rich vapor stream
182
, a vapor from the top of the third distillation column
166
.
Air stream
100
is compressed in a main air compressor
102
and purified in unit
104
to remove impurities such as carbon dioxide and water thereby forming a compressed and purified air feed
106
for the process. The pressure of the compressed air is generally between 75 psia and 250 psia and preferably between 100 psia and 200 psia. Stream
106
is split into two portions, stream
108
and stream
114
. Stream
108
is cooled in a main heat exchanger
110
to form cooled air stream
112
, which subsequently is introduced to the bottom of the first distillation column
130
. Stream
114
, which is typically 25% to 30% of the incoming air, is further compressed in a booster compressor
115
to form a pressurized stream
116
. Stream
116
is cooled in the main heat exchanger
110
to form stream
118
. Stream
118
is eventually reduced in pressure across valve
121
to form stream
122
, which constitutes a feed to the third distillation column
166
.
The first distillation column
130
produces an oxygen-lean fraction from the top, vapor stream
132
, and a first oxygen-enriched liquid stream
168
from the bottom. Stream
132
is split into two portions, stream
134
and stream
140
. Stream
134
is condensed in reboiler-condenser
135
to form stream
136
; stream
140
is condensed in reboiler-condenser
141
to form stream
142
. In this embodiment, stream
136
and stream
142
are combined to form stream
144
. A portion of stream
144
is returned to the first distillation column
130
as reflux stream
145
. The other portion of stream
144
constitutes nitrogen-enriched liquid stream
150
, which eventually is split into stream
152
and stream
156
. Stream
152
is reduced in pressure across valve
153
to form stream
154
, which constitutes a feed to the top of the second distillation column
164
. Stream
156
is reduced in pressure across valve
157
to form stream
158
, which constitutes a feed to the top of the third distillation column
166
.
First oxygen-enriched liquid stream
168
, which has an oxygen content of approximately 35 to 40 mole %, is eventually reduced in pressure across valve
169
to form stream
170
, which constitutes a feed to the second distillation column
164
. The second distillation column
164
produces a first nitrogen-rich vapor stream
194
from the top and a second oxygen-enriched liquid stream
160
from the bottom. Upward vapor flow for distillation is provided by reboiler-condenser
141
. First nitrogen-rich vapor stream
194
is eventually warmed in the main heat exchanger
110
to form stream
196
.
Second oxygen-enriched liquid stream
160
has an oxygen content of approximately 50 to 80 mole % and more preferably about 55 to 70 mole %. Stream
160
is eventually reduced in pressure across valve
161
to form stream
162
, which constitutes a feed to the third distillation column
166
. The third distillation column
166
produces second nitrogen-rich vapor stream
182
from the top and liquid oxygen-rich stream
172
from the bottom. Upward vapor flow for distillation is provided by reboiler-condenser
135
. Second nitrogen-rich vapor stream
182
is eventually warmed to intermediate temperature in the main heat exchanger
110
. A portion of partially warmed stream
182
is removed at an intermediate temperature as stream
184
; the remainder is completely warmed to form stream
192
. Stream
184
is reduced in pressure across turbo-expander
185
to form stream
186
and thereby produce refrigeration for the process. Stream
186
is then fully warmed in the main heat exchanger to form stream
188
.
Liquid oxygen-rich stream
172
is elevated in pressure through pump
173
to form stream
174
. Stream
174
is warmed in the main heat exchanger
110
to form stream
176
. At least a portion of the energy needed to warm stream
174
is provided, through indirect heat exchange, by cooling pressurized stream
116
. The warming of oxygen-rich stream
174
may include vaporization, and cooling of pressurized stream
116
may include condensation. Pressurized stream
116
is cooled without being subjected to distillation.
A tabulation of representative temperatures, pressures and flows for selected streams in
FIG. 1
is provided in Table
1
below.
The term “eventually” when applied to streams such as streams
118
,
150
,
160
,
168
,
182
, and
184
is intended to signify that optional steps may be included. For example,
5
streams
118
,
150
,
160
, and
168
may be further cooled before being reduced in pressure, and streams
182
and
194
may be warmed before being introduced to the main heat exchanger
110
. Such cooling and warming often is performed in a subcooler (not shown), procedures commonly known in the field of cryogenics. For clarity, the optional use of single or multiple subcoolers is implied but not described.
A noteworthy feature of the embodiment shown in
FIG. 1
is that all of the first oxygen-enriched liquid stream
168
is eventually introduced to the second distillation column
164
, and all of the cooled pressurized stream
118
is eventually introduced to the third distillation column
166
. Alternatively, all of the first oxygen-enriched liquid stream
168
may be eventually introduced to the third distillation column
166
, and all of the cooled pressurized stream
118
may eventually be introduced to the second distillation column
164
. It has been discovered that efficient operation requires that at least a portion of one of streams
118
or
168
be introduced to the second distillation column and that at least a portion of one of streams
118
or
168
be introduced to the third distillation column.
FIG. 2
Illustrates another embodiment of the invention. This second embodiment shares many similarities with the embodiment of FIG.
1
. Streams in
FIG. 2
which are common with those of
FIG. 1
are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in FIG.
2
.
As shown in
FIG. 2
, a cooled pressurized stream
118
is divided into stream
220
and stream
222
. Stream
222
is eventually reduced in pressure across valve
223
to form
25
stream
224
, which constitutes a feed to the second distillation column
164
. Stream
220
is eventually reduced in pressure across valve
121
to form stream
122
, which constitutes a feed to the third distillation column
166
. This embodiment produces some improvement in efficiency by increasing the production of the first nitrogen-rich vapor stream
194
at the expense of decreasing the production of the second nitrogen-rich vapor stream
182
. In the more typical cases, when the pressure of the second distillation column is greater than the pressure of the third distillation column, nitrogen product compression power may be reduced.
As an alternative, all of the cooled pressurized stream
118
may eventually be introduced to the second distillation column
164
and first oxygen-enriched liquid stream
168
may eventually be split into two fractions, with one fraction forming a feed to the second distillation column
164
and the other fraction forming a feed to the third distillation column
166
. As a further alternative, both stream
118
and stream
168
may be split and eventually be introduced to both the second distillation column and the third distillation column.
FIG. 3
shows an embodiment of the invention which illustrates an alternative processing step for the cooled pressurized stream
118
. This embodiment shares many similarities with the embodiment of FIG.
1
. Streams in
FIG. 3
which are common with those of
FIG. 1
are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in FIG.
3
.
As shown in
FIG. 3
, cooled pressurized stream
118
is eventually reduced in pressure across valve
121
to form stream
122
. In this embodiment, stream
122
is first introduced as a feed to the first distillation column
130
. Liquid stream
318
is withdrawn from an intermediate location of the first distillation column and is eventually reduced in pressure across valve
321
to from stream
322
, which constitutes a feed to the second distillation column
164
. In this embodiment, first oxygen-enriched liquid stream
168
is withdraw from the bottom of the first distillation column
130
and is eventually reduced in pressure across valve
169
to form stream
170
, which constitutes a feed to the third distillation column
166
. As an alternative, stream
322
may be a feed to the second distillation column and stream
170
may be a feed to the third distillation column. As a further alternative, either or both of streams
168
and
318
may be split between both the second and third distillation columns.
Introducing the cooled pressurized stream
118
into the first distillation column
130
and then removing a quantity of liquid from an intermediate location, such as stream
318
, is a common technique in cryogenic air separation. This is done for simplicity of design as well as for improving efficiency, since some vapor may be present in stream
122
as it enters the distillation column system. Persons skilled in the art will recognize that the flow of stream
318
need not be the same as the flow of stream
122
; in fact, the flow of stream
318
is often approximately 50-75% of the flow of stream
122
. Persons skilled in the art also will recognize that stream
318
need not be removed from first column
130
from the same location as stream
122
is introduced.
As an alternative, stream
122
may be split into fractions outside the first distillation column
130
. In such an event, different fractions may be directed to any or all of the first, second or third distillation columns.
FIG. 4
illustrates how an additional nitrogen product may be recovered. This embodiment shares many similarities with the embodiment of FIG.
1
. Streams in
FIG. 4
which are common with those of
FIG. 1
are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in FIG.
4
.
As shown in
FIG. 4
, reboiler-condenser
135
and reboiler-condenser
141
condense different oxygen-lean vapors. Vapor stream
132
exits the top of the first distillation column
130
and is split into stream
440
and stream
134
. Stream
134
is condensed in reboiler-condenser
135
to form stream
136
, which is returned to the first distillation column as top reflux. Stream
440
is warmed in the main heat exchanger
110
to form nitrogen product stream
442
.
Vapor stream
140
is removed from an intermediate location of the first distillation column
130
, condensed in reboiler-condenser
141
to form stream
142
, and returned to the first distillation column as intermediate reflux. Nitrogen-enriched liquid stream
150
is removed from the first distillation column at a location at or near the location that intermediate reflux stream
142
enters the first distillation column.
This embodiment in
FIG. 4
is useful when it is desired to produce a high purity nitrogen product from the distillation column system. In this embodiment, such a high purity nitrogen product is represented by stream
440
. Typical purity requirement for such a stream may be as low as 1 parts per million (ppm), which is usually much more stringent than the purity requirement for the major nitrogen products such as streams
182
and
194
.
In such cases, it is advantageous to withdraw the nitrogen-enriched liquid stream
150
from a location near, but not at, the top of the first distillation column
130
. This embodiment also shows that high purity nitrogen stream
440
leaves the first distillation column as a vapor. Alternatively, stream
440
may be removed as a liquid, for example as a portion of stream
136
, then pumped to delivery pressure before being warmed in the main heat exchanger
110
.
A modification of the embodiment illustrated in
FIG. 4
would be to exchange the reboiler-condenser duties. For example, stream
134
could be condensed in reboiler-condenser
141
and stream
140
could be condensed in reboiler-condenser
135
.
FIG. 5
illustrates an embodiment which uses an alternative pressurized stream. This embodiment shares many similarities with the embodiment of FIG.
1
. Streams in
FIG. 5
which are common with those of
FIG. 1
are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in FIG.
5
.
As shown in
FIG. 5
, oxygen-lean vapor stream
132
from the first distillation column
130
is split into recycle stream
540
in addition to streams
134
and
140
. Recycle stream
540
is warmed to near ambient temperature to form stream
542
, compressed in booster compressor
115
to form stream
116
, then cooled in the main heat exchanger
110
to form cooled pressurized stream
11
8
. Stream
118
is eventually reduced in pressure across valve
121
to form stream
122
, which in this case is a second feed to the top of the third distillation column
166
.
The embodiment of
FIG. 5
may be attractive to employ when booster compressor
115
can be incorporated into other compression services. This is often the case since nitrogen-rich product streams
192
and
196
are typically compressed before being delivered to an end user. Since the composition of stream
542
is nominally the same as streams
192
and
196
, compression of stream
542
may be performed in the same compressor.
There are numerous modifications and alternatives to the embodiment shown in
FIG. 5
, including but not limited to: 1) recycle stream
540
may originate from a location below the top of the first distillation column
130
; 2) recycle stream
540
may originate from at, or below, the top of either the second distillation column
164
or the third distillation column
166
; 3) the recycle stream may be derived from any of streams
188
,
192
or
196
; and 4) cooled pressurized stream
118
may be introduced to any or all of the first, second, or third distillation columns.
As another alternative, one may combine elements of the embodiment of
FIG. 1
with the embodiment of FIG.
5
. In this case, two pressurized streams might be cooled to warm the oxygen-rich stream: one derived from further compression of feed air, and one derived from a recycle from the process such as described in FIG.
5
.
FIG. 6
is another embodiment of the invention, which shows the use of a fourth distillation column
646
. This embodiment shares many similarities with the embodiment of FIG.
1
. Streams in
FIG. 6
which are common with those of
FIG. 1
are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in FIG.
6
.
As shown in
FIG. 6
, oxygen-lean vapor stream
638
from first distillation column
130
is split into streams
640
and
644
. Stream
640
is condensed in reboiler-condenser
141
to form stream
642
, which is returned to the first distillation column as top reflux.
Stream
644
is introduced to the bottom of the fourth distillation column
646
. Fourth distillation column
646
produces a further oxygen-lean fraction from the top, stream
132
, and the nitrogen-enriched liquid stream
150
from the bottom. Stream
132
is split into two portions, stream
134
and stream
440
. Stream
440
is warmed in the main heat exchanger
110
to form stream
442
. Stream
134
is condensed in reboiler-condenser
135
to form stream
136
. In this embodiment, the entirety of stream
136
is returned to the fourth distillation column as reflux. Stream
150
is eventually split into stream
152
and stream
156
. Stream
152
is reduced in pressure across valve
153
to form stream
154
, which constitutes a feed to the top of the second distillation column
164
. Stream
156
is reduced in pressure across valve
157
to form stream
158
, which constitutes a feed to the top of the third distillation column
166
.
This embodiment is useful when it is desired to produce a high purity nitrogen product from the distillation column system. In this embodiment, such a high purity nitrogen product is represented by stream
440
. Typical purity requirement for such a stream may be as low as 1 ppm, which is usually much more stringent than the purity requirement for the major nitrogen products such as streams
182
and
194
. In such cases, it is advantageous to withdraw the nitrogen-enriched reflux stream
150
from the bottom of the fourth distillation column
646
.
This embodiment also shows that high purity nitrogen stream
440
is extracted from the distillation system as a vapor. Alternatively, stream
440
may be removed as a liquid, for example as a portion of stream
136
, then pumped to delivery pressure before being warmed in the main heat exchanger
110
.
A modification of the embodiment illustrated in
FIG. 6
would be to exchange the reboiler-condenser duties. For example, stream
134
could be condensed in reboiler-condenser
141
and stream
640
could be condensed in reboiler-condenser
135
.
FIG. 7
is another embodiment of the invention which shows an alternative use of a fourth distillation column
720
. This embodiment shares many similarities with the embodiment of FIG.
1
. Streams in
FIG. 7
which are common with those of
FIG. 1
are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in FIG.
7
.
As shown in
FIG. 7
, a third portion of feed air is withdrawn from booster compressor
115
as side stream
716
. Stream
716
is cooled in the main heat exchanger
110
to form stream
718
, which is the feed to the bottom of the fourth distillation column
720
.
First distillation column
130
produces a first oxygen-lean fraction from the top, vapor stream
132
, and a first oxygen-enriched liquid stream
168
from the bottom. Stream
132
is condensed in reboiler-condenser
135
to form stream
136
. In this embodiment, a portion of stream
136
is returned to the first distillation column
130
as reflux stream
145
. The other portion of stream
136
constitutes a first nitrogen-enriched liquid stream
750
.
Fourth distillation column
720
produces a second oxygen-lean fraction from the top, stream
140
, and a fourth oxygen-enriched liquid stream
722
from the bottom. Stream
140
is condensed in reboiler-condenser
141
to form stream
142
. In this embodiment, a portion of stream
142
is returned to the fourth distillation column
720
as reflux stream
752
. The other portion of stream
142
constitutes a second nitrogen-enriched liquid stream
754
.
In this embodiment, streams
750
and
754
are eventually combined to form a third nitrogen-enriched liquid stream
150
, and streams
168
and
722
are eventually combined to form stream
170
.
This embodiment is useful for adjusting the relative pressures of the nitrogen-rich streams produced from the second and third distillation columns.
There are numerous modifications and alternatives of the embodiment shown in FIG.
7
. For example, as illustrated, the pressure of the fourth distillation column
720
is greater than the pressure of the first distillation column
130
. As an alternative, the pressure of the fourth distillation column
720
may be less than the pressure of first distillation column
130
. In such a case: 1) air feed
716
could be at a lower pressure than air feed
108
; or 2) stream
718
could be derived by turbo-expanding a portion of air feed
108
, thereby providing refrigeration for the process and eliminating turbo-expander
185
.
Another modification of the embodiment illustrated in
FIG. 7
would be to exchange the reboiler-condenser duties. For example, stream
132
could be condensed in reboiler-condenser
141
and stream
140
could be condensed in reboiler-condenser
135
.
Persons skilled in the art will recognize that the two air feed streams
108
and
716
may be derived from different sources. For example, each of these two streams may be compressed and purified in separate unit operations. Such an operation would be appropriate when the oxygen production rate is so large as to make using two smaller compressors and/or purifiers economical. Furthermore, separate main heat exchangers could be used. Taken to the extreme, pairs of columns could be operated as separate processes. For example, referring to
FIG. 7
, the first distillation column
130
and the third distillation column
166
may be built as one plant, complete with a dedicated compressor, purifier, and main heat exchanger; the fourth distillation column
720
and the second distillation column
164
may be built as another plant, complete with a dedicated compressor, purifier, and main heat exchanger. In this alternative, the second oxygen-enriched stream
160
would be transferred from one plant to the other. Numerous additional alternatives can be derived and will be known to persons skilled in the art.
FIG. 8
is another embodiment of the invention which illustrates that first oxygen-enriched liquid stream
168
may be preprocessed outside either the second distillation column
164
or the third distillation column
166
. This embodiment shares many similarities with the embodiment of FIG.
1
. Streams in
FIG. 8
which are common with those of
FIG. 1
are denoted with the same stream numbers and, for clarity, are not described in the discussion below regarding the embodiment shown in FIG.
8
.
As shown in
FIG. 8
, the first oxygen-enriched stream
168
is eventually reduced in pressure across valve
169
to form stream
170
. Stream
170
is introduced to a vessel
841
which encloses reboiler-condenser
141
. Stream
170
is at least partially vaporized by the reboiler-condenser
141
to produce vapor stream
842
and liquid stream
840
. Vapor stream
842
is introduced to the bottom of the second distillation column
164
. The bottom liquid from the second distillation column, stream
844
, is combined with liquid stream
840
to form second oxygen-enriched stream
160
.
The mode of operation suggested by
FIG. 8
is essentially equivalent to operating the process of
FIG. 1
with the bottom section removed from the second distillation column
164
of FIG.
1
. It is therefore within the spirit of the present invention to equate vaporizing a liquid feed outside a column and transferring the vapor to the column with transferring the liquid to the column and vaporizing within the column.
Persons familiar with distillation will understand that it is also possible to pass streams
844
and
840
separately to the third distillation column
166
. It also will be understood that a fraction of stream
170
may be split, prior to being introduced to vessel
841
, and sent directly to either the second distillation column
164
or the third distillation column
166
. Finally, the use of vessel
841
is illustrative and it is known in the field of heat transfer that stream
170
may be sent directly to reboiler-condenser
141
.
In
FIGS. 1
to
8
the mode of refrigeration supply is through expansion of stream
184
in turbo-expander
185
. Other alternatives exist and are known in the field of cryogenic air separation, including but are not limited to: 1) turbo-expansion of a portion of the nitrogen-rich vapor from the second distillation column; 2) turbo-expansion of a portion of pressurized stream
116
to either of the first, second or third distillation columns; 3) turbo-expansion of a portion of incoming air stream
108
to either of the second or third distillation columns; and 4) turbo-expansion of a vapor stream taken from either of the first, second, or third distillation columns, said vapor stream being withdrawn from any location in said columns.
As illustrated in
FIG. 1
, pressurized stream
118
is shown as being eventually reduced in pressure across a valve
121
. It will be known to persons familiar with cryogenics that valve
121
may be replaced with a work producing device, such as a dense fluid expander.
In
FIGS. 1
to
8
only one oxygen product is produced. It will be known to persons skilled in the art that multiple oxygen products may be produced. These oxygen products may differ in their pressure and/or purity. Examples of ways to make multiple purity oxygen products include, but are not limited to: 1) withdraw the lower purity oxygen product from a location above the bottom of the third distillation column and withdraw the higher purity oxygen product from the bottom of the third distillation column; and 2) withdraw the lower purity oxygen product from the bottom of the second distillation column and withdraw the higher purity oxygen product from the bottom of the third distillation column.
In
FIGS. 3 and 6
it is shown that an additional nitrogen-rich product is made from the first distillation column
130
. Persons skilled in the art will recognize that an additional nitrogen-rich product may be made from the first distillation column in any of the embodiments of the present invention. Persons skilled in the art also will recognize that none of the nitrogen-rich products need be the same composition. For example, it is found that in some cases it is advantageous to produce stream
196
and
192
at different purities, so that when combined, they meet the specification of the process. Conversely, all the nitrogen products may be of similar purity and compressed in a common product compressor.
In
FIGS. 1
to
8
the main heat exchanger
110
is shown as a single heat exchanger. Persons skilled in the art will recognize that such a depiction is not limiting to the invention. Typically, large plants require multiple heat exchangers in parallel. Furthermore, one may elect to pass different streams to different parallel heat exchangers. One common example, with reference to
FIG. 1
, would be to pass oxygen-rich stream
174
, pressurized stream
116
, and a portion of either stream
192
or stream
196
to a first parallel heat exchanger and to pass the remaining streams to a second parallel heat exchanger.
Finally, persons skilled in the art will recognize that one need not recover both streams
192
and
196
as products. For example, referring to the embodiment of
FIG. 1
, if the quantity of nitrogen desired is not large, one may elect to operate the third distillation column
166
at a reduced pressure and pass all of partially warmed stream
182
to turbo-expander
185
. The resultant flow of stream
192
would thereby become zero. In this case, the only nitrogen product produced by the process would be stream
196
, along with any optionally produced nitrogen-rich product from the first distillation column
130
. In another example, the third distillation column may be operated at near atmospheric pressure and the second nitrogen-rich vapor stream
182
may constitute a waste byproduct rather than a nitrogen product. In such a case, an alternative means of provided refrigeration, such as those previously discussed, would be applied.
In the application of the embodiment of
FIGS. 1
to
5
it is possible to spatially locate the three columns in a number of different ways. For example, if minimization of plot size is key, the three columns may be stacked on top of one another. In such a case, six combinations are possible. One configuration of note would be to install the second distillation column
164
on top of the third distillation column
166
and to install the third distillation column on top of the first distillation column
130
. This particular configuration is advantageous because stream
160
, the second oxygen-enriched stream from the second distillation column, may easily flow downward to the third distillation column.
Alternatively, if minimization of equipment height is key, all three columns may be located along side one another. In such a case, such as in
FIG. 1
, a pump would be needed to transfer liquid reflux stream
145
to the top of the first distillation column
130
. In some circumstances it may be advantageous to locate one of the reboiler-condensers on top of the first distillation column. In such an event a pump would be needed to transfer liquid from the bottom of one or both of the second distillation column
164
and/or third distillation column
166
.
An intermediate configuration strategy could install one of the columns on top of the other and have the remaining column located along side. There are six possible combinations of this type. One configuration of note would be to install the third distillation column
166
on top of the first distillation column
130
and to install the second distillation column
164
along side the first distillation column. In principle, any liquid made in reboiler-condenser
141
of the second distillation column would need to be pumped if it was necessary to return liquid to the top of the first distillation column. In the practice of this invention, it is possible to operate in such a manner that the reflux needed for the first distillation column is provided entirely by reboiler-condenser
135
of the third distillation column and it would not be necessary to pump reflux from reboiler condenser
141
. Analogously, a configuration may call for installing the second distillation column on top of the first distillation column and installing the third distillation column along side the first distillation column. This configuration is most appropriate when reboiler-condenser
141
of the second distillation column provides all the necessary reflux to the top of the first distillation column.
For the case where the second distillation column
164
and the third distillation column
166
are stacked on one another with the first distillation column
130
installed along side, the preferred configuration would install the second distillation column on top of the third distillation column. This configuration has two advantages: 1) stream
160
may be freely transferred to the third distillation column; and 2) reboiler-condenser
141
may supply all the reflux to the first distillation column and, if elevated properly, said reflux could be transferred without a pump. As with the case where all columns are located along side one another, in some circumstances it may be advantageous to locate one of the reboiler-condensers on top of the first distillation column. In such an event a pump may or may not be needed to transfer liquid from the bottom of one of the second or third distillation columns.
In the application of the embodiments of
FIGS. 6 and 7
it is possible to spatially locate the four columns in even more different ways. Although the number of combinations is relatively large, the combinations are easily enumerated. In one possible arrangement, all four columns are installed along side one another. For the case where three columns are stacked on top of one another and one column is installed along the side, there are
24
possible combinations: six configurations with the first distillation column
130
installed along the side, six configurations with the second distillation column
164
installed along the side, and so on.
For the case where two of the columns are stacked on one another and the other two columns are stacked on one another, and the stacked pairs are installed along side of one another, there are twelve possible combinations. For example, as implied by
FIG. 6
, the third distillation column
166
may be stacked on top of the fourth distillation column
646
and the second distillation column
164
may be stacked on top of the first distillation column
130
.
For the case where all four distillation columns are stacked on top of one another, there are
24
possible combinations. For example, referring to
FIG. 6
, the second distillation column
164
may be on top of the third distillation column
166
which may be on top of the fourth distillation column
646
which may be on top of the first distillation column
130
.
Persons skilled in the art will recognize that a reboiler-condenser associated with a column pair may be physically installed: 1) in the bottom of the column receiving the boilup; 2) in the column receiving the reflux; or 3) external to either column. Thus, the spatial location of a reboiler-condenser is also a variable for construction. For example, referring to
FIG. 8
, reboiler-condenser
141
is shown to be external to the second distillation column
164
. In this case, one may elect to place vessel
841
, and its contained reboiler-condenser
141
, near or below the second distillation column
164
, on near or above the first distillation column
130
, or even near or above the third distillation column
166
.
In the application of the embodiments illustrated in
FIGS. 1
to
8
, and those alternatives discussed in the text, the selection of the proper spatial location is a cost optimization exercise. Factors which play a role in selecting the optimal configuration include but are not limited to: 1) individual column diameters and column heights; 2) shipping and installation limitations on maximum height; 3) allowable plot space; 4) avoiding the use of liquid pumps; 5) whether the equipment enclosures are shop-fabricated or field-erected; and 6) the existence of other major equipment items, such as main heat exchanger
110
. Although, the number of possible options can be large, they are finite and can be readily identified . Therefore, persons skilled in the art may easily evaluate the cost of each configuration and select the optimal arrangement.
EXAMPLE
In order to demonstrate the efficacy of the present invention and to compare the present invention to more conventional processes, the following example is presented. The basis for comparison follows.
The prior art process is a standard elevated pressure, double-column, pumped-LOX cycle as illustrated in FIG.
9
. As shown in
FIG. 9
, air stream
100
is compressed in a main air compressor
102
and purified in unit
104
to remove impurities such as carbon dioxide and water, thereby forming a compressed and purified air feed stream
106
for the process. Stream
106
is split into two portions, stream
108
and stream
114
. Stream
108
is cooled in a main heat exchanger
110
to form cooled air stream
112
, which is subsequently introduced to a higher pressure column
130
. Stream
114
is further compressed in a booster compressor
115
to form pressurized stream
116
. Stream
116
is cooled in the main heat exchanger
110
to form stream
118
. Stream
118
is eventually reduced in pressure across valve
121
to form stream
122
, which constitutes a feed to a lower pressure column
166
.
The higher pressure column
130
produces an oxygen-lean fraction from the top, stream
132
, and a first oxygen-enriched liquid stream
168
from the bottom. Stream
132
is condensed in reboiler-condenser
135
to form stream
136
. A portion of stream
136
is returned to the higher pressure column
130
as reflux stream
145
. The other portion of stream
136
constitutes a nitrogen-enriched liquid stream
150
. Stream
150
is eventually reduced in pressure across valve
157
to form stream
158
, which constitutes a feed to the top of the lower pressure column
166
. First oxygen-enriched liquid stream
168
is eventually reduced in pressure across valve
169
to form stream
170
, which constitutes a feed to the lower pressure column
166
.
The lower pressure column
166
produces a nitrogen-rich vapor stream
182
from the top and a liquid oxygen-rich stream
172
from the bottom. Upward vapor flow for distillation is provided by reboiler-condenser
135
. Nitrogen-rich vapor stream
182
is eventually warmed to an intermediate temperature in the main heat exchanger
110
. A portion of partially warmed stream
182
is removed at an intermediate temperature as stream
184
; the remainder of stream
182
is completely warmed to form stream
192
. Stream
184
is reduced in pressure across a turbo-expander
185
to form stream
186
and thereby produce refrigeration for the process. Stream
186
is then fully warmed in the main heat exchanger to form stream
188
.
Liquid oxygen-rich stream
172
is elevated in pressure through pump
173
to form stream
174
. Stream
174
is warmed in the main heat exchanger
110
to form stream
176
. A portion of the energy needed to warm stream
174
is provided, through indirect heat exchange by cooling pressurized stream
116
.
The embodiment of the present invention chosen for comparison with the prior art process corresponds to FIG.
1
. The production basis is: 1) Oxygen=4,210 lb mole/hr at >95 mole % and 400 psia; 2) Nitrogen=12,960 lb mole/hr at >99 mole % and 150 psia.
Computer simulations of the two processes were developed. Selected results are presented in Table 1. A summary of the power consumed by the two processes is presented in Table 2. The results show that the present invention saves almost 1,000 kW or nearly 6% of the main air compressor power.
TABLE 1
|
|
HEAT AND MATERIAL BALANCE
|
Prior Art -
Present Invention -
|
FIG. 9
FIG. 1
|
Pres-
Pres-
|
Circuit
Flow lb
sure
Temp.
Flow lb
sure
Temp.
|
No.
mole/hr
psia
° F.
mole/hr
psia
° F.
|
|
Air Feed
108
13,663
116
67
14,231
115
67
|
Air Feed
116
5,628
960
90
5,542
980
90
|
1
st
196
—
—
—
6,037
58
64
|
Nitrogen
|
2
nd
192
12,966
33
65
6,929
33
64
|
Nitrogen
|
Waste
188
2,079
15
65
2,591
15
64
|
Oxygen
176
4,214
400
65
4,214
400
64
|
N2 Reflux
154
—
—
—
3,120
60
−297
|
N2 Reflux
158
5,963
35
−306
3,208
35
−305
|
O2-
168
7,369
113
−271
7,691
113
−271
|
enriched
|
O2
160
—
—
—
4,766
60
−287
|
enriched
|
|
TABLE 2
|
|
POWER SUMMARY - kW
|
Prior Art
Present Invention
|
FIG. 9
FIG. 1
|
|
Main Air Compressor
17,855
18,285
|
Booster Compressor
5,195
5,196
|
Nitrogen Compressor
8,238
6,817
|
Total
31,288
30,298
|
|
Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown or described. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.
SEQUENCE LISTING
Not Applicable.
Claims
- 1. A process for separating air to produce oxygen and nitrogen, said process using a distillation column system having at least three distillation columns, including a first distillation column, a second distillation column, and a third distillation column, wherein each distillation column has a top and a bottom, comprising the steps of:providing a stream of compressed air having a first nitrogen content; feeding at least a first portion of the stream of compressed air to the first distillation column; withdrawing a first oxygen-enriched liquid stream from the bottom of the first distillation column and feeding at least a portion of the first oxygen-enriched liquid stream to the second distillation column and/or the third distillation column; withdrawing a first oxygen-lean vapor stream from or near the top of the first distillation column, feeding at least a first portion of the first oxygen-lean vapor stream to a first reboiler-condenser of the second distillation column or of the third distillation column, and at least partially condensing the at least a first portion of the first oxygen-lean vapor stream, thereby forming a first nitrogen-enriched liquid; feeding at least a first portion of the first nitrogen-enriched liquid to the top of the first distillation column; feeding a second nitrogen-enriched liquid and/or at least a second portion of the first nitrogen-enriched liquid to the top of the second distillation column; withdrawing a second oxygen-enriched liquid stream from the bottom of the second distillation column and feeding the second oxygen-enriched liquid stream to the third distillation column; withdrawing a first nitrogen-rich vapor stream from the top of the second distillation column; withdrawing a second nitrogen-rich vapor stream from the top of the third distillation column; withdrawing a liquid oxygen stream from the bottom of the third distillation column, wherein said liquid oxygen stream is elevated in pressure before being warmed at least in part by indirect heat exchange with a pressurized stream having a nitrogen content at least equal to the first nitrogen content, said pressurized stream being cooled without being subjected to distillation; and feeding at least a portion of the cooled pressurized stream eventually to any or all of the first distillation column, the second distillation column, or the third distillation column.
- 2. A process as in claim 1, wherein the pressurized stream is the first portion of the stream of compressed air.
- 3. A process as in claim 1, wherein the pressurized stream is another portion of the stream of compressed air.
- 4. A process as in claim 3, comprising the further step of compressing further the another portion.
- 5. A process as in claim 1, wherein the pressurized stream is a compressed portion of an oxygen-lean vapor stream withdrawn from the distillation column system.
- 6. A process as in claim 1, wherein a boilup for the second distillation column is provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapor, and wherein a boilup for the third distillation column is provided at least in part by indirect heat exchange with another portion of the first oxygen-lean vapor.
- 7. A process as in claim 1, wherein the first distillation column is at a first pressure, the second distillation column is at a second pressure lower than the first pressure, and the third distillation column is at a third pressure lower than the second pressure.
- 8. A process as in claim 1, comprising the further steps of:providing a fourth distillation column having a top and a bottom; feeding a second portion of the first oxygen-lean vapor stream from the first distillation column to the bottom of the fourth distillation column; withdrawing a third nitrogen-enriched liquid stream from the bottom of the fourth distillation column and feeding at least a portion of the third nitrogen-enriched liquid to the second distillation column and/or the third distillation column; withdrawing a second oxygen-lean vapor stream from or near the top of the fourth distillation column, feeding at least a first portion of the second oxygen-lean vapor stream to a second reboiler-condenser of the second distillation column or of the third distillation column, at least partially condensing the first portion of the second oxygen-lean vapor stream, thereby forming a fourth nitrogen-enriched liquid, and feeding at least a portion of the fourth nitrogen-enriched liquid to the top of the fourth distillation column; and withdrawing a high purity nitrogen stream from the second oxygen-lean vapor stream or the fourth nitrogen-enriched liquid.
- 9. A process as in claim 1, comprising the further steps of:providing a fourth distillation column having a top and a bottom; feeding another portion of the stream of compressed air to the bottom of the fourth distillation column; withdrawing a third oxygen-enriched liquid stream from the bottom of the fourth distillation column, and feeding at least a portion of the fourth oxygen-enriched liquid stream to the second distillation column and/or the third distillation column; withdrawing a second oxygen-lean vapor stream from or near the top of the fourth distillation column, feeding at least a portion of the second oxygen-lean vapor stream to a second reboiler-condenser of the second distillation column or of the third distillation column, and at least partially condensing the second oxygen-lean vapor stream, thereby forming the second nitrogen-enriched liquid; and feeding at least a first portion of the second nitrogen-enriched liquid to the top of the fourth distillation column.
- 10. A process as in claim 9, wherein the fourth distillation column is at a fourth pressure greater than a first pressure of the first distillation column.
- 11. A process as in claim 9, wherein the fourth distillation column is at a fourth pressure less than a first pressure of the first distillation column.
- 12. A process as in claim 8, wherein a boilup for the second distillation column is provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapor stream, and wherein a boilup for the third distillation column is provided at least in part by indirect heat exchange with the first portion of the second oxygen-lean vapor stream.
- 13. A process as in claim 9, wherein a boilup for the third distillation column is provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapor stream, and wherein a boilup for the second distillation column is provided at least in part by indirect heat exchange with the second oxygen-lean vapor stream.
- 14. A process as in claim 1, comprising the further steps of:withdrawing a vapor stream from the first distillation column at an intermediate location, feeding the vapor stream to a second reboiler-condenser of the second distillation column or of the third distillation column, and at least partially condensing the vapor stream, thereby forming an intermediate reflux stream; feeding the intermediate reflux stream to the first distillation column at or near the intermediate location; and withdrawing the second nitrogen-enriched liquid from the first distillation column at or near the intermediate location for feeding at least a portion to the top of the second distillation column or the third distillation column.
- 15. A process as in claim 14, wherein a boilup for the second distillation column is provided at least in part by indirect heat exchange with the vapor stream withdrawn at the intermediate location, and wherein a boilup for the third distillation column is provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapor stream.
- 16. A process as in claim 14, wherein a boilup for the third distillation column is provided at least in part by indirect heat exchange with the vapor stream withdrawn at the intermediate location, and wherein a boilup for the second distillation column is provided at least in part by indirect heat exchange with the first portion of the first oxygen-lean vapor stream.
- 17. A cryogenic air separation unit using a process as in claim 1.
US Referenced Citations (8)