Information
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Patent Grant
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6568209
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Patent Number
6,568,209
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Date Filed
Friday, September 6, 200222 years ago
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Date Issued
Tuesday, May 27, 200321 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
- 062 643
- 062 646
- 062 620
- 062 644
- 062 903
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International Classifications
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Abstract
A cryogenic air separation system having a main heat exchanger for processing feed air, said heat exchanger having a reversing heat exchanger function in an upward feed airflow countercurrent section and having a desuperheating function in a downward feed air flow cocurrent section.
Description
TECHNICAL FIELD
This invention relates generally to cryogenic air separation and, more particularly, to cryogenic air separation employing a reversing heat exchanger to clean and cool feed air prior to its passing into the cryogenic air separation plant.
BACKGROUND ART
In the practice of cryogenic air separation to produce one or more products such as nitrogen and oxygen, the feed air to the cryogenic air separation plant must be cleaned of high boiling impurities, such as carbon dioxide and water vapor, before it enters the cryogenic air separation plant because such high boiling impurities will freeze within the plant at the very low temperatures at which the plant operates, thus reducing the operating efficiency of the cryogenic air separation plant.
One very important system for cleaning the feed air is by the use of a reversing heat exchanger wherein the high boiling impurities freeze out onto the heat exchanger passages as the feed air is cooled against return streams such as product and waste streams, and periodically the flow of the feed air stream and a waste stream are alternated in the heat exchanger passages so that the deposited high boiling impurities are passed out of the heat exchanger with the waste stream.
Another important function in the operation of a cryogenic air separation system is the desuperheating of the compressed feed air before it enters the column(s) of the cryogenic air separation plant.
The heat exchangers employed to cool, clean and desuperheat the feed air for a cryogenic air separation plant involve considerable capital costs. Any improvement to such heat exchanger arrangement would be highly desirable.
Accordingly, it is an object of this to provide an improved heat exchanger system for treating feed air to a cryogenic air separation plant.
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 cryogenic air separation method comprising:
(A) cooling feed air by upward flow in a main heat exchanger to produce cooled feed air, and dividing the cooled feed air into a first portion and a second portion;
(B) partially condensing the first portion of the cooled feed air by downward flow in the main heat exchanger, and passing the partially condensed first portion of the cooled feed air and the second portion of the cooled feed air into a cryogenic air separation plant;
(C) separating the feed air by cryogenic rectification in the cryogenic air separation plant to produce at least one product; and
(D) warming said at least one product by downward flow in the main heat exchanger, and recovering said at least one product.
Another aspect of the invention is:
Cryogenic air separation apparatus comprising:
(A) a main heat exchanger and a cryogenic air separation plant comprising at least one column;
(B) means for passing feed air upwardly through a first section of the main heat exchanger, and means for passing feed air downwardly through a second section of the main heat exchanger;
(C) means for passing feed air from the main heat exchanger to the cryogenic air separation plant; and
(D) means for passing product from the cryogenic air separation plant to the main heat exchanger, and means for recovering product from the main heat exchanger.
As used herein, the term “feed air” means a mixture comprising primarily nitrogen and oxygen, such as ambient air.
As used herein, the terms “turboexpansion” and “turboexpander” mean respectively method and apparatus for the flow of high pressure fluid through a turbine to reduce the pressure and the temperature of the fluid thereby generating refrigeration.
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 or the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements which may be structured packing and/or random packing elements. For a further discussion of distillation columns, see the Chemical Engineers' Handbook fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process.
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 more 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 she vapor and liquid phases is adiabatic and can include integral or differential 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 “cryogenic air separation plant” means the column or columns wherein feed air is separated by cryogenic rectification, as well as interconnecting piping, valves, heat exchangers and the like.
As used herein, the terms “upper portion” and “lower portion” of a column means those portions respectively above and below the midpoint of the column.
As used herein, the term “top condenser” means a heat exchange device that generates column downflow liquid from column vapor.
As used herein, the term “product nitrogen” means a fluid having a nitrogen concentration equal to or greater than 99 mole percent.
BRIEF DESCRIPTION OF THE DRAWING
The sole FIGURE is a schematic representation of one preferred embodiment of the invention wherein the cryogenic air separation plant comprises a single column with a top condenser for producing product nitrogen.
DETAILED DESCRIPTION
In general the invention combines the reversing heat exchanger function in a countercurrent section of a vertically oriented main heat exchanger for treating feed air for a cryogenic air separation plant with the desuperheating function in a cocurrent section of the vertically oriented main heat exchanger.
The invention will be described in detail with reference to the Drawing. Referring now to the FIGURE, compressed feed air
1
, generally at a pressure within the range of from 100 to 200 pounds per square inch absolute (psia), is passed through valve
20
A and as stream
60
into pass A of vertically oriented main heat exchanger
61
which comprises a countercurrent or first section
10
and a cocurrent or second section
15
. Countercurrent section
10
is at the lower end of vertically oriented main heat exchanger
61
and cocurrent section
15
is at the upper end of vertically oriented main heat exchanger
61
.
The feed air is cooled by upward flow in countercurrent section
10
by countercurrent indirect heat exchange with return streams as will be more fully described below. High boiling impurities such as water vapor and carbon dioxide freeze out onto the inner surface of pass A as the cooling feed air passes upwardly in countercurrent section
10
. The resulting cooled feed air is withdrawn from main heat exchanger
61
in cooled feed air stream
2
which is passed through valve
25
A and as stream
62
to gel trap
45
wherein it is further cleaned of any residual contaminants down to trace levels.
The cooled feed air in stream
3
emerging from gel trap
45
is divided into first portion
4
and second portion
6
. First portion
4
generally comprises from 3 to 20 percent, preferably from 5 to 15 percent, of the cooled feed air, and second portion
6
generally comprises the remainder of the cooled feed air, i.e. from 80 to 97 percent, preferably from 85 to 95 percent, of stream
3
. First portion
4
is passed to main heat exchanger
61
and is cooled and partially condensed by downward flow through cocurrent section
15
by cocurrent indirect heat exchange with return streams as will be more fully described below, thereby serving to desuperheat the feed air. The partially condensed feed air first portion is withdrawn from main heat exchanger
61
in stream
5
. Generally stream
5
comprises from about 20 to 90 percent liquid and from about 80 to 10 percent vapor, preferably from 30 to 70 percent liquid and from 70 to 30 percent vapor, most preferably from 40 to 60 percent liquid and from 60 to 40 percent vapor. In a particularly preferred embodiment, the partially condensed feed air first portion comprises about 55 percent liquid and about 45 percent vapor. Partially condensed feed air first portion
5
is combined with cooled feed air second portion
6
, which has been passed through valve
63
, to form combined feed air stream
7
for passage into the cryogenic air separation plant. Alternatively the first and second portions of the feed air may be passed separately into the cryogenic air separation plant.
In the embodiment of the invention illustrated in the FIGURE, the cryogenic air separation plant comprises a single column
64
and a top condenser
50
. The feed air is passed into the lower portion of column
64
of the cryogenic air separation plant. Column
64
is operating at a pressure generally within the range of from 90 to 190 psia. Within column
64
the feed air is separated by cryogenic rectification into nitrogen-enriched vapor and oxygen-enriched liquid.
Oxygen-enriched liquid is withdrawn from the lower portion of column
64
in stream
8
, throttled through valve
65
and passed as stream
9
into top condenser
50
. Nitrogen-enriched vapor is withdrawn from the upper portion of column
64
as stream
12
. A portion
12
of stream
66
is warmed by downward flow through main heat exchanger
61
by cocurrent indirect heat exchange with the partially condensing first feed air portion and by countercurrent indirect heat exchange with the cooling feed air stream. The warmed nitrogen-enriched vapor is withdrawn from main heat exchanger
61
as stream
16
, passed through valve
67
and recovered as product nitrogen in stream
68
.
Another portion
13
of the nitrogen-enriched vapor
66
is passed into top condenser
50
wherein it is condensed by heat exchange with boiling oxygen-enriched liquid introduced into top condenser
50
in stream
9
. Resulting nitrogen-enriched liquid is withdrawn from top condenser
50
in stream
14
. Some or all of the nitrogen-enriched liquid is passed into the upper portion of column
64
as reflux. In the embodiment of the invention illustrated in the FIGURE a portion
27
of the nitrogen-enriched liquid is passed into column
64
as reflux, and another portion is passed through valve
69
and recovered as product nitrogen liquid in stream
11
. A small portion of-the oxygen-enriched liquid provided into top condenser
50
is removed through valve
70
in stream
29
so as to remove contaminants that may accumulate in the condenser pool.
Oxygen-enriched vapor, generally comprising from about 25 to 40 mole percent oxygen, is withdrawn from top condenser
50
in stream
17
and is divided into stream
18
and stream
71
. Stream
18
is warmed by downward flow cocurrent heat exchange against the partially condensing first portion of the feed air and then by countercurrent heat exchange with the cooling feed air in the countercurrent section of main heat exchanger
61
. The warmed oxygen-enriched vapor stream is withdrawn from main heat exchanger
61
in stream
19
. Stream
71
is passed through valve
72
and as stream
26
is combined with stream
19
to form oxygen-enriched vapor stream
31
. Stream
31
is passed to turboexpander
55
wherein it is turboexpanded to generate refrigeration. Thereafter resulting refrigeration bearing turboexpanded stream
21
is passed from turboexpander
55
to main heat exchanger
61
.
The turboexpanded oxygen-enriched vapor is warmed by downward flow through cocurrent section
15
by indirect cocurrent heat exchange with the partially condensing first feed air portion. Resulting warmed oxygen-enriched vapor
22
is passed through valve
35
B and is then further warmed by downward flow in pass B through countercurrent section
10
by indirect countercurrent heat exchange with the cooling upwardly flowing feed air in pass A. Resulting further warmed oxygen-enriched vapor, which contains contaminants picked up by passage through pass B which were deposited there in a previous cycle, is then removed from main heat exchanger
61
in stream
73
, passed through valve
30
B and removed from the system as waste stream
28
.
Periodically the flow of the feed air in pass A and the flow of the waste stream in pass B is reversed. In such periods, feed air
1
flosses through valve
20
B and as stream
74
is passed into pass B of main heat exchanger
61
and then is passed through valve
25
B for passage to gel trap
45
. Simultaneously, warmed oxygen-enriched vapor
22
is passed through valve
35
A and then through pass A wherein it is further warmed and picks up the contaminants deposited therein by the feed air which passed through pass A in the previous period. The resulting waste stream is removed from main heat exchanger
61
in stream
74
, passed through valve
30
A and removed from the system in stream
28
.
Although the invention has been described in detail with reference to a certain preferred embodiment those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims. For example, another expansion device such as a Joule-Thomson valve may be used in place of turboexpander
55
. In addition liquid nitrogen may be added to the top condenser arid/or the column to help sustain the cryogenic rectification. Moreover, the internal passage configuration of stream
4
in cocurrent section
15
may be of the cocurrent cross-flow type instead of the simple cocurrent flow arrangement shown in the Drawing.
Claims
- 1. A cryogenic air separation method comprising:(A) cooling feed air by upward flow in a main heat exchanger to produce cooled feed air, and dividing the cooled feed air into a first portion and a second portion; (B) partially condensing the first portion of the cooled feed air by downward flow in the main heat exchanger, and passing the partially condensed first portion of the cooled feed air and the second portion of the cooled feed air into a cryogenic air separation plant; (C) separating the feed air by cryogenic rectification in the cryogenic air separation plant to produce at least one product; and (D) warming said at least one product by downward flow in the main heat exchanger, and recovering said at least one product.
- 2. The method of claim 1 wherein the first portion comprises from 3 to 20 percent of the cooled feed air.
- 3. The method of claim 1 wherein the partially condensed first portion of the feed air comprises from 20 to 90 percent liquid.
- 4. The method of claim 1 wherein said at least one product comprises product nitrogen.
- 5. The method of claim 1 further comprising turboexpanding a waste stream taken from the cryogenic air separation plant, and warming the turboexpanded waste stream by downward flow in the main heat exchanger.
- 6. The method of claim 1 wherein the partially condensed first portion of the cooled feed air and the second portion of the cooled feed air are combined and passed together into the cryogenic air separation plant.
- 7. Cryogenic air separation apparatus comprising:(A) a main heat exchanger and a cryogenic air separation plant comprising at least one column; (B) means for passing feed air upwardly through a first section of the main heat exchanger, and means for passing feed air downwardly through a second section of the main heat exchanger; (C) means for passing feed air from the main heat exchanger to the cryogenic air separation plant; and (D) means for passing product from the cryogenic air separation plant to the main heat exchanger, and means for recovering product from the, main heat exchanger.
- 8. The apparatus of claim 7 wherein the cryogenic air separation plant comprises a single column and a top condenser.
- 9. The apparatus of claim 8 wherein the means for passing product from the cryogenic air separation plant to the main heat exchanger communicates with the upper portion of the single column.
- 10. The apparatus of claim 8 further comprising a turboexpander, means for passing fluid from the top condenser to the turboexpander, and means for passing fluid from the turboexpander to the main heat exchanger.
US Referenced Citations (13)