Cryogenic rectification system for producing dual purity oxygen

A cryogenic rectification system employing a double column and two auxiliary reboilers associated with one or two auxiliary columns wherein both lower purity oxygen and higher purity oxygen is produced.

TECHNICAL FIELD 
This invention relates generally to the cryogenic rectification of feed air 
and, more particularly, to the cryogenic rectification of feed air to 
produce oxygen. 
BACKGROUND ART 
The demand for lower purity oxygen is increasing in applications such as 
glassmaking, steelmaking and energy production. Lower purity oxygen is 
generally produced in large quantities by the cryogenic rectification of 
feed air in a double column wherein feed air at the pressure of the higher 
pressure column is used to reboil the liquid bottoms of the lower pressure 
column and is then passed into the higher pressure column. 
Some users of lower purity oxygen, for example integrated steel mills, 
often require some higher purity oxygen in addition to the lower purity 
oxygen. Such dual purity production cannot be efficiently accomplished 
with a conventional lower purity oxygen plant. 
Accordingly, it is an object of this invention to provide a cryogenic 
rectification system which can effectively and efficiently produce both 
lower purity oxygen and higher purity oxygen. 
SUMMARY OF THE INVENTION 
The above and other objects, which will become apparent to one 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 lower purity oxygen and higher purity oxygen 
comprising: 
(A) at least partially condensing a first feed air portion and passing the 
resulting first feed air portion into a higher pressure column of a double 
column which also comprises a lower pressure column; 
(B) at least partially condensing a second feed air portion, having a 
pressure less than that of the first feed air portion, and passing the 
resulting second feed air portion into the higher pressure column; 
(C) separating feed air within the double column by cryogenic rectification 
to produce first oxygen-richer fluid and nitrogen-richer fluid; 
(D) passing first oxygen-richer fluid from the double column into a first 
auxiliary column and separating the first oxygen-richer fluid within the 
first auxiliary column by cryogenic rectification to produce second 
oxygen-richer fluid having an oxygen concentration greater than that of 
the first oxygen-richer fluid; 
(E) passing second oxygen-richer fluid from the first auxiliary column into 
a second auxiliary column and separating second oxygen-richer fluid within 
the second auxiliary column by cryogenic rectification into lower purity 
oxygen, having an oxygen concentration which is greater than that of the 
first oxygen-richer fluid and less than that of the second oxygen-richer 
fluid, and into higher purity oxygen, having an oxygen concentration 
greater than that of the lower purity oxygen; and 
(F) recovering lower purity oxygen and higher purity oxygen from the second 
auxiliary column. 
Another aspect of this invention is: 
Apparatus for producing lower purity oxygen and higher purity oxygen 
comprising: 
(A) a double column comprising a higher pressure column and a lower 
pressure column, a first auxiliary column having a first reboiler, and a 
second auxiliary column having a second reboiler; 
(B) means for passing feed air to the second reboiler and from the second 
reboiler into the higher pressure column; 
(C) means for passing feed air to the first reboiler at a pressure less 
than that of the feed air passed to the second reboiler, and means for 
passing feed air from the first reboiler into the higher pressure column; 
(D) means for passing fluid from the double column into the first auxiliary 
column; 
(E) means for passing fluid from the first auxiliary column into the second 
auxiliary column; and 
(F) means for recovering lower purity oxygen from the second auxiliary 
column and means for recovering higher purity oxygen from the second 
auxiliary column. 
A further aspect of the invention is: 
A method for producing lower purity oxygen and higher purity oxygen 
comprising: 
(A) at least partially condensing a first feed air portion and passing the 
resulting first feed air portion into a higher pressure column of a double 
column which also comprises a lower pressure column; 
(B) at least partially condensing a second feed air portion, having a 
pressure less than that of the first feed air portion, and passing the 
resulting second feed air portion into the higher pressure column; 
(C) separating feed air within the double column by cryogenic rectification 
to produce oxygen-richer fluid and nitrogen-richer fluid; 
(D) passing oxygen-richer fluid from the double column into an auxiliary 
column and separating the oxygen-richer fluid within the auxiliary column 
by cryogenic rectification into lower purity oxygen, having an oxygen 
concentration greater than that of the oxygen-richer fluid, and higher 
purity oxygen, having an oxygen concentration greater than that of the 
lower purity oxygen; and 
(E) recovering lower purity oxygen and higher purity oxygen from the 
auxiliary column. 
Yet another aspect of the invention is: 
Apparatus for producing lower purity oxygen and higher purity oxygen 
comprising: 
(A) a double column comprising a higher pressure column and a lower 
pressure column, and an auxiliary column having a first reboiler and a 
second reboiler; 
(B) means for passing feed air to the second reboiler and from the second 
reboiler into the higher pressure column; 
(C) means for passing feed air to the first reboiler at a pressure less 
than that of the feed air passed to the second reboiler, and means for 
passing feed air from the first reboiler into the higher pressure column; 
(D) means for passing fluid from the double column into the auxiliary 
column; and 
(E) means for recovering lower purity oxygen from the auxiliary column and 
means for recovering higher purity oxygen from the auxiliary column. 
As used herein, the term "feed air" means a mixture comprising primarily 
nitrogen and oxygen, such as ambient air. 
As used herein, the term "lower purity oxygen" means a fluid having an 
oxygen concentration with the range of from 70 to 98 mole percent. 
As used herein, the term "higher purity oxygen" means a fluid having an 
oxygen concentration equal to or greater than 99 mole percent. 
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 is generally 
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 
fluid streams into heat exchange relation without any physical contact or 
intermixing of the fluids with each other. 
As used herein the term "reboiler" means a heat exchange device which 
generates column upflow vapor from column liquid. A reboiler is generally 
within a column but may be physically outside a column. 
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. 
As used herein, the term "recovered" means passed out of the system, i.e. 
actually recovered, in whole or in part, or otherwise removed from the 
system.

DETAILED DESCRIPTION 
The invention will be described in detail with reference to the Drawings. 
Referring now to FIG. 1, feed air 80 is compressed to a pressure within 
the range of from 70 to 300 pounds per square inch absolute (psia) in 
compressor 81. Resulting pressurized feed air 10 is cleaned of high 
boiling impurities such as carbon dioxide and water vapor by passage 
through purifier 82 and resulting feed air stream 11 is cooled by indirect 
heat exchanger with return streams in main heat exchanger 5. 
The cryogenic rectification plant for the practice of the embodiment of the 
invention illustrated in FIG. 1 comprises a double column which includes 
lower pressure column 1 and higher pressure column 2, a first auxiliary 
column 3 having a first reboiler 7, and a second auxiliary column 4 having 
a second reboiler 8. A first portion 13 of the feed air, generally 
comprising from about 20 to 30 percent of feed air 80, is passed to second 
reboiler 8 wherein it is at least partially condensed against boiling 
column 4 bottom liquid. Resulting first feed air portion 16 is then passed 
through valve 83 and into higher pressure column 2. A fraction of stream 
16 may also be passed into lower pressure column 1. A second portion 12 of 
the feed air, generally comprising from about 70 to 80 percent of feed air 
80, is turboexpanded by passage through turboexpander 6 to a pressure less 
than that of first feed air portion 13 and within the range of from 50 to 
90 psia to generate refrigeration. Turboexpanded second feed air portion 
14 is passed to first reboiler 7 wherein it is at least partially 
condensed against boiling column 3 bottom liquid, and resulting second 
feed air portion 15 is passed into higher pressure column 2. If desired, a 
third feed air portion 70, generally within the range of from 1 to 5 
percent of feed air 80, may be cooled by indirect heat exchange in heat 
exchanger 50 and resulting stream 71 passed through valve 84 and into 
higher pressure column 2. 
Higher pressure column 2 is operating at a pressure within the range of 
from 50 to 90 psia. Within higher pressure column 2 the feed air is 
separated by cryogenic rectification into oxygen-enriched and 
nitrogen-enriched fluids which are passed respectively in streams 22 and 
20 through heat exchanger 50 and into lower pressure column 1 which is 
operating at a pressure less than that of column 2 and within the range of 
from 15 to 25 psia. High pressure nitrogen-richer vapor, having a nitrogen 
concentration of at least 97 mole percent, is passed as stream 17 into 
main condenser 9 wherein it is condensed against boiling column 1 bottom 
liquid. Resulting high pressure nitrogen-richer liquid is returned to 
column 2 in stream 19 as reflux. If desired, a portion 18 of stream 17 may 
be recovered as nitrogen gas product and/or a portion 24 of stream 19 may 
be recovered as nitrogen liquid product. 
Within lower pressure column 1 the input fluids are separated by cryogenic 
rectification into low pressure nitrogen-richer fluid, having a nitrogen 
concentration of at least 97 mole percent, and oxygen-richer fluid, having 
an oxygen concentration within the range of from 70 to 90 mole percent. 
Low pressure nitrogen-richer fluid is withdrawn from column 1 as vapor 
stream 40, warmed by passage through heat exchangers 50 and 5, and 
recovered as gaseous nitrogen stream 42. 
Oxygen-richer fluid is passed as first oxygen-richer fluid stream 25 from 
the lower portion of column 1 into the upper portion of first auxiliary 
column 3 which is operating at a pressure within the range of from 18 to 
30 psia. Within first auxiliary column 3 the first oxygen-richer fluid is 
separated by cryogenic rectification into second oxygen-richer fluid, 
having an oxygen concentration greater than that of the first 
oxygen-richer fluid and within the range of from 80 to 97 mole percent, 
and into remaining vapor which is passed from column 3 into column 1 in 
stream 26. 
Second oxygen-richer fluid is withdrawn from the lower portion of first 
auxiliary column 3 in stream 27, pumped to a higher pressure through 
liquid pump 51 and passed as stream 28 into the upper portion of second 
auxiliary column 4, which is operating at a pressure greater than that of 
first auxiliary column 3 and within the range of from 25 to 100 psia. 
Within second auxiliary column 4 the second oxygen-richer fluid is 
separated by cryogenic rectification into lower purity oxygen, having an 
oxygen concentration greater than that of the first oxygen-richer fluid 
and less than that of the second oxygen-richer fluid, and into higher 
purity oxygen, having an oxygen concentration greater than that of the 
lower purity oxygen. Lower purity oxygen is withdrawn from column 4 as 
stream 29, warmed by passage through main heat exchanger 5 and recovered 
as lower purity oxygen gas 30. If desired, a portion of stream 29 may be 
recycled back into first auxiliary column 3. Also, ballast tanks may be 
used, such as on lines 16 and/or 28, to enable flow variation in the lines 
without impacting the columns. 
Higher purity oxygen may be recovered from second auxiliary column 4 as 
either gas and/or liquid. FIG. 1 illustrates the recovery of higher purity 
oxygen in both gaseous and liquid forms. Gaseous higher purity oxygen is 
withdrawn from second auxiliary column 4 as stream 31, warmed by passage 
through main heat exchanger 5 and recovered as higher purity oxygen gas 
32. Liquid higher purity oxygen is withdrawn from second auxiliary column 
4 as stream 33, subcooled by passage through heat exchanger 50, and 
recovered as higher purity liquid oxygen 86. Some or all of stream 33 may 
be further processed to recover its rare gas, e.g. krypton and xenon, 
content. If desired some lower purity oxygen at lower pressure may be 
recovered from first auxiliary column 3. 
FIG. 2 illustrates another embodiment of the invention wherein added 
equipment is employed to gain enhanced flexibility. The numerals of FIG. 2 
correspond to those of FIG. 1 for the common elements and these common 
elements will not be described again in detail. 
Referring now to FIG. 2 cleaned, compressed feed air 11 is divided upstream 
of main heat exchanger 5 in streams 111 and 112. Stream 111 is compressed 
to a higher pressure, generally within the range of from 100 to 300 psia, 
by passage through compressor 87, cooled of heat of compression in cooler 
88 and passed as stream 113 through main heat exchanger 5 wherein it is 
cooled against return streams. The resulting stream forms first feed air 
portion 13 which is passed to second reboiler 8 and processed as 
previously described in conjunction with the embodiment illustrated in 
FIG. 1. Stream 112 is cooled by passage through main heat exchanger 5 
against return streams and resulting stream 72 is divided into stream 70, 
which is processed as previously described, and into second feed air 
portion 12 which is turboexpanded through turboexpander 6 and further 
processed as previously described. Flexibility is enhanced by this 
embodiment because the oxygen product pressure and refrigeration 
production are more independent. The air condensing pressure in reboiler 8 
and the inlet pressure to turboexpander 6 can be significantly different. 
FIG. 3 illustrates another embodiment of the invention which may be 
particularly useful with high energy costs. The numerals of FIG. 3 
correspond to those of FIGS. 1 and 2 for the common elements and these 
common elements will not be described again in detail. 
Referring now to FIG. 3, further compressed feed air stream 113, after 
passing through main heat exchanger 5 is not passed entirely to second 
reboiler 8. Rather this stream 133 is divided into first feed air portion 
13, second feed air portion 12 and additional feed air portion 70 which 
are processed as previously described in conjunction with the embodiment 
illustrated in FIG. 1. Stream 72 is passed into stream 14 downstream of 
turboexpander 6 and this resulting combined stream 89 comprising the 
second feed air portion is passed to first reboiler 7. In the practice of 
the embodiment illustrated in FIG. 3, feed air stream 11 may be at a lower 
pressure or oxygen stream 27 may be at a higher purity than those of the 
previously described embodiments. 
FIG. 4 illustrates another embodiment of the invention which is 
particularly advantageous when the oxygen product is required at a low 
pressure. In this embodiment a single large auxiliary column 90 is 
employed rather than two smaller auxiliary columns at separate pressures 
as are employed in the previously describe embodiments. Column 90 is 
operating at a pressure within the range of from 18 to 30 psia and has 
both first reboiler 7 and second reboiler 8. The numerals of FIG. 4 
correspond to those of FIG. 1 for the common elements and these common 
elements will not be described again in detail. 
Referring now to FIG. 4, oxygen-richer fluid is passed as stream 25 from 
the lower portion of lower pressure column 1 into the upper portion of 
auxiliary column 90 wherein it is separated by cryogenic rectification 
into lower purity oxygen, having an oxygen concentration which is greater 
than that of oxygen-richer fluid in stream 25, into higher purity oxygen 
having an oxygen concentration which exceeds that of the lower purity 
oxygen, and into remaining vapor which is returned to column 1 in stream 
26. Liquid and vapor flow directly between the upper portion and lower 
portion of column 90. Lower purity oxygen and higher purity oxygen are 
recovered as previously described. Lower purity oxygen may be recovered 
from column 90 either from a point above first reboiler 7, as illustrated 
in FIG. 4, or from a point below first reboiler 7, so long as it is from a 
point above the point where higher purity oxygen is recovered from 
auxiliary column 90. 
Now, by the use of this invention, one can effectively produce both higher 
purity oxygen and lower purity oxygen by the cryogenic rectification of 
feed air. 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.