Separation of fluid mixtures in multiple distillation columns

A fluid mixture is separated by distillation in a two column system in which the feed is prefractionated in a first column having at least one separation stage above the feed and the prefractionator bottoms provides feed to a second column operating at a lower pressure. Cooling for condensing the overhead vapor of the first column is provided by heat exchange with flashed prefractionator bottoms or with an intermediate fluid in the second column. The two-column system is readily combined with a high pressure column in a three-column distillation system for separating air which is particularly useful for integration with a gasification combined cycle combustion turbine system. Optionally, three nitrogen products can be produced at three different pressures.

TECHNICAL FIELD 
The invention pertains to the separation of fluid mixtures by multiple 
distillation columns, and in particular to the separation of air and the 
integration of the air separation process with a combustion turbine. 
BACKGROUND OF THE INVENTION 
Distillation is an important method for the separation of fluid mixtures in 
industries such as petroleum refining, organic and inorganic chemicals 
production, and the separation of atmospheric gases. Distillation is an 
energy intensive process, especially in the separation of low-boiling gas 
mixtures associated with cryogenic air separation, nitrogen rejection from 
natural gas, synthesis gas separation, and the separation of light 
hydrocarbons. In these separations, mechanical or electrical energy is 
utilized to supply the large amounts of driving force required to operate 
the separation equipment at temperatures far below ambient. It is 
desirable to improve the energy efficiency in such separations in order to 
improve the economics of recovering low-boiling gas products. 
Improved separation of such low-boiling mixtures has been achieved in the 
art by the use of multiple heat-integrated distillation columns. For 
example, in many cryogenic air separation processes two columns are 
operated at different pressures and are thermally linked such that 
condensing vapor at the top of the higher pressure column provides heat 
through indirect heat exchange for vapor boilup at the bottom of the lower 
pressure column. Such a method requires that the temperature difference 
between the heat source provided at the bottoms reboiler of the higher 
pressure column and the refrigeration provided to remove heat at the top 
condenser of the lower pressure column be much greater than the same 
temperature difference in a single column distillation process. 
Double-column distillation systems are well-known and widely used in the 
cryogenic separation of air. 
The use of a high pressure column with a low pressure column to reduce heat 
duty in distillation has been studied extensively in the art. 
Representative descriptions of such systems are given by A. W. Westerberg 
in a review article entitled "The Synthesis of Distillation-Based 
Separation Systems" in Computers and Chemical Engineering Vol. 9, No. 5, 
pp. 421-429, 1985 and in an article by N. A. Carlberg and A. W. Westerberg 
entitled "Temperature-Heat Diagrams for Complex Columns. 2. Underwood's 
Method for Side Strippers and Enrichers" in Ind. Eng. Chem. Res. 1989, 28, 
pp. 1379-1386. A characteristic of the systems described in these articles 
is that heat duty in the distillation columns is reduced by providing heat 
to the higher pressure column at a higher temperature and heat to the 
lower pressure column at a lower temperature. This characteristic results 
in the operation of the higher pressure column at temperatures greater 
than the temperatures in the lower pressure column. 
Cryogenic air separation systems can be integrated readily with combustion 
turbines, particularly in combination with the generation of electric 
power in combined cycle processes. The combustion turbine air compressor 
can provide compressed air for the turbine combustor as well as for the 
air separation system, and pressurized waste gas (typically nitrogen-rich) 
from the air separation system can be introduced into the combustor or 
expansion turbine to recover pressure energy and increase the overall 
system efficiency. Double-column air separation systems have been 
integrated with combustion turbines as disclosed in representative U.S. 
Pat. Nos. 4,224,045, 5,081,845, 5,251,451, and 5,257,504. 
Further improvement to the efficiency of cryogenic air separation systems 
can be realized by utilizing three integrated columns operating at three 
different pressures. Such systems are particularly useful when oxygen 
and/or nitrogen products are required at elevated pressures. U.S. Pat. No. 
5,231,837 discloses a triple column air separation system in which an 
intermediate pressure column operates between the high and low pressure 
columns. The intermediate pressure column is a stripping column fed at the 
top with partially vaporized liquid from the bottom of the high pressure 
column; reboiler duty to the intermediate pressure column is supplied by 
indirect heat exchange with vapor from the top of the high pressure 
column. Vaporized bottoms and flashed overhead condensate from the 
intermediate pressure column are fed to the low pressure column. A low 
pressure oxygen product, a high pressure nitrogen product, and a low 
pressure nitrogen product are recovered from the process. The process 
optionally is integrated with a combustion turbine wherein the two 
nitrogen product streams are compressed and introduced into the turbine 
combustor. 
U.S. Pat. No. 5,341,646 discloses a triple column air separation system in 
which feed to the intermediate pressure column is provided by both the 
overhead and bottom streams from the high pressure column and by a stream 
of cooled and partially condensed air. The intermediate column contains 
both rectification and stripping sections, and reboiler duty to the column 
is provided by compressed overhead vapor from the high pressure column. 
The potential for efficiency improvement in the separation of low-boiling 
gases is particularly favorable when the feed composition is such that the 
mole fraction of the desired lighter (more volatile) component to be 
recovered is significantly different from that of the desired heavier 
(less volatile) component to be recovered from the feed mixture, and when 
the products are required at elevated pressures. This combination of 
conditions is particularly applicable, for example, to air separation 
systems which supply oxygen at elevated pressures to hydrocarbon 
gasification processes operated in conjunction with combined cycle power 
generation systems. Multiple-column air separation systems with increased 
operating efficiency are desirable for use with such combined cycle power 
generation systems, and an improved triple-column air separation system 
for such an application is described in the following specification and 
appended claims. 
SUMMARY OF THE INVENTION 
The invention is a method for separating a fluid mixture containing at 
least one more volatile component and at least one less volatile component 
in which the fluid mixture is introduced at a first pressure as a first 
feed stream into a first distillation column having at least one 
separation stage above the feed point, and withdrawing from the column a 
first overhead vapor and a first bottoms liquid. The first bottoms liquid 
is flashed to a second pressure, and the resulting flashed first bottoms 
stream is fed to a second distillation column. A second overhead vapor 
enriched in the more volatile component and a second bottoms liquid 
enriched in the less volatile component are withdrawn from the second 
column. At least a portion of the first overhead vapor is condensed, and 
at least a portion of the resulting first condensate is returned to the 
first column as reflux. The cooling duty for condensing the first overhead 
vapor is provided by indirect heat exchange with either (1) fluid at an 
intermediate point in the second column or (2) at least a portion of the 
resulting flashed first bottoms stream prior to the second distillation 
column. 
Either (1) the temperature at the bottom of the first column is equal to 
the temperature at the bottom of the second column and the temperature at 
any other point in the first column is between the highest and lowest 
temperatures in the second column or (2) the temperature at any point in 
the first column is between the highest and lowest temperatures in the 
second column. No temperature in the first column is greater than the 
highest temperature or less than the lowest temperature in the second 
column. Typically the combined molar flow rate of the first and second 
overhead vapors is more than 50% of the molar flow rate of the first feed 
stream. 
Alternatively, an intermediate product consisting of a portion of the first 
condensate, a portion of the first overhead vapor, or portions of the 
first condensate and the first overhead vapor can be withdrawn from the 
first column. In this embodiment, the total molar flow rate of the first 
and second overhead vapors and the intermediate product is more than 50% 
of the molar flow rate of the first feed stream. A portion of the first 
condensate can be flashed and introduced into the second column at a 
location above the point at which the flashed first bottoms stream is 
introduced. Preferably, the more volatile component is nitrogen and the 
less volatile component is oxygen. 
In another general embodiment of the invention, a fluid mixture containing 
at least one more volatile component and at least one less volatile 
component is separated by introducing the fluid mixture at a first 
pressure as a first feed stream into a first distillation column having at 
least one separation stage below the feed point, and withdrawing therefrom 
a first overhead vapor and a first bottoms liquid. The first overhead 
vapor is condensed and least a portion of the resulting first condensate 
is returned to the first column as reflux; the remaining portion of the 
first condensate is pumped to a second pressure and introduced into a 
second distillation column. A second overhead vapor enriched in the more 
volatile component and a second bottoms liquid enriched in the less 
volatile component are withdrawn from the second column. Either (1) the 
temperature at the top of the first column is equal to the temperature at 
the top of the second column and the temperature at any other point in the 
first column is between the highest and lowest temperatures in the second 
column or (2) the temperature at any point in the first column is between 
the highest and lowest temperatures in the second column. No temperature 
in the first column is greater than the highest temperature or less than 
the lowest temperature in the second column. Typically the molar flow rate 
of the second overhead vapor is less than 50% of the molar flow rate of 
the first feed stream. 
In a specific embodiment of the invention, the first and second columns are 
operated in combination with a third distillation column wherein the first 
column operates as an intermediate pressure column, the second column 
operates as a low pressure column, and the third column operates as a high 
pressure column. The low, intermediate, and high pressure columns operate 
in a cycle in which a compressed feed stream containing oxygen and 
nitrogen, essentially free of additional components which would freeze in 
the cycle, is cooled to near its dew point. The feed optionally contains 
some argon. The resulting stream is fed into the high pressure column, and 
a high pressure vapor overhead and a high pressure bottoms liquid are 
withdrawn from the column. The high pressure bottoms liquid is cooled and 
flashed, and the flashed stream is fed into the intermediate pressure 
column at a point such that there is at least one separation stage above 
the feed. An intermediate pressure overhead vapor and an intermediate 
pressure bottoms liquid are withdrawn from the intermediate pressure 
column. The intermediate pressure bottoms liquid is flashed, and the 
resulting flashed stream is fed into the low pressure column at a point 
below the top of the column; a nitrogen-rich overhead vapor product is 
withdrawn from the low pressure column. Optionally, the intermediate 
pressure bottoms liquid is cooled before flashing. An oxygen-enriched 
product is withdrawn from the bottom of said low pressure column. 
Optionally, a portion of the high pressure overhead vapor can be withdrawn 
as a high pressure nitrogen-rich product. A portion of the intermediate 
pressure overhead vapor can be recovered as an intermediate pressure 
nitrogen-rich product. 
In a further embodiment, the separation process described above is utilized 
to separate air and is integrated with a combined cycle combustion turbine 
system. An air stream is compressed to a first pressure, a portion of the 
resulting pressurized air is combusted with fuel in a combustor, and the 
resulting combustion products are passed through an expansion turbine to 
produce shaft power; at least a portion of the shaft power drives a 
compressor to compress the air. Another portion of the resulting 
pressurized air is treated to remove essentially all impurities which 
would freeze in the air separation cycle, thereby providing the compressed 
feed air stream containing oxygen and nitrogen described earlier. The low 
pressure nitrogen-rich overhead vapor product is warmed to near ambient 
temperature, compressed to the first pressure, and combined with the 
portion of pressurized air at the first pressure prior to said combustor. 
In this manner the pressure energy in the low pressure nitrogen-rich 
product is recovered in the expansion turbine. 
Alternatively, a portion of the intermediate pressure overhead vapor is 
recovered as an intermediate pressure nitrogen-rich product which is 
warmed, compressed, and combined with the first compressed nitrogen-rich 
stream and the portion of pressurized air prior to the combustor. In this 
manner the pressure energy in the low pressure and intermediate pressure 
nitrogen-rich products is recovered in the expansion turbine. In another 
alternative embodiment, a portion of the high pressure overhead vapor is 
recovered as a high pressure nitrogen-rich product which is warmed, 
compressed, and combined with the other two nitrogen-rich product streams 
and the portion of pressurized air prior to said combustor. In this manner 
the pressure energy in all three nitrogen-rich products is recovered in 
the expansion turbine. 
In a variation of the above embodiment, air is separated in a three-column 
distillation system in which the overhead vapor from the intermediate 
pressure column is condensed by heat exchange with a flashed bottoms 
stream from the same column, and the resulting flashed stream is fed to 
the low pressure column. The three-column distillation system can be 
integrated with a combined cycle combustion turbine system in manner 
analogous to that discussed above.

DETAILED DESCRIPTION OF THE INVENTION 
The invention is an improved method for the separation of fluid mixtures, 
particularly low-boiling gas mixtures, by the use of integrated multiple 
distillation columns. In the most general embodiment of the invention, a 
multicomponent fluid mixture is initially separated in a prefractionation 
column followed by further fractionation in a second column, and the two 
columns are linked thermally in various configurations. The two columns 
can be integrated further with additional distillation columns. The 
multicomponent feed mixture is typically a vapor-liquid mixture from which 
a lighter or more volatile component and a heavier or less volatile 
component are recovered as products enriched in the respective components. 
For example, the more volatile component can be nitrogen and the less 
volatile component can be methane, wherein these components are recovered 
from nitrogen-light hydrocarbon mixtures in petroleum production. In 
another application, the more volatile component is nitrogen and the less 
volatile component is oxygen, for example in the cryogenic separation of 
air. The method of the present invention can be applied to other types of 
mixtures, but is particularly well-suited for the separation of 
low-boiling gases at below-ambient temperatures requiring large amounts of 
refrigeration. Typically the total molar flow rate of the product(s) 
enriched in the more volatile component is greater than 50% of the molar 
flow rate of the multicomponent feed mixture. 
The first embodiment of the invention utilizing two fractionation columns 
is illustrated in FIG. 1. Feed 101 is introduced into prefractionator 
column 103 which contains one or more separation stages in rectification 
section 105 above the feed point and reboiler section 107 below the feed 
point. Prefractionator column 103 is operated in combination with main 
distillation column 109, wherein column 103 operates at a higher pressure 
than column 109. Either (1) the temperature at the bottom of column 103 is 
equal to the temperature at the bottom of column 109 and the temperature 
at any other point in column 103 is between the highest and lowest 
temperatures in column 109 or (2) the temperature at any point in column 
103 is between the highest and lowest temperatures in column 109. No 
temperature in column 103 is greater than the highest temperature or less 
than the lowest temperature in column 109. At least a portion of overhead 
vapor 111 is condensed by indirect heat exchange with liquid in exchanger 
113 in the stripping section of column 109, and at least a portion of 
condensate 115 is returned as reflux 117 to column 103. A liquid product 
119 enriched in the more volatile component optionally can be withdrawn as 
desired from condensate 115. Optionally a portion 120 of overhead vapor 
111 can be recovered as a vapor product. If desired, both liquid product 
119 and vapor product 120 can be withdrawn. Optionally a portion 121 of 
condensate 115 is flashed or reduced in pressure across expansion valve 
123 and introduced into the top section of column 109 either at the top or 
a few stages below the top of the column. Liquid bottoms stream 125 is 
flashed across expansion valve 127 and introduced into column 109 at an 
intermediate point. Overhead vapor 129 from column 109 is condensed in 
condenser 131, a portion 133 of the condensate is returned to the column 
as reflux, and a portion is withdrawn as overhead product 135 which is 
enriched in the desired more volatile component in feed 101. Bottoms 
liquid product 139, which is enriched in the desired less volatile 
component, is withdrawn from column 109. Boilup vapor for column 109 is 
provided by reboiler section 137. Heat for reboiler sections 107 and 137 
can be provided in common by warm stream 141, or alternatively a separate 
heat source can be provided for each reboiler. Optionally, column 103 can 
have one or more stages in a stripping section below the feed point. 
Alternatively, a portion of feed 101 can bypass column 103 and pass 
directly into main column 109. Typically the total molar flow rate of one 
or more product streams 119, 120, and 135 (which are enriched in the more 
volatile component) is greater than 50% of the molar flow rate of feed 
101. 
A second embodiment of the invention is illustrated in FIG. 2. Feed 101 is 
introduced into prefractionator column 201 which contains one or more 
separation stages in rectification section 203 above the feed point and 
reboiler section 205 below the feed point. Prefractionator column 201 is 
operated in combination with main distillation column 207, wherein column 
201 operates at a higher pressure than column 207. Either (1) the 
temperature at the bottom of column 201 is equal to the temperature at the 
bottom of column 207 and the temperature at any other point in column 201 
is between the highest and lowest temperatures in column 207 or (2) the 
temperature at any point in column 201 is between the highest and lowest 
temperatures in column 207. No temperature in column 201 is greater than 
the highest temperature or less than the lowest temperature in column 207. 
Bottoms stream 209, which is enriched in the desired less volatile 
component in feed 101, is flashed across expansion valve 211 and at least 
a portion of the flashed stream is introduced into condenser section 213 
in which overhead vapor from column 201 is condensed by indirect heat 
exchange with the flashed bottoms stream. A portion 215 of the resulting 
condensate provides reflux to column 201, and another portion 217 can be 
withdrawn as desired as intermediate product 217 which is enriched in the 
desired more volatile component. Optionally, vapor product 218 can be 
withdrawn from column 201. If desired, both intermediate product 217 and 
vapor product 218 can be withdrawn. Optionally, another portion 219 of the 
condensate can be flashed across expansion valve 221 and introduced into 
column 207. Alternatively, no product 217 is withdrawn. Vapor 223 and 
liquid 225 from the boiling side of condenser section 213 are combined and 
introduced into column 207 at an intermediate point. Overhead vapor 227 
from column 207 is condensed in condenser 229, a portion 231 of the 
condensate is returned to the column as reflux, and a portion withdrawn as 
overhead product 233 which is further enriched in the desired more 
volatile component in feed 101. Bottoms liquid product 235, which is 
enriched in the desired less volatile component, is withdrawn from column 
207. Boilup vapor for column 207 is provided by reboiler section 237. Heat 
for reboiler sections 205 and 237 can be provided in common by warm stream 
239, or alternatively a separate heat source can be provided for each 
reboiler. Optionally, column 201 can have one or more stages in a 
stripping section below the feed point, but in such an option 
rectification section 203 typically will have more stages than the 
optional stripping section. Optionally, a portion of feed 101 can bypass 
column 203 and pass directly into main column 207. Typically the total 
molar flow rate of one or more product streams 217, 218, and 233 (which 
are enriched in the more volatile component) is greater than 50% of the 
molar flow rate of feed 101. 
An alternative embodiment of the invention is shown in FIG. 3. Feed 101 is 
introduced into prefractionator column 301 which contains one or more 
separation stages in stripping section 303 below the feed point and 
reboiler section 305 at the bottom of the column. Prefractionator column 
301 is operated in combination with main distillation column 307, wherein 
column 301 operates at a lower pressure than column 307. Either (1) the 
temperature at the top of column 301 is equal to the temperature at the 
top of column 307 and the temperature at any other point in column 301 is 
between the highest and lowest temperatures in column 307 or (2) the 
temperature at any point in column 301 is between the highest and lowest 
temperatures in column 307. No temperature in column 301 is greater than 
the highest temperature or less than the lowest temperature in column 307. 
Bottoms stream 309, which is enriched in the desired less volatile 
component in feed 101, is withdrawn as product from column 301. Overhead 
vapor from column 301 is condensed in condenser 313, a portion 315 of the 
resulting condensate is returned to column 301 as reflux, and another 
portion 317 is pumped to a higher pressure by pump 319 and fed into column 
307 at an intermediate point. Boilup in reboiler section 305 is provided 
by reboiler 321 which utilizes warmer intermediate vapor stream 323 from 
column 307 which is returned to column 307 as partially or totally 
condensed stream 325. Overhead vapor 327 from column 307 is condensed in 
condenser 329, a portion 331 of the resulting condensate is returned to 
column 307 as reflux, and the remainder 333 is withdrawn as overhead 
product 333 which is enriched in the desired more volatile component in 
feed 101. Boilup vapor for column 307 is provided in reboiler section 335 
by warm stream 337, and bottoms liquid product stream 339 is withdrawn 
which is enriched in the desired less volatile component in feed 101. If 
bottoms stream 309 of prefractionator column 301 contains a higher 
concentration of the desired more volatile component than bottoms stream 
339, then at least a portion of bottoms 309 can be pumped and introduced 
into the stripping section of column 307. Common cooling for condensers 
313 and 329 is provided by refrigerant stream 341; alternatively 
condensers 313 and 329 can be operated with separate refrigeration 
streams. Optionally, column 301 can have one or more stages in a 
rectification section above the feed point. Typically the molar flow rate 
of the product enriched in the more volatile component, i.e. stream 333, 
is less than 50% of the molar flow rate of feed 101. 
The general embodiments of the invention described above are particularly 
useful for the separation of low-boiling gas mixtures, for example in the 
separation of air to recover oxygen and/or nitrogen products. The first 
embodiment of the invention as discussed above with reference to FIG. 1 is 
utilized integrated separation process shown in FIG. 4. Compressed feed 
401 at a pressure of at least 50 psia containing oxygen and nitrogen, 
preferably air which has been compressed and subjected to pretreatment by 
known methods to remove essentially all contaminants which would freeze at 
cryogenic temperatures, is cooled and at least partially condensed against 
cold process streams in heat exchange zone 402 to yield cold high pressure 
feed 403. Refrigeration to the separation system is provided for example 
by compressing, cooling, and expanding a portion 404 of compressed feed 
401 in compander system 405 to provide a cold low pressure feed 406. 
Alternate arrangements are known in the art for providing refrigeration to 
the system for sufficient cooling of high pressure feed 403, and the 
present invention is not limited to any specific refrigeration method. 
The separation system of FIG. 4 comprises a high pressure distillation 
column 407, medium pressure fractionation column 408, and low pressure 
distillation column 410. Columns 407, 408, and 410 are fitted with trays, 
structured packing, or combinations thereof to promote vapor-liquid 
contacting and mass transfer within the columns. Medium pressure column 
408 corresponds to prefractionator column 103 of FIG. 1 and low pressure 
column 410 corresponds to main column 109 of FIG. 1. The integration of 
columns 407,408, and 410 will be clear from the following process 
description. Cold high pressure feed 403 at a temperature near its dew 
point enters the bottom of high pressure column 407 and liquid bottoms 
stream 409 is withdrawn therefrom and cooled against cold process streams 
in heat exchange zone 411. Cold stream 413 is flashed or reduced in 
pressure across expansion valve 415 and enters medium pressure column 408 
which contains one or more separation stages above the feed point. Liquid 
bottoms stream 417, which is enriched in oxygen, is cooled in heat 
exchange zone 411, flashed across expansion valve 419, and fed to an 
intermediate point of low pressure column 410. Boilup vapor for medium 
pressure column 408 is provided by indirect heat exchange in reboiler 421 
with portion 423 of high pressure column vapor overhead 425. Another 
portion 427 is withdrawn and warmed in heat exchange zone 402 to yield 
high pressure nitrogen-rich product 429. 
At least a portion of overhead vapor 431 from medium pressure column 408 is 
condensed in condenser 433 against liquid at an intermediate point in low 
pressure column 410, and portion 435 of the resulting condensate is 
returned to medium pressure column 408 as reflux. Another portion 437 of 
the resulting condensate is cooled in heat exchange zone 411, flashed 
across expansion valve 439, and fed as reflux at the top of low pressure 
column 410. Alternatively, condensate 437 can be warmed, flashed across 
expansion valve 438, and fed to column 410 at a point below the top of the 
column. Optionally, nitrogen-rich vapor 441 is withdrawn from medium 
pressure column 408, warmed in heat exchange zones 411 and 402, and 
withdrawn as medium pressure nitrogen-rich product 443. Overhead vapor 
from high pressure column 407 is condensed in reboiler-condenser 445 to 
provide boilup vapor in the bottom of low pressure column 410, and a 
portion 447 of the resulting condensate is returned to high pressure 
column 407 as reflux. Another portion 449 of the condensate is cooled in 
heat exchange zone 411, flashed across expansion valve 451, combined with 
medium pressure column condensate 437, and the combined stream is flashed 
across expansion valve 439 and fed into low pressure column 410. 
Cold low pressure feed 406 is introduced into low pressure column 410 which 
provides direct refrigeration for the integrated three-column process. 
Nitrogen-rich vapor 453 is withdrawn from low pressure column 410, warmed 
in heat exchange zones 411 and 402, and withdrawn as low pressure 
nitrogen-rich product 455. Oxygen-rich vapor 457 is withdrawn from low 
pressure column 410, warmed in heat exchange zone 402, and withdrawn as 
oxygen-rich product 459. The integrated columns 407, 408, and 410 operate 
in the respective pressure ranges of 50-350, 30-250, and 15-150 psia, and 
at any given operating condition the pressure in the high pressure column 
is higher than the medium pressure column which in turn is higher that in 
the low pressure column. Either (1) the temperature at the bottom of 
medium pressure column 408 is equal to the temperature at the bottom of 
low pressure column 410 and the temperature at any other point in medium 
pressure column 408 is between the highest and lowest temperatures in low 
pressure column 410 or (2) the temperature at any point in medium pressure 
column 408 is between the highest and lowest temperatures in low pressure 
column 410. No temperature in medium pressure column 408 is greater than 
the highest temperature or less than the lowest temperature in low 
pressure column 410. 
An alternative embodiment of the invention is given in FIG. 5, which is a 
modification of the process described above in reference to FIG. 4. One 
difference in this alternative embodiment compared with that of FIG. 4 is 
that reflux for the medium pressure column is provided in a different 
manner. A portion 507 of the liquid bottoms from medium pressure column 
501 is flashed across expansion valve 509 and provides refrigeration for 
condenser 511 in which at least a portion of overhead vapor from medium 
pressure column 501 is condensed as condensate 513. A portion 515 of the 
condensate provides reflux to medium pressure column 501 and the remaining 
portion 517 is warmed, flashed across expansion valve 519, and fed as 
impure reflux to low pressure column 503. Optionally, the liquid flashed 
across valve 519 can be fed as stream 520 to the top of low pressure 
column 503. The warmed liquid bottoms, after providing refrigeration for 
condenser 511, is introduced as stream 521 to an intermediate point of low 
pressure column 503. The alternative process of FIG. 5 is the same as the 
process of FIG. 4 in all other respects. Either (1) the temperature at the 
bottom of medium pressure column 501 is equal to the temperature at the 
bottom of low pressure column 503 and the temperature at any other point 
in medium pressure column 501 is between the highest and lowest 
temperatures in low pressure column 503 or (2) the temperature at any 
point in medium pressure column 501 is between the highest and lowest 
temperatures in low pressure column 503. No temperature in medium pressure 
column 501 is greater than the highest temperature or less than the lowest 
temperature in low pressure column 503. 
A number of alternatives are possible in the operation of the processes of 
FIGS. 4 and 5. Medium pressure column 408 and 501 are shown as 
rectification columns with one or more separation stages above the feed 
point; optionally these columns may include additional stages below the 
feed point. In FIG. 5, pure liquid nitrogen reflux optionally could be 
produced instead of impure reflux 517 and sent to the top of low pressure 
column 503 with reflux stream 449. Optionally, multiple reboilers could be 
used in low pressure columns 410 and 503 and/or medium pressure columns 
408 and 501 to improve efficiency. In another option, liquid nitrogen can 
be produced by expansion of one or more of the pressurized nitrogen-rich 
product streams. In yet another option, compander 405 or 523 can be 
operated to produce expanded air stream 406 or 525 at a medium pressure, 
and this air feed can be introduced into the medium pressure column. 
Either of the process cycles of FIGS. 4 and 5 can be integrated with a 
combustion turbine as used for example in a gasification combined cycle 
power generation system. FIG. 6 illustrates such an integration of gas 
turbine system 601 with cryogenic air separation system 603, wherein the 
air separation system operates according to the process of FIGS. 4 or 5. 
Air 605 is compressed by compressor 607, a portion 609 of the compressed 
air is reacted with fuel 613 in combustor 611, and the resulting hot, 
pressurized gas 615 is expanded in expansion turbine 617 to generate shaft 
power to drive compressor 607 and optionally electric generator 619. 
Another portion 621 of compressed air, optionally supplemented by 
auxiliary compressed air 623, is cooled and purified in front end cleanup 
system 625 wherein water, carbon dioxide, and other minor contaminants are 
removed by methods known in the art to eliminate freezing in air 
separation system 603. Purified compressed air feed 627 is separated 
therein as earlier discussed with reference to FIGS. 4 or 5, and oxygen 
product (typically greater than 80 vol % oxygen) is withdrawn for use for 
example in the gasification of carbonaceous material to produce fuel 613. 
Low pressure nitrogen 455, medium pressure nitrogen 443, and high pressure 
nitrogen 429 are withdrawn and utilized in combustion turbine system 601 
to recover residual pressure energy in the nitrogen streams. This is 
accomplished by compressing low and medium pressure nitrogen steams in 
first stage compressor 629, the discharge of which is combined with high 
pressure nitrogen 429 in second stage compressor 631 to yield compressed 
nitrogen stream 633 which is combined with compressed air 609 to combustor 
611. Alternatively, compressed nitrogen 633 can be introduced directly 
into combustor 611 or combined with hot, pressurized gas 615 prior to 
expansion turbine 617. Alternatively, only low pressure nitrogen 455 is 
produced and compressed in compressor 629. A combination of low pressure 
nitrogen 455 with intermediate pressure nitrogen 443 or high pressure 
nitrogen 429 can be produced if desired. Depending upon the operating 
pressures in air separation system 603 relative to combustion turbine 
system 601, only a single nitrogen compressor may be required. 
EXAMPLE 
To illustrate the advantage of using the triple column process of the 
present invention with combined cycle combustion turbine system of FIG. 6, 
the cycle of FIG. 4 of the present invention and the triple column process 
of earlier-cited U.S. Pat. No. 5,231,837 as illustrated in FIG. 7 were 
simulated by heat and mass balances using available simulation methods. 
Referring to FIG. 6 and utilizing the air separation cycle in FIG. 4 of the 
present invention, 100 lbmol/hr of contaminant-free pressurized air feed 
401 at 232 psia is separated to yield 21.9 lbmol/hr of oxygen 459 at 80.5 
psia which is used in a gasification process (not shown) to generate fuel 
gas 613 for gas turbine system 601. Nitrogen product streams 429, 443, and 
455 of FIG. 4 are produced at 77.6, 148.6, and 225.5 psia respectively. 
The three nitrogen streams are compressed by compressors 629 and 631 as 
shown to yield 78.1 lbmol/hr of pressurized nitrogen 633 at a 232 psia 
which is combined with combustion air 609 and introduced into combustor 
611. Product purity requirements for operation with the combustion turbine 
system of FIG. 6 are oxygen at 95 vol % purity and nitrogen containing 
less than 1 vol % oxygen. Thus oxygen product 459 at 80.5 psia contains 95 
vol % oxygen and nitrogen products 429, 443, and 455 contain less than 1 
vol % oxygen. 
The prior art triple column process of FIG. 7 utilizes high pressure column 
701, intermediate pressure column 703, and low pressure column 705 which 
are thermally linked by reboiler-condensers 707,709, and 711 respectively 
to separate 100 lbmol/hr of contaminant-free pressurized air feed 713 at 
232 psia. Air compander system 715 operating on portion 717 of air feed 
713 provides refrigeration to the system and cooled air stream 719 is fed 
to low pressure column 705. Nitrogen products 721 and 723 withdrawn from 
the system at 225.5 and 77.6 psia respectively; no intermediate pressure 
nitrogen is produced in this cycle. The two nitrogen streams are 
compressed as in FIG. 6 by compressors 629 and 631 to yield 78.1 lbmol/hr 
of pressurized nitrogen 633 at 232 psia which is combined with combustion 
air 609 and introduced into combustor 611. Oxygen product 725 at 80.5 psia 
contains 95 vol % oxygen and nitrogen products 721 and 723 contain less 
than 1 vol % oxygen. 
The results of the simulation indicate the key difference between the 
method of the present invention as shown in FIG. 4 and the method of U.S. 
Pat. No. 5,231,837 as shown in FIG. 7, namely, the relative flow rates of 
the nitrogen product streams. The present invention produces 61.9 lbmol/hr 
of low pressure nitrogen at 77.6 psia, 15.0 lbmol/hr of medium pressure 
nitrogen at 148.4 psia, and 1.2 lbmol/hr of high pressure nitrogen at 
225.5 psia. By comparison, the method of FIG. 7 produces 77.9 lbmol/hr of 
low pressure nitrogen at 77.6 psia and 0.2 lbmol/hr of high pressure 
nitrogen at 225.5 psia; no medium pressure nitrogen is produced. Both 
methods as applied to the combustion turbine cycle of FIG. 6 yield 78.1 
lbmol/hr of pressurized nitrogen 633 at 232 psia which is combined with 
combustion air 609 and introduced into combustor 611. Both methods also 
provide 21.9 lbmol/hr of oxygen 459 at 80.5 psia. 
The relative saving in total power required to provide pressurized nitrogen 
stream 633 and oxygen stream 459 for the present invention of FIG. 4 is 
readily calculated relative to the prior art method of FIG. 7 by 
##EQU1## 
which yields a relative power saving (RPS) of 0.039 or 3.9% for the 
present invention of FIG. 4. In the above expression, the numerator is 
proportional to the incremental nitrogen compression power saved by the 
method of the present invention compared with the method of FIG. 7 and the 
denominator is proportional to the total compression power required to 
compress atmospheric feed air to 232 psia for the air separation methods 
of the present invention or FIG. 7. 
This power saving is realized in the present invention chiefly because 
medium pressure nitrogen 441 of the required purity (i.e. containing less 
than 1 vol % oxygen) is withdrawn from medium pressure column 408, and 
this is possible because column 408 has one or more trays above feed 413. 
In the method of FIG. 7, however, a medium pressure nitrogen product of 
required purity is not possible because feed 727 enters medium pressure 
column 703 above the trays in that column. In addition, the method of the 
present invention produces more high pressure nitrogen than does the 
process of FIG. 7. 
Thus the present invention offers an improved air separation system which 
is more efficient than prior art methods for integration with a combustion 
turbine cycle. The operation of an intermediate pressure column in which 
the feed enters below the trays allows the withdrawal of a relatively high 
purity intermediate nitrogen product stream which requires less 
recompression for introduction into the turbine combustor. In addition, 
the method of the present invention produces a larger flow of 
high-pressure nitrogen which results in a reduced flow of low pressure 
nitrogen requiring recompression prior to introduction into the combustor. 
The essential characteristics of the present invention are described 
completely in the foregoing disclosure. One skilled in the art can 
understand the invention and make various modifications thereto without 
departing from the basic spirit thereof, and without departing from the 
scope of the claims which follow.