Consolidated heat exchanger air separation process

The present invention relates to the heat exchanger system in a process for the cryogenic distillation of air. In particular, the present invention is an improvement to the heat exchanger system to increase the operational efficiency of the process.

FIELD OF THE INVENTION 
The present invention relates to the heat exchanger system in a process for 
the cryogenic distillation of air. 
BACKGROUND OF THE INVENTION 
Processes which separate air via cryogenic distillation require a heat 
exchanger system in order to make the process workable and/or to achieve a 
power savings. The conventional heat exchanger system employs separate 
heat exchangers for each type of heat exchange service. For example, the 
heat exchanger system will at the very least include (1) a main or primary 
heat exchanger for cooling the feed air to a temperature near its dew 
point against other warming process streams and (2) a reboiler/condenser 
for condensing a nitrogen-rich gaseous overhead stream against a 
vaporizing oxygen-enriched liquid bottoms stream. The heat exchanger 
system will often further comprise a subcooler for subcooling a liquid 
process stream to a temperature lower than its bubble point. 
The problems with the conventional heat exchanger system include the high 
cost of purchasing separate heat exchangers as well as the pressure drop 
and costs associated with the piping connecting the heat exchangers. It is 
an object of the present invention to minimize these problems associated 
with the conventional heat exchanger system. 
SUMMARY OF THE INVENTION 
The present invention is an improvement to a process for the cryogenic 
distillation of air. In the process to which the improvement pertains, a 
feed air is compressed, cooled to near its dew point in a primary heat 
exchanger against other warming process streams and fed to a distillation 
column system having at least one distillation column. Also in the process 
to which the improvement pertains, a second heat exchange is performed in 
a reboiler/condenser between at least a portion of a nitrogen-rich gaseous 
overhead stream and at least a portion of an oxygen-enriched liquid 
bottoms stream whereby the nitrogen-rich gaseous overhead stream is 
condensed in the reboiler/condenser and the oxygen-enriched liquid bottoms 
stream is vaporized in the reboiler/condenser. The improvement is for 
increasing the operational efficiency of the process and comprises 
performing the reboiler/condenser's heat exchange service in the primary 
heat exchanger. 
Where the process further comprises subcooling a liquid process stream in a 
subcooler, the improvement can further comprise performing the subcooler's 
heat exchange service in the primary heat exchanger as well. Alternatively 
where the process further comprises a subcooler, the improvement can 
instead comprise performing the reboiler/condenser's heat exchange service 
in the primary heat exchanger and/or the subcooler.

DETAILED DESCRIPTION OF THE INVENTION 
To better understand the present invention, it is important to understand 
the prior art with respect to the heat exchanger system in a process for 
the cryogenic distillation of air. The conventional heat exchanger system 
employs separate heat exchangers for each type of heat exchange service. 
For example, the heat exchanger system will at the very least include (1) 
a main or primary heat exchanger for cooling the feed air to a temperature 
near its dew point against other warming process streams and (2) a 
reboiler/condenser for condensing a nitrogen-rich gaseous overhead stream 
against a vaporizing oxygen-enriched liquid bottoms stream. At least a 
portion of the condensed overhead stream is typically returned to the 
distillation column system as a reflux stream. The heat exchanger system 
will often further comprise a subcooler for subcooling a liquid process 
stream to a temperature lower than its bubble point. 
The problems with the conventional heat exchanger system include the high 
cost of purchasing separate heat exchangers as well as the pressure drop 
and costs associated with the piping connecting the heat exchangers. The 
present invention minimizes these problems by performing the 
reboiler/condenser's heat exchange service in the primary heat exchanger. 
Where a subcooler is present, the improvement can further comprise 
performing the subcooler's heat exchange service in the primary heat 
exchanger. Alternatively in the situation where a subcooler is present, 
the improvement can instead comprise performing the reboiler/condenser's 
heat exchange service in the primary heat exchanger and/or the subcooler. 
FIG. 1 is representative of an air separation process which incorporates 
the conventional heat exchanger system. As shown in FIG. 1, separate heat 
exchangers E1, E2, and E3 are used for the primary heat exchanger, the 
reboiler/condenser and the subcooler respectively. Referring now to FIG. 
1, a compressed feed air 10 which has been cleaned of impurities which 
will freeze out at cryogenic temperatures is cooled to near its dewpoint 
in primary heat exchanger E1 against other warming process streams. The 
resultant stream is fed to distillation column D1 in which the compressed, 
cooled feed air is rectified into a nitrogen-rich gaseous overhead stream 
12 and an oxygen-enriched liquid bottoms stream 14. A portion of stream 12 
is warmed in heat exchanger E1 and subsequently removed as a nitrogen-rich 
gaseous product in stream 16. The remaining portion of stream 12 is 
condensed in reboiler/condenser E2 and subsequently returned to the 
distillation column as reflux in stream 18. Stream 14 is subcooled in 
subcooler E3, reduced in pressure across valve V1, vaporized in 
reboiler/condenser E2, expanded in expander C1 to provide refrigeration 
for the process, warmed in subcooler E3, further warmed in primary heat 
exchanger E1 and subsequently removed as an oxygen-enriched gaseous 
product in stream 20. 
FIG. 2 is a first embodiment of the present invention as applied to the 
flowsheet depicted in FIG. 1. Similar streams and equipment in FIG. 2 
utilize common numbering with FIG. 1. Comparing FIG. 2 to FIG. 1, it can 
be seen that FIG. 1's reboiler/condenser E2 and subcooler E3 have been 
consolidated into FIG. 2's primary heat exchanger E4. 
FIG. 3 is a second embodiment of the present invention as applied to the 
conventional dual distillation column system comprising a high pressure 
column and a low pressure column. Referring now to FIG. 3, a compressed 
feed air 10 which has been cleaned of impurities which will freeze out at 
cryogenic temperatures is cooled to near its dewpoint in primary heat 
exchanger E1 against other warming process streams. The resultant stream 
is fed to high pressure column D1 in which the compressed, cooled feed air 
is rectified into a nitrogen-rich gaseous overhead stream 1 and a crude 
liquid oxygen bottoms stream 14. Stream 14 is reduced in pressure across 
valve V2 and subsequently fed to low pressure column D2 in which stream 14 
is distilled into a high purity nitrogen overhead stream 12 and an 
oxygen-enriched liquid bottoms stream 13. Stream 12 is warmed in the 
primary heat exchanger and subsequently removed as a high purity gaseous 
nitrogen product in stream 16. Stream 11 is condensed in the primary heat 
exchanger and subsequently split into streams 17 and 18. Stream 17 is used 
as reflux for the high pressure column while stream 18 is reduced in 
pressure across valve V3 and subsequently used a reflux for the low 
pressure column. Stream 13 is partially vaporized in the primary heat 
exchanger and flashed in flash drum F1. The vapor resulting from the flash 
is returned to the low pressure column as feed while the liquid resulting 
from the flash is reduced in pressure across valve V1, vaporized and 
partially warmed in the primary heat exchanger, expanded in expander C1 to 
provide refrigeration for the process, further warmed in the primary heat 
exchanger E1 and subsequently removed as an oxygen-enriched gaseous 
product in stream 20. 
The present invention provides a capital cost savings for air separation 
plants due to a reduction in the number of heat exchangers and 
interconnecting piping. A power savings is also achieved by the reduction 
of pressure drop associated with the interconnecting piping. 
The present invention has been described with reference to two specific 
embodiments thereof. These embodiments should not be viewed as limitation 
to the present invention, the scope of which should be ascertained by the 
following claims.