Cryogenic air separation system for enhanced liquid production
A system for separating air by cryogenic rectification whereby liquid production is increased by employing two separate turboexpanders, one which exhausts at a pressure no higher than that sufficient to feed the lower pressure column, the other which exhausts at a pressure no lower than that sufficient to feed the higher pressure column, and wherein one of the turboexpanders is fed with ambient temperature or modestly cooled feed air and preferably operates intermittently depending upon whether greater or lesser amounts of liquid product are desired.
1. A method for operating a cryogenic air separation plant employing a double column having a higher pressure column and a lower pressure column for rectifying feed air to produce a liquid product, said method comprising:
compressing a main feed air stream composed of the feed air to produce a compressed main feed air stream, cooling a part of the compressed main feed air stream in a main heat exchanger and introducing the compressed main feed air stream into the higher pressure column;
further compressing a first gas stream composed of another part of the main feed air stream, partially cooling the first gas stream within the main heat exchanger, passing at least the first gas stream at a first temperature of from 125K to 200K to a cold turbine, turboexpanding the first gas stream in the cold turbine to a pressure no greater than 3 psi higher than the operating pressure of the lower pressure colunm to produce a turboexpanded gas stream, and passing the turboexpanded gas stream into the lower pressure column;
further compressing a second gas stream, composed of yet another part of the compressed main feed air stream, the second gas stream being further compressed, at least in part, in a booster compressor and without being cooled in the main heat exchanger, removing heat of compression from the second air stream after passage through the booster compressor and then passing the second air stream at a second temperature of from 200K to 320K to a warm turbine driving the booster compressor, turboexpanding the second gas stream in the warm turbine to a pressure no lower than the operating pressure of the higher pressure column, and passing the turboexpanded second gas stream into least one of the cold turbine along with the first gas stream and an intermediate location of the main heat exchanger and thereafter the higher pressure column; and
modulating flow of the second gas stream or pressure ratio across the warm turbine to vary production of the liquid product.
2. The method of claim 1 wherein at least some oxygen product is recovered as the liquid product from the cryogenic air separation plant.
3. The method of claim 1 wherein at least some nitrogen product is recovered as the liquid product from the cryogenic air separation plant.
4. The method of claim 1 further comprising an argon column, passing fluid from the lower pressure column to the argon column, and recovering argon product from the argon column.
5. The method of claim 4 wherein at least some of the recovered argon product is recovered as the liquid product.
6. The method of claim 1 wherein the second temperature is from 280K to 320K.
7. The method of claim 1 wherein the operation of the warm turbine is turned on and off during the time the cold turbine is operating to modulate the flow to the warm turbine.