Oxygen is produced commercially in large quantities by the cryogenic rectification of feed air, generally employing the well known double column system, wherein product oxygen is taken from the lower pressure column. At times it may be desirable to produce oxygen at a pressure which exceeds its pressure when taken from the lower pressure column. In such instances, gaseous oxygen may be compressed to the desired pressure. However, it is generally preferable for capital cost purposes to remove oxygen as liquid from the lower pressure column, pump it to a higher pressure, and then vaporize the pressurized liquid oxygen to produce the desired elevated pressure product oxygen gas.
Cryogenic rectification requires refrigeration in order to operate. The requisite refrigeration is increased when oxygen is withdrawn from the column as liquid and pumped prior to vaporization because the pump work is added to the system. Refrigeration may be provided to the cryogenic process by the turboexpansion of a stream fed into the rectification column system. However, the compression of a stream for the turboexpansion consumes a significant amount of energy.
The problem is more acute when liquid product is also desired because the recovery of product as liquid removes a significant amount of refrigeration from the air separation plant.
Accordingly, it is an object of this invention to provide a cryogenic rectification system which can produce elevated pressure gaseous oxygen by the vaporization of pressurized liquid oxygen withdrawn from the cryogenic rectification plant while also enabling improved production of liquid product.