Air separation units (ASUs) have successfully been applied to support gasification projects worldwide. ASU technology has ranged from traditional, low pressure, standalone facilities supplying products only to the gasification island, to highly integrated, elevated pressure facilities that obtain air feed from and inject excess nitrogen into a gas turbine. The near-term direction of ASUs is increased single unit capacity, process optimizations that will benefit integration with the new generation of higher pressure ratio and increased capacity gas turbines, and overall ASU facility optimization for the specialized requirements of shipboard units for remote gas conversion processes. Longer-term development is proceeding on ion transport membrane technology to support cost improvements for highly efficient production of oxygen, power, and steam.

Cryogenic air separation processes are selected based on an optimization of the capital cost and power consumption required to meet customer production specifications. Compressed air is cooled and cleaned prior to cryogenic heat exchange and distillation into oxygen, nitrogen, and optionally, argon rich streams. Pressurizing these streams for delivery is accomplished by gas compression, liquid pumping or combinations of pumping followed by compression. Product storage can be provided as backup or for "peaking" duty, supplying higher than design rates of product delivery for short periods of time.

Numerous options for the configuration of heat exchange, distillation, pumping and compression equipment are possible. These process alternatives are driven by the purity and number of product and byproduct streams to be produced, by tradeoffs between capital and power consumption, and by the degree of integration between the ASU and other facility units. Characterizing the types of cryogenic ASU processes, hereinafter referred to as cycles, can be based on the method of pressurizing the product stream(s) or on the air feed pressure to the ASU.

Traditional cycles are based on removing gaseous products from the distillation section, warming and then compressing the gaseous streams to delivery pressure. Many hundreds of the these plants have been built worldwide. Oxygen typically leaves the main heat exchangers at fairly low pressure, resulting in compressors with high inlet volumetric flows and sometimes requiring two or more compressor casings or bodies to delivery the product at the required pressure.

Liquid products may be removed from the distillation section and pumped prior to entering the cryogenic heat exchangers for vaporization and warming. Products may be pumped to their delivery pressure or pumped to an intermediate pressure and compressed to their final delivery pressure. Producing a liquid product from the distillation system requires three to five times the power of producing a gaseous product. Therefore, the refrigeration contained in a pumped product stream must be efficiently recovered. This is accomplished by condensing an air or nitrogen feed stream at high pressure against the vaporizing product stream in the cryogenic heat exchangers. The liquefied air or nitrogen feed returns the refrigeration to the distillation section.

Low pressure (LP) ASU cycles are based on compressing the feed air only to the pressure required to reject the majority of the nitrogen byproduct at atmospheric pressure. Feed air pressures will vary between 3.5 to 6 barg depending on the oxygen purity and the level of energy efficiency desired. Elevated pressure (EP) ASU cycles produce all product and byproduct streams at pressures well above atmospheric pressures. An EP cycle is chosen when all or nearly all of the nitrogen byproduct will be compressed as a product stream.