Simtronics’ SPM-8300 Air Separation Plant simulator represents a typical commercial plant to produce very pure oxygen for the use in various process industries. Fairly pure nitrogen is produced as a byproduct. Oxygen and nitrogen are produced by chilling ambient air to very low temperatures where it liquefies. The liquefied air is then distilled at high and then low pressure to separate the oxygen and the nitrogen. The cold oxygen and nitrogen streams are warmed by exchanging heat with the feed air and the oxygen is delivered as vapor to a pipeline at battery limits.
Low temperatures are produced in the process by:
- compressing the air
- cooling the air with cooling water
- cooling the air with cold oxygen and nitrogen distillation streams
- extracting work from the high pressure air
- depressuring the air which causes it to auto-refrigerate (chill down)
No external refrigeration is required to chill the air.
Detailed Process Description
Air Compression Section (refer to Simulation PFD No. 1)
Ambient air is compressed to high pressure by Air Compressors K-101A/B which are identical 3-stage centrifugal compressors with water-cooled intercoolers between the first and second stages and the second and third stages (intercoolers not shown on PFD No. 1). The A compressor is driven by a steam turbine KT-101A and the B compressor is driven by an electric motor KM-101B. Only one compressor is normally in operation. Hot compressed air leaving the compressors is cooled in Aftercooler E-101 using cooling water. Condensate is separated from the cooled air in Condensate K.O. Drum D-101.
Cooled air at high pressure is then sent through one of the Purification Adsorbers R-101A/B to remove residual water, carbon dioxide and trace hydrocarbons present in the air. The Purification Adsorbers contain a bed of molecular sieves. Removal of these compounds is vital to prevent icing up of the downstream equipment and to prevent a potentially explosive situation within the process. The treated air then passes on to the Cold Section to be chilled. If an Adsorber bed becomes fully loaded and begins passing unacceptable concentrations of water, carbon dioxide or hydrocarbons, operation can be switched to the other Adsorber and the fully loaded bed can be regenerated using warm product nitrogen heated in Regeneration Heater E-102, which is electrically heated. Normally, E-102 is out of service.
The Deriming Heater E-104 uses low pressure steam to heat air from the Adsorbers that is used to de-ice the downstream equipment at shutdown or in case of suspected icing during normal operation. Normally, E-104 is out of service.
Cold Section (refer to Simulation PFD No. 2)
Air from Purification Adsorbers R-101A/B is first cooled in Main Exchanger E-104 by transferring heat with the product nitrogen stream from Subcooler E-105 and the product liquid oxygen stream from Liquid Oxygen Pumps P-101A/B (Simulation PFD No.3). The cooled air is moderately depressured in Expander KT-102 by driving Booster Air Compressor K-102. By depressuring and doing work to compress some of the feed air in K-102, the temperature of the air leaving KT-102 decreases significantly. A portion of this air is routed back to E-104 to cool down some more and is combined with bypass air and modestly compressed in K-102. The recompressed air from Booster Compressor K-102 is sent to the bottom of High Pressure Column C-101 and acts as a stripping agent to bring nitrogen upward in C-102. A large fraction of the air that leaves Expander KT-102 and sent to E-104 continues chilling and liquefying in E-104. This liquefied air is then sent to the bottom of High Pressure Column C-101.
In High Pressure Column C-101, nitrogen is stripped from the feed liquefied air and driven up the column which is trayed. Vapor rich in nitrogen reaching the top of C-101 is routed through the HP Condenser/LP Reboiler E-106 where it condenses by boiling liquid oxygen in the base of Low Pressure Column C-102. E-106 is immersed in the liquid oxygen which boils from the heat given up by the condensing nitrogen from C-101. The condensed nitrogen from E-106 returns by gravity as reflux to the top tray of C-101. A portion of the down-flowing liquid nitrogen reflux is removed at a point a few trays down from the top tray of C-101. This nitrogen-rich liquid is cooled in Subcooler E-105 using cold nitrogen product vapor from the top of Low Pressure Column C-102. The nitrogen-rich liquid is depressured which chills the stream further through auto-refrigeration and is used as reflux at the top of C-102.
Oxygen-rich liquid from the base of C-101 is sent to Subcooler E-105 to be chilled using cold vapor nitrogen from the top of C-102. It also is depressured thereafter, auto-refrigerates and is sent as feed to C-102 a few trays from the top of C-102. The flashed vapor from the two auto-refrigerated streams is mainly nitrogen which flows upward to the top of C-102, The remaining liquid in the auto-refrigerated reflux and feed streams fall toward the bottom and contact warmer oxygen-rich vapors rising up from E-106. Oxygen, being less volatile than nitrogen, concentrates as liquid in the bottom of C-102. The net oxygen product is taken off to the Liquid Oxygen Storage Tank T-101 (Simulation PFD No.3). Nitrogen product, meanwhile, leaves the top of C-102 as a vapor and is routed to E-105 and then to E-104 and then vented to atmosphere (Simulation PFD No. 1). If desired, a portion of the nitrogen can be taken off as liquid from the condenser return line of E-106 and sent to the Liquid Nitrogen Storage Tank T-102 (Simulation PFD No. 3).
Storage Section (refer to Simulation P&ID No. 3)
Product liquid oxygen is taken off from the base of Low Pressure Column C-102 and accumulated in Liquid Oxygen Storage Tank T-101. Liquid oxygen is pumped by Liquid Oxygen Pumps P-101A/B and mainly sent to Main Heat Exchanger E-104 (Simulation PFD No. 2) to be heated up and vaporized with warm air feed to E-105. A small portion of the liquid oxygen can be sent to sales storage at battery limit for bottling. In case of an upset of the unit, liquid oxygen can be vaporized in LOX Vaporizer E-108 using medium pressure steam, compressed in O2 Compressor K-103 and delivered to the oxygen pipeline (Simulation PFD No. 2) to meet contractual delivery obligations.
Liquid Oxygen Storage Tank T-101 is outfitted with LOX Storage PBU (Pressure Build-up Unit) E-107, which is a finned heat exchanger that will vaporize a small portion of the liquid oxygen in T-101 using heat from ambient to keep the tank pressure from falling too low (e.g. if the level in T-101 falls during vaporization of liquid oxygen while the Cold Section is not operating).
Product nitrogen is taken off from the condenser liquid return of E-106 and accumulated in Liquid Nitrogen Storage Tank T-102. This flow is normally very small compared to the net nitrogen flow from the top of Low Pressure Column C-102. T-102 is outfitted with LIN Storage PBU (Pressure Build-up Unit) E-109 to keep its pressure from falling too low. Liquid nitrogen is delivered as a vapor to utilities at battery limits by vaporization in Nitrogen Vaporizer E-110 which exchanges heat with ambient to vaporize the nitrogen.