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Mixed Purity Oxygen Facility

IP.com Disclosure Number: IPCOM000019411D
Publication Date: 2003-Sep-12
Document File: 3 page(s) / 74K

Publishing Venue

The IP.com Prior Art Database

Abstract

Large, multi-train ASU facilities enjoy significant cost savings through the purchase and erection of identical sets of equipment. In a prototypical study for a partial oxidation facility, six identical air compressors were headered together to feed four air pretreatment units. Air from these units split to feed four cold boxes and four booster air compressors for a total facility production of 18,000 short tons per day of oxygen at 97% purity. Although the predominant need was for 97% oxygen, some processes or facility locations may require the production of some oxygen at higher purities (99.5 to 99.8% purity). The high purity oxygen may coincide with production of some argon or other high purity byproducts such as LOX and LIN.

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Mixed Purity Oxygen Facility

         Large, multi-train ASU facilities enjoy significant cost savings through the purchase and erection of identical sets of equipment. In a prototypical study for a partial oxidation facility, six identical air compressors were headered together to feed four air pretreatment units. Air from these units split to feed four cold boxes and four booster air compressors for a total facility production of 18,000 short tons per day of oxygen at 97% purity. Although the predominant need was for 97% oxygen, some processes or facility locations may require the production of some oxygen at higher purities (99.5 to 99.8% purity). The high purity oxygen may coincide with production of some argon or other high purity byproducts such as LOX and LIN.

         Low pressure ASUs that reject most of the nitrogen byproduct to the atmosphere have air feed pressure requirements determined by the temperature approach of the reboiler/condenser located between the high and low pressure distillation columns. Assuming a constant temperature difference in the reboiler/condenser, as oxygen purity in the low pressure column increases the pressure of the high pressure column has to be increased so that the nitrogen will continue to condense against the boiling liquid oxygen. Therefore, as oxygen purity increases, high pressure column pressure increases, and air feed pressure also increases. For equivalent equipment and design constraints the air feed pressure for 97% oxygen is about 77 psia, while the air feed pressure for 99.5% oxygen is about 83 psia. A simplistic approach is to operate all main air compressors at the highest pressure required for the higher oxygen purity requirement and throttle the remaining air to the required lower pressure into the majority of cold boxes producing lower purity oxygen. Although workable in theory, the main air compressor or its driver is often a limiting factor in the design of these large facilities. Increased discharge pressure may reduce total throughput due to power or other limitations in the compressor or driver.

         One approach to maintain the main air compressor (MAC) discharge pressure at the level required for lower purity oxygen production is depicted in Figure 1.

         Figure 1 - Booster Air Compressor Modification

Pfl, the outlet pressure from the air pretreatment system, is maintained at the minimum value to produce 97% oxygen. A portion of this air flows to the booster air compressors (BACs). The BACs are sized to compress the flow of air required for operating the pumped liquid cycles for 97% oxygen production, plus the flow to feed a higher purity cold box and its pumped heat exchanger. About 75% of the higher purity cold box air feed is withdrawn after one stage of compression in the BACs. The higher feed pressure allows production of higher purity oxygen and potentially other byproducts such as argon, high purity liquid nitrogen and liquid oxygen.. Potential advantages of this option include:

1. MACs are iden...