Exergy Analysis for Air Separation Process Under Off-Design Conditions

Abstract

An air separation unit (ASU) represents the largest overall energy consumption (about 15–20%) in a steel production facility. Therefore, improving the operating efficiency of an ASU is an effective way to achieve energy savings and emission reductions. The exergy calculation program for an air separation process is developed, and the detailed exergy calculations and analysis for an actual ASU with capacity of 40,000 Nm3/h in service at Tangshan Tangsteel Gas Co. Ltd. are performed. The results show that the molar exergy contained in oxygen is the largest among all gaseous products, liquid argon contains the largest molar exergy among all liquid products, and liquid products of the same type have larger exergy values than their gaseous equivalents. In a same condition scenario (same environmental condition, same air feed mass flow at rated load operation of the expander), increasing liquids production is an effective way to enhance the process efficiency, especially by increasing liquid argon production at the rated load operation of the expander. The object efficiency of the process from the cleaning unit to production in an actual 40,000 m3/h ASU is 46.84%, while the simple efficiency of the cold box of the ASU is 64.31%. The largest amount of exergy loss is caused by the air compressor (AC), the packed-type air cooling tower (PACT), and the molecular sieve (MS) purifier. The cryogenic ASU itself is well operated from an exergetic viewpoint. On the basis of exergy analysis conducted, this study provides a reference for the improvement of the ASU analyzed and provides a reference for ASUs in general.