Flowsheet Optimization for Multi-Product Air Separation Units - Sponsored Whitepaper

FLOWSHEET OPTIMIZATION FOR MULTI-PRODUCT AIR SEPARATION UNITS

Bruce K. Dawson, Scott C. Siegmund, Zhang Yonggui, Air Products and Chemicals, Inc.

The First Baosteel Annual Academic Conference, Shanghai, China, May 27-28 2004

Abstract: An Air Separation Unit (ASU) produces oxygen and nitrogen by the cryogenic distillation of air. The majority of modern ASUs produce oxygen by the pumped LOX process (internal compression process), where the product oxygen is produced at an elevated pressure by boiling high-pressure liquid oxygen against high-pressure air. This flowsheet substitutes a Booster Air Compressor (BAC) for a product oxygen compressor. Some applications require a large quantity of nitrogen at an elevated pressure, as well as high- pressure oxygen. It is possible to produce nitrogen as a low pressure gas from the upper column of the ASU, as a medium pressure gas from the lower column, or as a high-pressure gas by pumping liquid nitrogen and warming it against high-pressure. A turbo expander is typically used to expand air or nitrogen from a higher pressure to a lower pressure to produce refrigeration for the process. Various expander configurations are possible. The best choice depends on the quantity and pressure of the high-pressure gaseous products, as well as whether or not liquid co-production is required.

The machinery selection has a significant effect on the total cost of the ASU, and therefore must be carefully considered when evaluating competing ASU flowsheets. When nitrogen is produced at elevated pressure, the optimum ASU flowsheet may require additional feed air compared to when the nitrogen is produced at low pressure and compressed externally. Minimizing the total number of compression stages often is the minimum capital cost solution. The applicability of vendor standard compression equipment may also be an important factor. This paper discusses the advantages and disadvantages of co-producing nitrogen in varying quantities via each of the methods listed. Possible turbo expander configurations are considered. Operability advantages and disadvantages of various alternatives are discussed. A recent case study for a large fertilizer project is presented to illustrate the method of evaluating several competing ASU flowsheets.