Dynamic Design of a Cryogenic Air Separation Unit

Abstract

We present the dynamic design of an air separation unit (ASU), which is used to separate oxygen, nitrogen, and argon products from air at very low temperatures. This unit can be used as an isolated plant to produce the products to be sold or as a unit joined to a larger plant, also called a "piggy back" plant, to produce products to be used in an additional process [1].  These gases are commonly used in refineries and oil recovery efforts. This process is convenient because the raw material is available at no cost and in limitless amounts. The primary cost in the process is compression. In order to minimize costs, heat integration is used throughout the process. This particular ASU will produce 1500 metric tons of 99.5% oxygen, 5000 metric tons of 99.5% nitrogen, and 58 metric tons of 95% crude argon each day to a customer. Heat integration will eliminate all costs from heating and cooling in the process. The compressor capital cost will be $16.5 MM¹. The venture guidance appraisal for this process is $118.5 MM. When running at full capacity, the plant will sell $113.9 MM worth of products. With a total annual cost for equipment and utilities of $39.0 MM, this plant will yield a yearly profit of $73.4MM. An integrated control structure has been designed to maintain product purity and reject atmospheric (temperature and pressure) disturbances, as well as flowrate disturbances. An overview of both a feed control and on demand scheme is included in the following pages, along with disturbance testing for both schemes.