Managing Trace Contaminants in Cryogenic Air Separation - Sponsored Whitepaper

Atmospheric air contains many trace components which are process contaminants or equipment fouling agents in a cryogenic air separation process unit (ASU). Based on the potential problems that they may cause, the trace contaminants are categorized as corrosive, plugging or reactive species. This paper discusses a three step process to manage the trace contaminants as they move through the ASU: identify the hazard, abate the hazard and verify the abatement.

Guidelines that contribute to safe and efficient operation of ASU's are presented and summarized. Focus is placed on the reboiler sump, where high boiling components concentrate in high purity oxygen. The special needs of oxygen vaporizers, where the oxygen is boiled to dryness, are also discussed.

Nitrous oxide (N2O) is one of the trace contaminants present in the air and it has received special attention in the past few years. It has a low solubility at cryogenic temperatures, and can be present in sufficient quantities to plug equipment. Various methods used to abate N2O accumulation are discussed, including a novel adsorbent, together with relevant plant operating data.

Technology for separating air into its primary components (oxygen, nitrogen & argon) by cryogenic distillation has been practiced for over 100 years. A high degree of thermal integration is required for efficient production. Figure 1 is a basic flow diagram for the separation of air by cryogenic distillation. Air is compressed in the Main Air Compressor (MAC) to between 4 and 10 atm. It is then cooled to ambient temperature and passed through the Pre-Purification Unit (PPU). This consists of a pair of vessels containing a fixed bed of adsorbent, typically either or both activated alumina or molecular sieve. As the air passes over the adsorbent, many of the trace contaminants are removed, especially water, carbon dioxide, and the heavy hydrocarbons. The purified air then enters the main heat exchanger, where it is cooled to near its liquefaction temperature (approximately 100°K) before entering the distillation system. The products are produced from the Low Pressure (LP) column (the top column in Figure 1). The high pressure column's main function is to allow thermal integration by producing the boil-up and reflux for the low pressure column.

Oxygen is the highest boiling of the three main components, so it is taken from the bottom of the low pressure column. Nitrogen is taken from the top of the low pressure column. (Argon splits between the oxygen and nitrogen, and can be recovered as a pure product by adding a third distillation column.) The product streams are warmed to ambient temperature against incoming air to recover the refrigeration. It is also possible to remove the products from the distillation system as liquid if sufficient refrigeration is provided. Liquid may be retained (for back-up or merchant sales).