Cryogenic Air Separation

Cryogenic air separation process is one of the most popular air separation process, used frequently in medium to large scale plants. It is the most preferred technology for producing nitrogen, oxygen, and argon as gases and/ or liquid products and supposed to be the most cost effective technology for high production rate plants. In today's market scenario, all liquefied industrial gas production plants make use of cryogenic technology to produce liquid products.

What is cryogenic air separation process?
Cryogenic air separation is a process to produce highly purified gases or liquids and it is done by taking large volumes of air from the atmosphere, which is then compressed, cooled, liquefied. This is then separated into its major components by distillation.
In the cryogenic gas processing, various equipment is used like the distillation columns, heat exchangers, cold interconnecting piping etc. which operate at very low temperatures and hence must be well insulated. These items are located inside sealed "cold boxes". Cold boxes are tall structures with either round or rectangular cross section. Depending on plant type, size and capacity, cold boxes may have a height of 15 to 60 meters and 2 to 4 meters on a side.

Cryogenic air separation flow diagram
The cryogenic air separation flow diagram given below does not represent any particular plant and shows in a general way many of the important steps involved in producing oxygen, nitrogen, and argon as both gas and liquid products.

Steps in Cryogenic Air Separation
•    First Step: The first step in any cryogenic air separation plant is filtering and compressing air. After filtration the compressed air is cooled to reach approximately ambient temperature by passing through air-cooled or water-cooled heat exchangers. In some cases it is cooled in a mechanical refrigeration system to a much lower temperature. This leads to a better impurity removal, and also minimizing power consumption, causing less variation in plant performance due to changes in atmospheric temperature seasonally. After each stage of cooling and compression, condensed water is removed from the air.

•    Second Step: The second step is removing the remaining carbon dioxide and water vapor, which must always be removed to satisfy product quality specifications. They are to be removed before the air enters the distillation portion of the plant. The portion is that where the very low temperature can make the water and carbon dioxide to freeze which can be deposited on the surfaces within the process equipment. There are two basic methods to get rid of water vapor and carbon dioxide - molecular sieve units and reversing exchangers.

•    Third Step: The third step in the cryogenic air separation is the transfer of additional heat against product and waste gas so as to bring the air feed to cryogenic temperature. The cooling is usually done in brazed aluminum heat exchangers. They let the heat exchange between the incoming air feed and cold product and waste gas streams leave the air separation process. The very cold temperatures required for distillation of cryogenic products are formed by a refrigeration process comprising expansion of one or more elevated pressure process streams.

•    Fourth Step: This step involves the use of distillation columns to separate the air into desired products. For example, the distillation system for oxygen has both "high" and "low" pressure columns. Nitrogen plants can have one or two column. While oxygen leaves from the bottom of the distillation column, nitrogen leaves from the top. Argon has a boiling point similar to that of oxygen and it stays with oxygen. If however high purity oxygen is needed, it is necessary that at an intermediate point argon must be removed from the distillation system. Impure oxygen produced in the higher pressure distillation column is further purified in the lower pressure column. Plants which produce high purity oxygen, nitrogen or other cryogenic gases require more distillation stages.

•    Fifth Step: The fifth step involves refrigeration which is formed at cryogenic temperature levels. Refrigeration compensate for imperfect heat exchange and for heat leak into the cold equipment. The refrigeration cycle is almost similar like the one used in home and automobile air conditioning systems. One or more elevated pressure streams are reduced in pressure, which chills the stream. To maximize chilling, the pressure expansion or reduction takes place inside an expander. Gaseous products usually come out from the plant at relatively low pressures. In general, the lower the delivery pressure, the higher the plant efficiency. It is always cost effective to produce the cryogenic gas at low pressure and use a blower or compressor to achieve required delivery and gaseous storage pressures.

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