Air was considered not liquefiable a century ago. But nowadays, with the advanced technologies, not only air is liquefied, but it is also separated into nitrogen and oxygen under a cryogenic temperature of -320 F. In this video, you will be able to learn the process to separate oxygen and nitrogen from air with cryogenic method.
A typical air separation process includes a Main Air Compressor, a Booster Air Compressor, a Com-Pander that is a compressor driven by an expander, a Main Heat Exchanger, a subcooler, a MP Column, a LP Column, a Main Vaporizer to thermally link MP and LP column, a liquid nitrogen pump and a liquid oxygen pump.
Air is first compressed in the main air compressor to about 80 psia. After CO2 and water are removed in the purification beds, one part of the air is fed into the main heat exchanger, being cooled to its dew point of -280 F and then it is introduced to the MP column. The other fraction is further boosted in the MP Booster to about 280 psi. One portion is further boosted in the HP Booster to 950 psi and then it is liquefied into liquid form before sending to the MP and LP column. The other portion is compressed to about 390 psi in the compressor, which is driven by the expander. It is then sent to the main heat exchanger, cooled down to -178 F and then expanded to the MP column.
There are three outputs from the MP column: the pure liquid nitrogen, the lean liquid and the rich liquid. As in this example, pure liquid nitrogen is withdrawn at the top of the MP column. Part of the liquid nitrogen is pumped to about 610 psi and then the high pressure liquid nitrogen is vaporized into high pressure gaseous nitrogen product. The other portion of pure liquid nitrogen is subcooled in the subcooler before entering the LP column as the reflux. The lean liquid is also subcooled in the subcooler before sending to the appropriate location of the LP column. With the same way, the Rich Liquid is sent to the LP column as well. Besides, a portion of the liquid air is sent to the LP column to enhance oxygen recovery.
There are also three outputs from the LP column: the pure low pressure nitrogen, the waste nitrogen and the liquid oxygen. Both the low pressure nitrogen and waste nitrogen are sent to the subcooler and the main heat exchanger to recover the cold. The liquid oxygen is pumped to about 950 psi and then vaporized into high pressure gaseous oxygen product in the main heat exchanger.
Air Separation Unit is very energy intensive; the key to optimize the process is to optimize the heat exchange in the main heat exchanger and the distillation in the MP and LP column.
As you can see, the hot and cold curves are pretty much in parallel and there are three pinch points, and it is a good sign that the heat exchanger is optimized. In addition, the composition curves in the MP and LP column are very smooth and it is considered a good distillation.
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