Compression in a cryogenic air separation unit has three main functions

One is to overcome the resistance to realize the transportation of distillation raw material air.

The second is the compression of the refrigerant in the expansion refrigeration cycle. In essence, it is reheated and compressed at room temperature. If the distillation raw gas of the cryogenic air separation unit is selected as the working medium of the expansion refrigeration cycle, the reheating process can be combined with the heat exchange process between the distillation raw gas and the distillation product. The working medium reheating normal temperature compressor of the expansion refrigeration cycle can also be used in combination with the distillation feed air delivery compressor (of course, it can also be set separately).

The third is the open heat pump cycle working fluid compression. There are many options for the open heat pump cycle working medium used in the cryogenic air separation unit. It can be air, nitrogen as a low-boiling product, or an intermediate product of the distillation process.

If air is selected as the working medium of the open-type heat pump cycle, when the circulation volume of the open-type heat pump is lower than the delivery volume of distilled air (the amount of air used for expansion and refrigeration needs to be deducted), the reheating process of the open-type heat pump cycle working medium can be compared with the deep In the cold air separation unit, the heat exchange process of the distillation raw material air and the distillation product is combined, and the heat pump cycle refrigerant compressor can also be combined with the air raw material to deliver the compressor, but it is still the reheating of the open heat pump cycle refrigerant at room temperature in essence. compression. In this case, the cryogenic compression of open heat pump circulation refrigerant air is generally not selected (in the current double air compressor single tower nitrogen production process scheme, since the air heat pump circulation volume is greater than the raw material gas delivery volume, the second air compression machine is cryogenic compression). If the open heat pump chooses a circulating working fluid other than air, then there are two options for the compression of the open heat pump circulating working medium: reheating normal temperature compression and low temperature compression, and there are only two options!

Until now, when I introduced the new single-tower process, the nitrogen was reheated and then compressed, of course to avoid unnecessary wrangling. The fact that nitrogen can be compressed at low temperatures is both theoretically and practically possible.

In fact, cryogenic compression is not uncommon. In the cryogenic air separation unit using LNG cold energy, nitrogen cryogenic compression is used to realize the conversion of LNG cold energy to the cryogenic air separation unit. This process is essentially an open heat pump liquefaction process. The nitrogen compression here can also be reheated and compressed at room temperature, combining its process with LNG gasification and reheating to improve the utilization efficiency of LNG cold energy (all LNG cold energy can be used).

It is very difficult to produce high-purity nitrogen by the twin-tower method. The main reason is that the purity of nitrogen is limited by the purity of liquid nitrogen in the lower tower, and the purity of liquid nitrogen in the lower tower is limited by the theoretical plate number of the lower tower and the reflux liquid-gas ratio! In order to realize the simultaneous production of high-purity nitrogen products by the double-tower process, one solution is to add a distillation section at the top of the upper tower. The nitrogen at the top of the newly added distillation section is compressed and condensed into liquid nitrogen at a low temperature in the condenser arranged at the inlet of oxygen-enriched liquid air or the inlet of liquid nitrogen in the lower tower, and then sent to the top of the tower to use the newly added rectifying section as reflux liquid, A high-purity nitrogen product is obtained at the top of the newly added rectifying section.

The comparison between reheated normal temperature compression and low temperature compression is a relatively complicated issue, which will be further analyzed later.