In conventional air separation units, oxygen and nitrogen products are drawn from the low-pressure oxygen and nitrogen streams in the upper column. After being reheated in heat exchangers, they exit the cold box at an absolute pressure of approximately 0.12 MPa. An oxygen compressor then compresses the gas to the required pressure (3.1 MPa) for delivery to users.
The liquid oxygen internal compression process, by contrast, withdraws liquid oxygen product from the condenser-reboiler, compresses it to the required pressure (approximately 3.1 MPa) using a liquid oxygen pump, and then reheats and vaporizes it in heat exchangers before supplying it to users. In other words, compression to the required pressure occurs inside the cold box.
Compared to the conventional process, the internal compression process has the following characteristics:
No oxygen compressor required. Compressing liquid to the same pressure consumes significantly less power than compressing an equivalent quantity of gas. Additionally, liquid oxygen pumps are compact, structurally simple, and considerably less expensive than oxygen compressors.
Safer than gaseous oxygen compression. Liquid oxygen compression presents fewer safety risks compared to compressing gaseous oxygen.
Enhanced operational safety. The continuous withdrawal of substantial quantities of liquid oxygen from the main condenser prevents the accumulation and concentration of hydrocarbons, thereby supporting safer equipment operation.
Higher heat exchanger costs. Since the oxygen vaporizes and is reheated at elevated pressure, the oxygen passages in the heat exchangers must withstand high pressure, increasing equipment costs compared to the conventional process. Design must also carefully address structural integrity and safety considerations.
Increased air compressor energy consumption. The ample cold release during liquid oxygen vaporization results in a larger temperature difference at the warm end of the heat exchangers, representing relatively greater cold loss. To maintain cold balance, higher feed air pressure is required, increasing energy consumption in the main air compressor.
Generally speaking, the increased energy consumption of the air compressor roughly offsets-or slightly exceeds-the energy savings from the liquid oxygen pump. Equipment costs are also broadly comparable, or marginally lower. However, from the perspective of safety and reliability, the internal compression process offers clear advantages. With the application of variable-frequency liquid pumps, oxygen and nitrogen product flow rates can be adjusted with great flexibility, and product purity stability is improved. Consequently, this process is widely adopted internationally.
