In a conventional air separation unit, oxygen and nitrogen products are withdrawn as low-pressure gases from the upper column. After rewarming 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) before delivery to the user.
In the liquid oxygen internal compression process, liquid oxygen product is withdrawn from the condenser-evaporator (main condenser), pressurized to the required pressure (~3.1 MPa) by a liquid oxygen pump, and then rewarmed and vaporized in heat exchangers before being supplied to the user. In other words, compression to the required pressure takes place inside the cold box.
Compared with the conventional process, the internal compression process offers the following characteristics:
No oxygen compressor required. Compressing liquid to the same pressure consumes far less power than compressing an equivalent quantity of gas. Moreover, the liquid oxygen pump is compact, simple in construction, and significantly less expensive than an oxygen compressor.
Safer than gaseous oxygen compression. Compressing liquid oxygen is inherently safer than compressing gaseous oxygen.
Enhanced safety against hydrocarbon accumulation. Because a substantial quantity of liquid oxygen is continuously withdrawn from the main condenser, hydrocarbons are less likely to concentrate there, contributing to safer operation of the equipment.
Higher heat exchanger cost. Since the oxygen stream is at high pressure during rewarming and vaporization, the oxygen passages in the heat exchangers must withstand high pressure. This increases heat exchanger costs compared with the conventional process, and the design must fully account for mechanical strength and safety.
Increased air compressor energy consumption. The vaporization of high-pressure liquid oxygen provides ample refrigeration, resulting in a larger temperature difference at the warm end of the heat exchangers-i.e., relatively higher cold loss. To maintain the cold balance, the feed air must be supplied at a higher pressure, which increases the energy consumption of the air compressor.
Generally speaking, the increased energy consumption of the air compressor is roughly offset by the reduced energy consumption of the liquid oxygen pump, or may result in a slight net increase. Equipment costs are also broadly comparable or slightly lower. However, from the perspectives of safety and reliability, the internal compression process offers clear advantages. With the application of variable-frequency liquid pumps, product oxygen and nitrogen flow rates can be adjusted with great flexibility, and product purity stability is excellent. Consequently, this process is widely adopted internationally at present.
