In a conventional air separation unit, oxygen and nitrogen products are obtained as low-pressure gaseous streams from the upper column. After being rewarmed in heat exchangers, they exit the cold box at an absolute pressure of approximately 0.12 MPa. An oxygen compressor is then required to compress the gas to the desired pressure (3.1 MPa) for delivery to the user.
The liquid oxygen internal compression process, by contrast, extracts liquid oxygen product from the condenser-reboiler (main condenser). This liquid oxygen is compressed to the required pressure (approximately 3.1 MPa) by a liquid oxygen pump, then rewarmed and vaporized in heat exchangers before being supplied to the user. In other words, the compression to the required pressure takes place inside the cold box.
Compared with the traditional process, the internal compression process has the following characteristics:
No oxygen compressor is required. Compressing a liquid to the same pressure consumes significantly less power than compressing the same quantity of gas. Furthermore, the liquid oxygen pump is compact, structurally simple, and considerably less expensive than an oxygen compressor.
Compressing liquid oxygen is safer than compressing gaseous oxygen.
Since a large quantity of liquid oxygen is continuously withdrawn from the main condenser, hydrocarbons are less likely to concentrate there, contributing to safer operation of the equipment.
Because the oxygen vaporizes and is rewarmed at high pressure, the oxygen passages in the heat exchangers must withstand high pressure, which increases heat exchanger costs compared with the traditional process. In addition, the structural strength and safety of the heat exchangers must be fully considered during design.
The refrigeration available during liquid oxygen vaporization is abundant, resulting in a relatively large warm-end temperature difference in the heat exchangers-meaning the cold loss is comparatively high. To maintain the cold balance, a higher feed air pressure is required, which increases the energy consumption of the air compressor.
In general, 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 overall increase. Equipment costs are also roughly comparable, or may be slightly lower. However, from the perspective of safety and reliability, the internal compression process offers clear advantages. With the application of variable-frequency liquid pumps, the flow rates of oxygen and nitrogen products can be adjusted very flexibly, and product purity stability is also good. For these reasons, it is currently one of the most widely adopted processes internationally.
