Since the 1990s, several mature advanced oxygen production technologies have been applied to small-scale air separation units. These innovations reduce cold loss, improve efficiency, lower operating pressure, save energy, and simplify operation. The main improvements are as follows:
Plate-fin heat exchangers replace wound-tube heat exchangers. This enhances heat transfer performance, reduces metal material consumption, and lowers the hot-end temperature difference of the heat exchanger from the original 5–7 °C to 2–3 °C. The reduced cold loss from incomplete heat exchange allows the unit operating pressure to be decreased. After adopting narrow-channel plate-fin heat exchangers for the condenser-reboiler, the heat transfer temperature difference in the main condenser can be reduced from 2.5 °C to 1.25 °C, and the condensation pressure in the lower column can be reduced by 0.04 MPa, which helps lower the discharge pressure of the air compressor.
A direct-cooling air precooling system is adopted. To improve the adsorption performance of molecular sieves, air is precooled by a refrigeration unit before entering the purifier. To enhance heat exchange efficiency, the refrigerant directly exchanges heat with air in the evaporator, simplifying the cooling system and making operation more convenient.
Long-cycle adsorber sets are adopted. The switching time of the adsorbers is extended from 8 h to 24 h, greatly simplifying operation. The adsorbent charge increases by less than double, while the consumption of regeneration gas and regeneration power is also reduced.
The distillation column adopts a structured packing design (upper column), or a hybrid structure combining copper (circular-flow tray distillation column) with aluminum (heat exchangers, piping). However, the copper-to-aluminum welding issue must be resolved.
Vertical oil-free air compressors and oxygen compressors are adopted, greatly extending the service life of wear-prone parts and improving mechanical performance.
Gas-bearing turboexpander units replace piston expanders, improving expander efficiency.
