The expansion and throttling of large air separation unit are the needs of refrigeration, and they play different roles respectively.
The refrigeration capacity is mainly produced by the expander, which is a low-pressure air separation unit. When the gas is expanded by external power through the expander, the internal energy will be consumed, and the effect of temperature drop is much greater than that of throttling. The higher the inlet temperature of the expander, the greater the effect. However, no liquid is allowed in the expander to avoid damaging the blades.
For medium pressure air separation unit, low temperature air enters the lower column, and throttling expansion is adopted to ensure that the air entering the column has a certain moisture content. For the expansion of cryogenic liquid, the energy loss of liquid throttling is small, and there is no significant difference between expander expansion and throttling expansion. The structure and operation of throttling valve are much simpler than expander. Therefore, throttling expansion is adopted when the liquid from lower column expands to upper column.
Therefore, the expansion and throttling refrigeration of large air separation unit are used together to meet the requirements of refrigeration capacity. The refrigeration capacity is adjusted by adjusting the refrigeration capacity of the expander, and the lowest temperature (- 193 ℃) in the air separation column is achieved by liquid throttling.
