Small air separation units and large air separation units not only have the distillation and separation process of air under the condition of deep cooling, but also the process of keeping the air separation unit in the deep cold state and making the cryogenic liquid products. Therefore, in addition to distillation process, air separation unit has a very important refrigeration process.
Unlike the general refrigeration process of heat pump, the refrigeration principle of air separation unit is isothermal enthalpy difference and expansion refrigeration. In the early stage of the development of deep cooling air separation technology, the refrigeration capacity required by air separation unit is completely from the isothermal enthalpy difference. Correspondingly, the air needs to be pressurized to a very high pressure. Later, adsorption purification and expansion refrigeration appear, and the air pressure is constantly reduced. At present, the air pressure of the double column process is generally 5-6αtm, the cooling capacity generated by the enthalpy difference of isothermal accounts for about 15% of the total cooling capacity, and the cooling capacity of expansion refrigeration accounts for 85% of the total cooling. If we want to increase the refrigeration capacity or make a large number of liquid products, the method is to increase the outlet pressure of the air compressor in small air separation units and large air separation units. The increase of air pressure increases the refrigeration capacity in two aspects, one is to improve the expansion ratio of expansion refrigeration, and the other is to improve the temperature before expansion of expansion refrigeration. However, due to the limitation of the air volume and expansion ratio of the expansion refrigeration, the quantity of liquid products can be produced is limited, which can only reach 10% of the oxygen output of the air separation unit. To further improve the quantity of liquid products, the double column process needs to be adjusted.
The outlet pressure of the air compressor in the new single column process is 5-6αtm, distillation and refrigeration will achieve high efficiency. In this case, the liquid products of air separation unit with new single column process reach 20-25% of the quantity of oxygen products. If the number of liquid products in the product plan is lower than this number, the simplest way is to reduce the outlet pressure of air compressor, but this method will greatly reduce the expansion refrigeration efficiency, This energy consumption advantage of distillation compared with the double column process will be offset by low refrigeration efficiency, so it is not feasible to realize the low liquid product scheme by directly reducing the outlet pressure of the air compressor.
In fact, from the perspective of distillation, the new single column process adopting the double heat pump process scheme is not essentially different from the upper column of the two column process, but the two column process adopts one-stop two heat pump distillation, while the new single column process adopts the direct double heat pump process. However, the efficiency of the two columns will decrease greatly. Before the end of expansion refrigeration, the two column process is reasonable. After the adsorption and purification technology of expansion refrigeration, the defects of the two column process have been exposed.
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