In the field of air separation, entropy is the core thermodynamic parameter to measure the energy quality and irreversibility of the system, which directly determines the energy consumption level and operation efficiency of the unit.
Entropy, with the symbol s, is essentially a measure of the degree of chaos and energy unavailability of a system, in kj/(kg ・ K). The second law of thermodynamics clearly states that the total entropy of an isolated system only increases but does not decrease, that is, the principle of entropy increase. The higher the entropy, the lower the energy quality, and the more difficult it is to convert into cold energy or useful work; The smaller the entropy, the more orderly the energy, and the more sufficient the available energy.
The essence of air separation unit is the combination of cryogenic refrigeration and distillation separation, and the whole process is closely related to entropy. The ideal reversible process is an isentropic process, and the entropy change Δ s=0. If the ideal expander operates without friction and heat loss, the refrigeration capacity reaches the theoretical maximum. In actual operation, irreversible factors such as pipeline resistance, heat transfer temperature difference of heat exchanger, equipment friction and gas leakage will cause entropy increase, which directly means the loss of cooling capacity and the increase of energy consumption.
In engineering, isentropic efficiency is commonly used to evaluate the performance of expanders. The closer the value is to 1, the smaller the irreversible loss and the higher the refrigeration efficiency. The core objective of the optimization of the air separation unit is to reduce the entropy increase in the whole process as much as possible: reduce the heat transfer temperature difference between the main heat exchanger and the subcooler, reduce the resistance of pipes and valves, improve the isentropic efficiency of the expander, and reduce the cold running and gas leakage.
Understanding the essence of entropy is the key to control the energy consumption of air separation. The smaller the entropy increase, the smaller the entropy loss, the lower the power consumption of the air compressor, and the more economical the operation of the device.
