Issues regarding the cooling capacity of air separation expanders

That is, when it expands to the same outlet pressure, the specific enthalpy drop is greater (in kJ/kg), which means that more heat is removed per unit mass of gas

Textbooks refer to it as “high temperature and high enthalpy drop”.

Reply to 1# freeballll: This is probably a property of the fluid itself; just take a look at a TS diagram to understand it

The expert pretends to be confused; there are annotations in the questions and answers for the oxygen producer； 108. What factors affect the cooling capacity of expansion mechanism refrigeration? Answer: The total cooling capacity Qp (kJ/h) of the expander is related to the expansion volume V (m3/h) and the specific cooling capacity △h (kJ/kmol): Qp = V△h/22.4 = V△ht•ηp/22.4. Here, the specific cooling capacity △h equals the product of the specific theoretical cooling capacity △ht and the efficiency ηp of the expander. The theoretical cooling capacity per unit depends on the pressure and temperature before expansion, as well as the pressure after expansion. Therefore, the relationship between the cooling capacity of the expander and various factors is as follows: 1) The greater the expansion volume, the greater the total cooling capacity. However, in low-pressure air separation units, the expanded air is directly fed into the upper column for distillation; an excessive amount of expanded air can affect the efficiency of distillation. This is undesirable in the separation process. 2) When the inlet and outlet pressures are constant, the higher the temperature before the machine, the greater the cooling capacity per unit. For example. When the absolute pressure before the expander is 0.55 MPa and the pressure after it is 0.135 MPa, the theoretical cooling capacity per unit volume at different temperatures before the expander is shown in Table 13: Table 13 Effect of temperature before the expander on theoretical cooling capacity. Temperature before expander, T1/K: 303, 273, 243, 213, 183, 163, 143; Theoretical cooling capacity per unit volume, △h/t, in kJ•kmol-1: 2850, 2470, 2300, 2010, 1720, 1510, 1300. However, as the temperature before the expander increases, the temperature after expansion also rises, and if the gas enters the upper column directly, it will disrupt the distillation process. During normal production, the increase in temperature is limited. 3) When the temperature before the machine and the pressure after it remain constant, the higher the pressure before the machine, the greater the unit cooling capacity. For example, when the inlet temperature of the expander is 160 K and the absolute outlet pressure is 0.135 MPa, the unit theoretical cooling capacity at different inlet pressures is shown in Table 14. Table 14 Effect of pressure before the expander on specific cooling capacity. Pressure before the expander, p1/MPa: 1.0, 0.9, 0.8, 0.7, 0.6. Specific theoretical cooling capacity, △ht/kJ•kmol-1: 1970, 1890, 1800, 1605, 1570. For low-pressure air separation units, the inlet pressure of the expander in the original process depends on the pressure in the lower column, that is, it is close to the pressure at the outlet of the air compressor. By adopting a booster turbine system, the expander is used to generate external work in order to drive the booster turbine, which compresses the expanded air from the air compressor; this allows the inlet pressure of the expander to be raised to around 1.0 MPa, thereby increasing the cooling capacity per unit volume. With a constant total cooling capacity required, the amount of air expanded can be reduced, which is beneficial for the distillation in the upper column. 4) The lower the pressure after the expander, the greater the pressure drop within the expander, and the higher the cooling capacity per unit. However, since the expanded gas enters the distillation tower, there is little room for pressure changes. 5) The higher the adiabatic efficiency of the expander, the greater the cooling capacity. 325. What is the effect of changes in the inlet temperature of the expansion machine on the cooling capacity? Answer: The inlet temperature of the expansion machine affects its cooling capacity per unit volume. For a given expander, with its flow channel dimensions fixed, the volume of gas that can flow through it (converted to standard conditions) will decrease as the temperature rises. Since the density of a gas is inversely proportional to the thermodynamic temperature T1, at a constant volumetric flow rate, the higher the temperature, the lower the mass flow rate. The gas flow rate within the deflector is proportional to the square root of the thermodynamic temperature T1; in other words, the higher the temperature, the greater the volumetric flow rate. Therefore, the actual amount of expansion is inversely proportional to the square root of the absolute temperature. For example, in a expander with Vo=7000 m3/h, the expansion amount refers to the conversion of the volumetric flow rate at a design inlet pressure of 0.55 MPa (absolute pressure) and an inlet temperature of to=-145°C (To≈128 K) into the volumetric flow rate V1 under standard conditions. If the inlet temperature is changed to T1=140K, then the expansion volume is V1=Vo(To/T1)1/2=7000×(128/140)1/2=6700 m3/h. The theoretical cooling capacity per unit of the expander increases as the inlet temperature rises. For example, when the inlet pressure is 0.55 MPa (absolute pressure) and the outlet pressure is 0.135 MPa (absolute pressure), the unit theoretical cooling capacity at different inlet temperatures is shown in Figure 73. It is basically proportional to the thermodynamic temperature. The total cooling capacity of the expander is proportional to the expansion amount and the unit theoretical cooling capacity. Although the expansion amount decreases in proportion to the square root of the intake air temperature, the unit theoretical cooling capacity increases in direct proportion to the increase in intake air temperature. Therefore, the total cooling capacity still increases in proportion to the square root of the intake air temperature. Therefore, increasing the temperature before the expander is beneficial for increasing the cooling capacity.

A high temperature in front of the machine means that the molecular kinetic energy is high, giving them a greater capacity to do work.

Simply put, the higher the temperature, the greater the volume work of the gas, and thus the stronger its capacity to do work. As the ordinary person in the field of chemistry says, essentially, the higher the temperature, the greater the internal energy contained, and thus the stronger its ability to be converted into kinetic energy.