In the mixture, isopentane, propane, ethylene – what are their properties?

They serve different purposes, operate in different temperature ranges, and there is no way to compare their advantages and disadvantages

The general selection criteria for refrigerants are as follows: 1. Moderate pressure and pressure ratio (not too high pressure, no negative pressure at low pressure, and not too large a pressure ratio). 2. High cooling capacity per unit volume and per unit mass. 3. Not too high exhaust temperature. 4. High coefficient of performance. 5. Strong thermal stability. All types of refrigerants provide cooling capacity, but they differ in terms of the temperature range they are suitable for. Ultimately, these were also selected after comparison by the original inventor, Mixed Refrigeration Company. Why are considerations based solely on the above 5 main points?

The composition of the mixed refrigerant is determined through calculations of material and heat balance, based on the properties of the raw gases and the process requirements. Typically, the mixed refrigerant is composed of nitrogen, methane, ethylene, propane, and isopentane. (1) Nitrogen: The main functions of the nitrogen component are, first, to increase the temperature difference at the cold end of the main cooling heat exchanger, and second, to raise the vaporization rate of the refrigerant, that is, to increase the flow rate of refrigerant to the low-temperature area in order to meet the cooling requirements of that area. (2) Methane: The main functions of the methane component are, first, to increase the vaporization rate of the refrigerant, and second, to increase the proportion of low-boiling-point components in the liquid refrigerant. Although nitrogen also has a low boiling point, its latent heat of vaporization is small; it is difficult to meet the cooling requirements in low-temperature areas relying solely on nitrogen. (3) Ethylene: The main function of the ethylene component is to meet the cooling capacity requirement at the cold end of the main heat exchanger. The second is to increase the amount of refrigerant liquefied. (4) Propane: The main functions of the propane component are, first, to meet the cooling capacity requirements at the hot side of the main heat exchanger, and second, to increase the amount of refrigerant that liquefies. To reduce power consumption, the proportion of propane in the mixed refrigerant cannot be too high; however, to provide enough cooling capacity to meet the process requirements without resulting in an excessively high gas flow rate of the refrigerant, the proportion of propane in the refrigerant cannot be too low either. (5) Isopentane: The main functions of the isopentane component are, first, to meet the cooling capacity requirements at the hot side of the main heat exchanger, and second, to increase the amount of refrigerant that liquefies. Generally, the higher the molecular weight, the greater the specific heat and the latent heat of vaporization, as well as the lower the adiabatic index. Therefore, isopentane has a strong heat absorption capacity, which results in a smaller amount of refrigerant circulation, reduced compression work, and also less compression work required for C5 itself. But there is a problem here: C5 has three isomers, n-pentane, isopentane, and neopentane. Why is it isopentane? It’s because only isopentane has a freezing point below -150 degrees; if another isomer were used, the cold box would definitely get clogged ； If isopentane is so good, then why can’t we use more of it? This is because at certain temperatures and pressures, the throttling effect of isopentane results in an increase in temperature rather than a decrease.

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