Selection of LNG refrigerant

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 excessive 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 heat of vaporization, and 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 chosen, 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.