Natural gas hybrid refrigeration: I have a question – is the refrigerant a mixture of multiple components?

Methane, ethylene, nitrogen, isobutane, and isopentane (propane in some cases) – these five refrigerants are combined based on the gas composition and the heat exchange area of the equipment, to be used as a mixed refrigerant

Yes, in fact, a mixed refrigerant is composed of five different refrigerants, and cooling is generated through phase changes that occur separately for each component. So, inside the cold box, the refrigerants are mixed. In other words, five types of refrigerants are mixed together and circulated in the same chemical pipeline.

Actually, I don’t understand why so many things need to be mixed together – what’s the advantage of that?

The hybrid refrigeration process is the result of the development of refrigeration technologies. The original refrigeration processes were relatively simple, with a single type of refrigerant, but they were not suitable for large-scale chemical production, mainly due to efficiency and cost issues. To address this issue, the refrigerant was first improved; that is, since no ideal refrigerant could be found, people opted for a method of mixing multiple refrigerants instead of using a single ideal one, which led to the creation of mixed refrigerants. It meets people’s ideal needs such as energy conservation and reduced consumption, as well as gradual temperature reduction. If you need a table comparison, I suggest looking up a comparison between cascade refrigeration and hybrid refrigeration processes by searching on Baidu.

The components and proportions used vary depending on the operating conditions; if you want to know more in detail, you can refer to the cryogenic manual.

The composition of the mixed refrigerant is determined through material and heat balance calculations, based on the properties of the feed gas and 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 reduced amount of refrigerant circulation, less compression work, and also lower compression work 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 ； Since isopentane is so good, 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.

You guessed right; there are indeed 5 types of refrigerants, but 4 types of refrigerants can also be liquefied

The TP refrigeration process uses a mixed refrigerant; the refrigerant components in AP equipment are methane, nitrogen, propane, and isopentane.