The process of refrigerating a gas to a temperature below its critical temperature so that liquid can be formed at some suitable pressure, also below the critical pressure.
Gas liquefaction is a special case of gas refrigeration. The gas is first compressed to an elevated pressure in an ambient-temperature compressor. This high-pressure gas is passed through a countercurrent heat exchanger to a throttling valve or expansion engine. Upon expanding to the lower pressure, cooling may take place, and some liquid may be formed. The cool, low-pressure gas returns to the compressor inlet to repeat the cycle. The purpose of the countercurrent heat exchanger is to warm the low-pressure gas prior to recompression, and simultaneously to cool the high-pressure gas to the lowest temperature possible prior to expansion. Both refrigerators and liquefiers operate on this same basic principle. See also Compressor; Critical phenomena; Gas; Heat exchanger; Refrigeration.
An important distinction between refrigerators and liquefiers is that in a continuous refrigeration process, there is no accumulation of refrigerant in any part of the system. This contrasts with a gas-liquefying system, where liquid accumulates and is withdrawn. Thus, in a liquefying system, the total mass of gas that is warmed in the countercurrent heat exchanger is less than the gas to be cooled by the amount that is liquefied, creating an unbalanced flow in the heat exchanger. In a refrigerator, the warm and cool gas flows are equal in the heat exchanger. This results in balanced flow condition. The thermodynamic principles of refrigeration and liquefaction are identical. However, the analysis and design of the two systems are quite different due to the condition of balanced flow in the refrigerator and unbalanced flow in liquefier systems.
The prerequisite refrigeration for gas liquefaction is accomplished in a thermodynamic process when the process gas absorbs heat at temperatures below that of the environment. A process for producing refrigeration at liquefied gas temperatures usually involves equipment at ambient temperature in which the gas is compressed and heat is rejected to a coolant. During the ambient-temperature compression process, the enthalpy and entropy, but usually not the temperature of the gas, are decreased. The reduction in temperature of the gas is usually accomplished by heat exchange between the cooling and warming gas streams followed by an expansion of the high-pressure stream. This expansion may take place either through a throttling device (isenthalpic expansion) where there is a reduction in temperature only (when the Joule-Thomson coefficient is positive) or in a work-producing device (isentropic expansion) where both temperature and enthalpy are decreased. See also Enthalpy; Entropy; Isentropic process.
