Liquefaction of gases
The process of refrigerating a gas to a temperature below its critical temperature so that liquid can be formed at somesuitable 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 orexpansion engine. Upon expanding to the lower pressure, cooling may take place, and some liquid may be formed.
Thecool, low-pressure gas returns to the compressor inlet to repeat the cycle. The purpose of the countercurrent heat exchangeris to warm the low-pressure gas prior to recompression, and simultaneously to cool the high-pressure gas to the lowesttemperature possible prior to expansion. Both refrigerators and liquefiers operate on this same basic principle.
Phenomena
An important distinction between refrigerators and liquefiers is that in a continuous refrigeration process, there is noaccumulation of refrigerant in any part of the system. This contrasts with a gas-liquefying system, where liquid accumulatesand is withdrawn. Thus, in a liquefying system, the total mass of gas that is warmed in the countercurrent heat exchanger isless than the gas to be cooled by the amount that is liquefied, creating an unbalanced flow in the heat exchanger.
In arefrigerator, the warm and cool gas flows are equal in the heat exchanger. This results in balanced flow condition. Thethermodynamic principles of refrigeration and liquefaction are identical. However, the analysis and design of the twosystems 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 gasabsorbs heat at temperatures below that of the environment. A process for producing refrigeration at liquefied gastemperatures usually involves equipment at ambient temperature in which the gas is compressed and heat is rejected to acoolant.
During the ambient-temperature compression process, the enthalpy and entropy, but usually not the temperature ofthe gas, are decreased. The reduction in temperature of the gas is usually accomplished by heat exchange between thecooling and warming gas streams followed by an expansion of the high-pressure stream. This expansion may take placeeither 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 enthalpyare decreased.