Discussion on the law of Liquefaction of Gases
Introduction
This refers to 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 ambien-temperature compressor. This high pressure gas is passed through a counter-current heat exchange 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 counter-current heat exchanger is to warm the low pressure gas prior to re-compression, and simultaneously to cool the high pressure gas to the lowest temperature possible prior to expansion. Both refrigerators and liquefies operate on this same basic principle.
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 This contrasts with a gas liquefying system, where liquid accumulates and is withdrawn.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 counter current 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 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 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.
The laws of Liquefaction of Gases
Liquefaction of gases is the condensation of gases into a liquid form, which is neither anticipated nor explained by the kinetic molecular theory of gases. Both the theory and the ideal gas law predict that gases compressed to very high pressures and cooled to very low temperatures should still behave like gases, albeit cold, dense ones. As gases are compressed and cooled, however, they invariably condense to form liquids, although very low temperatures are needed to liquefy light elements such as helium (for He, 4.2 K at 1 atm pressure).
Liquefaction can be viewed as an extreme deviation from ideal gas behavior. It occurs when the molecules of a gas are cooled to the point where they no longer possess sufficient kinetic energy to overcome intermolecular attractive forces. The precise combination of temperature and pressure needed to liquefy a gas depends strongly on its molar mass and structure, with heavier and more complex molecules usually liquefying at higher temperatures. In general, substances with large van der Waals aa coefficients are relatively easy to liquefy because large a coefficients indicate relatively strong intermolecular attractive interactions. Conversely, small molecules with only light elements have small a coefficients, indicating weak intermolecular interactions, and they are relatively difficult to liquefy. Gas liquefaction is used on a massive scale to separate O2, N2, Ar, Ne, Kr, and Xe. After a sample of air is liquefied, the mixture is warmed, and the gases are separated according to their boiling points.
References
Law of Liquefaction of gases. Accesed from https://chem.libretexts.org/LibreTexts/United_Arab_Emirates_University/UAEU_CHEM_111_(Ashraf)/Full_Chapters/Full_Chapter_10%3A_Gases.
https://archive.org/stream/liquidairandthel033169mbp/liquidairandthel033169mbp_djvu.txt
0 Comments