Liquefaction of Gases

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Liquefaction of gases

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 Critical phenomena

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 This contrasts with a gas-liquefying system, where liquid accumulates and is withdrawn.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 Enthalpy, Entropy, Isentropic process, Thermodynamic principles, Thermodynamic processes

Liquefaction of Gases

 

the transition of a substance from the gaseous state to the liquid. Liquefaction is accomplished by cooling gases below the critical temperature Tc with subsequent condensation as a result of the removal of the heat of vaporization (condensation). Cooling the gas below Tc is necessary to reach the range of temperatures in which the gas can condense (when T > Tc, a liquid cannot exist). Gas (ammonia) was first liquefied in 1792 by the Dutch physicist M. van Ma-rum. Chlorine was obtained in the liquid state in 1823 by M. Faraday, and oxygen in 1877 by the Swiss scientist R. Pictet and the French scientist L. P. Cailletet. Nitrogen and carbon dioxide were liquefied in 1883 by Z. F. Wróblewski and K. Olszewski. Hydrogen was first liquefied in 1898 by J. Dewar, and helium in 1908 by H. Kamerlingh Onnes.

Figure 1. Temperature-entropy diagram for an ideal cycle of gas liquefaction

An ideal process for the liquefaction of gases is illustrated in Figure 1. Isobar 1–2 corresponds to the cooling of the gas to the start of condensation, and isotherm 2–0 to the condensation of the gas. The area below l–2–0 is equivalent to the amount of heat that must be removed from the gas during its liquefaction, and the area enclosed by the curve l–2–0–3, where 1–3 is the isothermal compression of the gas and 3–0 the adiabatic expansion, characterizes the thermodynamically minimum amount of work Lmin required for liquefaction of the gas:

Lmin = T0(SGSL) – (JGJL)

where T0 is the temperature of the surroundings, SG and SL are, respectively, the entropies of the gas and liquid, and JG and JL are the heat content (enthalpies) of the gas and liquid.

The values of Lmin and the work Lact actually expended in the liquefaction of a number of gases are given in Table 1.

The industrial liquefaction of gases having critical temperatures Tc above the temperature of the surroundings, for example, ammonia and chlorine, is accomplished through compression of the gas in a compressor and a subsequent condensation of the gas in a heat exchanger cooled with water or brine. The liquefaction of a gas having Tc significantly below ambient temperature is accomplished by copious-cooling methods. More often, for the liquefaction of gases with low Tc, refrigeration cycles based on the throttling of a compressed gas (use of the Joule-Thomson effect), the expansion of the compressed gas with the production of external work in an expander, and the expansion of a gas from a constant volume without performing external work (heat-pump method) are used. In laboratory practice, multistage refrigeration (liquefaction) is sometimes used.

Figure 2. Scheme and temperature-entropy diagram for a gas-liquefaction cycle based on the Joule-Thomson effect: (C) compressor, (H1), (H2), and (H3) heat exchangers, and (Th) throttling valve

A design scheme and a diagram of the throttling cycle of gas liquefaction are given in Figure 2. After compression in the compressor (1–2), the gas is successively cooled in the heat exchangers (2–3-4) and then expanded (throttled) through the valve (4–5). Here, part of the gas is liquefied and accumulates in a collector, and the unliquefied gas is passed into the heat exchangers, where it cools fresh batches of compressed gas. To liquefy a gas by a throttling cycle, the temperature of the compressed gas before admission to the main heat exchanger H3 must be lower than the inversion temperature. A heat exchanger with a foreign cooling agent (exchanger H2) performs this cooling. If the inversion temperature of the gas lies above room temperature (nitrogen, argon, oxygen), then the scheme is basically feasible even without heat exchangers H1 and H2. The use of foreign coolants in this case has the purpose of increasing the yield of liquid. But if the inversion temperature of the gas is below room temperature, then a heat exchanger with a foreign coolant is mandatory. For example, liquid nitrogen is used in the liquefaction of helium.

Figure 3. Scheme and temperature-entropy diagram for a gas-liquefaction cycle using an expander: (C) compressor, (E) expander, and (Th) throttling valve

For liquefaction of gases on an industrial scale, cycles with expanders (Figure 3) are most often used, since the expansion of gases with the performance of external work is the most efficient cooling method. Here, the liquid usually is not obtained in the expander itself because it is technically simpler to carry out the liquefaction in an additional throttling stage. After compression in a compressor (1–2) and precooling in a heat exchanger (2–3), the stream of compressed gas is divided into two parts. Part M is diverted to the expander where, on expanding, it performs external work and is cooled (3–7). The cooled gas is fed into the heat exchanger, where it lowers the temperature of the remaining part of the compressed gas 1–M. This remaining part is then throttled and liquefied. In theory, expansion in an expander should be carried out at constant entropy (3–6). Because of losses, however, the expansion proceeds along line 3–7. To increase the thermodynamic efficiency of the process of gas liquefaction, several expanders operating at different temperature levels are sometimes used.

Cycles with heat pumps are usually used (in addition to expansion and throttling cycles) in liquefying gases by means of gas refrigerators, which make it possible to obtain temperatures as low as 12°K. These temperatures are low enough to liquefy all gases except helium (see Table 1). An additional throttling stage is attached to the refrigerator to liquefy helium.

Table 1. Values of the boiling point Tb (at 760 mm Hg), critical temperature Tc, and minimum (Lmin) and actual (Lact) work performed in the liquefaction of certain gases
GasTb (°K)TC (°K)L-min (kW-hr/kg)Lact (kW-hr/kg)
Nitrogen........77.4126.20.2201.2-1.5
Argon........87.3150.70.1340.8-0.95
Hydrogen .......20.433.03.3115-40
Air............78.8132.50.2051.25-1.5
Helium..........4.25.31.9315-25
Oxygen...........90.2154.20.1771.2-1.4
Methane.............111.7191.10.3070.75-1.2
Neon...........27.144.50.373-4
Propane...........231.1370.00.04∼0.08
Ethylene..........169.4282.60.119∼0.3

The gases subjected to liquefaction must be free of water vapor, oil, and other impurities. For example, carbon dioxide must be removed from air, and air from hydrogen. This removal is necessary because on cooling the impurities may solidify and block the heat-exchange apparatus. The unit that removes outside impurities from gas is therefore a necessary part of gas liquefaction facilities.

REFERENCES

Fastovskii, V. G., Iu. V. Petrovskii, and A. E. Rovinskii. Kriogennaia tekhnika, 2nd ed. Moscow, 1974.
Spravochnik po fiziko-tekhnicheskim osnovam kriogeniki, 2nd ed. Moscow, 1973.

A. B. FRADKOV

Liquefaction of gases

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 Compressor, 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 Thermodynamic principles, Thermodynamic processes

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