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 T_c with subsequent condensation as a result of the removal of the heat of vaporization (condensation). Cooling the gas below T_c is necessary to reach the range of temperatures in which the gas can condense (when T > T_c, 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 L_min required for liquefaction of the gas:

L_min = T₀(S_G – S_L) – (J_G – J_L)

where T₀ is the temperature of the surroundings, S_G and S_L are, respectively, the entropies of the gas and liquid, and J_G and J_L are the heat content (enthalpies) of the gas and liquid.

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

The industrial liquefaction of gases having critical temperatures T_c 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 T_c significantly below ambient temperature is accomplished by copious-cooling methods. More often, for the liquefaction of gases with low T_c, 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.

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.