Thermodynamics

At temperatures above the critical-point value, a substance exists in thegas phase only. The critical temperatures of helium, hydrogen, and nitrogen(three commonly used liquefied gases) are 268, 240, and 147°C,respectively. Therefore, none of these substances exist in liquid form atatmospheric conditions. Furthermore, low temperatures of this magnitudecannot be obtained by ordinary refrigeration techniques. Then the questionthat needs to be answered in the liquefaction of gases is this: How can welower the temperature of a gas below its critical-point value?Several cycles, some complex and others simple, are used successfully forthe liquefaction of gases. Below we discuss the Linde-Hampson cycle,which is shown schematically and on a T-s diagram in Fig. 11–15.Makeup gas is mixed with the uncondensed portion of the gas from theprevious cycle, and the mixture at state 2 is compressed by a multistagecompressor to state 3. The compression process approaches an isothermalprocess due to intercooling. The high-pressure gas is cooled in an aftercoolerby a cooling medium or by a separate external refrigeration system tostate 4. The gas is further cooled in a regenerative counter-flow heatexchanger by the uncondensed portion of gas from the previous cycle tostate 5, and it is throttled to state 6, which is a saturated liquid–vapor mixturestate. The liquid (state 7) is collected as the desired product, and thevapor (state 8) is routed through the regenerator to cool the high-pressuregas approaching the throttling valve. Finally, the gas is mixed with freshmakeup gas, and the cycle is repeated.Chapter 11 | 6274Heatexchanger 3MultistagecompressorT5Q9Regenerator21Makeupgas54329168Vaporrecirculated768s7Liquid removedFIGURE 11–15Linde-Hampson system for liquefying gases.