08 - L iquefaction of Gases If the temperature and pressure...

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Liquefaction of Gases If the temperature and pressure of a gas can be brought into the region between the satur- ated liquid and saturated vapour lines then the gas will become ‘wet’ and this ‘wetness’ will condense giving a liquid. Most gases existing in the atmosphere are extremely superheated, but are at pressures well below their critical pressures. Critical point data for common gases and some hydrocarbons are given in Table 8.1. Table 8.1 Critical temperatures and pressures for common substances Substance Critical temperature Critical pressure, T,OC [KI Pc (bar) 374 [647] 32 [305] 96 [369] 153 [426] 31 [304] -82 [191] -130 [143] -243 [30] -147 [126] 221.2 46.4 49.4 43.6 36.5 89 51 13 34 Figure 8.1 depicts qualitatively the state point of oxygen at ambient conditions and shows that it is a superheated gas at this pressure and temperature, existing at well above the critical temperature but below the critical pressure. If it is desired to liquefy the gas it is necessary to take its state point into the saturated liquid-saturated vapour region. This can be achieved in a number of ways. First, experience indicates that ‘heat’ has to be taken out of the gas. This can be done by two means: (i) cooling the gas by heat transfer to a cold reservoir, i.e. refrigeration; (ii) expanding the gas in a reversible manner, so that it does work. This method is satisfactory if the liquefaction process does not require very low temperatures. A number of common gases can be obtained in liquid form by cooling. Liquefaction by cooling - method
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136 Liquefaction of gases Fig. 8.1 Entropy, S State point of oxygen at ambient temperature and pressure Examples of these are the hydrocarbons butane and propane, which can both exist as liquids at room temperature if they are contained at elevated pressures. Mixtures hydrocarbons can also be obtained as liquids and these include liquefied petroleum gas (LPG) and liquefied natural gas (LNG). A simple refrigerator and the refrigeration cycle are shown in Figs 8.2(a) and (b) respectively. 4 k Condenser Throttle Evaporator Id* b gj k! E c” I I IQin Entropy, S Fig. 8.2 A simple refrigerator: (a) schematic of plant; (b) temperature-entropy diagram Consider the throttling process in Fig 8.2, which is between 4 and 1. The working fluid enters the throttle at a high pressure in a liquid state, and leaves it at a lower pressure and temperature as a wet vapour. If the mass of working fluid entering the throttle is 1 kg then the mass of liquid leaving the throttle is (1 - x,) kg. If this liquid were then withdrawn to a vessel, and the mass of fluid in the system were made up by adding (1 - kg of gas at
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Liquefaction by cooling - method (i) 137 state 3 then it would be possible to liquefy that gas. The liquefaction has effectively taken place because, in passing through the throttle, the quality (dryness fraction) of the fluid has been increased, and the energy to form the vapour phase has been obtained from the latent heat of the liquid thus formed: the throttling process is an isenthalpic one, and hence energy has been conserved. If the working fluid for this cycle was a gas, which it was
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This note was uploaded on 03/09/2010 for the course MECHANICAL ME9802701 taught by Professor Prof.william during the Spring '10 term at Institut Teknologi Bandung.

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08 - L iquefaction of Gases If the temperature and pressure...

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