Methane latent heat

This concept is now applied to the liquefaction of methane initially at atmospheric pressure and 105°F, 105°F being selected because it is a common industrial heat rejection temperature. The theoretical quantity of work (expressed in Btu of work equal to 778 ft-lb, of work) required to cool 1 lb of methane down to its liquefaction point and then to absorb the 219.7 Btu of latent heat of liquefaction at -258°F, is shown in Figure 3-2. It amounts to 510.8 Btu of work per pound of methane and is not to be confused with Btu of heat, although the quantities in this case are not very different. This amount of work per pound of methane is equivalent to 352 hp/MMcfd. An actual process with its expected inefficiencies would require twice this much work. 

In the case of methane combustion around 1.61 kg of water vapor is produced per m of methane (see Figure 34.4). The specific latent condensation heat of water is 2.418 MJ/kg at 25°C, so per m of methane 4.00 MJ of condensation heat is available. Compared to the HHV of methane of 39.8 M]/w , this is 10.04%. Compared to the LHV it adds an extra 11.05%. As natural gas consists mostly of methane, the same numbers apply roughly to natural gas. For other fuels, the stechiometric combustion equations are different and therefore the water vapor and the maximum amount of latent heat are different. For oil-fired boilers this is around 6% and for propane it is 8-9%. 

One kmol/s of a gas consisting of 75 mol% methane and 25% n-pentane at 300 K and 1 atm is to be scrubbed with 2 kmol/s of a nonvolatile paraffin oil entering the absorber free of pentane at 308 K. Estimate the number of ideal trays for adiabatic absorption of 98.6% of the pentane. Neglect the solubility of methane in the oil, and assume operation to be at constant pressure. The pentane forms ideal solutions with the paraffin oil. The average molecular weight of the oil is 200 and the heat capacity is 1.884 kJ/kg-K. The heat capacity of methane over the range of temperatures to be encountered is 35.6 kJ/kmol-K for liquid pentane, is 177.5 kJ/kmol-K for pentane vapor, is 119.8 kJ/kmol-K. The latent heat of vaporization of n-pentane at 273 K is 27.82 MJ/kmol (Treybal, 1980). 

A carefully studied example is the transition which takes place in liquidheliumat about 2.2 K. Theheat capacity and density of liquid helium as functions of the temperature are shown in Fig. 22. In this instance at least it is known that the two phases are able to co-exist, not merely at a particular temperature and pressure, but along a p-T equilibrium curve, as in the ordinary phase transitions already discussed. Similar effects are known to occur in many solids, notably in alloys, in the crystalline ammonium salts, in polymers and in solidified methane and hydrogen halides. For example, in ammonium chloride there is a sharp break in the heat capacity curve at —30.4 GL Now in any given example, if it were knotm with certainty that the latent heat and volume change were vanishingly small, the entropy... 

The fuel conversion efficiency for methane conversion to hydrogen is 93.9 percent at the thermoneutral point, x = 0.44 (an ATR reaction) and 91.7 percent at x = 0 (the SR reaction). The difference between the two efficiency values is exactly equivalent to the loss represented by the latent heat of vaporization of the H2O that escapes with the combustions products in the SR burner exhaust. The concentration of hydrogen is 53.9 percent at x = 0.44 (ATR) and 80 percent at X = 0 (SR). 

The ratio of available sensible heat/latent heat varies from 0.79 for methane, down to 0.25 for propane. In general, if the ratio is less flian about 0.25, the vapour cooling effect becomes insignificant, and this is so for propane, the higher hydrocarbons and LPGs. 

Let us now look more closely at an element of superheated liquid mixture at the liquid/vapour interface during surface evaporation. The spent Uquid, after evaporating a methane rich vapour, has increased in density due to both evaporative latent heat of cooling and an increase in more dense ethane composition. The spent element is convectively unstable and sinks away from the surface. 

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