Latent heat of liquefaction

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. 

The assumption that the adsorbate has liquid-like properties after the first layer is difficult to reconcile, because both porous and nonporous (planar) adsorbents exposed to a saturated vapor sometimes adsorb strictly a limited amount and not the infinitely large quantity as postulated by BET. Another drawback of the BET model is essentially the coordination number of the molecules in the higher layers. The BET model assumes that each molecule that is adsorbed in any layer after the first layer, gives out its full latent heat of liquefaction, whether it has horizontal neighbors or not, and shall show a coordination number of 12. But in the absence of horizontal neighbors, the coordination number is much less that 12, and, therefore, the heat evolved (i.e., the heat of adsorption) should be only a fraction of the latent heat of condensation. 

CH3OH vapor, obtained by distilling in vacuo liquid methanol (Sigma-Aldrich), was rendered gas-free by several freeze-pump-thaw cycles. The vapor pressure of CH3OH at T = 303 K is 164 Torr, and the standard molar enthalpy of liquefaction (i.e. the latent heat of liquefaction, qi) is -AlH = 38kJ moP. ... 

Condensation is the process of reduction of matter into a denser form, as in the liquefaction of vapor or steam. Condensation is the result of the reduction of temperature by the removal of the latent heat of evaporation. The removal of heat shrinks the volume of the vapor and decreases the velocity of, and the distance between, molecules. The process can also be thought of as a reaction involving the union of atoms in molecules. The process often leads to the elimination of a simple molecule to form a new and more complex compound. 

Figure 1. A. Differential heat of adsorption of water at 303 K on (a) A50, (b) Tri, (c) Qzpl (d) Cris and (e) Qzm outgassed at 423 K for 2 h. B. Differential heat of adsorption of water at 303 K on (a) Qzm outgassed at 1073K (b) Qzpl, (c) QmHF and (d) Qzm. Dotted line indicates the latent enthalpy of liquefaction of H20, -AHL= 44 kj mol-1.

Molecules in condensing from a gas to a liquid come to a state of comparative rest and lose kinetic and latent energy, the energy being liberated as heat of liquefaction or condensation. Similarly, heat is liberated when a vapor condenses on a solid surface.1,2 Stronger forces are involved in condensation on a solid because this can occur under conditions that do not allow normal liquefaction to take place. Consequently, one might expect a greater quantity of heat to be liberated in adsorption than in normal liquefaction, and this is found to be so except in a few cases, e.g., mercury or water on charcoal.58... 

The liquid state in the reaction zone causes, as a rule, little concern from the viewpoint of heat loss, since the latent heat of fusion is small. It can, however, cause a form of convective heat loss that may have unfortunate consequences. In a flare that burns with the flame upward, such as a railroad fusee, a molten slag is sometimes normally permitted to drop off. By faulty formulation, an excessive liquefaction may take place so that not only the burned-out slag but the molten material in the burning zone itself may slide off and prematurely end the burning of the flare. Similar conditions may occur with flame-down burning candles under violent motion. 

Liqu action. The energy required for liquefaction is the sum of flie sensible heat removed from fiie gas in cooling it to the liquefaction temperature and the latent heat of condensation ... 

The molar latent heats of state change (vaporization or liquefaction) are equal for the two components of the mixture. 

Primitive considerations convince us that such secondary forces exist. For example, gases consist of disordered molecules, whether they be polyatomic like chlorine or ether vapour, or single atoms like helium or mercury vapour. But all gases, even helium, ultimately condense to liquids — and then to solids — if they are cooled and/or compressed sufficiently. When the molecules are forced into close proximity and have their kinetic energies diminished, the weak intermolecular forces are able to take control. Liquefaction results. The strength of these forces can be measured by the latent heat necessary to evaporate the liquid, or to sublime the solid. The equation,... 

Q = energy removed in liquefaction of a given quantity of gas, qv = sensible heat removed to reduce temperature to point of liquefaction, and A. = latent heat removed at constant temperature to condense gas. 

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