Evaporation and condensation

The formation of a liquid phase from the vapour at any pressure below saturation cannot occur in the absence of a solid surface which serves to nucleate the process. Within a pore, the adsorbed film acts as a nucleus upon which condensation can take place when the relative pressure reaches the figure given by the Kelvin equation. In the converse process of evaporation, the problem of nucleation does not arise the liquid phase is already present and evaporation can occur spontaneously from the meniscus as soon as the pressure is low enough. It is because the processes of condensation and evaporation do not necessarily take place as exact reverses of each other that hysteresis can arise. 

Thus, as pointed out by Cohan who first suggested this model, condensation and evaporation occur at difi erent relative pressures and there is hysteresis. The value of r calculated by the standard Kelvin equation (3.20) for a given uptake, will be equal to the core radius r,. if the desorption branch of the hysteresis loop is used, but equal to twice the core radius if the adsorption branch is used. The two values of should, of course, be the same in practice this is rarely found to be so. 

As Everett points out, however, the analogy of a pore as a narrownecked bottle is over-specialized, and in practice a series of interconnected pore spaces rather than discrete bottles is more likely. The progress of capillary condensation and evaporation in pores of this kind (cf. Fig. 3.13) has been discussed by de Boer, and more recently by Everett. ... 

K. T. Semrau and C. L. Witham, "Condensation and Evaporation Effects ia Particulate Scmbbiag," Preprint 75-30.1, 68th APPCA Annual Meeting, Boston, Mass., June 15—20, 1975. 

In the Philippines, the sale of filled milk had become 85% of the dairy products market by the early 1970s, reflecting convenience as a purchase incentive rather than price. Filled condensed and evaporated milk has a market share of 10, 27, 54, 69, and 77% in the Netherlands, Mexico, Malaysia, Phillipines, and Thailand respectively (30). Imitation cream has an 8% market share in the United States, 11% in Spain, and 33% in Canada (30). Areas in the world expected to show the greatest growth in the sale of substitute and imitation dairy products are Canada, Ireland, and Mexico (30). 

The optimization of heat-transfer surfaces also plays a role. At the optimum, the lifetime cost of a surface is approximately equal in value to the lifetime cost of power used to overcome the temperature differential in the condenser and evaporator. Additionally, condensation on insulation is a sign of questionable insulation (see Insulation, thermal). Frost is a certain signal that insulation can be improved. 

Thermal and mechanical design of heat exchangers (condensers and evaporators) is presented earlier in this section. 

Whenever corrosion resistance results from the accumulation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be either to prevent their normal formation or to remove them after they have been formed. Either effect allows corrosion to proceed unhindered. This occurs frequently in smaU-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of oends in pipe hnes, and on propellers, agitators, and cen-trifiig pumps. Similar effects are associated with cavitation and impingement corrosion. 

Figure 7.22 Pitting on a 304 stainless steel coupon caused by acidic, chloride-containing water condensating and evaporating. High chloride concentrations were produced locally.

L. Holland. Vacuum Deposition of Thin Films, Chapman and Hall, London (1966) TS 695 H6. J.P. Hii th and M. Pound. Condensation and Evaporation, Pergamon Piess New York (1963). J.M. Howe. Intetfaces in Materials, J. Wiley and Sons, New York (1997) QC 173.4.157 H68. A.V. Goldstein (ed.). Handbook of Nanomaterials, Marcel Dekker, New York (1997) TA418.9. 

Fig. 6 shows both the actual cycle (shown in dashed lines) and the idealised cycle, which consists of two isosteres and two isobars. Heat flows in J/kg adsorbent q) are shown as shaded arrows. For most purposes, analysis of the ideal cycle gives an adequate estimate of the COP and cooling or heating per kg of adsorbent. An accurate calculation of the path of the actual cycle needs information on the dead volume of the whole system and of the heat transfer characteristics of the condenser and evaporator. General trends are more apparent from an analysis of the idealised cycle. 

The balance equations for water vapor flows are similar to balance equations for contaminant flows, but in addition possible condensation and evaporation must be calculated. Also they must be considered in heat flow equations. 

Room air conditioner (self-contained) A rcxjm air conditioner complete with a direct expansion (dx) system condenser and evaporator fans, filtration, and thermostatic control. 

Despite these qualifications copper and its alloys are used extensively and successfully in much chemical equipment. Uses include condensers and evaporators, pipelines, pumps, fans, vacuum pans, fractionating columns, etc. Tin-bronzes, aluminium-bronzes and silicon-bronzes are used in some circumstances because they present better corrosion resistance than copper or brasses. 

Figure 9-3 shows this schematically. If the partial pressure of the vapor is less than the equilibrium value (as in Figure 9-3A), the rate of evaporation exceeds the rate of condensation until the partial pressure of the vapor equals the equilibrium vapor pressure. If we inject an excess of vapor into the bottle (as in Figure 9-3Q, condensation will proceed faster than evaporation until the excess of vapor has condensed. The equilibrium vapor pressure corresponds to that concentration of water vapor at which condensation and evaporation occur at exactly the same rate (as in Figure 9-3B). At equilibrium, microscopic processes continue but in a balance that yields no macroscopic changes.

Hirth, J.P. Pound, G.M. "Condensation and Evaporation Nuc-leation and Growth Kinetics", MacMillan Co. New York, 1963. 

If a gas molecule strikes an empty space on the elementary crystalline space lattice it condenses and is held for a period of time (t) on the surface before it evaporates. This period of time between condensation and evaporation or the mean life of the molecule on the surface may vary from a small fraction of a second to an almost indefinite period. In those cases where the strength of union between adsorbate and adsorbing agent is weak and where the adsorbed molecule readily receives sufficient kinetic energy from shocks, provided by bombarding gas molecules and pulses from the solid adsorbate, to equal the latent heat of evaporation. 

The rate of condensation of the gas on the uncovered portions of the surface will be a/t, whilst the rate of evaporation from the covered area will be v6 where v is the rate of evaporation from unit area. At equihbrium the rate of condensation and evaporation will be equal or... 

Interfacial transfer is the transport of a chemical across an interface. The most studied form of interfacial transfer is absorption and volatilization, or condensation and evaporation, which is the transport of a chemical across the air-water interface. Another form of interfacial transfer would be adsorption and desorption, generally from water or air to the surface of a particle of soil, sediment, or dust. Illustration of both of these forms of interfacial transfer will be given in Section l.D. 

The ideas of Wheeler that condensation and evaporation occur within a center core during adsorption and desorption and that an adsorbed film is present on the pore wall has led to the proposal of various methods for pore size analysis. In addition to the methods of Pierce and the BJH technique, other schemes have been proposed, including those by Shull, Oulton, Roberts, Innes, and Cranston and Inkley. These ideas are all based upon some assumption regarding the pore shape. 

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