Dense evaporation-condensation processes

The first separation example is seawater desalination. Traditionally, desahnation was done by distillation or simple evaporation/condensation [55]. Today, thermally driven desalination has been largely replaced by the membrane process reverse osmosis. In reverse osmosis an applied pressure exceeding the osmotic pressure of the salt solution causes water to permeate through a dense membrane. Hydrated salt ions are relatively large compared to water and have a lower permeability through the membrane resulting in relatively salt-free water being collected as the reverse osmosis permeate. 

In powder metallurgy, the powdered material to be worked is pressed in a mold, then heated to increase the rate of diffusion. The temperature required to obtain flow of the material may be significantly below the melting point. As the powder becomes more dense and less porous, the vacancies move to the surface to produce a structure that is even less porous and more dense. In addition to diffusion, plastic flow and evaporation and condensation may contribute to the sintering process. As sintering of a solid occurs, it is... 

Earlier in this chapter, we discussed isotopic fractionation during evaporation. Under appropriate conditions, where the condensed phase remains isotopically well mixed and the gas phase is removed from the system to prevent back reaction, Rayleigh distillation will occur (Box 7.2), resulting in a condensed phase that is isotopically heavy relative to the starting composition (Fig. 7.9). Isotopic fractionation can occur during both condensation and evaporation, as demonstrated by experiments (Richter el al., 2002). But it is not necessary that isotopes fractionate during evaporation or condensation. It depends on the details of the process. If evaporation occurs into a gas phase that is sufficiently dense, back reactions between gas and liquid can reduce the isotopic fractionation to near the equilibrium value, which is very small. For example, sulfur in chondrules does not show the isotopic fractionation (Tachibana and Huss, 2005) expected during evaporation from a liquid. Also, evaporation from a solid does not produce isotopic fractionation in the solid because diffusion is much too slow to equilibrate the few layers of surface atoms that are fractionated with the bulk of the material. 

The sodium sulfite precipitates out of the bisulfite solution and builds a dense slurry of crystals in the evaporator. A portion of this slurry is withdrawn from the evaporator and sent to a dissolving tank where water from the condenser system is added to dissolve the sulfite crystals. The resulting solution is sent to another surge tank and is then fed back into the absorber to complete the process loop. 

As explained in Sect. 38.2, to produce particles using a conventional spray pyrolysis (CSP) process, the precursor is first atomized into a reactor where the aerosol droplets undergo evaporation and solute condensation drying and thermolysis of the precipitate particles at higher temperature forms micro- or meso-porous particles, and, finally, sintering of these porous particles forms dense particles. However, sub-micrometer to micrometer-sized particles traditionally are formed using the CSP process based on the one-droplet-to-one-particle (ODOP) principle due to the difficulty of generating very fine droplets (below 1 pm) [1-3]. 

At this stage different situations can be found. In the case that solvent evaporation finishes before the coalescence begins, the film is formed by a monolayer of ordered pores with similar sizes (F). If the precipitating polymer employed stabilizes the condensed water droplets preventing the coagulation and there is still solvent available to be evaporated, the first layer of condensed droplets can sink into the solution (G). Besides, the density of the solvent used, if is more (i.e., carbon disulfide (CS2)) or less (i.e., benzene, toluene) dense than water, can be also a key point on the development of monolayer or multilayered stractures. As a consequence, the surface covered now with solvent can condense a new layer of water droplets. The result of this repetitive process is the formation of multilayers of pores (H). 

For evaporation, a sharp transition is possible because at T y the vapor pressure reaches the external pressure. A gradual transition from a dense fluid to a gas is possible above the critical temperature. Note also, that at pressures less than equilibrium, the condensed phases can gradually evaporate under nonequilibrium conditions. The equations of Fig. 3.7 do not apply to such irreversible processes (see Sect. 2.1.2). 

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