Principles of Vapor Permeation

Vapor permeation differs from pervaporation, as stated above, insofar as the feed mixture to be separated is supplied as a vapor. At least the more-permeable component is kept as close to saturation conditions as possible. Thermodynamically there is no difference between a liquid and ifs equilibrium vapor, the partial vapor pressure and thus the driving force for the transport through the membrane are identical and the same solution-diffusion mechanism is valid. However, the density of the vaporous feed and thus the concentration of molecules per volume is lower by two to three orders of magnitude than that of the liquid. As a consequence the membrane is usually less swollen than when in contact with a liquid feed. As the feed mixture getting in contact with the membrane is already in the vapor phase no phase change occurs across the membrane and thus no temperature polarization will be observed. Concentration polarization, however, is still an issue. Although the diffusion coefficient is much higher in a vapor than in a liquid, this is at least partially outbalanced by the lower density of the vapor, and therefore concentration polarization effects may be observed at all concentrations of the component to be removed. Minimum 

Today vapor-permeation processes are widely used in the dehydration of organic solvents, or in the removal of methanol from other organic components, or in the removal of VOCs from gas streams. In the literature the term Vapor permeation is often related to the removal of organic vapors ( VOCs ) from air or gas streams only. In these applications the more-permeable component is brought close to saturation by cooling, compression, or both pretreatment steps. Thus there is no real reason for such a narrow definition and the means by which the vapor has been produced has no influence either on the nature of the membrane or the mechanism of the separation process. 

The evaporator may be part of the plant, in many applications the saturated vapor comes from the top of an upstream distillation column. Thus a vapor permeation step may be coupled with one or more distillation columns in a so-called hybrid system. 

As can be seen from comparison of Figs. 3.8 and 3.10 the permeate-side arrangement remains unchanged and the same means for maintaining a sufficiently low partial pressure at the permeate side are used. 

Superheating of the vapor should be strictly avoided. When a vapor is superheated at constant pressure the partial pressures of its components are not increased. At low degrees of superheating the vapor in contact with the membrane will behave as if it were supplied at the respective lower saturation temperature. Larger degrees of superheating will result in a drop of the performance of the membrane even below the values observed at the equivalent saturation temperature, as the density of the vapor and the activity coefficients (or fugacity coefficients) of the components drop. 

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