Principal Mass-Transport Phenomena

Vapor permeation (VP) and pervaporation (PV) are membrane separation processes whose only difference lies in the feed fluid being a vapor (VP) or a liquid (PV), respectively. This difference has impHcations for feed fluid handling as well as the nature of the transport phenomena occurring in the feed stream, as in VP the feed fluid is compressible whilst in PV it is effectively not however, this does not in any way affect the transport phenomena across and after the membrane barrier. For this reason, vapor permeation and pervaporation will be discussed simultaneously, with differences being expHcitly emphasized where necessary. 

The principle of VP/PV is illustrated in Fig. 3.6-9. A non-porous membrane separates a vapor or liquid feed from a downstream compartment to which a vacuum is applied. The difference in total pressure between the two sides of the non-porous membrane, and pp , creates a chemical potential gradient of the compounds, Ap = - ppermeate which is the driving force of the process. Contrary to 

Solution-diffusion transport mechanism When the feed contacts the membrane, the solutes (denoted i in Fig. 3.6-10) adsorb on and subsequently absorb in the membrane surface by solute-polymer interactions (Fig. 3.6-lOA). Preferential solute-polymer interactions imply that the solvating power of the polymer is higher for the solutes than for the bulk solvent. 

Under ideal conditions, a thermodynamic equilibrium will be reached when the chemical potential of the solute i is equal at the membrane surface and the feed phase adjacent to it. The sorption of these solutes at the membrane surface creates a solute concentration gradient across the membrane, resulting in a diffusive net flux of solute across the membrane polymer (Fig. 3.6-lOB). In vapor permea-tion/pervaporation, any solute that has diffused toward the membrane downstream surface is ideally instantaneously desorbed and subsequently removed from the downstream side of the membrane (Fig. 3.6-lOC). This can be achieved either by applying a vacuum (vacuum vapor permeation/pervaporation), or by passing an inert gas over the membrane downstream surface (sweeping-gas vapor permea- 

Permeability According to the aforementioned, both the sorption and the diffusion of a solute determine the mass transfer across, and hence the separation characteristics of the membrane. The product of the diffusion coefficient and the sorption coefficient Sf is denoted the permeabihty of the membrane for component i and is commonly designated as Pf. In order to avoid confusing permeability and pressure, however, here it will be denoted (used commonly for the phenomenological constant) 

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