Pervaporation enrichment factor

In the case of reverse osmosis, the enrichment factors (E and Ea) are less than 1.0, typically about 0.01, because the membrane rejects salt and permeates water. For other processes, such as dehydration of aqueous ethanol by pervaporation, the enrichment factor for water will be greater than 1.0 because the membrane selectively permeates the water. 

In the case of pervaporation of dissolved volatile organic compounds (VOCs) from water, the magnitude of the concentration polarization effect is a function of the enrichment factor. The selectivity of pervaporation membranes to different VOCs varies widely, so the intrinsic enrichment and the magnitude of concentration polarization effects depend strongly on the solute. Table 4.2 shows experimentally measured enrichment values for a series of dilute VOC solutions treated with silicone rubber membranes in spiral-wound modules [15], When these values are superimposed on the Wijmans plot as shown in Figure 4.12, the concentration polarization modulus varies from 1.0, that is, no concentration polarization, for isopropanol, to 0.1 for trichloroethane, which has an enrichment of 5700. 

Table 4.2 Enrichment factors measured for the pervaporation of VOCs from dilute solutions with silicone rubber spiral-wound modules...

FIGURE 9.10 Effect of feed sulfur content on sulfur enrichment factor for binary n-heptane-sulfur mixtures. (Reprinted from Separation and Purification Technology, 63, Zhao, C., Li. J., Qi, R., Chen, J., and Luan, Z., Pervaporation separation of n-heptane/sulfur species mixtures with polydimethylsiloxane membranes, 220-225, Copyright (2008), with permission from Elsevier.)... 

In pervaporation the components of a liquid solution permeate at different rates by dissolving and diffusing across a non-porous ( dense ) membrane and evaporating downstream from the membrane. This mechanism explains the term pervaporation, which derives from the permeation and evaporation steps of the process. In this way, a feed stream is separated into two streams the permeate and the retentate. The separation performance is determined by the flux and the selectivity, this latter being expressed as a separation factor or, alternatively, by an enrichment factor (Boddeker, 1990). 

DMMDA), to synthesize the polyimides (Scheme 3.23). Asymmetric polyimide membranes were prepared by phase inversion and the inner structure was analyzed by scanning electron microscopy (SEM). As shown in Fig. 3.14, the cross-sectional SEM image of the 6FDA-MDA membrane consisted of an ultrathin skin layer and a porous finger-like structure. The pervaporation properties of the prepared polyimides asymmetric membranes for n-heptane/thiophene mixtures were investigated at 40-77 °C. The permeation flux and sulfur enrichment factor of the polyimide membranes... 

The reactions were carried out over polymeric catalytic membranes consisting of poly(ether block amide) (PEBA) or blends of PEBA and poly(vinylpyrrolidone) (PVP) loaded with palladium nanoclusters. These membranes were used to simultaneously hydrogenate and concentrate the organic compounds in the permeate side by catalytic pervaporation. The pervaporative enrichment of the organics was achieved by a factor of 100, and a significant conversion was only obtained when the catalytic membranes were employed in pervaporation mode. 

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