Combined multistage membrane

Figure 19.16. Combined multistage membrane operations. (Hofflnan, 2003)...

Nanofiltration is used for water pretreatment in both thermal and membrane desalination processes. Desalination processes may rely solely on membrane processes (e.g., reverse osmosis in combination with nanofiltration, microfiltration or ultrafiltration [126]) or a combination of membrane and thermal (e.g., multistage flash [127]). 

Multistep and multistage membrane systems of the type shown in Figure 21.5 have been used to treat a number of vapor-gas streams. However, hybrid processes in which membrane separation is combined with another separation process, are attractive because they may enable each unit operation to operate in its preferred range, improving overall process efficiency. The most important of these hybrid processes is the combination of condensation under pressure with membrane separation, illustrated in Figure 21.6. In this process, a vapor-permeable membrane unit is combined with a vapor condensation-flash unit (Baker and Wijmans, 1994 Baker et al., 1998 Wijmans, 1993/1992). 

Ultrafiltration was applied to examine the size fractionation of Al, Ca, Cu, Fe, K, Na, and Pb in white and red wines [91]. Metal determinations were performed on the unfiltered wine, the 0.45 p,m membrane-filtered wine and each ultrafiltrate fraction. Aluminum was determined by ET-AAS, while FAAS was employed for Cu and Fe. An electroanalytical technique, stripping potentiometry, was selected for Pb measurement, whereas flame photometry was chosen for K and Na quantification. Fractionation patterns were evaluated and discussed. Castineira et al. [92] combined on-line tangential-flow multistage ultrafiltration with a home-built carbon analyzer and ICP-MS for size fractionation of nonvolatile dissolved organic compounds and metal species in three German white wines. The study showed that the major part of the elements investigated (up to 25) were dissolved in the size fraction of < 1 kDa, with the exception of Ba, Pb, and Sr, which also appeared in other fractions. 

It is theoretically possible to accomplish a complete separation of a pair of gases by membrane separation, either by very highly selective membranes combined with very low partial pressure of the permeating component on the permeate side of the membrane, or by multistage cascades. However, it usually is not economical to accomplish high purity and high recovery of both components of a binaiy by membrane separation alone. Therefore, it usually b necessary to couple membrane gas separation widi other processing steps when neatly complete separation te required. 

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