Page 352 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering
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 Encyclopedia of Physical Science and Technology  EN009K-419  July 19, 2001  20:57







              Membranes, Synthetic, Applications                                                          287

              salt-containing feed and almost salt-free permeate  transmembrane osmotic pressure difference between the
              streams, 
π, must be overcome to drive water to the per-  feed and product water (∼50 psi), the extrapolated water
              meate side. The osmotic pressure difference, 
π, between  flux is essentially zero.
              two solutions of different concentration is the pressure dif-  Application of RO for water production is now a
              ference that exists when there is no difference in chemical  well-accepted and economical process even for higher
              potential of water on the two sides of the membranes.  concentration seawater with osmotic pressures of over
                Neglecting convection effects, the solution-diffusion  300 psi. Malta, for example, has evolved an economical
              model gives the following expressions for water (1) and  reliable application of this technology to produce 60%
              salt (2) molar fluxes through a membrane with a selective  of  its  potable  water  supply  (Lamendolar  and  Tua,
              layer thickness of L and a transmembrane pressure drop  1995).
                                                                                              +
              
p (Merten, 1966):                                  Rejections of other ions besides Na and Cl are tun-
                                                                                                     −
                                 ˆ
                      J A = D A K A V A [
p − 
π]/LRT,  (9a)    able characteristics of the reverse osmosis membranes that
                                                                depend upon the intrinsic nature of polymer separating
                      J B = D B K B 
C B /L,            (9b)    layer and how it has been processed. In general, bivalent
                                                                ions like Ca 2+  and SO 2−  are more easily rejected than are
              where D A and K A and D B and K B are the diffusion coef-          4
                                                                monovalent ones like Na and Cl .
                                                                                    +
                                                                                           −
              ficient and partition coefficients for water and salt in the
              membrane, respectively. The partial molar volume of wa-
                 ˆ
              ter, V A , is generally well approximated by the pure com-
              ponent molar volume. The observed salt rejection coeffi-  II. MEMBRANE MATERIALS, GEOMETRY,
              cient is given in terms of external bulk salt concentrations  AND PACKAGING
                      2
              (moles/cm ) and known fluxes as shown below:
                                                                A. Membrane Material Selection
                       C B 		      j B         j B
               R o = 1 −   = 1 −       ≈ 1 −         .  (10)
                      C  bulk   C bulk      C bulk  ˆ           Membranes used for separation are thin selective barriers.
                        B         B  j v     B  j A V A
                                                                They may be selective on the basis of size and shape,
                Increasing the (
p − 
π) term in Eq. (9a) clearly in-
                                                                chemical properties, or electrical charge of the mate-
              creases rejection, since the flux of solvent (water) in-
                                                                rials to be separated. As discussed in previous sections,
              creases proportionally to this factor, while the flux of salt
                                                                membranes that are microporous control separation pre-
              is essentially independent of it, within the accuracy of the
                                                                dominantly by size discrimination, charge interaction, or
              approximations of the model. A typical example of such
                                                                a combination of both, while nonporous membranes rely
              behavior is shown in Fig. 4 as a function of feed pressure
                                                                on preferential sorption and molecular diffusion of indi-
                  ◦
              at 25 C for a brackish water feed with low salt concen-
                                                                vidual species. This permeation selectivity may, in turn,
              tration (0.5 wt % or 0.16 mol %). As expected based on
                                                                originate from chemical similarity, specific complexation,
              Eq. (9a), when the applied transmembrane 
p equals the
                                                                and/or ionic interaction between the permeants and the
                                                                membrane material, or specific recognition mechanisms
                                                                such as bioaffinity.
                                                                  A membrane material should meet several criteria: it
                                                                should be chemically and physically stable under anti-
                                                                cipated operating conditions, have the permselectivity re-
                                                                quired for a given process design, and be conveniently
                                                                fabricated into membrane form. Polymers are the most
                                                                frequently used membrane materials as they offer a wide
                                                                spectrum of properties. Specialty membranes made of in-
                                                                organic materials such as ceramics, metals, and carbon are
                                                                also available. Their ability to withstand extreme tempera-
                                                                tures and harsh chemical conditions enables their deploy-
                                                                ment in applications not addressed by polymeric mem-
                                                                branes. Membranes used in the life sciences are designed
                                                                to contact delicate biological or biochemical materials; a
                                                                high degree of biocompatibility and hydrophilicity is nec-
                                   2
              FIGURE 4 Flux in GFD (gal/ft /day) and rejection of NaCl at 25 C
                                                         ◦
              for atmospheric pressure permeate with increasing applied feed  essary to minimize nonspecific interaction and the conse-
              pressure with a 5000 mg/L salt feed. The membrane is an asym-  quent degeneration in membrane performance or damage
              metric polyamide.                                 to the biological material.
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