Mass transfer reverse osmosis

Electrodialysis. In reverse osmosis pressure achieves the mass transfer. In electro dialysis (qv), dc is appHed to a series of alternating cationic and anionic membranes. Anions pass through the anion-permeable membranes but are prevented from migrating by the cationic permeable membranes. Only ionic species are separated by this method, whereas reverse osmosis can deal with nonionic species. The advantages and disadvantages of reverse osmosis are shared by electro dialysis. 

Most theoretical studies of osmosis and reverse osmosis have been carried out using macroscopic continuum hydrodynamics [5,8-13]. The models used include those that treat the wall as either nonporous or porous. In the nonporous models the membrane surface is assumed homogeneous and nonporous. Transport occurs by the molecules dissolving in the membrane phase and then diffusing through the membrane. Mass transfer across the membrane in these models is usually described using the solution-diffusion... 

This can be further integrated from the wall to the boundary layer thickness y = 8, where the component is at the bulk concentration Cj,. Substituting / = - o and k = D/o, the mass-transfer coefficient yields the stagnant film model [Brian, Desalination by Reverse Osmosis, Merten (ed.), M.I.T. Press, Cambridge, Mass., 1966, pp. 161-292] ... 

A phenomenon that is particularly important in the design of reverse osmosis units is that of concentration polarization. This occurs on the feed-side (concentrated side) of the reverse osmosis membrane. Because the solute cannot permeate through the membrane, the concentration of the solute in the liquid adjacent to the surface of the membrane is greater than that in the bulk of the fluid. This difference causes mass transfer of solute by diffusion from the membrane surface back to the bulk liquid. The rate of diffusion back into the bulk fluid depends on the mass transfer coefficient for the boundary layer on feed-side. Concentration polarization is the ratio of the solute concentration at the membrane surface to the solute concentration in the bulk stream. Concentration polarization causes the flux of solvent to decrease since the osmotic pressure increases as the boundary layer concentration increases and the overall driving force (AP - An) decreases. 

Nomenclature, 17 384-413 basic scheme of, 17 384-385 biochemical, 17 401-402 computerized approaches to, 17 400-401 elastomer, 21 761t enzyme, 10 258-260 for ionic liquids, 26 840-841 glossaries related to, 17 404 inorganic, 17 387-394 macromolecular (polymers), 17 403 404 organic, 17 394-401 polymer, 20 390-395 pump, 21 88 quinone, 21 236-237 reactor technology, 21 358 related to mass transfer, 15 731-737 reverse osmosis, 21 674-676 Society of Rheology, 21 704 spray-related, 23 199t systematic, 17 394... 

The reverse osmosis membranes were tested in the standard experimental set-up (10). The experiments were carried out at three different pressures 17.4, 40.8 and 102 bars the corresponding sodium chloride concentrations were 3500 ppm, 5000 ppm and 29000 ppm. Before the reverse osmosis runs, membranes were thermally shrunk for 10 minutes in water and subsequently pressurized at 15-20% higher pressures than those used during the reverse osmosis experiments. A feed flow rate of 400 ml/mln was used giving a mass transfer coefficient k = 40 x 10 cm/s on the high pressure side of the membrane. 

Plate and frame systems offer a great deal of flexibility in obtaining smaller channel dimensions. Equations 4 and 5 show that the Increased hydrodynamic shear associated with relatively thin channels Improves the mass-transfer coefficient. Membrane replacement costs are low but the labor involved is high. For the most-part, plate and frame systems have been troublesome in high-pressure reverse osmosis applications due to the propensity to leak. The most successful plate and frame unit from a commercial standpoint is that manufactured by The Danish Sugar Corporation Ltd. (DDS) (Figure 15). 

Srinivasan and Tien (18) have made an analytical study on the mass-transfer characteristics of reverse osmosis in curved tubular membranes. The increase in mass-transfer due to secondary flow resulted in a substantial reduction in the wall concentration (the polarization modulus) for Np =100 and a/R=0.01 (see Figure 39). Further, the production capacity (permeation rate) was markedly increased (see Figure 40). 

Fluidized beds have also been used to promote mass-transfer in both ultrafiltration and reverse osmosis. Smolders et al (22) ran 18mm i.d. UF tubes and 12mm i.d. RO tubes with and without fluidized beds (Ballotinl glass spheres). ... 

Goosen M.F.A., Sablani S.S., Al-Maskari S.S., Al-Belushi R.H., and Wilf M., Effect of feed temperature on permeate flux and mass transfer coefficient in spiral-wound reverse osmosis systems. Desalination 144 2002 367-372. 

Our main concern here is to present the mass transfer enhancement in several rate-controlled separation processes and how they are affected by the flow instabilities. These processes include membrane processes of reverse osmosis, ultra/microfiltration, gas permeation, and chromatography. In the following section, the different types of flow instabilities are classified and discussed. The axial dispersion in curved tubes is also discussed to understand the dispersion in the biological systems and radial mass transport in the chromatographic columns. Several experimental and theoretical studies have been reported on dispersion of solute in curved and coiled tubes under various laminar Newtonian and non-Newtonian flow conditions. The prior literature on dispersion in the laminar flow of Newtonian and non-Newtonian fluids through... 

Membrane operation is a specific, but not exotic, operation. In fact it is a hybrid of classical heat and mass transfer processes (Figure 4.1). Direct contact mass transfer operations tend to reach equilibrium due to a difference of chemical potential between two phases that are put into contact. In the same way, temperature equilibrium is aimed at during heat transfer operations, for which driving force is a temperature gradient. In contrast, for membrane operations, by using the specific properties of separation of the thin layer material that constitutes the membrane, under the particular driving force that is applied, it is possible to deviate from the equilibrium that prevails at fluid-to-fluid interphase with classical direct contact mass exchange systems and to reorientate the mass transfer properties. In particular, this is the case with classical operations such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), gas separation (GS), pervaporation (PV), dialysis (DI) or electrodialysis (ED), for which a few characteristics are recalled in Table 4.1. 

However, the use of permeable and semipermeable membranes in microfilters, ultrafilters, osmosis, reverse osmosis, dialysis (which are comparatively newer methods of separation) has problems like high capital costs, low mass transfer rate, low selectivity, and large equipment size. 

It is required to design a reverse osmosis unit to process 2500 mVh of seawater at 25°C containing 3.5 wt% dissolved salts, and produce purified water with 0.05 wt% dissolved salts. The pressure will be maintained at 135 atm on the residue side and 3.5 atm on the permeate side, and the temperature on both sides at 25°C. The dissolved salts may be assumed to be NaCl. With the proposed membrane, the salt permeance is 8.0 x 10 m/h and the water permeance is 0.085 kg/rn-.h.atrn. The density of the feed seawater is 1020 kg/m ( of the permeate, 997.5 kg/nv and of the residue (with an estimated salt content of 5 wt%), 1035 kg/rnc Assuming a perfect mixing model and neglecting the mass transfer resistances, determine the required membrane area and calculate the product flow rates and compositions. 

The field of membrane separations is radically different from processes based on vapor-liquid or fluid-solid operations. This separation process is based on differences in mass transfer and permeation rates, rather than phase equilibrium conditions. Nevertheless, membrane separations share the same goal as the more traditional separation processes the separation and purification of products. The principles of multi-component membrane separation are discussed for membrane modules in various flow patterns. Several applications are considered, including purification, dialysis, and reverse osmosis. 

Dialysis involves the mass transference between two miscible liquid phases (the donor and acceptor solutions) separated by a liquid membrane through which some chemical species are likely to pass. Miscibility between the donor and acceptor solutions is inherent to dialysis and distinguishes it from e.g., liquid—liquid extraction, osmosis and ultrafiltration [271], These latter two membrane-based separation approaches tend to occur concomitantly with dialysis and involve the solvent rather than the solute crossing the membrane. In osmosis, the driving force towards separation is the concentration difference involved whereas in ultrafiltration, also called reverse osmosis, the driving force is an applied pressure that forces the solution across the membrane. 

Spiral-wound modules of the type used for reverse osmosis (see Fig. 26.19) are widely used for UF. They are not as prone to plugging as hollow-fiber units, since the entrance is a narrow slit about 1 mm wide, but prefiltration of the feed solution is recommended. The velocity in the feed channels corresponds to laminar flow, but the flow disturbances caused by the spacers make the pressure drop and the mass transfer greater than for true laminar flow. 

In Chap. 26, concentration polarization in reverse osmosis was treated using a simple mass-transfer equation, Eq. (26.48), which is satisfactory where the surface concentration is only moderately higher than the bulk concentration. For UF, the large change in concentration near the surface requires integration to get the concentration profile. The basic equation states that the flux of solute due to convection plus diffusion is constant in the boundary layer and equal to the flux of solute in the permeate ... 

Hollow-fiber membrane modules are the mass-transfer equivalent of shell-and-tube heat exchangers. As fluids flow through the shell and lumen, mass is transferred from one stream to the other across the fiber wall. In contrast to heat exchangers, though, mass transfer may involve a combination of diffusion and convection, depending on the nature of the membrane. These modules are used for a wide range of membrane processes, including gas separation, reverse osmosis, filtration, and dialysis. 

Calculate mass-transfer rates for dialysis, reverse osmosis, gas permeation, and ultrafiltration. 

What is important to recognize from the discussion is that the boundary condition for mass transfer through a semipermeable membrane is directly analogous to that for a mixed heterogeneous reaction. A consequence of this is that what is said about the one problem can be translated to the other, despite the somewhat different physics and chemistry. The example of reverse osmosis is therefore used as an illustration of a mixed heterogeneous reaction. The major part of the discussion will, however, be confined to the developing layer, where... 

In an ideal case, in the absence of any concentration polarization (CP), the concentration of the permeants should not change from bulk-feed phase to the membrane-feed interface. However, due to finite mass transfer effects, there could be a decrease in concentration of the permeants over the diffusion film. In the case of ultratiltration, microfiltration, or reverse osmosis, the retained solute on... 

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