Reverse osmosis component transport

Nonporous Dense Membranes. Nonporous, dense membranes consist of a dense film through which permeants are transported by diffusion under the driving force of a pressure, concentration, or electrical potential gradient. The separation of various components of a solution is related directiy to their relative transport rate within the membrane, which is determined by their diffusivity and solubiUty ia the membrane material. An important property of nonporous, dense membranes is that even permeants of similar size may be separated when their concentration ia the membrane material (ie, their solubiUty) differs significantly. Most gas separation, pervaporation, and reverse osmosis membranes use dense membranes to perform the separation. However, these membranes usually have an asymmetric stmcture to improve the flux. 

Dense membranes are used for pervaporation, as for reverse osmosis, and the process can be described by a solution-diffusion model. That is, in an ideal case there is equilibrium at the membrane interfaces and diffusional transport of components through the bulk of the membrane. The activity of a component on the feed side of the membrane is proportional to the composition of that component in the feed solution. 

The composition at the permeate-phase interface depends on the partial pressure and saturation vapour pressure of the component. Solvent composition within the membrane may vary considerably between the feed and permeate sides interface in pervaporation. By lowering the pressure at the permeate side, very low concentrations can be achieved while the solvent concentration on the feed-side can be up to 90 per cent by mass. Thus, in contrast to reverse osmosis, where such differences are not observed in practice, the modelling of material transport in pervaporation must take into account the concentration dependence of the diffusion coefficients. 

Stack, and a potential difference sufficient to force current through the stack is applied between the two electrodes placed at each end of the stack. For current to pass between the electrodes, ions must be transported through each of the membranes. By arranging the feeds to the various intermembrane compartments, it is possible to force ionic salts to pass from the dilute stream to the concentrated stream. In this way, a salt can also be split into its acid and base components. By combination of several cell pairs that comprise an anion- and a cation-selective membrane sheets in parallel, a stream concentrated in the original salts may be prepared. This configuration is the common method for industrial use, in which electrodialysis gives broadly the same result as reverse osmosis and has found very similar applications to general water treatment. 

Summary of Physicochemical Parameters. In the previous section steric parameters 4 . and were introduced to describe the effective size of solution components and the average size of the transport corridor, respectively. A variety of quantities that could be used to represent

dense membranes and the skin layer of reverse osmosis membranes in which the transport corridors are beyond the resolution capabilities of modern instruments and may be dynamic in nature. Therefore, any discussion of membrane material selection based on steric considerations must be qualitative. 

It is understood that the economical success of any membrane process depends primarily on the quality of the membrane, specifically on flux, selectivity and service lifetime. Consideration of only the transport mechanisms in membranes, however, will in general, lead to an overestimation of the specific permeation rates in membrane processes. Formation of a concentration boundary layer in front of the membrane surface or within the porous support structure reduces the permeation rate and, in most cases, the product quality as well. For reverse osmosis. Figure 6.1 shows how a concentration boundary layer (concentration polarization) forms as a result of membrane selectivity. At steady state conditions, the retained components must be transported back into the bulk of the liquid. As laminar flow is present near the membrane surface, this backflow is of diffusive nature, i.e., is based on a concentration gradient. At steady state conditions, the concentration profile is calculated from a mass balance as... 

Membrane processes are used to accomplish a separation since the membrane has the ability to transport one component more readily than another. For convenience, let us consider a solution consisdng of a solvent and a solute as commonly found in pressure-driven membrane processes such as roiciofiltraiion, ultrafUtration and reverse osmosis. When a driving force acts on the feed solution, the solute is (partly) retained by the. membrane whereas the solvent permeates through the membrane. Thus, the membrane has a certain retentivity for the solute while the solvent can permeate more or less freely. This implies that the concentration of the solute in the penneate(Cp) is lower than the concemrarion in the bulk (c, ). which is in fact the basic concept of membrane separations. This is shou n in figure Vn - 3. 

Let us now review reverse osmosis transport. Both feed and permeates are in the liquid phase, and we do not need to convert the concentration to partial vapor pressure to calculate the chemical potential difference as the driving force. Instead, Equations 5.241 and 5.242 are used as given. Let us define component B as the solvent and component A as the solute. Then Equations 5.242 and 5.241 can be written as... 

The above calculation steps were applied to the experimental data obtained using a cellulose acetate reverse osmosis membrane dried by the solvent exchange method. The details of the condition of the membrane preparation are reported in the literature [236]. The transport parameters obtained from the above experimental data were A = 0.1277 x 10 m", Aj = 0.1500 x 10 kmoI/m sPa, K = 4.0 x 10 m, and (7 = 1.3 x 10" m for CO2 gas. Note that an average pore size R as low as 4 x 10" m was obtained. Furthermore. flux components, Qsh and Qs were calculated and the contribution of the component flux to the total flux was determined for different gases under different operating pressures. The results are illustrated in Figure 6.20. 

Both Eq. 18.3-19 and Eq. 18.3-20 describe the solute flux, and Vci and All both reflect changes in solute concentration. The quantities v and Jy represent the amount of convection. At the same time, there are differences between Eqs. 18.3-18 and 18.3-19 for reverse osmosis and Eq. 18.3-20 for binary diffusion. These differences are reflected by the number of transport coefficients involved. For binary diffusion, there is one D. For reverse osmosis, there are four Lp, a, co, and 1 - because there is only one force, the concentration gradient, responsible for binary diffusion. For reverse osmosis, there are two independent forces, the concentration difference and the pressure difference. Reverse osmosis is more like ternary diffusion, where the three components are the solute, the solvent, and the membrane. 

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