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Diffusion from permeate/membrane surface

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. [Pg.197]

As described above, the initial cause of membrane fouling is concentration polarization, which results in deposition of a layer of material on the membrane surface. The phenomenon of concentration polarization is described in detail in Chapter 4. In ultrafiltration, solvent and macromolecular or colloidal solutes are carried towards the membrane surface by the solution permeating the membrane. Solvent molecules permeate the membrane, but the larger solutes accumulate at the membrane surface. Because of their size, the rate at which the rejected solute molecules can diffuse from the membrane surface back to the bulk solution is relatively low. Thus their concentration at the membrane surface is typically 20-50 times higher than the feed solution concentration. These solutes become so concentrated at the membrane surface that a gel layer is formed and becomes a secondary barrier to flow through the membrane. The formation of this gel layer on the membrane surface is illustrated in Figure 6.6. The gel layer model was developed at the Amicon Corporation in the 1960s [8],... [Pg.243]

In a facilitated transport membrane process, carriers react or coordinate reversibly with a solute which is transported through the membrane. The principles for such a membrane process for separation of ethene from ethane are shown in Figure 1. On the feed side of the membrane, ethane and ethene are dissolved in the membrane surface. Only ethene can form a complex with the silver ions within the membrane. Diffusion of the ethene-silver ion complex across the membrane to the permeate side takes place according to a concentration gradient. On the permeate side the partial pressure of ethene must be low so that a decomplexation reaction occurs and ethene is released from the membrane surface. Ethane can only diffuse through the membrane according to the concentration gradient across the membrane. [Pg.115]

In the methanol permeation study using a diffusion cell set-up, the positioning of the working elechode is very important. Attaching the working electrode on the membrane enhances methanol oxidation current detection. Even a small distance away from the membrane surface can affect the results because the methanol oxidation current may not be detected. The best way is to attach the working electrode on the membrane or to prepare it on the membrane by a suitable method, such as elecholess deposition. [Pg.381]

SPAN module. It was mentioned at the beginning that the special polyacrylonitrile fibers of SPAN have a wall thickness of 30 gm, which is considerably thicker than the 8 gm wall thickness of the SMC modules [19]. As a consequence, the presence of stronger capillary effects from the special porous fiber material of the SPAN module would be a reasonable conclusion. Furthermore, the texture of the special polyacrylonitrile fibers is expected to have better surface properties, supporting the permeation of molecules as compared with synthetically modified cellulose. In conclusion, both convection and diffusion effectively contribute to the filtration efficiency in a SPAN module, whereas for the SMC membrane, diffusion is the driving force for molecular exchange, the efficiency of which is also considerable and benefits from the large surface-to-volume ratio. [Pg.469]

Membranes can also be used as a reactor where catalysts are used frequently. The membrane may physically segregate the catalyst in the reactor, or have the catalyst immobilized in the porous/microporous structure or on the membrane surface. The membrane having the catalyst immobilized in/on it acts almost in the same way as a catalyst particle in a reactor does, except that separation of the product(s) takes place, in addition, through the membrane to the permeate side. All such configurations involve the bulk flow of the reaction mixture along the reactor length while diffusion of the reactants/products takes place generally in a perpendicular direction to/from the porous/microporous catalyst. [Pg.168]

Fig. 19.3 The solution-diffusion transport model in pervaporation. a Solution of compounds from the feed phase into the membrane surface, b Diffusion across the membrane barrier, c Desorption from the membrane permeate (downstream) side into the permeate phase... Fig. 19.3 The solution-diffusion transport model in pervaporation. a Solution of compounds from the feed phase into the membrane surface, b Diffusion across the membrane barrier, c Desorption from the membrane permeate (downstream) side into the permeate phase...
The transport of gas in polymers has been studied for over 150 years (1). Many of the concepts developed in 1866 by Graham (2) are still accepted today. Graham postulated that the mechanism of the permeation process involves the solution of the gas in the upstream surface of the membrane, diffusion through the membrane followed by evaporation from the downstream membrane surface. This is the basis for the "solution-diffusion model which is used even today in analyzing gas transport phenomena in polymeric membranes. [Pg.95]

The separation factor, /3pervap, contains contributions from the intrinsic permeation properties of the membrane, the composition and temperature of the feed liquid, and the permeate pressure of the membrane. The contributions of these factors are best understood if the pervaporation process is divided into two steps, as shown in Figure 9.3 [18]. The first step is evaporation of the feed liquid to form a saturated vapor in contact with the membrane the second step is diffusion of this vapor through the membrane to the low-pressure permeate side. This two-step description is only a conceptual representation in pervaporation no vapor phase actually contacts the membrane surface. Nonetheless, the representation of the process shown in Figure 9.3 is thermodynamically completely equivalent to the actual pervaporation process shown in Figure 9.F... [Pg.358]

If transport through a membrane involving both surface reaction (dissociation) and diffusion was limited by surface reactions, then n = 1. If transport was diffusion-limited, then n = 0.5. Intermediate values of n (0.5proton transport membranes was modeled with the same form of the equations used to model diffusion membranes, Eqns. 3 and 4. Values of k H2 and n were determined from Eqn. 3. The concentration of hydrogen on the permeate-side was insignificant relative to the concentration on the retentate-side. Therefore was equated to... [Pg.99]

The application of the Maxwell-Stefan theory for diffusion in microporous media to permeation through zeolitic membranes implies that transport is assumed to occur only via the adsorbed phase (surface diffusion). Upon combination of surface diffusion according to the Maxwell-Stefan model (Eq. 20) with activated-gas translational diffusion (Eq. 12) for a one-component system, the temperature dependence of the flux shows a maximum and a minimum for a given set of parameters (Fig. 15). At low temperatures, surface diffusion is the most important diffusion mechanism. This type of diffusion is highly dependent on the concentration of adsorbed species in the membrane, which is calculated from the adsorption isotherm. At high temperatures, activated-gas translational diffusion takes over, causing an increase in the flux until it levels off at still-higher temperatures. [Pg.562]

The stages of heat transfer in AGMD (Figure 19.7) include heat flux from the feed boundary layer to the membrane surface, vapor permeation through the membrane, and the diffusion in air gap, then condensation at the cold surface and finally heat transfer to the cooling water. [Pg.526]

The positive effect of velocity on the permeate flux is a result of enhanced hydrodynamic effects at the membrane surface, since high velocities lead to high shear and turbulent flow, which results in the formation of vortices and eddies that minimize the concentration polarization effects and the development of a fouling layer. The bigger the thickness of this layer, the higher its flow resistance and the smaller the permeate flux through the membrane becomes. Under turbulent flow conditions, shear effects induce hydrodynamic diffusion of the particles from the boundary layer back into the bulk, with a positive effect on the permeate flux. [Pg.563]

Fat or lipid materials and calcium-lipid complexes also contribute to fouling and flux decline in membrane processing of milk or whey. The transport properties of the feed stream and the changes they undergo as the concentration process proceeds also affect the rate of permeation. At high concentrations, the increased fluid viscosity near the membrane surface limits back-diffusion of solids from the polarized layer to the bulk phase, thereby, depressing flux rate [46]. [Pg.652]

Zelsmann and co-workers [88] have reported tracer diffusion coefficients for water in Nafion membranes exposed to water vapor of controlled activity. These were determined by various techniques, including isotopic exchange across the membrane. They reported apparent self-diffiision coefficients of water much lower than those determined by Zawodzinski et al. [64], with a weaker dependence on water content, varying from 0.5 x 10 cm to 3 x 10 cm /s as the relative humidity is varied from 20 to 100%. It is likely that a different measurement method generates these large differences. In the experiments of Zelsmaim et al., water must permeate into and through the membrane from vapor phase on one side to vapor phase on the other. Since the membrane surface in contact with water vapor is extremely hydrophobic (see Table 7), there is apparently a surface barrier to water uptake from the vapor which dominates the overall rate of water transport in this type of experiment. [Pg.267]


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See also in sourсe #XX -- [ Pg.313 ]




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Membrane diffusivity

Membranes diffusion

Membranes permeate

Membranes permeation

Surface diffusion

Surface diffusion Diffusivity

Surface diffusivity

Surface membranes

Surface permeation

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