Membrane selective layer thickness

Another parameter to consider is membrane thickness and the membrane selective layer thickness. This is a key parameter in membrane processes as it influences both membrane selectivity and membrane resistance against flux. 

Sqiaration selectivity obtained at laboratory scale on composite membranes (selective layer thickness 0.3. 5 im) by MTR Inc. with the following operating conditions feed = 60% methane/20%carbon dioxide/20 /o nitrogen feed pressure = 200 psig. 

Neglecting convection effects, the solution-diffusion model gives the following expressions for water (1) and salt (2) molar fluxes through a membrane with a selective layer thickness of L and a transmembrane pressure drop Ap (Merten, 1966) ... 

Example 30.5. Ultrafiltration tests with a 1.5-cm tubular membrane at — 25,000 gave a permeate flux of 40 L/m -h and 75 percent rejection for a 5 percent polymer solution. The polymer has an average molecular weight of 30,000, and the estimated diffusivity is 5 x 10 cm s. (fl) Neglecting the effect of molecular diffusion in the pores, predict the fraction rejected for a flux of 20 L/m -h, and predict the maximum rejection, (b) Estimate the fraction rejected for the low-molecular-weight fraction of the pol3mier with M 10,000. (c) If the selective layer thickness is 0.2 fan, does molecular diffusion have a significant effect on the rejection for case (a) ... 

Developer Substrate Support Membrane selective layer Selective layer thickness (pm) Membrane surface (m") Manufacturing method Geometry... 

The Energy Research Centre of the Netherlands (ECN) that produces membranes of Pd on alumina support with a surface area equal to 0.4 m and selective layer thickness of 3-9 pm. 

Concerning hydrogen selective membranes, industry is mainly focused on the manufacture of composite membranes made of a thin Pd film on porous substrates. Reducing the selective layer thickness allows membrane cost to be decreased (decreasing the Pd thickness by a factor two reduces the total Pd cost by a factor four) and increasing the hydrogen flux, which is in inverse proportion with the film thickness. On the other side, the porous substrates provides the mechanical... 

Three key elements determine the potential and applications of a hollow-fiber membrane (1) pore size and pore size distribution, (2) selective layer thickness, and (3) inherent properties (chemistry and physics) of the membrane material. Pore size and its distribution usually determine membrane applications, separation factor, or selectivity. The selective layer thickness determines the membrane flux or productivity. Material chemistry and physics govern the intrinsic permselectivity for gas separation and pervaporation, fouling characteristics for RO (reverse osmosis), UF (ultrafiltration), and MF (microfiltration) membranes, chemical resistance for membranes used in harsh environments, protein and drug separation, as well as biocompatibUity for biomedical membranes used in dialysis and biomedical and tissue engineering. 

Figure 31.13 Visual estimation of the dense-selective layer thickness of dual-layer hollow-fiber membranes with different heat treatment methods (left) 15°C for 3 h and right) 150°C for 1 h. (Li et al., 2004b).

A stream at 220 atm and 100 °C containing 27.2% NH3, 54.5% H2, and 18.2% NH3 is currently being recycled to an ammonia synthesis reactor. You want to feed it through a hollow-fiber module with a fiber volume fraction of 0.5 to recover 90% of the ammonia. The module s membranes are 240 pm in diameter, have a permeability P of 4.0 10 cm /sec, and a selective layer thickness I equal to 35 pm. How long should gas spend in this module ... 

In ISFETS utilizing polymeric ion-selective membranes, it has been always assumed that these membranes are hydrophobic. Although they reject ions other than those for which they are designed to be selective, polymeric membranes allow permeation of electrically neutral species. Thus, it has been found that water penetrates into and through these membranes and forms a nonuniform concentration gradient just inside the polymer/solution interface (Li et al., 1996). This finding has set the practical limits on the minimum optimal thickness of ion-selective membranes on ISFETS. For most ISE membranes, that thickness is between 50-100 jttm. It also raises the issue of optimization of selectivity coefficients, because a partially hydrated selective layer is expected to have very different interactions with ions of different solvation energies. 

These observations have several practical consequences for membrane processes where the selective layers are as thin as or even thinner than the low end of the range studied here. First, it is clear that use of thick film data to design or select membrane materials only gives a rough approximation of the performance that might be realized in practice. Second, because the absolute permeability of a thin film may be severalfold different than the bulk permeability, use of the latter type of data to estimate skin thickness from flux observations on asymmetric or composite membranes structures is also a very approximate method. Finally, these data indicate that one could expect... 

According to the method, a relatively thick silicone rubber layer is coated on a thin selective layer of an asymmetric PS membrane. The thickness of silicone rubber is about 1 pm while the effective thickness of the selective PS layer is 1/10 of 1pm. While being coated, silicone rubber penetrates into the pores to plug them (Fig. 5). Thus, the feed gas is not allowed to leak through the defective pores. The selectivity of the membrane approaches that of the defect-free PS layer. Moreover, because the permeabilities of silicone rubber for gases are orders of magnitude higher than those of PS, the permeation rate is not affected very much even when a relatively thick silicone rubber layer is coated. 

An asymmetric membrane has a very thin dense top layer (or skin) with a thickness of 0.1-0.5 pm. A porous sublayer with a thickness of approximately 50-150 pm supports the dense top layer. The thin dense skin facing the feed solution acts as the selective layer, allowing water passage but rejecting dissolved solids. The resistance to mass transfer across the membrane is also mainly determined by the thin top layer. In asymmetric membranes, the selective top layer and the porous support layer are made of the same polymer material. Asymmetric membranes can be obtained by phase inversion, a technique in which a polymer in solution is transformed in a controlled manner from a liquid into a solid form. The top skin layer and the porous support layer are formed in a single-step process. 

A boundary layer with a gradient in caustic soda concentration also forms at the surface of the membrane facing the catholyte based on a similar principle, resulting in a caustic soda concentration on the membrane surface which is higher than that in the bulk phase. Since this tends to reduce the current efficiency and electric conductivity of the membrane, it is necessary to minimize the boundary layer thickness or reduce the caustic soda concentration in the bulk phase. It is also essential to purify the brine with ion-exchange resin of high selectivity, in order to prevent precipitation of metal ions as hydroxides in the membrane and the boundary layer (74). 

Approach One approach to the development of a very thin membrane with gt mechanical strength is to integrate a structure with distmet selective and stqrpotting elements. An example of this would be a thin selective layer supported by a thick porous layer. Since the functionality of the two elements are now sqrarated, the flux and mechanical strength can be manipulated independently to meet the application requirements. 

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