Porous sublayer

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-fHm composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has Httle effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich soHd phase that forms the membrane and a polymer-poor Hquid phase that forms the membrane pores or void spaces. 

Figure 16. Schematic representation of binder droplet spreading and penetration into a porous sublayer.

Jonsson, G., The influence of the porous sublayer on the salt rejection and reflection coefficient of asymmetric CA membranes. 

In this chapter, pressure driven processes involving porous inorganic or hybrid membranes are particularly examined. As recently shown by Bhave [1], differences with traditional organic elements mainly result from the structure and intrinsic properties of materials, either regarding flow (with ceramic membranes, the transport occiu-s through the intergranular spaces within the top layer, porous sublayers and support, while across polymeric barriers it develops... 

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. 

With the rational assumption of negligible resistance to flow offered by the porous sublayer and with the result that the pore resistance is much greater Chan Che skin-layer resistance, the relation for the total resistance simplifies to... 

In the porous sublayer, the net ethanol flux is the difference between convective permeation and back-diffusion ... 

At steady state, the ethanol flow across the skin layer equals that across the porous sublayer. Thus, combining Equations 10 and 12 yields the overall ethanol balance across the entire membrane ... 

The TFC-801 membranes did not exhibit any measurable CCRO effect under equivalent test conditions. These results are consistent with our previous work on osmotic membranes. (9,) Thin-film-composite desalination membranes derived from polyamine precursors (such as TFC-801) contain a gel layer within the porous sublayer formed by thermal crosslinking of the polyamine.(10) This gel layer retards the diffusion of ethanol into the membrane from the recirculation solution. Thus the ethanol concentration inside the porous sublayer is not increased effectively by increasing the concentration of the recirculation solution. By contrast, Loeb-Sourirajan-type asymmetric membranes or composite membranes derived from monomeric precursors do not contain any gel layer. Such membranes are better suited to use in the CCRO process. 

The symmetric membrane has a uniform composition throughout its thickness whereas the asymmetric membrane has a dense, but thin, top layer, and a strong, more porous sublayer beneath it. 

Reverse osmosis was not commercially practical until techniques for increasing productivity were developed. The principal discovery (32) involved a casting procedure that results in asymmetric membranes having a thin dense layer of polymer, approximately 0.2)a thick, supported on a porous sublayer as Illustrated in Figure 8. These membranes are called Loeb membranes (33). Current commercial membranes of this type are made of cellulose acetate, aromatic polyamides, and certain composites that achieve water fluxes of the order of 1.0 m /m day with NaCl rejections of 99% or more (27). As seen in Equation 25, rejection increases with applied pressure. 

Figure 7b. SEM picture of the porous sublayer of an asymmetric capillary membrane with entrapped cells. Reproduced with permission from Ref. 14.

There are constant effective" permeabilities frit the various gaseous components that is, either the resistances of the porous sublayer ned the support are neglected or a lumped value besed ou the overall thickness is.used without any presesre and composition dependencies. 

The diffusion model [5, 6] has been developed for mass transfer study within the system bipolar plate - gas diffusion layer (with micro-porous sublayer) - electrocatalytic layer . It was shown that the current density distribution is a complex function and depends mainly on the electrochemical parameters of the MEA (electrocatalytic layer activity) and... 

Most membranes used in industries have an asymmetric structure. Figure 2.1 shows schematically a typical cross-sectional view of an asymmetric membrane [3]. It consists of two layers the top one is a very thin dense layer (also called the top skin layer), and the bottom one is a porous sublayer. The top dense layer governs the performance (permeation properties) of the membrane the porous sublayer only provides mechanical strength to the membrane. The membranes of symmetric structures do not possess a top dense layer. In the asymmetric membrane, when the material of the top... 

CA films by using the phase inversion process. These CA films were cast from solvent/nonsolvent solutions to yield size exclusion membranes consisting of a thin permselective outer layer and a more porous sublayer. These membranes permitted the rapid permeation of a 1500-dalton poly (ethylene glycol) ester of ferrocene however the reproducibility of results presents a problem with these CA mem-branes. Christie et demonstrated that thin films of plasticized polyvinylchloride (PVC), normally used for potentiometric ion-selective electrode applications, applied to electrodes over a polycarbonate dialysis membrane offered improved selectivity ratios for the amperometric detection of phenolic compounds and H2O2 in the presence of the common biological interferents, ascorbic acid and uric acid, over those observed at the dialysis membrane alone or at a composite dialysis/membrane. 

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