Membranes operation

D. L. Peet and J. H. Austin, Nafion Pefluorinated Membranes, Operation in Chlor—Alkali Plants, Chlorine Institute Plant Managers Seminar, Tampa, The Chlorine Institute, Feb. 1986. 

Ultrafiltration may be distinguished from other membrane operations by example When reverse osmosis is used to process whey, it passes only the water and some of the lactic acid (due to the solubihty of lactic acid in RO membranes). Nanofiltration used on whey will pass most of the sodium salts while retaining the calcium salts and most of the lactose. Microfiltration will pass everything except the particulates and the bacteria. 

Process Description Gas-separation membranes separate gases from other gases. Some gas filters, which remove hquids or sohds from gases, are microfiltration membranes. Gas membranes generally work because individual gases differ in their solubility and diffusivity through nonporous polymers. A few membranes operate by sieving, Knudsen flow, or chemical complexation. 

Modules Eveiy module design used in other membrane operations has been tried in peivaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is veiy low (O.I-I Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for neat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construc tion was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capiUaiy bundles. The capillaiy device with the feed on... 

Flux Decline Plugging, Fouling, Polarization Membranes operated in NFF mode tend to show a steady flux decline while those operated in TFF mode tend to show a more stable flux after a short initial decline. Irreversible flux decline can occur by membrane compression or retentate channel spacers blinding off the membrane. Flux decline by fouling mechanisms (molecular adsorption, precipitation on the membrane surface, entrapment within the membrane structure) are amenable to chemical cleaning between batches. Flux decline amenable to mechanical disturbance (such as TFF operation) includes the formation of a secondary structure on the membrane surface such as a static cake or a fluid region of high component concentration called a polarization layer. 

Asymmetric membranes have a tight, low-permeability, retentive zone that performs the desired separation and a more open, high-permeability zone that provides mechanical strength to the overall membrane. This structure is particularly critical to the economic viability of reverse-osmosis membranes. Asymmetric membranes operated in TFF mode must have the tight side facing the feed channel so that particles are retained on its surface and can be acted upon by the tangential flow. Asymmetric membranes operated in NFF mode can... 

Balannec, B., Gesan-Guiziou, G., Chaufer, B., Rabiller-Baudry, M., and Daufin, G., Treatment of dairy processing waters by membrane operations for water reuse and milk constituents concentration, Desalination,147, 89-94, 2002. 

Because Pd-alloy membranes operate at high temperatures in the range of WGS reaction and on the lower end of methane reforming reaction, they can be used in a membrane reactor configuration for the simultaneous separation of hydrogen. As discussed earlier,... 

Peet, D.L. Austin, J.H. (1986) Nafion perfluorinated membranes operation in chlor-alkali plants. 

The slurry flow management scheme to the cells has large numbers of parallel flow paths through the hydrocyclones and through individual electrode cavities. Upsets in these paths can lead to upsets in the quality and quantity of slurry flowing to the electrode cavities, with possible impact on membrane operation. 

We will adopt the assumption of thermal equilibrium under considered conditions of membrane operation this implies uniform temperature and zero heat flux in the system. 

Continuity of fhe wafer flux fhrough the membrane and across the external membrane interfaces determines gradients in water activity or concentration these depend on rates of water transport through the membrane by diffusion, hydraulic permeation, and electro-osmofic drag, as well as on the rates of interfacial kinetic processes (i.e., vaporization and condensafion). This applies to membrane operation in a working fuel cell as well as to ex situ membrane measuremenfs wifh controlled water fluxes fhat are conducted in order to study transport properties of membranes. 

An external gas pressure gradient applied between anode and cathode sides of the fuel cell may be superimposed on the internal gradient in liquid pressure. This provides a means to control the water distribution in PEMs under fuel cell operation. This picture forms the basis for the hydraulic permeation model of membrane operation that has been proposed by Eikerling et al. This basic structural approach can be rationalized on the basis of the cluster network model. It can also be adapted to include the pertinent structural pictures of Gebel et and Schmidt-Rohr et al. ... 

The lipid part of the membrane is essentially a two-dimensional liquid in which the other materials are immersed and to which the cytoskeleton is anchored. This last statement is not totally correct, as some membrane bound enzymes require the proximity of particular lipids to function properly and are thus closely bound to them. Simple bilayers formed from lipids in which both hydrocarbon chains are fully saturated can have a highly ordered structure, but for this reason tend to be rigid rather than fluid at physiological temperatures. Natural selection has produced membranes which consist of a mixture of different lipids together with other amphiphilic molecules such as cholesterol and some carboxylic acids. Furthermore, in many naturally occurring lipids, one hydrocarbon chain contains a double bond and is thus kinked. Membranes formed from a mixture of such materials can retain a fluid structure. The temperature at which such membranes operate determines a suitable mixture of lipids so that a fluid but stable structure results at this temperature. It will be seen that the lipid part of a membrane must, apart from its two-dimensional character, be disordered to do its job. However, the membrane bound proteins have a degree of order, as will be discussed below. 

The allowable current density—normality ratio for electric membrane operation has been approximately doubled by an improved tortuous path spacer with strap turbulence promoters and by operation at higher pressures up to 60 p.s.i. As a result, twice as much water can now be demineralized per square foot of membrane utilized and/or greater demineralization achieved per pass in electric membrane units. One practical result of this development is a new continuous-flow, two-stage single-stack demineralizer which will provide 93% demineralization at a capacity of 5000 gallons per day and 72% demineralization at a capacity of 30,000 gallons per day. These units produce from 67 to 150% more water per unit membrane area than previously used automatic batch-recirculating units and are far simpler in construction and operation. 

Catalytic reactivity results (H202 productivity and selectivity) summarized in Figure 8.9 are comparable, taking into account the differences in the reaction conditions, with patented results reported in Table 8.3. Although, for the direct synthesis of H202, the costs of membrane operations are still higher than those of conventional catalytic reactors, the possibility of safer operations is an incentive, particularly for smaller scale applications. 

Bailly, M., Roux-de Balmann, H., Aimar, P., Lutin, F., and Cheryan, M. 2001. Production processes of fermented organic acids targeted around membrane operations Design of the concentration step by conventional electrodialysis. J. Membr. Sci. 191, 129-142. 

Seawater and brackish water desalination membranes operated with 0.5 to 5 wt% salt solutions at pressures of 200-1000 psi. 

Low-pressure nanofiltration membranes operated with 200-5000 ppm salt solutions at pressures of 100-200 psi. 

Figure 6.10 Effect of solute type and concentration on flux through the same type of ultrafiltration membrane operated under the same conditions [15]. Reproduced from M.C. Porter, Membrane Filtration, in Handbook of Separation Techniques for Chemical Engineers, P.A. Schweitzer (ed.), p. 2.39, Copyright 1979, with permission of McGraw-Hill, New York, NY...

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