Reverse osmosis membrane pore size

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

This is considered too large for a reverse osmosis membrane pore. Presumably, a significant reduction in the size of the spherical macromolecule takes place during the evaporation and gelation steps. The macromolecular radius calculated above is, however, far smaller than the radius of the macromolecular nodule observed by Panar et al., under the electron microscope [63]. Kesting... 

Fig. 25. Reverse osmosis, ultrafiltration, microfiltration, and conventional filtration are related processes differing principally in the average pore diameter of the membrane filter. Reverse osmosis membranes are so dense that discrete pores do not exist transport occurs via statistically distributed free volume areas. The relative size of different solutes removed by each class of membrane is illustrated in this schematic.

Hyperfiltration (Reverse Osmosis) is a form of membrane distillation or desalination (desalting) operating with membrane pore sizes of perhaps 1 to 10 Angstrom units. The various individual RO component technologies have improved tremendously over the last 20 to 25 years, and resistance to fouling and permeate output rates have benefited. Nevertheless, all RO plants remain susceptible to the risk of fouling, and adequate pretreatment and operation is essential to minimize this problem. 

Much effort has been expended in attempting to use membranes for separations. Reverse osmosis membranes are used worldwide for water purification. These membranes are based on size selectivity depending on the pores used. They do not have the ability to selectively separate target species other than by size. Incorporation of carrier molecules into liquid membrane systems of various types has resulted in achievement of highly selective separations on a laboratory scale. Reviews of the extensive literature on the use of liquid membrane systems for carrier-mediated ion separations have been published [15-20]. A variety of liquid membranes has been studied including bulk (BLM), emulsion (ELM), thin sheet supported (TSSLM), hollow fiber supported (HFSLM), and two module hollow fiber supported (TMHFSLM) types. Of these liquid membranes, only the ELM and TMHFSLM types are likely to be commercialized. Inadequacies of the remaining... 

The deposition of a plasma polymer on an appropriate porous substrate to form a composite reverse osmosis membrane is a typical example of the second case. In both cases, however, the selection of the substrate membrane is the crucially important factor, particularly in the second case. In the application of nanofilm, the pore size of the substrate membrane must have the uniformity of pore size in nanometer scale, which is an extremely difficult requirement. 

Direct flow filtration has certain Umitations. The flux (filtration flow rate per unit membrane area) decreases over time as the process continues because the filtering media is loaded with more contaminant particles, as illustrated in Figure 14.1. Moreover, when the concentration of the contaminant in the feed stream is high, the filtering media must be replaced very frequently, which can be economically impractical. Also when the contaminant matter to be separated is small in size, requiring ultrafiltration or reverse osmosis membranes with much smaller pores, then direct filtration is less feasible as the flux declines very rapidly over time, again requiring frequent filter replacement. 

Ultrafiltration operates at lower pressures (0.2-1 MPa) than reverse osmosis and with higher permeate fluxes. It uses more porous membranes, pore size of 0.001-0.1 pm. In such a case, low-molecular weight dissolved compounds pass through the membrane, while colloid and suspended matters are rejected by UF membrane. 

Membranes are used for a wide variety of separations. A membrane serves as a barrier to some particles while allowing others to selectively pass through. The membrane pore size, shape, and electrostatic surface charge are fundamental to particle removal. Reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) relate to separation of ions, macromolecules and particles in the 0.001-10 pm range. 

Perhaps because much attention has centered on reverse osmosis membranes, the fine pores present in their skins were observed prior to the discovery of the functionally larger pores of ultrafiltration (UF) membranes. Recently, pores of v30 A have been observed by Zeman (35) in the skins of UF membranes. Their density, uniformity and diameters leave no doubt that these are actually the pores which function during UF. Our ability to actually "see" the intermicellar defect pores (the population of larger size pores) in the skins of RO membranes extends to the 10 X range. Therefore, it is reasonable to expect that at some point we shall be able to extend this ability to the population of smaller sized pores, whose existence is predicted by Sourlrajan s pore theory (36). 

The advantage of the preferential sorption-capillary flow approach to reverse osmosis lies in its emphasis on the mechanism of separation at a molecular level. This knowledge is useful when it becomes necessary to predict membrane performance for unknown systems. Also, the approach is not restricted to the so-called "perfect", defect-free membranes, but encompasses the whole range of membrane pore size. Until recently, the application of a quantitative model to the case of solute preferential sorption has been missing. Attempts to change this situation have been made by Matsuura and Sourirajan (21) by using a modified finely porous model. In addition to the usual features of this model (9-12), a Lennard-Jones type of potential function is Incorporated to describe the membrane-solute interaction. This model is discussed elsewhere in this book. 

T. D. Nguyen, K. Chan, T. Matsuura, S. Sourirajan, Effect of shrinkage on pore size distribution of different ceUulosic reverse osmosis membranes, Ind. Eng. Client. Prod. Res. Dev., 23, 501-508 (1984). 

The phenomeiion above describes reverse osmosis. Here, a liquid with a higher concentration of electrolyte is driven through a membrane (pore sizes are on the order of 3 nm), and the exiting solvent contains much less electrolyte. The reverse osmosis membranes usually have an of 0.995. Calculate the corresponding ipj. The present treatment is from Jacazio et al. (1972), who also compared theory to experiments. 

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