Membranes rejection

Recently developed softening membranes reject most of the hardness in water while passing sodium salts, and operate at pressures of about 5 bar. This can be used to provide substantially softened make-up water for shell... 

Maximum recoveries can be significantly affected by permeation losses. Control of permeation losses must be achieved through the proper choice of membrane type. A membrane with high rejection of most organics must be selected. The membrane rejection (R) for a solute is defined in terms of the permeate solute concentration (Cp) and feed solute concentration (CF) ... 

The exponential nature of the recovery response is represented graphically in Figure 1. The necessity for selecting membranes with high organic rejections is quite apparent. For example, in a 50-fold concentration, the recovery of a compound will not exceed 70% unless membrane rejection of that solute exceeds 0.9 (90 rejection). 

Improved organic rejections result from the chemical nature of the newer polymeric materials in the nylon and thin-film composite membranes. Data by Chian and Fang (6) represented in Figure 2 illustrate different membrane rejections for a variety of organic com-... 

Figure 3. Theoretical recoveries for batch and continuous processing modes (membrane rejection — 90%).

Membrane Concentration Test. Process potential was demonstrated by concentrating 500 L of synthetic tap water spiked with trace levels of the model compounds. A 50X volumetric concentration was achieved by reducing the sample volume from 500 to 10 L. The recovery of model compounds and membrane rejection of compounds were evaluated, and the location of system losses was approximated. 

Membrane Rejection. Both cellulose acetate and FT-30 composite membranes were evaluated for rejection of solutes. Sodium chloride rejections were confirmed and listed in Table III. Typical organic rejections of model compounds are listed in Tables IV and V for cellulose acetate and FT-30 composite membranes, respectively. Rejections were measured during screening and concentration tests solute levels were in the parts-per-billion range. Measurement of feed and permeate stream solute concentrations provided the necessary information to calculate solute rejection. Eq 1 was used to calculate rejection values. 

The overwhelming conclusion supported by data is the superiority of the FT-30 composite membrane for the majority of organic compounds tested. From arguments presented earlier, improved recovery of organic compounds on the basis of these higher rejection properties would be expected. Data from selected literature sources (6, 10-20) on membrane rejections of organics in water at parts-per-million levels were reviewed. Results are presented by chemical class in Table VI. Data are compiled for cellulose acetate and a cross-linked NS-1-type composite membrane. Differences in the rejection of various compound classes by the two membrane types determined at higher solute levels are similar to those observed and reported here at parts-per-billion levels. 

The results of field trials, based on total organic carbon (TOC) levels, show excellent recovery of the majority of organics present (21). Recovery data from a typical field RO concentration are shown in Table XIII. These results reflect high membrane rejections and recoveries found with higher molecular weight organics. Reductions in the total amount of adsorbed material in relation to the total sample... 

If the concentration polarization is negligible, the solute concentration on the membrane surface C is equal to the bulk concentration Cb Since the membrane rejects all solute, the total solute (m = CbV) is constant. Therefore,... 

In the case of reverse osmosis, the enrichment factors (E and Ea) are less than 1.0, typically about 0.01, because the membrane rejects salt and permeates water. For other processes, such as dehydration of aqueous ethanol by pervaporation, the enrichment factor for water will be greater than 1.0 because the membrane selectively permeates the water. 

Although some nanofiltration membranes are based on cellulose acetate, most are based on interfacial composite membranes. The preparation procedure used to form these membranes can result in acid groups attached to the polymeric backbone. Neutral solutes such as lactose, sucrose and raffinose are not affected by the presence of charged groups and the membrane rejection increases in proportion to solute size. Nanofiltration membranes with molecular weight cut-offs to neutral solutes between 150 and 1500 dalton are produced. Typical rejection curves for low molecular weight solutes by two representative membranes are shown in Figure 5.13 [35],... 

The neutral nanofiltration membrane rejects the various salts in proportion to molecular size, so the order of rejection is simply... 

To achieve a high membrane rejection towards the substrate it is important that the pore size of the membrane is smaller than the size of the molecules to be retained. Nevertheless, other factors influence the separation properties of a membrane, such as the shape and flexibility of the substrate and its acid-base properties, as well as the concentration-polarization phenomenon and the membrane fouling. 

MWCO), usually defined as the molar mass at which the membrane rejects 90% of solute molecules. However, as in microfiltration, the molecular shape can affect permeability through the membrane pores. For example, a membrane with a nominal cut-off of 100 kDa, which does not allow globular molecules with a molar mass of 100 kDa to flow through, may allow fibrous molecules with higher molar masses to flow across the pores. As in microfiltration, the membrane pore size is not uniform, with a normal distribution around an average value. 

Unlike CA membranes, polyamide membranes cannot tolerate free chlorine or any other oxidizers. Some manufacturers quote 200 - 1,000 ppm-hrs of exposure until the membrane rejection is lost.21 This means after 200 - 1,000 hours of exposure to 1 ppm free chlorine, the membrane rejection will be unacceptably low. Chlorine attack is faster at alkaline pH than at neutral or acidic pH. 

Boron rejection membranes exhibit up to 90+% rejection of boron, while standard membranes reject about 50-70%.(20, 29) These membranes are typically used for seawater applications where boron removal is a concern. Boron is difficult to remove with membranes because boron, which exists as boric acid, is not ionized a typical seawater pH, 7.0 - 8.0, whereas the pKa of boric acid is 9.14 - 9.25.20... 

Scaled membranes exhibit lower productivity and lower salt rejection. This lower salt rejection is a function of the concentration polarization phenomenon (see Chapter 3.4). When membranes are scaled, the surface of the membrane has a higher concentration of solutes than in the bulk solution. Since the membrane rejects when the membrane "sees," the passage of salts will be higher, even though the absolute or true rejection stays constant. 

Figure 9.2 Reverse osmosis membrane rejection as a function of feed water total dissolved solids. Assumes constant applied feed pressure.

Because of the carbon dioxide present in most waters, the pH of RO product water is generally lower than the pH of feed water, unless the carbon dioxide is completely removed from the feed water. If carbon dioxide is present in feed water, it will be present in permeate, as gases are not rejected by RO membranes (see Chapter 3.2). However, the membrane rejects carbonate and bicarbonate. Passage of carbon dioxide upsets the equilibrium among these compounds in the permeate. A new equilibrium occurs in the permeate, hence lowering its pH ... 

Potential of non-specific binding (NSB) to filter membrane or plastic devices. Low recovery from either protein-filtrate or buffer indicates adsorptive losses and/or membrane rejection... 

Figure 8.6e shows the effect of operating time on percent rejection. As shown, this particular membrane rejects divalent ions better than it does the monovalent ions. Generally, percent rejection increases with the value of the ionic charge. 

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