Hollow fibers, solute rejection

Major problems inherent in general applications of RO systems have to do with (1) the presence of particulate and colloidal matter in feed water, (2) precipitation of soluble salts, and (3) physical and chemical makeup of the feed water. All RO membranes can become clogged, some more readily than others. This problem is most severe for spiral-wound and hollow-fiber modules, especially when submicron and colloidal particles enter the unit (larger particulate matter can be easily removed by standard filtration methods). A similar problem is the occurrence of concentration-polarization, previously discussed for ED processes. Concentration-polarization is caused by an accumulation of solute on or near the membrane surface and results in lower flux and reduced salt rejection. 

A hollow-fiber reverse-osmosis module consists of a shell which houses the hollow fibers (Fig. 11.3). The fibers are grouped together in a bundle with one end sealed and the other open to the atmosphere. The open ends of the fibers are potted into Ml epoxy sealing head plate after which the permeate is collected. The pressurized feed solution (denoted by the shell side fluid) flows radially from a central porous tubular distributor. As the feed solution flows around the outer side of the fibers toward the shell perimeter, the permeate solution penetrates through the fiber wall into the bore side by virtue of reverse osmosis. The permeate is collected at the open ends of the fibers. The reject solution is collected at the porous wall of the shell. 

Albany International Research Co. has developed an advanced hollow fiber composite reverse osmosis membrane and module under the name of Quantro II . This composite membrane is comprised of a porous hollow fiber substrate on which has been deposited a rejection barrier capable of fluxes of commercial importance at high rejection of dissolved salts at elevated temperatures. Resistance to active chlorine has been demonstrated. Proprietary processes have been developed for spinning of the fiber, establishment of the rejection barrier and processing of the fiber to prepare modules of commercial size. Prototype modules are currently in field trials against brackish and seawater feed solutions. Applications under consideration for this membrane include brackish and seawater desalination as well as selected industrial concentration processes. 

Polysulfone. Polysulfone homo- (19) and copolymers have been formed into hollow fiber membranes for hemofiltration by solution extrusion, followed by coagulation and washing. Equilibrium water absorption by these membranes varies between 0.85 and 2.1 percent. The water-equilibrated polymers are glassy at room temperature. Sieving properties of copolymer hemofilters prepared with a 2-phenyl-2-phenoxy propane segment have been characterized (2). The hydrophobicity of this polymer is reflected in a significantly altered rejection spectrum in the presence of protein when compared to saline solutions. 

The low ethanol rejection and the instability of the hollow-fiber NS-100 membranes preclude the use of this membrane for practical ethanol enrichment. Nevertheless, for the purpose of demonstrating the concept of CCRO using hollow-fiber membranes, CCRO experiments were conducted at the reduced ethanol concentration of 10 vol%. The permeate fluxes of NS-100 modules were measured at 250 psi in the absence and presence of recirculation with a 10-volX ethanol solution. The results were varied recirculation brought about flux increases ranging from 5% to about 20%. The limited flux increase may again be explained in terms of the formation of a polyamine gel during NS-100 membrane fabrication. Nevertheless, the flux increase shows that the hollow-fiber geometry is a viable one for CCRO operation. 

Ultrafiltration Rates and Rejection of Solutes by Cellulosic Hollow Fibers... 

The boundary-layer theory used here to correct observed rejection coefficients is an improvement over thin-film theory, but it ag ears limited to filtrate velocities, J, below about 0.5 X 10 cm/sec for highly rejected solutes. Xn exact theory for incomplete rejection by hollow fibers is needed to define the validity of Equation 23, over the range of conditions of the experiments. 

Solute rejection for four solutes and ultrafiltration rates for two protein solutions have been measured for high-flux cellu-losic hollow-fiber bundles of three lengths. 

Example 26.4. A hollow-fiber permeator with d, = 300 /rm and d-, 200 /an gives a water flux of 10 gal/day-ft with O.I Af NaCl solution at 20°C, and the salt rejection is 97 percent. Feed solution flows normal to the fibers at an average superficial velocity of 0.5 cm/s. Is concentration polarization significant ... 

Ismail, A.F. and Lau, W.J. 2009b. Influence of feed conditions on the rejection of salt and dye in aqueous solution by different characteristics of hollow fiber nanofiltration membranes. Dpsalin. Water Treat. 6 281-288. 

The previous discussion brings us to one of the most important features of UF gel polarization, which is important when the separation of macromolecules is involved in either flat sheet, tubular or hollow-fiber UF membrane configurations. As permeate containing the smaller molecules passes through the membrane, a layer of solution containing the larger rejected molecules accumulates adjacent to the membrane surface... 

Consider a protein solution having the characteristics of the feed solution in Examples 7.2.5 and 7.2.6. This solution has to be desalted via continuous diajiltration (Section 6.4.2.1) in the hollow fiber unit of Example 7.2.6. The diafiltration configuration is illustrated in Figure 7.2.5(e). For 2000 liters of feed protein solution to be desalted, determine the crossflow membrane area required if the following levels of salt removal are desired in 2 hours (a) 99% of the salt removed (b) 99.9% of the salt removed (c) 99.99% of the salt removed. It is known that salt has zero rejection. Specify the volume of buffer solution required in each case. 

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