Ultrafiltration membrane flux

The final parameter in Equation (4.9) that determines the value of the concentration polarization modulus is the diffusion coefficient A of the solute away from the membrane surface. The size of the solute diffusion coefficient explains why concentration polarization is a greater factor in ultrafiltration than in reverse osmosis. Ultrafiltration membrane fluxes are usually higher than reverse osmosis fluxes, but the difference between the values of the diffusion coefficients of the retained solutes is more important. In reverse osmosis the solutes are dissolved salts, whereas in ultrafiltration the solutes are colloids and macromolecules. The diffusion coefficients of these high-molecular-weight components are about 100 times smaller than those of salts. 

Figure 6.7 The effect of pressure on ultrafiltration membrane flux and the formation of a secondary gel layer. Ultrafiltration membranes are best operated at pressures between p2 and p3 at which the gel layer is thin. Operation at high pressures such as p4 leads to formation of thick gel layers, which can consolidate over time, resulting in permanent fouling of the membrane...

Limitations Some apphcations which seem ideal for MF, for example the clarification of apple j mce, are done with UF instead. The reason is the presence of deformable sohds which easily plug and blind an MF membrane. The pores of an ultrafiltration membrane are so small that this phigging does not occur, and high fluxes are maintained. UF can be used because there is no soluble macromolecule in the juice that is desired in the filtrate. There are a few other significant applications where MF seems obvious, but is not used because of dejormable particle plugging. 

Fig. 5-4. (a) Separation of d,1-phenylalanine by an amino aeid immobilized in the pores of a polysulfone ultrafiltration membrane, (b) Effeet of volume flux on the separation faetor, Jv = volume flux, T = 37 °C [32],... 

Subsequently, a clear Juice is obtained by ultrafiltration. A serious problem in this process is the fouling of the ultrafiltration membrane, causing a reduced flux rate. For apple processing, the material responsible for this effect has been isolated and extensively characterized [2-4]. It appeared to consist mainly of ramified pectic hairy regions (MHR), which were not degraded by the pectolytic enzymes present in the technical pectinase preparation. 

Figure 8.8. Dependence of membrane flux J on (a) Applied pressure difference AP, (b) Feed solute concentration Cf, (c) Cross-flow velocity (u) for ultrafiltration...

Membrane Properties. The performance range of ammonia-modified membranes in low pressure operation is indicated in Figure 6 along with the performance of the reference membrane (I, reference membrane IV, ammonia-modified membrane). The lower boundary of the performance range refers to a solvent-to-polymer ratio of 3, the upper boundary to a ratio of 4. While the salt rejection towards univalent ions of the ammonia-modified membrane is limited to below 80 %, the maximum low pressure flux is over 15 m /m d (approaching 400 gfd) at a sodium chloride rejection of the order of 10 %. This membrane thus exhibits the flux capability of an ultrafiltration membrane while retaining the features of reverse osmosis membranes, viz. asymmetry and pressure resistance. 

The same nanofiltration experiments were performed with a 50-A ultrafiltration membrane (available from US Filter/Membralox, Warrendale, PA,USA), this time with a monodentate phosphite ligand (24) used for comparison and toluene as the solvent (Table V). Both higher retentions and flux rates for the dendrimers were obtained relative to what was observed with the reverse osmosis membranes. Dendrophite G4 was used in three subsequent reactions carried out with this procedure. 

Rhodium Retention and Flux Rate Measurements using a 50 A Ultrafiltration Membrane... 

Cabral and coworkers [253] have investigated the batch mode synthesis of a dipeptide acetyl phenylalanine leucinamide (AcPhe-Leu-NH2) catalyzed by a-chymotrypsin in a ceramic ultrafiltration membrane reactor using a TTAB/oc-tanol/heptane reverse micellar system. Separation of the dipeptide was achieved by selective precipitation. Later on the same group successfully synthesized the same dipeptide in the same reactor system in a continuous mode [254] with high yields (70-80%) and recovery (75-90%). The volumetric production was as high as 4.3 mmol peptide/l/day with a purity of 92%. The reactor was operated for seven days continuously without any loss of enzyme activity. Hakoda et al. [255] proposed an electro-ultrafiltration bioreactor for separation of RMs containing enzyme from the product stream. A ceramic membrane module was used to separate AOT-RMs containing lipase from isooctane. Application of an electric field enhanced the ultrafiltration efficiency (flux) and it further improved when the anode and cathode were placed in the permeate and the reten-tate side respectively. 

The transition between reverse osmosis membranes with a salt rejection of more than 95 % and molecular weight cutoffs below 50 and ultrafiltration membranes with a salt rejection of less than 10% and a molecular weight cutoff of more than 1000 is shown in Figure 2.42 [74], The very large change in the pressure-normalized flux of water that occurs as the membranes become more retentive is noteworthy. Because these are anisotropic membranes, the thickness of the separating layer is difficult to measure, but clearly the permeability of... 

The production by Loeb and Sourirajan of the first successful anisotropic membranes spawned numerous other techniques in which a microporous membrane is used as a support for a thin, dense separating layer. One of the most important of these was interfacial polymerization, an entirely new method of making anisotropic membranes developed by John Cadotte, then at North Star Research. Reverse osmosis membranes produced by this technique had dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Souri-rajan process. Almost all reverse osmosis membranes are now made by the interfacial polymerization process, illustrated in Figure 3.20. In this method, an aqueous solution of a reactive prepolymer, such as a polyamine, is first deposited in the pores of a microporous support membrane, typically a polysul-fone ultrafiltration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, such as a diacid chloride in hexane. The amine and acid chloride react at the interface of the two immiscible... 

Figure 4.11 shows that ultrafiltration and pervaporation for the removal of organic solutes from water are both seriously affected by concentration polarization. In ultrafiltration, the low diffusion coefficient of macromolecules produces a concentration of retained solutes 70 times the bulk solution volume at the membrane surface. At these high concentrations, macromolecules precipitate, forming a gel layer at the membrane surface and reducing flux. The effect of this gel layer on ultrafiltration membrane performance is discussed in Chapter 6. 

The formation of a gel layer of colloidal material at the ultrafiltration membrane surface produces a limiting or plateau flux that cannot be exceeded at any particular operating condition. Once a gel layer has formed, increasing the applied... 

The effect of the gel layer on the flux through an ultrafiltration membrane at different feed pressures is illustrated in Figure 6.7. At a very low pressure p, the flux Jv is low, so the effect of concentration polarization is small, and a gel layer does not form on the membrane surface. The flux is close to the pure water flux of the membrane at the same pressure. As the applied pressure is increased to pressure p2, the higher flux causes increased concentration polarization, and the concentration of retained material at the membrane surface increases. If the pressure is increased further to p3, concentration polarization becomes enough for the retained solutes at the membrane surface to reach the gel concentration cgel and form the secondary barrier layer. This is the limiting flux for the membrane. Further increases in pressure only increase the thickness of the gel layer, not the flux. 

Experience has shown that the best long-term performance of an ultrafiltration membrane is obtained when the applied pressure is maintained at or just below the plateau pressure ps shown in Figure 6.7. Operating at higher pressures does not increase the membrane flux but does increase the thickness and density of retained material at the membrane surface layer. Over time, material on the membrane surface can become compacted or precipitate, forming a layer of deposited material that has a lower permeability the flux then falls from the initial value. 

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...

Experiments with different ultrafiltration membranes and the same feed solution often yield very different ultrafiltration limiting fluxes. But according to the model shown in Figure 6.6 and represented by Equation (6.2), the ultrafiltration limiting flux is independent of the membrane type. 

In the absence of suspended solutes or colloids, the pure solvent flux through an ultrafiltration membrane is directly proportional to the applied pressure difference and inversely proportional to the viscosity of the solvent and the membrane thickness. Transport within the pores occurs in the creeping flow regime, since kinematic viscosities of liquids are sufficient to make Re < C 1 for practical pore sizes. In the simplest case, the membrane can be considered to be a packed array of straight, equal diameter nonintersecting capillary tubes. The observed volumetric flux, nAvA (cc/sec cm2), equals the product of the mass flux of solvent based on the total membrane area, nA... 

Koschuh, W., V.H. Thang, S. Krasteva, S. Novalin and K.D. Kulbe, Flux and Retention Behaviour of Nano filtration and Fine Ultrafiltration Membranes in Filtrating Juice from a Green Biorefinery A Membrane Sreening, J. Membr. Sci., 261, 121-128 (2005). 

---