Ultrafiltration of macromolecular solutions

R.W. Baker and H. Strathmann, Ultrafiltration of Macromolecular Solutions with High-flux Membranes, J. Appl. Polym. Sci. 14, 1197 (1970). 

In the ultrafiltration of macromolecular solutions, a large number of investigators have observed that as pressure is increased, permeate flux first increases and then remains more or less pressure independent. Blatt, (1970), among others,... 

Gel polarized ultrafiltration was recently analyzed for cross flow and unstirred batch cell systems by Trettin and Doshi (1980 a,b). We have shown in these papers that the widely used film theory does not predict the limiting flux accurately. The objective of this paper is to derive an expression for the permeate flux when the pressure independent ultrafiltration of macro-molecular solutions is osmotic pressure limited. We will also attempt to distinguish between gel and osmotic pressure limited ultrafiltration of macromolecular solutions. 

An early work considering osmotic pressure in the ultrafiltration of macromolecular solutions was done by Blatt, et al,. (1970), who employed a theory which had been developed for cross flow reverse osmosis systems. They essentially suggested that the film theory relationship given by Eq. (2) could be solved simultaneously with Eq. (1) to predict permeate rates, where the... 

With the ultrafiltration of macromolecular solutions in cross flow systems such as thin channel or tubular systems it is usually the procedure to measure average flux rates at steady state. Therefore, Eq. (81) may be integrated to give... 

In macromolecular ultrafiltration, 2is pressure is increased, permeate flux first Increases and then In a large number of cases levels out and remains more or less pressure Independent. This could be due to the increase In solute concentration at the membrane surface such that either gel formation occurs or the corresponding osmotic pressure approaches the applied pressure. Limiting flux for the gel polarized case was recently analyzed for cross flow and unstirred batch cell systems by Trettln and Doshi (1980,a, b). In this paper we have analyzed the osmotic pressure limited ultrafiltration for the two systems. Our unstirred batch cell data and the literature cross flow data agree quite well with the theory. We have further shown that an unstirred batch cell system can be used to determine whether pressure Independent ultrafiltration of macromolecular solution is gel or osmotic pressure limited. Other causes for the observed pressure Independence may be present but are not considered in this paper. 

Probstein, R. F., J, S. Shen, and W. F, Leung, "Ultrafiltration of Macromolecular Solutions at High Polarization in Laminar Channel Flow," Desalination, Volume 24, p. 1 (1978). 

Trettin, D. R., and M. R. Doshi, "Limiting Flux in Ultrafiltration of Macromolecular Solutions," To be Published, Chemical Engineering Communications (1980a). 

Rautenbach, R. and Schock, G., Ultrafiltration of macromolecular solutions and cross-flow microfiltration of colloidal suspensions. A contribution to permeate flux calculations , J. Membr. Sci., 30, 231 (1988)... 

For ultrafiltration, the macromolecular solutes and colloidal species usually have insignificant osmotic pressures. In this case, the concentration at the membrane surface (C ) can rise to the point of incipient gel precipitation, forming a dynamic secondary membrane on top of the primary structure (Figure 7). This secondary membrane can offer the major resistance to flow. 

Membrane processes, such as reverse osmosis and dialysis, are already used for certain effluent treatment applications and desalination. They can be operated continuously, and they allow recovery of the dissolved values. Membrane processes have benefited enormously from recent advances in membrane materials that can withstand high-pressure gradients and harsh chemical environments. Ultrafiltration of macromolecular complexes of metal ions, which shows more chemical specificity, was more promising for detoxifying effluents, and the work is still ongoing. Particulate matter, which is present in many environmental and processing solutions, can dramatically reduce the permeability of... 

The effect of osmotic pressure in macromolecular ultraflltra-tlon has not been analyzed in detail although many similarities between this process and reverse osmosis may be drawn. An excellent review of reverse osmosis research has been given by Gill et al. (1971). It is generally found, however, that the simple linear osmotic pressure-concentration relationship used in reverse osmosis studies cannot be applied to ultrafiltration where the concentration dependency of macromolecular solutions is more complex. It is also reasonable to assume that variable viscosity effects may be more pronounced In macromolecular ultra-filtration as opposed to reverse osmosis. Similarly, because of the relatively low diffuslvlty of macromolecules conqiared to typical reverse osmosis solutes (by a factor of 100), concentration polarization effects are more severe in ultrafiltration. 

Mazid M.A. (1988), Separation and fractionation of macromolecular solutions by ultrafiltration. 

Ultrafiltration is often applied for the concentration of macromolecular solutions where the large molecules have to be retained by the membrane while small molecules (and the solvent) should permeate freely. In order to choose a suitable membrane, manufacturers often used the concept of cut-off but this concept should be considered critically (see chapter IV). 

As mentioned above, concentration polarisation can be very severe in ultrafiltration because the flux through the membrane is high, the diffusivity of the macromolecules is rather low and the retention is normally very high. This implies that the solute concentration at the membrane surface attains a very high value and a maximum concentration, the gel concentration (Cg), may be reached for a number of macromolecular solutes. The gel concentration depends on the size, shape, chemical structure and degree of solvation but is independent of the bulk concentration. The two phenomena, concentration polarisation and gel formation are shown in figure VII -12. 

Ultrafiltration Asymmetric microporous membrane, 1 to 10 lA pore radius Hydrostatic pressure difference, 0.5 to 5 bar Sieving mechanism Separation of macromolecular solutions... 

A key factor determining the performance of ultrafiltration membranes is concentration polarization due to macromolecules retained at the membrane surface. In ultrafiltration, both solvent and macromolecules are carried to the membrane surface by the solution permeating the membrane. Because only the solvent and small solutes permeate the membrane, macromolecular solutes accumulate at the membrane surface. The rate at which the rejected macromolecules can diffuse away from the membrane surface into the bulk solution is relatively low. This means that the concentration of macromolecules at the surface can increase to the point that a gel layer of rejected macromolecules forms on the membrane surface, becoming a secondary barrier to flow through the membrane. In most ultrafiltration appHcations this secondary barrier is the principal resistance to flow through the membrane and dominates the membrane performance. 

A concentration process involves removal of a solvent, typically water from a macromolecular solution. Ultrafiltration is the method of choice for large-scale concentration. The selectivity issue involving removal of water from a macromolecular solution using ultrafiltration is trivial. The main challenge in a concentration process is maintaining a high productivity on account of the increased macromolecular concentration in the feed solution. Some of the main applications of macromolecular concentration using ultrafiltration are listed below [2] ... 

Moulin, P. Manno, P. Rouch, J.C. Serra, C. Clifton, M.J. Aptel, P. Flux improvement by Dean vortices ultrafiltration of colloidal suspensions and macromolecular solutions. J. Membr. Sci. 1999, 156, 109. 

Goldsmith (1971) pointed out that developed osmotic pressures for macromolecular solutions were not necessarily negligible. The ultrafiltration of Carbowax 20M (polyethylene oxide) and various Dextrans was studied in thin channel and tube flow as well as stirred batch cell. Both turbulent and laminar flow regimes were considered. Data were analyzed with the use of Eq. (2) and the phenomenological relationship of Eq. (1) with Rg = 0. From Eq. (1) it was possible to calculate an average... 

The success of the Leveque and Dittus-Boelter relationships in indicating the variation (power dependence) of ultrafiltrate flux with channel geometry and fluid velocity for macromolecular solutions is gratifying. The more crucial test of the theory, of considerable interest to the design engineer, is whether these relationships can be used to calculate quantitatively the ultrafiltrate flux knowing the channel geometry, fluid velocities and solute characteristics. 

Colloidal Suspensions. The agreement between theoretical and experimental ultrafiltration rates for macromolecular solutions can be said to be within 15 to 30%. For colloidal suspensions, experimental flux values are often one to two orders of magnitude higher than those indicated by the Leveque and Dittus-Boelter relationships. 

PROBSTEIN, R.F., LEUNG, W-F. ALLIANCE, Y. 1979. Determination of diffusivity and gel concentration in macromolecular solutions by ultrafiltration. J. Phys. Chem. 83, 1228-1232. 

This latter equation is the basic equation of the boundary layer resistance model [17-19]. The boundary layer can be considered as a concentrated solution through which solvent molecules permeate, with the permeability of this stagnant layer depending very much on the concentration and the molecular weight of the solute. The resistance exerted by thislayer is far much greater for macromolecular solutes (ultrafiltration) relative to for low molecular weight. solutes (reverse osmosis). Because there is a concentration profile in the boundary layer, the permeability P of the solvent may be written as a function of the distance coordinate x w ith the boundaries x = 0 and x = 6. 

Ultrafiltration is used in a wide range of applications, mainly in the food, dairy, textile, metallurgy and pharmaceutical industries. The feed is generally an aqueous solution containing macromolecular solutes, emulsions or suspended solids. Flux decline due to concentration polarisation and fouling presents a serious problem. To reduce this phenomenon, high cross-flow velocities are required. 

A key factor determining the performance of ultrafiltration membranes is concentration polarization, which causes membrane fouling due to deposition of retained colloidal and macromolecular material on the membrane surface. The pure water flux of ultrafiltration membranes is often very high— more than 1 cm /(cm min) [350 gal/(ft day)]. However, when membranes are used to separate macromolecular or colloidal solutions, the flux falls within seconds, typically to the 0.1 cm /(cm min) level. This immediate drop in flux is caused by the formation of a gel layer of retained solutes on the membrane surface because of the concentration polarization. The gel layer forms a secondary barrier... 

The addition of water-soluble polymers followed by ultrafiltration, named as polyelectrol)de-enhanced ultrafiltration (PEUF), can be efficiently exploited to remove ionic species from aqueous solutions. This process is based on the use of a polyelectrol)de having an opposite charge to that of the target ions and the formation of macromolecular complexes between pollutant ions and polymer due to electrostatic attractions. These complexes are too large to pass through a UF membrane so they are retained in the retentate streams. Examples of separation of both cationic and anionic metal ions by PEUF have been extensively reported (Christian et al., 1995 Tabatabai et al, 1995a Tangvijitsri et al, 2002). 

Ultrafiltration is a membrane separation process, used for the concentration and purification of macromolecular dissolved solids and very fine suspended solids (colloids), in which the solution is caused to flow under pressure across the membrane surface. Solubles and colloids are rejected at the semi-permeable membrane barrier, while solvents and microsolutes below the molecular weight cut-off (MCWO - usually in the range of 1000 to 1,000,000) pass through the membrane as the permeate. The materials retained at the membrane surface are carried on downstream by the flowing process liquid as retentate (or concentrate). The pro-cess is directly comparable with reverse osmosis, but because of the looser, more open membranes used for ultrafiltration, operation pressures of only 0.6-6 bar are needed (as against 20 to 100 bar for reverse osmosis). 

As discussed previously, the technique of microfiltration is effectively utilized to remove whole cells or cell debris from solution. Membrane filters employed in the microfiltration process generally have pore diameters ranging from 0.1 to 10 pm. Such pores, while retaining whole cells and large particulate matter, fail to retain most macromolecular components, such as proteins. In the case of ultrafiltration membranes, pore diameters normally range from 1 to 20 nm. These pores are sufficiently small to retain proteins of low molecular mass. Ultrafiltration membranes with molecular mass cut-off points ranging from 1 to 300 kDa are commercially available. Membranes with molecular mass cut-off points of 3,10, 30, 50, and 100 kDa are most commonly used. 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

Fig. 4.4 Scheme of pulsed ultrafiltration-mass spectrometry (PUF-MS) to screen chemical mixtures for compounds that bind to a macromolecular receptor. The ultrafiltration membrane traps a receptor in solution, but allows low molecular weight... 

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