Microfiltration polarization

Early ultrafiltration membranes had thin surface retentive layers with an open structure underneath, as shown in Fig. 20-62. These membranes were prone to defects and showed poor retention and consistency. In part, retention by these membranes would rely on large retained components in the feed that polarize or form a cake layer that plugs defects. Composite membranes have a thin retentive layer cast on top of a microfiltration membrane in one piece. These composites demonstrate consistently high retention and can be integrity-tested by using air diffusion in water. 

Metal oxides, used for manufacturing of ceramic nanofiltration membranes, are intrinsically hydrophilic. This limits the use of these membranes to polar solvents filtration of nonpolar solvents (n-hexane, toluene, cyclohexane) usually yields zero fluxes. Attempts have been made to modify the pore structure by adding hydrophobic groups, for example, in a silane coupling reaction [38, 43]. This approach is similar to modifications of ultrafiltration and microfiltration membranes... 

A membrane is usually seen as a selective barrier that is able to be permeated by some species present into a feed while rejecting the others. This concept is the basis of all traditional membrane operations, such as microfiltration, ultrafiltration, nanofil-tration, reverse osmosis, pervaporation, gas separation. On the contrary, membrane contactors do not allow the achievement of a separation of species thanks to the selectivity of the membrane, and they use microporous membranes only as a mean for keeping in contact two phases. The interface is established at the pore mouths and the transport of species from/to a phase occurs by simple diffusion through the membrane pores. In order to work with a constant interfacial area, it is important to carefully control the operating pressures of the two phases. Usually, the phase that does not penetrate into the pores must be kept at higher pressure than the other phase (Figure 20.1a and b). When the membrane is hydrophobic, polar phases can not go into the pores, whereas, if it is hydrophilic, the nonpolar/gas phase remains blocked at the pores entrance [1, 2]. 

Qualitatively similar polarization responses are apparent for both microfiltration and ultrafiltration processes, but for microfiltration, additional more complex can occur as is discussed below (Belfort, Davis, and Zydney, 1994 Segre and Silberberg, 1962). The rejection parameter mentioned earlier can be written as either an observed, R0, or an intrinsic, R , value ... 

Gan, Q., Xue, M., and Rooney, D. (2006) A study of fluid properties and microfiltration characteristics of room temperature ionic liquids [C10-min][NT 2] and N8881[NTf2] and their polar solvent mixtures. Sep. Purif. Technol, 51 (2), 185-192. 

Astaire, J.C., Ward, R., German, J.B., Jimenez-Flores, R. 2003. Concentration of polar MFGM lipids from buttermilk by microfiltration and supercritical fluid extraction../ Dairy Sci. 86, 2297-2307. 

Many models have been published to calculate the microfiltration process. One important factor is the concentration polarization, which represents the most important limiting physical obstacle. At high particle concentration and with time, a layer is formed on the membrane. This layer is responsible for the flux reduction. A comprehensive overview on this technique is given by Ripperger52 and Staude.53 Often similar or identical module types are used in microfiltration and ultrafiltration. 

The relationship between permeate flux and the flow characteristics in microfiltration processes is often described using the him model, which is based on the concentration polarization concept [38] ... 

Blatt WF, Dravid A, Michaels AS, and Nelson L. Solute polarization and cake formation in membrane ultrafiltration Causes, consequences and control techniques. In Membrane Science and Technology, ed., Fhnn JE, Plenum Press, New York, 1970, pp. 47-97. 40. Blanpain-Avet P, Doubrovine N, Lafforgue C, and Lalande M. The effect of oscillatory flow on crossflow microfiltration of beer in a tubular mineral membrane system—membrane fouhng resistance decrease and energetic considerations. J. Membr. Sci., 1999 152(2) 151-174. 

Jonsson, G. and Wetten, I.G., Control of concentration polarization, fouling and protein transmission of microfiltration processes within the agro-hased industry. Proceedings of the ASEAN-EU Workshop on Membrane Technology in Agro-Based Industry, Kuala Lumpur, Malaysia, 1994, p. 157. 

B.B. Gupta, P. Blanpain and M.Y. Jaffrin, Permeate flux enhancement by pressure and flow pulsations in microfiltration with mineral membranes. /. Membr. Sci., 70 (1992) 257. M.Y. Jaffrin, L.H. Ding and J.M. Laurent, Kinetics of concentration polarization formation in crossflow filtration of plasma from blood experimental results. /. Membr. Sci., 72 (1992) 267. 

A theoretical analysis has been carried out for galvanostatic and potentiostatic pulse regimes [27]. The idea that developed is a bit the same as backflushing with pressure driven-membrane operation such as microfiltration or ultrafiltration. The time dependencies of the extent of the concentration polarization near the membrane surface during the pulse are described theoretically for both pulse regimes and a qualitative discussion of the pause duration is presented. The main characteristic of the non-stationary process is the transition time between the state without polarization and the state with stationary polarization. 

In practice, however there could be differences between the observed and estimated flux. The mass transfer coefficient is strongly dependent on diffusion coefficient and boundary layer thickness. Under turbulent flow conditions particle shear effects induce hydrodynamic diffusion of particles. Thus, for microfiltration, shear-induced difflisivity values correlate better with the observed filtration rates compared to Brownian difflisivity calculations.Further, concentration polarization effeets are more reliably predicted for MF than UF due to the fact diat macrosolutes diffusivities in gels are much lower than the Brownian difflisivity of micron-sized particles. As a result, the predicted flux for ultrafiltration is much lower than observed, whereas observed flux for microfilters may be eloser to the predicted value. 

Although the driving potential in microfiltration is the hydraulic pressure gradient, the microfiltration flux is often also affected by the fluid velocity along the membrane surface. This is invariably due to the accumulation of filtered particles on the membrane surface in other words, the concentration polarization of particles. 

Equation 8.7 [6] was obtained to correlate the experimental data on membrane plasmapheresis, which is the microfiltration of blood to separate the blood cells from the plasma. The filtrate flux was affected by the blood velocity along the membrane. Since, in plasmapheresis, all of the protein molecules and other solutes will pass into the filtrate, the concentration polarization of protein molecules is inconceivable. In fact, the hydraulic pressure difference in plasmapheresis is smaller than that in the UF of plasma. Thus, the concentration polarization of red blood cells was assumed in deriving Equation 8.7. The shape of the red blood cell is approximately discoid, with a concave area at the central portion, the cells being approximately 1-2.5 pm thick and 7-8.5 pm in diameter. Thus, a value of r ( = 0.000 257 cm), the radius of the sphere with a volume equal to that of a red blood cell, was used in Equation 8.7. 

Some investigators [8] have suspected that the rate of microfiltration of blood is controlled by the concentration polarization of the platelet (another type of blood cell which is smaller than the red blood cell), such that the effective diffusivity of platelets is affected by the movements of red blood cells. 

Screening Equipment Filtration Cake Filters Centrifugal Filters Principles of Cake Filtration Clarifying Filters Liquid Clarification Gas Cleaning Principles of Clarification Crossflow Filtration Types of Membranes Permeate Flux for Ultrafiltration Concentration Polarization Partial Rejection of Solutes Microfiltration... 

All types of tangential microfiltration produce higher quality clarification than those achieved by filtration through a kieselguhr precoat. However, the filtrates are not always sterile, particularly when unclogging by reversed flow has destroyed the polarization layer. 

Akay G., Odirile P.T., Keskinler B., Wakeman R.J. 2000. Crossflow microfiltration characteristics of surfactants The effects of membrane physical chemistry and surfactant phase behavior on gel polarization and rejection. In, Surfactant Based Separations Science and Technology, Scamehorn J.F. and Harwell J.H. (Eds.), ACS Symposium Series, 740, 175-200. 

In an ideal case, in the absence of any concentration polarization (CP), the concentration of the permeants should not change from bulk-feed phase to the membrane-feed interface. However, due to finite mass transfer effects, there could be a decrease in concentration of the permeants over the diffusion film. In the case of ultratiltration, microfiltration, or reverse osmosis, the retained solute on... 

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