Microfiltration filtration flux

Blood cells are separated from blood (hematocrit 40%) by microfiltration, using hollow-fiber membranes with an inside diameter of 300 pm and a length of 20 cm. The average flow rate of blood is 5.5 cm s T Estimate the filtrate flux. 

Fig. 6. Normalized microfiltration permeate flux vs time for different concentrations of yeast in secondary feed reservoir ( ) 1.36 g/L ( ) 0.68 g/L ( ) 0.34 g/L. The cycle conditions were tf = 280 s, = 10 s, and th = 10 s, with Pf=Ph = 7.5 psi. The primary feed contained a mixture of yeast (with the same concentration of yeast as in the secondary feed) and 2.0 g/L of BSA. The dashed line represents the normalized permeate flux during filtration of protein alone without deposition of SMY and without backflushing. Error bars represent SD for two to three repeats.

Surface water can also be processed to become drinking water but it requires some pretreatment prior to the microfiltration step. A filtrate flux of 1,000-1,500 L/hr-m can be realized [Guibaud, 1989]. 

Two process modes, namely, dead-end and cross-flow modes, are widely used for microfiltration (14). For the dead-end mode, the entire solution is forced through the membrane. The substances to be separated are deposited on the membrane, which increases the hydraulic resistance of the deposit. The membrane needs to be renewed as soon as the filtrate flux no longer reaches the required minimum values at the maximum operation pressure. This mode is mostly used for slightly contaminated solutions, e.g., production of ultra-pure water. For the cross-flow mode, the solution flows across the membrane surface at a rate between 0.5 and 5.0 m/s, which prevents the formation of a cover layer on the membrane surface. A circulation pump produces the cross-flow velocity or the shear force needed to control the thickness of the cover layer. The system is most widely used for periodic back flushing, where part of the filtrate is forced in the opposite direction at certain intervals, and breaks up the cover layer. The normal operating pressure for this mode is 1-2 bars. 

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. 

Depending on the size of cells and debris, and the desired clarity of the filtrate, microfiltration membranes with pore sizes ranging from 0.01 to 10 pm can be used. In cross-flow filtration (CFF see Figure 9.2b), the liquid flows parallel to the membrane surface, and so provides a higher filtration flux than does dead-end filtration (Figure 9.2a), where the liquid path is solely through the membrane. In CFF, a lesser amount of the retained species will accumulate on the membrane surface, as some of retained species is swept from the membrane surface by the... 

We wish to concentrate and achieve a solvent switch for a solution by batch crossflow microfiltration. The flux, jy, for the ceramic microfiltration membrane is 10 gal/(h-tf). The initial solution volume is 1800 gal the final volume is 360 gal. The amount of protein present is 18.0 kg, and the molecular weight is 1,213.43 g/mole. The pressure drop is 30psi (essentially 2 aim) and the operating temperature is 277 K. Calculate the area. A, required to complete the filtration in 2 h. 

Let us focus first on cake filtration and microfiltration for the case where the fluid is a liquid. In the configuration of Figure 6.3.21, the techrtique is called deadend filtration. The same configuration is routinely employed in lahoratories with a filter paper on, say, a Buchner funnel and a partial vacuum on the side of the permeate/filtrate a precipitate/ deposit builds up quickly on the filter paper as the slurry is filtered. As time passes, a particle based deposit continues to build up on the filter paper it is called a cake. This cake provides an additional resistance to the flow of the filtrate through the membrane/filter/cloth in deadend filtration. As time passes, deposition of the particles onto/in the cake continues. Therefore the resistance to the flow of the filtrate increases with time. If one wants to maintain a constant eflae of the filtrate flux, the applied pressure difference AP has to increase with time. Alternatively, for a constant applied pressure difference, the flux of the filtrate will decrease with time (Figure 6.3.22). 

Consider first the case of constant filtration flux in cake filtration/microfiltration. Since the volume flux Vs is... 

We observed earlier in dead-end cake filtration (equation (6.3.135k)) that Rcs varies inversely with the square of the particle radius therefore, in effect, the filtration flux varies with the square of the particle radius for cake dominated filtration. The larger the particle radius, the higher the filtration flux. As shown in equation (7.2.145), in cross-flow microfiltration also the averaged filtration flux increases with particle radius, here as Romero and... 

Recent research into the reduction of filtration flux that occurs as soon as the microfiltration process is started (Figure 7.2.7(a)) indicates that it is very much dependent on the level of the flux and therefore the applied AP. Beyond a critical flux, Vs,a, deposits form on the membrane since the forces dragging the colloidal particles in suspension toward the membrane are larger than the forces causing the particles to move away from the membrane. If this critical flux value is reached at the end of the membrane channel, where the boundary layer is the thickest, then the... 

A. Zydney and C. Colton. A concentration polarization model for filtrate flux in cross-flow microfiltration of particulate suspension. Chem. Eng. Commun., 47 1, 1986. 

Ideally, cross-flow microfiltration would be the pressure-driven removal of the process liquid through a porous medium without the deposition of particulate material. The flux decrease occurring during cross-flow microfiltration shows that this is not the case. If the decrease is due to particle deposition resulting from incomplete removal by the cross-flow liquid, then a description analogous to that of generalised cake filtration theory, discussed in Chapter 7, should apply. Equation 8.2 may then be written as ... 

After extraction, the solute-laden CLAs need to be separated from the mother liquor so that they can be back stripped. Hence attempts were made to filter the solute-rich CLAs from the aqueous phase using cross-flow microfiltration [70]. The filtration characteristics of the CLAs as indicated by the flux, CLA size, and concentration showed that they are completely retained by the membrane and do not foul the membrane surface. Using this system, the CLAs could easily be concentrated up to 30% w/v at low pressures, and the permeate stream remained totally clear. The CLAs appear to maintain their structural integrity because only 3 mg dm of SDS was... 

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... 

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

In the current work, we employed a modified approach, with predeposition of a secondary membrane of yeast (SMY) before starting the filtration of protein. Backflushing was employed periodically to remove the deposited secondary membrane to recover the flux, and a new secondary membrane was deposited subsequently with the start of each new cycle, prior to restarting the filtration of protein. Microfiltration experiments were performed with yeast as the secondary membrane and BSA-only solutions and yeast-BSA mixtures as the feed. Ultrafiltration experiments were performed with yeast as the secondary membrane deposition medium and cellulase enzyme solutions, used in the conversion of biomass into ethanol, as the feed. In this article, we also present direct visual observation images (19) of the formation of the secondary membrane and its subsequent removal. 

Fig. 5. Permeate flux during microfiltration of mixture of 2.0 g/L of BSA and 1.34 g/L of yeast with deposition of SMY and with backflushing (A), without SMY but with backflushing (x), and without deposition of SMY and without backflushing ( ) permeate flux for filtration of BSA without deposition of SMY and with backflushing...

Fig. 7. Normalized recovered flux plotted after 600 s ( , cycle 2), 900 s ( , cycle 3), and 1200 s (O, cycle 4) of microfiltration with deposition of SMY. The SMY was deposited for tsf= 25 s, followed by forward feed filtration of 2.0 g/L of BSA for tf= 275 s and then th = 0.1,0.2,0.5,1.0,2.0, or 5.0 s of backflushing. The points are joined by straight lines for clarity. The error bars represent SD for three repeats.

High-purity WPC (i.e., 70-95% proteins or total solids) can be produced by thermocalcic aggregation, followed by microfiltration, ultra filtration and diafiltration of whey proteins. Ultraflltration has been practiced since early 1970s. It appears that zirconia membranes on carbon supports with a MWCO of 10,000 to 20,000 daltons and zirconia membranes on alumina supports with a pore diameter of 0.05 to 0.1 pm are suitable for this purpose. A permeate flux of as high as 60 L/hr-m for processing acid whey to a protein content of 25 to 37% using a zirconia membrane with a MWCO of 10,000 daltons has been reported [Merin and Daufin, 1989]. 

Microfiltration and ultrafiltration are the two main filtration techniques for which ceramic membranes have been widely used to date. As described in Section 6.2.1.2, MF and UF ceramic membranes exhibit macro- and mesoporous structure, respectively, which result from packing and sintering of ceramic particles. Liquid flow in such porous media is convective in nature and the simplest description of permeation flux, J, is given by the Darcy s equation [20] ... 

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