Caked layer

Mlcrofiltra.tlon, Various membrane filters have been used to remove viral agents from fluids. In some cases, membranes which have pores larger than the viral particle can be used if the filtration is conducted under conditions which allow for the adsorption of the viral particle to the membrane matrix. These are typically single-pass systems having pore sizes of 0.10—0.22 lm. Under situations which allow optimum adsorption, between 10—10 particles of poHovims (28—30 nm) were removed (34—36). The formation of a cake layer enhanced removal (35). The titer reduction when using 0.10—0.22 p.m membrane filters declined under conditions which minimized adsorption. By removal standards, these filters remove vimses at a rate on the low end of the desired titer reduction and the removal efficiency varies with differences in fluid chemistry and surface chemistry of viral agents (26). 

Concurrent with the liquid-shiny interface moving radially outward, the cake layer builds up with the cake-slurry interface moving radiaUy inward, with radial position given by ... 

The cyliudricaTsection provides clarification under high centrifugal gravity. In some cases, the pool should be shallow to maximize the G-force for separation. In other cases, when the cake layer is too thick inside the cylinder, the settled solids—especially the finer particles at the cake surface—entrain into the fast-moving liqmd stream above, which eventually ends up in the centrate. A slightly deeper pool... 

The additive should provide a thin layer of solids having high porosity (0.85 to 0.90) over the filter medium s external surface. Suspension particles will ideally form a layered cake over the filter aid cake layer. The high porosity of the filter aid layer will ensure a high filtration rate. Porosity is not determined by pore size alone. High porosity is still possible with small size pores. 

Figure 7. Distribution of static pressure p j in liquid and p along the cake thickness and filter plate I, II -boundaries between the cake and sludge at x" and x III, IV-boundaries between cake layers or cake and filter plate at x" and x V- boundary line between the cake and filter plate or free surface of filter plate 1,3-curves Ps,=f(ho,) and p=f(h J at x 2, 4 -curves Ps,=f(hgj.) and P=f(hoc)...

Cake layer formation builds on the membrane surface and extends outward into the feed channel. The constituents of the foulant layer may be smaller than the pores of the membrane. A gel layer can result from denaturation of some proteins. Internal pore fouling occurs inside the membrane. The size of the pore is reduced and pore flow is constricted. Internal pore fouling is usually difficult to clean. 

Component Transport Transport through membranes can be considered as mass transfer in series (1) transport through a polarization layer above the membrane that may include static or dynamic cake layers, (2) partitioning between the upstream polarization layer and membrane phases at the membrane surface, (3) transport through the membrane, and (4) partitioning between the membrane and downstream fluid. 

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. 

The formation of a cake layer can be neglected (e.g., biocatalytic reaction without macromolecules) in this case the concentration-polarization layer can also exist ... 

Equation 14.22 takes into account that the membrane and cake layers could have different partition coefficient. Thus, you can get an algebraic equation system with 6 equations, which can be solved relatively easily (as will be discussed elsewhere). Obviously, for prediction of the filtration efficiency, the values of transport parameters have to be known. 

The basic hydrodynamic equations are the Navier-Stokes equations [51]. These equations are listed in their general form in Appendix C. The combination of these equations, for example, with Darcy s law, the fluid flow in crossflow filtration in tubular or capillary membranes can be described [52]. In most cases of enzyme or microbial membrane reactors where enzymes are immobilized within the membrane matrix or in a thin layer at the matrix/shell interface or the live cells are inoculated into the shell, a cake layer is not formed on the membrane surface. The concentration-polarization layer can exist but this layer does not alter the value of the convective velocity. Several studies have modeled the convective-flow profiles in a hollow-fiber and/or flat-sheet membranes [11, 35, 44, 53-56]. Bruining [44] gives a general description of flows and pressures for enzyme membrane reactor. Three main modes... 

In this case the fluid phase is aerated (in the case of aerobic bioreactor) that maintains the turbulent hydrodynamic conditions on the one hand, and prevents the forming of the cake layer on the immersed membrane module, on the other hand. The reactor description is also well known [67], and is not discussed here. 

External fouling is caused by the formation of a cake layer of cells or other materials on the membrane surface, leading to a reduction in permeate flux (defined as the volume of permeate produced per time and membrane area). Internal fouling is caused mainly by proteins and particles smaller than membrane pores. Proteins and protein aggregates can adsorb or deposit at the pore entrance or inside the pores and cause pore blockage or narrowing, leading to increased hydraulic resistance (2). 

Marcinkowsky et al. (16) were the first to use dynamic secondary membranes in reverse osmosis for rejection of salts. Giiell et al. (17) later investigated protein transmission and permeate fluxes in microfiltration of protein mixtures using yeast to form a predeposited secondary membrane, and they observed higher flux and protein transmission in the presence of the secondary layer. Kuberkar and Davis (18) also observed higher flux and transmission of BSA in the presence of a cake layer of yeast,... 

If you re making a layer cake, place the cooled cakes upside down on cake rounds. Using a serrated knife, slice the cakes in half horizontally and place each layer on a cake round, so that you have four cake layers. 

To add a layer of cake to the frosted layer, pick up one of the other cake layers on a cake round, bend one edge of the cake round 2 inches away from the cake, and place the exposed corner of cake evenly on top of the frosted layer. With a knife or a clean spatula, slowly remove the layer from the cake round onto the frosted layer, dropping it gently on top. 

For a three-layer cake Place the cake layers on cake rounds. Spread V4 cup frosting across the top of one layer, then press V3 cup chopped pineapple evenly into the frosting. Place the second layer on top and add frosting and pineapple as with the first layer. For the final cake layer, frost the top but omit the pineapple. 

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