The Formation of Filter Cake

Filtration operations are capable of handling suspensions of varying characteristics ranging from granular, incompressible, free-filtering materials to slimes and colloidal suspensions in which the cakes are incompressible. These latter materials tend to contaminate or foul the filter medium. The interaction between the particles in suspension and the filter medium determines to a large extent the specific mechanisms responsible for filtration. 

The structure of the cake formed and, consequmtly, its resistance to liquid flow depends on the properties of the solid particles and the liquid phase suspension, as well as on the conditions of filtration. Cake structure is first established by hydrodynamic factors (cake porosity, mean particle size, size distribution, and particle specific surface area and sphericity). It is also strongly influenced by some factors that can conditionally be denoted as physicochemical. These factors are  

the influence of electrokinetic potentials at the interphase in the presence of ions, which decreases the effective pore cross section, and 

the presence of solvate shells on the solid particles (this action is manifested at particle contact during cake formation). 

Due to the combining effects of hydrodynamic and ysicochemical factors, the study of cake structure and resistance is extremely complex, and any mathematical description based on theoretical considerations is at best only descriptive. 

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Cross-flow-elec trofiltratiou (CF-EF) is the multifunctional separation process which combines the electrophoretic migration present in elec trofiltration with the particle diffusion and radial-migration forces present in cross-flow filtration (CFF) (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears later in this section) in order to reduce further the formation of filter cake. Cross-flow-electrofiltratiou can even eliminate the formation of filter cake entirely. This process should find application in the filtration of suspensions when there are charged particles as well as a relatively low conduc tivity in the continuous phase. Low conductivity in the continuous phase is necessary in order to minimize the amount of elec trical power necessaiy to sustain the elec tric field. Low-ionic-strength aqueous media and nonaqueous suspending media fulfill this requirement. 

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). 

MFI measurements are based on the formation of a cake or gel when water containing colloids are filtered through a membrane filter. Recently the use of the MFI parameter has been studied in relation to slow sand filtration performance 5 and in relation to clogging of injection wells [6 ]. 

Cake Space The volumetric space available in a filter to support the formation of a cake. 

External filter cake formation causes a fast and sharp drop in permeability or injectivity of the formation. Internal filter cake formation cases a steady drop in permeability. Increasing solids concentration or particle size will cause more damage to formation. 

In similar manner, excessive stretch caused by the mass of filter cakes in filter press operations could result in the port holes in the fabric moving out of alignment with the corresponding holes in the filter plate, thereby restricting the flow of filtrate out of the press. Likewise, in other filtration systems, such as pressure leaf filters, the same stretch could result in the formation of creases and ultimately mechanical damage to the fabric. 

In the process of filtration, the medium resistance over the fluid flow increases with the formation of a cake or with time as the filter medium is obstructed. The main variables of interest are the flow velocity through the filter and the pressure drop in the unit. 

As wastewater permeates through the membrane there is a tendency for a cake of solids to form on the feed side of the membrane. The thickness of the cake is typically limited by the shearing action of the cross-flow, such that the flux undergoes an initial decay, and then stabilizes to a nearly steady-state value. With continued use, the cake may begin to density, at which point the membrane typically requires cleaning to prevent further decline in the flux rate. If, however, some of the solids or dissolved chemicals in the feed water load the pore stmemre of the filters, the flux may decline in an unrecoverable manner. In addition to the tendency for the formation of a cake on the feed side of the membrane concentration polarization may also limit the water flux through the membrane. If the flux is held at a constant value, the transmembrane... 

The cross-flow filtration device (Figure 14.6(b)) aims to avoid the deposition of particles on the upstream side of the membrane and the associated resistance to filtration. Parallel to the surface of the filter, a flow is maintained, whose velocity is much higher than the velocity of the permeate flow induced by the pressure difference through the filter. The fluid and particles essentially just travel along the surface of the filter, and a small fraction of the fluid flow rate passes through the membrane. The friction of the tangential flow on the filter wall is used to re-entrain the particles that are deposited on the filter, in order to prevent the formation of a cake which would diminish the filtration flow rate. Cross-flow filtration reduces the phenomenon of membrane clogging. 

Table 3.1 is organized primarily by the type of filtration used in each filter. Relatively pure surface filtration comes first, exemplified by screens, followed by depth filtration. The bulk of the table is taken up with those filters that work initially by surface screening, but which move quickly into the formation of a cake of accumulated solids on the surface of the filter medium. Once a thin cake is formed, the remaining filtration occurs through this cake, which grows in thickness until the pressure differential across it becomes too great. 

The very feature that distinguished the plate filter, i.e. the formation of a cake on the upper surface of the element only, makes this type that much more difficult to operate when it comes to dry cake removal. Wet discharge is as easy as with a leaf... 

In situations where a low concentration of suspended solids needs to be separated from a liquid, then cross-flow filtration can be used. The most common design uses a porous tube. The suspension is passed through the tube at high velocity and is concentrated as the liquid flows through the porous medium. The turbulent flow prevents the formation of a filter cake, and the solids are removed as a more concentrated slurry. 

Surfactants aid dewatering of filter cakes after the cakes have formed and have very Httle observed effect on the rate of cake formation. Equations describing the effect of a surfactant show that dewatering is enhanced by lowering the capillary pressure of water in the cake rather than by a kinetic effect. The amount of residual water in a filter cake is related to the capillary forces hoi ding the Hquids in the cake. Laplace s equation relates the capillary pressure (P ) to surface tension (cj), contact angle of air and Hquid on the soHd (9) which is a measure of wettabiHty, and capillary radius (r ), or a similar measure appHcable to filter cakes. 

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