Cake permeability

Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the plates and the filtrate flows to further processing. When the solids are fine and slow to filter a body-aid is added to the feed slurry in order to enhance cake permeability. However, it should be kept in mind that the addition of body-aid increases the solids concentration in the feed so it occupies additional volume between the plates and increases the amount of cake for disposal. Likewise, for all those applications when the cake is the product, precoat and filter-aid may not be used since they mix and discharge together with the cake. 

Blinding A situation wherein fabric and dust cake permeability cannot be maintamed. The collected dust so adheres to or is so embedded m the fabric substrate that it cannot be removed by the cleaning method employed. This is evidenced by continually increasing pressure drop. 

The expansion index [51,102,103], defined as the ratio of cell fresh weight to dry weight evaluated at the time of maximum fresh weight, is a more qualitative indicator of changes in cell/aggregate size, under various cultivation conditions. Wongsamuth and Doran [58] identified the filtration characteristics of suspensions of Atropa belladonna, specifically cake permeability, which is at least partially related to morphology, as a useful indicator of shear effects. 

In dead-end filtration, a cake forms on the surface of the pad as the filtration proceeds. The cake permeability is the most important physical property of a porous medium and the hydraulic properties of the flow and the specific cake resistance are described by Darcy s Law ... 

The primary factor in the design of filters is the cake resistance or cake permeability. Because the value of the cake resistance can be determined only on the basis of experimental data, laboratory or pilot-plant tests are almost always necessary to supply the information needed for a filter design. After the basic constants for the filter cake have been determined experimentally, the theoretical concepts of filtration can be used to establish the effects of changes in operating variables such as filtering area, slurry concentration, or pressure-difference driving force. 

Filter aids are inert materials that can be used in filtration pretreatment. The principle characteristics of filter aids are (1) increasing cake permeability, (2) increasing cake porosity, and (3) increasing cake rigidity (decreasing cake compactibility). 

Pautz and Crocker (20) adapted Barkman and Davidson s internal cake permeability equation at a constant injection flow rate and derived... 

It is well known that the compressibility and permeability of clay filter cakes (and clay compacts in general) are a strong function of clay composition (particularly exchange cation) and saturating solution. Figure 35, for example, shows the dependence of filter cake permeability kc as a function of ec for various ratios of the exchange cations potassium and sodium (134). The permeability was observed to scale as... 

Figure 35. Dependence of filter cake permeability kc on void fraction for various ratios Na/(Na + K) of exchange cations. (Reproduced with permission from reference 134. Copyright 1991 Mineral Society.)...

Sethi and Wiesner (1997) predicted a most unfavourable size of 0.4 pm where the backtransport, considering all mechanisms, was at a minimum and thus the resulting flux lowest. When also considering cake permeability as a function of particle size, this minimum shifted to 0.01 - 0.1 pm. The dependence on membrane pore size and initial flux was not addressed in this study. Chellam and Wiesner (1998) pointed out the complicating effects of polydispersity on the use of such models. Most natural systems are polydisperse. 

Table 5.4 Particle diameter and cake permeability and resistance.

The cake permeability, cake volume firaction concentration and solid density will all be constants for an jnconq>ressible filter cake and can, therefore, be replaced by a single constant called specific resistance a which has units m kg ... 

Calculate cake permeability k = 4.25x10 m" Calculate Zeitsch drainage number D= 10 Estimate S = 0.28... 

Cake permeability, k is derived from the ecific re ance 1 1... 

These expressions include k, the filter cake permeability (m ) P , the thrediold pressure below v hich air will not penetrate the filter cake pores (N/m ) the equilibrium, irreducible cake saturation, a fimction of the capillaiy number iV (defined below) s the porosity of the filter cake. 

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