Cake porosity

The constant given the value 5 in equation 1 depends on particle size, shape, and porosity it can be assumed to be 5 for low porosities. Although equation 1 has been found to work reasonably well for incompressible cakes over narrow porosity ranges, its importance is limited in cake filtration because it cannot be used for most practical, compressible cakes. It can, however, be used to demonstrate the high sensitivity of the pressure drop to the cake porosity and to the specific surface of the soHds. 

Flocculants and surfactants (qv) are used frequently as filter aids, particularly when slimes are present or when the particles to be filtered are very fine and difficult to filter. Low molecular weight polymers are more commonly used. These form small, dense floes which provide higher cake porosity. 

Cake Dryness In dewatering, usually the cake needs to be as diy as possible. Cake diyness is commonly measured by the solids fraction by weight W or by volume ,. The moisture content is measured by the complement of W or ,. The volume fraction of the pores and void in the wet cake is measured by the cake porosity (= 1 — ,) whereas the volume fraction of the liquid in the pores of the cake is measured by the saturation S. For welf-defined solids in the cake with sohd density (bone diy) p, and hquid density Pl, and given that the cake volume... 

Wlien the cake is saturated S = 1, the cake porosity can be determined from Eq. (18-86) as... 

The structure of the cake formed and, consequently, 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 ... 

Distance measured from filter cloth Dimensionless distance Cake porosity... 

Fig. 10. Dependence of the soot cake porosity for different soot aggregate sizes at the prevailing Peclet number.

Contact angles on compressed cakes have also been determined by the h — t method introduced by Kossen and Heertjes (103.104]. This method is based on the assumption that the powder consists of identical spheres. After the powder is compressed into a cake, it is saturated with the measuring liquid and a drop is placed onto the surface. The contact angle is calculated from the height of the drop (e.g.. as measured by a eathetometer). a knowledge of the cake porosity, the liquid density, and the surface tension. 

The cake porosity, obtained from the steady-state flux, is shown with the surface charge density in Fig. 2. It was observed that the porosity of a cake layer tended to increase as the surface charge density increased. The stirring effect was investigated with monodisperse microspheres of different surface charge density. It was observed that the stirrer speed was proportional to the porosity of a cake layer. 

Average cake porosity (or solidosity) Deliquoring time... 

Ruth s equation in mass basis required knowledge of density of solids. To get a better view of cake solids, and cake porosity, volume of cake solids instead of mass is used in Tiller s theory ... 

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). 

It is necessary to calculate the volumetric slurry concentration (()j and the average cake porosity Sja,. Using Equation (22.8) with and 5 replacing and leads to... 

Filtration characteristics Volume of filtrate versus time, filtrate rate, volume of filter cake, cake porosity, clarity of filtrate, clogging of filter media, effect of slurry concentration on filtrate rate and filtrate clarity, effect of slurry viscosity on filtrate rate and media clogging, etc. 

Eylander (16) modified Barkman and Davidson s (2) equation for internal formation plugging and introduced the filter cake porosity parameter (0.2). His equation is based on internal filter cake formation that grows towards the wellbore. 

The determination of cake porosity and fractal dimension of membrane deposits is difficult. While filtration laws cart be applied, most assume a constant porosity. 

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