Interception cake filtration

It is advisable to start a constant-pressure filtration test, like a comparable plant operation, at a low pressure, and smoothly increase the pressure to the desired operating level. In such cases, time and filtrate-quantity data shoulci not be taken until the constant operating pressure is reahzed. The value of r calculated from the extrapolated intercept then reflec ts the resistance of both the filter medium and that part of the cake deposited during the pressure-buildup period. When only the total mass of diy cake is measured for the tot cycle time, as is usually true in vacuum leaf tests, at least three runs of different lengths should be made to permit a rehable plot of 0/V against W. If rectification of the resulting three points is dubious, additional runs should be made. 

In the case of multiparticle blockage, as the suspension flows through the medium, the capillary walls of the pores are gradually covered by a uniform layer of particles. This particle layer continues to build up due to mechanical impaction, particle interception and physical adsorption of particles. As the process continues, the available flow area of the pores decreases. Denoting as the ratio of accumulated cake on the inside pore walls to the volume of filtrate recovered, and applying the Hagen-Poiseuille equation, the rate of filtration (per unit area of filter medium) at the start of the process is ... 

The determination of the specific cake and medium resistance can be extracted from the slope and axis intercept of the graph for the cake formation using Equation 6. ft can be seen that, the inverse slope of Equation 6, 1/Kc, can be taken as the indication of the filtration rate, dVi/dt. When 1/Kc increases the filtration rate increases (4). 

In liquid-solid systems, the impaction or interception mechanism is referred to as filtration. Filtration can be classified in several ways vacuum, pressure, belch, continuous, cake, or depth. Filtration is generally considered to be a separation involving passage of liquid through a porous medium that retains most of the panicalates contained in the liquid. The porous medium mey be a wire screen, cloth, paper, or a bed oF... 

If a period of constant pressure filtration then follows, the parabolic rate law (Equation 2.23) can be applied, but care should be taken in intetpreting the meaning of the intercept fi om this figure. It is no longer solefy a function of medium re ance, but is now a fimction of medium resistance and the resistance due to filter cake formed under conditions of constant rate. Usefid data can still be obtained from the intoccqtt but an average cake resistance value must be calculated first, winch can then be subtracted firom the resistance obtained from the new intercept. 

Assuming that the cake is incompressible, Ky and C are constants characteristic of the slurry, cake, rate of filtrate flow, and so on. Hence, a plot of pressure, — Ap, versus the total volume of filtrate collected, V, gives a straight line for a constant rate dVfdt. The slope of the line is Ky and the intercept is C. The pressure increases as the cake thickness increases and the volume of filtrate collected increases. 

Machac and Crha developed plots which, like in Newtonian liquid filtration, have to be linear. The cake and medium resistances are then determined from the slope and the intercept, as is usual in filtration. The problem here is that, in filtration at constant pressure, the filtration velocity may vary widely and this requires the parameters K and n also to apply in a wide interval of the corresponding rate of shear. The way to relate the shear rates to filtration velocities is via the following equation ... 

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