Filtration compression-permeability

Parameters q and W are variables when filtration conditions are changed. Coefficient (rj, is a function of pressure (rj, = f(P). The exact relationship can be derived from experiments in a device called a compression-permeability cell. Once this relationship is defined, the integral of the right hand side of the above equation may be evaluated analytically. Or, if the relationship is in the form of a curve, the evaluation may be made graphically. The interrelation between W and P, is established by the pump characteristics, which define q = f(W) in the integral. Filtration time may then be determined from dq/dt = W, from which we may state ... 

A compressibility-permeability test cell has been developed by Ruth(7) and Grace(8) for testing the behaviour of slurries under various conditions of filtration. A useful guide to the selection of a filter type based on slurry characteristics is given in Volume 6. 

The constants a0, k, and n are determined most simply in compression-permeability cells as explained in Section 11.4, but those found from filtration data may be more appropriate because the mode of formation of a cake also affects its resistivity. Equations (11.14) and (11.2) together become... 

Compression-Permeability Tests Instead of model leaf tests, compression-permeability experiments may be substituted with advantage for appreciably compressible solids. As in the case of constant-rate filtration, a single run provides data equivalent to those obtained from a series or constant-pressure runs, but it avoids the data-treatment complexity of constant-rate tests. 

Figure 1 show the compression permeability cell used in the experiment. The vacuum filtration apparatus comprises a principal base, a cylinder of 60mm diameter and a perforated plate. Figure 2 shows the vacuum filtration equipment. To produce the filtered cakes of FGM, a mixed slurry consisting of two components was sucked continuously by a vacuum pump through a filter paper extended over the perforated plate. 

Figure 1. Schematic diagram of a vacuum filtration apparatus and a compression permeability cell...

Tiller, F.M., S. Haynes, and W.M. Lu. 1972. The role of porosity in filtration vii effect of side-wall friction in compression-permeability cells. AIChE Journal 18 13-19. 

Alternatively, compression-permeability experiments can be performed. A filter cake at a low pressure drop and atm pressure is built up by gravity filtering in a cylinder with a porous bottom. A piston is loaded on top and the cake compressed to a given pressure. Then filtrate is fed to the cake and a is determined by a differential form of Eq. (14.2-9). This is then repeated for other compression pressures (Gl). 

W.M. Lu, Theoretical and Experimental Analyses of Variable Pressure Filtration and the Effect of Side Wall Friction in Compression-Permeability Cells, PhD dissertation. University of Houston, Texas, 1968. 

Filter aids as well as flocculants are employed to improve the filtration characteristics of hard-to-filter suspensions. A filter aid is a finely divided solid material, consisting of hard, strong particles that are, en masse, incompressible. The most common filter aids are applied as an admix to the suspension. These include diatomaceous earth, expanded perlite, Solkafloc, fly ash, or carbon. Filter aids build up a porous, permeable, and rigid lattice structure that retains solid particles and allows the liquid to pass through. These materials are applied in small quantities in clarification or in cases where compressible solids have the potential to foul the filter medium. 

Wells, S. A. and Dick, R. I. (1993) "Permeability, Solid and Liquid Velocity, and Effective Stress Variations in Compressible Cake Filtration," Proceedings, American Filtration Society Conference on System Approach to Separation and Filtration Process Equipment, Chicago, Illinois, May 3-6, pp. 9-12... 

Such equipment consists of a hollow cylinder fitted with a permeable bottom and a permeable piston under controlled pressure. Slurry is charged to the slurry, cake is formed with gentle suction, and the piston is lowered to the cake level. The rate of flow of filtrate at low head through the compressed cake is measured at a series of pressures on the piston. From the results the resistivity of the cake becomes known as a function of pressure. The data of Figures 11.4(b) and (c) were obtained this way those of Figure 11.4(a) by filtration tests. 

Filtration. Filtration can include filter presses, rotary drum vacuum filters (RDVF), belt filters, and variations on synthetic membrane filtration equipment, such as filter cartridges, pancake filters, or plate and frame filter presses. These processes typically operate in a batch mode when the filter chamber is filled up or the vacuum drum cake is exhausted, a new batch must be started. This type of filtration is also called dead-end filtration because the only fluid flow is through the membrane itself. Due to the small size of cells and their compressible nature, typical cell cakes have low permeability and filter aids, such as diatomaceous earths, perlite, or other mined materials are added to overcome this limitation. Moreover, the presence of high solids and viscous polymeric fermentation byproducts can limit filtration fluxes without the use of filter aids. 

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