Compression permeability, cake filtration

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... 

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... 

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. 

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). 

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. 

It is beyond the scope of this entry to review the basic principles governing filtration. However, it is interesting to note that filtration produces a more concentrated and dewatered cell sludge (20-35% w/v) or cell solids (>40% w/v) than settling. A variety of filter media, membranes, and equipment are commercially available. In the case where the deposited cake is compressible with low permeability and thereby adds more resistance to filtration, filter aids or precoats often alleviate the problem. Two of the most widely used filter aids are diatomaceous earth and perlite. 

The effects of compression on a filter cake or suspension can be measured using a piston press as illustrated in Figure 4.10. The press comprises an upright cylinder mounted on a base plate, where both the cylinder and plate are usually made from stainless steel. The base of the plate is recessed to accommodate a loosely fitting sinter drainage plate which is used to support the planar filter medium. The drainage plate must be sufficiently permeable to allow unrestricted passage of the filtrate. Liquid is forced from the cyhnder by a piston which is actuated either hydraulically or pneumatically. In more sophisticated units a force transducer can be mounted in the piston shaft to measure... 

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