Pressure effect, cake filtration

The scale-up of conventional cake filtration uses the basic filtration equation (eq. 4). Solutions of this equation exist for any kind of operation, eg, constant pressure, constant rate, variable pressure—variable rate operations (2). The problems encountered with scale-up in cake filtration are in estabHshing the effective values of the medium resistance and the specific cake resistance. 

In the filtration of a slurry, the resistance of the filter cake progressively increases and consequently, in a constant pressure operation, the rate of filtration falls. If the build-up of solids can be reduced, the effective cake thickness will be less and the rate of flow of filtrate will be increased. 

Cake filtration could be used for some of these materials, but a cake of l-/rm particles would have a high resistance to flow, and the filtration rate would be very low. Ultrafiltration (UF) covers a wider size range, from 1-pm particles down to molecules about 10 /rm in size (Af= 300). The term hyperfiltration is sometimes used for separation of small molecules or ions, but reverse osmosis is a more descriptive term, because the osmotic pressure has a major effect on the flux. Furthermore, the separation in reverse osmosis occurs by a solution-diffusion mechanism in the dense polymer rather than by a screening action at the membrane surface (see Chap. 26). 

The resistance of the cake of accumulated particulates, Rc, is more complicated it is a variable which increases as filtration proceeds resulting in a progressively lower filtration rate at constant pressure. This is due to the continually increasing thickness of the cake and its compaction under the pressurized conditions of filtration. If Rm is defined as above, Rc must also include the effect of pore plugging within the membrane. 

Tien C., Teoh S.K. and Tan R.B.H., 2001. Cake filtration analysis - the effect of the relationship between the pore liquid pressure and the cake compressive stress, Chem. Eng. Sci., 56, 5361-5369. 

It does not matter, from the fundamental point of view, how the pressure drop is generated in the filter. In the case of the centrifugal filters there is an additional phenomenon of the mass forces acting on the Hquid within the cake. The conventional filtration theory must be amended to include this effect (2). 

Dislodging of Cake by Reverse Flow. Intermittent back-flushing of the filter medium can also be used to control cake growth, leading to filtration through thin cakes in short cycles. Conventional vacuum or pressure filters can be modified to counter the effects of the forces during the back-flush (23,24). 

The resulting suspension is placed in a round flask equipped with a scraper-agitator device, and agitation is effected for 48 hours in an ice water bath. The antibiotic is isolated from the mother liquor by filtration through a Buchner filter. The filter cake is washed with 5 liters of a methyl alcohol and water mixture (1/2.5 by volume) cooled to 4°C. After drying in an oven at reduced pressure, 2.805 kilograms of a greyish-yellow crude product is obtained. 

Effect of bed support. The structure of the bed, and hence K", is markedly influenced by the nature of the support. For example, the initial condition in a filtration may affect the whole of a filter cake. Figure 4.3 shows the difference in orientation of two beds of cubical particles. The importance of the packing support should not be overlooked in considering the drop in pressure through the column since the support may itself form an important resistance, and by orientating the particles as indicated may also affect the total pressure drop. 

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