Batch pressure filtration

Figure 2. Pressure filtration batch concentration system.

In cake or surface filtration, there are two primary areas of consideration continuous filtration, in which the resistance of the filter cake (deposited process solids) is veiy large with respec t to that of the filter media and filtrate drainage, and batch pressure filtration, in which the resistance of the filter cake is not veiy Targe with respect to that of the filter media and filtrate drainage. Batch pressure filters are generally fitted with heavy, tight filter cloths plus a layer of precoat and these represent a significant resistance that must be taken into account. Continuous filters, except for precoats, use relatively open cloths that offer little resistance compared to that of the filter cake. 

Since most batch-type filters operate under pressure rather than vacuum, the following discussion will apply primarily to pressure filtration and the various types of pressure filters. 

In many types of processes such as batch constant-pressure filtration or fixed-bed ion exchange, the production rate decreases as a function of time. At some optimal time /opt, production is terminated (at P°pt) and the equipment is cleaned. Figure E4.12a illustrates the cumulative throughput P(t) as a function of time t for such a process. For one cycle of production and cleaning, the overall production rate is... 

Batch Expression Equipment In batch expression equipment, the cake is initially formed by pressure filtration just as in other pressure filters. After the filtration stage, a squeezing device such as a diaphragm is inflated with gas or liquid to compress the cake. Batch expression equipment allows longer compression time and higher compression pressure. The cake can be very dry. 

Figure 13.9 Agitation (tip speed of three-propellor agitator) affects filtration resistance and liquid entrainment in pressure filtration (5 bar) of milk fat crystals from a batch crystallizer at 28 °C (Adapted from Patience et al. 1999 Hartel 2001 with permission.)...

Cake thickness is an important factor in determining the capacity and design of a batch filter and it determines the cycle of operation. In constant-pressure filtration, the integration of Equation 58.1, with the medium resistance neglected, yields... 

A. Hakkinen, K. PoUanen, M. Kaijalainen, J. Rantanen, M. Louhi-Kultanen, and L. Nystrom. Batch cooling crystallization and pressure filtration of sulphathiazole The influence of solvent composition. Biotechnol. Appl. Biochem. 41 17-28, 2005. 

Crystallization. The crystallization procedure is taken from that described by Fujii et For crystallization experiments, the Sulfolobus sp. ferredoxin solution obtained from a preparative Sephadex G-50 gel filtration column (Amersham Pharmacia Biotech) is concentrated by pressure filtration through an Amicon YM3 or YMIO membrane at 4° and made to 5 mg/ml in 0.5 M Tris-maleate-NaOH buffer, pH 5.0, containing 1% 2-methyl-2,4-pentanediol. Crystals suitable for X-ray diffraction analysis are obtained by a batch method performed under aerobic conditions. Fine-powdered ammonium sulfate is slowly added to 300 p.1 of 5 mg/ml protein solution until the turbidity is observed to persist (1.9-2.1 M). The crystallization solution is stored at 37° in an incubator. Dark brown crystals with appropriate dimensions of 0.3 x 0.3 x 0.5 mm are obtained in 3-5 weeks. Fujii etaO reported that reproducibility of the crystallization is enhanced by seeding a drop of the mother liquor containing microcrystals into the crystallization solution just before the crystallization begins. 

The punq) characteristic is again required to evaluate variable rate and pressure filtration. The result normally required is a calculation of the time taken to filter a knorvn volume or mass of slurry. The cake depth is also inportant as most filters are of the batch type and this d th must be less than the clearance within the filter. 

Pressure filtration, Part I—Batch pressure filtration 369... 

Various designs are used to achieve either a clarification of liquors, or a significant increase in solids concentration resulting in the production of a wet solid filter cake of minimal moisture content. This can be driven by either an applied pressure to the substrate over a porous filter medium (i.e. pressure filtration), or a vacuum applied behind the filter medium (vacuum filtration). The process can be on a batch basis, or via a continuous operation where, by a mechanical rotary means, new filtration areas are offered to the substrate. Chemical conditioning, including flocculation and coagulation, can be used to increase throughput, cake solids or fines capture. 

Optimization of Cycle Times. In batch filters, one of the important decisions is how much time is allocated to the different operations such as filtration, displacement dewatering, cake washing, and cake discharge, which may involve opening of the pressure vessel. Ah. of this has to happen within a cycle time /. which itself is not fixed, though some of the times involved may be defined, such as the cake discharge time. 

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