Filtration in Solid-Liquid Separation

In filtration, suspended solid particles in a fluid of liquid or gas are physically or mechanically removed by using a porous medium that retains the particles as a separate phase or cake and passes the clear filtrate. Commerical filtrations cover a very wide range of applications. The fluid can be a gas or a liquid. The suspended solid particles can be very fine (in the micrometer range) or much larger, very rigid or plastic particles, spherical or very irregular in shape, aggregates of particles or individual particles. The valuable product may be the clear filtrate from the filtration or the solid cake. In some cases complete removal of the solid particles is required and in other cases only partial removal. 

The feed or slurry solution may carry a heavy load of solid particles or a very small amount. When the concentration is very low, the filters can operate for very long periods of time before the filter needs cleaning. Because of the wide diversity of filtration problems, a multitude of types of filters have been developed. 

Industrial filtration equipment differs from laboratory filtration equipment only in the amount of material handled and in the necessity for low-cost operation. A typical laboratory filtration apparatus is shown in Fig. 14.2-1, which is similar to a Buchner funnel. The liquid is caused to flow through the filter cloth or paper by a vacuum on the exit end. The slurry consists of the liquid and the suspended particles. The passage of the particles is blocked by the small openings in the pores of the filter cloth. A support with relatively large holes is used to hold the filter cloth. The solid particles build up in the form of a porous filter cake as the filtration proceeds. This cake itself also acts as a filter for the suspended particles. As the cake builds up, resistance to flow also increases. 

In the present section 14.2 the ordinary type of filtration will be considered where a pressure difference is used to force the liquid through the filter cloth and the filter cake that builds up. 

In Section 14.4E centrifugal filtration will be discussed, where centrifugal force is used instead of a pressure difference. In many filtration applications, ordinary filters and centrifugal filters are often competitive and either type can be used. 

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Zeitsch, K., 2000. Centrifugal Filtration. In Solid-liquid separation, 4th edition. Ed. L. Svarovsky. Oxford Butterworth-Heinemann. 

Sedimentation classifiers, 76 619-620 in depth filtration theory, 11 339 in particle size measurement, 78 142-144 in solid-liquid separation, 76 656-657 Sedimentation rate... 

Tiller, F.M., Filtration Theory Today, Chem. Eng., 73,13, 151, 1966. Svarovsky, L., Advanced in Solid Liquid Separations I, Chem. Eng., 86,... 

In general, flocculants are used in solid-liquid separation processes such as thickening and filtration. Inorganic salts are also used sometimes to aggregate fine particles. Flocculation technique has been developed further for special applications of selective flocculation, selective dispersion and agglomeration flotation. 

Hardman E (2000c), Textiles in filtration , in Horrocks AR and Anand SC,Handbook of Technical Textiles, Cambridge, Woodhead Publishing Limited, 348-350. Hardman E (1994), Some aspects of the design of filter fabrics in solid/liquid separation processes . Filtration and Separation, 31(8) 813-818. 

Fitch E.B., 1986. Gravity Separation Equipment, in Solid/Liquid Separation Equipment Scale-Up , Eds. D.B. Purchas and R.J. Wakeman, Uplands Press and Filtration Specialists Ltd, London. 

Wakeman R.J., 1993. Scale-up procedures and test methods in solid/liquid separation 2. Filtration of binary particle mixtures and flocculated suspensions, IFPRl Annual Research Report ARR 25-02. 

Efficient and rapid removal of solvent and antisolvent is a vital component of the successful MBPprocess. Once the precipitation has been completed (concurrently for continuous process and at the end of the solvent addition for the semi-batch process), the MBP is filtered from the suspension. This is generally performed by any of the standard filtration processes such as vacuum filtration ranging from Buchner funnel to Nutsche filter or centrifugal filter. Centrifugal filters are preferred for filtration of MBP suspension because of the particle size, the hydrogel nature of the polymer, and the effectiveness in solid/liquid separation... 

In conclusion to cross-flow filtration as a unit operation, it is probably the most exciting development in solid-liquid separation yet to be fuUy explored. Its advantages are in high filtration rates due to minimized particle deposition on the medium without a strong effect of particle size on performance, and in the absence of chemical additives or filter aids in the products. 

Zeitsch, K., Centrifugal filtration , ch. 14 in Solid-Liquid Separation, ed. by L. Svarovsky, Butterwortbs, London (1977)... 

Belfor G. Fluid mechanics and cross-flow membrane filtration. In Muralidhara HS, ed. Advanees in Solid Liquid Separation. Columbus Battelle Press, 1986, pp 182 183. 

In this subsection, basic design theory for preliminary sizing and specifying equipment are reviewed. Some sample design calculations are included. References cited at the end of tlie chapter can be consulted for more detailed information and design methods. For solid-liquid separation methods, the reader should refer to Liquid Filtration, 2" edition, by N. P. Cheremisinoff, Butterworth-Heinemarui Publishers (1998). 

Moody, G. M., 1994. In Selection and Scale - Up for Solid/Liquid Separation, The Filtration Society 30th Anniversary Symposium, p.9-29... 

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