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Tuesday, March 5, 2013

FILTRATION: DEFINITION AND CLASSIFICATION


DEFINITIONS AND CLASSIFICATION OF FILTRATION
Filtration is the separation of a fluid-solids mixture involving passage of most of the fluid through a porous barrier which retains most of the solid particulates contained in the mixture. This subsection deals only with the filtration of solids from liquids; gas filtration is treated in Sec. 17. Filtration is the term for the unit operation. A filter is a piece of unit-operations equipment by which filtration is performed. The filter medium or septum is the barrier that lets the liquid pass while retaining most of the solids; it may be a screen, cloth, paper, or bed of solids. The liquid that passes through the filter medium is called the filtrate.

Filtration and filters can be classified several ways:
  1. By driving force The filtrate is induced to flow through the filter medium by hydrostatic head (gravity), pressure applied upstream of the filter medium, vacuum or reduced pressure applied downstream of the filter medium, or centrifugal force across the medium. Centrifugal filtration is closely related to centrifugal sedimentation,  and both are discussed later under “Centrifuges.”
  2. By filtration mechanism.    Although the mechanism for separation   and  accumulation  of  solids  is  not  clearly  understood,   two models are generally considered and are the basis for the application of  theory  to the  filtration process. When  solids are  stopped  at the surface of a filter medium and pile upon one another to form a cake of increasing thickness, the separation is called cake filtration.  When solids  are  trapped  within the  pores  or  body of the  medium,  it is termed  depth,  filter-medium, or clarifying filtration.
  3. By objective.    The process goal of filtration may be dry solids (the cake is the product  of value), clarified liquid (the filtrate is the product of value), or both. Good solids recovery is best obtained by cake  filtration,  while clarification of the  liquid  is  accomplished  by either depth or cake filtration.
  4. By operating cycle.   Filtration  may be intermittent (batch) or continuous.  Batch  filters  may  be  operated  with  constant-pressure driving force,  at  constant  rate,  or  in cycles that  are  variable with respect  to  both  pressure  and  rate.  Batch  cycle can  vary greatly, depending on filter area and solids loading.
  5. By nature of the solids.   Cake filtration may involve an accumu- lation of solids that is compressible or substantially incompressible, corresponding roughly in filter-medium filtration to particles that are deformable   and  to  those  that  are  rigid.  The  particle  or  particle-aggregate size may be of the same order of magnitude as the mini- mum pore size of most filter media (1 to 10 µm and greater), or may be smaller (1 µm down to the dimension of bacteria and even large molecules).  Most filtrations involve solids of the  former  size range; those of the latter range can be filtered, if at all, only by filter-medium-type filtration or by ultrafiltration unless they are converted to the for- mer range by aggregation prior to filtration.
  6. These methods of classification are not mutually exclusive. Thus filters usually are divided first into the two groups of cake and clarifying equipment,  then into groups of machines using the same kind of driv- ing force, then further into batch and continuous classes. This is the scheme of classification underlying the discussion of filters of this sub-section. Within it, the other aspects of operating cycle, the nature of the solids, and additional factors (e.g., types and classification of filter media) will be treated explicitly or implicitly.

SOURCE: 
Genk, Wayne J., dkk. 2008. Perry’s chemical engineers handbook,  Section 18: Liquid-solid Operations and equipment. New York: The McGraw-Hill Companies, Inc

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