A. FILTRATION THEORY
While research has developed a significant and
detailed filtration theory, it is still so difficult to define a given
liquid-solid system that it is both
faster and more accurate to determine
filter requirements by performing
small-scale tests. Filtration
theory does, however, show how
the test data can best be correlated, and extrapolated when necessary, for use
in scale-up calculations.
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 very large with respect to
that of the filter media and filtrate drainage, and batch pressure filtration,
in which the resistance of the filter cake is not very large 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.
Simplified theory for both batch and continuous
filtration is based on the time-honored Hagen-Poiseuille equation:
where V is the volume of filtrate collected, Θ is the filtration time,
A is the filter area, P is the total pressure across the system, w is the
weight of cake solids/unit volume of filtrate, µ is the filtrate viscosity, α
is the cake-specific resistance, and r is the resistance of the filter cloth
plus the drainage system.
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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