The operation of separating a solid from a liquid by means of a porous medium (use a wire or fabric filter cloth) is called filtration. The medium retains the solid in the form of a spongy cake, while the liquid passes through it.
Filtration: The mechanical separation of a solid from a liquid by passage through a foraminal medium which retains the concrete and allows the liquid to pass is called filtration.
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Filtration Explanation
In the case of filtration operation, the volume of suspensions to be handled may vary from extremely large quantities to relatively small quantities the suspensions may contain small or large proportions of solids and the valuable product may be the solid the liquid, or both, or something none of them.
-min.jpg "Cake Filtration")
The driving force required for separation by filtration based on the nature of a suspension may be divided into four categories, namely gravity, vacuum, pressure, and centrifugal.
Some Examples of Filtration
Separation of suspended impurities (solids) from water ( water purification), separation of solid organic and inorganic materials from their slurry such as calcium carbonate, ammonium sulfate, sugar, para nitro aniline, etc.
Types of Filtration
There are two types of Filtration
(i) Cake filtration.
(ii) Deep bed filtration.
(i) Cake filtration
In the case of cake filtration, the proportion of solids in the suspension is large and most of the solid particles are collected in the cake which can subsequently be detached from a filter medium.
(ii) Deep bed filtration
In the case of deep filtration, the proportion of solids is very small and the particles of the solid being smaller than the pores of a filter medium will penetrate a considerable depth and ultimately get trapped inside the filter medium and usually no layer of solids will appear on the surface of the medium.
Accordingly, filters are divided into two main groups, clarifying filters and cake filters. Clarifying filters also called liquid wheres cake filters are used to remove small amounts of solids in the form of cake crystals. Clarifying filters find application in water treatment.
Principle of Cake Filtration
In cake filtration, the feed to be handled (two-phase mixture) is termed as a slurry the bed of deposited solids on a foraminal membrane is termed as cake, and clear liquid leaving the filter medium is termed as filtrate.
A cake filtration operation is shown in the upper diagram. During the initial period of flow, solid particles are trapped within the pores of a medium forming the true filter medium. The liquid passes through the bed of the solids through the filter medium. In the early stages of filtration, the rate of Filtration is high.
As the cake thickness increases the rate of filtration decreases for a given pressure differential across the filter medium. This is because as the cake gradually builds upon the medium the resistance to flow progressively increases.
Specific cake resistance definition
A specific cake resistance α can be defined by the equation (formula)
α = ΔPc A/μuMc
Where,
ΔPc = is the Pressure drop over the cake.
A = is the filter area measured perpendicular to the direction of flow.
u = is the linear velocity of the filtrate based on the filter area.
μ = is the viscosity of the filtrate.
Mc = is the total mass of solids in a cake.
Specific cake resistance unit
Specific cake resistance units in the SI units system, the units of α are m/kg and dimensions of M¯¹L¹.
Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane
Factors Affecting the Rate of Filtration
The rate at which the filtrate is obtained in a Filtration operation, the rate of filtration depends in the following factors:
- Pressure drops across the feed inlet and far side of the filter medium.
- Area of the filtering surface.
- The viscosity of the filtrate.
- Resistance of the filter medium and initial layers of cake.
- Resistance of the filter cake.