FILTRATION – Universiti Teknologi Malaysia

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• Separation of solids from liquids by passing a suspension through a
permeable medium which retains the particles

FILTRATION

Filter cake

Filter medium

Slurry
flow

Filtrate

• a pressure drop has to be applied across the medium (screen/cloth)
•

fluid flow through small holes of a screen/cloth

• retains the large solid particles as a separate phase (porous cake)
•
•
•

passes the clear filtrate
porous filter cake acts as a filter for the suspended particles
flow resistance increases as filter medium becomes clogged or cake builds
up

FILTRATION

• The valuable product may be:

– the clear filtrate from the filtration or

– the solid cake ( solid particles build up)

• essentially a mechanical operation
•
•

less demanding in energy than evaporation or drying
Used to remove solid particles from or to :

1) Clarify juices
2) Extracts
3) Vegetable and fish oils.
4) Fermented beverages.
5) Recirculated cooking oil
6) Flume water, milk, and soy milk.
7) Separate potato starch from potato fruit water,
8) High-melting fats from vegetable oils in fractionation processes.
9) Crystals from mother liquors.
10) Chemically precipitated impurities.

PRINCIPLE OF FILTRATION

• The cake gradually builds up on the medium and the resistance

to flow progressively increases.

• During the initial period of flow, particles are deposited in the
surface layers of the cloth to form the true filtering medium.

• The most important factors that influence the

(a) The drop in pressure from the feed to the far side of the

rate of filtration are:

filter medium.

(b) The area of the filtering surface.
(c) The viscosity of the filtrate.
(d) The resistance of the filter cake.
(e) The resistance of the filter medium
and initial layers of cake.

FILTRATION

As time passes during filtration, either
– the filtrate flow rate diminishes or

– pressure drop rises

Constant-pressure filtration

– pressure drop is held constant
– flow rate allowed to fall with time

Constant -rate filtration (less common)

– pressure drop is progressively increased

Liquid passes through 2 resistance in series:

– cake resistance (zero at start & increases with time)
– filter medium resistance (impt. during early stages of filtration)

during washing, both resistances are constant, and filter medium resistance is
usually negligible

BASIC THEORY OF FILTRATION

Rate of filtration = driving force/resistance

where

L = thickness of cake (m)
p = total pressure drop = pcake + pfilter medium (N/m2)
A = filter cross section area (m2)
 = specific cake resistance (m/kg)
 = viscosity of filtrate ( Pa.s)
cs = dry mass of cake deposited per unit volume of filtrate (kg
solids/m3 filtrate)
V = volume of filtrate (m3)
Rm = resistance of filter medium to filtrate flow (m-1)

dVdtpAcSVARm CONSTANT PRESSURE FILTRATION (BATCH)

t/V = (Kp/2)V + B  plot as a graph

Time of filtration:

where

t/V (s/m3)

Intercept = B

Slope = Kp/2

Filtration volume , V (m3)

Determination of constants in a constant-pressure filtration run

p = total pressure drop = pcake + pfilter (N/m2)
A = filter cross section area (m2)
 = specific cake resistance (m/kg)
 = viscocity of filtrate ( Pa.s)
cs = dry mass of cake deposited per unit volume of filtrate (kg solids/m3 filtrate)
V = volume of filtrate (m3)
Rm = resistance of filter medium to filtrate flow (m-1)

dtdVcs2ApVApRmKpVBtKpV22BVKpcs2ApBApRm Example 1 ( lab scale)

Data for the laboratory filtration of CaCO3 slurry in water at 298.2 K are reported as
follows at a constant pressure (-∆p) of 338 kN/m2. The filter area of the plate and frame
press was A=0.0439 m2 and the slurry concentration was cs=23.47 kg/m3. Calculate the
constant α and Rm from the experimental data given, where t is time in s and V is filtrate
volume collected in m3.