Filtration

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Filtration
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Introduction

• Filtration may be defined as the separation of
  solids from liquids by passing a suspension
  through a permeable medium which retains the
  particles.

Figure 1. Schematic diagram of filtration system
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• The fine apertures necessary for filtration are
  provided
• by fabric filter cloths,
• by meshes and screens of plastics or metals,
• by beds of solid particles.
• In some cases, a thin preliminary coat of cake,
  or of other fine particles, is put on the cloth
  prior to the main filtration process.

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Types of filtration

• Surface filters
• Depth filters

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1.Surface filters

• used for cake filtration in which the solids
  are deposited in the form of a cake on the
  up-stream side of a relatively thin filter medium.

Figure2. Mechanism of cake filtration
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2.Depth filters

• used for deep bed filtration in which particle
  deposition takes place inside the medium and cake
  deposition on the surface is undesirable.

Figure 3. Mechanism of deep bed filtration
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• The fluid passes through the filter medium, which
  offers resistance to its passage, under the
  influence of a force which is the pressure
  differential across the filter.

rate of filtration driving force/resistance
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• The filter-cake resistance is obtained by
  multiplying the specific resistance of the filter
  cake, that is its resistance per unit thickness,
  by the thickness of the cake.
• The resistances of the filter material and
  pre-coat are combined into a single resistance
  called the filter resistance.
• It is convenient to express the filter resistance
  in terms of a fictitious thickness of filter
  cake.
• This thickness is multiplied by the specific
  resistance of the filter cake to give the filter
  resistance.

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Factor affected on filtration

• Pressure drop ( ?P )
• Area of filtering surface ( A )
• Viscosity of filtrate ( v )
• Resistance of filter cake ( a )
• Resistance of filter medium ( Rm )
• Properties of slurry ( µ , ??? )

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Filtration Equation

• Flow of fluid through packed bed Application of
  Carman-kozenys equation
• ??? k1 constant 4.17 for particles with
  definite size and shape
• ? viscosity of filtrate (Pa.s)
• v linear velocity based on filter area (m/s)
• ? void fraction ???? porosity of cake
• L thickness of cake (m)
• S0 specific surface area of particle area per
  volume of solid particle (m2/m3)
• ?Pc pressure drop in cake (N/m2)

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• Substitute v in term of volume (V)
• A filter area (m2)
• V volume of filtrate at t sec
• L thickness of filter cake
• Cs kg of solid/m3 of filtrate
• ?P density of solid particle in cake (kg/m3)

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• Substitute L in term of height of cake (L)
• Obtain

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Specific cake resistance (?)

• ??? ? Void fraction
• S0 specific surface area of particle
  (m2)

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Specific cake resistance ?

• Their specific resistance change with pressure
  drop across the cake ?pc. In such cases, an
  average specific cake resistance ?av should can
  be determine from
• If the function ? ?(?pc) is known from pilot
  filtration tests, bomb filter test or from the
  use of a compressibility cell.

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• An experimental empirical relationship can be
  used over a limit pressure range
• Where ?0 the resistance at unit applied
  pressure drop
• n a compressibility index obtained from
  experiments
• (n 0 for incompressible substance)

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Filter medium resistance R

• Normally be constant but may vary with time (as a
  result of some penetration of solid into the
  medium) and sometimes may also change with
  applied pressure (because of the compression of
  fiber in the medium).

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• As the overall pressure drop across an installed
  filter include losses not only in the medium but
  also in the associated piping and in the inlet
  and outlet ports
• It is convenient in practice to include all these
  extra resistances in the value of the medium
  resistance R.

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Constant pressure filtration

• Equation is useful because it covers a situation
  that is frequently found in a practical
  filtration plant.
• We could predict the performance of filtration
  plant on the basis of experimental results.
• If a test is carried out using constant pressure,
  collecting and measuring the filtrate at measured
  time intervals

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Filtration Equations for Constant Pressure
Filtration
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Determine ? and Rm
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Filtration equation for Constant rate Filtration
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For incompresible cake Kv and C are constant
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Slope
Y-intercept
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• From constant rate equation the pressure drop
  required for any desired flow rate can be found.
  Also, if a series of runs is carried out under
  different pressures, the results can be used to
  determine the resistance of the filter cake.

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Ex1. Constant pressure Filtration area 0.01
m2 A Solution density 1,062
kg/m2 ? Solution viscosity 1.6?10-3
Pa.s ? Filtration pressure 200 kPa ?P Solid
concentration 3 kg/m3 Cs Determine specific
filter cake resistance and filter medium
resistance
Time (sec) Volume (cm3)
0 0
14 400
32 800
55 1200
80 1600
107 2000
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The solution
Y aX C Y axis tA/V X axis
V/A Slope ??Cs/2?P Y intercept ?Rm/?P
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Time (sec) Volume (cm3) Volume (m3) tA/V V/A
0 0 0 0 0
14 400 0.0004 350 0.04
32 800 0.0008 400 0.08
55 1200 0.0012 458.33 0.12
80 1600 0.0016 500 0.16
107 2000 0.0020 535 0.20
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• Ex2. Constant rate
• A slurry containing 25.7 kg dry solids/m3 of
  filtrate across the filter medium area 2.15 m2 at
  a constant rate of 0.00118 m3/s. If the pressure
  drop was observed 4,000 and 8,500 Pa after 150
  and 450 seconds of filtration, respectively. The
  viscosity of filtrate was 0.001 Pa.s
• Determine the specific cake resistance and
  filter medium resistance.

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The solution
(1)
(2)
(3)
(4)
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?P (Pa) t (sec)
4,000 150
8,500 450
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Filtration Equipment

• The basic requirements for filtration equipment
  are
• mechanical support for the filter medium
• flow accesses to and from the filter medium
• provision for removing excess filter cake.

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• In some instances, washing of the filter cake to
  remove traces of the solution may be necessary.
• Pressure can be provided on the upstream side of
  the filter, or a vacuum can be drawn downstream,
  or both can be used to drive the wash fluid
  through.

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• Filtration equipment (a) plate and frame press
  (b) rotary vacuum filter (c) centrifugal filter

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1.Plate and frame filter press

• In the plate and frame filter press, a cloth or
  mesh is spread out over plates which support the
  cloth along ridges but at the same time leave a
  free area, as large as possible, below the cloth
  for flow of the filtrate.
• The plates with their filter cloths may be
  horizontal, but they are more usually hung
  vertically with a number of plates operated in
  parallel to give sufficient area.

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• In the early stages of the filtration cycle, the
  pressure drop across the cloth is small and
  filtration proceeds at more or less a constant
  rate.
• As the cake increases, the process becomes more
  and more a constant-pressure one and this is the
  case throughout most of the cycle.
• When the available space between successive
  frames is filled with cake, the press has to be
  dismantled and the cake scraped off and cleaned,
  after which a further cycle can be initiated.

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• The plate and frame filter press is cheap but it
  is difficult to mechanize to any great extent.
• Filtration can be done under pressure or
  vacuum.
• The advantage of vacuum filtration is that the
  pressure drop can be maintained whilst the cake
  is still under atmospheric pressure and so can be
  removed easily.
• The disadvantages are the greater costs of
  maintaining a given pressure drop by applying a
  vacuum and the limitation on the vacuum to about
  80 kPa maximum.
• In pressure filtration, the pressure driving
  force is limited only by the economics of
  attaining the pressure and by the mechanical
  strength of the equipment

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• BAS stainless steel plate and frame filter press

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2.Rotary filters

• In rotary filters, the flow passes through a
  rotating cylindrical cloth from which the filter
  cake can be continuously scraped.
• Either pressure or vacuum can provide the
  driving force, but a particularly useful form is
  the rotary vacuum filter.

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• The rotary vacuum drum filter

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• A suitable bearing applies the vacuum at the
  stage where the actual filtration commences and
  breaks the vacuum at the stage where the cake is
  being scraped off after filtration. Filtrate is
  removed through trunnion bearings.
• Rotary vacuum filters are expensive, but they do
  provide a considerable degree of mechanization
  and convenience.

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3.Centrifugal filters

• Centrifugal force is used to provide the driving
  force in some filters.
• These machines are really centrifuges fitted with
  a perforated bowl that may also have filter cloth
  on it.
• Liquid is fed into the interior of the bowl and
  under the centrifugal forces, it passes out
  through the filter material.

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• Centrifugal filters

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4.Air filters

• Filters are used quite extensively to remove
  suspended dust or particles from air streams.
• The air or gas moves through a fabric and the
  dust is left behind. These filters are
  particularly useful for the removal of fine
  particles.
• The air passing through the bags in parallel. Air
  bearing the dust enters the bags, usually at the
  bottom and the air passes out through the cloth

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• A familiar example of a bag filter for dust is to
  be found in the domestic vacuum cleaner. Some
  designs of bag filters provide for the mechanical
  removal of the accumulated dust.