Filtration

Contents
o Introduction
o Type of Filtration
o Filter media
o Design Equation for Batch Filtration
o Specific Cases of Filtration
o Industrial Filter Equipment
o Selection Criteria of various type of filter

FILTERATION
The separation of solids from a suspension in a liquid by means
of a porous medium or screen which retains the solids and
allows the liquid to pass is termed filtration.

Filtration
In the laboratory, the suspension is
poured into a conical funnel fitted with
a filter paper.

In the industrial equivalent, difficulties
are encountered in the mechanical
handling of much larger quantities of
suspension and solids. A thicker layer
of solids has to form and, in order to
achieve a high rate of passage of
liquid through the solids, higher
pressures are needed, and a far
greater area has to be provided.

Steps involved in filtration
1. Draining the liquor
2. Filtration
3. Filling with wash water
4. Washing
5. Draining the wash water
6. Opening, dumping and reassembling
7. Filling with slurry.

Principle of Filtration
Since the filter medium is permeable only to the fluid, it retains the
solid particles and permits only the fluid to pass through which is
collected as the filtrate. The volume of filtrate collected per unit
time (dV/dt) is termed as the rate of filtration.
As the filtration proceeds, solid particle accumulate on the filter
medium forming a packed bed of solids, called filter cake.

As the thickness of the cake increases
 resistance to flow of filtrate increases
 rate of filtration gradually decreases.
If rate is maintained to be constant then
pressure difference driving force (-P) will
increase.

Therefore, a batch filter is operated either at constant pressure or at constant rate.

Constant rate and Pressure Filtration

Cake Filtration
• Cake filtration consists of passing a solid
suspension (slurry) through a porous medium
or septum (e.g., a woven wire). The solids in
the slurry are retained on the surface of the
medium where they build up, forming an
increasing thicker cake.
• As more slurry is filtered the solids retained on
the medium provide most of filtering action. In
cake filtration the cake is the real filtering
element.

Cake Filtration (continued)
• As time goes by the thickness of the cake
increases, as more solids are filtered. This
results in a corresponding increase of the
pressure resistance across the cake.

• If the cake is incompressible (i.e., it does not
change its volume as pressure builds up) the
pressure resistance increases proportionally
to the cake thickness.

• However, since most cakes are compressible
the pressure across the cake typically
increases even faster than the cake build-up.

Examples of Cake-Forming Filters
• Filter presses
• Belt filters
• Vacuum filters:
- Rotary vacuum belt filters
- Rotary vacuum precoat filters
- Vacuum disk filters

Note:
• Cake filtration is intrinsically a batch process.
Hence, it can be expected that as filtration
proceeds the cake will build up and the
pressure drop across the cake will increase.
• Mathematical modelling of batch cake filtration
is based on the determination of the rate of
formation of the cake and the calculation of
pressure drop at any given time.
• Continuous filtration is often modelled as a
succession of batch processes carried out
over infinitesimally small time intervals.

Depth (or Deep-Bed)
Filtration
• Depth filtration consists of passing a liquid,
typically containing only a small amount of
solids, through a porous bed where the solids
become trapped.
• Solid entrapment occurs within the entire
filter bed or a significant part of it.
• Depth filtration is typically a batch process

Direction of Flow in Deep-Bed Filters
• Up flow
• Down flow (most common)
Examples of Deep-Bed Filters
• Granular-bed filters
• Deep-bed up flow filter
• Pulsed-bed filter
• Traveling-bridge filter

Backwashing
• During backwashing water is pumped
upward, i.e., in the opposite direction of the
suspension during normal operation
• The backwashing flow expands the bed to
dislodge all the particles removed during
filtration
• In order for backwashing to be effective the
filter medium must be fluidized

Type of Filter
• Cake Filter

• Clarifying Filter

• Cross Flow

• Ultra Filter cake filter

Cake Filter
• A filter cake is formed by the substances that are retained on
a filter .
• The filter cake grows in the course of filtration, becomes
"thicker" as particulate matter is being retained.
• With increasing layer thickness the flow resistance of the filter
cake increases
• After a certain time of use the
filter cake has to be removed
from the filter,
e.g. by back flushing.

Filter cake

Clarifying Filter
• Any filter, such as a sand filter or a cartridge
filter, used to purify liquids with a low solid-liquid
ratio; in some instances colour may be removed as
well.

Disk-and-plate clarifying filter. N-pin series of clarifying filter for electrolytic
aluminium flue gas

Cross flow Filters
 Cross flow filters – feed suspension flows under
pressure at high velocity across filter medium

• Thin layer of solids may form on surface ,but high
velocity keeps layer from building up
• Medium is ceramic, metal, or polymer with pores
small enough to exclude most of suspended particles
• Some liquid passes through as clear filtrate, leaving
more concentrated suspension behind

Ultrafiltration (UF)
• Ultrafiltration (UF) is a variety of membrane filtration in
which hydrostatic pressure forces a liquid against
a semipermeable membrane. Suspended solids and solutes of
high molecular weight are retained, while water and low
molecular weight solutes pass through the membrane.

inclined ultrafiltration downward ultrafiltration

Design Equation for Batch Filtration
Cake Filter
Pressure Drop During Cake Filtration
Lc
p  pa  pb  ( pa  p' )  ( p' pb )
 pc  pm
pa

Upstream face of cake
p
Filtrate Direction of
flow of slurry
Where p = overall pressure drop
pb p’
pc = pressure drop over cake
pm = pressure drop over medium

dL
L

Design Equation for Batch Filtration
Since the cake forms a porous bed over the filter medium, the flow of
filtrate through the accumulated cake is analogous to fluid flow
through a packed bed of granular solids.
• If the particles in the cake are uniformly wet by the filtrate then
Kozeny’s equations can be used to compute the pressure
drop across the cake (-P).
The velocity of fluid through the bed is:

Where, K = Kozeny’s constant
= 25/6, for random packed particles of
definite size and shape
RH = Hydraulic radius
= flow area/wetted perimeter

Assumptions:
• Flow of filtrate through the cake is laminar.
• Particles in the cake are uniformly wet by the filtrate.
• There is no channeling of the liquid through the cake.
RH = Void volume/Total surface area of particle
=  / sp (1- )
Where,
 = Void fraction of the bed = Void volume/total volume
sp = Specific surface (surface area per unit volume) of the particles
Using value of RH
we have ,

.
The superficial velocity of the liquid Usup is defined as the volumetric flow rate of
the liquid divided by the total (or empty) cross-sectional area and can be
related U’ as,
Usup/U’ = Void area / Total area
= Void volume / Total volume = 

(-Pc) = pressure drop across the cake
Lc = thickness of the cake

Where,

Design…

The factor  is called the specific cake resistance and is a measure of
the resistance offered by the cake to the flow of filtrate. The average
value of  is determined experimentally for each sludge.

 is independent of
Incompressible pressure drop and
Filter cake position in the cake

Compressible
Formed when cake is not made up of individual rigid
particles
, sp/vp vary from layer to layer
 varies with distance from filter medium
Cake nearest the surface of the media is subjected to the
greatest compressive force and has the lowest 
Pressure gradient is non-linear and local value of  may
vary with time

Design…
For the computation of Lc

Lc is expressed in terms of the volume of filtrate V, cake voidage and
concentration of feed slurry. If filtrate is the solid-free liquid, then
Mass of solids in the cake = Mass of solids in the feed slurry
If x is the mass fraction of solids in the feed slurry, then,
Mass of solids in the feed slurry

Mass of solids in the cake

v is the volume of cake deposited by passage of unit volume of filtrate.
Then,

Filter Media…
When choosing a filter material we must consider cost, particle
size, operating temperature, and chemical resistance.
1. PO (PONG) - Polypropylene Felt
This non-woven material is our popular workhorse. Polypropylene offers excellent
chemical resistance. Its cost effectiveness makes it ideal for applications up to 200°
F. This felt material can come plain (untreated felt) or glazed (high heat applied to
exterior surface fibers).
2. PMO - Polypropylene Mesh
Similar to nylon mono filament mesh, polypropylene mono filament mesh has better
acid resistance than nylon and is more cost effective for temperatures up to 200° F.
3. PEMF - Polyester Microfiber Felt
This material is grown from raw microscopic fibers. Its long life and ultra-fine micron
rating make it possible to use bag filters for applications that once required expensive
high maintenance cartridges. It has a higher melting point than polypropylene
microfiber making it ideal for hot oil and other applications up to 325° F.
4. NMO - Nylon Mesh
This monofilament mesh is a woven material in which each thread is a single filament.
This material's strength enables it to be used in a variety of different applications

<- Nylon
Monofilament
(60x zoom

Polyester
Multifilament
60x zoom

Design…

Pressure drop across the filter medium is:

Rm is to be determined experimentally.
 Rm may vary with P, as solid particles can be forced into the filter medium

 Can also vary with age and cleanliness of filter medium

 But, since it is important only during early stages of filtration, can usually be
assumed as constant
Therefore, the total resistance of filtration is

Then ultimate filtration equation is:

Design…

Empirical Equation for Resistance of Compressible
Cakes
Sperry correlation
   0 (p) s
 Where s is the compressibility coefficient of the cake
 0 for incompressible sludges
 + for compressible ones
 Usually 0.2 <  < 0.8
It has a inherent limitation that it predicts zero resistance when the (P) is
zero.

Ruth correlation

Both these correlations are the functions of (-P) only. Therefore, for
compressible sludge also,  is constant if the filtration is being conducted
at constant pressure.

Empirical Equation for Resistance of Compressible
Cakes

Donald and Hunneman correlation

0 < n < 20

Generalized correlation

Specific cases of filtration
Final filtration equation

Constant pressure and incompressible sludge (=const.)

where ,

For practice purpose,

Slope of line = KP which gives 
Y-Intercept of line = B which gives Rm

Constant rate and incompressible sludge (=const.)



Where, and

Also

where,

-P

Slope of line = Kr or Kr’ which gives 
Y-Intercept of line = Br which gives Rm

V or t

Constant pressure and compressible sludge

If    0 ( p) s
Then,

Assuming v to be constant (v is constant for small values of  and for
most of filter cakes particularly compressible cakes,  is small).
-P1

Where, -P2

and -P3

Slopes of lines = Kp’ values
Y-Intercept of line = B which gives average
value of Rm

Constant pressure and compressible sludge

log-log plot

Kp’

Slope of line = (1-s) which gives value of s
Y-Intercept of line = (f 0v/A2) which gives
value of0
-P

Constant rate and compressible sludge

If  = 0 (-Pc)s Then

since

If

Then

Continuous Filtration
Continuous removal of the deposited cake.
The cake thickness is thus not allowed to increase to large
values and therefore the filtration process can be
conducted at a constant rate employing a constant
pressure difference.
• Filling Rotary drum filter
• Cake formation (vacuum filter)
• Dewatering
• Washing
• Dewatering
• Cake removal

A full rotation of the drum is equivalent to a complete
batch cycle. Thus, the equation developed for batch
filtration can be used for continuous filtration as
well, keeping in mind that the equation stands for one
full rotation of drum.

The filtration equations

Where, t is the time for cake formation. If tc is
the time for one full rotation of the drum,

t = f tc where f is fraction of the cycle
available for cake formation.

f = fractional submergence of
the drum in the slurry.
V = volume of filtrate collected
during one rotation of the drum
and (V/tc) stands for the rate of
filtration.
For compressible cake,    0 (p) s

Industrial Filtration Equipment

1. Discontinuous Pressure Filters:
Apply large P across septum to give economically rapid filtration
with viscous liquids or fine solids.
• Plate and Frame Filter Press
• Shell and leaf Filters

2. Continuous Vacuum Filter:
Vacuum filters are simple and reliable machines and therefore have gained
wide acceptance in the chemical, food and pharmaceutical industries.
• Rotary Drum filter
• Horizontal Belt filter

1. Plate and Frame Filter Press
• Filter presses work in a "batch" manner.
• The plates are clamped together, then a pump starts feeding
the slurry into the filter press to complete a filtering cycle and
produce a batch of solid filtered material, called the filter
cake.
• A filter press uses increased pump pressure to maximize the
rate of filtration

Plate and Frame filter parts…

A medium scale frame and filter press..

Shell and leaf filter:
Description:
In these horizontal pressure leaf filter Jacketting and cloth enveloped filters are
optional. There are 2 hydraulic cylinders to open and close the special bayonet
wedge lock closure, which is provided at the lid. The retractable filtered shell is
mounted on four external wheels and all nozzle connections are mounted on fixed
head of filter vessel. Appropriate inter-locking (preventing opening under
pressure) is also a key feature of this type of horizontal pressure leaf filter.
Range: Available in 5 sq. m to 250 sq. m.
Advantages:
Horizontal Leaf Filter is also a multi utility device that has the following
advantages: -
» Nil spillage, as a result of close and compact operation.
» No use of filtered cloth reduces operational expenses.
» High productivity due to high rate of filtration.
» Cheap and economical operational costs.
» Highly user friendly.

Shell and leaf filter

Internal Structure

Rotary Drum Filter
• Horizontal drum that turns at 0.1-2 r/min in an agitated slurry trough
• Filter medium covers face of drum, which is partially submerged
• Vacuum and air are alternately applied as the drum rotates
• As panel leaves slurry zone, a wash liquid is drawn through filter, then cake
is sucked dry with air, and finally cake is scraped off
• From 30% up to 60-70% of filter area can be submerged
• Cakes usually 3-40 mm thick
• Drum sizes range from 0.3 m in diameter to 3 m in diameter

Rotary Drum Filter
Parts of a Rotary Drum filter

• The Drum:
The drum is supported by a large diameter trunion on the valve end
and a bearing on the drive end. The drum face is divided into
circumferential sectors each forming a separate vacuum cell. The
internal piping that is connected to each sector passes through the
trunion and ends up with a wear plate having ports that correspond to
the number of sectors.

Parts of Rotary Drum Filter
• The Valve: A valve with a bridge setting controls the sequence of the cycle so
that each sector is subjected to vacuum, blow and a dead zone. When a sector
enters submergence vacuum commences and continues through washing, if
required, to a point that it is cut-off and blow takes place to assist in discharging
the cake.
• The valve has on certain filters adjustable blocks and on others a fixed bridge ring.
Adjustable bridge blocks enable the optimization of form to dry ratio within the
filtration cycle as well as the "effective submergence" of the drum when the slurry
level in the tank is at the maximum.

a. cake formation
b. cake washing & drying
c. cake blow discharge b

a c

Parts of Rotary Drum Filter
The Internal piping:

The Filter cloth:
The filter cloth retains the cake and is fastened to the drum face by
inserting special caulking ropes into the grooved division strips.
Nowadays, with some exceptions, they are made from synthetic materials
such as polypropylene or polyester with monofilament or multifilament
yarns and with sophisticated weaves and layers. The image on the right
shows the method of joining the cloth ends with clippers and to retain the
fines from passing through to the filtrate multifilament strings are
threaded across the entire cloth width. Another option quite often used
on belt discharge filters is to join the ends with a special sewing machine.

The Horizontal belt Filter
Operation:
The feed sludge to be dewatered is introduced from a hopper between two
filter cloths (supported by perforated belts) which pass through a convoluted
arrangement of rollers. As the belts are fed through the rollers, water is
squeezed out of the sludge. When the belts pass through the final pair of
rollers in the process, the filter cloths are separated and the filter cake is
scraped off into a suitable container.[1] A belt filter is generally used in
phosphate fertiliser plants to separate the solid from slurry. It comprises
washing to different zone to minimise the product losses. Belt filters use a
vacuum system to minimise off gas and effluent during operations. It has
applications in many other areas of industry.

Horizontal Belt Filter
Advantages
• Continuous operation (except for a Nutsche filter)
• Intensive soluble recovery or removal of contaminants from the
cake by counter-current washing (specially on Horizontal
Belt, Tilting Pan and Table Filters)
• Producing relatively clean filtrates by using a cloudy port or a
sedimentation basin (on Horizontal Belt, Tilting Pan and Table
Filters)
• Polishing of solutions (on a Precoat Filter)
• Convenient access to the cake for sampling or operator's activities
• Easy control of operating parameters such as cake thickness or
wash ratios
• Wide variety of materials of construction
Disadvantages
• Higher residual moisture in the cake
• Untight construction so it is difficult to contain gases
• Difficult to clean (mainly as required for food grade applications)
• High power consumption by the vacuum pump

Tray Filter
Description:
• The Tray Filter, as opposed to the Horizontal Belt Filter.
• It belongs to the group of top feed filters and is primarily applied to
the finer chemical compounds handling thin cakes although in
recent years large machines suitable for thick cakes may be seen on
the bulkier processes.
• Sizes may vary from 0.25 to 3.0 meter wide units and lengths of 2
meters for pilot units and up to 25 meters for industrial machines.
• The cloth moves continuously over reciprocating trays which move
forwards with "vacuum on" in the forward stroke and retract with
"vacuum off" in the backwards stroke.
• The cloth moves intermittently over fixed trays and stops with
"vacuum on" in the filtration phase and "vacuum off" to
enable its movement forwards.

Tray Filter
The Reciprocating Trays Type
• This filter consists of a series of trays which are allocated for cake
formation, washing and drying.
• The trays are each connected through a flexible hose to a manifold that
runs along the filter and collects the filtrate to vacuum receivers.
• To meet such requirements the manifold is separated by blind flanges
which can be set at different positions
• The number of receivers is determined by the process requirements with
one unit collecting the mother filtrate and others the wash filtrate and the
cake drying.

The Reciprocating Trays Type
• The reciprocating trays are designed so that, while under vacuum, they
can move freely in the forward direction together with the cloth and at
the same velocity .
• When they reach the end of stroke point the vacuum is cut-off, the trays
are purged by an atmospheric release and pulled back by an air driven
pneumatic cylinder.
• During the backstroke the filter cloth
keeps moving forwards since there is
no vacuum between the retracting
trays and the backside of the cloth
which holds them together.

two co-current washing stages->

Tray Filter
The Fixed try type
• The trays of this filter are fixed to the frame and the cloth moves forwards
in short increments during "vacuum off" and remains stationary at the
"vacuum on" mode.
• The vacuum modes are controlled by solenoid actuated valves arranged in
a similar method described in the section on the Reciprocating
trays filter.
Cake Discharge Roll

Cloth Locking
Roll
Cloth
Tensioning Roll
Fixed Trays
3-Way Valves
Cloth Wash
Manifold
Pneumatic
Cylinder

Fixed roll with "on-off" motor actuator:
• In this system the discharge roll is mounted on the frame in a fixed
position and a timer controlled pulley moves the cloth at "vacuum off" to
allow cake discharge and "vacuum on" during filtration. A special tilting
feed box pours the slurry onto the filter deck when the cloth moves at
"vacuum off".

Selection Criteria for Tray Filter
• They may be built from synthetic materials of construction which makes them
suitable to withstand highly corrosive applications without the use of exotic
and expensive alloys.
• The sealing against loss of vacuum is simple as opposed to the rubber belt
filters which use sacrificial moving belts to seal between the underside of the
main belt and the top flanges that run along the vacuum box.
• They are built in a modular construction which enables expansion when
circumstances so require.
• The separation of mother and wash filtrates is sharp and accurate since, as
opposed to partitions in a vacuum box of a rubber belt filter, the filtrate is
contained in a tray.
• They lend themselves better than rubber belt filters when two vacuum zones
are required such as high vacuum for the feed and wash zones and low
vacuum with high air rates for the drying zone.
• The power consumption is lower since the rubber belt filters require special
arrangements to support the heavy belt and reduce friction.
• Features are found more often on tray filters such as gas tight
enclosures, compression rolls and blankets, thermal drying, vibrating trays to
seal cracks in the cake and ultrasonic or chemical cloth cleaning.
• They are however less in use for very thick and heavy cakes since, contrary to
rubber belt filters, the friction during indexing due to the cake weight between
the supporting grids that cover the trays and the backside of the cloth may
cause extensive wear.

The Disc Filter
• A disk filter is a type of water filter used primarily in
irrigation, similar to a screen filter , except that the filter cartridge is
made of a number of disks stacked on top of each other like a pile
of poker chips. The water passes through the small grooves in
between and the impurities are trapped behind. Some types of disk
filters can be backflushed in such a way that the disks are able to
separate and spin during the cleaning cycle.

Single Disk Filter Multi Disk Filter

Disk Filter
MAIN ELEMENTS:
• Filtering Cylinders: made totally out of Stainless Steel, quality Aisi-
304 or 316, this cylinder is built in a specially designed machine and
this element has two side lids which support the filtering material
together with a metallic framework, which is easy to dismount or
adjust.
• Filtering mesh of 10 to 1.000 microns made in polyester. Also
available in Stainless Steel AISI 316L ( from 10 to 1.000 microns).
• Modular design.
Easy access to the filtering cylinder from the exterior of the
filter, quickly and easily.
• Shell or frame: In strong meccano construction welded in Stainless
Steel, quality Aisi-304 or 316, supplied with entrance and exit
connections, anchoring legs, water-tightness elements, etc…
System for elimination of residue: This is carried out by means of a
combination of water sprinklers, the rotation of the filtering
cylinder and the residue conveying hopper found inside the filtering
cylinder.

Disk Filter
COMMON APPLICATIONS:
• Tertiary treatment of municipal wastewater.
• Pre-treatment of drinking water.
• Treatment of storm water.
• Treatment of water from industrial processes.
CHARACTERISTICS:
• Adequate for filtering small particles.
• deal system for applications which recycle municipal water.
• Equipment available with different diameters
0.5, 0.8, 1.2, 1.6, 2.0, 2.4 and 2.8 metres.
• Automatic cleaning system with filtered pressurized
water, activated with a level sensor.
• Different versions and standard models which can be adapted to
multiple needs.
• Manufactured in Stainless Steel (AISI 304 or AISI 316L) and
available in other special materials depending on the application.

Disk Filter
ADVANTAGES & BENEFITS:
• Wide range in solids concentration in the water to be
treated.
• Filtering mesh from 10 to 1.000 microns, according to
application.
• Equipment with production flows from 10 to 5.400 m3/h.
• Compact design which requires up to 75% less implantation
surface, compared with a conventional sand filter.
• Low investment thanks to its compact design.
• Reduced energy cost as the filtering is through gravity.
• Continuous working , including in washing phase.
• Very easy access to all filter elements.
• Easy operating and maintenance.

Disk filter
Selection Criteria
• The main considerations in selecting a Disc Filter are:
• When they suit an application that meets the following
requirements:
• The form to dry time ratio is approximately ½ to 1.
• No cake washing is required.
• The cake parts easily from the cloth.
• The cloth does not clog.
• When a cloth on one of the sectors tears the entire sector
may be replaced within a very short downtime.
• The filtration area may be expanded by adding more discs
to a barrel that has unused discs.
• The Disc Filter provides for maximum area at minimum cost
and floor space

Disk filter
Operational Sequence
• The operation sequence of a Disc Filter is, except for
washing, similar to a Drum Filter.
• Vacuum commences when the sector is fully submerged in the
slurry and the port of the rotating barrel passes the dead zone
bridge.
• The cake forms until the leading edge of the sector emerges from
the slurry and drying commences.
• The sector continues to dry the cake under vacuum until the port in
the rotating barrel fully covers the bridge in the valve that separates
the vacuum from the blow compartments.
• The port in the barrel passes the bridge and opens to constant low
pressure air blow or snap blow and the cake falls off to the
discharge chute.
• Once the barrel port passes the blow opening of the valve the
sector enters a dead zone that continues until the port opens to
vacuum with the sector fully submerged.

The Table Filter
Description:
• The Table Filters belong to the top feed group, introduced in the early 40's and
were rather small and of a simple design.
• limitation was at the discharge zone ,since the cake was contained in a fixed rim
and special sealing arrangements had to be provided in order to avoid the spillage
of brine at the table's circumference.
• Another problem was that the thin heel left between the scroll and the surface of
the table was dislodged by applying a back blow but not removed from the surface
of the passing cell. So, as it reached the feed zone it was mixed with the incoming
slurry without the cloth being washed.
• This has caused progressive media blinding which effected filtration rate and
required frequent stoppage of the operation for cloth washing.

Table filter
The major subassemblies of the Table Filter are:
• A series of fixed trapezoidal cells that form a rotating table and each
connected to a stationary valve in the centre of the filter. The cell is
designed with steep sloped bottom for fast evacuation of the filtrate.
• A valve that may be raised from the top and has a bridge setting and
compartments to control the various zones.
• An internal rim fixed to the table at the inner circumference and a
continuous rubber belt that surrounds the table at the periphery and
confine the slurry, wash liquids and the cake during the filtration cycle.
• Rollers that support the vertical loads, centering thrust and others that
move the rim away from the table in the discharge zone and maintain it
under tension.
• Radial rubber dams that separate between the feed, wash stages, cake
discharge and cloth wash, and cloth drying zones to prevent the mixing of
filtrates.
• A variable pitch screw that transports the cake radially towards the point
of discharge.

Table filter
Feed+Cloudy Port
Zone
Mother
Filtrate Zone
1st Wash
Filtrate Zone
2nd Wash
Filtrate Zone
3rd Wash
Filtrate +
Drying Zone
Cake
Discharge
Zone
Cloth Washing
Zone
Cloth Drying Zone
Cake Discharge
Scroll
Cake Retaining
Rubber Rim
Central Filtrate
Valve

Table filter
Selection Criteria:
• When the process downstream requires a de-lumped cake since the screw
disintegrates the solid lumps while conveying them to the periphery.
• When the solids are fast settling and cannot be kept as a homogenous
slurry in bottom or side feed filters such as Drum or Disc Filters.
• When very short cycle times are required for fast dewatering cakes such as
phosphate slurry.
• When a clear filtrate is required right from the start it is good practice to
form a thin heel that serves as a filter medium over the exposed cloth.
This is done by either a "cloudy port outlet" that is recirculated or, if solids
are settling fast, by allocating a portion of the table after the cloth drying
dam and prior to entering the vacuum zone to act as a "sedimentation
pool".
• When intensive cake washing is required.
• When a large filtration area is required but a Horizontal Belt Filter does
not fit into the layout.
• When cakes tend to crack under vacuum measures such as a flapper or
pressure roll may assist in sealing the cracks thus avoiding loss of vacuum.

Table filter
Operational Sequence
• The cycle of a Table Filter that includes three counter-current washing
stages consists of the following zones:
• With cells under vacuum during filtration:
– Cloudy Port Recycle or Sedimentation Pool (before applying vacuum).
– Cake Formation.
– Cake Predrying.
– First Washing.
– First Predrying.
– Second Washing.
– Second Predrying.
– Third Washing.
– Final Drying.
With cells purged to atmosphere:
– Cake discharged dry and conveyed by the screw to the cake hopper.
– Cloth wash and sluicing for the removal of the heel.
With cells under low vacuum:
– Evacuation of cloth wash water.
– Cloth drying (for such applications that dilution of mother liquor must be
avoided)

The candle filter
Description:
The Candle Filters are, as all pressure filters,
operating on a batch cycle and may be seen
in process lines handling titanium dioxide,
flue gas, brine clarification, red mud,
china clay, fine chemicals and many other
applications that require efficient low
moisture cake filtration or high degree
of polishing.

Candle filter
The Candle Filter consists of three major components:
• The vessel
• The filtering elements
• The cake discharge mechanism

1. The Vessel:
There are two types of vessel configuration:
• Vessels with conical bottom for cake filtration and polishing.
• Vessels with dished bottom for slurry thickening.

Candle filter
The Cake Discharge Mechanism:
There are two methods to discharge the cake at the end of the
cycle:
• Snap blow
. • Vibrating mechanism
For cakes that discharge readily a snap blow from the backside of
the medium is sufficient to release the cake but cakes that are
difficult to discharge require a mechanism that assists release by
vibrating the entire pack of candles. In this instance it is good
practice to incorporate special headers with high impact sprays
in the upper part of the vessel to clean the candles and dislodge
entrained particles.

Candle filter
Candle Filters are best selected in the following instances:
• When minimum floor space for large filtration areas is required.
• When the liquids are volatile and may not be subjected to vacuum.
• When there is a risk of environmental hazard from toxic, flammable
or volatile cakes specially secured discharge mechanisms may be
incorporated.
• When high filtrate clarity is required for polishing applications.
• When handling saturated brines that require elevated temperatures
the tank may be steam jacketed.
• When the cake may be discharged either dry or as a thickened
slurry.

They should be selected with care:
• When the cake is thick and heavy and the pressure is not sufficient
to hold it on the candle.
• When coarse mesh screens are used the filtration step must be
preceded with a precoat to retain cakes with fine particles.
Precoating with a thin layer of diatomite or perlite is not a simple
operation and should be avoided whenever possible.

Candle filter
Advantages:
• Excellent cake discharge.
• Adapts readily to slurry thickening.
• Minimum floor space.
• Mechanically simple since there are no complex sealing
glands or bearings.
Disadvantages:
• High headroom is required for dismantling the filtering
elements.
• The emptying of the vessel in between cake
filtration, washing and drying requires close monitoring
of the pressure inside the vessel to ensure that the
cake holds on to the candles.

Centrifugal filtration
The removal of a liquid from a slurry by introducing the slurry into a rapidly
rotating basket, where the solids are retained on a porous screen and the
liquid is forced out of the cake by the centrifugal action.

A highly accelerated form of sedimentation,
centrifugation is a process used to separate
or concentrate materials suspended in a liquid
medium. Centrifugation uses gravity and
centrifugal force to separate particles heavier
than the liquid medium. Centrifuges spin the
material at high rotation speeds and separate
the particulate from the liquid. Centrifugal
force can reach many thousand times that of
gravity, quickly separating the liquid/solid
material, sometimes even to the
nano-particle level.

Consider a centrifugal of radius R and height b rotating at an angular speed ω
as shown figure. Let us assume that the liquid surface inside the basket is
vertical and the cake is also deposited parallel to the wall of the basket, as
shown in the figure.

Since the centrifugal forces are many-fold larger in magnitude than the
gravitational forces, the effectively pressure difference driving force for
filtration may be assumed to be due to centrifugal action only. Thus

We can therefore use an equation for constant pressure provided the
inherent assumptions such as the flow of filtrate through the cake is laminar
and the voids of the cake are completely filled with the filtrate ( no
channeling ) are kept alive. Thus,

Where (- Δ P) is obtained from previous (above ) equation. It must be noted
that the cross-sectional area of the cake Ac perpendicular to the direction of
flow of filtrate changes as the thickness of the cake increases. A specific value
of Ac cannot be therefore used in the above equation since Ac is a function
of time.

Where, = arithmetic mean cake area,

= logarithmic mean cake area,

(R-Rc) = final thickness of cake formed.
Am is the area of the filter medium which can be assumed to be roughly
equal to the inside surface surface area of the basket. Thus,
Am = 2πRb.

selection criteria
• The properties of the fluid, particularly its viscosity, density and
corrosive properties.
• The nature of the solid—its particle size and shape, size
distribution, and packing characteristics.
• The concentration of solids in suspension.
• The quantity of material to be handled, and its value.
• Whether the valuable product is the solid, the fluid, or both.
• Whether it is necessary to wash the filtered solids.
• Whether very slight contamination caused by contact of the
suspension or filtrate with the various components of the
equipment is detrimental to the product.
• Whether the feed liquor may be heated.
• Whether any form of pre treatment might be helpful.

Filter aid
1. Diatomaceous earth:
A naturally occurring, soft, siliceous sedimentary rock that is easily crumbled
into a fine white to off-white powder. It has a particle size ranging from less
than 1 micrometre to more than 1 millimetre, but typically 10 to 200
micrometres. This powder has an abrasive feel, similar to pumice powder, and
is very light as a result of its high porosity. The typical chemical composition
of oven-dried diatomaceous earth is 80 to 90% silica, with 2 to 4% alumina
(attributed mostly to clay minerals) and 0.5 to 2% iron oxide.

Examples…
Question : A leaf filter with 1.0 m2 of filtering surface operated
at constant pressure of 1.8 bar(gage) gave the following results:

Filtrate volume(m3) 3.99 6.09 7.65 9.63 11.33

Time(min) 10 20 30 45 60

The original slurry contained 10% by weight of solid calcium carbonate
(specific gravity = 2.72) in water and the cake formed is essentially
incompressible.
a)Determine the time required to wash the cake formed at the end of 70
minutes of filtering at the same pressure using 3.0 m3 of wash water.
b)If the time for dumping the cake and reassembling the press is 60
minutes, what is optimum cycle time and what is the volume of filtrate
collected per cycle? Assume wash water is used in the same proportion to the
final filtrate as in (a).

Solution:
%input
fs=1;%filtering surface=1 m^3
p=1.8; %constant pressure= 1.8 bar
x=0.1; % weight % of calcium cabonate
sp=2.72; % specific gravity =2.72
tf=70; %70 mins of filtering
tf=tf*60;
vw=3; %volume of wash water m^3
tetc=3600;

t=[10,20,30,45,60];%t time in mins
fv=[3.99,6.09,7.65,9.63,11.33]; %fv= filterate volume (m^3)
n=5;% n=no. of values of filterate volume
dt=size(n-1);
dfv=size(n-1);
dtf=size(n-1);

av=size(n-1);

for i=1:n
t(i)=t(i)*60;
end

for i=1:4
Continue…
dt(i)=t(i+1)-t(i); tw=vw/rw;
dfv(i)=fv(i+1)-fv(i); disp('t wash=');disp(tw);disp('sec');disp('
dtf(i)=dt(i)/dfv(i);
av(i)=fv(i)+.5*dfv(i);
');disp((tw/60));disp('min');
end

disp('time in secs=');disp(t); r=vw/vf;
disp('Filtrate volume (m^3)=');disp(fv); disp(r);
disp('delta t=');disp(dt);
disp('delta V=');disp(dfv);
disp('delta t/delta V=');disp(dtf);
disp('V avg=');disp(av);
vo=tetc/(r*kp+.5*kp);
vo=sqrt(vo);
u=polyfit(av,dtf,1);
b=u(2); %intercept disp('Volume final optimum
kp=u(1); %slope
(m^3)=');disp(vo);
disp('slope (sec m^-6)=');disp(s);
disp('intercept (sec m^-3)=');disp(b);
tc=(r*kp+.5*kp)*vo*vo+(b+r*b)*vo+tetc;
vf=b*b+4*.5*tf*kp;
vf=sqrt(vf);
disp('optimum cycle time (sec)=');
vf=vf-b; disp(tc);
vf=vf/kp;
disp('Volume final (m^3)=');disp(vf); disp(tc/3600);disp('hr');
rw=vf*kp+b;
rw=1/rw;
disp('rate of washing (m^3/s)=');disp(rw);

Output:
Output:
time in secs=
600 1200 1800 2700 3600

Filtrate volume (m^3)=
3.9900 6.0900 7.6500 9.6300 11.3300

delta t=
600 600 900 900

delta V=
2.1000 1.5600 1.9800 1.7000

delta t/delta V=
285.7143 384.6154 454.5455 529.4118

Output:
V avg= 5.0400 6.8700 8.6400 10.4800

slope (sec m^-6)= 44.2873

intercept (sec m^-3)= 70.0130

Volume final (m^3)= 12.2817

rate of washing (m^3/s)= 0.0016

t wash= 1.8418e+003sec

30.6967 min

ratio= 0.2443

Volume final optimum (m^3)= 10.4507

optimum cycle time (sec)= 8.1104e+003

2.2529 hr

Filtration

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Filtration

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