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PIERO M. ARMENANTENJIT
Cake Filtration
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PIERO M. ARMENANTENJIT
Cake Filtration Cake filtration consists of passing a solid
suspension (slurry) through a porous mediumor septum (e.g., a woven wire). The solids inthe slurry are retained on the surface of themedium where they build up, forming anincreasing thicker cake.
As more slurry is filtered the solids retained onthe medium provide most of filtering action. Incake filtration the cake is the real filteringelement.
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PIERO M. ARMENANTENJIT
Cake Filtration (continued) As time goes by the thickness of the cake
increases, as more solids are filtered. Thisresults in a corresponding increase of thepressure resistance across the cake.
If the cake is incompressible (i.e., it does notchange its volume as pressure builds up) thepressure resistance increases proportionallyto the cake thickness.
However, since most cakes are compressiblethe pressure across the cake typicallyincreases even faster than the cake build-up.
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PIERO M. ARMENANTENJIT
Cake Filtration (continued) The cake is removed intermittently during
batch filtration processes. This is done bytaking the filter off line and manually orautomatically collecting the cake.
The cake is removed continuously incontinuous processes, for example byscraping the cake with blades, as in rotatingfilters.
Cake washing and drying operations can alsobe incorporated in the operation of mostfilters.
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PIERO M. ARMENANTENJIT
Examples of Cake-Forming Filters Filter presses Belt filters Vacuum filters:
- Rotary vacuum belt filters
- Rotary vacuum precoat filters
- Vacuum disk filters
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PIERO M. ARMENANTENJIT
Example of a Filter Press
After Metcalf and Eddy, Wastewater Engineering, 1991, p. 869
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PIERO M. ARMENANTENJIT
Cross Section of a Filter Press
After Freeman, Standard Handbook of Hazardous Waste Treatment andDisposal, 1989, p. 7.9
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PIERO M. ARMENANTENJIT
Example of a Belt Filter
After Freeman, Standard Handbook of Hazardous Waste Treatment andDisposal, 1989, p. 7.10
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PIERO M. ARMENANTENJIT
Schematic of a Belt Press Filter
After Vesilind, Treatment and Disposal of Wastewater Sludges, 1979, p.156.
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PIERO M. ARMENANTENJIT
Example of a Rotary Vacuum Belt Filter
After Freeman, Standard Handbook of Hazardous Waste Treatment andDisposal, 1989, p. 7.8
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PIERO M. ARMENANTENJIT
Example of a Rotary Vacuum Precoat Filter
To Vacuum
Precoat
CakeDischarge
Slurry
Knife
Filtrate
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PIERO M. ARMENANTENJIT
Precoats and Filter Aids A precoat is a layer of fine particulate material
(e.g., perlite) added on to the filter septumbefore filtration to form a coating cake
During filtration the filtered solids in the slurrymay clog the filter and reduce the rate offiltration. This happens especially if theresulting cake is very compressible
In such cases a filter aid made of fine particlesof a hard but porous material (such as perlite)having good filtering properties is added to theslurry to prevent and filtered with the slurrysolids
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PIERO M. ARMENANTENJIT
Common Types of Precoats and FilterAids
Diatomaceous earth (diatomite)A light siliceous material derived primarilyfrom sedimented diatoms (minute planktonicunicellular or colonial algae with silicifiedskeletons). Typical bulk density: 0.32 g/cm3
PerliteA volcanic glass made of siliceous rock havinga concentric shelly structure. Typical bulkdensity: 0.16 g/cm3
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PIERO M. ARMENANTENJIT
Analysis ofCake Filtration
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PIERO M. ARMENANTENJIT
Analysis of Suspended SolidsRemoval During Cake Filtration
As the suspension moves through the filtermedium (septum) the suspended solids arestopped by the filter septum forming a filter cakeon top of the filter septum
As more solids suspension passes through thefilter the cake builds up providing most of thefiltering action for the incoming suspension
Equations can be written to describe the removalof the particles in suspension by the filter (i.e.,the formation of the cake) and the pressure dropof the fluid as it passes through the cake
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PIERO M. ARMENANTENJIT
Important Variables in Cake Filtration Available pressure drop across cake, DP (Pa) Area of filtration, A (m2) Specific resistances of cake, a (m/kg) Specific resistances of medium (septum), Rm (1/m) Fluid superficial velocity, us (m/s) Size of cake particles, Dp (m) Shape factor for particles, fs Type of solids in suspension Cake void fraction, e (void volume/total bed volume) Time, t (s)
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PIERO M. ARMENANTENJIT
Important Variables in Cake Filtration Cake thickness, L (m) Concentration of solids in wastewater, Xw (g/L) Residual concentration of solids in filtrate, XF (g/L) Mass fraction of solids in cake, XC (g/g) Cumulative volume of wastewater fed to filter, VW (L) Cumulative volume of filtrate generated, VF (L) Cumulative mass of wet cake, mc (g) Mass of solids in the cake per volume of filtrate, XS (g/L) Density of wastewater, rw (g/L) Density of filtrate, rf (g/L)
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PIERO M. ARMENANTENJIT
Approach to Cake Filtration as aBatch Process
Cake filtration is intrinsically a batch process.Hence, it can be expected that as filtrationproceeds the cake will build up and thepressure drop across the cake will increase.
Mathematical modeling of batch cake filtrationis based on the determination of the rate offormation of the cake and the calculation ofpressure drop at any given time.
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PIERO M. ARMENANTENJIT
Approach to Cake Filtration as aBatch Process (continued)
Integral quantities (such as the cumulativevolume of filtrate produced during a timeinterval, or the mass of the cake generatedduring the same interval) can be calculated byintegration of the basic instantaneous massbalances. In these equations the pressuredrop is typically a function of time.
Continuous filtration is often modeled as asuccession of batch processes carried outover infinitesimally small time intervals.
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PIERO M. ARMENANTENJIT
Mass Balance Around a FilterFor a filter operating in a batch mode thefollowing diagram can be drawn:
(Wet) Cake
WastewaterFiltrate
VW XW VF XF
mc X'c
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PIERO M. ARMENANTENJIT
Definition of Xs
Xs is defined as the mass of (dry) solids in thecake per volume of filtrate generated.
From this definition it is that:
XX m
Vsc c
F
= =mass of solids in cakevolume of filtrate
'
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PIERO M. ARMENANTENJIT
Relationship Between SolidConcentrations Around a Filter
Mass balances around the filter give:
X V X m X Vw W c c F F= +' (solids)
r rw W c F FV m V= + (overall)
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PIERO M. ARMENANTENJIT
Relationship Between SolidConcentrations Around a Filter (cont.d)
After an explicit expression for VF has beenobtained it can be substituted in the equationdefining Xs to get:
X XX X
X Xs cW F F W
W W c
= --
''
r rr
If the densities of the wastewater and the filtrateare the same, then:
r r rW F= =
X XX X
X Xs cF W
W c
= --
rr
''
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PIERO M. ARMENANTENJIT
Relationship Between SolidConcentrations Around a Filter (cont.d)
Special case: A common situation is that inwhich all the solids contained in the suspensionare removed by the filter and contribute to theformation of the cake. In other words, the filtratedoes not contain any solids. In such a case it is:
XF 0
and the expression for Xs becomes:
X XX
X Xs cW
c W
= -
rr
''
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PIERO M. ARMENANTENJIT
Relationship Between SolidConcentrations Around a Filter (cont.d)
Note that, in general, Xs is different from XWOnly if:
r X Xc W'>>
it would then be that:
X XX
X XX
XX
Xs cW
c Wc
W
cW= -
@ @rr
rr
''
''
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PIERO M. ARMENANTENJIT
Cumulative Mass Balance for theSolids in the Cake
At a generic time t a cumulative mass balance forthe solids in the cake (i.e., the solids that havecontributed to the formation of the cake in thetime interval 0-t) gives:
solids accumulated in the cake during time t
solids removed from suspension during time t
=
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PIERO M. ARMENANTENJIT
Cumulative Mass Balance for theSolids in the Cake
The previous equation can be rewrittensymbolically, for a generic time t, as:
( ) ( )L A X L A X Vs W s F1- + =e r eThe first term represents the mass of solids in thesolid component of the cake at time t; the secondterm is the amount of solids still in suspension inthe water contained in the cake; and the thirdterm is the amount of solids removed from thefiltrate (and now held in the cake).
Remark: L, e, and VF can all be functions of time.
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PIERO M. ARMENANTENJIT
Cumulative Mass Balance for theSolids in the Cake (continued)
The volume of water contained in the cake (eLA) istypically much smaller than the volume of filtrate,VF, produced during the time interval 0-t.Furthermore, Xs and Xw are of the same order ofmagnitude. Then, one can safely assume that:
( )X V X L As F W>> eHence, the cumulative mass balance for thesolids in the cake becomes:
( )L A X Vs s F1- @e r
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PIERO M. ARMENANTENJIT
Cake Thickness, L, as a Function ofVolume of Liquid Passed Through
the FilterThe previously derived mass balance equation forthe solids in the cake:
( )L A X Vs s F1- =e rcan be rewritten as:
( )LX V
As F
s
=-1 e r
where L, e, and VF can all be functions of time.
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PIERO M. ARMENANTENJIT
Shape Factor of Particles in CakeThe particle shape factor, fp, is defined as:
fp =Surface area of sphere having same volume as particle
Surface area of particlei.e.,
fp
ppp
p
p
sph p
D
DVA
VD A
sph
sph
= =6 62
3
where Dsph is the diameter of a sphere having thesame volume as the particle.
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PIERO M. ARMENANTENJIT
Relationship Between Dp, Dsph, and fpSince:
DVAp
p
p
=6
and:
fpsph
p
pDVA
= 16
then:
D Dp p sph= f
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PIERO M. ARMENANTENJIT
Approximate Relationship BetweenDp and Sieve Opening
The assumption is often made that:
D Dsph p
where Dp is the average size of the particleswhose size is between two sieve openings
D D Dp s s= 1 2and where Ds1 and Ds2 are the sieve openings.Then:
D D Dp p sph p p= @f f
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PIERO M. ARMENANTENJIT
Pressure Drop During Cake Filtration
Filter Cake L(t)
Filter Medium
Filtrate
Suspension
DPc(t)
DPm
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PIERO M. ARMENANTENJIT
Pressure Drop During Cake FiltrationAt any time, t, the pressure drop experienced atthat time by a suspension passing through a filtercake supported by a filter medium (or septum) is:
( ) ( )D D DP t P t Pc m= +where:DP(t) = total pressure drop across filterDPc(t) = pressure drop due to filter cakeDPm = pressure drop due to filter mediumRemark: during batch filtration the cake can beexpected to build up, and the pressure drop toincrease as time passes.
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PIERO M. ARMENANTENJIT
Pressure Drop Across Filter CakeSince the liquid passing through the filter cakemoves in laminar flow (because of the small poresof the cake and the slow fluid velocity) the Blake-Kozeny equation can be used (instead of the moregeneral Ergun equation) to describe thedependence of the pressure drop through thecake with the superficial velocity, us:
( )DP L
Duc
p pL s=
-
150 1 23
2
Ree
e r Blake-Kozeny equation
where DPc is the pressure drop through the cake,and is, in general, a function of time.
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PIERO M. ARMENANTENJIT
Pressure Drop Across Filter CakesSubstituting the expression for Re in the Blake-Kozeny equation gives:
( )DP L
Duc
ps=
-
1501 2
3 2me
e
As before, DPc is typically a function of time, sincethe cake thickness, L, the superficial velocity, us,and the void fraction, e, can all change with time.
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PIERO M. ARMENANTENJIT
Pressure Drop Across Filter Cakes(Carman-Kozeny Equation)
For filter cakes the constant 150 may not beappropriate since the cake particles arecompressible.
Therefore the Blake-Kozeny equation is oftenrewritten to produce to so-called Carman-Kozenyequation:
( )DP k L
Duc
ps=
-
1
2
3 2
1m ee Carman-Kozeny equation
where: k1 = proportionality constant.
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PIERO M. ARMENANTENJIT
Superficial Velocity in Cake FiltrationAs before, the superficial (or approach) velocity isdefined as the velocity of the liquid as it flowsthrough a cross section equal to that of the tank(or filter vessel) in the absence of the cake. It isalso equal to the filtrate flow rate, QF, divided bythe total cross-sectional area normal to flow, i.e.:
uQA
dVd t As
F F= = 1
where:A = cross sectional area or empty filter vessel
VF = volume of filtrate passed through the cakeduring time t
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PIERO M. ARMENANTENJIT
Equation for Pressure Dropin the Cake
Combining together the equations:
( )DP k L
Duc
ps=
-
1
2
3 2
1m ee uQA
dVd t As
F F= = 1
and: ( )LX V
As F
s
=-1 e r
the following expression for DPc is found:( )
( )DPk
DX VA
dVd tc s p
s F F=-
-
1
2
3 2 2
1 11r
ee e
m
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PIERO M. ARMENANTENJIT
Equation for Pressure Dropin the Cake
The previous equation can be re-arranged to givethe final equation for the pressure drop in thecake:
DP X VA
dVd tc
s F F= a m 2
where a = specific cake resistance to filtration, isgiven by:
ar
ee
= -kDs p1
2 3
1
DPC, L, a, and VF can all be functions of time.
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PIERO M. ARMENANTENJIT
Equation for Pressure Dropin Filter Medium (Septum)
The pressure drop across the filter medium(septum) can also be expressed using theCarman-Kozeny equation that can be rewritten as:
( )DP k LD
umm
m
m
pms=
-
2
2
3 2
1m
ee
where the subscript m refers to the medium andthe superficial velocity is given by:
uQA
dVd t As
F F= = 1
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PIERO M. ARMENANTENJIT
Equation for Pressure Drop in FilterMedium (continued)
The resulting expression of the pressure drop inthe medium is:
DP RA
dVd tm m
F= m 1
where:
( )R k
LDm
m
m
m
pm
=-
2
2
3 2
1 ee
with Rm = specific resistance of medium tofiltration
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PIERO M. ARMENANTENJIT
Equation for Total Pressure DropDuring Cake Filtration
Recalling that the total pressure drop in a filter is:( ) ( )D D DP t P t Pc m= +
it is:
( ) ( ) ( )DP t t X V tA
RA
dVd t
s Fm
F= +
a m m2
1
Since by definition it is: QF(t) = dVF/dt, then:
( ) ( )( ) ( )[ ]dVd t
Q tA P t
t X V t ARF
Fs F m
= =+
2 Dm a
This is the main design equation for cake filters.
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PIERO M. ARMENANTENJIT
Specific Cake Resistance andCake Compressibility
From the expression for a:a
re
e= -k
Ds p1
2 3
1
one can incorrectly assume that the pressureacross the cake has no impact on specific cakeresistance. In fact, the void fraction e for mostcakes can be significantly affected by pressure,since the cake is often compressible. Since thepressure drop changes with time the void fractione can also be a function of time, at least inprinciple.
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PIERO M. ARMENANTENJIT
Specific Cake Resistance andCake Compressibility
In practice, it is convenient to carry outexperiments to determine:
the specific cake resistance under no pressuredifference (no compression). Cake is built upby gravity filtering;
the effect of pressure difference across thecake on the specific cake resistance. Cake isbuilt up first and then compressed to a knowpressure with a piston provided with a porousbottom. Filtrate is passed trough the cake.
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PIERO M. ARMENANTENJIT
Specific Cake Resistance andCake Compressibility
Piston
Suspension
Filtrate
Cake
CakeBuildup
CompressedCake
Suspension
Filtrate
Filtrate
PP
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PIERO M. ARMENANTENJIT
Specific Cake Resistance andCake Compressibility (continued)Possible results of cake compression experiment:
cake is incompressible. Cake resistance, a isindependent of DP;
cake is compressible. Cake resistance isexpressed as:
( )a a= o sPDwith: ao = empirical constant
s = coefficient of compressibility (typicalrange for most domestic sludges: 0.4-0.9; lime sludges: 1.05; sand: 0).
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PIERO M. ARMENANTENJIT
Specific Cake ResistanceTypical values of the specific cake resistance, a,are in the following ranges:
1013-1015 m/kg for raw sludges; 1011-1012 m/kg for well conditioned sludges.
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PIERO M. ARMENANTENJIT
Typical Specific Cake Resistancewith Chemical Conditioning
0 0.5 1 1.5 2 2.5
Chemical Conditioner (%)
1E+010
1E+011
1E+012
1E+013
1E+014
Spe
cific
Cak
e R
esis
tanc
e, a
(m/k
g)
After Vesilind, Treatment and Disposal of Wastewater Sludges, 1979, p.156.
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PIERO M. ARMENANTENJIT
Compressibility of Sludges as Measuredby the Specific Resistance Test
1 10 100
Vacuum Pressure (kPa)
1E+010
1E+011
1E+012
Spe
cific
Cak
e R
esis
tanc
e, a
(m/k
g)
After Vesilind, Treatment and Disposal of Wastewater Sludges, 1979, p.156.
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PIERO M. ARMENANTENJIT
Batch Filtration OperationsBatch cake filtration is typically carried out underone of the following conditions:
Constant filtrate flow rate. Since the pressuredrop across the filter increases as a result ofcake buildup this condition implies that theupstream pressure must be increased withtime.
Constant pressure drop across the filter. Thiscondition implies that the filtrate flow ratedeclines as the cake builds up.
Variable flow rate and variable pressure drop.
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PIERO M. ARMENANTENJIT
Batch Cake Filtration atConstant Filtrate Flow Rate
In some cases cake filtration is carried out usinga positive displacement pump. This results in aconstant flow rate process. Then:
QF = constantRecalling the design equation for cake filters it is:
( )( ) ( )[ ]
dVd t
QA P t
t X V t ARF
Fs F m
= =+
2 Dm a
Important: although QF = dVF/dt is a constant, VF(the total filtrate at time t) is not. In fact, it is:
dV Qdt V Q tF F F= =
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PIERO M. ARMENANTENJIT
Batch Cake Filtration atConstant Filtrate Flow Rate (cont' d)
Assuming that the cake is not compressible (i.e.,a is independent of DP) the pressure buildupwhile operating at constant filtrate flow rate (i.e.,constant QF) is given by:
( ) ( )[ ]DP t X V t ARA
Qs F m F=+m a
2
i.e., recalling that VF = QFt:
( )DP t X QA
tR Q
As F m F= +ma m
2
2 ( )for constantQF =
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PIERO M. ARMENANTENJIT
Applications of Batch Cake Filtrationat Constant Filtrate Flow Rate
Batch cake filtration at constant filtrate flowrate is used primarily in sludge dewatering;
The type of filters that utilizes filtration methodis the filter press;
Positive displacement pumps are used to forcethe suspension through the filter;
Gauge pressures up to 225 psi (15 atm) areused.
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PIERO M. ARMENANTENJIT
Batch Cake Filtration at Constant DPIf the pressure across the filter is constant thegeneral filtration equation:
( ) ( )( ) ( )[ ]dVd t
Q tA P t
t X V t ARF
Fs F m
= =+
2 Dm a
becomes:
( ) ( )[ ]dVd t
Q tA P
X V t ARF
Fs F m
= =+
2 Dm a
Note that the coefficient a is constant (but notnecessarily equal to ao) even if the cake iscompressible, since DP = constant.
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PIERO M. ARMENANTENJIT
Batch Cake Filtration atConstant DP (continued)
Since DP = constant, the previous equation can beintegrated by separating variables:
( )m a X V ARA P
dV d ts F m F
Vt
F '' '
+
= 2
00D
Integration of this equation yields:
( ) ( )ma mXA P
V tR
A PV t ts F
mF2 2
2
D D+ = ( )for constantDP =
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PIERO M. ARMENANTENJIT
Batch Cake Filtration atConstant DP (continued)
The previous equation can be rewritten as:
( ) ( )h gV t V t tF F2 + =where DP is constant, and the parameters h and gare given by the equations:
h ma= XA P
s
2 2 D and gm= RA P
m
D
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PIERO M. ARMENANTENJIT
Batch Cake Filtration at Constant DP:Determination of Filtration Parameters
The determination of h and g from batchexperiments conducted at constant DP can bemade by rearranging the equation:
( ) ( )ma mXA P
V tR
A PV t ts F
mF2 2
2
D D+ =
as: ( ) ( )t
V tX
A PV t
RA PF
sF
m= +ma m2 2 D D
with: slope and intercept= = = =hma g mXA P
RA P
s m
2 2 D Dand t/VF = y-coordinate and VF = x-coordinate
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PIERO M. ARMENANTENJIT
Determination of the Specific Cake ResistanceThrough Batch Filtration Experiments atConstant DP: Buchner Funnel Apparatus
Buchner Filter
Vacuum Gauge
Graduated Cylinder
To Vacuum
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PIERO M. ARMENANTENJIT
Determination of the Specific Cake ResistanceThrough Batch Filtration Experiments at
Constant DP: Filter Leaf Apparatus
VacuumGauge
GraduatedCylinder
To Vacuum
FilterLeaf
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PIERO M. ARMENANTENJIT
Example of ExperimentalDetermination of Filtration Constants
0 0.001 0.002 0.003 0.004 0.005 0.006
Cumulative Filtrate Volume, VF (m3)
0
5,000
10,000
15,000
20,000
25,000
t/VF
(s/m
3 )
ExperimentalRegression
Intercept = 6840.95 s/m3
Slope = 2.85378E+006 s/m6
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PIERO M. ARMENANTENJIT
Batch Cake Filtration at Constant DP:Approximate Equations
If the resistance of the filter medium, Rm, is verysmall in comparison to the cake resistance, a, thebatch filtration equation can be rewritten as:
( )t XA P
V ts s@ma2 2
2
D ( )V t A P
Xt
tF
F
@ =22 D
ma hi.e.:
( )Q t dVd t
A PX t tF
F
F
= @ =12
2 12
12 Dma h
Note that QF(t) 0 for t .
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PIERO M. ARMENANTENJIT
Batch Cake Filtration at Constant DP:Plot of Approximate Expression
for VF(t)
Time, t
Cum
ulat
ive
Filtr
ate
Vol
ume,
VF
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PIERO M. ARMENANTENJIT
Applications of Batch Cake Filtrationat Constant DP
Batch cake filtration at constant DP is usedprimarily in sludge dewatering;
The types of filters using this filtration methodinclude:
- Filter presses
- Belt filter presses
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Rotary Vacuum Filters
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Rotary Vacuum Filter
CakeDewatering
CakeDischarge
Suspension
Solids
Cake
Blade
CakeFormation
q
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Rotary Vacuum Filters
In continuous filtration operations, such asthose involving vacuum rotary filters, eachfilter element undergoes a batch cakefiltration, followed by a cake dewatering phase,and a cake discharge phase with each rotation.
The filter cake is formed under a constant DPdriving force generated by a vacuum.
The filter cake is formed only during the timeperiod when the filter surface is immersed inthe suspension.
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Cycle Time
The immersion period in each cycle (i.e., for eachfull rotation of the filter drum) is given by:
t f t fk c k= = = =2
22p
wqp
pw
qw
where: tc = cycle time (time for one full rotation)fk = fraction of cycle time available for cake
formation = fraction submergence of drumsurface
q = angle comprising the sector immersed insuspension (rad)
w = rotational (angular) velocity (rad/s)
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Filtration EquationDuring the time period t = fk tc the filter cake in arotary filter is formed just as in a batch operation.The (batch) filtration equation for part of thecontinuous process is:
( ) ( )ma mXA P
V tR
A PV t t f ts F
mF k c2 2
2
D D+ = =
since DP is constant.
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Filtrate Generated
During a CycleThe previous quadratic expression is an equationin VF that can be solved for VF and rearranged togive:
( )V t AX
R Rf t X P
Fs
m mk c s= - + +
a
am
2 2 D
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Filtrate Flux
The previous equation can be rearranged to give:
Filtrate flux = =
= = - + +
QA
VAt X
Rt
Rt
f X Pt
F
F
c s
m
c
m
c
k s
c
1 222a
am
D
which predicts the filtrate flux, i.e., the amount offiltrate VF produced per unit filter area during acycle (or full rotation) lasting a time interval tc.QF = average filtrate flow rate during the interval tc
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Filter Loading
This equation can be rewritten to give the amountof filter solids loading, Gk, produced during thesame cycle:
G V XAt
Rt
Rt
f X Ptk
F s
c
m
c
m
c
k s
c
= = - + +
1 222a
am
D
where:
Gk = filter solids loading (kg/m2 s) = amount ofsolids filtered per unit filter area over a timeinterval tc.
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Continuous Operation
Since a continuous process is nothing more thana sequence of cycles, each one lasting tc, then:
Filtrate flux = = - + +
QA X
Rt
Rt
f X Pt
F
s
m
c
m
c
k s
c
1 222a
am
D
The equation:
G V XAt
Rt
Rt
f X Ptk
F s
c
m
c
m
c
k s
c
= = - + +
1 222a
am
D
can be used to describe the continuous operationof a rotary (vacuum) filter.
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Rotational VelocityIf the angular velocity is expressed in rpm, i.e.:
N = 602wp
then the cycle time and the rotational (angular)velocities (in rad/s or rpm) are related by:
tNc
= =2 60pw
where: w = rotational (angular) velocity in rad/stc = cycle time (to complete a rotation) in secondsN = rotational (angular) velocity in rpm (rotations
per minute).
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Continuous Filtrate Flux
The continuous filtrate flux can be convenientlyexpressed in terms of the agitation velocity:
( )Filtrate flux = = - + +
QA X
R R f X PFs
m m k s12 2
2 2
2aw
pw
pw a
mpD
( )Filtrate flux = = - + +
QA X
R N R N f N X PFs
m m k s160 60
260
2 2
2aa
mD
where: tc = cycle time (to complete a rotation), in sN = rotational (angular) velocity in rpm (rotations
per minute).
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Simplified EquationsIf the resistance of the filter medium, Rm, is verysmall then the equations for continuous filtrationcan be simplified and rewritten as:
Filtrate flux = = @QA
VAt
f PX t
F F
c
k
s c
2 Dma
G V XAt
f X Ptk
F s
c
k s
c
= @ 2 Dma
These equations are especially useful tounderstand the relationships between the variousvariables affecting a continuous filtration process.
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PIERO M. ARMENANTENJIT
Continuous Cake Filtration atConstant DP: Simplified Equations
If the resistance of the filter medium, Rm, is verysmall the equations for continuous filtration can beexpressed as a function of the rotational velocity as:
Filtrate flux = = @ @QA
VAt
f PX
F F
c
k
s
D wpma
@ 260
f P NX
k
s
Dma
where: tc = cycle time (to complete a rotation) in sN = rotational (angular) velocity in rpm (rotations
per minute).
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PIERO M. ARMENANTENJIT
Applications of Continuous CakeFiltration at Constant DP
Continuous cake filtration at constant DP is themost widely used method of sludgedewatering;
The types of filters using this filtration methodinclude:
- Rotary vacuum belt filter
- Rotary vacuum precoat filters
- Rotary vacuum drum filters
- Rotary vacuum disc filters
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Comparison of Different Types of FiltersRotary
Drum BeltFilters
Belt FilterPresses
FilterPresses
GranularDeep-Bed
Filters
Size 1-70 m2 1-2 m beltwidth
0.02-16 m3 0.2-10 m2
Solids inFeed (%)
2-5 2-8 -- --
Solids inCake (%)
15-20 15-25 28-40
SolidsLoading
10 kg/m2 h 190-270kg/m h
-- 0.12-0.5m3/m2 min
After Freeman, Standard Handbook of Hazardous Waste Treatment andDisposal, 1989, p. 7.12
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Design Information for PressureCake Filters
Cycle time: of the order of hours
Solids loading: 0.2-2 lb/fth
Solids in cake: up to 50%
Remark: although solids loading in pressurefilters (e.g., filter presses) is typically smaller thanthat of vacuum filters the percentage of solids inthe cake is typically higher. This is the result ofthe higher pressure that can be used in theoperation of pressure filters (as opposed to amaximum of 1 atm in vacuum filters).
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Design Information for Rotary Vacuum FiltersDiameter: up to 5 mLength: up to 6 mVacuum levels: typically 20 in. Hg (68 kPa)Submergence: 15-25% of drum area
PrimarySludges
Waste-activatedsludges
Solids loading 20-60 kg/m2h(4-12 lb/ft2h)
5-20 kg/m2h(1-4 lb/ft2h)
Solids in cake 25-40%(typically 20-25%)
10-15%
After Sundstrom and Klei, Wastewater Treatment, 1979, p. 234.
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PIERO M. ARMENANTENJIT
Operation of Rotary Vacuum Filters Solids loading increases with increasing drum
submergence, drum rotational speed, pressuredifference across cake, solids concentration infeed.
Percentage of solids in cake decreases withincreasing drum submergence, and drumrotational speed.
Rotary vacuum filters can be used to dewatersludges from activated sludge plants(biological sludges), chemical sludges, andsludges from precipitation operations.
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PIERO M. ARMENANTENJIT
Additional Information and Exampleson Cake Filtration
Additional information and examples on can be found inthe following references:
Sundstrom, D. W. and Klei, H. E., 1979, WastewaterTreatment, Prentice Hall, Englewood Cliffs, NJ, p.229-234.
Geankoplis, C. J., Transport Processes and UnitOperations, 3rd Edition, 1993, Allyn and Bacon,Boston, pp. 800-815.
Freeman, H. M. (ed.), 1989, Standard Handbook ofHazardous Waste Treatment and Disposal, McGraw-Hill, New York, pp. 7.3-7.19.
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PIERO M. ARMENANTENJIT
Additional Information and Exampleson Cake Filtration
Haas, C. N. and Vamos, R. J., 1995, Hazardous andIndustrial Waste Treatment, Prentice Hall, EnglewoodCliffs, NJ, pp. 75-78.
Wentz, C. W., 1995, Hazardous Waste Management,Second Edition, McGraw-Hill, New York. pp. 196-200.
Vesilind, P. A., 1979, Treatment and Disposal ofWastewater Sludges, Ann Arbor Science, Ann Arbor,MI, pp. 140-161.
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