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The fifth lecture in the module Particle Technology, delivered to second year students who have already studied basic fluid mechanics. Filtration covers the modification of Darcys law to predictive filtration design equations as well as ones used for test data analysis. Examples of industrial equipment for filtration are included.
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FiltrationChapter 4 in Fundamentals
Professor Richard Holdich
[email protected] Course details: Particle Technology, module code: CGB019 and CGB919, 2nd year of study.
Watch this lecture at http://www.vimeo.com/10201620
Visit http://www.midlandit.co.uk/particletechnology.htm
for further resources.
Filtration
Types Cake filtration mechanism Modification of Darcy's law Constant pressure filtration Constant rate filtration Variable rate & pressure
filtration Industrial equipment
Types of filtration
Normally batch (in duplicate) but some continuous ones:
Deep bed - clarification
Image supplied by DynaSand and Hydro International (Wastewater) Ltd.
Types - membrane
Clarification on filtering membranes
Types - Clarification
Cartridge and candle filtration
Cake filtration mechanism
Multifilament filter cloth p. 40
Cake filtration mechanism
Monofilament filter cloth
Cake filtration mechanism
Monofilament open filter cloth/mesh
Cake filtration mechanism p.31
Why can’t we simply measure Rm for each medium?Ideal
Filtrate
Bridgingover pores
Filter m edium
Filter cake
sharp interface m edium /cake - uniform spheresin cake easy to m odel
Cake filtration mechanism – reality p 41
Why can’t we simply measure Rm for each medium?
RealFilter cake
Filter m edium
i.e. Rm = f(material to be filtered)
Modification of Darcy's law
Porosity or voidage
and Concentration
dV
dt
1
A
Porous m edia
void + solid = unityfraction fraction
Volum e fractions:
U =o
U o
U
Superficial velocity:
+ C = 1
Modification of Darcy's law
Darcy’s law:
At
V
kL
P 1
d
d
Kozeny-Carman equation:
At
VS
L
P v 1
d
d)1(53
22
Pressure/L
Flow rate
xSv /6use:
Modification of Darcy's law
Darcy’s law/Kozeny:
At
V
kL
P 1
d
d
Pressure/L
Flow rate
What do the graphs tell us about these equations?
How will this vary for filtration?
Think about a given material and filter in these equations – what is constant, what varies, look at the graph…
What are the independent and dependent variables?
A
QSv
3
22)1(5
Time
Volume liquid
Modification of Darcy's law – p.29
Darcy’s law:
At
V
kL
P 1
d
d
Q is constant - permeation
Time
Filtrate volume
Q decreases - filtration
At constant pressure drop:
Modification of Darcy's law – p. 32
Build up of incompressible filter cake:
Filter medium
Filter cakeformation
Modification of Darcy's law
20 kPa
P = dV 1 L k dt A
1.5 V
V = R I
0.75 V10 kPa
0 kPa 0 V
Modification of Darcy's law
Pressure drops are additive:
Pcake
Pmedium
At
V
k
L 1
d
d
At
V
k
L
m
m 1
d
d
Modification of Darcy's law
Pressure drops are additive:
PAt
V
k
L 1
d
d
At
VRm
1
d
d
00
G rad ie n t:
Cak
e vo
lum
e
F iltra te v o lu m e
= L A V
Ratio: cake volume:filtrate = constant =
PA
RV
kC
C
PAV
t m
s
s
2d
d
PA
RV
PA
c
V
t m
2d
d
Modification of Darcy's law
00
G rad ie n t:
Cak
e vo
lum
e
F iltra te v o lu m e
= L A V
Ratio: cake volume:filtrate = constant =
1sC
What does
Represent – in English, see the graph…
skC1
What does
Represent – in English
Modification of Darcy's law – p.36
where c is the dry cake mass per unit volume of filtrate:
and is the specific resistance to filtration (m/kg).
sm
sc
1
s is feed slurry mass fraction and m is the moisture ratio of the cake (mass cake wet/mass cake dry - or sample). In some instances one can assume m=1; i.e. neglect liquid in cake.
Modification of Darcy's law – p.36
AQRRP mc /)(
w
Rc alpha = Rc/w
Considering Rc & alpha some more:
w is dry mass/unit area solids:
A
cVw
so:
AQRA
cVP m /)(
Modification of Darcy's law – equation (4.11)
sm
sc
1
PA
RV
PA
c
V
t m
2d
d
General filtration equation:
Constant pressure filtration
Constant P filtration - integrate general equation:
to give:
PA
RV
PA
c
V
t m
2d
d
PA
RV
PA
c
V
t m
22
baVV
t
i.e:
Time over filtrate volume
Filtrate volume
b
a
Constant pressure filtration
summary:
Need to know:
PA
RV
PA
c
V
t m
22
viscosity, pressure, and filter area
& slurry mass fraction, liquid density (and cake moisture - if poss.)
Time over filtrate volume
Filtrate volume
b
a
Need to calculate:c then
and Rm
Constant pressure filtration
General filtration equation:
Constant pressure:
PA
RV
PA
c
V
t m
22
PA
RV
PA
c
V
t m
2d
d
y = m x + c
Constant pressure filtration
Filtration Testing in the Laboratory:
effect of pressure, different cloths or media, slurry agitation, filter aids and flocculants effect of slurry pre-concentration
High permeability: vacuum leaf
Low permeability: pressure bomb
Tests:
Constant pressure filtration
Filtration Testing in the Laboratory:
specific resistance - possibly as f(pressure),
medium resistance cake concentration - possibly as
f(pressure) or moisture ratio
High permeability: vacuum leaf
Low permeability: pressure bomb
To obtain values of:
Constant pressure filtration
Filtration Testing in the Laboratory:
Liquid viscosity filtration
pressure filter area
High permeability: vacuum leaf
Low permeability: pressure bomb
Also required for scale-up or simulation:
Slurry mass fraction
liquid density solid density - if
cake height is required
Constant pressure filtration p. 41 – vacuum filter leaf
To v a cu u mp u m p
C a lib ra tedf il tra te
rec e iv e r
D ra in
L e af o r
M e ch an ica la g ita tio n
Ve n t
Va lv e - fu lly o p e n in te st
S lu rry tan k
F ilte r in g s id e
B u ch n e r
S tir re r
fu n n e l
Experimental characterisation
Constant pressure filtration
Constant rate filtration – p. 36
Constant rate:
General filtration equation:
PA
RV
PA
c
V
t m
2d
d
t
V
A
RV
t
V
A
cP m
2
V
t
t
V
d
d
Filtration pressure
Filtrate volume
b
a
Variable rate & pressure filtration
General filtration equation:
Variable pressure and rate equation:
PA
RV
PA
c
V
t m
2d
d
A
RV
A
c
Q
P m
2
plot numerical integration of:V
Q &
1
Q
Vt
d
Industrial equipment – p. 35
Rotary vacuum filter (continuous) Stages
• cake formation in slurry tank (F)
• drying and/or washing (D and W)
• discharge - then back to formation (D & Di)
F
DW
D & Di
Industrial equipment
Constant pressure:
PA
RV
PA
c
V
t m
22
Rearrange for a quadratic:
02
22
tV
PA
RV
PA
c m
Industrial equipment – p. 36
Simulation of Rotary Vacuum Filter:
02
22
tV
PA
RV
PA
c m
i.e. aV2 + bV - t = 0
a
atbbV
2
42
where ‘form’ time t = F/n (submergence/speed)
Industrial equipment
per cycle of drum:
• Mass dry cake deposited = cV (kg)
• Mass wet cake deposited = mcV (kg)
• mass slurry filtered = mcV + V (kg)
a
atbbV
2
42
Calculate volume, hence:
All above is per cycle, hence 3600/t for output per hour.
Industrial equipment
Vacuum belt filter (continuous)
Image appears courtesy of Polyfilters UK Limited www.polyfilters.com
Industrial equipment
Vacuum belt filter (continuous)
Image supplied courteousy of BHS-Sonthofen GmbH, Germany www.bhs-sonthofen.de
Industrial equipment
Vacuum disc filter (continuous)
Image courtesy of FLSmidth, Inc.
Industrial equipment
Tube pressure filter (batch)
Image courtesy of Mesto Minerals (Sala) AB
Filtration
Types Cake filtration mechanism Modification of Darcy's law Constant pressure filtration Constant rate filtration Variable rate & pressure
filtration Industrial equipment
This resource was created by Loughborough University and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme.
Slide 3. Image of a DynaSand® is provided courtesy of Hydro International (wastewater) Limited. See http://www.hydro-international.biz/irl/wastewater/dynasand.php for more details.
Slide 37. The image of a vacuum belt filter (continuous is provided with the permission of Polyfilters (UK) Limited. See http://www.polyfilters.com/process.html for more details.
Slide 38. Image provided courtesy of BHS-Sonthofen GmbH. See www.bhs-sonthofen.de for more details.
Slide 39. Image provided courtesy of FLSmidth Inc. See http://www.flsmidthminerals.com/Products/Filtration/Vacuum+Filtration/Vacuum+Disc+Filters/Agidisc+Vacuum+Filters/Agidisc+Vacuum+Filters.htm for more details.
Slide 40. Image of a tube press discharge, provided courtesy of Mesto Minerals (Sala) AB. See http://www.metso.com/miningandconstruction/MaTobox7.nsf/DocsByID/C44A6B216E52C95142256AF6002D6148/$File/Tube_Press_ES.pdf for more details.
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