particle technology- membranes and colloids
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The sixth lecture in the module Particle Technology, delivered to second year students who have already studied basic fluid mechanics. Membranes and Colloids covers the different types of particle related pressure driven membrane separations and models of flux decay and fouling. Colloidal behaviour using the DLVO theory is also covered, including colloid stability.TRANSCRIPT
Membranes & ColloidsChapters 4 & 13 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/10202852
Visit; http://www.midlandit.co.uk/particletechnology.htm
for further resources.
Membranes & Colloids
Types, configurations and permeate flux
Surface and internal fouling Polarisation flux models &
enhancement Colloidal interaction – DLVO
theory
Introduction
Particle size - colloids
Bacteria 0.2 to 8 microns viruses 0.05 to 0.5 microns colloidal silica 0.02 to 1 micron macromolecules 0.01 to 0.5 microns ions <0.01 microns no concentration limit in MF & UF
processes
Membrane types
Microfiltration• 0.05 to 10 microns generally
Ultrafiltration• 1 to 50 nano-metres
Nanofiltration & Reverse Osmosis
Membrane cartridges
Cells - unstirred and stirred
Filter Media - Pressure
start
Medium
Rm
Po
P
v1
later
Medium
Rm
Po
P
v2
Crossflow filtration
Crossflow filtration
Flux variation and resistance
Resistances to membrane filtration:
Flux variation and resistance
Darcy’s law:
At
V
kL
P 1
d
d
Darcy’s law modified:
Jk
LP
)( mC RR
PJ
Flux variation and resistance
Darcy’s law:
)( mC RR
PJ
J is permeate flux, in conventional units of litres per metre squared of membrane area per hour. It is the same as superficial velocity.
Crossflow filtration
Permeate flux decay
Filtration fundamentals
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
Membranes & Colloids
Types, configurations and permeate flux
Surface and internal fouling Polarisation flux models &
enhancement Colloidal interaction – DLVO theory
Filter Media - Pore Size?
What do we mean by pore size?
Filter Media - Pore Size?
Metal fibre microfiltration medium - rated at 3 microns
Filter Media - Pore Size!
Equivalent pore size
Membrane media - PTFE
0.2 micron rated membrane filter
Membrane internal fouling?
Membrane secondary membrane
Membranes & Colloids
Types, configurations and permeate flux
Surface and internal fouling Polarisation flux models &
enhancement Colloidal interaction – DLVO theory
Membrane models
Rejection
R = 1 - Np/Nb
Equilibrium flux response
Membrane film theory
Membrane simple circuit
Membrane feed & bleed
Diafiltration
Diafiltration
Stirred tank displacement washing only:
)/exp( VJAtCC o
e.g. washing times given a flux rate of 50 l m-2 h-1 and tank volume of 1000 litres
Area (m2): 1 10 20t(hrs) C(ppm) C(ppm) C(ppm)0 500 500 5001 476 304 1842 452 184 684 409 68 96 370 25 1
Membrane cleaning
Membrane & other SLS
Membrane surface filter
Membrane surface filter - slots
Membranes & Colloids
Types, configurations and permeate flux
Surface and internal fouling Polarisation flux models &
enhancement Colloidal interaction – DLVO
theory
Colloidal interaction
Stokes’ law tells us about settling?
Increase diameter but decrease density – net enhanced rate
COAGULATION v
FLOCCULATION
Colloidal interaction
Floc bed clarifier
Electrical interaction
Surface –ve charge Fixed layer +ve ions Diffuse layer after… Shear layer Zeta potential –
measured by moving particle in field,
Typically -50 to 0 mV
Van der Waal’s attraction
H
H
HH
A
s
s
ss
HA 2
ln)2(
)H2(1
12 = s
x
zH s
2
where AH is the Hamaker* constant for a given system and Hs is the ratio of the separation distance (z) between the particles and the particle radius.
So, in terms of particle diameter
*5x10-20 J for water
Electrical repulsion
zΚzΚ
zΚxZZZZ
pR 2exp(1ln)(
exp(1
)exp(1ln2
82
2:2
1:2:1:
where the Zeta potential is used extensively above, together with the particle diameter, dielectric constant of the system and the Debye-Hückel function –which is a function of ionic conditions.
Net forces - DLVO
Total
RAT
Dimensionless interaction energy
Tk B
T
Force
zF
d
d
Net forces - DLVO
Curve 1 Primary minimum and maximum
Curve 2 Primary and Secondary
minimum Single maximum
Curve 3 Primary minimum
Which colloid is the most stable?
Net forces - DLVO
Silting of estuaries Click image for
XLS
Stokes’ settling equation
Colloid stability important in filtration and sedimentation.
Often assessed by the Zeta potential
Surface forces can predominate at iso-electric point.
Membranes & Colloids
Types, configurations and permeate flux
Surface and internal fouling Polarisation flux models &
enhancement Colloidal interaction – DLVO theory
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