particle technology- membranes and colloids

Membranes & ColloidsChapters 4 & 13 in Fundamentals

Professor Richard Holdich

R.G.Holdich@Lboro.ac.uk Course details: Particle Technology, module code: CGB019 and CGB919, 2nd year of study.

Watch this lecture at http://www.vimeo.com/10202852

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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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