MEMBRANE SEPARATION PROCESSES

• Membrane: The layer ofmaterial that acts as a selectivebarrier between two phasesand remains impermeable tospecific substance when adriving force is applied.

Fundamentals of Membrane Process

specific substance when adriving force is applied.

• Membrane Separation Processprinciple: membrane acts as aselective filter under a drivingforce such as hydrostaticpressure/ concentrationgradient, etc.

ADVANTAGES

� Low energy alternative to evaporation.

� Chemical and mechanical stresses on the product are

� Product concentration and purification in a single step

� Selectivity is good.

�Method can be easily the product are minimal.

�No phase change involved(except in pervaporation), hence modest energy requirement.

�Method can be easily scaled up.

� In bioprocess industry: used in the recovery of extracellular products (proteins, enzymes) and biomass from fermentation broths

CLASSIFICATION OF MEMBRANE

SEPARATION PROCESSES

(on the basis of the driving force)

�Microfiltration (MF)

�Ultrafiltration (UF)

� Reverse osmosis (RO) or

I. Driving force:

hydrostatic pressure

� Reverse osmosis (RO) or

Hyperfiltration

�Dialysis

� Electrodialysis

II. Driving force:

concentration

gradient

III. Driving force: Applied

electric field

CLASSIFICATION OF MEMBRANE

SEPARATION PROCESSES

Micro filtration(MF) Ultrafiltration(UF)

Reverse osmosis(RO) Dialysis

• Change of phase

• Azeotropic mixture

• An inert carrier or

vacuum is provided

PERVAPORATION

vacuum is provided

on other side of

membrane

• Due to difference in

solubility and

diffusion.

Characteristics of Membrane Separation

ProcessesPROCESS DRIVING FORCE CHARACTERISTIC

FEATURES OF

MEMEBRANE PORE

SIZE

SEPARATION

MECHANISM

MF Pressure 0.1-1 bar 0.02-10 micrometre Sieving/filteringMF Pressure 0.1-1 bar 0.02-10 micrometre Sieving/filtering

UF Pressure 2-10 bar 0.001-0.02

micrometre

Sieving/filtering

RO Pressure 10-100

bar

Non – porous Solution diffusion

Dialysis Concentration

difference

1-3 nm Sieving and

diffusion

Electrodialysis Electrical potential Mol. Wt < 200 Ion migration

THEORETICAL MODELS FOR

MEMBRANE PROCESSES

CAPILLARY FLOW MODEL:

• Membrane is loose and microporous , capable of retaining particles larger than 10 A.

• The flow of the feed • The flow of the feed occurs through the pores by convective flow

• The solvent moves as viscous flow

• The solute molecules /smaller particles carried by convection with the solvent. Dominates in MF and UF

SOLUTION DIFFUSION MODEL

• The dissolution of the molecular species being transported in the material of the membrane followed by molecular diffusion.

THEORETICAL MODELS FOR

MEMBRANE PROCESSES

of the membrane followed by molecular diffusion.

• Driving force- concentration gradient in the membrane set up by applied pressure difference.

• Obeys Fick’s diffusion law.

• Membrane is tight and capable of retaining solutes less or about 10 Angstrom.

Dominates in RO

RETENTION COEFFICIENT

� It explains the separating ability of membrane in MF,UF and RO.

R = Cm - Cp

Cm

R is the theoretical retention coefficient and Cm and Cp represent the concentrations of the solute at the membrane surface and in the permeate respectively

� Actual retention coefficient R’ is� Actual retention coefficient R’ is

R’ = Cb – Cp

Cb

Where Cb is the concentration in the bulk

� R’= 1- ( 1 - R)(Cm / Cb)

� Due to Concentration polarization at the membrane surface Cm/Cb>1 and R’< R.

� Concentration polarization increases solute leakage in RO while build up of solute particles on the membrane surface in MF and UF.

FACTORS AFFECTING THE SEPARATION

PROCESSES

CONCENTRATION POLARISATION

FACTORS AFFECTING THE SEPARATION

PROCESSES

CONCENTRATION POLARISATION

• Short term and reversible.

• It occurs when the non permeating particles have Cm> Cb, the concentration polarisation is set up at membrane surface.

• Increases osmotic pressure and reduces flux.

• From the mass balance of the solute:• From the mass balance of the solute:

• Rate of convection towards the membrane= rate of diffusion back into the bulk liquid+ rate of permeation

Js C = D dC/dx + Js Cp

• Concentration of solute in the membrane is given by

Js=k’ ln((Cm-Cp)/(Cb-Cp))

• k’=mass transfer coefficient

• Js=flux

• k’ depends on Diffusivity, mol.wt, viscosity and the thickness of boundary layer.

FOULING

The flux through the membrane decreases slowly with time in all the membrane processes due to fouling caused by e.g.� Slime formation

� Microbial growth

� Deposition of macro molecules (particularly in UF)

� Colloidal deposition and physical compaction in membrane (particularly in RO due to high pressure operation)

Fouling is long term and irreversible.

� Inhibited by careful selection of membrane material(hydrophilic surface is less prone to fouling by proteins)

� Pretreatment of feed (such as pH adjustment or precipitation to remove salt)

� Frequent cleaning with chemicals and backwashing

2 types membrane fouling2 types membrane fouling

�Surface (temporary) fouling

•Foulant appears an evenly deposited layer on the membrane surface

•Can be easily removed by cleaning solution

•Permeation rate of membrane can be regenerated by cleaning

•Most common type of fouling in UF plant

•Most studies dealt with this type of fouling

�Pore (permanent) fouling

•Particulate matter diffuses into the membrane

•Could be caused by the poor quality of the cleaning water

•Flux cannot be regenerated by cleaning

•Determines the lifetime of the membrane

•Received much less attention in literature

Cross flow filtration overcomes both concentration polarization and fouling.

Cross flow membrane filtration

OPERATIONAL REQUIREMENT OF

MEMBRANES

1) Selectivity and high separation efficiency

2) High permeate flux rate

3) Mechanical/physical strength to withstand high pressure upto 50-60atm.pressure upto 50-60atm.

4) Durability and consistency of performance over prolonged periods

5) Resistance to corrosion

6) Ease of fabrication in appropriate shape

7) Low cost and readily available

STRUCTURE OF MEMBRANES

• Semipermeable membrane used in RO and UF have generally two phases.

• Top layer: thin(0.5-10um) dense layer, microporous structure;responsible for basic microporous structure;responsible for basic separation characteristics

• Bottom layer: thick(50-125um) macroporous material;gives strength to the membrane

• Both casted in a single membrane of 0.1 -0.2mm thickness.

PREPARATION OF MEMBRANES

Step I:base material is dissolved in solvent with additives- give homogeneous solution.

Step II: a film is casted by spreading solution over a glass plate or hollow tube.

Step III: controlled atmosphere maintained for Step III: controlled atmosphere maintained for evaporation of solvent.

Step IV: membrane dipped in water bath(273-281K), solvent and additives leachout, thus forming micropores.

Step V: membrane is annealed at 340-360K to cause pore shrinkage.

EQUIPMENT

Components of a typical membrane separation plant

Membrane ModulesMembrane Modules

� Spiral wound: Flexible permeate spacer between 2 flat membrane

sheets

� Hollow fibre: inside out / outside in flow

� Tubular : either single or in a bundle, inside – outside

� Plates and frame: Series of flat membrane sheets and support plates

Membrane ModulesMembrane Modules

Comparison of Membrane ModulesComparison of Membrane Modules

Type of Membrane Type of Membrane

Schematic diagrams of types of membranes

Applications of Membrane Separation

ProcessesSPECIES MOLECULAR

WEIGHT

SIZE(nm) TECHNIQUE

Inorganic salts 10 - 100 0.1 -0.2 RO

Simple organic

substances(acids ,

sugars)

100 - 500 0.4 -1.0 RO

sugars)

Antibiotics 400 - 1000 0.8 -1.2 RO

Biopolymers

(proteins, enzymes,

polysaccharides)

10^4 - 10^6 2 – 10 UF + D

virus 30 – 300 UF + D

Colloids 100 – 1000 UF + MF + D

Bacterial cells 300 – 10^4 UF + MF + D

Yeast and fungi 10^3 – 10^4 MF

CASE STUDIES• Separation of protein from

suspended particles using submerged membrane filtration:

� The main purpose of this study is to understand how to enhance the filtration flux and protein recovery in submerged membrane filtration by using hydrodynamic methods.

� PMMA particles and BSA were selected as the sample cells and

• Separation of ethanol from ethanol–water mixture and fermented sweet sorghum juice using pervaporation membrane reactor :

• Pervaporation, an effective and economical membrane technology, has been proven to substitute distillation process for ethanol separation. In this work, the separation of two types of mixtures; ethanol–water mixture and fermented sweet sorghum were selected as the sample cells and

proteins

� The filtration flux and BSA production in the SPI operation was much higher than in the other techniques, especially when SPI was combined with a periodic backwash.

� Refrence: Journal of Membrane Science, Volume 362, Issues 1-2, 15 October 2010, Pages 427-433Kuo-Jen Hwang, Hung-Pin Lo, Tung-Wen Cheng, Kuo-Lun Tung

fermented sweet sorghum were investigated using cellulose acetate membranes. The pervaporation performances of the two mixtures were carried out under various operating conditions such as ethanol concentrations, operating times and temperatures

• Ethanol flux is affected to a greater extent than water in the fermentation broth

• Refrence: Desalination, Volume 271, Issues 1-3, 15 April 2011, Pages 88-91P. Kaewkannetra, N. Chutinate, S. Moonamart, T. Kamsan, T.Y. Chiu

• Removal of char particles from fast pyrolysis bio-oil by microfiltration:

• Microscopic and ash content analysis of the feed and permeate streams were conducted to determine the efficacy of the process. The results demonstrated the removal of the major quantity of char particles with a significant reduction in overall ash content of the bio-oil.

Journal of Membrane Science, Volume 363, Issues 1-2, 1 November 2010, Pages 120-127Journal of Membrane Science, Volume 363, Issues 1-2, 1 November 2010, Pages 120-127Asad Javaid, Tatiana Ryan, Gayla Berg, Xiaoming Pan, TusharVispute, Surita R. Bhatia, George W. Huber, David M. Ford

GlossaryGlossaryFeed

� The solution to be concentrated or fractionated

Flux

� The rate of extraction of permeate measured in litres per square

meter of membrane surface per hour (L/m2/h)

Membrane fouling

� Deposition of solids on the membrane, irreversible during

processingprocessing

Permeate

•The filtrate, the liquid passing through the membrane

Retentate

•The concentrate, the retained liquid

Transmembrane pressure

•Pressure gradient between the upstream (retentate side) and downstream

(permeate side)