ERT 313 : BIOSEPARATION ENGINEERING “Membrane Separation Process” By; Mrs Haf iza Bint i Shu kor ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011)

ERT 313 : BIOSEPARATION ENGINEERING

“Membrane Separation Process”

By; Mrs Hafiza Binti Shukor

ERT 313/4 BIOSEPARATION ENGINEERINGSEM 2 (2010/2011)

Students should be able Students should be able to; to;

ANALYZE the separation of gases and liquids using membranes.

ANALYZE Microfiltration, Ultrafiltration, Reverse-Osmosis and Nanofiltration, membrane configurations and mode of operations.

CALCULATE and ANALYZE membrane permeability, concentration polarization and membrane fouling.

DEVELOP basic design of membrane unit.



WHAT IS A MEMBRANE?WHAT IS A MEMBRANE?• Membranes are materials

which have voids in them, letting some molecules pass more conveniently than some other molecules.

• A semi-permeable membrane is a VERY THIN film that allows some types of matter to pass through while leaving others behind


WHAT IS A MEMBRANE?WHAT IS A MEMBRANE?


HOW SEPARATION OCCURSHOW SEPARATION OCCURSDifference in permeabilities through a

membrane:– Difference in size,– Affinity to the membrane,– Charge, etc.

DRIVING FORCESDRIVING FORCES• Pressure difference,• Concentration difference,• Voltage difference, etc.


ADVANTAGESADVANTAGES

• Continuous separation• Low energy requirement• Meet various separation demands

DISADVANTAGESDISADVANTAGES• Fouling• Service periods


•Membranes may be composed of natural (e.g modified natural cellulose polymers ) or synthetic polymers (plastic materials) or inorganic ceramic materials be good film formers, manage high permeate flows, have high selectivity, have good chemical and bacteriological resistance, be resistant to detergents and disinfectants, be inexpensive.

Membrane material


•Reverse osmosis (RO)Concentration of solution by removal of water

•Nanofiltration (NF)Concentration of organic components by removal of part of monovalent ions like sodium and chlorine (partial demineralisation)

•Ultrafiltration (UF)Concentration of large and macro molecules

•Microfiltration (MF)Removal of bacteria, separation of macromolecules

Membrane filtration applications


OSMOSISOSMOSIS•Pure water flows from a dilute solution through a semipermeable membrane (water permeation only) to a higher concentrated solution•Rise in volume to equilibrate the pressure (osmotic pressure)

•If pressure greater than the osmotic pressure is applied to the high concentration the direction of water flow through the membrane can be reversed.

REVERSE REVERSE OSMOSISOSMOSIS

Osmotic pressure- P required to equalize the solvent activities if pure solvent is on one side of membrane


REVERSE OSMOSISREVERSE OSMOSIS• only remove some suspended materials larger than 1

micron• the process eliminates the dissolved solids, bacteria,

viruses and other germs contained in the water • only water molecules allowed to pass via very big

pressure • almost all membranes are made polymers, cellulosic

acetate and matic polyamide types rated at 96%-99+% NaCl rejection


• extensive applications:– Prepare pure water from dilute aqueous solutions– Purify organic solvent– potable water from sea or brackish water– ultrapure water for food processing and electronic

industries– water for chemical, pulp & paper industry

REVERSE OSMOSISREVERSE OSMOSIS


MICROFILTRATIONMICROFILTRATION

Is a filtration process which removes contaminants from a fluid (liquid & gas) by passage through a microporous membrane



• a sterile filtration with pores 0.1-10.0 microns• micro-organisms cannot pass through them• operated at low pressure differences• used to filter particles.


• wide array of applications:– sterile water for pharmaceutical industry– food & beverages – chemical industry– microelectronics industry– fermentation– laboratory/analytical uses



ULTRAFILTRATIONULTRAFILTRATIONIs a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semi permeable membrane.

Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane.

This separation process is used in industry and research for purifying and concentrating macromolecular (103 - 106 Da) solutions, especially protein solutions.


ULTRAFILTRATIONULTRAFILTRATION

• has smaller pores than microfiltration membranes • driving force → pressure differential (2-10 bars to 25-30 bars)• used to separate species with pore sizes 10-1000 Å (103-0.1

microns) • Can be obtained down to a a molecular weight cutoff

(MWCO) level of 1000 Daltons (Da) and up to as high as 1 000 000 Da.

• assymmetric; the pores are small

Assymmetric - membrane in which the pore size and structure vary from one side of the membrane to the other


• wide range of applications :– oil emulsion waste treatment– treatment of whey in dairy industries– concentration of biological macromolecules– electrocoat paint recovery– concentration of textile sizing– concentration of heat sensitive proteins for food

additives

ULTRAFILTRATIONULTRAFILTRATION


NANOFILTRATIONNANOFILTRATIONRelatively recent membrane filtration process used most often with low total dissolved solids water such as surface water and fresh groundwater, with the purpose of softening (polyvalent cation removal) and removal of disinfection by-product precursors such as natural organic matter and synthetic organic matter.


NANOFILTRATIONNANOFILTRATION• Range between UF and RO membranes• the mass transfer mechanism is diffusion & separate

small molecules from the solution (assymmetric)

• cellulosic acetate and aromatic polyamide type membranes (salt rejections; 95% for divalent salts to 40% for monovalent salts)

• can typically operate at higher recoveries; conserving total water usage due to a lower concentrate stream flow rate.


• typical applications:– desalination of food, dairy and beverage products

or byproducts – partial desalination of whey, UF permeate or

retentate as required – desalination of dyes and optical brighteners – purification of spent clean-in-place (CIP) chemicals – color reduction or manipulation of food products – concentration of food, dairy and beverage products

or byproducts – fermentation byproduct concentration

NANOFILTRATIONNANOFILTRATION



SUMMARYSUMMARY


Membrane Configurations / modulesMembrane Configurations / modules

This refers to the packing of the membrane in the module so that it can be installed in the system.

Common configurations include:•Plate and frame•Tubular•Spirally wound•Hollow fine fibre


Flat membranes

• Mainly used to fabricate and used • In some cases modules are stacked together like a

multilayer sandwich or plate-and-frame filter press• Three basic structures are commonly uses for membranes:

- homogeneous (no significant variation in pore diameter from the filtering surface to the other side)

- asymmetric (has a thin layer next to the filtering surface that has very pores)

- composite (containing very small pores next to the filtering surface)


•Membrane sandwiched between membrane support plates which are arranged in stacks similar to a plate heat exchanger

•Typically polymers (e.g polyethersulfone) with polypropylene or polyolefin support

•UF (<1 to 1000 kDaMWCO)

•MF (0.1 to 0.16 um diameter)

Plate and frame design


Plate and frame membrane systems


waste water system


Spiral wound membranes

• Constructed from flat sheet membranes separated by spacer screens

• Consists of a sandwich of four sheets wrapped around a central core of a perforated collecting tube

• The four sheets consist of a i) top sheet of an open separator grid for feed channelii) a membraneiii) a porous felt backing for the permeate channeliv) another membrane

• The feed solution is fed into one end of the module and flows through the separator screens along the surface of the membranes

• The retentate is the collected in the other end of the module• The permeate spirals radially inward, eventually to be collected

through a central tube


Spiral-wound elements and assembly


spiral-wound membrane - cross section


3. Hollow-fiber membranes• The membranes are in the shape of

very-small-diameter hollow fibers• Typically, the high-pressure feed

enters the shell side at one end and leaves at the other end

• The hollow fibers are closed at one end of the tube bundles

• The permeate solution inside the fibers flows countercurrent to the shell-side flow and is collected in a chamber where the open ends of the fibers terminate

• Then the permeate exits the device

Hollow-fiber separator assembly.


Comparison of membrane module


Comparison of membrane module


1. Osmotic pressure of solutions• Experimental data shows that the osmotic pressure π of a solution is

proportional to the concentration of the solute and temperature T• Van’t Hoff originally showed that the relationship is similar to that for

pressure of an ideal gas• For example, for dilute water solutions,

• If a solute exists as two or more ions in solution, n represents the total number of ions

• For more concentrated solutions, Eq. (1) is modified using the osmotic coefficient Φ, which is the ratio of the actual osmotic pressure π to the ideal π calculated from the equation

Flux Equation for Reverse OsmosisFlux Equation for Reverse Osmosis

(1)

n: number of kg mol of solute, Vm: volume of pure solvent water in m3 associated with n kg mol of solute, R: gas law constant 82.057 x 10-3 m3·atm/kg mol·K, T: temperature in K.


Calculate the osmotic pressure of a solution containing 0.10 g mol NaCl/1000 g H20 at 25°C.

Solution: The density of water = 997.0 kg/m3 and then, n = 2 x 0.10 x 10-3 = 2.00 x 10-4 kg mol (NaCl gives two ions). Also, the volume of the pure solvent water Vm = 1.00 kg/(997.0 kg/m3). Substituting into

Eq. (1),

This compares with the experimental value in Table 1 of 4.56 atm.

Example 1Example 1


Example 1Example 1

TABLE 1. Osmotic Pressure of Various Aqueous Solutions at 25°C


2. Diffusion-type models• For the diffusion of the solvent through the membrane,

• Nw is the solvent (water) flux in kg/s·m2; • Pw the solvent membrane permeability, kg solvent/s·m·atm; • Lm the membrane thickness, m;• Aw the solvent permeability constant, kg solvent/s·m2·atm; • ΔP = P1 - P2 (hydrostatic pressure difference with P1 pressure

exerted on feed and P2 on product solution), atm; • Δπ = π1 - π2 (osmotic pressure of feed solution - osmotic pressure of

product solution), atm.


(2)

(3)


Flux Equation for Reverse OsmosisFlux Equation for Reverse Osmosis• For the diffusion of the solute through a membrane, an approximation for the flux

of the solute is

• Ns is the solute (salt) flux in kg solute/s·m2; • Ds the diffusivity of solute in membrane, m2/s; • Ks = cm/c (distribution coefficient), concentration of solute in

membrane/concentration of solute in solution;• As is the solute permeability constant, m/s; • c1 the solute concentration in upstream or feed (concentrate) solution, kg

solute/m3; • c2 the solute concentration in downstream or product (permeate) solution, kg

solute/m3. • The distribution coefficient Ks is approximately constant over the membrane.

(4)

(5)


• Making a material balance at steady state for the solute, the solute diffusing through the membrane must equal the amount of solute is leaving in the downstream or product ( permeate) solution:

• where cw2 is the concentration of solvent in stream 2 (permeate), kg solvent/m3. If the stream 2 is dilute in solute, cw2 is approximately the density of the solvent.

• In reverse osmosis, the solute rejection R is defined as the ration concentration difference across the membrane divided by the bulk concentration on the feed or concentrate side (fraction of solute remaining in the feed stream):


(6)

(7)



• This can be related to the flux equation as follows, by first substituting Eq. (2) and (4) into (6) to eliminate Nw and Ns in Eq. (6)

• Then, solving for c2/c1 and substituting this result to Eq. (7)

(8)

(9)

where B is in atm-1


Example 2Example 2Experiments at 25°C were performed to determine the permeabilities of a cellulose acetate membrane. The laboratory test section shown in Fig. 3 has membrane area A = 2.00 x 10-3 m2. The inlet feed solution concentration of NaCl is c1 = 10.0kg NaCl/m3 solution (10.0 g NaCI/L, ρ1 = 1004 kg solution/m3). The water recovery is assumed low so that the concentration c1 in the entering feed solution flowing past the membrane and the concentration of the exit feed solution are essentially equal. The product solution contains c2 = 0.39 kg NaCl/m3 solution (ρ2 = 997 kg solution/m3) and its measured flow rate is 1.92 x 10-8 m3 solution/s. A pressure differential of 5514 kPa (54.42 atm) is used. Calculate the permeability constants of the membrane and the solute rejection R.

FIGURE 3. Process flow diagram of experimental reverse-osmosis laboratory unit.


Example 2Example 2Solution: Since c2 is very low (dilute solution), the value of c2 can

be assumed as the density of water (Table 1), or cw2 =

997 kg solvent/m3. To convert the product flow rate to water flux, Nw, using an area of 2.00 x 10-3 m2,

Substituting into Eq. (6),


To determine the osmotic pressures from Table 1, the concentrations are converted as follows. For c1, 10 kg NaCl is in 1004 kg solution/m3 (ρ1 = 1004). Then, 1004 – 10 = 994 kg H2O in 1 m3 solution. Hence, in the feed solution, where the molecular weight of NaCl = 58.45, (10.00 X 1000)/ (994 x 58.45) = 0.1721 g mol NaC1/kg H2O. From Table 1, π1 = 7.80 atm by linear interpolation. Substituting into Eq. (1), the predicted = 8.39 atm, which is higher than the experimental value. For the product solution, 997 - 0.39 = 996.6 kg H2O. Hence, (0.39 x 1000)/(996.6 x 58.45) = 0.00670 g mol NaCl/kg H2O. From Table 1, π2 = 0.32 atm. Then, Δπ = π1 - π2 =7.80 - 0.32 = 7.48 atm and ΔP = 54.42 atm.Substituting into Eq. (2),

Example 2Example 2


Solving, (Pw/Lm) = A = 2.039 x 10-4 kg solvent/s • m2• atm.

Substituting into Eq. (4),

Solving, (DsKs/Lm) = As = 3.896 x 10-7 m/s.

To calculate the solute rejection R by substituting into Eq. (7),

Also, substituting into Eq. (9) and then Eq. (8),

Example 2Example 2


Factors affecting membrane performanceFactors affecting membrane performance

Concentration polarisationDifferential solute conc between membrane surface and bulk streamReversibly affected by operation parameters

FoulingFormation of depositsIrreversibly affected by operation parameters


Concentration Polarization in Reverse- Osmosis Concentration Polarization in Reverse- Osmosis Diffusion ModelDiffusion Model

• In desalination, localized concentrations of solute build up at the point where the solvent leaves the solution and enters the membrane

• The solute accumulates in a relatively stable boundary layer next to the membrane

• Concentration polarization, β, is defined as the ratio of the salt concentration at the membrane surface to the salt concentration in the bulk feed stream c1

• Concentration polarization causes the water flux to decrease, since the osmotic pressure π1 increases as the boundary layer concentration increases.

• Also, Solute concentration increases at the boundary• Hence, often the ΔP must be increased to compensate, which results

in higher power costs


2 types membrane fouling2 types membrane foulingSurface (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


•More energy consumption•Duration of continuous operation without need for cleaning•Membrane durability•Properties and quality of concentrate•Overall economy of the membrane process

Implications of membrane fouling


Glossary

•FeedThe solution to be concentrated or fractionated

•FluxThe rate of extraction of permeate measured in litres per square meter of membrane surface per hour (L/m2/h)

•Membrane foulingDeposition of solids on the membrane, irreversible during processing


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)•Average pressure at the inlet and outlet of the equipment


The End

Documents

ERT 313 : BIOSEPARATION ENGINEERING “Membrane Separation Process” By; Mrs Haf iza Bint i Shu kor ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011)