Membrane separation techniques

1. Dialysis and electrodialysis2. Reverse osmosis3. Gas permeation4. pervaporation

Dialysis

(wash)

DialysisFeed: Liquid at P1, containing solvent, solutes of type A, solutes of

type B, and/or insoluble colloids.Wash: solvent at P2 .

Product: liquid diffusate (permeate) containing solvent, solute A, small amounts of solute B

Retentate: dialysate solvent containing remaining A,B and retained colloidal matter.

Membrane: thin, microporous. Size of pores allowing solutes of type A to pass, for larger solutes of type B reduced or no passage.

Example: Recovery of H2SO4 from an aqueous stream containing sulfates.

streams in streams out feed wash dialysate diffusate

Flow rate, gph 400 400 420 380

H2SO4, g/L 350 0 125 235

CuSO4, g/L as Cu 30 0 26 2

NiSO4, g/L as Ni 45 0 43 0

Commercial applications of dialysis

1. Recovery of sodiumhydroxide from waste streams,2. Recovery of chromic, hydrochloric and hydrofluoric acids from

contaminating metal ions,3. Recovery of sulfuric acid from aqueous solutions containing

NiSO4,

4. Removal of alcohol from beer to produce low-alcohol beer,5. Recovery of nitric acid and hydrofluoric acid from spent

stainless-steel pickle liquor.6. Removal of mineral acids from organic compounds,7. Removal of low-molecular-weight contaminants from polymers,8. Purification of pharmaceuticals.

Of great importance is hemodialysis: Urea, creatine, uric acid, phosphates and chlorides are removed from blood without removing essential higher-molecular-weight compounds and blood cells (artificial kidney).

Estimation of membrane areaAt a differential location in a dialyzer, the rate of mass transfer of

solute across the dialysis membrane is:

dni = Ki (ciF-ciP) dAM

Ki : overall mass transfer coefficient.

In terms of individual coefficients Ki is given by:

(1/Ki) = (1/kiF) + (lM/PMi) + (1/kiP)

kiF , kiP : mass-transfer coefficients for the feed side and permeate-side boundary layers.

The necessary membrane area can be found by integrating the above equation.

Electrodialysis

• It is an electrolytic process for separating an aqueous electrolyte feed solution into a concentrate (brine) and a dilute or desalted water (diluate) by means of an electric field and ion-selective membranes.

• Ion-selective membranes are of two types, cation selective and anion selective, arranged in an alternating-series pattern. Both types do not allow water to pass.

• A direct current voltage is applied across the anode and cathode which are made of chemically neutral materials. Electrons are metalically conducted through wiring from anode to cathode then through the cell by ionic conduction from the cathode back to the anode.

• The acidic electrode rinse solution that circulates through compartments 1 and 5 neutrilizes the remaining OH- ions and prevents precipitation of compounds such as CaCO3 and Mg(OH)2

Electrodialysis; C, cation transfer membrane; A, anion transfer membrane

Reactions:

At the cathode:Reduction of water: 2H2O + 2e- 2OH- + H2(g)

At the anode:Oxidation of water: H2O 2e- + ½ O2 (g) + 2H+

If Cl- are present: 2 Cl- 2e- + Cl2 (g)

• In commercial electrodialysis systems, 100-600 cell pairs are used.

• Typically, 50-90% brackish water is converted to potable water. Main application is desalination of water for salt conc. of 500-5000ppm. Below this range, ion exchange, above this range reverse osmosis is more economical. There are some other commercial applications similar to applications of dialysis.

Estimation of the required membrane area

Current density rather than permeability is used.Applying Faraday’s Law:

AM = zFQc / iAM = total area of cell pairs, m2

z = electrochemical valance of the ions being transported through the membranes,

F = Faraday’s Constant (96520 amp-s/equivalent)Q = Volumetric flow rate of the diluate (potable water), m3/sc = difference between feed and diluate ion concentration in

equivalents/m3

i = current density, amps/m2 of a cell pair = current efficiency 1.00

Power consumption is given by; P = I E , where P = power, wattsI = electric current flow through the stack, ampsE = voltage across the stack, volt

Osmosis and reverse osmosis

initial condition at equilibrium after osmosis

reverse osmosis

Reverse Osmosis• Separation technique used to partially remove a solvent from a

solute-solvent mixture applying a pressure gradient.

• The feed is a liquid at high pressure P1, containing solvent (water), solubles (inorganic salts) and colloidal matter. No sweep liquid is used, permeate side of membrane kept at a much lower pressure, P2.

• A dense membrane is used that is permselective for the solvent. To withstand large pressure difference, thick, asymmetric, composite membranes are utilized.

• Products of RO are a permeate of almost pure solvent and a retentate of solvent-depleted feed.

• Main application of RO is for desalination and purification of seawater, brackish water and wastewater. Over 750 million gallons/day drinkable water is is produced using RO.

• In a typical RO for desalination; salt content is 3.5%w, is 350psi, feed pressure is 800-1000psi, transmembrane water flux is 0.365m3/m2-day. 45% of feed water with 99.95% purity is obtained.

Reverse osmosis

• A typical cylindirical module is 20cm in diameter by 100cm long, contains 33.9 m2 of membrane surface.

• Other uses of RO in industry are:1. Treatment of industrial wastewater to remove heavy metal

ions, nonbiogradable substances and other components of commercial value,

2. Treatment of rinse water from electroplating processes to obtain a metal ion concentrate and a permeate that can be used as rinse,

3. Separation of sulfites and bisulfits from effluents in pulp and paper processes,

4. Treatment of wastewater in dyeing processes,5. Recovery of constituents having food value from wastewaters

in food processing plants (lactose, lactic acid, sugars, starches, proteins).

6. Treatment of municipal water to remove inorganic salts, low-molecular-weight organic compounds, viruses and bacteria.

7. Concentration of certain food products, coffee, tea, soups, milk, fruit juices and tomato juice,

8. Concentration of amino acids, alkaloids, enzymes and like.

Estimation of solvent flux

When RO takes place with solute on each side of the membrane, at equilibrium:

( P1 - 1 ) = ( P2 - 2 )

The driving force for the transport of solvent through the membrane is: (P - ) and the rate of mass transport is:

NH2O = ( PMH2O / lM ) (P - )

P = hydraulic pressure difference across the membrane (PF – PP),

= osmotic pressure difference across the membrane, (P - F), often P 0, since pure solvent,

The flux of solute (salt) through the membrane is given by:

Ni = ( Di / lM ) ( c’io – c’iL )

Di is the diffusivity of solute in the membrane, c’io and c’iL are feed side and permeate side solute concentrations in the membrane. The flux of solute is independent on pressure. The higher P, the purer the permeate water.

For RO of seawater and of solutions with low salt content, the osmotic pressure can be estimated by:

= 1.12 T mi

= osmotic pressure, psi T = temperature, K

mi = summation of molalities of all dissolved ions and nonionic species in the solution, mol/liter

Concentration polarization

• A phenomenon, called CP is particularly important on the feed side of the RO-membrane. CP is the buildup or depletion of species in the boundary layer due to mass-transfer resistance.

• For concentrations for water and salt, cW , cS ; (cSl – cSF) causes mass transfer of salt by diffusion from the membrane surface back to the bulk feed. The lower the mass-transfer coefficient, the higher cSl. The value of cSl fixes the osmotic pressure. A salt balance at the upstream membrane surface gives:

NH2O cSF = kS (cSl – cSF)

cSl = cSF ( 1 + NH2O / kS )

CP effect is seen to be most significant for high water fluxes and low mass-transfer coefficients.

Concentration polarization effects in reverse osmosis

cWP

cWFcWl

cSl

cSF

cSMcSP

Reverse osmosis process for production of potable water

PF PC

PC 85-90% PF

abt. 50% of feed,purity = 99.95%w

Gas permeation

• Gaseous feed at high pressure, contains some low-MW species (MW50), to be separated from small amounts of higher-MW species.

• Permeate side of the membrane kept at a much lower pressure.

• Membrane used is often dense, sometimes microporous, permselective for certain low-MW species in the feed gas. Permselectivity depends on both membrane absorption and membrane transport rate.

• Products are a permeate enriched in A-type species and a retentate enriched in B-type species.

• Transport through the membrane modeled by solution-diffusion model.

• If membrane microporous, pore size extremely important to block passage of B-type species.

• Since 1980, applications of GP with dense polymeric membranes have increased dramatically.

Industrial Applications of GP:

1. Separation of H2 from CH4, (H2 permeation rate through dense membrane is very high)

2. Adjustment of H2-to-CO ratio in synthesis gas,

3. O2 enrichment of air,

4. Removal of CO2

5. Drying of natural gas and air,

6. Removal of He and organic solvents from air.

• At low temperature processes GP is preferred to absorption and pressure-swing adsorption separation tecniques due to low capital investment, ease of installation and operation, absence of rotationg parts, high flexibility, low weight and less space requirements.

• Available GP membranes for bulk separation of air achieve separation factors of 3-7 for O2-N2. Product purities are 95-99% N2 retentate, and 30-45% O2 permeate.

• In GP usually spiral wound or hollow fiber modules are used because of their higher packing density. Typical feed side pressures are 300-500 psia, may be as high as 1650 psia.

Pervaporation

• The term pervaporation is a combination of terms permselective and evaporation.

• Different phases on two sides of the membrane. Feed is a liquid mixture at a pressure high enough to maintain a liquid phase as the feed is depleted of species A and B to produce vapor phase permeate.

• A composite membrane is used that is selective for species A, selectivity for B is a lot less. The retentate is enriched in species B.

• Permeate pressure is maintained below the dew point of the permeate to rocover it as a vapor, often under vacuum. The vapor permeate is enriched in species A.

• Overall permeabilities of species A and B depend upon their solubilities in and diffusion rates through the membrane.

• Major commercial applications of PV are: 1. Dehydration of ethanol, 2. Dehydration of other organic alcohols, ketones and esters, 3. Removal of organics from water. Separation of organic mixtures by PV is receiving much

attention

• PV is best applied when the feed solution is dilute in the main permaent since the latent heat of vaporization of the permaent is provided by the sensible heat of the feed.

Pervaporation integrated with distillation for removal of water from ethanol

60%w ethanol

95%w ethanol99,5 %w ethanol

25%w ethanol

Ethanol-water separability for polyvinylalcohol membrane at 60oC and 15 torr (0.02atm.) vacuum.

Enthalpy balance in PV

A PV module typically operates adiabatically. The enthalpy of vaporization is supplied by sensible enthalpy of the feed. For PV of a binary liquid mixture of components A and B; assuming:

1. constant pure component liquid specific heats, 2. neglicible heat of mixing, an enthalpy balance in terms of mass flow rates m, liquid sensible

heats CP, and heats of vaporization H gives:

(mAFCpA + mBFCpB) (TF-TR) = (mAPCpA + mBPCpB) (TP-TR)

+ (mAP HA + mBP HB ) TP = temperature of the permeate = permeate dew point at

permeate vacuum,

Estimation of flux• Transport of a permeant through a membrane by PV is different

from dialysis or gas permeation since there is a phase change and there are nonideal solution effects in the liquid feed. The driving force for permeation is expressed in terms of partial vapor pressure difference.

• Since pressures on both sides of the membrane are low, the gas phase obeys the ideal gas law.

At the upstream membrane surface (1), permeant activity for i is:

ai(1) = pi

(1) / PiS(1) Pi

S(1) = vapor pressure of i at TF

The liquid on the upstream side is generally nonideal:

ai(1) = i

(1)xi(1)

Combining: pi(1) = i

(1)xi(1)Pi

S(1)

At the downstream vapor side (2), the partial pressure is:

pi(2) = yi

(2) PP(2) PP = total permeate pressure

The driving force will be: i(1)xi

(1)PiS(1) - yi

(2) PP(2)

The permeant flux: Ni = (PMi/lM) i(1)xi

(1)PiS(1) - yi

(2) PP(2)

In PV, the permeability PMi depends on permeant, on the polymer, on the temperature but also on the concentrations of permeants in the polymer, which can be large enough to cause polymer swelling.