Membrane Separation Process

8/17/2019 Membrane Separation Process

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Membrane Separation Processes

By

Dr. Nadeem Feroze

DEPARTMENT OF CHEMICAL ENGINEERING


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Introduction

• Effective product separation is very crucial to

economic operation in the process industry.

Some materials are very much difficult to separate

examples are

• Dispersed solids ( compressible or density close to

liquid phase)

•

Low molecular weight, non-volatile organics,pharmaceuticles, dissolved salts

• Biological materials


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Introduction

• The processing of these categories of materials hasbecome important

• Newer biotechnological industries

• Sophisticated nature of processing in food industries

• Nature has the answer

•

Highly effective

• Energy efficient though slow in process terms

• Nature separates biologically active materials by meansof membranes


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• Membranes are defined as ‘‘ an interphaseseparating two phases and selectively

controlling the transport of materials between

those phases‘‘

• A membrane is an interphase rather than

interface because it occupies it a finite,

though normally small, element of space.


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• Human beings are surrounded by membranes

Skin

Kidney

Liver

• Membranes control the separation ofmaterials at all levels of life, down to the outer

layers of bacteria and subcellular components.


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• Synthetic membranes have been developed

for conventially difficult separations.

• It offers advantage of ambient temperature

operation, relatively low capital and runningcosts, modular construction.


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Classification of Membrane Processes

• Industrial membrane processes may be classified according to the

size range of materials which they are to separate and the driving

force used in separation.

• Discussion will be concerned with pressure driven processes

Microfiltration

Ultrafiltration

Nanofiltration

Reverse Osmosis

• These are already well established large scale industrial processes.

• RO is a widely used separation process.


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Classification of Membrane Separation Processes for

Liquid Systems


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Nature of Synthetic Membranes

• Membranes used for pressure driven separationprocesses are most commonly made of polymericmaterials.

• Initially most membranes were cellulosic in naturebut are now replaced by advance polymers such aspolyamide, polysulphone, polycarbonate.

• These synthetic polymers have better resistance tomicrobial degradation and improved Chemicalstability


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Membrane development process

a) Polymer is dissolved in a solvent 10-30 percentby mass

b) Resulting solution is cast on a suitable support

as a film of thickness 100 micrometer

c) Film is quenched by immersion in a non-solventbath, typically water or non aqueous solution

d) Resulting membrane is annealed by heating

This process is known as immersion precipitation


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Contd ….

• The ultimate membrane structure results as acombination of phase separation and masstransfer, variation of the production

conditions giving membranes with differentseparation characteristics.

•

Most microfiltration membranes have asymmetric pore structure, and they can have aporosity as high as 80 per cent.


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• Ultrafiltration and reverse osmosis membranes have an

asymmetric structure comprising a 1 –2 μm thick top layer offinest pore size supported by a ∼100 μm thick more openly

porous matrix, as shown in Figure.

• Such an asymmetric structure is essential if reasonable

membrane permeation rates are to be obtained


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• Another important type of polymeric membrane is

the Thin-film Composite membrane.

• This consists of an extremely thin layer, typically

∼1 μm, of finest pore structure deposited on amore openly porous matrix. The thin layer is

formed by phase inversion or interfacialpolymerization on to an existing microporous

structure.


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Inorganic Membranes

• Recent advancement has been the development of

Microfiltration and Ultrafiltration membranes

composed of inorganic oxide materials presently

produced by two main techniques

1. Depositon of collodial metal oxide on to a

supporting material such as carbon

2. Puerly ceramic materials by high temperature

sintering of spray dried oxide microspheres.


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Advantages of inorganic membranes

• Higher temperature stability

• Steam sterilisation

• Increased resistance o fouling

• Narrow pore size distribution


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• A parameter often quoted in manufacturer’s literature isthe nominal molecular weight cut-off (MWCO) of amembrane.

• This is based on studies of how solute molecules arerejected by membranes.

• A solute will pass through a membrane if it is sufficientlysmall to pass through a pore, if it does not significantlyinteract with the membrane and if it does not interactwith other, larger solutes


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• It is possible to define a solute rejection

coefficient R by:

R = 1 - (Cp/Cf )

• Cf = concentration of solute in the feed stream• Cp = concentration of solute in permeate


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General Membrane Equation

• The general membrane equation is an attempt to state the

factors which may be important in determining the

membrane permeation rate for pressure driven processes

Where

J is the membrane flux∗, expressed as volumetric rate per unit area

|P| is the pressure difference applied across the membrane.

is the difference in osmotic pressure across the membrane

Rm is the resistance of the membrane

Rc is the resistance of layers deposited on the membrane, the filter cake and gel foulants.


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Membrane Modules

• In industry the equipment for MF, UF, and RO is

applied in form of modules

•

Membrane area is in the range of 1-20 m2

• Modules can be connected in parallel or in series

• Commonly used modules are

Tubular

Flat sheet

Spiral wound

Hollow fibre


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Tubular modules

• Used in turbulent flow regime

• Concentration of high solids content feed

• Membrane is cast on the inside of a porous support whichis often housed in a perforated stainless steel pipe

• Individual modules contain a cluster of tubes in series heldwith in a stainless steel permeate

• Diameter in 10-25 mm , 1-6 m length

• Feed is pumped through the tubes with Reynold Numbergreater than 10,000

• Can be easily cleaned

• Main disadvantages are the relatively low membranesurface area and higher volumeric hold up


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Flat sheet modules

• Similar to conventional filterpresses

• Consists of annular membranediscs of outer diameter 0.3 m

• Suitable for laminar flow• A single module contains 19 m2

of membrane area

• Permeate is collected from eachmembrane so that damaged

membranes can be easilyidentified

• Replacement of membranerequires dismantling of thewhole stack


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Spiral-wound modules

• Consist of several flat membranesseparated by turbulence-promotingmesh separators and formed into aSwiss roll

• This produces a cylindrical modulewhich can be installed within a

pressure tube.

• Feed enters at one end of thepressure tube and encounters anumber of narrow, parallel feedchannels formed between adjacentsheets of membrane.

• Permeate spirals towards theperforated central tube forcollection.

• Up to six such modules may beinstalled in series in a single pressuretube.


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Hollow-fibre modules

• Consist of bundles of finefibres, 0.1 –2.0 mm indiameter, sealed in a tube.

• For desalination

applications, feed flow isusually around the outsideof the unsupported fibreswith permeation radiallyinward.

• Capable of high pressureoperation

• Readily fouled and difficultto clean


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Plant Configuration

• Membrane modules can be configured in various

ways to produce a plant of the required separation

capability.

• A simple batch recirculation system arrangement is

most suitable for small-scale batch operation.

• Larger scale plants will operate either as feed and

bleed or continuous single-pass operations


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Feed and bleed

• Retentate is initially totally recycled

• When the required solute concentration is reached within the loop, a fraction ofthe loop is continuously bled off.

•The main advantage is that the final concentration is then continuously available asfeed is pumped into the loop.

• The main disadvantage is that flux is lower than the average flux in the batch mode,with a correspondingly higher membrane area requirement.


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Feed and bleed

• Large-scale plants usually use multiple stages operated in series to overcome thelow-flux disadvantage of the feed and bleed operation.

• Only the final stage is operating at the highest concentration and lowest flux,while the other stages are operating at lower concentrations with higher flux.

• Thus, the total membrane area is less than that required for a single-stageoperation.


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Continuous single pass

• Concentration of the feed stream increases gradually along the length ofseveral stages of membrane modules arranged in series.

• The feed only reaches its final concentration at the last stage.

• There is no recycle and the system has a low residence time.


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Reverse Osmosis


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Introduction

• When miscible solutions of different concentration are

separated by a membrane that is permeable to the

solvent but nearly impermeable to the solute, diffusion

of the solvent occurs from less concentrated to the more

concentrated solution, where solvent activity is lower.

• The diffusion of solvent is called Osmosis.

• Osmotic transfer of water occurs in many plant and

animal cells


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Introduction

• The transfer of solvent can be stopped by increasing the

pressure of the concentrated solution until the activity of

solvent is same on both sides of the membrane.

• If pure solvent is on one side of the membrane, the

pressure required to equalize solvent activities is the

osmotic pressure of the solution.

• If pressure higher than the osmotic pressure is applied,

solvent will diffuse from concentrated solution to dilute

solution. This Phenomenon is called Reverse Osmosis


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Osmosis and Reverse Osmosis Illustration


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Basic Terminology

•Feed water: Supply water that is fed into the RO system to betreated

• Permeate: A portion of the feed water that passes through a seriesof membranes and is returned as purified water.

• Concentrate: A portion of the feed water that is rejected by themembrane and contains the solution of impurities that have beenfiltered out of the permeate.

• Water flux: The rate of permeate production typically expressed asthe rate of water flow per unit area of membrane (e.g., gallons persquare foot per day)

• Recovery rate: The ratio of permeate flow to feed water flow, whichindicates the overall water efficiency of the system


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Basic Information


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RO Membrane Properties

• More than 50% of RO modules use cellulose acetate

membrane.

• It has high permeability for water and low permeability for

dissolved salts.

• Its limitation are:I. Smaller allowable pH range of 4.5-7.5 (beyond this range, cellulose

acetate becomes prone to hydrolysis)

II. Susceptibility to biological attack (degradation due to growth of

microbes)

III. Reduction of solvent flux because of compaction or mechanical

compression of membrane at high pressure difference


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RO Membrane Properties

• Another common membrane materials in RO systemsare thin film composite (TFC) membranes.

• TFC membranes are not chlorine-tolerant but can

tolerate harsh chemical environments and wide ranges inwater temperature and pH, and are less vulnerable tocompaction than CA membranes.

• TFC membranes generally have higher water flux than CAmembranes because the layers are extremely thin, whichcreates more water transport through the membranematerial.


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Membrane Configuration

• Spiral Wound

• Plate and Frame

• Hollow fibre

• Tubular


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Mechanism

• The mechanism of water and salt transport in

reverse osmosis is not completely understood.

• One theory is that water and solutes diffuseseparately through the polymer by a solution-

diffusion mechanism.

• In this mechanism concentration of water in dense

polymer is assumed to be proportional to the activity

of water in the solution


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Mechanism

• On the low-pressure side of the dense layer, activity isessentially unity if nearly pure water is produced at 1 atm.

• On the high-pressure side, activity would be slightly less than 1.0 at atmospheric pressure

1.0 at atmospheric pressure

slightly greater than 1.0 at higher pressures

• The upstream pressure is generally set at 20 to 50 atm abovethe osmotic pressure of the feed solution.

• At these pressures, activity of water “aW“ is only a few percentgreater than for pure water at 1 atm and change in activityand concentration across the membrane are small


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• The flux of solute is assumed proportional to the

difference in solution concentration, the diffusivity

and a solubility or distribution coefficient.


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Concentration Polarization

• Nearly complete rejection of solute by the membrane

leads to a higher concentration at the membrane surface

than in bulk solution and this effect is called

Concentration polarization.

• Concentration polarization reduces the flux of water

because the increase in osmotic pressure reduces the

driving force for water transport.

• The solute rejection decreases both because of the lower

water flux and the greater salt concentration at the

surface increases the flux of solute


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Factors that impact on RO performance

Basic effects of:

• Temperature

• Pressure

• Recovery rate


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Temperature Effects

• RO permeate flow is strongly dependent on the

temperature of the feed water.

• The higher the temperature the higher the permeate

flow rate.

• Why? Lower viscosity makes it easier for the water to

permeate through the membrane barrier

• RULE OF THUMB – for every 1˚C the permeate flowwill increase ~ 3%


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Temperature Variation on Salt flux

• Solute rejection declines with a temperature rise because

of the osmotic pressure increase with temperature.

•

Increasing temperature increases salt passage more thanwater passage

• Generally you will get better rejections at lower

temperatures

• RULE OF THUMB – salt flux increases 6% for 1˚C increase


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Pressure Effects

• Water passage increases with pressure. Soluterejection rises with pressure, since solvent fluxincreases and solute diffusion does not.

• Higher flow of water through the membrane will tendto promote more rapid fouling, the single greatestcause of membrane failure.

• Membrane element manufacturers usually providelimits with regard to the maximum applied pressure tobe used, as a function of feed water quality.


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Recovery rate Effects

• As recovery is increased, concentration of solute in theconcentrate stream increases, resulting in increasedosmotic pressure which must be overcome.

• Membrane flux declines with increasing soluteconcentration at high recovery rate.

• Large systems typically have recovery rates between40% and 60%. In other words, for every 10 gallons offeed water entering the system, 4 to 6 gallons ofpurified permeate water are produced


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Steps to design RO membrane system

1. System Design Information and Feed Water

2. Selection of Element Type and Average Permeate

Flux

3. Calculation of Number of Total RO Elements4. Decision of Recovery Rate

5. Decision of Number of Stages

6. Decision of Number of RO Elements per PressureVessel

7. Decision of Element Arrangement


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1. System Design Information and Feed Water

• The RO membrane system highly depends on the availablefeed water.

• Therefore, the system design information (required product flow rate, expected recovery rate, annual water temperature,water source, application, pretreatment, required product waterquality, operating pressure limit, etc.) and the feed wateranalysis should be thoroughly studied and considered inselection of the RO system design.

• If the required permeate water quality is so high that thequality cannot be achieved by 1pass RO system, and then a2 pass RO system should be considered


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2. Selection of Element Type and Average

Permeate Flux

• According to the feed water source, pretreatment and feed watersalinity, the type of RO membrane element is selected.

• Consider the following characteristics while selecting particular

membrane for an application:

Membrane fouling rates

Water flux specification

Solid rejection rate

System pressure requirements Membrane response to cleaning operations and tolerance of

cleaning procedures

Tolerance of pH

Temperature range

Chemical abrasion resistance


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Contd …

• Once the water source, pretreatment and RO

element type are fixed by the designer, the

recommended value of the average permeate flux

(also called “design flux”) is given.

• In some cases, the design flux value is determined by

pilot experiment data or customer’s experience.


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3. Calculation of Number of Total RO Elements

• The relationship between the number of total elements, the product

flow rate and the average permeate flux is expressed as follow equation:

In which:

NE = total element numbers

Q p = product flow rate

JV, ave = average permeate flux

(MA)E = membrane area of element

• The calculated number of RO elements may be a slightly changed based on the

decision of element arrangement, that is, the number of pressure vessel and RO

elements per pressure vessel.


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4. Decision of Recovery Rate

• In an RO membrane system, a recovery rate as high as

possible is desirable, but a high recovery rate can also cause

some problems as follows

Possibility of scale formation increase because of the increase

of concentration factor

Osmotic pressure increase because of the increase of

concentration factor

Concentrate flow rate decrease

Permeate water quality deterioration because of average feed

concentration increase


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Contd …

• The relationship between recovery rate and concentration

factor is shown in Table

• Usually in brackish water desalination, the recovery rate is

decided by scale formation, and in seawater desalination, by

feed pressure limit.


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5. Decision of Number of Stages

• The number of RO stages defines how

many pressure vessels are in series in

the RO membrane system.

• Every stage consists of a certainnumber of pressure vessels in parallel

• The number of stages is a function of

the system recovery rate, the number

of elements per vessel, and the feed

water quality as shown

6 D i i f N b f RO El


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6. Decision of Number of RO Elements per

Pressure Vessel

• RO membrane elements can be coupled together in seriesin the pressure vessel, typically 1-8 elements per onepressure vessel.

• In deciding the number of RO elements per pressure vessel,plant size is usually considered first.

• In a large-scale plant (> 40 m3/h), 6-8 elements perpressure vessel are usually adopted, and in a smaller plant,3- 5 elements per pressure vessel.

• In all cases, the space required to install or remove the ROelements should be considered in the plant design.


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7. Decision of Element Arrangement

• The RO element arrangement (array) means elementnumbers per vessel, vessel numbers per stage and stagenumbers per pass.

• For the decision of element arrangement, the systemdesign parameters should be consistent with the designflux guideline.

•

In the case that not every parameter is in accordance withthe design guideline, it is necessary to make a priority inthe parameters. Usually average permeate flux,concentrate flow rate and pressure drop per vessel shouldbe of higher priority.


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RO System Components

RO systems consist of the following basic components, which are

common to every RO system. The specifications for each

component vary by application, source water quality, and the

required permeate quality.

• Pre-filter(s): It is common for pre-filters to pretreat the feed

water supply before it enters an RO system. Multiple pre-

filters may be used in an RO system. The most commonly used

pre-filters are sediment filters used to remove sand, silt, dirt,and other sediment. Carbon filters also may be used to

remove chlorine and organic compounds


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RO System Components

• Valves: Valves are required to control the flows and pressuresof an RO system for the system to operate correctly andoptimally. There are generally two valves in an RO unit on thefeed water piping and on the concentrate piping.

• Storage tank: Permeate is stored in tanks. Industrial andcommercial storage tanks may hold up to 9000 gallons ofwater.

• Drain line: This line runs from the outlet end of the ROmembrane housing to the drain. This line is used to dispose ofthe concentrate rejected by the membrane element

Diagram of an RO System with Basic


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Diagram of an RO System with Basic

Components


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REVERSE OSMOSIS WATER TREATMENT PLANT

• The largest scale applications of membrane separation processesare those which form the key step in the desalination, or moregenerally demineralization, of brackish water in the production ofdrinking water.

• An outline is given of such a plant capable of producing 70,000m3/day of drinking water for a large city in the Middle East

• The water to be processed is obtained from a deep well with a totaldissolved solids (TDS) content of 1.4 kg/m3; that is, it is of moderatesalinity and hardness.

• The plant specification required that the product water should havea maximum TDS of 0.5 kg/m3

Flow diagram of reverse osmosis plant for


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Flow diagram of reverse osmosis plant for

demineralization of Brackish water


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Main Process Steps

Pretreatment

a. Evaporative cooling to reduce the feed water temperature from50 –55◦C to 30 –35◦C which is more compatible with satisfactoryoperation of the reverse osmosis unit and more suitable for final

use.

b. Precipitation softening by addition of slaked lime (Ca(OH)2) andsodium aluminate or ferric chloride. The net result is part-removalof calcium, silica and especially colloids. The clarifiers used ensurecompletion of these processes within the tank.

c. Acidification to optimise removal of residual coagulant.

d. Prechlorination to ensure a disinfected supply to the reverseosmosis plant


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Pretreatment

e. Rapid gravity filtration to reduce further the content of particulate

material.

f. Acidification to reduce the pH to 5.0 for optimum life of the

reverse osmosis membrane.

g. Sequestering, addition of sodium hexametaphosphate to retard

the precipitation of calcium sulphate which otherwise will exceed

its solubility limit in the reject stream.

h. Cartridge filtration with elements rated at 25 μm to protect thehigh pressure pumps and reverse osmosis membranes in the

event of a break-through of particulate material.

Demineralization by reverse osmosis


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• Reverse osmosis was chosen for the demineralization step as

it gave an economic solution in terms of both capital andrunning costs.

• It allowed a high water recovery rate, was modular in

construction and so could be easily extended, could cope withreasonable variations in feed salinity.

• The pre-booster pumps, cartridge filters and high pressurepumps are arranged in seven parallel streams, one of which ison standby

• A total of thirteen reverse osmosis stacks is installed, anytwelve of which will meet the required throughput


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• Each stack contains 210 reverse osmosis modulesaccommodated in 35 pressure vessels arranged in aseries –parallel array of 20 –10 –5 to achieve the desiredwater recovery

• These modules are of the spirally wound type.

• The permeate from the unit is blended with theslipstream flow, with pH adjustment if necessary, to

maintain a final water TDS < 0.5 kg/m3.

• The reject is discharged to evaporation ponds.


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Separation of Gases


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Porous Membranes

• When a gas mixture is allowed to diffuse through aporous membrane to a region of lower pressure, gaspermeating the membrane is enriched in low molecular-weight components since they diffuse more rapidly.

• When the pores are much smaller than mean free path inthe has phase, the gases diffuse independently byKnudsen Diffusion.

•Diffusivity in the pore is proportional to the pore size andaverage molecular velocity, which varies inversely withthe square root of molecular weight.


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Contd …

• The flux per unit membrane area depends on aneffective diffusivity De that is lower than the pore

diffusivity by the factor ( ԑ/τ ) where ԑ is the porosityand τ the tortuosity.

• For membranes with about 50% porosity, this factor

is generally 0.2 to 0.3

• The flux of each gas is proportional to the concentration


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• The flux of each gas is proportional to the concentration

gradient, which is linear if the membrane structure is

uniform and the gases do not interact.

• Usually the gradient is expressed as a partial pressure

gradient and ideal gases are assumed

• The composition of the permeate depends on the fluxes

of all species. For a binary system, mole fraction of A in

the permeate is


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Typical Pressure gradients in a porous membrane


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Polymer Membranes

• The transport of gases through dense (non-porous) polymer

membranes occurs by a solution-diffusion mechanism.

•The gas dissolve in the polymer at the higher pressure side ofmembranes, diffuses through the polymer phase, and desorbs

or evaporates at the low pressure side.

•The rate of the mass transfer depends on the concentrationgradient in the membrane, which is proportional to the partial

pressure gradient if solubility is proportional to the pressure.


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Gradients in a dense Polymer Membrane

• Henry’s law is assumed toapply for each gas and

equilibrium is assumed at

the interface.

• The gas-film resistances

are neglected for this

case, so the partialpressures at the gas-

polymer interface are

same as those in the bulk


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• The flux for gas A is

• The concentrations are related to the partial pressures by a

solubility coefficient S, which has units such as (mol /cm3.atm)

• Replace the concentration gradient with a pressure gradient

gives

The product DASA is the flux per unit pressure gradient, which is called

permeability coefficient ‘ q A’ and often expressed in Barrers.

• Since the actual membrane thickness is not always known or


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• Since the actual membrane thickness is not always known orspecified for commercial membranes, it is customary to usethe flux per unit pressure difference, which will be called the

permeability (Q A )

•The ratio of permeabilities for a binary mixture is themembrane selectivity α (also called the ideal separation

factor)

• A high selectivity can be obtained from either a favorable diffusivity ratioor large difference in solubilities.

• The diffusivities in the membrane depend more strongly on the size andshape of molecules than do gas phase diffusivities


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• Permeabilities reported for light gases in a few polymers are

listed in table

•

The permeabilities are much greater for rubber and siliconethan for polysulfone and other glassy polymers.

• The permeability goes through a minimum with increasing

molecular weight or size of the gas molecule.


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Pervaporation

I t d ti


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Introduction

• Pervaporation is a separation process in which one ormore components of a liquid mixture diffuse through

a selective membrane, evaporate under low pressure

on downstream side and are removed by a vacuum

pump or a chilled condenser.

• The process differs from other membrane processes

in that there is a phase change from liquid to vaporin the permeate.

I t d ti


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Introduction

• The driving force in the membrane is achieved bylowering the activity of the permeating components atthe permeate side.

• Components in the mixture permeate through themembrane and evaporate as a result of the partialpressure on the permeate side being held lower than thesaturation vapor pressures.

• The driving force is controlled by applying a vacuum onthe permeate side

S h i Di f P i i


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Schematic Diagram of Pervaporation unit


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• Composite membranes are used with dense layer in

contact with the liquid and the porous supporting layer

exposed to the vapor.

• Pervaporation is favored when the feed solution is dilute

in the main permeant because sensible heat of the feedmixture provides the permeant enthalpy of vaporization

• The phase change occurs in the membrane and the heat

of vaporization is supplied by the sensible heat of theliquid conducted through the thin dense layer.


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A li ti


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Applications

• Dehydration of ethanol or the production of high purityethanol by a hybrid process which also incorporates

distillation.

• Such separations use cellulose-acetate-based composite-membranes, with an active layer of polyvinyl alcohol.

• Membranes used for ethanol purification are also suitable for

dehydration of many other organic solvents includingmethanol, isopropanol, butanol, MEK, acetone and

chlorinated solvents.

A li ti


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Applications

• Removal of volatile organic contaminants from waterusing silicone rubber or organophilic polymers for themembrane.

• The separation of close-boiling organic mixtures likebenzene –cyclohexane is receiving much attention.

• Separating benzene from cyclohexane consisting of acellulose acetate support matrix and incorporatingpolyphosphonates to improve the preferentialpermeability of benzene.

C i f th l t biliti


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Comparison of ethanol – water separabilities


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Pervaporation Processes

H b id f l f t f th l


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Hybrid process for removal of water from ethanol

Dehydration of dichloroethylene


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Dehydration of dichloroethylene.

Removal of volatile organic compounds (VOCs) from


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f g p ( ) f

wastewater

Membrane Selection and Modules


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Membrane Selection and Modules

• Membrane selection is critical in the commercialapplication of PV.

• For water permeation, Hydrophilic membrane materialsare preferred.

• For example, a three-layer composite membrane is used forthe dehydration of ethanol, with water being the mainpermeating species.

• The support layer is porous polyester, which is cast on amicroporous polyacrylonitrile or polysulfone membrane.The final layer, which provides the separation, is dense PVAof 0.1 mm in thickness.


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• Hydrophobic membranes, such as silicone rubber and Teflon,

are preferred when organics are the permeating species.

• Commercial membrane modules for PV are almost exclusively

of the Plate-and-frame type because of the ease of using

gasketing materials that are resistant to organic solvents and

the ease of providing heat exchange for vaporization and

high-temperature operation.

• Hollow-fiber modules are used for removal of VOCs from

wastewater. Because feeds are generally clean and operation

is at low pressure, membrane fouling and damage is minimal,

resulting in a useful membrane life of 2 –4 years.

Mechanism


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Mechanism

•The flux of each component is proportional to the concentrationgradient and the diffusivity in the dense layer.

• However, the concentration gradient is often non-linear becausethe membrane swells appreciably as it absorbs liquid.

• Diffusion coefficient in the fully swollen polymer may be 10 to 100times the value in unswollen polymer.

• When the polymer is swollen mainly by absorption of onecomponent, the diffusivity of other components is increased also.

• This interaction makes it difficult to develop correlations formembrane permeability and selectivity.

Mechanism


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Mechanism

•Models for transport of permeant through a membrane bypervaporation have been proposed, based on solution-diffusion.

• They assume equilibrium between the upstream liquid and

the upstream membrane surface & between thedownstream vapor and its membrane side.

• Membrane transport follows Fick’s law, with a permeantconcentration gradient as the driving force.

• However, because of phase change and non ideal-solutionfeed, simple equations do not apply.

Mechanism


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Mechanism

• A convenient PV model is that of Wijmans and Baker who express the driving force for permeation in terms ofa partial-vapor-pressure difference.

• Pressures on both sides of the membrane are low, thegas phase follows the ideal-gas law.

• Therefore, at the upstream membrane surface (1),

permeant activity for component i is

where Psi is the vapor pressure at the feed temperature


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Mechanism


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Mechanism

•If non-linear effects are neglected than the corresponding permeant flux , after dropping unnecessary superscripts, is

where

PMi is called permeability coefficient expressed in Barrers units

is termed as Permeability

• In commercial applications of PV, liquid feed usually has a low concentration ofthe more permeable species, so swelling of the membrane and resulting non-linear effects are not as pronounced as when testing solutions of highconcentration.

Gradients in a Pervaporation membrane


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Gradients in a Pervaporation membrane


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DIALYSIS

Introduction


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Introduction

• In a dialysis membrane-separation process, the feed is a liquid,at pressure P1, containing solvent, solutes of type A, and solutes

of type B and/or insoluble, but dispersed, colloidal matter.

•A sweep liquid or wash of the same solvent is fed at pressure P2to the other side of the membrane.

• The membrane is thin with micropores of a size such that solutes

of type A can pass through by a concentration driving force.

• Solutes of type B are larger in molecular size than those of type

A and pass through the membrane only with difficulty or not at

all.

Introduction


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Introduction

• Colloids do not pass through the membrane.

• With pressure P1 = P2, the solvent may also pass throughthe membrane, but by a concentration-driving force

acting in the opposite direction.

• The transport of the solvent is called osmosis.

• By elevating P1 above P2, solvent osmosis can bereduced or eliminated if the difference is higher than theosmotic pressure.

Contd


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Contd …

• The products of a dialysis unit (dialyzer) are a liquiddiffusate ( permeate) containing solvent, solutes of

type A, and little or none of type B solutes; and a

dialysate (retentate) of solvent, type B solutes,

remaining type A solutes, and colloidal matter.

• Ideally, the dialysis unit would enable a perfect

separation between solutes of type A and solutes oftype B and any colloidal matter


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Example


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Example

• When dialysis is used to recover sulfuric acid (type Asolute) from an aqueous stream containing sulfate salts

(type B solutes), the following results are obtained

• Thus, about 64% of the H2SO4 is recovered in the

diffusate, accompanied by only 6% of the CuSO4, and no

NiSO4.

Applications


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Applications

• Recovery of chromic, hydrochloric, and hydrofluoric acids fromcontaminating metal ions.

• Removal of alcohol from beer to produce a low-alcohol beer.

• Recovery of nitric and hydrofluoric acids from spent stainless steel pickleliquor.

• Removal of mineral acids from organic compounds.

• Removal of low-molecular-weight contaminants from polymers.

• Hemodialysis, in which urea, creatine, uric acid, phosphates, and chloridesare removed from blood without removing essential higher-molecularweight compounds and blood cells in a device called an artificial kidney

Membrane Types & Modules


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Membrane Types & Modules

•Typical microporous-membrane materials used in dialysis arehydrophilic, including cellulose, cellulose acetate, various acid-resistant polyvinyl copolymers, polysulfones, and

polymethylmethacrylate.

•Typically less than 50 mm thick and with pore diameters of 15to 100 Ȧ

• The most common membrane modules are plate-and-frame and hollow-fiber.

• Dialysis membranes can be thin because pressures on eitherside of the membrane are essentially equal.


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Contd…


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Contd …

• In a plate-and-frame dialyzer, the flow pattern is nearlycountercurrent.

• Because total flow rates change little and solute

concentrations are small, it is common to estimate solute

transport rate by assuming a constant overall mass-transfer coefficient with a log-mean concentration-

driving force

Rate Equation


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Rate Equation

• The differential rate of solute mass transfer acrossthe membrane is

• where Ki is the overall mass-transfer coefficient, in terms of

the three coefficients

• where kiF and kiP are mass-transfer coefficients for the feed-

side and permeate-side boundary layers (or films)

Contd…


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Contd …

•If a solute does not interact with the membrane material,effective diffusivity De , is the ordinary molecular-diffusioncoefficient, which depends only on solute and solventproperties.

• In practice, the membrane may have a profound effect onsolute diffusivity if membrane –solute interactions such ascovalent, ionic, and hydrogen bonding; physical adsorptionand chemisorption; and increases in membrane polymer

flexibility occur.

• Thus, it is best to measure PMi experimentally using processfluids.


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•Transport of solvents such as water, usually in a directionopposite to the solute, can be described in terms of Fick’s law

• It is common to measure the solvent flux and report a so-called water-transport number , which is the ratio of thewater flux to the solute flux , with a negative value indicatingtransport of solvent in the solute direction

• Design parameters for dialyzers are best measured in the

laboratory using a batch cell with a variable-speed stirringmechanism on both sides of the membrane so that externalmass-transfer resistances, 1/kiF and 1/kiP are made negligible


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• A common dialyzer is the plate-and-frame type.For dialysis, the frames are vertical and a unitmight contain 100 square frames, each 0.75 m0.75 m on 0.6-cm spacing, equivalent to 56 m2 of

membrane surface.

• Recent dialysis units utilize hollow fibers of 200-

mm inside diameter, 16-mm wall thickness, and28-cm length, packed into a heat exchanger- likemodule to give 22.5 m2 of membrane area.

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Membrane Separation Process

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