3.3 Separations-part i

Chemical Engineering Design

CHAPTER 3CHAPTER 3PROCESS DESIGN AND SAFETYPROCESS DESIGN AND SAFETY

3.1 Safety

3.2 Case study on process design and safety

3.3 Chemistry and separations

3.4 Unit ratio material balance

3.5 Detailed flow sheet


Course OutcomeCourse Outcome

Ability to explain and identify process design and safety.

Chemical Engineering Design© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

3.3 CHEMISTRY & 3.3 CHEMISTRY & SEPARATIONSEPARATION

PART IPART I


Separation ProcessesSeparation Processes

• Separation processes are needed for feed pretreatment, Separation processes are needed for feed pretreatment, product recovery and waste processingproduct recovery and waste processing

• Most separations are based on moving a component from Most separations are based on moving a component from one phase to another and then segregating the two phasesone phase to another and then segregating the two phases– Driven by activity gradient as phases try to reach equilibriumDriven by activity gradient as phases try to reach equilibrium

– Affected by rates of mass transfer and heat transferAffected by rates of mass transfer and heat transfer

© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy


Separation SpecificationsSeparation Specifications

• RecoveryRecovery: How much of the desired component made it : How much of the desired component made it to the stream it was supposed to be in:to the stream it was supposed to be in:


F, zA, zB

R, xA, xB

P, yA, yB Product enriched in A

AA

A

A

A

xRyP

yP

zF

yP

A ofRecovery


Separation SpecificationsSeparation Specifications

• PurityPurity: The concentration of desired component in the : The concentration of desired component in the stream it was supposed to be in:stream it was supposed to be in:


F, zA, zB

R, xA, xB

P, yA, yB Product enriched in A

Purity of A in product = yA


Impact of Separation SpecificationsImpact of Separation Specifications• Tighter specifications lead to higher cost:Tighter specifications lead to higher cost:

• Final product must meet purity specificationsFinal product must meet purity specifications

– Set by ASTM, USP, etc.Set by ASTM, USP, etc.

• Recycles sometimes have purity specificationsRecycles sometimes have purity specifications

– e.g. to protect catalyst from contaminants or poisonse.g. to protect catalyst from contaminants or poisons

• Product that is not recovered is lost profit and also increased Product that is not recovered is lost profit and also increased waste cost: separation recovery factors into plant yieldwaste cost: separation recovery factors into plant yield

Purity or Recovery (%)

Cos

t

90 99 99.9 99.99



Vapor-Vapor SeparationsVapor-Vapor Separations

MembraneBased on differences in relative permeability of gasesUsed for H2/CH4, CO2 removal,air separation

AbsorptionUsing a liquid solvent in an absorber-stripper loopUsed for acid gases, drying, water wash

AdsorptionAdsorb components selectively on a solidRegenerate sorbent by temperature swing (TSA) or pressure swing (PSA)Used for air separation, H2/CH4, most separations involving low concentrations



AdsorptionAdsorption

• One component from vapor phase preferentially adsorbs One component from vapor phase preferentially adsorbs onto the surface of a solid adsorbentonto the surface of a solid adsorbent

• Two types of adsorption:Two types of adsorption:– Reversible: Reversible:

• Usually physisorptionUsually physisorption

• Adsorbed component can be released by decreasing pressure or increasing Adsorbed component can be released by decreasing pressure or increasing temperaturetemperature

• Sorbent can be regenerated and used in multiple cycles, hence temperature-Sorbent can be regenerated and used in multiple cycles, hence temperature-swing adsorption (TSA) and pressure-swing adsorption (PSA)swing adsorption (TSA) and pressure-swing adsorption (PSA)

– Irreversible:Irreversible:• Usually chemisorptionUsually chemisorption

• Adsorbed component usually reacts irreversibly with solidAdsorbed component usually reacts irreversibly with solid

• Low concentrations can be achieved, but solid is difficult to regenerateLow concentrations can be achieved, but solid is difficult to regenerate

• Used for contaminant removal guard bedsUsed for contaminant removal guard beds



tB

t1

Concentration of B on sorbent

Distance down sorbent bed

Gas mixture A + B

Purified gas A

t2

Concentration Profiles During Concentration Profiles During AdsorptionAdsorption

• At time At time ttBB

breakthrough of breakthrough of the adsorbed the adsorbed component component occurs and it occurs and it begins to appear begins to appear in the outlet gasin the outlet gas

• Concentration Concentration profile moves profile moves down the bed down the bed during adsorptionduring adsorption



Product

Feed

= open valve

= closed valve

Irreversible AdsorptionIrreversible Adsorption

• Two guard beds can be used in parallel so that when Bed 1 Two guard beds can be used in parallel so that when Bed 1 nears breakthrough the process flow can be switched to Bed nears breakthrough the process flow can be switched to Bed 22

• Some adsorbent will be wasted, as beds cannot be run close Some adsorbent will be wasted, as beds cannot be run close to breakthrough for fear of contaminant slippageto breakthrough for fear of contaminant slippage



Product

Feed

= open valve

= closed valve

Irreversible Adsorption: Lead-Lag Irreversible Adsorption: Lead-Lag Guard Bed SystemGuard Bed System

• Alternative arrangement has beds in seriesAlternative arrangement has beds in series• When upstream bed reaches breakthrough, downstream bed is still OK. When upstream bed reaches breakthrough, downstream bed is still OK.

Upstream bed can be taken offline, reloaded and brought back into Upstream bed can be taken offline, reloaded and brought back into downstream service, etc.downstream service, etc.

• Makes more efficient use of adsorbentMakes more efficient use of adsorbent



Guard Beds for Mercury CaptureGuard Beds for Mercury Capture

• Mercury occurs in Mercury occurs in natural gas and natural gas and light oilslight oils

• It must be It must be removed to removed to protect protect equipment and equipment and catalystscatalysts

Source: UOP



p1

T1

Par

tial

pre

ssur

e

Mass adsorbed (g/g sorbent)

p2

m1m2

T2

T2 > T1

Reversible Adsorption: IsothermsReversible Adsorption: Isotherms

• Reversible adsorption exploits changes in loading with Reversible adsorption exploits changes in loading with pressure or temperaturepressure or temperature



p1

T1

Par

tial

pre

ssur

e


p2

m1m2

T2

T2 > T1


• PSA: cycle between high and low pressure to load and PSA: cycle between high and low pressure to load and regenerate the adsorbentregenerate the adsorbent

Adsorb at (p1, T1) gives loading m1

Pressure Swing:Decrease pressure to p2 and loading decreases to m2

Delta loading = m1 – m2

(kg/kg sorbent)



p1

T1

Par

tial

pre

ssur

e


p2

m1m2

T2

T2 > T1


• TSA: cycle between low and high temperature to load TSA: cycle between low and high temperature to load and regenerate the adsorbentand regenerate the adsorbent

Adsorb at (p1, T1) gives loading m1

Temperature Swing:Increase Temperature to T2 and loading decreases to m2

Delta loading = m1 – m2

(kg/kg sorbent)



PSA and TSA SystemsPSA and TSA Systems

PSAPSA• Shorter cycle time (no Shorter cycle time (no

heating or cooling)heating or cooling)– Typically 5 – 60 minsTypically 5 – 60 mins

• Multiple beds needed for Multiple beds needed for high recovery, purityhigh recovery, purity– Use pressure balancing Use pressure balancing

and purge to get better and purge to get better recovery and purityrecovery and purity

– 8, 10, 12, 16 bed plants8, 10, 12, 16 bed plants

• Applications: hydrogen Applications: hydrogen purification, air separationpurification, air separation

TSATSA• Longer cycle time for heating Longer cycle time for heating

and cooling of bed and vesseland cooling of bed and vessel– Typically 60 – 200 minsTypically 60 – 200 mins

• Additional equipment needed Additional equipment needed for heating & coolingfor heating & cooling– Often use a purge gas for regen, Often use a purge gas for regen,

e.g. steam, Ne.g. steam, N2 2 or a slip-stream of or a slip-stream of

productproduct

• Fewer beds (no need to Fewer beds (no need to pressure equalize)pressure equalize)

• Applications: gas drying, VOC Applications: gas drying, VOC capture, COcapture, CO22 removal in cryo removal in cryo

plantsplants© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy


12-Bed PSA Unit12-Bed PSA Unit

Surge TankAdsorber Vessels

Valve Skid

Source: UOP



PSA CyclePSA Cycle

• Pressure equalization steps reduce the amount of gas lost during Pressure equalization steps reduce the amount of gas lost during depressurization and hence improve recoverydepressurization and hence improve recovery

• Repressurization is done using product gas to improve purityRepressurization is done using product gas to improve purity

• Some steps are co-current, some counter-current, to exploit Some steps are co-current, some counter-current, to exploit concentration profiles in the bedconcentration profiles in the bed

• Many different cycles have been invented – see patent literature for Many different cycles have been invented – see patent literature for examples examples

Bed

Time

A absorb1,2,3 equalizePP provide purgeD desorbP purgeR repressure

From U.S. 4,381,189 “Pressure Swing Adsorption System and Process”



Preliminary Design of PSA UnitsPreliminary Design of PSA Units1.1. Delta loading across cycle depends on the adsorbent selected, the Delta loading across cycle depends on the adsorbent selected, the

temperature of operation and the pressure cycle – use isotherms to temperature of operation and the pressure cycle – use isotherms to determine delta loadingdetermine delta loading

2.2. Select number of beds (more beds = more equalization steps, higher Select number of beds (more beds = more equalization steps, higher recovery, higher purity)recovery, higher purity)

3.3. Select cycle time and time in adsorption step, Select cycle time and time in adsorption step, ttaa

4.4.

5.5. Size each bed as a cylindrical pressure vesselSize each bed as a cylindrical pressure vessel

6.6. Add costs for valve skids, surge tankAdd costs for valve skids, surge tank

(Detailed design – need to consider mass transfer rates and dynamics – (Detailed design – need to consider mass transfer rates and dynamics – much more complex analysis)much more complex analysis)

factor loading bed loading delta

component adsorbed of rate flow Massbedper adsorbent of Mass

at

Bed loading factor = fraction of bed loaded at end of adsorption stage ~ 0.8 to 0.9



Membrane SeparationMembrane Separation

• Thin membranes of polymer or metal can be used to separate Thin membranes of polymer or metal can be used to separate gases:gases:– Different species diffuse through a thin membrane at different rates:Different species diffuse through a thin membrane at different rates:

– Different gases have different solubility in metal or polymerDifferent gases have different solubility in metal or polymer

• PermeatePermeate passes through the membrane and becomes passes through the membrane and becomes enriched in faster or more soluble speciesenriched in faster or more soluble species

• RetentateRetentate does not pass through membrane and becomes does not pass through membrane and becomes enriched in slower or less soluble speciesenriched in slower or less soluble species

• Membranes have relatively low cost, but cannot obtain high Membranes have relatively low cost, but cannot obtain high purity or recoverypurity or recovery

• Membranes are therefore widely used for bulk separation of Membranes are therefore widely used for bulk separation of gasesgases



Dense layer 0.1 to 1.0 m

(not to scale)

Porous support

0.1 to 1.0 mm

Asymmetric MembraneAsymmetric Membrane

• Polymer membranes are usually cast as asymmetric membranesPolymer membranes are usually cast as asymmetric membranes• Thin, dense, active layer is supported on a thicker stronger porous layerThin, dense, active layer is supported on a thicker stronger porous layer• Backing cloth is used in some cases as support for active layerBacking cloth is used in some cases as support for active layer



Feed

Permeate

Retentate

One hollow fiber (of thousands)

Potting

Hollow Fiber MembranesHollow Fiber Membranes

• Membranes are cast as long thin fibersMembranes are cast as long thin fibers

• A bundle of fibers is set into a resin (potting) that effectively forms a A bundle of fibers is set into a resin (potting) that effectively forms a tubesheettubesheet

• Feed is fed shell-side and permeate withdrawn from inside the fibers Feed is fed shell-side and permeate withdrawn from inside the fibers

Membrane cross section



Flat Sheet (Spiral Wound) MembranesFlat Sheet (Spiral Wound) Membranes

• Membranes are cast as sheetsMembranes are cast as sheets• Sheets are glued back-to-back along edges to form an Sheets are glued back-to-back along edges to form an

envelope and attached to a perforated tubeenvelope and attached to a perforated tube• The assembly is then rolled up into a spiral-wound moduleThe assembly is then rolled up into a spiral-wound module

Retentate

Feed

Permeate

Enlarged cross section

DensePorousPorousDense

Membrane envelopes

Permeate

Perforated tube



• SEM, TEM, STEM can be used for SEM, TEM, STEM can be used for

microscopic analysismicroscopic analysis

• Note asymmetric structureNote asymmetric structure

– Thin selective skinThin selective skin

– Porous support layerPorous support layer

UOP 5565M-25

Gas Separation MembranesGas Separation Membranes



Membrane Flux and PermeabilityMembrane Flux and Permeability

• Flux of species Flux of species i i through the membrane is proportional to through the membrane is proportional to partial pressure gradient:partial pressure gradient:

• Proportionality constant is the Proportionality constant is the permeabilitypermeability divided by divided by membrane thicknessmembrane thickness

• Ratio of permeabilities of two species is the Ratio of permeabilities of two species is the selectivityselectivity of of the membrane for species i relative to species jthe membrane for species i relative to species j

pifii

i ppP

M ,,

Mi = molar flux of component i (mol/m2.s),Pi = permeability of membrane for omponent i (mol/m.s.bar), = membrane thickness (m),pi,f = local partial pressure of component i on feed side (bar),pi,p = local partial pressure of component i on permeate side (bar)

j

iij P

PS



Membrane Flow PatternMembrane Flow PatternFeed

Permeate

RetentateFeed

Permeate

Retentate

Feed

Permeate

Retentate

Permeate

Countercurrent Countercurrent

Cross-flowCross-flow

Cocurrent Cocurrent

• Integration of the flux Integration of the flux equation along the equation along the membrane depends on membrane depends on the flow patternthe flow pattern

• Note that only flat sheet Note that only flat sheet membranes can be used membranes can be used in cross-flow modein cross-flow mode

• Neither flat sheet nor Neither flat sheet nor hollow fiber membranes hollow fiber membranes can use a sweep gascan use a sweep gas



Membrane Process PerformanceMembrane Process Performance

• Membranes usually give low recovery of permeate Membranes usually give low recovery of permeate species (<95%, often <90%)species (<95%, often <90%)– Need to maintain a high enough partial pressure on retentate Need to maintain a high enough partial pressure on retentate

side to give a reasonable flux at the outlet of the unitside to give a reasonable flux at the outlet of the unit

– If outlet partial pressure is low, flux is low and area required If outlet partial pressure is low, flux is low and area required becomes large and costlybecomes large and costly

• Unless the selectivity is very high, membranes usually Unless the selectivity is very high, membranes usually give low purity on permeate side (<98%, often <95%)give low purity on permeate side (<98%, often <95%)

• Hence membranes are used for bulk separations:Hence membranes are used for bulk separations:– Air separation (hollow fiber)Air separation (hollow fiber)

– COCO22 rejection from natural gas (spiral wound) rejection from natural gas (spiral wound)

– HH22 recovery from mixtures with methane (hollow fiber) recovery from mixtures with methane (hollow fiber)



Permeate RecyclePermeate Recycle

• Permeate from 2Permeate from 2ndnd module is recycled to feed of 1 module is recycled to feed of 1stst module module

• First module can now run under conditions that maximize permeate First module can now run under conditions that maximize permeate purity (high selectivity) and we don’t have to worry about recoverypurity (high selectivity) and we don’t have to worry about recovery

• Second module can run under conditions that maximize recovery (high Second module can run under conditions that maximize recovery (high flux) and we don’t have to worry about purityflux) and we don’t have to worry about purity

• Partial pressure of desired component in 1Partial pressure of desired component in 1stst module is increased module is increased

Feed

Permeate

Retentate



Retentate RecycleRetentate Recycle

• Retentate from 2Retentate from 2ndnd module is recycled to feed of 1 module is recycled to feed of 1stst module module

• Permeate now goes through two membranes in series, so final permeate Permeate now goes through two membranes in series, so final permeate purity is increasedpurity is increased

• First module can run at higher flux, lower selectivity as it makes a rough First module can run at higher flux, lower selectivity as it makes a rough separationseparation

• An extra compressor is needed between the membrane stagesAn extra compressor is needed between the membrane stages

Feed

Permeate

Retentate



Membrane ModulesMembrane Modules

UOP SeparexUOP Separex modules for rejecting CO modules for rejecting CO22 from natural gas from natural gas



Vapor-Liquid SeparationsVapor-Liquid SeparationsFlashSingle stage thermal& phase eqbm

DistillationMultiple stage separationbetween identified light key& heavy key components

EvaporationSingle stage removal of volatile solute or solvent

StrippingMulti-stage removal of volatile solute from solvent

AbsorptionRemoval of vaporcomponent usingnon-volatile solvent

FractionationSeparation of multicomponentmixture into fractions by boiling ranges (e.g. in oil refining)


Mul

ti-st

age:

see

nex

t lec

ture

See heat exchange lectures


Vapor-Liquid Flash DrumsVapor-Liquid Flash Drums

• Flash or knockout drums are widely used in chemical plants:Flash or knockout drums are widely used in chemical plants:– Downstream of condensers and coolersDownstream of condensers and coolers

– Upstream of compressors and between compressor stagesUpstream of compressors and between compressor stages

– As reflux drums on columnsAs reflux drums on columns

– In relief systemsIn relief systems

• Design function is to separate liquid drops from vapor and Design function is to separate liquid drops from vapor and prevent vapor blowing out into liquid-filled lines by prevent vapor blowing out into liquid-filled lines by maintaining liquid level controlmaintaining liquid level control

• There will usually be ~1 to 2% liquid entrainment in the There will usually be ~1 to 2% liquid entrainment in the vapor from a knockout drum unless a demister is used.vapor from a knockout drum unless a demister is used.



Vertical Flash DrumVertical Flash Drum

• Vessel diameter is chosen to give Vessel diameter is chosen to give vapor velocity that is less than vapor velocity that is less than terminal velocity of dropsterminal velocity of drops

• Use 0.15 Use 0.15 uutt if there is no demister if there is no demister

• Allow 1 diameter above feed and Allow 1 diameter above feed and at least 0.6 diameters below feed at least 0.6 diameters below feed for settling, also allow 0.4 for settling, also allow 0.4 diameters for demisterdiameters for demister

• Height of liquid depends on level Height of liquid depends on level control control

ut 0.07[(L v)/v]1/2



Liquid Level Control BandsLiquid Level Control Bands• Level control needs to allow for some natural variation Level control needs to allow for some natural variation

in liquid level due to splashing, etc.in liquid level due to splashing, etc.

• Alarms must not be set too close to normal operating Alarms must not be set too close to normal operating level or they will be a nuisance (& will be ignored)level or they will be a nuisance (& will be ignored)

• Operators need time to respond to alarms before Operators need time to respond to alarms before shutdownshutdown

LAHH shutdown trip

LAH alarm

LAL alarm

LALL shutdown trip

Normal operating band

• A typical assumption is about 2 A typical assumption is about 2 minutes between alarm and trip, minutes between alarm and trip, so allow 10 minutes of liquid so allow 10 minutes of liquid residence time below feedresidence time below feed

• But note: midpoint of normal But note: midpoint of normal operating band should be > operating band should be > DDvv/2 /2

below feed point, so if 5 mins of below feed point, so if 5 mins of liquid holdup gives height < liquid holdup gives height < DDvv/2, /2,

use half a diameter to the use half a diameter to the midpoint and 5 min holdup below. midpoint and 5 min holdup below.

5 min of liquid

5 min of liquid or Dv/2



Horizontal Flash DrumHorizontal Flash Drum

• Bigger area for settling + more space for liquid holdupBigger area for settling + more space for liquid holdup

• Trade-off is higher plot space and stronger foundations needed to support Trade-off is higher plot space and stronger foundations needed to support vesselvessel

• Often used when process control demands some liquid inventory, e.g. reflux Often used when process control demands some liquid inventory, e.g. reflux drumsdrums

• Design is more complex than vertical drum – see Ch16Design is more complex than vertical drum – see Ch16



Liquid-Liquid SeparationsLiquid-Liquid Separations

ExtractionMulti-stage contacting of two liquid phases

DecantingSingle stage thermal& phase eqbm

Mixer-SettlerSingle theoretical stage extraction processOften 2 or 3 stages are still cheaper than a column

MembraneBased on differences in relative permeability of componentsMembrane can be used to keep two solvents from mixing



Feed

Light liquid

Dispersion band

Drain

Vent

Heavy liquid

Horizontal DecanterHorizontal Decanter

• Design is similar to knockout drum: allow droplets to settle and Design is similar to knockout drum: allow droplets to settle and provide adequate holdup for level control – see Chapter 16provide adequate holdup for level control – see Chapter 16

• Siphon take-off can control level without instruments if densities are Siphon take-off can control level without instruments if densities are constantconstant



Multistage Extraction: Sulfolane Multistage Extraction: Sulfolane ProcessProcess

• Used for L/L extraction of Used for L/L extraction of benzene and toluene from benzene and toluene from gasoline using sulfolane solventgasoline using sulfolane solvent



Recovery of Components from Recovery of Components from Liquid SolutionsLiquid Solutions

How many methods can you think of?How many methods can you think of?

DistillationDistillation

ExtractionExtraction

PrecipitationPrecipitation(a.k.a. salting out)(a.k.a. salting out)

Ion ExchangeIon Exchange

CrystallizationCrystallization

EvaporationEvaporation AdsorptionAdsorption(Chromatography)(Chromatography)

MembranesMembranes(Reverse osmosis)(Reverse osmosis)

You should have direct experience with many of these You should have direct experience with many of these in Organic Chem and Unit Ops labs!in Organic Chem and Unit Ops labs!



Circulating Magma CrystallizerCirculating Magma Crystallizer

• Most widely used industrial Most widely used industrial scale crystallizerscale crystallizer

• Supersaturation can be Supersaturation can be achieved by evaporation or achieved by evaporation or cooling (figure shows cooling (figure shows evaporative type)evaporative type)

• Usually designed in Usually designed in consultation with a vendorconsultation with a vendor

• Cost correlates with heat Cost correlates with heat transfer area as most of the transfer area as most of the metal is in the heat metal is in the heat exchangerexchanger



Oslo CrystallizerOslo Crystallizer

• Only the liquor circulates Only the liquor circulates through the exchangerthrough the exchanger

• Allows growth of larger Allows growth of larger and more regular and more regular crystals (less shear)crystals (less shear)

• Crystals are only Crystals are only agitated by circulating agitated by circulating liquorliquor

• Figure shows Figure shows evaporative type, but evaporative type, but can also use cooling to can also use cooling to achieve supersaturationachieve supersaturation



Industrial ChromatographyIndustrial Chromatography

• Chromatography is a very versatile separation process, Chromatography is a very versatile separation process, particularly forparticularly for– Mixtures of close-boiling compounds that are expensive to separate Mixtures of close-boiling compounds that are expensive to separate

by distillation or crystallization (e.g. xylenes, glucose-fructose)by distillation or crystallization (e.g. xylenes, glucose-fructose)

– Thermally sensitive compounds that cannot be distilled or Thermally sensitive compounds that cannot be distilled or crystallized (many biological products, natural extracts, flavors, etc.)crystallized (many biological products, natural extracts, flavors, etc.)

• Many process variations have been developedMany process variations have been developed• High recovery (>99%) and high purity (>99.9%) can be High recovery (>99%) and high purity (>99.9%) can be

achievedachieved• Cost depends on process scheme and whether sorbent Cost depends on process scheme and whether sorbent

(stationary phase) and eluent (mobile phase) can be reused(stationary phase) and eluent (mobile phase) can be reused– For quality control reasons used sorbent and eluent are often For quality control reasons used sorbent and eluent are often

discarded in the pharmaceutical industrydiscarded in the pharmaceutical industry



Batch ChromatographyBatch Chromatography

• A pulse or batch of feed is introduced into the column, then washed through using an eluentA pulse or batch of feed is introduced into the column, then washed through using an eluent

• The fraction that contains the desired product is retained and the rest discardedThe fraction that contains the desired product is retained and the rest discarded

• If sorbent cost is low, sorbent may be discarded instead of eluting heaviesIf sorbent cost is low, sorbent may be discarded instead of eluting heavies

Feed

“Lights”

Product

Eluent

“Heavies”

Metering pump (discontinuous operation)

Chromatography column

A

“Lights”Product

“Heavies”

tcycleTime

Conc. of dissolved material in solvent at “A”



Variations on Batch ChromatographyVariations on Batch Chromatography

• Solvent gradient chromatographySolvent gradient chromatography– Eluent composition is changed over time to change solute-sorbent Eluent composition is changed over time to change solute-sorbent

interaction and elute different speciesinteraction and elute different species

• HPLC (high performance liquid chromatography)HPLC (high performance liquid chromatography)– High pressure pumps and long packed columns of sorbentHigh pressure pumps and long packed columns of sorbent

• Gel permeation chromatographyGel permeation chromatography– Stationary phase pore structure excludes the product, so heavies come out Stationary phase pore structure excludes the product, so heavies come out

first: gives faster cycle timefirst: gives faster cycle time

• Affinity chromatographyAffinity chromatography– Highly specific interaction between solute and sorbent, e.g. antibody-antigenHighly specific interaction between solute and sorbent, e.g. antibody-antigen

– Example: Protein A chromatography for recovery of monoclonal antibodiesExample: Protein A chromatography for recovery of monoclonal antibodies

– Affinity chromatography is one of the most widely used methods in Affinity chromatography is one of the most widely used methods in recovering large biologically derived moleculesrecovering large biologically derived molecules



Feed

A

Net desorbentConcentration in liquid

B

hE

Raffinate

Extract

Solids recirculation

Fresh desorbent

S hF

hR

0

Hei

ght

Zone

I

II

III

IV

Continuous Countercurrent Continuous Countercurrent ChromatographyChromatography

• If solids move then more strongly adsorbed component A can be separated from less strongly adsorbed BIf solids move then more strongly adsorbed component A can be separated from less strongly adsorbed B



Countercurrent ChromatographyCountercurrent Chromatography

• Continuous movement of solids is difficult to accomplishContinuous movement of solids is difficult to accomplish– Good sorbents such as zeolites and gels do not flow well or Good sorbents such as zeolites and gels do not flow well or

suffer attritionsuffer attrition

• Instead, solids movement can be simulated using a Instead, solids movement can be simulated using a rotary valve or set of switching valvesrotary valve or set of switching valves

• UOP Sorbex process UOP Sorbex process – Developed for separating normal paraffins in 1960sDeveloped for separating normal paraffins in 1960s

– Now mostly used for recovering p-xylene from mixed xylenes Now mostly used for recovering p-xylene from mixed xylenes and separating glucose and fructose to make High Fructose and separating glucose and fructose to make High Fructose Corn SyrupCorn Syrup

• Simulated Moving Bed chromatography is now being Simulated Moving Bed chromatography is now being used more widely in pharmaceutical separationsused more widely in pharmaceutical separations



AC Adsorbent columnRV Rotary valveEC Extract columnRC Raffinate column

UOP Sorbex ProcessUOP Sorbex Process



Questions ?Questions ?


3.3 Separations-part i

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