Extraction in Chemical Technology Principle

8/10/2019 Extraction in Chemical Technology Principle

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EXTRACTIONEXTRACTION

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Introduction

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Extraction: refers to an operation in which one or

more component of a liquid or a solid aretransferred to another liquid

When liquid-liquid extraction is used, at least oneof the liquid components must be sucientlyinsoluble so that two liquid phases are formed

For example: the separation of certaincomponents of petroleum-based oils by the use

of acetone !f acetone is added to the oil, some

of the components of the oil dissol"e in theacetone solution, and the remainder of the oilforms an immiscible #non-mixin$% layer with theacetone solution &hese two layers can beseparated


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&he transfer of material from one phase toanother ta'es place by difusion

(i)usion may here be de*ned as thespontaneous intermin$lin$ of the particlesformin$ a solution

For example: when ammonia "apor isabsorbed in water, the ammonia molecules orparticles must di)use throu$h the $as to the

surface of the liquid water &hese ammoniamolecules must then enter the liquid solutionand di)use away from the water surface intothe main body of the liquid

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Liquid-liquid Extraction

iquid-iquid extraction #E% is a masstransfer operation in which a liquid solution#the feed% is contacted with an immiscible ornearly immiscible liquid #sol"ent% that exhibitspreferential anity or selecti"ity towards oneor more of the components in the feed

&wo streams result from this contact: theextract, which is the sol"ent rich solutioncontainin$ the desired extracted solute, andthe ranate, the residual feed solutioncontainin$ little solute

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E is often used to separate two mutually solubleliquids

/ third liquid #solvent%, ha"in$ a preferentialsolubility for one of the components of the mixture,must be added to the solution, and a separate liquidphase must be formed

&he sol"ent ta'es up part of the components of theori$inal solution and forms an immiscible layer withthe remainin$ solution

&he sol"ent layer is called extract

&he remainder of the ori$inal solution #may containsome of the sol"ent% is termed as ranate

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Factors to be considered when optimiin$ the

desi$n and operation of the extractionprocesses:

ol"ent selection

3peratin$ 4onditions

5ode of 3peration Extractor &ype

(esi$n 4riteria

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Solvent selection

ol"ents di)er in their extraction capabilitiesdependin$on theirown and the solute7schemical structure 8identify the desired unctional rou!"s#in the sol"entfor any $i"en solute

&he desired properties of sol"ents are a $i$distri%ution coecient& ood selectivit' towardssolute and little or no miscibility with feed solution/lso, the sol"ent should be easily reco"erable forrecycle

3ther factors a)ectin$ sol"ent selection are boilin$point, density, interfacial tension, "iscosity,corrosi"eness, 9ammability, toxicity, stability,compatibility with product, a"ailability and cost


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O!eratin conditions

(ependin$ on the nature of the extraction process, thetemperature, p; and residence time could ha"e an e)ecton the yield and selecti"ity

3ther parameters to be considered are selecti"ity, mutual

solubility, precipitation of solids and "apor pressure

&he p; becomes si$ni*cant in metal and bio-extractions

!n bio-extractions #e$,


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(ode o o!eration

Extractors can be operated in crosscurrent or counter-current mode

Cross-current

5ainly used in low capacity multi-product batchprocesses


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Extractor t'!e

4ommercially important extractors can beclassi*ed into the followin$ broad cate$ories:

(ixer-settlers

usually a series of static or a$itated mixersinterspersed with settlin$ sta$es &hese aremostly used in the metal industry whereintense mixin$ and hi$h residence time isrequired by the reacti"e extraction processes

!n batch mode of operation, these mixer-settlers could be simple batch "essels wherefeed and sol"ent are mixed and settled &hisoperation is repeated with fresh sol"ent

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MIXER SETTLER


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Centriual extractors

are hi$h-speed rotary machines that o)er

ad"anta$es of "ery low residence time &he number of sta$es in a centrifu$al de"ice

is usually limited to one, but currentlyde"ices with multiple numbers of sta$es arecommon

&hese extractors are mainly used inpharmaceutical industry

Colu)n contactors "static#

Colu)n contactors "aitated#

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KARRColumn


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SCHEIBELColumn


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1=

rotating-disk contacter

a. agitator; b. static disk


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Factors a)ectin$ the selection of extractors

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*esin criteria

&he basic function of extraction equipment is to mix twophases, form and maintain droplets of dispersed phaseand subsequently separate the phases

(ixin

&he amount of mixin$ required is determined byphysicalproperties such as "iscosity, interfacialtension and density di)erences between the twophases

!n a$itated batch extractors, the a$itator desi$n is

often optimied for reaction and heat transfer, notextraction

tatic extraction columns rely completely on thepac'in$@internals and 9uid 9ow "elocities past theinternals to create turbulence and droplets

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Settlin

&he settlin$ characteristics depend on the 9uidproperties #density di)erence, interfacial tension, and

continuous phase "iscosity% and the amount of mixin$ ettlin$ in a$itated batch "essels is carried out by

stoppin$ the a$itator

!n continuous columns, a settlin$ section is pro"idedeither as a part of theextractor or as a separate

piece of equipment downstream of the extractor

Selection o continuous and dis!ersed !$ases

!n column extractors, the phase with the lower"iscosity #lower 9ow resistance% is $enerally chosen

as the continuous phase &he phase with the hi$her 9ow rate can be dispersed

to create more interfacial area and turbulence

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Calculations or Extraction

4onsider a dilute solution of acetaldehyde intoluene, where part of the acetaldehyde is to beextracted usin$ water as the extractin$ sol"ent

&oluene and water are practically immiscible in

mixtures containin$ up to 10 wt A acetaldehydeWhen water is added to the solutions, some

acetaldehyde dissol"e in water and two liquidlayers are formed

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WA + T

A + W

A + T

Extract

Raffinate


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et

BoC '$s of acetaldehyde per '$ of toluene in the

initial solution

oC '$s of water #the extractin$ sol"ent% added

;oC '$s of toluene in the initial solution

D1C the equilibrium concentrations of acetaldehyde in

the extract in '$s of acetaldehyde per '$ of water

B1C the equilibrium concentrations of acetaldehyde

in the ranate in '$s of acetaldehyde per '$ oftoluene

/t equilibrium, the o"erall material balance on theacetaldehyde

Bo;oC ;oB1 oD1

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Exa)!le+

/ solution of acetaldehyde and toluene

containin$ 11 '$ of acetaldehyde and 1?? '$ oftoluene is to be extracted by =? '$ of water&he extraction is to be performed at 1 o4,where the equilibrium relationship between theacetaldehyde concentration in the extract and

ranate may be expressed as D1C 22B1 ;owmany 'ilo$rams of acetaldehyde will beremo"ed by the water

#/nswer: ?2 '$%

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Extraction in Chemical Technology Principle

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