Status of the Reactive Extraction as a Method of Separation

Review ArticleStatus of the Reactive Extraction as a Method of Separation

Dipaloy Datta,1 Sushil Kumar,2 and Hasan Uslu3

1Chemical Engineering Department, Thapar University, Patiala 147004, India2Department of Chemical Engineering, Motilal Nehru National Institute of Technology (MNNIT),Allahabad, Uttar Pradesh 211004, India3Chemical Engineering Department, Engineering and Architecture Faculty, Beykent University, Ayazaga, 34437 Istanbul, Turkey

Correspondence should be addressed to Hasan Uslu; [email protected]

Received 7 August 2014; Accepted 20 September 2014

Academic Editor: Saeid Azizian

Copyright 2015 Dipaloy Datta et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The prospective function of a novel energy efficient fermentation technology has been getting great attention in the past fifty yearsdue to the quick raise in petroleum costs. Fermentation chemicals are still limited in the modern market in huge part becauseof trouble in recovery of carboxylic acids. Therefore, it is needed considerable development in the current recovery technology.Carboxylic acids have been used as the majority of fermentation chemicals. This paper presents a state-of-the-art review on thereactive extraction of carboxylic acids from fermentation broths.This paper principally focuses on reactive extraction that is foundto be a capable option to the proper recovery methods.

1. Introduction

Liquid-liquid extraction (LLE) is a process in which a par-ticular solute is removed from a liquid phase (feed phase) byanother liquid phase (solvent or extract phase). Applicationof extraction in life science started in way back to about3500 BC to recover products from various natural resources.At a Sumerian text dated 2100 BC, production of perfumes,pharmaceutical oils, andwaxes was documented. Inmedievalages extraction was performed with ethanol and was appliedin the field of hydrometallurgy by making use of mineralacids [1]. With the developments in thermodynamics, par-ticularly the distribution law by Nernst in 1891 and design ofan apparatus for extraction, significant improvements wereaccomplished in the late 19th century. However, it was notuntil the early 1930swhen the first large-scale LLEprocesswasin operation. Lazar Edeleanu (a Romanian Chemist: 18611941) removed aromatic and sulphur compounds from liquidkerosene by the LLE process using liquid sulphur dioxide asa solvent at temperature as low as 6 to 12C. This yieldedclean kerosene suitable to be used as a fuel for residentiallighting. Nowadays, besides the extraction of almost allmetals in mining industries and environmental applications,extractants are widely used in the extraction of organic

and inorganic acids, organic chemistry intermediates, andpharmaceuticals for the purposes of separation, purification,or enrichment [2] of the product.

The main difference between reactive extraction andsolvent extraction is the reaction between the extractantand the solute in the organic phase. Aliphatic amines andphosphoric solvents are proposed as effective extractants byearlier researchers [3]. While extractants play the major rolein the reaction, diluents also have a significant effect onthe level of extraction. Nonaromatic, water immiscible, andpolar solvents with intermediate molecular weights and highboiling points are commonly preferred for the extractionto have high distribution and selectivity [4]. The solvents(diluents) control the physical properties (viscosity, density,surface tension, etc.) of the solvent phase and also affect thestability of the complex structure formed between the soluteand the extractant.

The reactive extraction is found to be a promisingmethodfor the recovery of the carboxylic acids from a dilute fermen-tation broth [510]. This separation method has advantagessuch as the following:

(i) effective at high concentration of substrate in theextractive fermentation,

Hindawi Publishing CorporationJournal of ChemistryVolume 2015, Article ID 853789, 16 pageshttp://dx.doi.org/10.1155/2015/853789

2 Journal of Chemistry

(ii) the acid can be reextracted and the solvent can bereused,

(iii) better control of pH in the bioreactor,(iv) better recovery of acid with higher product purity,(v) reduction of downstream processing load and recov-

ery cost,(vi) reactants are relatively immiscible,(vii) product(s) undergoes subsequent undesired reac-

tions in reaction phase,(viii) reaction products to be separated are immiscible with

the reaction phase,(ix) phase equilibrium can be positively influenced,(x) heat transfer is to be improved during the reaction,(xi) product-catalyst separation can be affected by a

liquid-liquid separation.

Kertes and King [3] categorize the extractants as threemajor types:

(i) carbon bonded oxygen bearing extractants,(ii) phosphorus bonded oxygen bearing extractants,(iii) high molecular weight aliphatic amines.

The first two categories are nonreactive in nature andextract the acid molecules by solvation. The distinctionbetween the first two categories is based on the strengthof the solvation bonds and the specificity of solvation. Thecoordinate bonds between carbon bonded oxygen donorextractant and the acid are too weak for a specific solvationbut it is significantly more for phosphorus bonded oxygenbearing extractants. The extractants in the second categorymake the solvation process more specific and the numberof solvating molecules per extracted acid molecule can beaccessible experimentally. The aliphatic amines in the thirdcategory can react with the carboxylic acid molecule andform acid-amine complexes by proton transfer or by ion pairformation. This causes a significant increase in the distribu-tion coefficient of the carboxylic acid [11]. Among aliphaticamines, primary alkyl amines are observed to be excessivelysoluble inwater at room temperature while secondary aminesform a gel phase (third phase) at the interface which createdifficulty in phase separation [3].The extractability of tertiaryamines is found to be more than that of the primary andsecondary amines [5]. Aliphatic tertiary amines having morethan six carbon atoms per chain are found to be effectiveextractants for the recovery of carboxylic acids [3].

Although a tertiary amine has good extractability, itmust always be used with a diluent due to its viscous andcorrosive nature. Further, the stability of the formed acid-amine complexes in the reactive extraction is affected by thebasicity of the amine which can be manipulated by usingdifferent types of diluents. Moreover, use of a diluent controlsthe physical properties such as density, viscosity, and surfacetension of the organic phase [12].

Diluents can be broadly divided into two groups: (i)active diluents and (ii) inactive diluents. Generally, the active

diluents are polar in nature due to the presence of functionalgroups. They are good solvating media for an ion-pairsuch as an acid-amine complex [13]. The category includeschlorinated hydrocarbon, ketone, alcohol, and halogenatedaromatic solvents. Inactive diluents being nonpolar providevery low distribution of the acid and poor solvation of thepolar complexes. Alkanes, benzene, alkyl substituted aromat-ics, and so forth fall in this category. These diluents limit theformation of the third phase at higher concentrations of acidin the organic phase and are useful in the stripping of acid.The equilibrium curve can be shifted towards the aqueousphase by increasing the concentration of the inert diluent inthe mixture of diluents [14].

Reactive extraction represents a reaction between the acid(solute) and extractant molecule at the interface of aqueousand organic phase where transfers of acid molecules takeplace by the diffusion and solubilizationmechanism. Reactiveextraction strongly depends on various parameters suchas aqueous phase composition, organic phase composition,types of complexes (1 : 1, 2 : 1, etc.) formed, properties of thesolvent (extractant and diluent), type of solvent, temperature,and pH [15]. The purpose should be achieving a highdistribution coefficient with higher selectivity. This can berealized by utilizing an appropriate organic phase at optimumconditions.

There are two stages in extractive separation (Figure 1).The first is the extraction of the solute to produce a solute-extractant complex and a relatively solute free aqueousraffinate.The second step is necessary for stripping the solutefrom the organic complex to obtain amine free aqueous soluteas a product and also for simultaneously regenerating theextractant recycled back to the extraction unit. In the secondstage of reactive extraction, it is necessary to regenerate theorganic phase. Tamada and King [16] have described twoapproaches for the regeneration of extractant-diluent system:(i) temperature swing regeneration and (ii) diluent swingregeneration. Yabannavar and Wang [17] have purified lacticacid from a loaded organic phase using sodium hydroxide(NaOH) and hydrochloric acid (HCl). Poole and King [18]have used an aqueous solution of lowmolecular weight aminefor complete regeneration of organic phase.

Reactive extraction has many applications in chemicaland pharmaceutical industries (intermediates, organic acids,vitamins. . .) or in hydrometallurgical and environmentalscience (heavy metals, inorganic acids. . .). The liquid ionexchangers are used to promote a certain reaction to accom-plish very selective separations [19]. There are many liquidion exchangers commercially available. They work basicallywith three different types of mechanisms: anion exchange,cation exchange, and solvation. It is practical to use ion-exchangers in a diluent which is immiscible with water sincethey are highly viscous or solid materials. The diluent makesthe organic phase easier to handle [1].

There are a significant number of studies on reactiveextraction of organic acids in the literature. These studiesinvestigate various aspects of reactive extraction of organicacids: chemical interactions between acids and extractants,reaction mechanisms, type of extractants and diluents, effectof temperature and pH, effect of aqueous and organic phase

Journal of Chemistry 3

Aqueous phase Organic phase Stripping phase

Extraction Back-extraction

HCaq + H2O HCorg + S (HCS)org HC + stripping agent

HCaq HCorg

Caq + H3O+

Figure 1: Schematic representation of the reactive extraction process (extraction and back-extraction).

compositions on extraction, effect of modifiers, extractivefermentation processes, kinetics of extraction, stripping theacid from the organic phase, and so forth.

2. Equilibrium Studies on Reactive Extractionof Carboxylic Acids

Several researchers [3, 8, 9, 13, 16, 21, 26, 31, 5659] havestudied theoretical and experimental aspects of the recoveryof carboxylic acids from their aqueous solutions by reactiveextraction. In their work, the investigators have examinedthe effects of various parameters on the distribution of theacid (solute) between the aqueous and organic phase. Someof these parameters are concentration and composition ofboth phases, types of the extractants and diluents, pH ofthe aqueous phase, temperature of the system, and toxicityof the solvent phase to the microorganism [3, 13, 57, 60,61]. In addition to these experimental studies, the possiblereaction mechanisms are also described and determinedusing different equilibrium and kinetic models [8, 9, 50,52]. Some experimental results have showed synergistic andantagonistic effects on the extraction of the carboxylic acidswhen there is more than one acid in the aqueous phaseor more than one extractant in the organic phase [21, 62,63]. Some researchers have tried to recover the carboxylicacids from production medium by extractive fermentationand studied the process conditions affecting recovery duringproduction [57, 60, 61, 64].

King and his group have performed the pioneeringstudies on the equilibriumof reactive extraction of carboxylicacids. Besides carboxylic acids, they have also studied theextraction of chlorinated hydrocarbons and aromatics [65],ethanol [66], ammonia [67], and low molecular weightaliphatic alcohols [3] from aqueous solutions. In one of theirearlier work, Wardell and King [11] studied the extractionof acetic and formic acids from their aqueous solutions[11] using phosphoryl solvents (tributyl phosphate, dibutylphosphonate, tributylphosphineoxide, and triphenylphos-phineoxide) and tertiary amine extractants (tri--octylamineand tri-iso-octylamine) dissolving in different solvents. Highdistribution coefficients are achieved with phosphoryl com-pounds which act as a Lewis base (presence of the phosphorylbond, PO). The results indicated that, with the increasein the electronegativity, a decrease in the electron-donatingability and disappearance of Lewis basicity were observed.This study also showed the advantage of using long chainamines as extractants in the recovery of carboxylic acids. It

was also observed that the extent of the extraction of aceticacid appeared to increase with an increase in the solubilityparameter of the diluent besides the polarity.

At the later stage, Kertes and King [3], in 1986, pub-lished a research article in which the authors discussed theimprovements in fermentation technology and its need ofcommercialization. They have reviewed 11 carboxylic acids(propionic, pyruvic, lactic, succinic, fumaric,maleic, itaconic,tartaric, citric, and isocitric) which are obtained by aerobicfermentation of glucose via the glycolytic pathway andglyoxylate bypass. The investigators pointed out that it is theundissociated part of a monocarboxylic acid which can onlybe extracted into carbon-bonded and phosphorus-bondedsolvent. This revealed that the initial pH of the aqueous solu-tion and the dissociation constant (p

) of the acid are two

very important and influential parameters in the extractionof particular acid. Mass action law and Nernst distributionlaw are used in order to evaluate the data. The authors haveconsidered the dimerization of acids in the organic phaseand proposed a relation between dimerization constant andpartition coefficient. The emphasis is given for the use ofaliphatic tertiary amines compared to primary and secondaryamines. In particular, monocarboxylic acids under compara-ble conditions are noted to be more easily extracted with anappropriate organic phase than di- or tri-carboxylic acids [3].

King and his coworkers [13, 16, 56] continued theirstudies on reactive extraction of carboxylic acids. In their firststudy [13], they have carried out the extraction equilibriumexperiments of carboxylic acids (acetic, lactic, succinic, mal-onic, fumaric, and maleic) with different p

values. They

have studied the effect of pof the corresponding acid

on the extent of extraction. It is found that the acid withhigher p

value is extracted inmore amounts. Furthermore,

they have also searched for the effect of functional groupspresent in acid other than the primary carboxyl groupon extraction. The reactive extraction is examined usingAlamine 336 dissolved in various diluents (active diluents:1-octanol, DCM, chloroform, MIBK, nitrobenzene; inertdiluent: heptane). These diluents are selected with differentchemical characteristics such as electron donating, electronaccepting, polar and nonpolar in order to observe the effect ofdiluent-complex interactions on the equilibrium conditionsand the possible formation of acid-amine complexes (1 : 1,2 : 1). They have indicated that solubility of the acid-aminecomplex in the solvent phase is decreased in the followingorder: alcohol nitrobenzene proton donating halogenatedhydrocarbon > ketone > halogenated aromatic > benzene >alkyl aromatic > aromatic hydrocarbon.


In their second study, Tamada and King [56] have inves-tigated the chemical interactions between the componentsby using the results of mass action law analysis and thespectroscopic studies. Organic phase is analyzed by infraredspectroscopy to examine the stoichiometry of the acid-aminecomplex. They emphasize that there is an ion pair formationbetween the amine and first acid molecule and there is ahydrogen bond formation between the carboxyl of the secondacidmolecule and the carboxylate of the first in the formationof 2 : 1 complex of acid-amine.

In the last part of the study [16], this group has carriedout the coextraction of water with the acids by Alamine 336dissolved in various diluents and found that amine had noeffect on the coextraction of water. Water coextraction wasdecreased in the order of 1-octanol > MIBK > nitrobenzene> methylene chloride > chloroform > heptane during theextraction of succinic acid. Coextraction of water for differentacids is also compared and it is revealed that monocarboxylicacids carry less water than dicarboxylic acids. In this study,the effect of the temperature on the reactive extractionof carboxylic acids by Alamine 336 is performed. It isobserved that the distribution coefficient decreased withthe increase in the temperature of the system as with theformation of the complex, the system became more orderedand entropy decreased. Consequently, the amount of acidextracted decreased with the increase in temperature. Finally,King and his coworkers regenerated the extractant throughback-extraction by two approaches: swing temperature andswing diluent methods.

The quaternary amines are first studied by Yang et al.[68]. They have investigated Alamine 336 and Aliquat 336,quaternary amine produced by the methylation of Alamine336 and composed of a large organic cation with a chlorideion, dissolved in kerosene or 2-octanol to extract lactic, acetic,and propionic and butyric acids. It has been reported thatwhile Alamine 336 can extract only the undissociated acid,Aliquat 336 can extract both dissociated and undissociatedacids and always gives higher distribution coefficients thanAlamine 336 at all pH values. It is also reported in this studythat the extraction power of Alamine 336 is increased by thepolar diluent, 2-octanol. On the other hand a diluent effectcould not be detected on the extraction power of Aliquat 336.

Prochazka et al. [69] have studied trialkylamine, a mix-ture of straight chain tertiary amine with 79 carbon atoms,dissolved in themixture of 1-octanol and n-heptane to extractlactic, malic and citric acids. It is reported that all systemsused in this study are sensitive to temperature and solventcomposition. In another study, Juang et al. [21, 70] useda tertiary amine based extractant, tri-n-octylamine (TOA)dissolved in xylene to extract succinic and tartaric acids[21, 70]. They obtained thermodynamic data on extractionand compared extraction equilibria of succinic and tartaricacids with TOA dissolved in xylene. They have reported thatthe equilibrium distribution coefficient of the acids extractedwith TOA increases with initial concentration of amine in theorganic phase and of the acid in the aqueous phase. TOA hasalso been investigated by Poposka et al. [71] as an extractantdissolved in an iso decanol/n heptane mixture for extractionequilibria of citric acid. The data were obtained as a function

of acid, amine and isodecanol concentrations at 298K and anappropriate mathematical model was proposed.

Most of the studies on reactive extraction in the literatureare focused on a single acid in the aqueous phase. However,Juang et al. [70] have reported that extraction characteristicsare affected by the presence of more than one acid in theaqueous phase. Separation mechanisms of lactic and citricacids in the aqueous phase with TOA dissolved in xylenehave been investigated via the supported liquid membranetechnique. They have reported large synergistic effects com-pared to single acid systems for low initial concentrationratio of citric to lactic acid () and an antagonistic effectat = 2. The presence of the second acid has a positiveeffect on the transport of citric acid but a negative effecton that of the lactic acid. Transport rate of the acids alsoincreases with TOA concentration and temperature. Newsynergistic extraction system for organic acids was developedby Matsumoto et al. [72]. The extraction equilibria wereinvestigated for acetic, glycolic, propionic, lactic, succinic,fumaric, malic, and itaconic acids by tri-n-octylamine (TOA)and/or tri-n-butylphosphate (TBP) dissolved in hexane. Itis reported that when a mixture of extractants are used asynergy is developed and a more effective extraction of allorganic acids is achieved.

The above studies clearly show that the reaction betweenthe extracted acid and the extractant present in the organicphase is dependent on the corresponding acid and the con-tents of the organic phase. To explore the possibilities of reac-tive extraction and its application and commercialization,further studies are carried out with different carboxylic acidsusing various extractants dissolved in different categoriesof diluents. A brief review of these extraction studies issummarized in a tabular form and shown in Table 1 to have aclear understanding of the various reactive systems used.

2.1. EquilibriumModels. The theories of equilibria of reactiveextraction are explained in this section. The model describesthe physical and chemical phenomena occurring in theextraction process in the mathematical forms. They alsoexplain interaction mechanism between the components ofaqueous (acid and water) and organic (extractant and/ordiluent) phases. In these model equations, assumption ismade that all the reactions are in thermodynamic equilibriumoccurring at the interface of aqueous and organic phases.

2.1.1. Mass Action Law Model. Equilibrium data are inter-preted byMass Action Law, which was proposed by GuldbergandWaage in 1864. Kertes and King in 1986 [3] applied MassAction Law for reactive extraction of carboxylic acids. Inthe Mass Action Law model, activities of the aqueous andorganic phase species are assumed to be proportional to therespective concentration of the species and the equilibriumconstant takes care of the constant of proportionality or thenonidealities associated with the reactive system. Therefore,the apparent equilibrium constant (written in terms of speciesconcentration) is used for the development of mathematicalmodel of the reaction equilibrium. This model can be sub-categorized in two types: (i) physical extraction where only


Table1:Ex

tractant/dilu

entsystem

forthe

recovery

ofcarboxylicacid

byreactiv

eextraction:equilib

rium

studies.

SL number

Carboxylicacid

Extractant

Dilu

ent

Parametersstudied

Find

ings

References

1Differentcarboxylic

acids

Organop

hospho

rous

and

aliphatic

amines

Alcoh

ols,ketones,ethers,and

hydrocarbo

nsVa

rious

aqueou

sand

organic

phasep

aram

eters

Review

edthee

xtraction

chem

istry

ofcarboxylicacids

[3]

2Lacticacid

TOA

Hexane

Type

ofacid

andinitialacid

concentration

Hydroph

obicity

ofthea

cid

controlsequilib

rium

constants

[20]

3Ac

etic,lactic,succinic,

malon

ic,fum

aric,and

maleic

Tri-a

lkylam

ine(Alamine3

36)

Methylisobu

tylketon

e(MIBK)

,n-heptane,dichloromethane

(DCM

),andnitro

benzene

Effectsof

diluent-c

omplex

interactions

Equilibriu

mconstants,partition

coeffi

cientand

dimerization

constant

determ

ined

[13]

4Citric

Alamine3

36-Xylene,toluene,benzene,

MIBK,

1-octanol,D

CM,and

chloroform

Effecto

fdilu

ents

11

,12

and

23

arec

orrelated

with

solvatochrom

icparameters

[12]

6Lactic

TOA

Xylene

Temperature,acidandTO

Aconcentration

(1:1),(1:2),and(3:1)acid-TO

Acomplexes

form

ed[21]

7(L+)

lactic

Tri-p

ropylamine(TP

A)and

TOA

1-octanolandheptane

Acid

andam

inec

oncentratio

nMixed

tertiary

amineo

fsho

rtandlong

chaincanfacilitatee

asy

phases

eparation

[22]

8Citric,lactic,and

malic

Tri-iso-octylam

ine(TIOA)

1-octanolandheptane

Effecto

fmod

ifier

andtype

ofacid

Extractio

nmechanism

isprop

osed

consideringph

ysical

andchem

icalextractio

n[23]

9Glyoxylic,glycolic,

acrylic,and

benzoic

Tri-a

lkylph

osph

ineo

xide

(TRP

O)

Kerosene

Aqueou

sand

organicp

hase

compo

sitions

Equilib

rium

mod

elfor11

isprop

osed

[24]

10Propionic

Aliq

uat336

Cyclo

hexane,hexane,toluene,

MIBK,

ethylacetate,hexane+

MIBK,

hexane

+toluene,and

MIBK+toluene

Effecto

fdilu

entand

diluent

mixture,acidandam

ine

concentration

Order

ofextractio

n:ethylacetate

>MIBK>MIBK+toluene>

toluene>

hexane

+MIBK>

toluene+

hexane>cyclo

hexane

>hexane

[25]

11Propionic

Tri-

-butylph

osph

ate(TB

P),

TOA,and

Aliq

uat336

1-Octanol

Acid

andextractant

concentration

Order

ofextractib

ilityisfoun

dto

beTO

A>Aliq

uat336>TB

P,1:1

complex

form

ed[26]


Table1:Con

tinued.

SL number

Carboxylicacid

Extractant

Dilu

ent

Parametersstudied

Find

ings

References

12Ita

conic,maleic,malic,

oxalic,tartaric

,and

succinic

TBP

Do-decane

pHandinitialacid

concentration

Mechanism

ofdi-carboxylic

acidsisp

ropo

sed

[27]

13Nicotinic

TOPO

andTB

P

Benzene,heptane,kerosene,

1-octanol,M

IBK,

diethylether,

decane,kerosene+

1-octanol,

andheptane+

1-octanol

Effecto

fdilu

ent,extractant

type,

initialacid,and

extractant

concentration

Solvationnu

mbera

ndequilib

rium

extractio

nconstants

ared

etermined

[28]

14Levulin

ic-Lauryltri-alkyl-m

ethylamine

(AmberliteLA

-2)

Dim

ethylphthalate,dim

ethyl

adipate,dimethylsuccinate,

dimethylglutarate,diethyl

carbon

ate,iso

amylalcoho

l,1-h

exanol,1-octanol,1-non

anol,

1-decanol,diisobutylketone

(DIBK)

,and

MIBK

Dilu

enteffect,and

amine

concentration

LSER

mod

elprop

osed

and

11

,

21

,and

31

ared

etermined

[29]

15Form

icAmberliteLA

-2

Dim

ethylphthalate,dim

ethyl

adipate,dimethylsuccinate,

dimethylglutarate,diethyl

carbon

ate,iso

amylalcoho

l,1-h

exanol,1-octanol,1-non

anol,

1-decanol,D

IBK,

andMIBK

Effecto

fdilu

entand

amine

concentration

Extractio

nconstants(

51

,61

,and

71

)wered

etermined

and

LSER

mod

elprop

osed

[30]

16Nicotinic

Organop

hospho

rous

and

aliphatic

amines

Alcoh

ols,ketones,ethers,and

hydrocarbo

nsVa

rious

aqueou

sand

organic

phasep

aram

eters

Review

edthee

xtraction

chem

istry

ofnicotin

icacid

[31]

17Glycolic

AmberliteLA

-21-O

ctanol,cyclohexane,

iso-octane,toluene,2-octano

ne,

andMIBK

Solventtype,am

inea

ndacid

concentration

Order

ofextractio

n:MIBK>

2-octano

ne>1-o

ctanol>toluene

>iso

-octane>

cyclo

hexane

[32]

18Citric

Tri-d

odecylam

ine(TD

DA)and

AmberliteLA

-2

MIBK,

1-octanol,toluene,

cyclo

hexane,1-octanol+toluene,

1-octanol+MIBK,

MIBK+

toluene,1-o

ctanol+toluene,

1-octanol+MIBK,

MIBK+

toluene,andiso

-octane

Dilu

enteffect,aminea

ndacid

concentration

1-Octanolprop

osed

tobe

most

effectiv

esolvent

[33]

19Glutaric

TOA

Isoamylalcoho

l,1-o

ctanol,

1-non

anol,1-decanol,m

ethyl

ethylketon

e(MEK

),diiso

butyl

ketone

(DIBK)

,hexan-2-one,

toluene,kerosene,and

hexane

Dilu

enteffect,aminea

ndacid

concentration

Kerosene

isfoun

dto

bethem

ost

effectiv

edilu

ent.Eq

uilib

rium

constantsfor

1:1and

2:1

complex

aree

stim

ated.

[34]


Table1:Con

tinued.

SL number

Carboxylicacid

Extractant

Dilu

ent

Parametersstudied

Find

ings

References

20Lactic

TBP

Dod

ecane

Acid

andsolventcom

position,

change

inthep

hase

volumes

and

pH

Apparent

equilibriu

mconstants

andthen

umbero

freacting

extractant

molecules

[35]

21Ac

rylic

AmberliteLA

-2Cy

clohexane,2-octanon

e,toluene,MIBK,

iso-octane,

hexane,and

1-octanol

Type

ofdiluent,andam

ine

concentration

1:1&

1:2acid-aminec

omplexes

forp

roton-do

natin

gdiluentsand

1:1&

2:3for

non-proton

-don

atingdiluents,

overallextractionconstants(

11

,

12

and

23

)

[36]

22Form

icTD

DAandTB

PEthylvalerate,diethyladipate,

diethylsebacate,1-o

ctanol,and

heptane

Effecto

fdilu

ent,acid

andam

ine

concentration

Com

parativ

estudy

ofph

ysical

andchem

icalextractio

n,TD

DA

suggestedto

betheb

est

extractant

[37]

23PenicillinG

Di-

-octylam

ine,TO

A,N

235(a

mixtureof

tertiary

amines),TB

P,anddi-(2-ethylhexyl)p

hospho

ricacid

(D2E

HPA

)

n-Bu

tylacetate,M

IBK,

2-ethyl

hexano

l,kerosene,and

heptane

Initialacid

concentration,pH

andtemperature

Effecto

fextractanto

nthe

stabilityof

penicillinGmainly

depend

sontemperature,

degradationof

penicillinGin

alkalisolutio

nisgoverned

bypH

,mechanism

ofdegradation

discussed

[38]

24Succinic

AmberliteLA

-2Hexane,cyclo

hexane,toluene,

iso-octane,MIBK,

2-octano

ne,

and1-o

ctanol

Initialextractant

concentration

Extractio

nconstants(1:1,1:2&

2:3)

foun

dou

t,ordero

fextractio

nof

diluents:

1-octanol

>2-octano

ne>MIBK>toluene

>iso

-octane>

hexane>

cyclo

hexane

[39]

25Ita

conic

TBPandAliq

uat336

Sunfl

ower

oil

Initialacid

andextractant

concentration

Non

toxics

ystem

prop

osed

[40]

26Propionic

TBP

Kerosene

and1-d

ecanol

Aqueou

sand

organicp

hase

compo

sitions,and

temperature

Mod

ifier

hasa

stron

geffecto

ndegree

ofextractio

n[41]


Table1:Con

tinued.

SL number

Carboxylicacid

Extractant

Dilu

ent

Parametersstudied

Find

ings

References

27L(+)T

artaric

AmberliteLA

-21-o

ctanol,cyclohexane,

isooctane,hexane,andMIBK

Organicph

asec

ompo

sitions

Extractabilityof

acid

ishigh

especiallywith

polarsolvents

(MIBKand1-o

ctanol)

[42]

28Ca

proic

TBP

MIBKandxylene

Phasec

ompo

sitions

MIBKisab

ettersolvent

than

xylene

[43]

29Ac

etic

TOA

DCM

,butylacetate,heptanes,

and1-o

ctanol

Organicph

asec

ompo

sitions

and

pH

Thes

olvent

polaritycontrolsthe

form

edstr

uctureof

the

interfa

cialacid

andTO

Acompo

unds

[44]

30Picolin

icTB

PSunfl

ower

andcasto

roil

Aqueou

sand

organicp

hase

compo

sitions

Differentm

odels

areu

sedto

representthe

equilib

rium

data

[45]

31Picolin

icTrialkylam

ine(N235),T

BPTetrachlorom

ethane,kerosene,

and1-o

ctanol

Aqueou

sand

organicp

hase

compo

sitions,and

pH

Distrib

utioncoeffi

cienth

ighly

depend

ento

npH

andthe

apparent

alkalin

ityof

N235/1-o

ctanol

[46]

32Ac

etic,propion

ic,

butyric

andvaleric

TBP

Cyclo

hexane,sulfonated

kerosene,and

1-octanol

Equilib

rium

time,temperature

andph

aser

atio

Con

ditio

nsof

extractio

nand

strip

ping

arefou

nd[47]

33Citric

TOA

Rice

bran

oil,sunfl

ower

oil,

soybeanoil,andsesameo

ilAq

ueou

sand

organica

ndph

ase

compo

sitions

Overallextractio

nconstantsa

ndassociationnu

mbers

[48]


diluent (pure form) is used for extraction and (ii) chemicalextraction where both extractant (phosphoric and aminic)and diluent take part in the extraction process.

(1) Physical Extraction.The process of physical equilibria withpure diluent takes place in three parts: (i) partial dissociationof the acid molecule in aqueous phase, (ii) distribution of theundissociated acid molecule between aqueous and organicphases, and (iii) dimerization of undissociated acid moleculein the organic phase.

(i) Carboxylic acid can exist in undissociated (HC) anddissociated (C) forms in the aqueous solution of water. Thedissociation of the acid in the aqueous solution depends uponthe strength of the acid (p

) and is described by

HC H+ + C (1)

The dissociation constant () is calculated using

=

[H+] [C][HC]

.

(2)

The total acid concentration in the aqueous phase (HC) andundissociated acid [HC] concentration can be expressed as(3) and (4), respectively:

HC = [HC] + [C

] (3)

[HC] =HC

(1 + (

/ [H+]))

. (4)

(ii) The distribution of the undissociated acid moleculebetween aqueous and organic phases is represented by (5) andthe corresponding partition coefficient is given by (6):

HC HC (5)

=

[HC][HC].

(6)

(iii) The undissociated extracted acid molecules candimerize in the organic phase due to the strong donor-acceptor interaction. This is due to the solute-solute inter-action through hydrogen bond which is stronger than thesolute-solvent interaction.The dimerization of the undissoci-ated extracted acid in the organic phase (HC) is representedby

2HC (HC)2

(7)

The dimerization constant () is expressed by

=

(HC)2

(HC)2

. (8)

The extraction efficiency (with pure diluent) of acid iscalculated by the physical distribution coefficient, diluent

:

diluent

=

diluentHC

diluentHC,

(9)

where diluentHC is the total (undissociated and dimer forms)concentration of acid in the organic phase and diluentHC isthe total (dissociated and undissociated) concentration inaqueous phase at equilibrium with pure diluent.

For a dilute concentration of acid (as used in this study),

diluent

in terms of dimerization constant () and partitioncoefficient () can be represented as [3]

diluent

= + 2

2

[HC] . (10)

To estimate the values of physical extraction parameters (and), the plots ofdiluent

versus [HC] can be fitted linearlyand value of from the intercept and from the slope canbe obtained.

(2) Chemical Extraction. The interaction of acid moleculewith the extractant molecule in the chemical extraction canoccur in two ways: (i) through hydrogen bonding of undisso-ciated acid molecule, (11) and (ii) by ion pair formation, (12)[73]:

HC + S (HC) (S) (11)

H+ + C + S H+CS (12)

The extractionmechanism described in (11) and (12) dependson the pH of aqueous solution, p

of acid, the acid and

extractant concentrations, and the basicity of the extractantwith respect to the acid (p

). The extraction of carboxylic

acid by phosphorous based extractants (TBP, TOPO, etc.) canbe described by (11) and that for amine based extractants(TOA, Aliquat 336, etc.) by both mechanisms (11) and (12).

An equilibrium extraction process is described as a set ofreactions (see (13)) between molecules of acid (HC) and molecules of extractant (S) to form various ( : ) complexeswith corresponding apparent equilibrium constant (

) as

given by (14):

HC + S (HC)()

(13)

=

[(HC)()

]

[HC] [S]. (14)

The distribution coefficient () can be written as

=

HCHC=

[(HC)()

]

HC.

(15)

Substituting the values of [HC] and [(HC)()

] from (4) and(15), respectively, in (14) results in

=

(1 +

/ [H+])

1

HC [S]. (16)

The equilibrium free extractant concentration ([S]) in theorganic phase is represented as

[S] = [S]in [(HC) (S)] (17)

[S] = [S]in

HC

.(18)


Now, putting the value of [S] from (18) in (16), (19) is derived:

=

([S]

in

HC

)

HC1

(1 +

/ [H+])

. (19)

The values of equilibrium extraction constant () and the

stoichiometry (, ) of the reactive extraction are determinedby minimizing the error between the experimental andpredicted values of

.

The equilibriummodel for the simultaneous formation ofvarious types of acid-extractant complexes (1 : 1, 2 : 1, 3 : 1, 1 : 2,etc.) can be represented by a system of equations (see (20)to (23)) depending upon the type of the acid, extractant, anddiluent and their concentrations used in the experiment.

HC + S (HC) (S) (20)

HC + (HC) (S) (HC)2(S) (21)

HC + (HC)2(S) (HC)

3(S) (22)

(HC) (S) + S (HC) (S)2

(23)

The corresponding extraction constants (11, 21, 31

and

12) are calculated using (24) to (27):

11=

C11(1 +

/ [H+])

HC [S](24)

21=

C21(1 +

/ [H+])

HCC11(25)

31=

C31(1 +

/ [H+])

HCC21(26)

12=

C12

C11[S]. (27)

C11, C21, C31, and C

12are the concentrations of 1 : 1, 2 : 1, 3 : 1,

and 1 : 2 complexes, respectively, in the organic phase. HCand [S] are given by (28) and (29), respectively:

HC = C11 + 2C21 + 3C31 + C12

=

11[S] HC

[1 +

/ [H+]]

+

2

11

21[S] 2HC

[1 +

/ [H+]]2

+

3

11

21

31[S] 3HC

[1 +

/ [H+]]3

+

11

12[S]2

HC

[1 +

/ [H+]]

(28)

[S] = [S]in (C11+ C21+ C31+ 2C12)

= [S]in (

11[S] HC

[1 +

/ [H+]]

+

11

21[S] 2HC

[1 +

/ [H+]]2

+

11

21

31[S] 3HC

[1 +

/ [H+]]3

+

2

11

12[S]2

HC

[1 +

/ [H+]]

) .

(29)

Using experimental results and by applying the mass actionlaw, the values of the individual equilibrium constants (

11,

21, 31and

12) and the concentration of complexes (C

11,

C21, C31, and C

12) can be estimated. An objective function

can be defined to determine individual equilibrium constantsbased on experimental values of HC.

Now, amodel based on the loading ratio () for formationof various types of complexes (1 : 1, 2 : 1, etc.) between acidand extractant can also be described. The extent to whichthe organic phase (extractant and diluent) may be loadedwith acid is expressed by the loading ratio. It is a ratio ofacid concentration in the organic phase at equilibrium to theinitial extractant concentration in the organic phase ([S]in):

=

HC

[S]in

(30)

The value of depends on the extractability of the acid(strength of the acid-base interaction) and its aqueous con-centration and the stoichiometry of the overall extractionequilibrium. It is found that when the organic phase is nothighly concentrated by acid, that is, at very low loading ratios( < 0.5), 1 : 1 acid-extractant complex is formed. A plot of/(1 ) versus [HC] yields a straight line passing throughorigin with a slope of complexation constant (

11) as given

by (31):

1

=

11[HC] . (31)

If the carboxylic acid concentration is high enough (at least > 0.5), this relation can be expressed as shown by

=

1[HC] . (32)

To account for the extraction only by extractant, a term isdefined for the distribution of acid by chemical extraction(chem

) as

chem

=

HC ]diluentHC

HC,

(33)

where ] is the volume fraction of diluent in the organic phase.Therefore, the overall distribution coefficient (total

) byphysical and chemical extraction is obtained by adding (9)and (33).

total

= ]diluent

+

chem

(34)


In general the diluent alone also solvates some amount ofsolute (acid) from aqueous solution by physical extractionwhich is described in the previous section.Therefore, in sucha case, expression for [(HC)

(S)] is represented as

[(HC)(S)] = HC ]

diluentHC . (35)

Therefore, (35) could be obtained by including the term forphysical extraction and rewriting (34) as

[HC] =

HC ]diluentHC

[[S]in (HC ]

diluentHC )]

. (36)

2.1.2. Linear Solvation Energy Relation (LSER). A linearsolvation energy relationship (LSER) approach has beenintroduced by Kamlet et al. [74] in 1983 and then improvedby Abraham [75]. It characterizes solvation effects in termsof nonspecific and H-bonding interactions. Thus, a solvationproperty of interest () for an organic solute is modeledby a linear solvation energy relationship of the form [76]

=

0

+ (

+ ) + + + H + , (37)

where H is the Hildebrands solubility parameter, a measureof the solvent-solvent interactions that are interrupted increating a cavity for the solute. , an index of solventdipolarity or polarizability, measures the ability of the solventto stabilize a charge or a dipole by virtue of its dielectric effect., a polarizability correction term, equals 0.0 for nonchlo-rinated aliphatic solvents, 0.5 for polychlorinated aliphatics,and 1.0 for aromatic solvents. Solvatochromic parameter ,representing scale of solvent HBD (hydrogen-bond donor)acidities, describes the ability of solvent to donate a protonin a solvent-to-solute H-bond. , scale of HBA (hydrogen-bond acceptor) basicities, provides a measure of the solventsability to accept a proton (donate an electron pair) in a solute-to-solvent H-bond.The parameter, a measure of coordinatecovalency, equals 0.20 for P=O bases, 0.0 for C=O, S=O,andN=Obases, 0.20 for single-bonded oxygen bases, 0.60 forpyridine bases, and 1.00 for 3-hybridized amine bases. Thecoefficients , , , , , , and are the regression coefficientsthat measure the relative susceptibilities of to indicatedsolvent property scale. Equation (37) is adopted to describethe effect of diluents on the values of distribution coefficients():

log10

= log

10

0

+ (

+ ) + + + H + ,

(38)

where the parameters , , , and refer to the diluents,and

0 represents the distribution coefficient for an idealdiluent. The fifth term of (38), which contains the solubilityparameter H, does not affect the values of the objectivefunction (log

10

) significantly and the value of = 0 is

considered for the diluents used in this study. Thus, (38)results in

log10

= log

10

0

+ (

+ ) + + .(39)

Equation (39) can be adopted to describe the effect ofdiluents on the values of distribution coefficients (

). In

case, a mixture of diluents is used with the extractant,the solvatochromic parameters of the solvent mixtures arecalculated as [77]

12=

1

1+ (1

1)

2, (40)

where 1is the mole fraction of the first solvent and

2=

1

1is the mole fraction of the second solvent.

1is

the solvatochromic parameter of the first solvent and 2

is the solvatochromic parameter of the second solvent insolvent mixtures.The solvatochromic parameters of differentdiluents can be found in the study by Kamlet et al. [74].

2.2. Kinetic Studies on Reactive Extraction of Carboxylic Acids.Kinetic studies are equally essential as equilibrium studies forthe complete design of a reactive extraction unit. Lewis typestirred cell [78], cylindrical stirring vessel with highly agitatedsystem [50], stirred cell with a microporous hydrophobicmembrane [79], and so forth were used to perform thekinetic studies on the reactive extraction of various carboxylicacids. In these studies, the investigators have describedand analyzed the kinetic mechanism of reactive extractionusing formal elementary kinetic model, a mechanism ofreactions of acid-amine complexes [50], theory of extractionaccompanied by a chemical reaction [79], and so forth. Theestimation of the intrinsic kinetic parameters such as rateconstants (forward and backward) and reaction order werealso carried out using experimental data. According to theirfindings, the reaction between the acid and the extractantnot only depends on the composition of the organic andaqueous phases, it also depends on the hydrodynamic param-eters (volume ratio of phases, interfacial area and speed ofagitation) of the system which also confirmed the region ofthe (mass transfer controlled or reaction controlled) reaction.In a study by Jun et al. [80] in 2007, it was observed that thereaction rates were affected by pH and contamination presentin the aqueous phase. At a pH greater than the p

of acid,

more dissociation took place leading to the reduction in theextraction efficiency. Therefore, it was recommended that,to have an effective separation of acid from the productionmedia, the pH of the fermentation broth should be kept ata value less than the p

of the acid. Further, a brief review

of the kinetic studies on reactive extraction is summarized inTable 2 to have an overviewof the reactive kinetics of differentcarboxylic acids.

2.2.1. Kinetic Model. A comprehensive study on the theory ofextraction accompanied with chemical reaction in a stirredcell is proposed by Doraiswamy and Sharma [79] in 1984 todetermine the effect of chemical reaction on the specific rateof reaction. With the help of the film and renewal theorieswith physico-chemical and hydrodynamic parameters, theyhave classified the reactive system into four reaction regimes(very slow, slow, fast, and instantaneous) depending on theirrelative diffusion and reaction rates (Table 3).

The value of physical mass transfer coefficient () is

required to confirm the regime of reaction. This is obtained


Table2:Ex

tractant/dilu

entsystem

forthe

separatio

nof

carboxylicacidsb

yreactiv

eextraction:kinetic

studies.

SLnu

mber

Carboxylic

acid

Extractant

Dilu

ent

Parameters

Find

ings

References

1PenicillinG

AmberliteLA

-2Ke

rosene

Agitatio

nspeed,interfacialareab

etween

twoim

misc

iblesolutio

ns,pHof

aqueou

sph

ase,initialcarrier,andpenicillinG

concentration

Arateequatio

nforthe

masstransferw

asprop

osed

with

theforwardandbackward

rateconstantsa

s1

=1.6

4L3mol2

s1

and

2

=6.56105

Ls1

,respectively

[49]

2Citric

TOA

iso-decanoland

n-paraffin

Con

centratio

nsof

acid

andam

inea

ndspeedof

agitatio

nForm

alelem

entary

kinetic

mod

elprop

osed

andreactio

nkineticse

valuated

[50]

3PenicillinG

AmberliteLA

-2Ke

rosene

andn-bu

tyl

acetate

Aqueou

sand

organicp

hase

compo

sitions,pH,and

temperature

Thefractionalresistanceso

faqu

eous

layerd

iffusion,interfa

cialchem

ical

reactio

n,andorganiclayer

diffu

sionwere

quantitatively

determ

ined

[51]

4Tartaric

TIOA

iso-decanol,and

kerosene

Con

centratio

nsof

acid,amine,and

iso-decanol

Mod

ified

Langmuirisotherm

was

prop

osed

andkinetic

parameters

interpretedby

aformalele

mentary

kinetic

mod

el

[52]

5Lactic

Aliq

uat336

Oleylalcoho

l

Initiallactatea

ndextractant

concentrations

inextractio

n,initial

chlorid

e,andextractant-la

ctatec

omplex

concentrations

instrip

ping

Extractio

nandstr

ipping

kineticsw

ere

investigated,diffusionthroughthe

organic-ph

asefi

lmwas

determ

ined

asthe

rate-determiningste

p

[53]

6Ph

enylacetic

Alamine3

36Ke

rosene

andMIBK

Acid

andam

inec

oncentratio

n,volume

ratio

ofph

ases,and

stirrer

speed

Intrinsic

kineticsw

ered

escribed;the

reactio

nfoun

dto

bezero

orderin

Alamine3

36andfirstorderinacid

with

arateconstant

of0.9s1

[54]

7PenicillinG

AmberliteLA

-2Ke

rosene

Acid

andam

inec

oncentratio

n,andpH

Disp

ersedliq

uid-liq

uidextractio

nsyste

mprop

osed;D

anckwertand

Biot

nosu

sed

todeterm

iner

ates

tep

[55]


Table 3: Classical limiting regimes for irreversible reaction in a stirred cell.

Regime Description Hatta Number (Ha) Effect on the specific rate of extraction (molm2s1)

[HC]molL1

[]

molL1Stirrer speed(, rpm)

Volume ratio ofphases

1 Very slow1

[HC](+1)/2 [S]/2 None None

4 Instantaneous None []Increases withincrease in thespeed of stirring

None

by conducting physical extraction of acid from aqueous phasewith pure diluent. For a batch process a differential massbalance yields the following equation:

aqorg

=

(

org org) , (41)

whereis the interfacial area (m2);aq is the aqueous phase

volume (m3);org is the equilibriumacid concentration in theorganic phase. The time-dependent concentration of acid inthe organic phase is obtained by integrating (42) as

ln(

org

org org) =

org. (42)

A plot of ln(org/(

orgorg)) versus time () yields a straightline and the slope is used to evaluate physical mass transfercoefficient (

).

The reaction between acid and extractant is reversible.This problem of reversibility can be avoided by measuringthe initial specific rate of reaction which is governed onlyby the forward reaction. Therefore, in this study, the initialspecific rate of extraction, HC,0 (molm

2

s1) is calculatedfrom experimental data using the following equation:

HC,0 =org

(

HC,org

)

=0

. (43)

(HC,org/)=0 is the initial slope of curve which is arepresentation of the concentration in the organic phaseversus time (). The values of HC,0 are determined withvarious experimental conditions and used to determine theprobable effect of the important variables and to draw aninference on the appropriate kinetics of reactive extraction.In this regard, the effects of the speed of agitation ()and volume ratio of the phases (org/aq) on the initialspecific rate of extractionmust be examined to determine thereaction regime. Therefore, based on the guidelines providedby Doraiswamy and Sharma [79], the reactive extraction ofacid with extractant in diluent is governed by the followingequation:

HC,0 = [HC]

[S]

,

(44)

where and are the orders of the reaction with respect toacid and extractant, respectively, and

is the rate constant

of the reaction.For a (, ) reaction taking place in the organic phase

with a rate law shown in (44) and with a high excessof extractant, Hatta number () is given by a generalexpression as

=

(2/ (

+ 1))

[HC]

1

[S]

HC

.

(45)

HC is the diffusion coefficient of acid into diluent.The valueof HC is estimated using Wilke-Change (1955) and Reddy-Doraiswamy (1967) equations which are given by (46) and(47), respectively:

HC = 7.4 1012

(

acid)

0.6

(46)

HC = 1011

(

diluent

acid)

1/3

, (47)

where denotes the diluent association factor; signifies themolar volume of the component; is temperature (K); and represent molecular weight (kgkmol1) and viscosity(kgm1s1) of the diluent, respectively.

To determine the effect of reaction on the pure masstransfer of acid from the aqueous to the organic phase, theenhancement factor for the reactive extraction of acid iscalculated using

=

HC,0

org. (48)

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

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