Journal of Scientific & Industrial ResearchVol. 62, September 2003, pp 926-930

Extraction of Hydroxycarboxylic Acids with MIBKlfolueneSolutions of Amines

Ismail Inci" and Ahmet Aydm

Istanbul University, Faculty of Engineering, Chemical Engineering Department, 34850,Istanbul, Turkey

Received: 27 January 2003; accepted: 31 May 2003

Studies are reported on the extraction of glycolic, malic, and citric acids by aIamine 336 and tri-n-octylamine dissolvedin MIBKltoluene mixture (I : 1 volume ratio). The effects of amine type and concentration on extraction of hydroxycarboxy-lic acids by amines are investigated. Formation of acid amine complexes is a dominating factor in the system underconsideration .. The distribution coefficients and loading factors are calculated and reported.

Key words: Hydroxycarboxylic acid, MIBK, Toluene, Amines

Introduction

Hydroxycarboxylic acids occur in nature invarious fruits. The simple hydroxycarboxylic acid isglycolic acid. It occurs in sugar beets and unrapegrape. Therefore, glycolic acid is also known as acidof grape. Malic acid occurs in apple and therefore it isknown as apple acid. Citric acid is the most widelyused hydroxycarboxylic acid in foods andpharmaceuticals. Its palatability and easy access forassimilation have led to its utility as an acidingredient. Recently, it has become an importantindustrial raw material for use with many varied andexpanding applications, as in the cleaning andpolishing of iron and steel, as a component in certainnon-ferrous plating solutions, and in the treatmentand conditioning of industrial water supplies.Production by the mold fermentation of sugarsolutions, extraction from lemon and lime juice, andextraction from pineapple-scanning residuesconstitute the three important methods of commercialmanufacture of citric acid. The traditional method ofpreparing citric acid was by extraction from the juiceof acidulous fruits of certain citrus species such as,citrus medica, and citrus bergamia. The citric acidcontent of these juices varies from 5 to 8 per cent.

Sometimes, hydroxycarboxylic acids are foundin natural product in at the most 5-10 per cent

* Author for correspondence

strength. Therefore, extraction of hydroxycarboxylicacids from aqueous solutions is very important.Subsequent separation, purification and. concentrationof the acid recovery of hydroxycarboxylic acids fromfermentation broth or other mixtures makeseparations difficult because of high affinities of theacids for water distillation of dilute, non-volatile acidwhich involves large energy consumption for the heatof vaporisation of water which must be considered.Furthermore, distillation cannot fractionate amongnon-volatile acid.

The low aqueous activity of hydroxycarboxylicacids results in low distribution coefficients of acidinto conventional solvents. Thus, solvent extractionwith conventional solvents requires very high solventflow rates and result in substantial dilution of acid.

Aliphatic tertiary amines dissolved in an organicsolvent are powerful extractants for carboxylicacids!". The amine binds the acid in the organicphase through reversible complexation. Often, watertakes part in complex formation, thus having a stronginfluence on the liquid-liquid equilibriurnt'", .

It has been found that diluents, especially thosewith functional groups, can affect the extractionbehaviour of amine significantly. The stoichiometryof solute:amine complex, loading of amine, as well asthe third phase formation are influenced by thediluent. The effect of diluent can be understood in

INCI & AYDIN: EXTRACfION OF HYDROXYCARBOXYLIC ACIDS 927

tenns of ability to solvate to organic phase species, Therefore, it is necessary to distinguish between general solvation from electrostatic, dispersion or other forces and specific solvation due to hydrogen b d· n on mg · .

The extraction process can be described by the reactions,

HA + R3N = HA . (R3N),

HA +2 R3N = HA . (R3N2>'

The resulting acid amine complexes are stabilizied due to the hydrogen bonding with the diluent I4

.15.

The structure of acid:amine complexes in diluents was determined by Yerger and Barrow l6

.

Yerger and Barrow l? proposed that the first acid

interacts directly with the amine to fonn an ion pair and the OH of the carboxyl of the second acid fonns a hydrogen bond with the conjugated CO of the carboxylate of the first acid to fonn a complex .

Various researhers l.12 have studied the

extraction of different carboxylic acids by amines dissolved in organic solvents. But data for MIDK I toluene solvents mixture could not be obtained. In this study, experimental results for liquid-liquid equilibrium involved in the reactive extraction of glycolic, malic, and citric acid with tri-n-octylamine and alamine 336 dissolved in MIDK I toluene mixture at 25 °C, are presented.

Theoretical Approach

An equilibrium description of acid-amine system can be represented by a set of reactions of p acid A, molecules and q amine B, molecules to fonn various (p, q) complexes with corresponding equili­brium constants, Kpq.,rue.

pA + qB = ApE q. (IrR •• ... ( 1)

Kpq.,rue = [ApB q. orxJI( [AJ p [BI q) , ... (2)

where the species activities are given in by brackets. For practical application the activities of the organic phase species are assumed to be proportional to the concentrations of the species, with the constants of the proportionality considered in the equilibrium constant. The apparent equilibrium constant for the overall reaction can be written as:

... (3)

Here species concentrations are denoted by square brackets and are expressed in molar tenns. The loading of the extractant, Z, is defined as the total concentration of acid in the organic phase, divided by the total concentration of amine in organic phase l8

.

The expression for the loading, Z, can be derived from Eq. (2) and (3) in the fonn :

... (4)

Distribution coefficients for acid, extracted from water into organic phase, are determined as,

. .. (5)

Experimental Procedure

Tri-n-octylamine, glycolic, malic, and cltnc acids and solvents were procured from Merck Company. Alamine 336 a commercial product (Henkel Co.) was used a mixture of straight-chain tertiary amines with seven to nine carbon atoms per chain containing 2.75 mole/kg of active amines (M = 392 g/mole). All chemjcals were used as such .

Hydroxycarboxylic acids was dissolved in water to prepare the solutions with initial concentrations of acid 10 mass per cent. The initial organic phases were prepared by the dissolution tri-n-octylamine and alamine 336 in the diluents to produce solutions with approximately constant concentrations (1.80 molelL, 1.40 molelL, 1.10 mole/L, 0.70 mole/l" and 0.40 molelL, respectively) .

For extraction experiments, equal volumes of aqueous hydroxycarboxylic acids solution and an organic solution of amine were stirred in glass flask in a shaker bath at 25°C for 2 h. Thereafter the mixture was kept in a bath for another 4-6 h to complete phase separation .

The concentration of the acid in the aqueous phase was determined by titration with aqueous sodium hydroxide (relative uncertainty: I per cent)19.22. Acid analysis was checked against a material balance. In most cases the deviation between the amount of acid analyzed and the amount of acid known by preparing the solutions by weighing did not exceed 4 per cent The solubilities of amine salts and solvents in the aqueous phase were negligible in variables examined.

928 J SCI IND RES VOL 62 SEPTEMBER 2003

Results and Discussion

Table 1 gives experimental results. Theconcentrations of amines in solvents were between0.40 mole/L and 1.80 mole/L, The glycolic, malic,and citric acids concentration in the initial aqueousphase was 10 mass per cent.

Table I-Experimental results of the extraction of hydroxy-carboxylic acids with tri-n-octylamine and alamine 336 in

MIBKltoluene mixture

Diluent Carnine Z D(molelL)

Glycolic acid by TOA 1.80 0.74 8.45

1.40 0.81 4.09

1.10 0.69 1.81

0.70 0.65 0.52

0.40 1.08 0..46

Glycolic acid by A336 1.80 0.73 7.65

1.40 0.46 3.62

1.10 1.15 3.58

0.70 0.90 1.46

Malic acid by TOA 0.40 1.05 0.45

1,80 0.45 83.61

1,40 0,58 63.52

l,lO 0,73 14.51

0,70 1.02 11.05

0,40 1.13 1.34

Malic acid by A336 1.80 0.45 72.40

1.40 0.57 35.67

1.10 0.71 11.55

0.70 0.96 6.00

0.40 1.12 1.30

Citric acid by TOA 1.80 0.31 72.33

1.40 0 ..37 66.87

1.10 0.50 26.40

0.70 0.77 21.83

0.40 0.78 1.33

Citric acid by A336 1.80 0.32 57.26

1.40 0.36 44.59

1.10 0.46 33.44

0.70 0.69 12.53

P.40 0.58 1.22

The equilibrium data on the distribution ofglycolic, malic, and citric acids between water andaliphatic amines (tri-n-octylamine and alamine 336)dissolved in MmKltoluene (l: 1 volume ratio) arepresented in Table 1. It can be seen that from Figure 1that the extraction concentration of tri-n-octylamineand alamine 336 changes with increase in initialconcentration of amine in the organic phase.

According to Table 1 and Figure 1, extraction ofhydroxycarboxylic acids by tri-n-octylamine the orderwas, as fallow: Malic > Citric > Glycolic andextraction of hydroxycarboxylic acids by alamine 336the order was as fallow: Malic> Citric> Glycolic.

Extraction increases with increase in acidity andhidrophobicity of the acid. Acidity had a strongerinfluence than hidrophobicity in the acids studied inthe present work.

Figure 2 shows the effect of amineconcentration on loading. The loading curve is a plotof Z vs. amine concentration. Overloading (loadinggreater then unity), indicates that complexes withmore than one acid per amine have been formed.With gycolic, malic and citric acids, overloading canbe observed at low amine concentrations (Figure 2).

Systems that include the diluent, specifically inthe complex stoichiometry, show decrease in loading

~~============~----~-----,

70

.. -O .. Glycolic byA336

••• /!r •• MaI ic by 1\336

.. -0 .. Citric by A336

~MalicbyTOA

-,-Glycolic by TOA

-+-Cltric byTOA

c.!!! 60u,850co:g40.c

~~

20

10

o~--~~~~~~~~--~o 0,5 1,5· 2

OnlenIIatimofanire ~ IL)

Figure 1- Variation of distribution coefficients withconcentration of amine in MIBKltoluene mixture

INCI & AYDIN: EXTRACTION OF HYDROXYCARBOXYLIC ACIDS 929

1,6 ·· O ·rGl y col ic byA336 "~"M a l ic byA336

·· a ··Citr i c by A336 ----'- Malic by TOA

1,4 -*-Gl yco lic by TOA -'-Citr i c byTOA

1,2

0 tl 1 III U. til

.5 0,8 -a

3 0,6

0,4

0,2

0

0 0,5 1,5 2

Cow*<6rJctarrine (IT'd IL)

Figure 2 - Vari ati on of loading factors with concentration of amine in MIBKJtoluene mixture

with increase in amine concentration. With all of the solvents at amine extraction loading decreases, thereby indicating that complexes include the diluent specifically (Figure 2). Systems that exhibit aggregation, formation of complexes with large number of acid and amine molecules, and an abrupt increase in loading. Glycolic acid at tri-n-octylamine extraction ( Figure 2) exhibit abrupt increase at low amine concentrations, thereby indicating that complexes include large number of acid and amine molecu les.

For systems with on ly one amine per complex, there is no effect of total amine concentration on the loading. If there is more than one amine per complex, loading Increases with Increase In anune concentration.

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Abbreviations and Notations Used

A Acid

B Amine

A/IB".,,,!: Complex in organic phase

[ApB".or!:} = Activity of complex in organic phase

[A} Acti vity of acid

[B} Activity of amine

Bp" Apparent equilibrium constant

CA336 Concentrati on of alamine 336, (molelL)

C A Concentration of acid in aqueous phase

CllIn inf Concentration of amine in organic phase

CAI'Bq.orli Concentrati on of complex in organic phase

C A.org Concentrati on of acid in organic phase

C TOA Concentration of tri-n-octylamine, (molelL)

CB./O l Concentration of total amine in organic phase

D = Distribution coeffi cient

HA Hydroxycarboxylic acids

K plf.1ntt' = Equilibrium constant

MIBK Methyl isobutyl ketone

p = Number of acid molecules

R3N Tertiary amine

Z Loading factor