Equilibria and kinetics for reactive extraction of lactic acid ... - ITM

Journal of Chemical Technology andBiotechnology J Chem Technol Biotechnol 77:1068±1075 (online: 2002)DOI: 10.1002/jctb.680Equilibria and kinetics for reactive extractionof lactic acid using Alamine 336 in decanolKailas L Wasewar, 1 A Bert M Heesink, 2 Geert F Versteeg 2 andVishwas G Pangarkar 1 *1 Mumbai University Institute of Chemical Technology, Matunga, Mumbai - 400 019, India2 Department of Chemical Engineering, University of Twente, 7500 AE, Enschede, The NetherlandsAbstract: Lactic acid is an important commercial product and extracting this from aqueous solution isa growing requirement in fermentation-based industries. The design of an amine extraction processrequires (i) equilibrium and (ii) kinetic data for the acid±amine (solvent) system used. The equilibriumcomplexation constants for ratios of (1:1) and (2:1) have been estimated. The kinetics of extraction oflactic acid by Alamine 336 in decanol has also been determined. The reaction between lactic acid andAlamine 336 in decanol in a stirred cell falls in Regime 3, ie extraction accompanied by a fast chemicalreaction occurring in the diffusion ®lm. The reaction has been found to be zero order in Alamine 336and ®rst order in lactic acid with a rate constant of 0.21s 1 . These data will be useful in the design ofextraction processes.# 2002 Society of Chemical IndustryKeywords: lactic acid; reactive extraction; Alamine 336; decanol; equilibria; kineticsNOTATIONA CCross-sectional area of stirred cell (m 2 )[B] Free Alamine concentration in the organicphase (kmol m 3 )[BHL] (1:1) Lactic acid±Alamine complexconcentration in organic phase (kmol m 3 )D ADiffusivity of solute A (lactic acid) insolvent (decanol) (m 2 s 1 )K E1(1:1) Lactic acid±Alamine equilibriumcomplexation constant (m 3 kmol1 )K E2(2:1) Lactic acid±Alamine equilibriumcomplexation constant ((m 3 kmol1 ) 2 )K E3(3:1) Lactic acid±Alamine equilibriumcomplexation constant ((m 3 kmol1 ) 3 )[HL] Lactic acid concentration (kmol m 3 )k 1First order rate constant (s 1 )k LMass transfer coef®cient (m s 1 )k mnRate constant for a reaction that is mthorder in species A and nth order in species BK DDistribution coef®cient (dimensionless)N Speed of agitation (rev s 1 )R ASpeci®c rate of extraction of lactic acid(kmol m 2 s 1 )t Time of extraction (s)V Phase volume (m 3 )z Loading ratio (kmol lactic acidkmolamine 1 )Subscriptsaq Aqueous phaseorgTOrganic phaseTotalSuperscript* Equilibrium1 INTRODUCTIONLactic acid is a commodity chemical utilized in thefood, chemical and pharmaceutical industries. Lacticacid is an important chemical, which can be convertedto ethanol, propylene glycol, acrylic polymers andpolyesters. In particular, an increasingly interestingapplication is the use of lactic acid as a monomer forthe synthesis of biodegradable homopolymers andcopolymers. 1,2 Lactic acid is the raw material for theproduction of biodegradable polylactic acid. A growingdemand for biodegradable polymers, both assubstitutes for conventional plastic materials and fornew materials for speci®c uses such as controlled drugdelivery or arti®cial prostheses, draws attention to theneed to improve conventional processes for lactic acidproduction. 3Lactic acid can be produced by the fermentation ofbiomass. However, the fermentation broth has a lowconcentration (

Reactive extraction of lactic acidprocesses, lactic acid has been recovered from thefermentation broth by precipitation of calcium lactatewith calcium hydroxide. In this, the separation and®nal puri®cation stages account for up to 50% of theproduction costs. 4,5 Thus, this method of recovery isexpensive 6,7 and unfriendly to the environment as itconsumes lime and sulfuric acid and also produces alarge quantity of calcium sulfate sludge as solid waste. 6Allowing accumulation of lactic acid product infermentation broth inhibits further product formation.Reactor productivities are low and the products areobtained in a dilute form. The effects of end productinhibition can be reduced by in-situ removal of lacticacid from fermentation broth by several methods.A number of processes for lactic acid recovery fromfermentation broth without precipitation have beenstudied: solvent extraction, 8±15 membrane bioreactor,16,17 liquid surfactant membrane extraction, 18,19adsorption, 20 direct distillation, 21 electrodialysis, 22±24chromatographic methods, 15 ultra®ltration, 15 reverseosmosis, 15,25 drying, 15 etc.Reactive extraction with a speci®ed extractant givinga higher distribution coef®cient has been proposed as apromising technique for the recovery of carboxylic andhydroxycarboxylic acids. 8,26 Solvent extraction withtertiary amines has been widely used to recover/fractionate metals such as uranium, iron, cobalt, etcfrom aqueous solutions. 27 Long chain tertiary amineswere used commercially for recovery of ethanoic acidfrom dilute aqueous streams. 14,28 Tertiary aminesoffer advantages over other extractants, on thegrounds of lower cost and generally higher equilibriumdistribution coef®cients (K D). 29 Tertiary amines werefound to be effective for extracting lactic acid, andalcohols were among the best diluents, with theadditional advantage that a lactic acid ester could beproduced after the extraction process was completed.30,31 Alamine 336 (mixture of C 8,C 9and C 10tertiary amines) yields a good combination of high K D,low solubility in water and good regenerability.The design of an amine extraction process requires(i) equilibrium and (ii) kinetic data for the acid±amine(solvent) system used. However, little information onthe equilibria and no information pertaining to kineticsare available. In view of this it was thought desirable toobtain the equilibria and kinetics for the extraction oflactic acid by Alamine 336 (a tertiary amine, withaliphatic chains of 8±10 carbon groups) dissolved indecanol, as diluent.range (0.005±1.5kmol m 3 ) of lactic acid concentrationwas used.The reactive component was Alamine 336 (straightchain tertiary amine containing C 8±C 10alkyl groups(Henkel Corp, USA)) with decanol as diluent.Alamine was used as supplied.2.2 Methods2.2.1 EquilibriaAll experiments were carried out at room temperatureof 25°C. Known volumes of aqueous (different concentrationsof lactic acid) and organic phases (30cm 3each) (both pure decanol and decanol containingdifferent concentrations of Alamine 336) of knownconcentrations were equilibrated in a temperaturecontrolledshaker bath for 24h. The two phases wereallowed to settle for at least 30min, which wassuf®cient time for a complete phase separation. 32 Todetermine the concentration of lactic acid, the aqueousphase was titrated with NaOH using phenolphthaleinas indicator. The acid concentrations in the organicphase were calculated by mass balance.2.2.2 KineticsA stirred cell (Fig 1) of 0.07m in diameter and 0.1mheight, with a ¯at bottom was used for the kineticstudies. 33 A 100cm 3 sample of an aqueous solution oflactic acid of known concentration was ®rst placed inthe vessel. The position of the four-blade paddle(0.058m in diameter and 0.01m in width) doublestirrer was adjusted to 0.01m below and above theinterface. A ®xed volume (100cm 3 ) of the organicextractant mixture was then added, and the mixturewas stirred. Using acid±base titration with NaOH andphenolphthalein as indicator, the acid concentration inthe aqueous phase was determined periodically. Theconcentration of lactic acid in the organic phase wasdetermined by mass balance.The reproducibility was checked by carrying out2 MATERIALS AND METHODS2.1 MaterialsAll the chemicals used (lactic acid, decanol, sodiumhydroxide) were of reagent grade and were usedwithout pretreatment. All solutions of lactic acid wereprepared by dissolving lactic acid of analytical purity indistilled water. In practical situations of acid recoveryfrom fermentation broths, the acid concentrations arenot expected to be high. Hence, a low concentrationFigure 1. Stirred cell used for kinetic study.J Chem Technol Biotechnol 77:1068±1075 (online: 2002) 1069

KL Wasewar et alTable 1. Procedure for discerning reaction mechanism: stirred cellRegimeEffect on the speci®c rate of absorption (kmol m 2 s 1 )[A*] [B o] Speed of agitation Volume of liquid (B phase)1 a [A*] m a [B o] n None a volume2 a [A*] None Increases with increase in the speed of stirring None3 a [A*] (m‡1)2 a [B o] n/2 None None4None a a [B o] Increases with increase in the speed of stirring in the same manneras in Regime 2Nonea Provided that [B o]z[A*].Reproduced from Ref 33 with permission from John Wiley & Sons.duplicate experiments in some selected cases. Theresults were found to be reproducible within 5%.2.3 Theory of extraction accompanied by achemical reactionDoraiswamy and Sharma (1984) have given anexhaustive discussion on the theory of extractionaccompanied by a chemical reaction. 33 Four regimesof extraction accompanied by reaction have beenidenti®ed depending upon the physico-chemical andhydrodynamic parameters. When the reaction isreversible the solute has a ®nite equilibrium concentrationin the bulk and the driving force needs to bemodi®ed by incorporating the same. The extractioninvolves the partitioning of the solute available in theaqueous phase to the organic phase:A aq ! A orgSolute A present in the aqueous phase combineswith the organic reactant (amine) B according toA ‡ zB , ComplexTable 1 gives the guidelines for discerning themechanism. The hydrodynamic factors (as signi®ed bythe speed of agitation in a stirred cell) are unimportantin Regimes 1 and 3 whereas they do affect the rate ofextraction in Regimes 2 and 4. The expressions for therate of extraction for various regimes are given byDoraiswamy and Sharma. 33 The expression forRegime 3, extraction accompanied by a fast generalorder chemical reaction occurring in the diffusion ®lm,isrR A ˆ‰A * Š2m ‡ 1 D Ak mn ‰A * Š m1 ‰B 0 Š n…1†extraction. The pH was measured for various concentrationsof lactic acid and the following correlationhas been obtained for the above range of pH values:pH ˆ 1:90:26 ln‰HLŠThe physical equilibrium distribution isotherm wasmeasured at 25°C in decanol. The results are shown inFig 2. The regression equation for physical equilibriaobtained by a statistical analysis of the equilibriumdata is:‰HLŠ * org ˆ 0:13‰HLŠ aq0:05‰HLŠ 2 aq…2†For a low range of lactic acid concentration there is alinear relationship between lactic acid concentration inthe two phases, and a parabolic relationship for higherconcentrations. It may be argued that for low concentrationsof lactic acid a Henry's law type isotherm isvalid, whereas at higher concentrations non-idealbehavior can prevail, causing the deviation fromHenry's law. The chemical equilibrium distributionisotherms were measured at 25°C for Alamine 336concentrations of 20%, 30% and 40% (v/v) indecanol. The chemical equilibrium isotherms areshown in Fig 3. It was observed that equilibriumconcentration in the organic phase increased withincreases in Alamine 336 concentration. This showsthat the extraction ef®ciency increases with increasinglactic acid concentrations. San-Martin et al 1992, have3 RESULTS AND DISCUSSION3.1 Extraction equilibriaExperiments were carried out to describe the physicaland chemical equilibria for lactic acid. The pHchanges with the progress of extraction of lactic acid.The range of pH used for extraction experiments was1.8±3.4. Equilibrium pH is always higher than initialpH and its value depends on the extent of lactic acidFigure 2. Physical equilibria for extraction of lactic acid with decanol.1070 J Chem Technol Biotechnol 77:1068±1075 (online: 2002)

Reactive extraction of lactic acidTable 2. Distribution coefficient for lactic acid extractionwith various concentrations of Alamine 336 in decanol% Alamine 336 Distribution coef®cient, K D0 0.1320 12.5730 16.4440 23.37Figure 3. Chemical equilibrium isotherms for reactive extraction of lacticacid with various concentrations of Alamine 336 in decanol.reported that the equilibrium concentration increasedup to a concentration of 40% (v/v) of the Alamine 336and then remained constant. 32,34The reactive liquid±liquid extraction of lactic acid(HL) with the tertiary amine Alamine 336 (B) gives areaction complex (BHL) which remains in the organicphase and may be represented by:HL aq ‡ B org , BHL orgThe distribution coef®cient, K D, is de®ned by:K D ˆ‰HLŠ org=‰HLŠ aq…3†…4†The regression equations for chemical equilibria oflactic acid at low concentration for various Alamine336 concentrations in decanol obtained by a statisticalanalysis of the equilibrium data as shown in Fig 4 are:20% Alamine; ‰HLŠ * org ˆ 12:57‰HLŠ aq…5†30% Alamine; ‰HLŠ * org ˆ 16:44‰HLŠ aq…6†40% Alamine; ‰HLŠ * org ˆ 23:37‰HLŠ aq…7†Distribution coef®cients of lactic acid for low concentrationin Alamine concentration in decanol forvarious Alamine concentrations are given in Table 2.A quantitative interpretation of the equilibrium forthe acid±amine extraction may be made by de®ning anequilibrium complexation constant, K E, as:K E ˆ‰BHLŠ org=‰HLŠ aq‰BŠ org…8†The concentration of free amine in the organic phasewould be:‰BŠ org ˆ‰BŠ T‰BHLŠ org…9†If 1:1 lactic acid ± Alamine 336 complex is formedthen [HL] org=[BHL] org; andhence, log K D ˆ log K E ‡ log [B] org…10†If the former assumption is valid, a plot of log K Dversus log [B] orgshould yield a straight line with a slopeof unity. As shown in Fig 5 the slope is less than unity(0.54), which implies that the organic phase extractsmore acid than would be expected on the basis of a 1:1complex. Hence, the extraction equilibrium of thelactic acid is not adequately represented by eqn (3).Generally, the simple stoichiometric reaction (eqn(3)) is not suitable for describing the formation of acomplex of acid and amine molecules. Distributiondata can be interpreted by a set of equilibria involvingthe formation of complexes with n acid molecules andFigure 4. Equilibrium relationships (Distribution coefficients) for reactiveextraction of low concentrations of lactic acid with various concentrations ofAlamine 336 in decanol.Figure 5. Distribution coefficient of lactic acid in Alamine dissolved indecanol as a function of free amine concentration in organic phase.J Chem Technol Biotechnol 77:1068±1075 (online: 2002) 1071

KL Wasewar et alone amine molecule: 11,12HL aq ‡ B K 11org !HL aq ‡ BHL K 21org !BHL orgHL aq ‡ B(HL) K n1n 1;org !B(HL) 2;orgB(HL) n;org…11†…12†…13†The equilibrium complexation constant for thereaction represented by eqn (13) is:‰K En ˆ BHL(HL) nŠ org‰ BHLŠ org‰ HLŠ n …14†aqThe extent to which the organic phase (Alamine336‡decanol) can be loaded with lactic acid isexpressed as the loading ratio, 34 z:z ˆ ‰ HLŠorg…15†‰ BŠ TThe value of z depends on the extractability of theacid (strength of the acid±base interaction) and itsaqueous concentration, and is independent of theamine content in an inert diluent. 10±12The stoichiometry of the overall extraction reactiondepends on the loading ratio in the organic phase, z.Ifthe organic phase is not highly concentrated ie at verylow loading ratios (z

Reactive extraction of lactic acidTable 3. Concentration range of lactic acid in the aqueous phase at whichone of the complexes formed in the organic phase in equilibriumComplexof lactic acid is not high enough. This situation isfrequently encountered in fermentation processes forproduction of lactic acid.Figures 6 and 7 allow the determination of theconcentration range of lactic acid in the aqueous phaseat which one of the reactants is present in a higherproportion. The results are shown in Table 3.When the lactic acid concentration in the aqueousphase is within the range indicated in Table 3, theequilibrium concentration in the organic phase is highenough to form either the (1:1) or the (2:1) complexes.For the experimental conditions used in this work thelactic acid concentration is not high enough to allowformation of the (3:1) complex.3.2 Kinetics3.2.1 Physical Mass Transfer CoefficientThe value of the physical mass transfer coef®cient, k L,is required for con®rming the regime of extraction.This was obtained by conducting physical extraction(diluent only) of lactic acid from water. For a batchprocess a differential mass balance yields the eqn (19):d[HL] orgV aq ˆ k L A C [HL] * org [HL]dtorg …19†Here, [HL] * org is the equilibrium concentration in thediluent only. Integration of this equation yields:k L ˆ VaqA C tComplexation constant(1:1) 45.2 m 3 kmol(2:1) 18.1 (m 3 kmolZ ‰HLŠorg01d[HL] org …20†[HL] org[HL] * orgConcentration range(kmol m 3 )

KL Wasewar et alFigure 11. Effect of phase ratio on the specific rate of extraction of lacticacid with Alamine 336 in decanol (initial lactic acid concentration=0.53kmol m 3 ; Alamine concentration in organic phase=0.41kmol m 3 ;speed of agitation =1.07rev s 1 ).Figure 13. Effect of initial Alamine 336 concentration on specific rate ofextraction of lactic acid with Alamine 336 in decanol (initial lactic acidconcentration =0.54kmol m 3 ; speed of agitation =1.07rev s 1 ).speci®c rate of extraction, R A. A regression analysis ofthe data yielded m =1 (as per eqn (1)). Thus, thereaction is ®rst order with respect to lactic acid.3.2.3.2 Order with respect to Alamine 336. Figure 13shows a plot of the speci®c rate of extraction of lacticacid against initial Alamine 336 concentration in theorganic phase. Evidently there is no effect of Alamine336 concentration on the rate of extraction, indicatingthat the reaction is zero order in Alamine 336 (n =0 ineqn (1)).3.2.4 Rate constantFor m =1 and n =0, eqn (1), the rate expression for theinitial part of the extraction is reduced topR A ˆ‰HLŠ * orgD A k 1…22†The data were ®tted to eqn (22) to obtain the valueof the ®rst order rate constant, k 1, as 0.21s 1To con®rmpthat Regime 3 holds, the value of theparameter D A k 1 =k L was evaluated. 34 It was foundpthat for the range of k Lvalues (Fig 9) D A k 1 /k L8orgreater than 3 which is the condition for the validity ofRegime 3. Thus, the above mentioned results re¯ectthe intrinsic kinetics of the extraction process.Further, Alamine 336 is a surface active solvent.Hence, Alamine molecules may concentrate at theinterface, with nitrogen lone pairs pointing towards thewater. Lactic acid would be distributed evenlythroughout the aqueous phase and would diffusethrough that phase on a ®rst order basis. Therefore,there is possibility of a reaction at the interface that iszero order with respect to Alamine and ®rst order withrespect to lactic acid.However, in the present case the solubility of lacticacid in the organic phase is higher than that in water.On the other hand the organic amine has practicallynegligible solubility (

Reactive extraction of lactic acidThe theory of extraction accompanied by chemicalreaction has been used to obtain the kinetics ofextraction of lactic acid by Alamine 336 in decanol.The reaction between lactic acid and Alamine 336 indecanol in a stirred cell falls in Regime 3, extractionaccompanied by a fast chemical reaction occurring inthe diffusion ®lm. The reaction has been found to bezero order in Alamine 336 and ®rst order in lactic acidwith a rate constant of 0.21s 1 . These data will beuseful in the design of extraction processes.REFERENCES1 Buchta K, Lactic acid. Biotechnology 3:409±412 (1983).2 Datta R, Tsai SP, Bonsignore P, Moon SH and Frank JR,Technology and economic potential of poly(lactic acid) andlactic acid derivatives. FEMS Microbio Rev 16:221±231 (1995).3 Lipinsky ES and Sinclair S, Is lactic acid a commodity chemical?Chem Eng Prog 82:26±32 (1986).4 Chaudhuri JB and Pyle DL, Emulsion liquid membraneextraction of organic acids. 1. 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