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

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