Theoretical and Experimental Study of Lactic Acid Stripping from

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Ind. Eng. Chem. Res. 2010, 49, 8238–8243

Theoretical and Experimental Study of Lactic Acid Stripping from Loaded Organic Phase G. Kyuchoukov and D. Yankov* Institute of Chemical Engineering, Bulgarian Academy of Sciences, Acad. G. BontcheV Street, Block 103, 1113 Sofia, Bulgaria

The process of stripping lactic acid with different stripping agents was investigated for two organic-phase systems containing trioctylamine (TOA). On the basis of theoretical considerations for extraction mechanism, equations for stripping were developed. The theoretical values of the extent of stripping and the concentrations of lactic acid in organic and aqueous phases were compared with experimental values in the cases of water, NaOH, NaCl, HCl, and carbonates as the stripping agents. The possibilities of repeated use of the extractants in extraction/stripping runs were investigated. The best results were obtained with a system containing 20% TOA, 30% dodecane, and 50% decanol with (NH4)HCO3 as the stripping agent. Introduction Lactic acid (2-hydrooxypropionic acid) is an important chemical with wide use in the food industry (as a food additive and preservative), in the pharmaceutical industry, for the production of biodegradable polymers, or for environmentally friendly solvents (lactate esters). Usually, in an effort to obtain optically pure L-(+)-lactic acid, the acid is produced by fermentation. Lactic acid fermentation is a typical example of an end-product-inhibited process. During the fermentation, simultaneously with the product inhibition, the pH of the medium decreases (often outside the optimum range for the specific microorganism). These undesirable effects can be overcome by in situ removal of the product. Among the different alternatives for lactic acid recovery, reactive extraction is considered as the most promising one. Numerous articles on lactic acid extraction with different extractants have been published in the literature. However, relatively little attention has been paid to extractant regeneration (stripping), despite the importance of the feasibility of this process. The main requirements for a stripping agent are a high degree of stripping; fast separation of the phases; no formation of a third phase; and most important, preservation of a high extraction capacity of the extractant after stripping. To successively regenerate the extractant, it is necessary to alternate the conditions in an organic/water two-phase system between being favorable for the acid partition in the organic phase (extraction) and being favorable for the acid partition in the water phase (stripping). This can be achieved by changing an important parameter of the process such as temperature, diluent composition, or pH. In an early work on lactic acid extraction, Ratchford et al.1 attempted to recuperate lactic acid from the organic phase by simple steam distillation of the amine. They reported that the process was partially workable in the cases of triamyl- and tributylamine. In their experiments with didecyl-, trihexyl-, trioctyl-, and dibenzylamine, steam distillation resulted in excellent solvent recovery, but negligible amine distillate. Tamada and King2 pointed out two possibilities for regeneration of the acid-loaded organic phase: so-called temperatureswing regeneration (TSR) and diluent-swing regeneration (DSR). In a TSR scheme, the acid-loaded organic phase is contacted * To whom correspondence should be addressed. E-mail: yanpe@ bas.bg.

with aqueous phase at higher temperature than in the extraction run. The amount of stripped acid in the aqueous phase depends on the change in the extraction equilibrium and is very sensitive to the natures of the diluent and acid. In a DSR process, the composition of the acid-loaded organic phase is changed either by removing the diluent or by adding a second diluent, and then, modified organic phase is contacted with an aqueous phase for acid removal. Both processes are energy-consuming because of the need to heat the aqueous stream (TSR) or distill the diluent (DSR). An interesting variant of the stripping process can be realized in the case when the diluent has a lower boiling point than water. In this case, adding a second diluent provides the possibility of combining both TSR and DSR by simple distillation.3,4 Another possibility for the stripping of lactic acid can be realized by contacting the acid-loaded organic phase with basic aqueous solution (pH swing). Yabannavar and Wang5 re-extracted lactic acid with 2 N NaOH at a phase ratio of 1:10 (NaOH/solvent). The authors reported 100% recovery of the acid. Choudhury and Swaminathan6 used 4 N NaOH for lactic acid recovery from the solvent phase (TOA in methyl isobutyl ketone). They reported 84-100% recovery, depending on phase ratio. In the case of TOA, the stripping agents used were HCl and NaCl, but the recovery of lactic acid was poor for HCl in comparison with NaOH. Harington and Hossain7 performed re-extraction with salts (Na2CO3 and NaCl) and observed better lactic acid recovery with sodium carbonate, because of a greater driving force (larger pH difference between the organic and aqueous phases). The degree of re-extraction was found to depend on the concentration of sodium carbonate solution. Maisuria and Hossain8 studied the re-extraction of lactic acid from 10% trioctylamine in tributylphosphate with Na2CO3, NaOH, NaCl, and distilled water. The efficiency of the lactic acid recovery decreased in the same sequence. Increasing the sodium carbonate concentration from 0.1 to 2 M led to an increase in the degree of reextraction from 43% to 91%. To avoid the formation of salt byproduct and chemical consumption, Poole and King9 proposed the use of a watersoluble and volatile base (trimethylamine, TMA) for the backextraction of lactic acid in the aqueous phase, followed by thermal decomposition of the acid-base complex. The authors reported some difficulties due to the solubility of the lactic acid in water and its tendency to polymerize in concentrated

10.1021/ie100914n  2010 American Chemical Society Published on Web 07/28/2010

Ind. Eng. Chem. Res., Vol. 49, No. 17, 2010

solutions. Heating of the TMA/lactate solution at 101-120 °C and 300 mmHg for 28 h gave about 60% recovery of TMA, leaving behind a viscous aqueous solution. In an attempt to overcome this obstacle, Tung and King10 proposed the use of butyl alcohol for the esterification of TMA/lactate. The advantage of esterification is that, in the distillation step, the lactate ester can be recovered from the top of column as a pure product. For an initial butanol-to-lactate ratio of 2.5, the obtained conversion was about 70%. Ja¨rvinen et al.11 also used an aqueous solution of TMA for the back-extraction of lactic acid from clarified fermentation broth. Studying the possibilities for the intensification of lactic acid production, Wasewar et al.12 investigated the equilibria and kinetics of the back-extraction of lactic acid with TMA. Near-stoichiometric recovery of lactic acid from the amine extract was reported. By heating the TMA/ lactate aqueous solution at 100-120 °C and 200 mmHg, 99% recovery of TMA was achieved. Pai et al.13 examined the performance of reactive extraction cascades for the back-extraction of lactic acid with TMA. They studied the influence of the number of reactor-extractor stages, the cascade configuration, the phase ratio, the pH, and the Damko¨hler number. Yabannavar and Wang5 used HCl to displace lactic acid from 30% Alamine 336 in oleyl alcohol. They investigated the influence of the HCl concentration and volumetric phase ratio and concluded that a greater-than-stoichiometric amount of HCl was necessary to replace the lactic acid. The authors assumed that an excess of HCl was used for the saturation of the free amine. Depending on the process conditions, the degree of reextraction varied from 32% to 83%. Brief descriptions of all of the methods used for the stripping of some organic acid, along with their advantages and drawbacks, have been provided by Wasewar et al.,14 Joglekar et al.,15 and Keshav et al.16 Additional information on other methods for organic acid removal (hollow-fiber modules, liquid membranes, pertraction, etc.) can be found in the review article of Schlosser et al.17 It is worth mentioning that, for some organic acids, the kinetics of formation/decomposition of acid complexes can significantly influence the overall mass-transfer rate and should be taken into account in process modeling.17,18 Usually, published results have presented data from a single extraction/re-extraction step, and no data on the use of an extractant in multiple extraction/re-extraction process have been presented. The aim of the present work was, therefore, to study the applicability of different chemicals for the stripping of lactic acid and to elucidate the influence of the organic-phase composition, type of reagent, and number of stripping steps on the extraction capacity of the extractant.

Two different extraction systems were used. The first was composed of 20% TOA in decanol and the second of 20% TOA, 30% dodecane, and 50% decanol. The extraction experiments were carried out in 125 mL separatory funnels. Equal volumes (25 mL) of aqueous phase containing lactic acid and organic phase were shaken for 15 min at ambient temperature on an IKA HS501 digital shaking machine (IKA Labortechnique). Our preliminary experiments showed that a maximum of 5 min provided sufficient time to attain equilibrium. After phase separation, the volume of the phases and the pH value of the aqueous phase were measured. The stripping experiments were carried out as follows: An organic phase that had already been loaded with lactic acid was brought into contact with a water solution (with the desired concentration) of stripping agent at a phase volume ratio of 1:1. In some cases, the acid-loaded organic phase was used for multiple consecutive stripping runs (up to five), whereas in others, it was used for consecutive extraction/stripping cycles (up to three). As stripping agents, boiled distilled water and solutions of HCl, NaCl, NaOH, Na2CO3, NaHCO3, (NH4)2CO3, and (NH4)HCO3 were used. After phase separation, the volumes of the phases and the corresponding pH values were determined, and the concentrations of chlorides, carbonates, and hydrogen carbonates were measured by potentiometric titration. Pure (98%) crystalline L-(+)-lactic acid (Sigma) was used as the standard for HPLC analyses. The lactic acid concentration of the aqueous phase (after the extraction or stripping step) was determined by HPLC, and the corresponding concentrations in the organic phase were calculated by mass balance. The HPLC system was composed of a pump (Knauer), a refractive-index (RI) detector (Perkin-Elmer), and the chromatographic software EuroChrom (Knauer). The column used was Aminex HPX-87H (Bio-Rad). As a mobile phase, 0.005 M H2SO4 solution was used at a flow rate of 0.6 mL/min. Theoretical Section In what follows, molar concentrations are used to describe the extraction and stripping equilibria by mass action law. For a precise thermodynamic description of the liquid-liquid equilibrium involved in the extraction of carboxylic acids, one can refer to the work of Maurer’s group, starting with a 2006 review article.19 In our opinion, to realize an effective stripping of the organic acid from the extract, the real mechanism of extraction must be considered. TOA is a Lewis base, and it interacts with organic acid through a reversible reaction. In the case of lactic acid, which is a monocarboxylic acid, interactions might occur (a) with the undissociated form of the acid E¯ + HL T E:HL

Materials and Methods The tertiary amine tri-n-octylamine (TOA; Fluka) was used as the extractant. The organic phase was composed of TOA dissolved in dodecane and/or n-decanol (both from Aldrich). A stock solution of lactic acid was prepared from 90% L-(+)lactic acid (Acros Organics). Because of the presence of dimers of the acid in concentrated lactic acid solutions (about 25% of the total concentration), a 10-fold-diluted solution was boiled under reflux for 8-10 h to achieve hydrolysis of the dimers. The presence or absence of dimers was determined by highperformance liquid chromatography (HPLC). The resulting solution, containing 100-120 g/L lactic acid, was used to prepare the aqueous phases (about 10 g/L lactic acid) for the extraction experiments.

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(1)

with the extraction constant (HL) KE(EHL) )

[E:HL] [E:HL] ) [E¯][HL] [E¯](1 - R)[CHL]

(2)

or (b) with the dissociated form of the acid E¯ + H+ + L- T EH+L-

(3)

with the corresponding extraction constant +

(H ) ) KE(EHL)

[EH+L-] [EH+L-] ) + [E¯][H ][L ] [E¯][H+]R[CHL]

(4)

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The presence of the interaction product with H-bond and/or ionpair formation might be detected by IR analyses of the organic phase. However, on the basis of the experimental results, it is not possible to reveal the mechanism of extraction,20 because, taking eqs 2 and 4 into account, the relation between the two extraction constants is +

) (HL) ) K(H KE(EHL) E(EHL)Ka

(5)

where Ka )([H+][L-])/[HL] is the dissociation constant of the acid, E represents the extractant, HL represents undissociated acid molecules, E:HL represents the interaction product with a H bond, EH+L- represents the interaction product with ionpair formation, H+ represents hydrogen ion, L- represents lactate anion, R represents the fraction of dissociated acid molecules, CHL ) HL + L-, square brackets indicate molar concentrations, and overbars indicate the organic phase. From eqs 1 and 4, it is obvious that, in the absence of other cations, the equilibrium acid concentration depends on the (H+) corresponding extraction constant [K(HL) E(EHL) or KE(EHL)], the total (initial) concentration of extractant, the initial concentration of j /V, because r[EHL] ) [CHL]in acid, and the phase ratio r ) V j j - [CHL], [E] ) [E]tot - [EHL], EHL ) E:HL + EH+L-, and R depends on [CHL] andKa {R ) [L-]/[CHL] ) Ka/(Ka + [H+])}. Therefore, according to eq 2 or 4 for the calculation of stripped organic acid after back-extraction, the value of corresponding extraction constant must be known. In the absence of other cations (treatment with water), there are three opportunities to effectuate the back-extraction: (i) without changing the temperature when the extraction constant is evaluated, (ii) changing the value of the extraction constant by changing the temperature, and (iii) changing the organicphase composition. The other way to strip the organic acid is to use an aqueous phase containing other anions aside from the anion of the stripped acid. There are two possibilities for this purpose: (i) the aqueous phase contains a substance with basic properties or (ii) the aqueous phase contains substances or their anions that, in the presence of hydrogen ion, are strongly extracted from the organic phase, such as hydrochloric acid or chloride salt. In the former case, decomposition of the interaction product occurs according to eq 3 because of the very low concentration of hydrogen ion in the aqueous phase. The latter case is more complicated. The process description as the exchange of anions EHL + Cl- T EHCl + L-

(6)

with extraction constant KE(EHCl/EHL) )

[EHCl][L-] [EHL][Cl-]

(7)

where Cl- represents chloride anion and EHCl is the corresponding interaction product in the organic phase, is not correct because it does not present the true extraction mechanism. The extraction of anions with an amine always demands the presence of hydrogen ion, or the extraction of chloride anion must be presented, as in the equation E¯ + H+ + Cl- T EH+Clwith extraction constant

(8)

(H+)

KE(EH+Cl-) )

[EH+Cl-] [E¯][H+][Cl-]

(9)

Both extraction constants (see eqs 4 and 9) express equilibrium in the presence of H+. The extraction constant KE(EH+Cl-/EHL) is the ratio between them, but it does not take into account the concentration of H+. Consequently, the concentration of H+ at equilibrium must be equal to the initial concentration ([H+] ) [H+]in), which is not observed in extraction experiments. Two examples are examined here: If the organic phase charged with EHL is treated with water containing only NaCl, there is no extraction of Cl- by the free part of extractant because of the very low concentration of H+, but there is the condition for the stripping of L- and H+. An increase of the H+ concentration ensures the extraction of Cl- up to equilibrium. At equilibrium, the H+ concentration reached according to the mass balance is [H+] ) [L-] + [Cl-] - [Cl-]in

(10)

The concentration of OH- was omitted, because it is very small in comparison to the concentrations of other species. Taking into account eqs 10, 4, and 9, it is not a problem to +) calculate the values of [H+], [L-], and [Cl-] because K(H E(EHL), (H+) j ]tot, and [Cl ]in are previously known. In KE(EH+Cl-), [EHL]in, [E addition, according to the material balance, r[EHCl] ) [Cl-]in j ] ) [E j ]tot - [Cl-], r[EHL] ) r[EHL]in - [CHL], and [E [EHCl] - [EHL]. When chloride salt is used, the stripping of the organic acid will depend on its initial concentration and on the free extractant concentration. In this case, independently of any increase in the chloride ion concentration, the free extractant concentration cannot be lower than the initial extractant concentration because there is essentially no additional concentration of hydrogen ions. If hydrochloric acid is used, then according to the mass balance, the value of H+ concentration reached will be [H+] ) [L-] + [Cl-]

(11)

It is obvious that the necessary condition to strip the organic acid is [Cl-]in g r[RHL]in. When the stripping is carried out with HCl, the necessary and sufficient condition will be [HCl] g r[E]tot, because the HCl is strongly extracted by the free extractant. In this case, the concentration of free extractant could be lower than the initial concentration. In both cases (stripping with HCl and stripping with neutral chloride salt), with an increase of chloride ions, the extent of stripping will increase, but with an increase in the initial free extractant concentration, the extent of stripping will decrease. What are the differences? In the case of stripping with neutral chloride salt r[EHCl] < r([EHL]in -[EHL]) ) CHL, independently of its initial concentration. In the case of stripping with HCl, when [Cl]in > r[EHL]in, then r[EHCl] > r([EHL]in [EHL]) ) CHL. Results and Discussion As mentioned in Materials and Methods, several methods were used to strip lactic acid from the organic phase. Stripping by Means of Distilled Water. The results obtained for both compositions of the organic phase are shown in Figure 1, where the equilibrium concentrations of lactic acid in the organic and aqueous phases are presented as functions of the corresponding run number. The theoretical lines were calculated according to eq 2. For this purpose, the extraction constant

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Figure 3. Lactic acid concentrations and extent of stripping for stripping with NaCl. Figure 1. Changes in lactic acid concentration during consecutive stripping runs with water.

Figure 4. Equilibrium lactic acid concentrations in the water phase for consecutive extraction (line 2)/stripping with NaCl (line 1) runs. Figure 2. Extent of stripping during consecutive stripping runs with water. (HL) [KE(EHL) ] was calculated from the data for the extraction stage. Its values for the first and second compositions were 69.88 and 23.48 L/mol, respectively. As could be supposed, according to the Theoretical Section, the stripping of acid depends on the extraction constant. The stripping is better when the extraction constant is lower. Figure 2 displays the total extent of stripping and the stripping for each run for both compositions of the organic phase. After the sixth run, the total extent of stripping reaches a value of 0.445 (theoretical value ) 0.465) for the second system and 0.198 (theoretical value ) 0.214) for the first. The extent of stripping for each run (corresponding stripping stage) decreases with increasing run number because of the increase in the free extractant concentration. This decrease is more pronounced for the second system (lower extraction constant). Having in mind the theoretically obtained influence of the extraction constant on the extent of stripping, it is worth mentioning the good coincidence between the theoretical predictions and our experimental results. Stripping by Means of Chloride Salt. These experiments were performed with the second extraction system. The organic phase was initially charged with 9.136 g/L (0.1015 mol/L) lactic acid. It was treated with an aqueous phase containing 0.5257 mol/L NaCl at a volume ratio between the two phases equal to 1. The points in Figure 3 show the concentration of lactic acid

in the aqueous phase and the total extent of stripping as functions of the stripping run. They fit very well with the theoretical curves calculated on the base of extraction constants for lactic acid +) (H+) ] and for hydrochloric acid [K(H [KE(EHL) E(EHCl)]. The value of the extraction constant for the dissociated form of lactic acid is 170145 L/mol and was calculated according to eq 5, taking into account the dissociation constant of lactic acid (Ka ) 1.38 × 10-4) and the extraction constant for the undissociated form of lactic acid [K(HL) E(EHL) ) 23.48 L/mol]. The value of the extraction constant for hydrochloric acid is about 3 × 105 and was determined on the stripping stage with NaCl. At this initial concentration of chloride ions in the aqueous phase, the total extents of extraction (Rtot) after the first, second, and third runs were 0.8968, 0.9988, and 0.9999, respectively. This means that two stages are sufficient for the total stripping of lactic acid. Here arises the question of what will happen to the extraction ability of the organic phase when it contains chloride ions. To elucidate this question, a few experiments were carried out. They comprise consecutive treatment of the organic phase (second system) with aqueous phase containing 10.72 ( 0.40 g/L lactic acid and then with a solution of NaCl (0.5257 g/L). The experimental results are shown in Figure 4. Theoretical curves 1 and 2 represent the concentration of lactic acid in the aqueous phase at equilibrium after the extraction and stripping stages, respectively. They were calculated taking into account the initial concentrations of extractant, lactic, and chloride anions in the aqueous phase and their extraction constants. What is

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Figure 5. Degree of extraction and stripping with HCl.

the reason for the increase of lactic acid in the aqueous phase after extraction and the decrease of its concentration after stripping? It is the decrease of the free extractant concentration resulting from the increase in the concentration of amine hydrochloride. Therefore, the organic phase after stripping must be regenerated by means of aqueous solution with basic property. The same problem occurred when the stripping was carried out with hydrochloric acid solution. The extent of stripping was very high (more then 90%), but the extraction ability of the organic phase decreased significantly when it was not regenerated. The concentration of amine hydrochloride was higher than after stripping with chloride salt because of the higher concentration of hydrogen ions (Figure 5). Stripping by Means of Aqueous Solution with Basic Property. As mentioned in Materials and Methods, various agents were used, including sodium hydroxide or sodium and ammonium carbonates or hydrogen carbonates. In this case, it is clear that the quantity of stripping agent must be more than the stoichiometric requirement needed for the neutralization of the organic acid. When carbonate or hydrogen carbonate was used, liberation of CO2 from the aqueous phase was observed, according to the reactions 2HL + CO32- T 2L- + H2CO3 T 2L- + H2O + CO2v (12) or HL + HCO3- T L- + H2CO3 T L- + H2O + CO2v (13) In the beginning, both extraction systems were tested. The first one (20% TOA and 80% decanol) showed better extraction properties but worse physicochemical ones concerning separation of the two-phase system. This was the reason to choose the second extraction system (20% TOA, 50% decanol, and 30% dodecane). Better results for the extraction and stripping stages were obtained when hydrogen carbonate was used. In this case, there was faster ( pKa1. 2. Regeneration and process considerations. Ind. Eng. Chem. Res. 1994, 33, 3224–3229. (11) Ja¨rvinen, M.; Myllykoski, L.; Keiski, R.; Sohlo, J. Separation of lactic acid from fermented broth by reactive extraction. Bioseparation 2000, 9, 163–166. (12) Wasewar, K. L.; Heesink, A. B. M.; Versteeg, G. F.; Pangarkar, V. G. Intensification of conversion of glucose to lactic acid: Equilibria and kinetics for back extraction of lactic acid using trimethylamine. Chem. Eng. Sci. 2004, 59, 2315–2320. (13) Pai, R. A.; Doherty, M. F.; Malone, M. F. Design of reactive extraction systems for bioproduct recovery. AIChE J. 2002, 48, 514–526. (14) Wasewar, K. L.; Yawalkar, A. A.; Moulijn, J. A.; Pangarkar, V. G. Fermentation of Glucose to Lactic Acid Coupled with Reactive Extraction: A Review. Ind. Eng. Chem. Res. 2004, 43, 5969–5982. (15) Joglekar, H. G.; Rahman, I.; Babu, S.; Kulkarni, B. D.; Joshi, A. Comparative assessment of downstream processing options for lactic acid. Sep. Purif. Technol. 2006, 52, 1–17. (16) Keshav, A.; Wasewar, K. L. Back extraction of propionic acid from loaded organic phase. Chem. Eng. Sci. 2010, 65, 2751–2757. (17) Schlosser, S.; Kertesz, Martak, J. Recovery and separation of organic acids by membrane-based solvent extraction and pertraction: An overview with a case study on recovery of MPCA. Sep. Purif. Technol. 2005, 41, 237–266. (18) Martak, J.; Schlosser, S.; Vlckova, S. Pertraction of lactic acid through supported liquid membranes containing phosphonium ionic liquid. J. Membr. Sci. 2008, 318, 298–310. (19) Maurer, G. Modeling the liquid-liquid equilibrium for the recovery of carboxylic acids from aqueous solutions. Fluid Phase Equilib. 2006, 241, 86–95. (20) Yankov, D.; Molinier, J.; Kyuchoukov, G. Extraction of tartaric acid by trioctylamine. Bulg. Chem. Commun. 1999, 31 (3/4), 446–456.

ReceiVed for reView April 19, 2010 ReVised manuscript receiVed July 8, 2010 Accepted July 12, 2010 IE100914N