Esterification of Lactic Acid with n-Butanol by Reactive Distillation

Sep 7, 2007 - Esterification of lactic acid with n-butanol may be performed to synthesize n-butyl lactate or to recover lactic acid from its aqueous s...
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Ind. Eng. Chem. Res. 2007, 46, 6873-6882

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Esterification of Lactic Acid with n-Butanol by Reactive Distillation Rakesh Kumar and Sanjay M. Mahajani* Department of Chemical Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India

Esterification of lactic acid with n-butanol may be performed to synthesize n-butyl lactate or to recover lactic acid from its aqueous solution. In the present work, the reaction is performed in the presence of cationexchange resins as a catalyst. Kinetic parameters like activation energy and the rate constants are estimated using the pseudohomogeneous model. The applicability of reactive distillation for this reaction is evaluated through experiments using batch and continuous reactive distillation techniques. An equilibrium stage model is formulated, and simulation results are compared with the experimental results. Further, the effects of operating parameters like feed mole ratio, catalyst loading, and boilup rate are evaluated on the conversion of lactic acid in batch reactive distillation. Experimental results of continuous reactive distillation are compared with the simulation results observed using the Aspen plus process simulator. Effects of operating variables were studied with the help of the experimentally validated model. A continuous reactive distillation process for the recovery of lactic acid is proposed. 1. Introduction Lactic acid, which is produced by the fermentation process, is widely used in food, chemical, and pharmaceuticals industries and is an important raw material for the production of biodegradable polymers. Esters of lactic acid are high boiling liquids with properties of interest as solvents and as plasticizers for cellulose plastics and vinyl resins. Ethyl lactate is a nontoxic biodegradable solvent that solubilizes several paints, whereas n-butyl lactate is used as a high boiling solvent in lacquer formulations and as a cosurfactant in the preparation of microemulsions with anionic surfactants. Apart from the various applications of esters, the esterification reaction is also important as regards to the separation and purification of lactic acid from its aqueous solutions. As mentioned before, lactic acid is currently produced by the fermentation process. The purification and recovery of the nonvolatile lactic acid from the fermentation broth is a difficult and expensive task. The highest purity levels are required for the production of polymer-grade lactic acid. Several strategies have been studied in the past for the recovery of lactic acid from the fermentation broth1-9. Esterification of lactic acid with the alcohols to form the ester and then hydrolizing the ester back to lactic acid is known to be the most practical method for the recovery of lactic acid. The reaction is as follows:

lactic acid + alcohol S ester + water

(1)

The conventional process for the production of ester involves the reaction in a reactor followed by separation in a number of distillation units. To enhance the equilibrium conversion of the reaction, either one of the reactants must be used in excess or one of the products must be removed continuously from the reaction mixture. The latter can be achieved through separation of water by simultaneous distillation. Reactive distillation (RD) offers several advantages over such a type of equilibrium-limited * Corresponding author. Tel.: +91-022-25767246. Fax: +91-02225726895. E-mail: [email protected].

liquid-phase reaction. Several investigators have studied such reactions by reactive distillation in the past.10-12 There are two alternate methods by which one can esterify lactic acid from its aqueous solution obtained by fermentation using reactive distillation. In the first approach, ester can be removed from the top of the esterifier as vapors, which leads to in situ separation of other organic acid impurities. In such case, esterification with methanol is more logical, because methyl lactate is the most volatile ester and methanol is the most reactive of all the alcohols. This approach has been discussed in our earlier work13,14 and a few other studies.15-18 In the second approach, ester from the esterifier can be withdrawn from the bottom of the column. In such a case, C4C5 alcohols (e.g., n-butanol) are the preferred alcohols for the esterification of lactic acid because they form a low boiling heterogeneous azeotrope with water. Removal of water (aqueous layer) from the azeotropic composition and recycling back the alcohol through the organic layer to the reaction mixture leads to an enhancement in the conversion. The present work investigates this approach in detail. Many methods were described previously for the conversion of lactic acid to butyl lactate. Garbiel and Bogin19 proposed a method for producing n-butyl lactate by dehydrating 70% lactic acid with excess n-butanol at 117 °C followed by addition of HCl catalyst. This was further followed by refluxing and esterification with addition of excess n-butanol and drawing the n-butanol-water azeotrope as an overhead product. Kaplan20 proposed a recovery process that uses a distillation column for continuous esterification of nearly 50% food-grade lactic acid. The overheads are condensed, and the n-butanol rich organic phase is refluxed back to the esterification column. The esterified column bottoms are fed directly to a second column where butyl lactate is separated from the high boiling impurities. The purified butyl lactate is hydrolyzed in the presence of HCl catalyst back to relatively pure lactic acid. HCl causes corrosion problems in the lactic acid hydrolysis columns and may contaminate the product with chloride ions. Dassy et al.21 studied the kinetics of the liquid-phase synthesis and hydrolysis of butyl lactate

10.1021/ie061274j CCC: $37.00 © 2007 American Chemical Society Published on Web 09/07/2007

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Figure 1. (a) Conversion profile of lactic acid at different temperatures, (b) conversion profile of lactic acid at different catalyst loadings, (c) effect of catalyst loading on initial reaction rate, and (d) parity plot for butyl lactate; mole ratio (BuOH/LA) )1.5; catalyst loading ) 4% w/w.

catalyzed by cation-exchange resin in a batch reactor in the presence of dioxane and toluene as solvents/entrainers. Cockrem and Johnson22 described the process for the recovery of lactic acid from a fermentation broth. They proposed the use of n-butanol as the esterifying alcohol. Though the idea of simultaneous removal of water through azeotropic distillation is not new, the systematic studies on the applicability of reactive distillation have not been performed, especially in view of the recent advances and understanding of reactive distillation systems. This reacting system in RD is partly similar to esterification of acetic acid with n-butanol in RD, which has been studied in detail in our earlier work.12 The main difference between the two reacting systems lies in the volatility and nature of the carboxylic acids used. Lactic acid is virtually nonvolatile and has a tendency to form oligomers that may further get esterified, leading to the formation of undesired side products. Hence, there is a need to perform feasibility studies with RD though rigorous experimentation and support through modeling and simulation. In the present work, esterification of a dilute solution of lactic acid (30% w/w) is studied in batch and continuous reactive distillation units using ion-exchange resin as a solid catalyst. As the first step of process development, the reaction kinetics of esterification of lactic acid with n-butanol was studied using Amberlyst-15 as a catalyst. The different kinetic parameters were estimated by regression analysis. These parameters were further used in a reactive distillation model to predict its performance. The batch and continuous reactive distillation experiments were performed to study the feasibility of reactive distillation, and the results obtained by simulation and experiments are compared. n-Butyl lactate thus formed in reactive distillation may

be hydrolyzed further to obtain relatively pure lactic acid. This work provides an energy-efficient and cost-effective alternative to the conventional, reactor-followed-by-distillation approach of making n-butyl lactate. 2. Experimental Section 2.1. Materials. Lactic acid (90% w/w) was obtained from Purac, Netherland, whereas n-butanol and propan-2-ol (99.9% analytical reagent (AR) grade) were supplied by Spectrochem Pvt. Ltd., Mumbai (India). n-Butyl lactate (>98% w/w) was obtained from Merck KGaA, Germany. The cation-exchange resin, Amberlyst-15 (Rohm and Haas, U.S.A.) was used as a catalyst for the reaction without any pretreatment. 2.2. Analysis. Analysis of n-butanol, water, and n-butyl lactate was performed on a gas chromatograph (Chemito, India) equipped with a thermal conductivity detector. A Porapack-Q column was used with hydrogen as a carrier gas. Injector and detector temperatures were maintained at 150 and 220 °C, respectively. The oven temperature was maintained at 220 °C. The concentration of free lactic acid was determined by titration method. Samples were titrated with standard 0.1 N sodium hydroxide solution using phenolphthalein as an indicator under sufficiently cold conditions. The absence of oligomers of lactic acid (e.g., lactoyl lactic acid), if any, was confirmed by analyzing some representative samples using high-performance liquid chromatography (HPLC) on a C18 column. 3. Batch Kinetics 3.1. Apparatus and Procedure. A kinetic study of the esterification of lactic acid (30% w/w) was performed in a batch

Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007 6875 Table 1. Estimated Kinetic Parameters (Esterification of Lactic Acid with n-Butanol) based on Pseudohomogeneous Activity-Based Model for the Confidence Interval of 95% parameter

value

Ea (kJ/mol) Eb (kJ/mol) ln(Kf0) (mol/kg‚h) ln(Kb0) (mol/kg‚h) objective function

53.40 ( 1.86 52.24 ( 6.01 25.26 ( 1.98 23.34 ( 0.63 2.02 × 10-4

reactor to obtain a suitable rate equation under the conditions of interest. Initially, a glass reactor of 300 mL capacity was charged with a measured amount of lactic acid and the catalyst. Once the desired temperature was attained, a known amount of n-butanol was added to the reactor, and this time was considered as the zero reaction time. The samples from the reactor were withdrawn at different time intervals. The experiments were conducted over a wide range of temperature (45-96 °C) and catalyst loadings (10-236 g/mol of lactic acid) to generate kinetic data under different conditions. 3.2. Kinetic Model. A pseudohomogeneous concentrationbased kinetic model (eq 2) has been proposed to explain the experimental data. It should be noted that, for relatively dilute aqueous streams, a simplified pseudohomogeneous concentration-based model is known to work well in the composition range of interest.26

rLA )

nLA dXLA ) kf0 exp(-Ea/RT)(xLAxBuOH) Wcat dt kb0 exp(-Eb/RT)(xBuLAxW) (2)

The four parameters of the model were estimated by minimizing the following objective function using the least-square method in Aspen Custom Modeler:23 NDyn NMeas Mj

min{

∑ ∑ ∑ (zj(tijk) - zikj)2} i)1 j)1 k)1

where NDyn and NMeas are the total number of dynamic experiments and number of measured species concentrations, respectively, and Mj is the number of measurements of zj in experiment i. The estimated values of parameters with confidence intervals are given in Table 1. Parts a and b of Figure 1 show the variation of reaction conversion with respect to time at different temperatures and catalyst loadings. As expected, the rate of reaction increases significantly with an increase in catalyst loading and temperature. The initial reaction rate is the linear function of catalyst loading, as shown in Figure 1c. Figure 1d gives a parity plot that shows a good agreement between experimental and predicted results. 4. Batch Reactive Distillation Batch reactive distillation (BRD) is a simple experimental tool to quickly evaluate the feasibility of reactive distillation for a reaction of interest. It consumes lesser amounts of chemicals and time. The following section describes the experimental observations and the comparison with the simulation. 4.1. Apparatus and Procedure. Figure 2 shows the experimental setup of a single-stage batch reactive distillation used for carrying out esterification. It consists of a glass reactor of 1 L capacity equipped with an electric heating mantle and the condenser. The reactor contains baffles, a thermocouple, and a high-speed motor for effective agitation and uniform mixing of the reaction mixture. A Dean and Stark apparatus was used to separate aqueous and organic layers and to recycle the

Figure 2. Batch reactive distillation (BRD) setup for esterification of lactic acid with n-butanol.

n-butanol-rich organic layer back to the reactor. Initially, a dilute solution of lactic acid (30% w/w) along with catalyst was charged to the reactor. Once the desired temperature was reached, n-butanol was added, and this time was considered as the zero reaction time. Samples were collected from the reactor at different time intervals and analyzed on GC. The aqueous layer (water concentration >90%) from the Dean and Stark apparatus was withdrawn continuously. 5. Mathematical Model for Batch Reactive Distillation 5.1. Assumptions. Following assumptions were made while formulating a single-stage batch reactive distillation model. (i) The reaction takes place only in the liquid phase that surrounds the catalyst. (ii) The vapor-phase composition is in equilibrium with the liquid-phase composition. (iii) Molar holdup of vapor phase is negligible as compared to the liquid-phase molar holdup. (iv) Heat of reaction for the esterification of lactic acid is low, and hence, this term from the energy balance equation is neglected. 5.2. Model Equations for Reactor. 5.2.1. Total and Component Material Balance.

dM ) LD - V dt

(3)

M dxi ) LD(xIi - xi) - V(yi - xi) + Wcatri dt (i ) 1...(c - 1))

(4)

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Figure 3. Comparison of experimental and simulation results: (a) composition profile of the reactor, (b) temperature profile of the reactor, and (c) amount of water withdrawn; (BuOH/LA) ) 2; catalyst loading ) 4% w/w. C

∑1 xIi ) 1.0

5.2.2. Kinetic Model. Pseudohomogeneous model given by eq 2 was used for the reaction kinetics in eq 4. 5.2.3. Vapor-Liquid Equilibria.

Pyi )

psat i γixi

(5)

5.3. Modeling Equations for Decanter. 5.3.1. Mass Balance.

zi ) βxIi + (1 - β)xIIi

(6)

where zi is the overall composition of the condensed liquid entering the decanter. This composition is the same as the vapor composition (yi) in eq 4. 5.3.2. Liquid-Liquid Equilibria.

xIi γIi ) xIIi γIIi 5.3.3. Summation Constraint.

(7)

(8)

Equations 6 and 8 imply that the summation of the compositions in the second liquid phase is also unity.

LD V

(9)

V ) Q/λ

(10)

β) 5.4. Energy Balance.

The UNIQUAC equation was used to calculate the activity coefficients in the liquid phase, whereas the vapor phase was assumed to be ideal. Table 2 gives the binary interaction parameters24 used in the VLE and LLE models. The equations were solved using MATLAB ode15s solver. 6. Results and Discussion: Batch Reactive Distillation The comparison of concentration and temperature profiles of the reactor obtained from experiments and by simulation is

Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007 6877 Table 2. Uniquac Binary Interaction Parameters (K) Used for Kinetics and VLE i/j

butanol

water

butyl lactate

lactic acid

van der Waals volume

van der Waals surface area

butanol water butyl lactate lactic acid

0.0 -292.443 22.819 -62.683

-34.223 0.0 -3.724 304.254

-43.348 -190.229 0.0 -30.538

-6.165 -522.320 -6.132 0.0

3.454 0.92 5.745 3.164

3.048 1.4 4.976 2.88

shown in parts a, b, and c of Figure 3. Results are in good agreement except for the small differences between experimental and predicted temperatures profiles. The concentration of lactic acid decreases slowly in the initial period, whereas a further drop in the concentration is rapid due to the efficient removal of water. In about 5 h, 92% conversion of lactic acid was observed under the conditions mentioned. The reaction temperature increases slowly in the initial period because of the presence of a substantial quantity of water. Toward the end of the batch, temperature also increases significantly because of a sharp decrease in water concentration. The experimentally validated model for BRD was further used to study the effect of different parameters. The initial rate of esterification increases with an increase in catalyst loading, which results in an increase in conversion, indicating the reaction

to be kinetically controlled under the conditions of interest. As anticipated, an increase in mole ratio results in an increase in the conversion. The higher concentration of n-butanol is favorable as it increases the rate and efficacy of water removal through the formation of a minimum boiling azeotrope. At high boilup, the rate of water removal increases, which results in an increase in the conversion of lactic acid. The parametric studies on BRD indicated that close to quantitative conversion is feasible and the performance strongly depends on the efficiency of water removal. 7. Continuous Reactive Distillation 7.1. Experimental Setup and Procedure. The reaction was further performed in a continuous reactive distillation column to demonstrate the feasibility of recovery of lactic acid through

Figure 4. Continuous reactive distillation (CRD) setup for esterification of lactic acid with n-butanol.

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Figure 5. Comparison of experimental and simulation results: (a) composition profile of n-butanol and water, (b) composition profile of butyl lactate and lactic acid, and (c) temperature profile of the column.

the formation of n-butyl lactate. The experimental setup of a laboratory-scale reactive distillation is shown in Figure 4. It consists of a glass column with 55 mm internal diameter and 2.7 m height. The column has three different zones viz. nonreactive rectifying section, nonreactive stripping section, and the middle reactive zone packed with sulzer KATAPAK-S structured packing filled with ion-exchange resin, Amberlyst 15 (NTSM ) 3). The nonreactive sections of the column are packed with a wire mesh packing supplied by Evergreen India Ltd. (NTSM ) 8). A Dean and Stark type arrangement was provided at the top of the column to remove the aqueous phase and recycle back the organic phase from the azeotropic mixture of n-butanol and water. To minimize heat losses to the surroundings, the column walls are covered with an insulating asbestos sheet. The calibration for heat losses was performed independently, and typically 62% of the heat supplied to the column is lost to the surroundings when the bottom temperature is close to 130 °C. The temperature sensors (Pt-100) are provided at different locations in the column (positions 1-6). Two feed streams were introduced continuously at different locations. Lactic acid solution was fed at the top of the reactive section, and pure n-butanol was fed at the bottom of reactive section of the column. Water was removed continuously from the top of the column. At steady state, flow rates of the bottom and top streams were measured and the overall material balance was verified. The constancy of the column concentration profile with respect to time confirms the attainment of the steady state. 7.2. Modeling and Simulation of Reactive Distillation Column. Simulation studies for esterification of lactic acid (LA) with n-butanol in continuous reactive distillation were carried out using the Aspen Plus steady-state simulator. The RADFRAC module, which has a rigorous model for carrying out equilibrium-stage modeling, was used for this purpose. The detailed MESH equations for the equilibrium-stage model can be found elsewhere.25 A decanter was added to the reactive distillation unit to separate the two-phase mixture of the top stream. The UNIQUAC model was used to estimate the vapor-liquid

Table 3. Experimental Details for Esterification of Lactic Acid (30% w/w) with n-Butanol total no. of stages

14

rectifying section reactive section stripping section lactic acid feed flow rate lactic acid feed composition (wt fraction) butanol water butyl lactate lactic acid butanol feed flow rate butanol feed composition (wt fraction) butanol water butyl lactate lactic acid distillate flow rate distillate composition (wt fraction) butanol water butyl lactate lactic acid bottom flow rate bottom composition (wt fraction) butanol water butyl lactate lactic acid catalyst loading per stage heat duty (excluding the heat losses) % conversion of lactic acid

2-4 stages 5-9 stages 9-13 stages 350 g/h 0.0 0.7 0.0 0.3 211 g/h 0.999 0.001 0 0 268 g/h 0.06 0.94 negligible negligible 292 g/h 0.391 0.008 0.601 negligible 30 g 369 W 99.5

equilibrium with all the binary interaction parameters from the Aspen data bank (Table 2). Because of the presence of a relatively large amount of water, the reactive zone and stripping section in the column may experience liquid-phase splitting. Both vapor-liquid equilibrium (VLE) and vapor-liquid-liquid equilibrium (VLLE) thermodynamic modeling approaches from Aspen were used to predict the results. There is not much difference in the predicted composition profiles by both modeling approaches under the conditions of interest. A pseudohomo-

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Figure 6. Effect of different parameters on conversion: (a) effect of catalyst loading, (b) effect of boilup, (c) effect of mole ratio, (d) effect of feed concentration, and (e) effect of number of reactive stages. All the design and operating parameters are as mentioned in Table 3, except the one that is varied in each case.

geneous concentration-based kinetic model described by eq 2, which worked well for the batch reactive distillation experiments, was used. The chemical reactions were assumed to occur only in the liquid phase in the reactive section. 8. Results and Discussion: Continuous Reactive Distillation The comparison of concentration and temperature profiles obtained from the experiment and simulations results are shown in Figure 5 for a representative run. The trends in composition and temperature profiles are in agreement. The VLLE modeling approach predicts phase split in the upper part of the reactive zone. The discrepancy especially in the upper part of the reactive zone is due to the possible error in VLLE/VLE predictions, as most of the interaction parameters are estimated by UNIFAC method. Lactic acid and n-butanol concentrations decrease whereas butyl lactate concentration increases as we move toward

the bottom of the column. With 30% lactic acid solution as feed and with a mole ratio of 2.44:1 (n-butanol/lactic acid), near to quantitative conversion of lactic acid was achieved. The experimental details for the run are given in Table 3. It is remarkable that a very high conversion level is realized with sufficiently high yield. The formation of high boiling oligomers and their esters was negligible because of the presence of a large amount of water throughout the column. Thus n-butanol has a distinct advantage over methanol and ethanol because reactive distillation with n-butanol, due to effective water removal, can handle large quantities of water in the feed, which in turn suppresses oligomer formation. 9. Effect of Operating Parameters The effect of different operating parameters has been studied through simulation with the help of the experimentally validated model. The run given by Table 3 is considered as a base case.

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Figure 7. Proposed process flow diagrams for the recovery of lactic acid. Table 4. Simulation Results for the Proposed Flow Sheet Shown in Figure 8 (Feed to Esterification Column: 30% w/w Lactic Acid in Water) column configurations total no. of stages no. of stages in rectifying section no. of stages in reactive section no. of stages in stripping section feed 1 flow rate (kmol/h) feed 2 flow rate (kmol/h) top product flow rate (kmol/h) bottom product flow rate (kmol/h) distillate composition (mole fraction) alcohol water ester acid bottom composition (mole fraction) butanol water ester acid catalyst loading per stage (kg) condensed duty (kW) reboiler duty (kW) % conversion

esterification

hydrolysis

14 3 5 4 138.06 21 141.26 17.79

21 2 15 2 10.90 32 19.40 23.49

0.0240 0.975 negligible negligible

0.556 0.443 negligible negligible

0.383 negligible 0.61 0.006 100 3095.91 3477.75 99

0.00 0.535 0.004 0.459 100 825.84 935.59 98.6

One parameter was varied at a time by keeping the values of all other parameters the same as that mentioned in Table 3. 9.1. Effect of Catalyst Loading. Catalyst loading was varied over a range 1-30 g/stage to examine its effect on the conversion of lactic acid (Figure 6a). Initially, the conversion of lactic acid increases with an increase in catalyst loading. At sufficiently high catalyst loading (25 g/stage), a quantitative conversion is realized. A further increase in catalyst loading is not recommended, as the reaction is no longer controlled by kinetics. 9.2. Effect of Boilup. The effect of a change in boilup on conversion is shown in Figure 6b. The conversion of lactic acid increases with an increase in boilup and reaches close to 100%. At high boilup, the temperature of the reactive zone increases, which leads to an increase in the rate of the reaction. The water content in the bottom product may be brought down to a minimal level by increasing the boilup. If n-butanol is used in excess, it finds an outlet through the bottom stream, and hence, the butyl lactate purity in the bottom is always 99.5%) conversion may be achieved, which increased only slightly with a further increase in the number of reactive stages. It can be concluded that four reactive stages are sufficient to get the desired conversion and bottom purity. 9.6. Effect of Rectifying and Stripping Stages. An increase in the number of rectifying and stripping stages does not result in a significant change in conversion and bottom concentration. This is because of the large differences in boiling points of the components involved. Hence, the presence of nonreactive zones does not alter the performance significantly. 10. Proposed Process for the Recovery of Lactic Acid On the basis of the results obtained in this study, the proposed process flow diagram for the recovery of lactic acid is shown in Figure 7. The process consists of two reactive distillation columns to carry out the esterification and hydrolysis reactions and a stripper to purify butyl lactate from the nonvolatile

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impurities and the heavy oligomers formed, if any. The lactic acid solution (10-30% w/w) is fed to the first column at the top of the reactive zone, and pure n-butanol is fed at the bottom of the reactive zone. The top stream of the column consists of a two-phase mixture of n-butanol and water. Water is removed continuously from the two-phase mixture of distillate, and n-butanol is recycled back to the column through the organic phase. n-Butyl lactate formed is removed from the bottom stream of the column along with nonvolatile impurities and heavy oligomers, if any. Butyl lactate is separated from the impurities in a stripper. It is then hydrolyzed in another reactive distillation column. n-Butyl lactate is fed at the top of the reactive section, and water is fed at the bottom of the reactive section. The overhead stream from the hydrolysis column is separated into two phases; the aqueous phase is refluxed back, and n-butanol is withdrawn as distillate through the organic layer. Lactic acid formed is taken out as a bottom stream along with excess water, if any. The continuous removal of lactic acid and n-butanol from the reaction zone may increase conversion; thus, pure lactic acid may be obtained from the bottom of the second reactive distillation. The formation of oligomers and their esters is negligibly small. Preliminary simulations have been performed for the proposed flow sheet, and the results are given in Table 4. The kinetics used for hydrolysis is taken from Dassy et al.21 Several simulations were performed systematically by varying various parameters, and the best results are reported in Table 4. The hydrolysis column needs a relatively large amount of catalyst as compared to that in the esterification column, because hydrolysis is intrinsically a much slower reaction than esterification. The detailed analysis of the feasibility studies for hydrolysis of n-butyl lactate in reactive distillation through experiments can be the subject of future investigation. We do not claim that the results in Table 4 represent the optimum design, and a rigorous flow sheet optimization considering the cost function may be necessary. 11. Conclusions The kinetics of esterification of lactic acid with n-butanol in the presence of acid catalyst is studied, and a kinetic model is proposed. A relatively dilute solution of lactic acid (30% w/w) is used as a feed, and ∼92% conversion of lactic acid was obtained in batch reactive distillation. Reactive distillation helps simultaneous separation of water and enhances the equilibrium conversion. The experiments are also conducted in the reactive distillation column in continuous mode and near to quantitative conversions are obtained. The column model that uses the developed kinetics explains the data reasonably well. This technique may prove to be an efficient and economic alternative for the synthesis of butyl lactate as well as for the recovery of lactic acid from its dilute aqueous solution. Under the operating conditions of interest, the losses to oligomers of lactic acid and their ester are negligible, making n-butanol a promising competitor to methanol and ethanol for the purpose of lactic acid recovery from the aqueous solutions. Acknowledgment The authors acknowledge CSIR, New Delhi, India, for the financial support. Notations CW ) cooling water Ea, Eb ) activation energy (kJ mol-1) Kf0, Kb0 ) preexponential factor (mol g-1 min-1)

L ) liquid flow (mol min-1) M ) molar holdup (mol) nLA ) initial moles of lactic acid Psat i ) saturated vapor pressure of component i P ) total pressure (Pa) Q ) heat duty (kJ s-1) rj ) rate of reaction (mol g-1 min-1) R ) gas constant (kJ mol-1 K-1) T ) temperature (K) t ) time (min) V ) vapor flow (mol min-1) Wcat ) weight of catalyst (g) XLA ) conversion of lactic acid xi ) liquid-phase mole fraction of component i yi, ) vapor-phase mole fraction of component i Greek Letters β ) aqueous-phase fraction defined by eq 9 γi ) activity coefficient of component i λ ) latent of vaporization of reactor mixture (kJ mol-1) List of AbbreViations BuOH ) n-butanol LA ) lactic acid BuLA ) n-butyl lactate W ) water Subscript eq ) equilibrium i ) component number j ) stage number aq ) aqueous phase D ) organic phase Superscript sat ) saturated I ) organic phase II ) aqueous phase Literature Cited (1) Baniel, A. M. Process for the extraction of organic acids from aqueous solution. EP 00,49,429, 1982. (2) Kawabata, N.; Yasuda, S.; Yamazaki, T. Process for recovering a carboxylic acid. U.S. Patent 4,323,702, 1982. (3) Kulprathipanja, S.; Oroshar, A. R. Separation of lactic acid from fermentation broth with an anionic polymeric absorbent. U.S. Patent 5,068,418, 1991. (4) King, C. J; Tung, L. A. Sorption of carboxylic acid from carboxylic salt solutions at pHs close to or above the pKa of the acid, with regeneration with an aqueous solution of ammonia or low-molecular-weight alkylamine. U.S. Patent 5,132,456, 1992. (5) Cao, X.; Yun, H. S.; Koo, Y. M. Recovery of L-(+)-Lactic Acid by Anion Exchange Resin Amberlite IRA-400. Biochem. Eng. J. 2002, 11, 189. (6) Hongo, M.; Nomura, Y.; Iwahara, M. Novel Method of Lactic Acid Production by Electrodialysis Fermentation. Appl. EnViron. Microbiol. 1986, 52, 314. (7) Boyaval, P.; Corre, C.; Terre S. Continuous Lactic Acid Fermentation with Concentrated Product Recovery by Ultrafiltration and Electrodialysis. Biotech. Lett. 1987, 9 (3), 207. (8) Datta, R.; Tsai, S. P.; Bonsignore, P.; Moon, S. H.; Frank, J. R. Technology and Economic Potential of Poly(lactic acid) and Lactic Acid Derivatives. FEMS Microbiol. ReV. 1995, 16, 221. (9) Heriban, V.; Skara, J.; Sturdik, E.; Ilavsky, J. Isolation of Free Lactic Acid Using Electrodialysis. Biotechnol. Tech. 1993, 7 (1), 63. (10) Saha, B.; Teo, H. T. R.; Alqahtani, A. Iso-amyl Acetate Synthesis by Catalytic Distillation. Int. J. Chem. React. Eng. 2005, 3, A11.

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ReceiVed for reView October 3, 2006 ReVised manuscript receiVed June 6, 2007 Accepted July 18, 2007 IE061274J