Reactive distillation with side draw

Reactive distillation with side draw Suman Thotla, Sanjay Mahajani Department of Chemical Engineering, Indian Institute of Technology, Bombay, Mumbai 400 076, India abstract Keywords: Reactive distillation Lactic acid Esterification Acetone Aldol condensation Fatty acid Biodiesel Mesityl oxide Diacetone alcohol We demonstrate the applicability of a new reactive distillation configuration, i.e. reactive distillation with side draw, for certain industrially important reactions. For the reacting systems which involve products with intermediate volatility, a side draw facilitates its in situ removal and enhances either conversion or selectivity. It further reduces the downstream processing in some cases. The concept is proved for three representative systems, viz. esterification of lactic acid, aldol condensation of acetone and for esterification of fatty acid by methanol. Experimental proof is also provided in some cases. 1. Introduction The applicability of reactive reactive distillation (RD) for many industrially important processes is well known [1]. In the case of equilibrium controlled reactions such as esterification and etherification, the objective is to surpass equilibrium conversion whereas in the case of multiple reaction systems, RD can be used to improve selectivity. In most of the studies reported in the past, a conventional reactive distillation configuration involves a reactive zone placed in the distillation column at an appropriate location. In addition to this section, two non-reactive zones viz. rectifying and stripping sections may also be employed. The products are withdrawn through distillate and/or the bottom streams depending on the volatilities. Thus, reactive distillation has been successfully applied for the mixtures wherein, at least one product is either the most volatile or the least volatile component. Sometimes minimum/maximum boiling azeotropes in the system can be exploited advantageously to achieve in situ removal. If the pure products can be separated efficiently then one can use close to stoichiometric feed mole ratio of the reactants. Some relevant examples are esterification of acetic acid by methanol or butanol, MTBE synthesis from methanol and isobutene, etc. However, in some cases, the volatilities are such that the separation of the product as either top product and bottom product is difficult or sometimes infeasible, e.g. a case in which the product is intermediate boiling and product does not form an azeotrope with other component(s). In a conventional reactive distillation configuration the reactant being more volatile, tends to move away from the reactive zone, leaving behind the intermediate boiling product, thereby creating unfavorable conditions for the reaction. In such cases one may need to use one of the reactants in excess to expedite the reaction and facilitate separation. The excess reactant thus may not serve the purpose of reactive distillation and RD option may be unattractive compared to the conventional approach of reaction followed by separation performed in a sequential manner. In this paper, we present an alternative to the conventional notion of removing products only through top or bottom streams. A configuration that involves removal of product(s) as side stream(s) has been suggested, which may provide more flexibility in design and as a result, improve the performance of the existing RD applications and encompass under its umbrella many more reactions which are otherwise written off as potential candidates. A list of some potentially important applications is given in Table 1. In most of the cases, water is the component with intermediate volatility, to be removed through the side draw. We have chosen three industrially important reactions as representative examples. In the first case i.e., esterification of lactic acid, water being an intermediate boiling component is removed as a side draw to facilitate much lower mole ratio of methanol to lactic acid and yet yield relatively pure methyl lactate in the bottom stream with enhanced conversion level. This also helps avoid unfa-

928 Table 1 Potentially important reactions for reactive distillation with side draw. Reactions Desired product Side draw Reference Esterifications Adipic acid (A) + methanol (B) monomethyl adiapate (C) + water (D) 1. Mono methyl adiapate (C) + methanol (B) dimethyl adiapate + water (D) Dimethyl adipiate Water [2] Glutaric acid (A) + methanol (B) monomethyl glutarate (C) + water (D) 2. Monomethyl glutarate (C) + methanol (B) dimethyl glutarate (E) + water (D) Dimethyl gluterate Water [2] Maleic acid (A) + ethanol (B) monoethyl maleate (C) + water (D) 3. Monoethyl maleate (C) + methanol (B) diethyl maleate (E) + water (D) Dimethyl maleate Water [3] 4. Fatty acid (A) + methanol (B) ester (C) + water (D) Ester of fatty acid Water [4] 5. Lactic acid (A) + methanol (B) methyl lactate (C) + water (D) Methyl lactate Water [5] Acetalizations Glyoxal (A) + methanol (B) monoacetal (C) + water (D) 1. Monoacetal (C) + methanol (B) diacetal (E) + water (D) Diacetal Water [6] 2. Diol (A) + acetone (B) acetal of diol (C) + water (D) Acetal of diol Water [7] Aldol condensation 2Acetone (A) diacetone alcohol (B) 1. Diacetone alcohol (B) mesityl oxide (C) + water(d) Cross Aldol condensation Acetaldehyde (A) + methyl ethyl ketone (B) methyl pentanone (C) + water (D) 1. 2Acetaldehyde (A) crotanaldehyde (E) + water (D) Diacetone alcohol Water [8,9] Methyl pentanone Water [10] vorable uncatalysed hydrolysis of lactate in the bottom section of the column. In the second case, i.e. aldol condensation of acetone to mesityl oxide (MO) and diacetone alcohol (DAA), removal of intermediate boiling water as a side stream is studied. In the third case, i.e. esterification of oleic acid with methanol to methyl oleate and water, the removal of intermediate boiling water as a side stream is studied. In the first two examples experimental support is provided to validate the simulation results. In all the cases simulations are performed using ASPENPLUS [11]. 2. An illustrative example A reacting system involving following irreversible liquid phase reaction is considered. 2A B + C The reaction is assumed to be elementary and the kinetics is given by the following equations: r 1 = k 1 x 2 A (1) In this case both the products, B and C, are less volatile than the reactant A. Hence, we choose hybrid reactive distillation unit operated under total reflux condition. For volatility of AC = 12.2, BC = 10.5 and for 100% conversion of A, the composition profile of the column is shown in Fig. 1a. It can be seen that the mole fraction of intermediate boiling component (B) which is to be removed as a side draw is close to 1 between the stages 2 and 6. Hence, by removing relatively pure B as side draw from any of these stages, the composition profile of the stages above this side draw is unaffected. This is mainly due to the total reflux. Fig. 1b shows the composition profile of RD column with side draw. Hence, for a given reaction system to remove intermediate boiling component through the side draw, one should provide sufficient number of non-reactive stages below the reactive zone to realize a region of maximum concentration of intermediate boiling component. Intermediate boiling component is removed as a side draw from this region. It can be seen from these figures that side draw does not influence the composition profiles in the upper part for the column. However, as expected, the composition profiles in the region below the side draw change significantly. Further, as shown in Fig. 2, for an insufficient number of stages one may not obtain a region of pure B. The number of nonreactive stages thus required to obtain the region of pure B depends on the relative volatility of B with respect to C and more the relative volatility, less is the number of non-reactive stages (Fig. 3). 3. Experimental work A multipurpose reactive distillation (RD) setup, used to study esterification of lactic acid and aldol condensation of acetone, is shown in Fig. 4. A column with 5.1 cm diameter contains reactive section (2.3 m) and non-reactive stripping section (1 m). It can be operated in batch, semi-batch or continuous modes. The setup has a provision to withdraw a side product from various positions as indicated. Reactive section is packed with Sulzer Katapak-S packing filled with pre-treated dry cation exchange resin Amberlyst 15. The non-reactive sections are packed with HYFLUX packing from Evergreen India Ltd. In between the two sections, a weir is provided to facilitate enough accumulation of liquid hold up (6 ml) for side withdrawal. Temperature sensors and sampling ports are provided for analyzing liquid composition and temperature on each stage to monitor the state of RD run periodically. The heights of the individual sections, location of feed and product withdrawal were varied as desired in each case. 3.1. Materials and catalyst l(+)-lactic acid (90% wt) was obtained from Spectrochem India Ltd. Methanol and acetone (AR grade, moisture < 0.02%) were supplied by Merck Ltd. The catalyst used in the experiments is commercial strong cation exchange resin Amberlyst 15 and was obtained from Rohm and Hass, Philadelphia, PA. Before its use, the fresh catalyst was carefully washed with pure water, isopropyl alcohol, dilute hydrochloric acid and then again with pure water sequentially. It was then dried at 70 C under vacuum for about 12 h to remove the moisture present in it. 3.2. Analysis A gas chromatograph (GC-911; Mak Analytica India Ltd.) equipped with thermal conductivity detector (TCD) was used to analyze the reaction mixture of methanol, water and methyl lactate. The analysis was carried out in a Porapack-Q column and elution was established using hydrogen as a carrier gas at a flow rate of 40 ml/min with isopropyl alcohol as external standard. The oven temperature was varied from 120 Cto240 C for the methyl lactate water system. The concentration of lactic acid was determined by titration method. Samples were titrated with standard 0.05 N alcoholic sodium hydroxide solution using phenolphthalein as an indicator.

929 Fig. 1. Liquid phase composition profiles in (a) conventional reactive distillation (b) reactive distillation with side draw for 2 kw of re-boiler duty; F A = 0.02 kmol/h and side draw = 0.01 kmol/h. A gas chromatograph (GC-911; Mak Analytica India Ltd.) equipped with flame ionization detector (FID) was used to analyze the reaction mixture of acetone, MO and DAA. The analysis was carried out in a 30 m long BP-5 column (SGE, Australia) with an inner diameter of 0.55 mm. Elution was established using nitrogen at a flow rate of 0.5 ml/min. Methyl ethyl ketone (MEK) was used as an external standard. Water concentration was calculated based on the material balance. The molar amount of water formed was assumed to be same as that of mesityl oxide because the formation of heavier side-products was negligible compared to the amount of acetone converted or mesityl oxide formed [8]. Fig. 2. Effect of non-reactive stages on liquid phase composition profile of B for 2 kw of re-boiler duty; F A = 0.02 kmol/h.

930 Table 2 UNIQUAC model binary interaction parameters [11]. Lactic acid Methanol Methyl lactate Water Lactic acid 0 452.35 288.41 357.47 Methanol 1040.75 0 7.37 165.26 Methyl lactate 208.41 41.42 0 394.57 Water 499.38 351.0861 219.62 0 4. Case 1: Esterification of lactic acid Lactic acid + methanol K 1 K 2 methyl lacate + water (2) Fig. 3. Effect of volatility on liquid phase composition profile of B for 2 kw of reboiler duty; F A = 0.02 kmol/h. Esterification of lactic acid is relevant in the synthesis of methyl lactate which finds applications as a solvent in chemical industry. The reaction can also be used to recover lactic acid in the form of its methyl ester from the aqueous solution obtained by fermentation of carbohydrates [5]. Recovery by normal distillation is expensive due to the large amount of water that exists in the feed. Moreover, the impurities (e.g. succinic acid) formed in the fermentor also get carried away with lactic acid. Esterification reactions are equilibrium controlled reactions and the presence of large amount of water poses limitations if one desires to obtain substantial conversion. Hence, excess methanol is required for close to 100% conversion of lactic acid. By using reactive distillation, the chemical equilibrium can be shifted towards the formation of methyl lactate through continuous removal of water and methyl lactate from the reactive zone. However, as mentioned in the earlier section, since water is an intermediate boiling component, its separation is not easy and large amount of methanol is required not only from the reaction view point but also to strip off water and remove it as an overhead product. Hence, a conventional reactive distillation configuration with close to stoichiometric ratio of lactic acid to methanol is not an attractive option. Moreover, the hydrolysis of methyl lactate which is a self-catalysed reaction may take place in the lower portion of column in the presence of small quantities of water. Lactic acid thus formed by hydrolysis may undergo polymerization, which further complicates the operation. Side removal of water not only drives the reaction to completion but also facilitates column operation at relatively less mole ratio of methanol to lactic acid in the feed. Here, we show the importance of side draw with three representative experimental and simulation results. All the binaries except water and methyl lactate are non-azeotropic and the volatility order is methanol (b.p.: 64.7 C) > water (b.p.: 100 C)>methyl lactate (b.p.: 144 C) > lactic acid (b.p.: 224 C), respectively. Water is an intermediate boiling component. In such cases, reactive zone is placed in the rectifying section and non-reactive stages are provided below the reactive zone thus leading to a hybrid reactive rectification configuration. Lactic acid is a relatively strong acid (pk a = 3.85) and selfcatalyses its esterification. Similarly, traces of lactic acid present in the lactate water mixture are known to catalytically trigger the hydrolysis [12]. In a conventional reactive distillation configuration, methyl lactate is expected to leave the column from the bottom. Relatively high temperature in the reboiler, presence of water and unconverted lactic acid would result in hydrolysis of lactate and Table 3 Kinetic parameters for esterification of lactic acid [5]. Parameter Value Fig. 4. A multipurpose reactive distillation setup with total reflux and side draw B: bottom product; SD: side withdrawal. E f (kj/mole) 53.1 ± 6.51 E b (kj/mole) 40.1 ± 16.066 Log k f0 (mole/(kg h)) 25.40 ± 2.25 Log k b0 (mole/(kg h)) 20.01 ± 5.56

931 Fig. 5. (a) Liquid phase mole fraction of methyl lactate and water in reboiler, (b) liquid phase mole fraction of lactic acid and (c) temperature of reboiler ( C) with respect to conversion of methanol. subsequent oligomerization of lactic acid making the operation unstable. We observed this behavior through the experiments conducted in semi-batch mode. The results are described in the next section. 4.1. Semi-batch reactive distillation (SBRD) As the first step to evaluate the feasibility of reactive distillation, a representative SBRD run was conducted under total reflux without removing any product from the RD column. SBRD runs are less time consuming and use less amount of chemicals. Hence, these experiments can be performed as the first step in process development to quickly ascertain the potential of reactive distillation and foresee undesired problems, if any. The setup shown in Fig. 4 was used to conduct the experiments at 0.5 kw reboiler duty by charging the reboiler with 650 ml methanol initially and feeding 80% (w/w) lactic acid at 1.5 ml/min continuously above the reactive zone. The samples were withdrawn at different time intervals (every 15 min up to first hr and then, every 30 min till the end of the experiment) from the re-boiler to assess the performance. It should be noted that unlike the conventional RD configuration that removes water as an overhead product, we allow the accumulation of both water and ester in the reboiler. In the continuous version as well, the column will be operated under total reflux. An equilibrium stage model based simulations are performed using Aspen Plus simulator (Aspen Custom Modeler) [11]. The heat losses from the column wall and reboiler were considered while simulating the column performance. Similarly, the number of theoretical stages for the reactive and non-reactive sections were determined independently by performing simple distillation of the standard methanol water mixture (NTSM for the reactive zone: 3; NTSM for the non-reactive zone: 6). The equations involved in a semi-batch reactive distillation model are well known and one may refer to our earlier work for more details [13]. The UNIQUAC binary interaction parameters used for the prediction of VLE are given in Table 2. A pseudo-homogeneous activity-based kinetic model was used to explain the experimental results. The kinetic model is taken from Fig. 6. Steady state (a) liquid phase composition profile (b) temperature profile along the height of the column for 0.5 kw; F LA = 150 ml/h; mole ratio (methanol/lactic acid) = 2.1; reflux rate 1.18 kg/h; side draw = 180 ml/h.

932 Fig. 7. Unsteady state experimental (a) liquid concentration profile of reboiler, (b) temperature profile of reboiler for 0.5 kw; F LA = 150 ml/h; mole ratio (methanol/lactic acid) = 2.1; reflux rate 1.18 kg/h; side draw = 120 ml/h. [5]. r = n 0 W cat dx dt k b0 exp = k f 0 exp ( E f RT ) (a LA a MeOH ) ( E ) b (a MLA a W ) (3) RT where a i is the activity of the ith component. Activity coefficients were calculated using the UNIQUAC method and UNIQUAC binary interaction parameters are given in Table 2. The values of kinetic parameters are given in Table 3. The experimental and simulation results of SBRD are shown in Fig. 5a c and are found to be in good agreement. Fig. 5a c show how reboiler concentration and temperature vary with respect to conversion of methanol in a typical SBRD run. As anticipated, the temperature of the reboiler and hold up increase over a period due to accumulation of reactants and products. As shown in Fig. 5a, SBRD run can be divided into two different regimes, region-i is below 80% conversion of methanol and region-ii is above 80% conversion. The simulation results match well with experimental results in the region-i that corresponds to relatively low conversion of lactic acid and high concentration of methanol. On the other hand, in the region-ii, i.e. at high conversion of lactic acid and low concentration of methanol the experimental results deviate from the predicted ones. The deviation between simulation and experimental results in region-ii is due to self-catalysed hydrolysis of methyl lactate that takes place leading to formation of lactic acid which further polymerizes due to relatively high temperature in the reboiler. The uncatalysed hydrolysis and polymerization reactions are not included in the simulation of RD. SBRD run shows that if one desires to achieve higher conversion levels in RD, water from the stripping section needs to be removed completely to avoid polymerization of lactic acid. A side withdrawal of water may be employed and in the next section, we demonstrate the importance of side draw through experiments. column and reboiler to the surrounding, was solved using Aspen Plus simulator (RADFRAC) [11]. The relevant model equations and underlying assumptions may be found elsewhere [14]. Fig. 6a and b show the steady state concentration and temperature profiles along the height of the column, obtained by experiments and simulation, in a representative CRD run. The side draw contains methanol, water and methyl lactate. In the setup used, it is not possible to obtain pure water or pure methyl lactate through the side draw as the height of the column is not sufficient to separate pure water and pure methyl lactate in the stripping section. From Fig. 6a, water concentration in the reactive zone is less and almost all the water is removed as a side draw from the bottom most reactive stage avoiding the self-catalysed hydrolysis of methyl lactate in the stripping section. Hence, a high conversion of lactic acid (i.e. 78%) is realized compared to a conventional equilibrium reactor (65%) [5] for the mole ratio of lactic acid to methanol as 1:2.1. Moreover, undesired reaction of oligomer formation that takes place in conventional RD without side draw is suppressed. We observed that with a decrease in the side draw rate, oligomerization is enhanced to reduce yield and conversion. Fig. 7a and b show unsteady state composition and temperature profiles of reboiler. Due to hydrolysis of methyl lactate in the stripping section and the reboiler, lactic acid concentration increases leading to continuous increase in the temperature that results in an unstable operation and the steady state is never achieved. Hence, in the esterification of lactic acid with methanol, removal of water from the column at a desired rate is crucial as water reacts with methyl lactate to give lactic acid that undergoes 4.2. Continuous reactive distillation (CRD) The set up shown in Fig. 4 was used to conduct CRD experiments under total reflux by continuously feeding lactic acid at the top of the reactive section and methanol below the reactive zone, respectively. The products, methyl lactate and water, are continuously withdrawn through the bottom product and side stream (from 6th stage), respectively. Samples at several locations in the column were analyzed at different time intervals (every 30 min) to monitor the performance. The attainment of steady state was ensured by constancy in temperatures and concentrations at different locations. Typically, it takes 9 10 h to attain the steady state. An equilibrium stage model, considering the heat losses from the Fig. 8. Comparison of reactive stripping and reactive distillation with side draw for 0.6 kw; F LA = 150 ml/h.

933 Fig. 9. Effect of mole ratio of methanol to lactic acid on concentration of water in the column for (a) reactive distillation with side draw and (b) reactive stripping for 0.6 kw; F LA = 150 ml/h. uncontrolled oligomerization and results in a rise in temperature of the reboiler. Hence, it is very important to control the side stream flow rate so as to remove most of the water that is formed in the reaction and that comes with the feed. 4.3. Comparison of conventional RD configuration and RD with side draw As mentioned before, water in conventional RD is stripped off using excess methanol and is removed as the top product. Excess methanol is used not only to expedite the reaction but also to increase the vapor flow rate that improves the efficacy of water removal. This configuration may also be called as reactive stripping. In this section, we compare the performances of such conventional RD configuration with that of RD with side draw. The experimentally validated simulator is used in both the cases and the results are compared. In both the configurations water is removed completely to avoid its presence in the stripping section. This removes the chances of possible instability due to self-hydrolysis. Fig. 8 shows the comparison of simulation results for reactive distillation with side draw and conventional reactive distillation with superheated methanol as feed. From Fig. 8, it can be concluded that reactive distillation with side draw offers much better performance than that of conventional reactive distillation. On increasing the mole ratio of methanol to lactic acid from 3 to 15, one can expect an increase in conversion of lactic acid. But in the case of reactive distillation with side draw the conversion of lactic acid does not change significantly after a certain mole ratio (7). This may be attributed to Table 4 UNIQUAC model binary interaction parameters [11]. Acetone DAA MO Water Acetone 0 79.56 30.80 642.7335 DAA 36.17 0 212.5 1188.5242 MO 61.65 102.1 0 56.94 Water 87.7160 351.0861 1044 0 the presence of small amount of water in the reactive zone. As can be seen from Fig. 9, in the conventional RD, since excess methanol (90 C) is used to strip water off from the reactive stages, concentration of water decreases with increase in mole ratio of methanol to lactic acid from 9 to 15. On the other hand, in reactive distillation with side draw, concentration of water on the reactive stages is not affected by changing the mole ratio of methanol to lactic acid and conversion cannot be improved beyond a certain level, i.e. 92% for the conditions given in Fig. 8. 5. Case 2: Aldol condensation of acetone DAA is produced by selective aldol condensation of acetone. Subsequent dehydration of DAA produces 4-methyl-3-penten-2-one, commonly known as mesityl oxide (MO). Both diacetone alcohol (DAA) and mesityl oxide (MO) are useful products derived from acetone. Condensation of mesityl oxide and acetone further produces heavier products, such as isophorone. Eqs. (4) and (5) show the reactions of interest. 2Acetone dicetonealcohol (4) Dicetonealcohol mesityloxide + water (5) The main reason behind the use of reactive distillation for this reaction is to achieve flexibility over the relative production rates of DAA and MO. An efficient removal of DAA from the reactive zone results in the suppression of its further reaction to MO and improves DAA to MO ratio. The reaction in RD has been well studied in the past [15]. It is known that the equilibrium constant for the reaction of acetone to DAA is very low and DAA concentration does not rise beyond a particular limit. Further, at relatively large conversion (>10%), the formation of high molecular weight compounds is significant. Water plays an important role in this reaction and its presence makes a favorable impact on the DAA formation and it can be advantageously exploited to further increase the selectivity of DAA.

934 Table 5 Kinetic parameters for aldol condensation of acetone [8]. Parameter Value E f (J/mole) 71056.8 ± 6668.58 E b (J/mole) 97539.5 ± 11272.8 E MO (J/mole) 61296 ± 6766.92 k f0 (mole/(kg h)) 1.26 10 12 ± 5.928 k b0 (mole/(kg h)) 3.04 10 18 ± 56.876 k MO0 (mole/(kg h)) 1.07 10 13 ± 11.081 K W 2.90 ± 0.474 In our earlier work [8] and [9], we proposed reactive distillation (RD) for simultaneous production of DAA and MO using cation exchange resin with and without water dosage. RD was found to offer better performance than the conventional reactors. It has distinct advantages of shifting the chemical equilibrium towards the formation of DAA and also suppressing the consecutive dehydration of DAA to MO. By changing operating conditions such as reboiler duty and catalyst concentration (kg of catalyst/meter of reactive zone), one may achieve DAA:MO ratio over a wide range. The combined effect of simultaneous separation of DAA and water inhibition can result in an enhanced performance. As acetone (boiling point of 56.3 C) is the lightest compound compared to water (b.p.: 100 C), MO (b.p.: 129.5 C) and DAA (b.p.: 169.2 C), the reactive section is placed in the rectifying section for the best performance [16]. It should be noted that MO and water form a minimum boiling azeotrope at atmospheric pressure. However, in the present case, since both MO and water are removed as bottom products, this azeotrope is not relevant in context to the process feasibility. Except for the MO water mixture, all the other binaries are nonazeotropic. The VLE data used for simulations is taken from Aspen Plus [11]. In reactive distillation experiments, acetone is present on reactive stages in large proportion thereby homogenising the reaction mixture and avoids a possible liquid liquid split on reactive stages. Whereas in the non-reactive section which is placed below the reactive section, a phase split occurs due to the miscibility gap between water and MO. A phase split in distillation column is known to exhibit instability in column operation. This phase split can be advantageously utilized to remove water as a side stream and obtain a water-free mixture of DAA and MO as bottom products. It is also possible to remove MO as a separate side stream and obtain pure DAA and MO. 5.1. Kinetics of aldol condensation of acetone The kinetic model developed in our earlier work [8] is used to explain the experimental reactive distillation run. According to the kinetic model, the first step (i.e. dimerization of acetone to DAA) is insensitive to the water content and can be best explained using a pseudo-homogeneous type model whereas, the second step (i.e. dehydration of DAA to MO) is explained using the Langmuir Hinselwood type model as it is strongly influenced by adsorption of water on the resin. r DAA = dn DAA dt r MO = dn MO dt = M cat (k f a 2 Ac k ba DAA ) r MO (6) = M catk MO a DAA (1 + K W a W ) 2 (7) where M cat is the weight of catalyst, K W is adsorption constant for water and a i is the activity of the ith component. The UNIQUAC model was used to determine the activities of the components and the UNIQUAC binary interaction parameters were determined by UNIFAC method using Aspen property plus [11]. The binary interaction parameters of UNIQUAC model are given in Table 4. The values of kinetic parameters are given in Table 5. Fig. 10. Steady state experimental and simulated (a) liquid phase composition profile, (b) temperature profile along length of column for 0.5 kw of reboiler duty; total catalyst loading = 280 g; F Acetone = 2 ml/min; reflux rate = 1.7 kg/h) and side draw = 1.2 ml/min.

935 Fig. 11. Steady state simulated liquid phase composition profile along length of column for 0.5 kw of re-boiler duty; total catalyst loading = 280 g; F Acetone = 2.5 ml/min; side draw for water = 0.4 ml/min; side draw for MO = 1.69 ml/min; total number of stages = 24; reactive stages = 8. 5.2. Continuous reactive distillation (CRD) The set up shown in Fig. 4 was used to conduct CRD experiments under total reflux condition by feeding acetone continuously at the top of the reactive section. The products are continuously withdrawn through the bottom stream and the side stream (from 4th stage). Samples at several locations in the column were analysed at different time intervals (30 min) to monitor the performance. An Fig. 12. Steady state simulated (a) configuration (b) liquid phase composition profile (c) temperature profile along length of column for 0.5 kw of re-boiler duty; total catalyst loading = 280 gms; F FA = 3 ml/min; F MeOH = 2 ml/min and side draw=3ml/min; shows side draw stage.

936 equilibrium stage model, considering the heat losses from the column and reboiler to the surrounding, was solved using Aspen Plus simulator (RADFRAC) [11]. Fig. 10a and b show the steady state concentration and temperature profiles along the length of the column, obtained by experiments and simulation in a representative CRD run. The side draw is a heterogeneous mixture of two liquid phases. There is no effect of side draw on the ratio of DAA:MO obtained but it adversely affects the conversion of acetone slightly due to the loss of acetone through the side stream, the conversion decreases from 99.5 to 96.2%. The motivation behind using side draw here is to remove only water effectively by taking advantage of phase split. However, it was not possible to conduct experiments in the present setup with selective aqueous phase removal as side draw because it requires relatively tall column with 25 stages (i.e. 2 m reactive section and 3 m non-reactive section). The experimentally validated model can be further used to obtain the conditions under which near to 100% conversion of acetone can be achieved by removing water using decanter on the side draw stage. Further, an additional side draw may be used to separate MO and DAA as shown in the Fig. 11a. The corresponding simulation results are shown in the Fig. 11b. 6. Case 3: Esterification of fatty acid Esterification of fatty acid such as oleic acid and palmitic acid is an industrially important class of reactions especially with an increasing interest in biodiesel as a renewable energy source. The product of this reaction i.e., fatty acid ester can be used as a substitute for diesel or blend in diesel. The reaction system is similar to the one in the esterification of lactic acid discussed before with the only difference that the self-hydrolysis of fatty acid ester in this case is not as fast as that of methyl lactate. Moreover, the miscibility gap between water and fatty acid ester results in phase splitting in the stripping section similar to that in the case of aldol condensation of acetone discussed in the earlier section. All the binaries are non-azeotropic and the volatility order is methanol (b.p.: 64.7 C) > water (b.p.: 100 C) > methyl oleate (b.p.: 344 C) > oleic acid (b.p.: 360 C). Here, we demonstrate that the phase splitting along with side draw can be advantageously utilized for the production of methyl oleate by removing water as a side stream in reactive distillation column. The phase split between methyl oleate, methanol and water can be effectively utilized to obtain pure methyl oleate as a bottom product in a single RD column. The reaction kinetics is given by Eqs. (9) and (10) and is taken from [4]. Oleic acid + methanol K 1 K 2 methyl oleate + water (9) ( ) r = C Cat (k f 0 exp E f (x FA x MeOH ) RT ( k b0 exp E ) b (x MFA x W )) (10) RT C cat is the catalyst concentration and x i is the mole fraction of the ith component. The UNIFAC-LL model was used to determine the activities of the components and the binary interaction parameters were determined by UNIFAC method using Aspen property plus [11]. The binary interaction parameters of UNIFAC-LL model and the values of kinetic parameters are given in Tables 6 and 7 Simulation results shown in the Fig. 12b indicate that it is possible to remove water along with methanol as a side draw and obtain pure methyl oleate as bottom product in a single reactive distillation column configuration shown in Fig. 12a. Table 6 UNIFAC-LL model binary interaction parameters [11]. Oleic acid Methanol Methyl oleate Water Oleic acid 0 405.09 83.44 377.08 Methanol 46.99 0 24.03 165.26 Methyl oleate 106.26 579.72 0 645.17 Water 232.04 351.0861 252.11 0 Table 7 Kinetic parameters for esterification of fatty acid [4]. Parameter Value E f (kcal/mole) 14 ± 0.99 E b (kcal/mole) 2.68 ± 0.05 Ln k f0 (mole/h) 12.93 ± 2.87 Ln k b0 (mole/h) 4.17 ± 0.04 7. Conclusion Reactive distillation with side draw is a promising alternate configuration which has not been used and investigated well in the past. In the case of esterification of lactic acid, it not only enhances the conversion level at a given methanol to acid mole ratio but also avoids possible ester hydrolysis leading to polymerization and hence, instability in column operation. The configuration can also be used successfully for many other industrially important reactions. It has been shown to work well for aldol condensation of acetone and esterification of fatty acids with methanol. Appendix A. Nomenclature Abbreviation CRD continuous reactive distillation DAA diacetone alcohol FID flame ionization detector MEK methyl ethyl ketone MO mesityl oxide MTBE methyl tertiary butyl ether NSTM number of theoretical stages per meter RD reactive distillation SBRD semi-batch reactive distillation TCD thermal conductivity detector Notation a i activity coefficient of component i E activation energy (kj/mole) k i rate constant (mole/min) M mass (gm) n number of moles (g mole) r i reaction rate (g-mole/min) of component i t time (min) Suffixes f b Ac DAA MO FA LA MFA MLA MeOH cat W forward reaction backward reaction acetone diacetone alcohol mesityl oxide fatty acid lactic acid methyl ester of fatty acid methyl lactate methanol catalyst water

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