Acetalization of Glycerol with Formaldehyde by Reactive Distillation

Article pubs.acs.org/Ind. Eng. Chem.

Acetalization of Glycerol with Formaldehyde by Reactive Distillation Amit Hasabnis and Sanjay Mahajani* Department of Chemical Engineering, IIT-Bombay, Powai, Mumbai 400076, India ABSTRACT: The feasibility of reactive distillation (RD) for the reversible acetalization of glycerol with formaldehyde is evaluated through experiments and simulations. Simultaneous removal of acetal and water from the reactive zone of the RD column helps shift the reaction in the forward direction and achieve close to quantitative conversion levels. The results of laboratory-scale RD experiments performed in this study are compared with the ones predicted by simulation using the kinetics developed in the present work. Since commercial formaldehyde is available in the form of its aqueous solution, a large amount of water has to be removed to achieve substantial conversion. An experimentally validated simulator is thus used to design an appropriate RD configuration that offers minimum energy consumption. Toluene is used as an entrainer to remove water from the RD column. The process is compared with the reported indirect route of transacetalization of glycerol with methylal.

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INTRODUCTION Glycerol is the coproduct in the process of biodiesel production by transacetalization of triglycerides. To improve the economics of the biodiesel process, it is desirable to convert glycerol into value-added products. Several industrially important reactions that use glycerol as a feedstock have been reported in the literature (e.g., see Perez-Pariente et al.1 and Zhou et al.2). The production of acetals of glycerol through acetalization or transacetalization is one such reaction that can add value. These acetals find applications as additives in diesel, as low-toxicity solvents for paints and pharmaceuticals, in the synthesis of insecticides, and in formulations of water-based inks.3 The acetals of glycerol can be produced via two different pathways: transacetalization of glycerol with other acetals and direct acetalization of glycerol with aldehydes. Transacetalization of glycerol with methylal was studied in detail in our earlier work.4 This route allows one to work under anhydrous conditions, and the removal of large amounts of water that otherwise comes with formaldehyde (37% w/w aqueous solution) is not necessary. The problem associated with the transacetalization route is that it requires an additional step to synthesize methylal. On the other hand, direct acetalization avoids this extra step but involves the removal of a large amount of water, which has a substantial latent heat of vaporization. It is therefore necessary to systematically evaluate the potential of the direct acetalization route in comparison with the tranacetalization route. It is with this intention that we undertook the present work. The reaction scheme for acetalization of glycerol with formaldehyde is given by eq 1:

which combines reaction and distillation, would therefore be expected to give enhanced performance. Since water has a high latent heat, its removal in reactive distillation has to be achieved through a proper choice of entrainer. Furthermore, the operating and design parameters for the RD process have to be optimized by using an experimentally validated simulator. In the present work, we not only evaluate the feasibility of reactive distillation for this reaction by experiments and simulations but also determine the energy consumption under near-optimal conditions. This helps us to compare this alternative with the transacetalization route. The article is organized as follows: First, the literature on this reaction is reviewed and the gaps are pointed out. The kinetic studies performed to obtain a suitable rate expression are then described. Further, studies of the use of RD and the possible use of an entrainer in RD by independent simulations using an appropriate vapor−liquid equilibrium (VLE) model from the literature and the kinetics developed in this work are discussed. We show a comparison between the experimental results and the predictions for a continuous RD column. A potentially important RD configuration giving close to quantitative yield of the product is proposed. Finally, the process based on this configuration is compared with the conventional route and with the transacetalization route from the viewpoint of energy consumption. Review of Previous Studies. A detailed literature survey on acetalization of glycerol with aldehydes is given in our earlier work.4 It may be concluded that much of the work reported is from a catalyst selection point of view, and the reaction engineering aspects and process design studies are not evident. Different types of acidic catalysts can be used for the reaction, and the maximum conversion obtained for glycerol was 80% in a batch reactor (Crotti et al.5). The glycerol conversion decreases with an increase in the size of the hydrocarbon chain

Pure formaldehyde is unstable, and thus, formaldehyde is commercially available in the form of its aqueous solution. Because water is one of the products of this reversible reaction, it must be removed during the course of the reaction in order to achieve substantial conversion. Reactive distillation (RD), © 2014 American Chemical Society

Received: Revised: Accepted: Published: 12279

April 17, 2014 May 29, 2014 June 9, 2014 June 9, 2014 dx.doi.org/10.1021/ie501577q | Ind. Eng. Chem. Res. 2014, 53, 12279−12287

Industrial & Engineering Chemistry Research

Article

Table 1. Summary of the Reported Work on Glycerol Acetalization with Aldehydes catalyst Amberlyst 15 (A-15) ion-exchange resin (A-15) and clays H2SO4, p-toluenesulfonic acid (pTSA), A-15, MSA iridium A-15 pTSA, zeolite, resins pTSA, H2SO4, HCl, ion-exchange resin zeolite, A-15, K-10, H-ZSM5, pTSA Amberlyst-47

summary

reference

dimethyl sulfoxide as the solvent; glycerol conversion decreases with the size of the hydrocarbon chain in aldehyde six-membered-ring compound selectivity up to 78%; DCM as the solvent and mild reaction temperature is good for the reaction homogeneous catalyst (H2SO4, pTSA) is more active (90% glycerol conversion) than A-15 (80%) At 40 °C, six-membered-ring selectivity of 83% reaction with acetaldehyde in an aqueous reaction environment, the zeolite catalyst is better; without a water environment; the pTSA catalyst is better proposed semibatch RD process; excess reactant is used to remove water; pTSA gives 58% yield, catalyst 0.01−0.5 mol % T = 70 °C, glycerol/ formaldehyde molar ratio of 1:1.2, zeolite gives 95% conversion in 1 h. pseudohomogeneous kinetic model proposed; glycerol conversion (75%) at 100 °C with 5 wt % catalyst

Paulo et al.6 Deutsch et al.7 Vijayalakshmi et al.8 Crotti et al.5 Hong et al.9 Ruiz et al.3 Bruchmann et al.10 Carolina et al.11 Agirre et al.12

in the aldehyde. The summary of the literature is given in Table 1.

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PROCEDURES, RESULTS, AND DISCUSSION Experiments on Reaction Kinetics. Materials. Glycerol (99%) was obtained from Qualigens Fine Chemicals, India.

Figure 2. Effect of catalyst loading on the conversion of glycerol at 1200 rpm. Conditions: T = 348 K; glycerol/formaldehyde molar ratio = 1:1.

Figure 1. Effect of stirring speed on conversion of glycerol. Conditions: T = 348 K; catalyst loading = 5 wt %; glycerol/ formaldehyde molar ratio = 1:1.

Table 2. Values of Kinetic Parameters with Standard Errors (95% Confidence Intervals) for the Acetalization of Glycerol with Formaldehyde in the Presence of Amberlyst-15 as the Catalyst parameter

value

Ea,f (J/mol) Ea,b (J/mol) ln(kf0) (kmol kg−1 h−1) ln(kb0) (kmol kg−1 h−1) SSE

39052 ± 2564.5 46301 ± 3652.5 18.53 ± 0.2 25.5 ± 0.35 2.1 × 10−4

Figure 3. Effect of molar ratio on the conversion of glycerol at 1200 rpm. Conditions: T = 348 K; catalyst loading = 5 wt %.

Analysis. A gas chromatograph (MAK-Analytica) equipped with a flame ionization detector (FID) was used to analyze the acetals and glycerol. The analysis was carried out using a 30 m long BP-5 column supplied by Restek Corporation. Nitrogen was used as a carrier gas at a flow rate of 30 mL/min. The oven temperature was varied over a range of 45−240 °C. A gas chromatograph (GC-911; Mak Analytica India, Ltd.) equipped

Aqueous formaldehyde was supplied by Merck Ltd., India. The commercially available cation-exchange resin catalyst (Amberlyst-15) was obtained from Rohm and Haas Pvt. Ltd., India. It was washed with distilled water, isopropyl alcohol (IPA), dilute hydrochloric acid, and again with distilled water prior to its use. To remove the moisture, it was heated under vacuum for 10 h at 70 °C. 12280

dx.doi.org/10.1021/ie501577q | Ind. Eng. Chem. Res. 2014, 53, 12279−12287


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Figure 4. Effect of temperature on the conversion of glycerol at 1200 rpm. Conditions: catalyst loading = 5 wt %; glycerol/formaldehyde molar ratio = 1:1.

Figure 5. Parity plot for the conversion of glycerol comparing the experimental and model-predicted results for the reaction kinetics under different conditions.

Figure 6. Experimental setup for the continuous reactive distillation.

Table 3. UNIQUAC Binary Interaction Parameters (T in K)a gly gly HCHO GAb H2O tol

0 −466.89 −442.72 −188.49 −811.26

HCHO 627.29 0 31.14 −149c −43.55

GAb 212.53 −51.35 0 761.09 −426.05

H2O −45.9 240c −1485.8 0 −950.6

quantities of glycerol, aqueous solution of formaldehyde, and catalyst (Amberlyst-15) were added to the reactor. The agitation was started when the desired temperature was attained. This time was considered as the zero reaction time. The samples were removed after specific time intervals and cooled immediately to prevent the possible loss of vapors during sampling. Several experiments were carried out to study the effect of different parameters including temperature, catalyst loading, stirring speed, mole ratio, etc., on the kinetics of the reaction. Effect of Speed of Agitation. To study the effect of external mass transfer on the reaction, experiments were performed at different stirring speeds, and the results are shown in Figure 1. It was observed that there was only a small change in the conversion of glycerol when the speed was increased from 400 to 1200 rpm. Hence, all of the other runs were carried out at 1200 rpm to obtain intrinsic kinetics that is free from external mass transfer limitations. Kinetic Modeling. The kinetics of the liquid-phase reaction in the absence of intraparticle diffusion limitations is explained by an activity-based pseudohomogeneous model as shown in eq 2:

tol 14.42 38.5 6.63 −350.21 0

a

Abbreviations: gly = glycerol, HCHO = formaldehyde; GA = glycerol acetal; tol = toluene. bUNIQUAC parameters derived from Unifac. c Taken from Albert et al.14

with a thermal conductivity detector (TCD) was used to analyze water separately. For this analysis, a Porapack-Q column was used with hydrogen as a carrier gas at a flow rate of 30 mL/min. The concentration of formaldehyde was determined by the sodium sulfite method.13 A weighed quantity of the sample was treated with an excess of sodium sulfite, and the liberated sodium hydroxide was titrated against 0.1 N hydrochloric acid using thymolphthalein as an indicator. Apparatus and Procedure. The reaction was performed in a 300 mL stainless steel batch autoclave (Parr, USA) equipped with temperature and speed monitoring facility. The measured 12281



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Figure 7. (a−e) Column composition and (f) temperature profiles for glycerol acetalization with formaldehyde. Total number of stages = 16, Reactive stages = 6 (10 to 15); FGlycerol = 2 ml/min; mole ratio of HCHO/glycerol = 1; Catalyst loading per stage = 60 gms; Feed stage of Glycerol is 15; Feed stage for aq. formaldehyde 3; Feed stage for toluene 2; reboiler duty = 2302.74 kJ/hr.

“nlinfit” used for nonlinear regression and are given in Table 2. The comparison of model predictions with the experimental results is given in Figures 2−4, where it can be seen that model predictions agree reasonably well with the experimental results. Figure 5 shows the parity plot covering all of the kinetic data points generated under different conditions. Vapor−Liquid Equilibrium. It is known that formaldehyde in aqueous solution is very active and reacts with water to form methylene glycol. Methylene glycol can further undergo polymerization to form poly(oxymethylenes). Hence, appropriate formaldehyde−water VLE data are needed for simulation

n 0 dX Wcat dt ⎛ Ea,f ⎞ = k f0 exp⎜ − a ) ⎟(a ⎝ RT ⎠ glycerol HCHO

ri =

⎛ Ea,b ⎞ − k b0 exp⎜ − a ) ⎟(a ⎝ RT ⎠ acetal water

(2)

where Wcat is the catalyst loading (in g), n0 is the stoichiometric coefficient, and ai is the activity of component i. The kinetic parameters were estimated using the MATLAB function 12282



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Figure 8. Effect of reboiler duty on glycerol conversion in the RD column. Glycerol = 10 mol/h; HCHO/glycerol molar ratio = 1; catalyst loading/stage = 80 g. Figure 10. Proposed column configuration for glycerol acetalization with formaldehyde. Total number of stages = 22; number of reactive stages = 20 (stages 2 to 21); Fglycerol = 10 mol/h; HCHO/glycerol molar ratio = 1; catalyst loading per stage = 120 g; feed stage of glycerol is stage 21; feed stage for aqueous formaldehyde is stage 5; reboiler duty = 3621.58 kJ/h.

Experimental Setup and Procedure. Figure 6 shows a schematic of the continuous RD setup used to perform the reaction of interest. It consists of a reactive section (2 m) along with a nonreactive stripping section with a height of 1 m. The reactive section is packed with Sulzer Katapak-S packing [number of theoretical stages per meter (NTSM) = 3] filled with the cation-exchange resin Amberlyst 15. The nonreactive sections are packed with HYFLUX packing (NTSM = 8) from Evergreen India Ltd. The NTSMs offered by the packings were determined by performing independent experiments. Nonreacting binary pairs from the reaction system were chosen, and batch distillation was performed under similar experimental conditions. The NTSM offered by Katapak S was then determined using the McCabe−Thiele method. The column is thus viewed as a multistage unit with total of 16 equilibrium stages including the reboiler. The reactive section runs from stage 10 to stage 15. In all, six temperature sensors and five sampling ports are provided at different locations, as shown in Figure 6. The attainment of a steady state is indicated by the constancy in all of the temperatures and concentrations with respect to time. Chilled water is used in the condenser to avoid loss of any component, especially formaldehyde, if present. The experimental run is performed at an equimolar feed ratio of glycerol and aqueous formaldehyde. Glycerol, being a highboiling component, is fed at the top of the column (stage 15), and formaldehyde solution is fed near the bottom (stage 3). As mentioned before, the main problem associated with this reaction is the presence of water in the feed (63 wt %), which is also the coproduct of the reaction. The large amount of water is detrimental to the progress of the reaction, as it favors the reverse reaction. Hence, removal of water from the reaction mixture is necessary. This can be facilitated by using an appropriate entrainer. In the present case, toluene is used as an entrainer because of its low mutual solubility with water. Initially, it is fed to the reboiler and circulated in the column. Also, to account for loss of toluene through water stream from the decanter, a very small dosage of toluene is given just above the reboiler. Water is removed from the top, and the acetal along with the unreacted glycerol exits from the bottom stream.

Figure 9. Effect of the number of reactive stages on glycerol conversion in the RD column. Glycerol = 10 mol/h; HCHO/glycerol molar ratio = 1; catalyst loading/stage = 100 g.

Table 4. Effect of Reflux Split Ratio on Conversion and Selectivity split ratio

split stream stage no.

energy required (kJ/kg)

1 0.8 0.5 0.3 0.15

− 5 5 5 5

3433.18 3372.47 3350.95 3316.78 3270.31

purposes. The data reported in the literature have been generated through experiments performed under different conditions.14−17 As will be explained later, in our case the UNIQUAC parameters proposed by Albert et al.14 fit the RD results well. The data used for the simulation are given in Table 3. The volatilities of the components decrease in the following order: formaldehyde (−17.8 °C) > 37 wt % formaldehyde aqueous solution (96 °C) > water (100 °C) > glycerol acetal (187 °C) > glycerol (287 °C). From the volatility order, it can be seen that the products (water and acetal) have intermediate volatilities and that the reactant formaldehyde is relatively less volatile in the presence of water but can escape through the overhead stream if not reacted fully. Continuous Reactive Distillation. In this section, we compare the experimental data (in the form of column profiles) with those obtained by independent simulations using the equilibrium stage model (RADFRAC) from Aspen Plus. 12283



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Figure 11. Profiles of (a) glycerol, (b) formaldehyde, (c) acetal, (d) water, (e) toluene, (f) temperature, and (g) reaction rate on the reactive stages. Total number of stages = 22; number of reactive stages = 20 (stages 2 to 21); Fglycerol = 10 mol/h; HCHO/glycerol molar ratio = 1; catalyst loading per stage = 120 g; feed stage of glycerol is stage 21; feed stage for aqueous formaldehyde is stage 5; reboiler duty = 3621.58 kJ/h.

Comparison with Simulation Results. The experimental results obtained from the laboratory column are compared with the simulations performed using RADFRAC in Aspen, which uses an equilibrium stage model. Although this model makes an assumption of phase equilibrium and does not consider hardware effects, it has been proven to be a good conceptual design tool for preliminary cost estimation and analysis. As shown in Figure 7, the simulation and experimental results agree reasonably well. The overhead stream mainly consists of water and toluene with traces of formaldehyde, whereas the bottom stream consists of unreacted glycerol and acetal. The conversion obtained from this configuration is close to 80% with respect to glycerol, which is much higher than the conversion obtained in a conventional batch reactor for the

same mole ratio (∼60%). The error analysis of the experimental results showed that the error involved due to manual components, GC analysis, and sampling put together was within ±4%. Effect of the Entrainer. In this section, we study the need for an entrainer in the RD column. As stated before, water is present in large proportion. As shown in our previous work,4 an entrainer can successfully remove water from the reactive section/column. In this case, the column temperature in both the configurations (i.e., with and without entrainer) is below the permissible thermal limit of the catalyst (120 °C for Amberlyst-15). However, the loss of formaldehyde with water through the overhead stream is higher (35 mol %) than when the column is operated with the entrainer (almost 0.5 mol %) 12284



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Figure 12. Process flow diagram corresponding to proposed RD configuration 2 (Hasabnis and Mahajani4) for the production of acetals of glycerol. A1: total number of stages = 25; number of reactive stages = 15 (stages 4 to 18); catalyst loading per stage = 180 g; reboiler duty = 7445.81 kJ/h. C1: total number of stages = 22; number of reactive stages = 18 (stages 2 to 19); Fglycerol = 0.541 kmol/h; glycerol/methylal molar ratio = 4.5; catalyst loading per stage = 167 g; feed stage of glycerol is stage 19; reboiler duty = 30960 kJ/h; feed stage for methylal is stage 2. C2: total number of stages = 6; reflux ratio = 0.35; reboiler duty = 15909.91 kJ/h; feed stage is stage 4.

stages was assessed, and it was found that there is no need of stripping and/or rectifying stages for this reaction. In the bottom part of the column, only acetal and water are present. Water can be stripped off with the help of toluene, and pure acetal can be drawn from the bottom stream. Also, the rectifying section is mainly dominated by toluene and water, which do not require separation. Hence, a fully reactive column was used for the further analysis. Effect of Reboiler Duty. Figure 8 shows that increasing the reboiler duty increases the conversion of glycerol because more water is removed from the reactive section, which shifts the reaction in the forward direction. Effect of the Number of Reactive Stages. The effect of the number of reactive stages was studied by keeping a catalyst loading of 100 g/stage. Figure 9 shows that more than 15 reactive stages are sufficient to obtain glycerol conversion above 98%. Though, it can be seen that by using 17 reactive stages, more than 99% conversion can be obtained and 20 stages are enough to ensure quatitative conversion. It is also observed from simulations that, by using excess reactive stages, reverse reaction of water and acetal is likely to take place and overdesign is not desirable. Effect of Reflux Splitting. A reflux-splitting configuration is useful especially when one needs efficient removal of water formed in the reaction. Here the reflux (i.e., the toluene-rich phase) is split into two parts; one part is sent to the top of the column, and the other is introduced at the bottom of the column to strip off water from the reactive zone. A reflux split ratio of unity means all of the reflux is given to the top of the column as in the normal case. Table 4 summarizes the effect of the reflux split ratio to get greater than 98% conversion of glycerol and the corresponding energy consumptions. The parametric study was carried out using a column with 20 reactive stages (catalyst loading of 100

Table 5. Kinetics of the Acetalization of Methanol with Formaldehyde (Drunsel et al.18) parameter

value

Ea,f (kJ/mol) Ea,b (kJ/mol) kf0 (mol mol−1 s−1) kb0 (mol mol−1 s−1)

54.65 54.74 0.322 0.0125

Table 6. VLE Parameters for the Acetalization of Methanol with Formaldehyde (Drunsel et al.18)a HCHO H2O MA MeOH MG

HCHO 0 −254.5 0 −128.6 59.2

H2O 867.8 0 501.053 −181.0 −191.8

MA 0 −7.1885 0 −71.2 −7.1885

MeOH 238.4 289.6 410 0 289.6

MG 189.2 189.5 501.053 −181 0

a

Abbreviations: HCHO = formaldehyde; MA = methylal; MeOH = methanol; MG = methylene glycol.

for a similar column design (i.e., 40 stages with eight nonreactive rectifying stages and 10 nonreactive stripping stages). We also performed batch experiments to confirm the loss of formaldehyde through the overhead water stream. It is evident from the experiments that formaldehyde loss can be significantly reduced using toluene as an entrainer. To obtain close to 100% conversion at a stoichiometric feed ratio of reactants, the effects of various parameters, including the number of rectifying and stripping stages, the number of reactive stages, the reboiler duty, and reflux splitting, were studied with the help of the experimentally validated simulator, and the results are presented in the next section. Parametric Study. Effect of Stripping and Rectifying Sections. The necessity of nonreactive stripping and rectifying 12285



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Figure 13. Flowsheet for conventional production of glycerol acetal. CSTR: feeds are 10 mol/h for glycerol, 10 mol/h for HCHO, and 30 mol/h for water. C1: total number of stages = 12; feed stage is stage 8; reflux ratio = 0.1; reboiler duty = 2160.56 kJ/h. C2: total number of stages = 6; feed stage is stage 4; reflux ratio = 0.11; reboiler duty = 441.29 kJ/h. C3: total number of stages = 8; feed stage is stage 5; reflux ratio = 0.15; reboiler duty = 426.64 kJ/h.

in detail and showed that reactive distillation works much better than the conventional sequential approach of reaction followed by distillation. A configuration with excess glycerol followed by distillation to separate the acetal was found to be the best among all (column C1 in Figure 12). The energy consumption in that case was about 2510 kJ/kg of the product, after consideration of the energy available with the hot glycerol stream. To compare this route with the direct acetalization route studied here, we include the synthesis of methylal in the transacetalization process (see Figure 12). Methylal is produced by acetalization of formaldehyde (in aqueous solution) with methanol in the presence of ion-exchange resin as the catalyst, and reactive distillation proved to be very beneficial in this case as well.17 For the simulation of this column (A1), the activitybased pseudohomogenoeus kinetic model proposed by Drunsel et al.18 is used (see Table 5). The VLE parameters used for this column are given in Table 6. The overhead stream in the first RD column is almost pure methylal, which goes further to the transacetalization step performed in another RD column that receives glycerol in excess. The bottom stream of this column contains acetal and excess glycerol, which are separated in a normal distillation column and glycerol is recycled back. We simulated this entire process in Aspen and found a total energy consumption of 3140 kJ/kg of acetal. For the RD configuration proposed in this work, the energy requirement is 3517.46 kJ/kg of acetal. Hence, transacetalization of glycerol with methylal appears to be a slightly better option than direct acetalization of glycerol with formaldehyde from an energy consumption point of view. However, since there is not much difference in the energy consumption for the two routes, it would be advisible to perform detailed economic analysis (including capital costs, etc.) before a final decision is made. Comparison with the Conventional Configuration. In the conventional process (Figure 13), the reaction is performed in a conventional reactor to obtain near equilibrium conversion, after which the reaction mixture is sent to the distillation columns for downstream separation. The first distillation

g/stage) with a nonreactive reboiler and condenser attached to it. Here it is observed that the split stream needs to be introduced below the feed stage of formaldehyde or else it does not serve the purpose of water removal from the column. From Table 4 it can be concluded that not much difference is observed in the energy required for the column configuration at higher split ratios, unlike glycerol esterification (the reaction of glycerol with acetic acid4), where water removal was very important for the reaction to shift in the forward direction. On the other hand, formaldehyde is very reactive, and the kinetics of the reaction is such that it does not need complete removal of water from the reactive section. Proposed Configuration. We performed various simulations to obtain an RD configuration that gives close to 100% conversion of glycerol with a stoichiometric feed ratio of glycerol and formaldehyde (see Figure 10). The proposed configuration is a fully reactive RD configuration. Glycerol is fed at the top of the reactive section and formaldehyde toward the bottom (at stage 5). The makeup toluene stream is introduced at stage 2, just above the reboiler, to account for its loss through the distillate. As discussed in the earlier section, by the use of toluene as an entrainer, the loss of formaldehyde through water can also be minimized. Figure 11 shows the column profiles for each of the components and the temperature profile along the height of the column. It can be seen that water is present predominantly in almost all the stages of the column and is successfully removed from the top of the column with the help of toluene. Almost pure acetal is obtained as a bottom product. From the rate profile, it can be seen that much of the reaction takes place at the top of the reactive section, although other stages are necessary to get close to 100% conversion. Comparison with the Transacetalization Route. As mentioned before, the acetal may be produced by two different routes: direct acetalization or transacetalization. We can use either conventional or reactive-distillation-based processes in both cases. In our previous work,4 we studied transacetalization 12286



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(4) Hasabnis, A. C.; Mahajani, S. M. Transacetalization of glycerol with methylal by reactive distillation. Ind. Eng. Chem. Res. 2012, 51, 13021−13036. (5) Crotti, C.; Farnetti, E.; Guidolin, N. Alternative intermediate for glycerol valorization. Green Chem. 2010, 12, 2225−2231. (6) Silva, P. H. R.; Gonca̧lves, V. L. C.; Mota, C. J. A. Glycerol acetals as anti-freezing additives for biodiesel. Bioresour. Technol. 2010, 101, 6225−6229. (7) Deutsch, J.; Martin, A.; Lieske, H. Investigation on heterogeneously catalyzed condensation of glycerol to cyclic acetals. J. Catal. 2007, 245, 428−435. (8) Vijayalakshmi, C.; Udaybhaskar, R.; Viswanath, K.; Satyavathi, B.; Prasad, R. B. N. Novel route of recovery of glycerol from aqueous solutions by reversible reactions. Int. J. Chem. React. Eng. 2009, 7, 38 DOI: 10.2202/1542-6580.2054. (9) Hong, X.; Kolah, A. K.; McGiveron, O.; Lira, C. T.; Miller, D. J. An improved approach to make cyclic acetals from glycerol. http:// www3.aiche.org/proceedings/Abstract.aspx?PaperID=163946 (accessed May 28, 2014). (10) Bruchmann, B.; Haberle, K.; Grunner, H.; Hirn, M. Prepration of cyclic acetals or ketals. U.S. Patent 5,917,059. (11) Carolina, X.; Valter, L.; Caludia, J. A. M. Water-tolerant zeolite catalyst for the acetalization of glycerol. Green Chem. 2009, 11, 38−41. (12) Agirre, I.; Garcia, I.; Barrio, V. L.; Güemez, M. B.; Cambra, J. F.; Arias, P. L. Glycerol acetals, kinetic study at the reaction between glycerol and formaldehyde. Biomass Bioenergy 2011, 35 (8), 3636− 3642. (13) Chopade, S. P.; Sharma, M. M. Acetalization of ethylene glycol with formaldehyde using cation-exchange resins as catalysts: Batch versus reactive distillation. React. Funct. Polym. 1997, 34, 37−45. (14) Albert, M.; García, B. C.; Kuhnert, C.; Peschla, R.; Maurer, G. Vapor liquid equilibrium of formaldehyde and methanol. AIChE J. 2000, 46, 1676−1687. (15) Brandani, V.; Giacomo, G. D.; Foscoio, P. U. Isothermal vapor liquid equilibria for the water−formaldehyde system. A predictive thermodynamic model. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 179−185. (16) Maurer, G. Vapor−liquid equilibrium of formaldehyde- and water-containing multicomponent mixtures. AIChE J. 1986, 32, 932− 948. (17) Kolah, A. K.; Mahajani, S. M.; Sharma, M. M. Acetalization of formaldehyde with methanol in batch and continuous reactive distillation columns. Ind. Eng. Chem. Res. 1996, 35, 3707−3720. (18) Drunsel, J.; Renner, M.; Hasse, H. Experimental study and model of reaction kinetics of heterogeneously catalyzed methylal synthesis. Chem. Eng. Res. Des. 2012, 90, 696−703.

column separates light components (water and formaldehyde) from the heavy ones (glycerol and acetal), which are further separated in the subsequent distillation columns. The total energy requirement for the process was found to be 4853.35 kJ/kg. The reflux ratio and the number of stages for each distillation column used in the process were determined using the DSTWU distillation package in Aspen Plus, which uses a short-cut method based on the Winn−Underwood−Gilliland correlation. It is known that short-cut methods work reasonably well for nonazeotropic mixtures at the conceptual design stage.

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CONCLUSION The present work demonstrates the applicability of RD for acetalization of glycerol with aqueous formaldehyde solution using Amberlyst-15 as the catalyst. A pseudohomogeneous activity-based model is proposed, and it successfully explains the data for the laboratory-scale column. Toluene is used an entrainer to remove water and to minimize loss of formaldehyde through the overhead water stream. A fully reactive distillation configuration is proposed for the best performance in the column. The proposed configuration is more energy efficient than the conventional process of acetal production (4853.35 kJ/kg). The proposed process was further compared with the alternate route of transacetalization, and from an energy consumption point of view, it was found that transacetalization of glycerol with methylal (3140 kJ/kg) consumes slightly less energy than the direct acetalization route (3517.46 kJ/kg).

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AUTHOR INFORMATION

Corresponding Author

*Phone: (022) 2576 7246. Fax: (022) 2572 6895. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS RD reactive distillation GC gas chromatography IPA isopropyl alcohol FID flame ionization detector TCD thermal conductivity detector

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NOMENCLATURE Wcat catalyst loading n0 stoichiometric coefficient Ea,f activation energy of forward reaction Ea,b activation energy of backward reaction kf forward reaction rate constant kb backward reaction rate constant

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REFERENCES

(1) Perez-Pariente, J.; Díaz, I.; Mohíno, F.; Sastre, E. Selective synthesis of fatty monoglycerides by using functionalised mesoporous catalysts. Appl. Catal., A 2003, 254, 173−188. (2) Zhou, C. H.; Beltramini, J. N.; Fan, Y. X.; Lu, G. Q. Chem. Soc. Rev. 2008, 37, 527. (3) Ruiz, V. R.; Velty, A.; Santos, L. L.; Leyva-Pérex, A.; Sabater, M. J.; Iborra, S.; Corma, A. Gold catalysts and solid catalysts for biomass transformations: Valorization of glycerol and glycerol−water mixtures through formation of cyclic acetals. J. Catal. 2010, 271, 351−357. 12287


Acetalization of Glycerol with Formaldehyde by Reactive Distillation

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