ARTICLE pubs.acs.org/IECR
Entrainer-Based Reactive Distillation for the Synthesis of 2-Ethylhexyl Acetate Prafull Patidar and Sanjay Mahajani* Department of Chemical Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai, 400 076, India
bS Supporting Information ABSTRACT: The production of 2-ethylhexyl acetate by esterification of 2-ethylhexanol with acetic acid in the presence of ionexchange resin (Amberlyst-15) as catalyst is investigated in a reactive distillation (RD) column using toluene as an entrainer. Toluene helps in removing water efficiently from the reactive zone and maintains the reactive zone temperature below the thermal stability limit of the catalyst. Experiments as well as simulations show that quantitative conversion is achievable using the proposed entrainer-based reactive distillation process. The effect of different parameters on the conversion is studied through simulations. The proposed RD process is compared with the conventional process that uses homogeneous p-toluenesulfonic acid (pTSA) as a catalyst and it is shown that the proposed process is more energy efficient and less capital intensive.
1. INTRODUCTION Reactive distillation (RD) is a combination of chemical reaction and distillation in a single unit. Various advantages of RD include increased conversion in case of equilibrium reactions, improved selectivity, heat integration, avoidance of azeotropes, etc. RD is well-known for the commercial production of many industrially important chemicals like MTBE, methyl acetate, etc. and is looked upon as an efficient tool of process intensification.1,2 Notable examples are esterification of acetic acid with lower alcohols such as methanol or n-butanol, wherein, the chemical equilibrium can be shifted toward the production of esters through continuous removal of water and ester from the reactive zone. However, for high boiling alcohols, the high temperature zone in the RD column needs special attention. The temperature of the reactive zone may cross the catalyst thermal stability limit which may adversely affect its activity and life. Moreover, above 100 °C, corrosion can be severe in the presence of acetic acid vapors. In such a case, the use of an external mass separating agent, commonly known as an entrainer, helps maintain the reactive zone temperature under control and protects the catalyst from thermal degradation. The entrainer may also enhance the separation efficiency by selectively increasing the relative volatility of one of the products and hence improve the per-pass conversion. 2-Ethylhexyl acetate is a high boiling retarder solvent with limited water solubility, commonly used to promote flow and retard blushing in formulations such as lacquers, lacquer emulsions, screen inks, baking enamels, and air-dry enamels. It is an excellent solvent for nitrocellulose and is widely used in the leather industry. Although synthesis of 2-ethylhexyl acetate by esterification of 2-ethylhexanol with acetic acid (see eq 1) has been known for a long time, there exists no literature on the application of RD for this reaction. Systematic investigations in this regard, which form the main objective of the present work, would help improve the existing process and make it more costeffective and compact. r 2011 American Chemical Society
The article is organized as follows: First we review the earlier investigations on entrainer-based reactive distillation (EBRD). The kinetics and vaporliquid equilibrium (VLE) data required to analyze the RD process is then presented. The data on the kinetics for the heterogeneous ion-exchange resin (Amberlyt-15) catalyst and the VLE for some of the binary pairs are available in literature. The experiments are performed to generate the kinetic data on homogeneous pTSA catalyst and the VLE data for the remaining binary pairs; the relevant parameters are estimated. Further, we describe the experimental work on a laboratory-scale continuous reactive distillation unit. The results are compared with steady state ASPEN simulations, and the experimentally validated simulator is used to carry out parametric studies. Finally, we present the comparison of the proposed entrainedbased RD process with the conventional process that uses pTSA catalyst.
2. PREVIOUS STUDIES Only a few investigations on the use of an entrainer in reactive distillation have been reported in the past, especially in the past decade. Esterification of alkanolamines with carboxylic acids like acrylic acid and methacrylic acid with simultaneous azeotropic removal of water using an appropriate solvent is carried out by Kimura et al.3 Entrainer-based reactive distillation for the Special Issue: CAMURE 8 and ISMR 7 Received: July 25, 2011 Accepted: September 27, 2011 Revised: September 26, 2011 Published: September 27, 2011 8748
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Industrial & Engineering Chemistry Research esterification of fatty acids with various entrainers has been investigated by Dimian et al.4 They have shown that by carrying out esterification of lauric acid with 1-propanol in the presence of n-propyl-acetate as an entrainer, the overall reaction rate increases substantially and that the catalyst loading can be reduced up to 50% as against the process without an entrainer, performed under similar conditions. Wang and Wong5 studied an entrainerbased reactive distillation process for the production of isopropyl palmitate. They have shown through plant-wide control studies that the entrainer inventory in the reactive distillation column is self-regulatory, provided that the organic phase liquid level is controlled without offset. Thotla et al.6 have reported a detailed study of esterification of ethylene glycol with acetic acid producing mono- and diacetates of ethylene glycol using ethylene dichloride (EDC) as the entrainer. They have listed some potentially important reactions that can benefit from the entrainerbased reactive distillation as well as some possible entrainers for RD to separate water. Recently, Hasabnis and Mahajani7 have also proved EDC to be an appropriate entrainer for the esterification of glycerol to achieve a quantitative yield toward triacetin. de Jong et al.8 have suggested that for the entrainer selection in entrainer-based RD, the rules applied are similar to the ones established for azeotropic distillation. Nevertheless, we believe that the kinetics of the reaction may be a deciding factor for the entrainer-based RD concept to be feasible. Following the entrainer selection criteria discussed in detail by various authors,4,6,8 we found toluene to be suitable as the entrainer for our system. The presence of an entrainer influences both concentration and temperature profiles along the height of the column thereby significantly affecting the local reaction rates. It is therefore necessary to study the catalytic distillation process for a given reaction through systematic experiments and simulation, and arrive at a design that gives the desired performance in a cost-effective way.
3. KINETICS AND VAPORLIQUID EQUILIBRIUM (VLE) 3.1. Experimental Work. Materials and Catalyst. 2-Ethylhexanol (99 wt %) and acetic acid (99.9 wt %) were obtained from s. d. Fine Chemicals, Ltd., India. Isopropyl alcohol (AR grade, moisture 125 °C due to the presence of high-boiling ester. The conventional process requires additional separation columns to obtain pure product. The product from the reactive reboiler is sent to the second column wherein 2-ethylhexyl acetate is separated from any unconverted reactants and other impurities. However, the bottom product from the second column still contains a catalyst impurity which makes it essential to draw pure ester in the vapor form from the reboiler, followed by a condenser to obtain a liquid product. This makes the conventional process capital and energy intensive. The spent catalyst is usually not recovered or recycled, but is purged and sent for effluent treatment. In this work, we compare the conventional process with the proposed process of catalytic distillation which uses a heterogeneous catalyst. The heterogeneous catalyst allows one to use a fully hybrid RD column. The conventional process is simulated using Aspen (RADFRAC), and it is found that the energy consumption is much higher (3.41 times in the case presented) than the consumption of the catalytic distillation process proposed in this work (Figure 11). Simulation conditions and the results for both the processes are compared in Tables 6 and 7. ’ CONCLUSION Entrainer-based reactive distillation (EBRD) can be successfully used for the synthesis of high purity 2-ethylhexyl acetate. Use of toluene as an entrainer results in efficient removal of water formed in the reaction, enabling us to achieve near complete conversion. Further, it controls the reaction temperature and prevents the catalyst from possible thermal degradation. The 8758
dx.doi.org/10.1021/ie2016027 |Ind. Eng. Chem. Res. 2012, 51, 8748–8759
Industrial & Engineering Chemistry Research proposed process offers significant energy savings and capital cost savings compared to the conventional process.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional equilibrium data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ NOMENCLATURE ai = activity of species i Ccat = catalyst concentration, (kg/m3) Ki = adsorption constant of species i Ef0,Eb0 = activation energy of forward and backward reaction, J/mol kf0,kb0 = Arrhenius pre-exponential factor for forward and backward rate constant, kmol /(kg 3 h) for heterogeneous model; kmol 3 m3/(kg 3 h) for homogeneous model kf,kb = forward and backward reaction rate constants: kmol/(kg 3 h) for heterogeneous model, kmol 3 m3/(kg 3 h) for homogeneous model Km = adsorption constant of acetic acid Kw = adsorption constant of water Mcat = mass of catalyst, kg n = initial molar holdup, kmol ri = rate of the reaction of species i, kmol.m3/(kg.hr) xi = mole fraction of species i
ARTICLE
(5) Wang, S. J.; Wong, D. S. H. Design and Control of EntrainerAdded Reactive Distillation for Fatty Ester Production. Ind. Eng. Chem. Res. 2006, 45, 9042–9049. (6) Thotla, S.; Srinivas, S.; Mahajani, S. M. Entrainer Based Reactive Distillation for Esterification of Ethylene Glycol with Acetic Acid. Ind. Eng. Chem. Res. 2009, 48, 9461–9470. (7) Hasabnis, A. C.; Mahajani, S. M. Entrainer-Based Reactive Distillation for Esterification of Glycerol with Acetic Acid. Ind. Eng. Chem. Res. 2010, 49, 9058–9067. (8) de Jong, M. C.; Zondervan, E.; Dimian, A. C.; de Haan, A. B. Entrainer Selection for the Synthesis of Fatty Acid Esters by EntrainerBased Reactive Distillation. Chem. Eng. Res. Des. 2010, 88, 34–44. (9) Gyani, V. C.; Mahajani, S. M. Reactive Chromatography for the Synthesis of 2-Ethylhexyl Acetate. Sep. Sci. Technol. 2008, 43, 2245– 2268. (10) Raal, J. D.; M€uhlbauer, A. L. Phase Equilibria: Measurement and Computation; Taylor and Francis: WA, 1998. (11) Forner, F.; Brehelin, D.; Rouzineau, D.; Meyer, M.; Repke, J.-U. Startup of a Reactive Distillation Process with a Decanter. Chem. Eng. Process 2008, 47, 1976–1985.
Greek Letters
νi = stoichiometric coefficient for component i Abbreviations
2EHAc = 2-ethylhexyl acetate 2EHOH = 2-ethylhexanol AA = acetic acid EBRD = entrainer-based reactive distillation GC = gas chromatograph MTBE = methyl tertiary butyl ether NTSM = number of theoretical stages per meter pTSA = p-toluenesulfonic acid RCM = residue curve map RD = reactive distillation VLE = vapor liquid equilibrium
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