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Kinetics of Catalytic Esterification of Propionic Acid with Methanol over Amberlyst 36 Yu-Ting Tsai, Ho-mu Lin, and Ming-Jer Lee* Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan
The kinetic behavior of heterogeneous esterification of propionic acid with methanol over an acidic cationexchange resin, Amberlyst 36, was investigated by using a fixed-bed reactor. The kinetic experiments were conducted at temperatures from 313.15 to 333.15 K, and molar ratios of methanol to propionic acid in the feed stream from 1 to 5. The reaction rate was found to increase with increasing temperature, but the equilibrium conversions of propionic acid changed only slightly over the entire range of reaction temperatures indicating that the heat effect of this reaction is minor. It was also found that the equilibrium conversion of propionic acid increases with the molar ratios of feed increasing from 1 to 5. The relative adsorption strength between any two reacting species was determined from the results of binary adsorption experiments. The magnitude of adsorption strengths follows the order of water > methyl propionate > propionic acid > methanol. The kinetic data were correlated with the ideal-quasi-homogeneous (IQH), the nonideal-quasi-homogeneous (NIQH), the Eley-Rideal (ER), and the Langmuir-Hinshelwood-Hougen-Waston (LHHW) models, respectively, to determine the kinetic parameters for each model. Among these investigated models, the LHHW yielded the best results. 1. Introduction To efficiently recover acetic acid and propionic acid from a waste stream in a caprolactam plant, a reactive distillation technique was used to convert the waste acids and methanol into methyl acetate and methyl propionate, simultaneously. The kinetic and thermodynamic models are essentially needed to implement the process simulation for the waste acid recovery system. Since the kinetic behavior closely depends on the catalyst to be used, the kinetic parameters should be determined from the kinetic data of acetic acid with methanol and propionic acid with methanol over the same type of catalyst. Although several previous papers1-7 reported the kinetic studies for the same reacting systems, the majority of the experimental runs1-5,7 used Amberlyst 15 as acidic catalyst, and no kinetic data for both esterification systems over the same advanced catalyst, Amberlyst 36, are available. The objective of this study is to investigate the kinetic behavior of the esterification of propionic acid with methanol by using a reactor packed with acidic ionexchange resin Amberlyst 36 beads propionic acid + methanol S methyl propionate + water
(1)
The experimental runs were conducted at temperatures ranging from 313.15 K up to near the normal bubble temperature of the reacting mixtures, 333.15 K, and at molar ratios of methanol to propionic acid in the feed stream from 1 to 5. As indicated by previous investigators,1-3,5-7 the adsorption effect on the kinetic behavior was often significant in the esterification systems over ion-exchange resin beads. In the present study, adsorption experiments of three constituent binaries were also conducted to determine the relative adsorption constants of these constituent compounds over the resin beads of Amberlyst 36. The kinetic data obtained from this study were correlated with four models, including the ideal-quasi-homogeneous (IQH), the * To whom correspondence should be addressed. E-mail: mjlee@ mail.ntust.edu.tw. Tel.: +886-2-2737-6626. Fax: +886-2-2737-6644.
nonideal-quasi-homogeneous (NIQH), the Eley-Rideal (ER), and the Langmuir-Hinshelwood-Hougen-Waston (LHHW). The correlated results from these four different kinetics models were compared. In the data correlation, a correlative solution model, the NRTL,8 was applied to calculate the activity coefficients of each constituent. 2. Experimental Section 2.1. Materials. Propionic acid (99%), methanol (99+%, water content methyl propionate > propionic acid > methanol. The ratio of the adsorption constants of water:methyl propionate:propionic acid:methanol is 1:0.943:0.282:0.246. These relative adsorption constants will be introduced in the ER and the LHHW models to reduce the number of undetermined parameters.
where ∆hf is the molar heat of esterification (∆hf ) E0,f - E0,r) and the rate constant of the forward reaction is kf ) Af exp(-E0,f/ RT) and that of the reverse reaction is kr ) Ar exp(-E0,r/RT). In the kinetic parameters determination process, the righthand-side integral of eq 5 was calculated with Simpson’s numerical integration method at a given XA. The NRTL model with binary parameters determined from the phase equilibrium data, as listed in Table 2, was applied to calculate the activity coefficients for each reacting species. The experimental kinetic data, T from 313.15 to 333.15 K and θΒ0 from 1 to 5, a total of 49 data points from the first 7 series of runs, were correlated simultaneously with each kinetic model. The optimal values of the kinetic parameters were obtained by minimizing the fol-
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Table 4. Correlated Results of Kinetic Data model
p
N
10-8 Af (mol min-1 kg-1)
E0,f (kJ mol-1)
Af/Ar
∆hf/R (K)
IQH NIQH ER LHHW
4 4 5 5
49 49 49 49
2.56 51.37 8855.44 9576.50
45.282 50.894 55.279 57.922
5035.12 186.06 148.99 141.28
-2241.02 -222.16 -159.58 -56.04
a
KD
rmsda(mol min-1 kg-1)
85.02 6.94
0.334 0.488 0.297 0.255
N [(1/RHS) - (103F /W) ]2)/(N - p)}1/2 where p is the number of parameters, N is the number of data points. RMSD ) {( ∑ i)1 i A i
lowing objective function with a modified Levenberg-Marquardt algorithm:
RMSD )
rate expression for this catalytic esterification over Amberlyst 36 is given by
∑ [(1/RHS)
i
( -6966.8 ) T 56.0 exp( a a ] T )
9.58 × 1011 exp
N
- (103FA /W)i]2
i)1
N-p
(10)
where N is the number of data points, p is the number of parameters in each model, RHS is the right-hand-side integral of eq 5. The optimal values of the kinetic parameters were obtained by changing the initial guess of the parameters repeatedly during the course of the parameter determination. Table 4 reports the correlated results. As seen from the tabulated values, the rmsd values are 0.334, 0.488, 0.297, and 0.255 from the IQH, NIQH, ER, and LHHW models, respectively. As an the illustrative example, Figure 5 compares the correlated results from different kinetics models with experimental values. In general, the LHHW model yields the best correlation. The smoothed curves on Figures 1 and 2 also show the results calculated from the LHHW model at different temperatures and at different feed compositions, respectively. According to the calculated results, the ER and the LHHW models appear to be better than the IQH and the NIQH models; it is suggested that the adsorption effect should be considered to properly represent the kinetics behavior of methyl propionate synthesis over Amberlyst 36. Among the tested models, the best
-rA )
[a a
A B
- 7.1 × 10-3
C D
(1 + 1.96aA + 1.71aB + 6.55aC + 6.94aD)2
(11) The activation energy of the esterification is about 57.9 kJ mol-1, while the effective molar heat of reaction is about 0.466 kJ mol-1. This indicates that this esterification is slightly endothermic. 5. Conclusions The kinetic behavior has been studied for the catalytic esterification of propionic acid with methanol, over Amberlyst 36, at temperatures from 313.15 to 333.15 K and feed compositions, θB0, from 1 to 5. At a given contact time, the conversion of proponic acid increases with increasing temperature and θB0. The equilibrium conversions XAe were found to be insensitive to reaction temperature, indicating that the heat effect of this reaction system is very slight. As also evidenced from experimental results, the mass transfer resistances resulting from film diffusion and pore diffusion are negligible over the experimental conditions. The results of adsorption experiments showed that the magnitudes of adsorption constants for the constituent species followed the order of water > methyl propionate > propionic acid > methanol. Four kinetics models, including the IQH, NIQH, ER, and LHHW models, have been applied, respectively, to correlate the kinetic data. The LHHW model yielded the best results. This model can be further used in the process simulation and design of the recovery of waste propoinic acid with reactive distillation technique. Acknowledgment Financial support from the Ministry of Economic Affairs, Taiwan, through Grant No. 97-EC-17-A-09-S1-019 is gratefully acknowledged and the authors also deeply thank Professor C.C. Yu for his valuable suggestions during regular discussions. Nomenclature
Figure 5. Comparison of calculated results from different models with experimental values at 323.15 K and θB0 ) 3.
Af ) Arrhenius preexponential factor of the forward reaction (mol min-1 kg-1) Ar ) Arrhenius preexponential factor of the reverse reaction (mol min-1 kg-1) a ) activity aij and bij ) model parameters in the NRTL (K) CA0 ) inlet concentration of propionic acid (mol cm-3) E0,f ) activation energy of the forward reaction (kJ mol-1) E0,r ) activation energy of the reverse reaction (kJ mol-1) F ) volumetric flow rate of feed (cm3 min-1) FA) molar flow rate of propionic acid in feed (mol min-1) ∆hf ) molar heat of methyl propionate synthesis (kJ mol-1)
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kf ) forward reaction rate constant (mol min-1 kg-1) kr ) reverse reaction rate constant (mol min-1 kg-1) KD ) adsorption equilibrium constant of water Ka ) equilibrium constant in terms of activity Kγ ) equilibrium constant in terms of activity coefficient Kx ) equilibrium concentration ratio K1,2 ) ratio of adsorption constants between components 1 and 2 m ) mass of absorbent (g) N ) number of data points n0) total initial number of moles in the liquid phase (mol) niS ) number of moles of component i adsorbed on the surface per unit mass of absorbent (mol g-1) S n ) the constant total number of moles which can be accommodated in the adsorbed phase by unit of solid (mol g-1) p ) number of parameters in the kinetics model -rA ) reaction rate of propionic acid (mol min-1 kg-1) R ) gas constant (kJ mol-1 K-1) T ) temperature (K) W ) catalyst loading (g) x ) mole fraction ∆x ) change in the mole fraction in the liquid phase XA ) conversion of propionic acid XAe ) equilibrium conversion of propionic acid R ) nonrandomness parameter of the NRTL model γ ) activity coefficient θB0 ) molar ratio of feed (methanol to propionic acid) τ ) contact time () W/F) (g min cm-3) Subscripts A, B, C, D ) propionic acid, methanol, methyl propionate, and water, respectively e ) at equilibrium state f ) forward reaction i ) component i ij ) i-j pair r ) reverse reaction Superscripts expt ) experimental value calc ) calculated value
Literature Cited (1) Xu, Z. P.; Chuang, K. T. Kinetics of Acetic Acid Esterification over Ion Exchange Catalyst. Can. J. Chem. Eng. 1996, 74, 493. (2) Song, W.; Venimadhavan, G.; Manning, J. M.; Malone, M. F.; Doherty, M. F. Measurement of Residue Curve Maps and Heterogeneous Kinetics in Methyl Propionate Synthesis. Ind. Eng. Chem. Res. 1998, 37, 1917. (3) Po¨pken, T.; Go¨tze, L.; Gmehling, J. Reaction Kinetics and Chemical Equilibrium of Homogeneously and Heterogeneously Catalyzed Acetic Acid Esterification with Methanol and Methyl Acetate Hydrolysis. Ind. Eng. Chem. Res. 2000, 39, 2601. (4) Kirbaslar, S. I.; Terzioglu, H. Z.; Dramur, U. Catalytic Esterification of Methyl Alcohol with Acetic Acid. Chin. J. Chem. Eng. 2001, 9, 90. (5) Lilja, J.; Aumo, J.; Salmi, T.; Murzin, D. Yu.; Ma¨ki-Arvela, P.; Sundell, M.; Ekman, K.; Peltonen, R.; Vainio, H. Kinetics of Esterification of Propanoic Acid with Methanol over a Fibrous Polymer-Supported Sulphonic Acid Catalyst. Appl. Catal. A: Gen. 2002, 228, 253. (6) Liu, Y.; Lotero, E.; Goodwin, J. G. A., Jr. Comparison of the Esterification of Acetic Acid with Methanol Using Heterogeneous Acid Catalysis. J. Catal. 2006, 242, 278. (7) Ali, S. H. Kinetics of Catalytic Esterification of Propionic Acid with Different Alcohols over Amberlyst 15. Int. J. Chem. Kinet 2009, 41, 432. (8) Renon, H.; Prausnitz, J. M. Local Compositions in Thermodynamic Excess Function for Liquid Mixtures. AIChE J. 1968, 14, 135. (9) Lee, M. J.; Wu, H. T.; Kang, C. H.; Lin, H. M. Kinetic Behavior of Amyl Propionate Synthesis Catalyzed by Acid Cation Exchange Resin. J. Chin. Inst. Chem. Eng. 1999, 30, 117. (10) Kipling, J. J. Adsorption from Solutions of Non-Electrolytes; Academic Press: New York, 1965. (11) Hsieh, C. T.; Ji, W. Y.; Lin, H. M.; Lee, M. J. Multiphase Equilibria for Mixtures Containing Water, Acetic Acid, Propionic Acid, Methyl Acetate and Methyl Propionate. Fluid Phase Equilib. 2008, 271, 69. (12) Gmehling, J.; Onken, U. Vapor-Liquid Equilibrium Data Collection; Chemistry Data Series; DECHEMA: Frankfurt, Germany, 1979; Vol. 1, Part 1, p 271. (13) Blanco, A. M.; Ortega, J. Isobaric Vapor-Liquid Equilibria of Methanol + Methyl Ethanoate+ Methyl Propanoate, and + Methyl Butanoate at 141.3 kPa. J. Chem. Eng. Data 1996, 41, 566. (14) Gmehling, J.; Onken, U. Vapor-Liquid Equilibrium Data Collection; Chemistry Data Series; DECHEMA: Frankfurt, Germany, 1979; Vol. 1, Part 1, p 60.
ReceiVed for reView January 18, 2010 ReVised manuscript receiVed February 26, 2010 Accepted March 4, 2010 IE1001179
