Ind. Eng. Chem. Res. 2009, 48, 9461–9470
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Entrainer Based Reactive Distillation for Esterification of Ethylene Glycol with Acetic Acid Thotla Suman, Seethamraju Srinivas, and Sanjay M. Mahajani* Department of Chemical Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
In this paper, we study the potential of entrainer in reactive distillation involving high boiling reactants to decrease the reactive stage temperature and for separation of one of the products to enhance the conversion. Esterification of ethylene glycol with acetic acid in the presence of Amberlyst 36 with 1,2-dichloro ethane (EDC), as an entrainer, is chosen as the model reaction. The effect of different parameters on selectivity of diacetate of ethylene glycol (DAEG) in entrainer based reactive distillation (EBRD) has been studied both through experiments and simulations. The results show that, by using entrainer, it is possible to obtain close to 100% selectivity toward diester even with a stoichiometric mole ratio, which is otherwise not possible in a conventional reactor. Introduction
k1
ethylene glycol(EG) + acetic acid y\z monoacetate of EG +
The applicability of reactive distillation (RD) is well-known for many industrially important processes.1 In the case of equilibrium controlled reactions such as esterification, the presence of water posseses limitations if one desires to obtain substantial conversion. Hence, for close to 100% conversion of alcohols, excess acid is required. By using reactive distillation, the chemical equilibrium can be shifted toward the production of esters through continuous removal of water and ester, from the reactive zone. If the product(s) can be separated efficiently, then one can use close to stiochiometric feed mole ratio of the reactants. Notable examples are esterification of acetic acid with lower alcohols such as methanol or n-butanol.2,3 In these cases, the temperature of the reactive zone is much lower than the catalyst deactivation temperature. However, for high boiling alcohols, the temperature of the reactive zone may cross the catalyst thermal stability limit. Further, at temperatures above 100 °C, corrosion can be severe in the presence of acetic acid vapors. For ideal systems, when water and acetic acid are more volatile than alcohol and ester, separation of water from acetic acid in a single reactive distillation column is difficult due to the presence of a tangent pinch in the vapor-liquid equilibrium (VLE) thereby limiting the conversion of acetic acid. The entrainer forms an azeotrope with water which facilitates the separation of acetic acid from water and further maintains the reactive zone temperature below a permissible limit. Very few systematic studies on the use of entrainer for such esterification reactions have been reported in the past. Esterification of alkanolamines with carboxylic acids like acrylic acid and methacrylic acid is carried out with simultaneous azeotropic removal of water using appropriate solvent.4,5 Entrainer based reactive distillation for esterification of fatty acids with various entrainers has also been investigated.6 We have listed some potentially important reactions that can benefit from entrainer based reactive distillation in Table 1.
k2
water k3
monoacetate of EG + acetic acid y\z diacetate of EG + water k4
(2) Esterification of ethylene glycol with acetic acid gives the monoacetate of ethylene glycol which further reacts with acetic
The present work investigates the use of entrainer in RD for the industrially important reaction of esterification of ethylene glycol with acetic acid. The reaction (eqs 1 and 2) takes place in two steps involving two products, viz. monoester and diester. * To whom correspondence should be addressed. Phone: +91-2225767246. Fax: +91-22-25726895. E-mail:
[email protected].
(1)
Figure 1. Semibatch reactive distillation setup with distillate rate.
10.1021/ie801886q CCC: $40.75 2009 American Chemical Society Published on Web 05/21/2009
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Table 1. Potentially Industrial Important Reactions for Application of Entrainer in RD reactions
desired product(s)
high boiling alcoholb (A) + acid (B) h ester (C) + water (D) high boiling di/trihydroxy alcoholc (A) + acid (B) h monoester of alcohol (C) + water (D) monoester of alcohol (C) + acid (B) h diester of alcohol (E) + water (D) diester of alcohol (E) + acid (B) h triester of alcohol (F) + water (D) high boiling acidd (A) + alcohole (B) h ester (C) + water (D)
1 2
3
high boiling dibasic acidf (A) + alcohole (B) h monoester (C) + water (D) monoester (C) + alcohole (B) h diester (E) + water (D) glyoxal (A) + alcohole (B) h monoacetal of glyoxal (C) + water (D) monoacetal of glyoxal (C) + alcohole (B) h diacetal of glyoxal (E) + water (D)
4
5
refsa
advantage reactive zone temperature below 100 °C, shifting equilibrium and separation of water and acetic acid reactive zone temperature below 100 °C, shifting equilibrium and selectivity
ester di/triesters
reactive zone temperature below 100 °C, breaking azeotrope between alcohol to reduce downstream processing and shifting equilibrium reactive zone temperature below 100 °C, breaking azeotrope between alcohol and water to reduce downstream processing and shifting equilibrium
ester diesters
diacetal of glyoxal
breaking azeotrope between alcohol and water, shifting equilibrium and selectivity
4, 5 7, 8
6 9, 10
11
a Though some of the reactions are not conducted in RD, more information about the kinetics is available in these references. b P/O tert-butyl cyclohexanol, cellosolve, alkanolamines. c Ethylene glycol, diethylene glycol, glycerol. d Fatty acids. e Ethanol, butanol, isoamyl alcohol, ethyl hexanol. f Maleic acid, glutaric acid, glyoxalic acid.
Table 2. Set of Possible Entrainers for Reactive Distillation to Separate Water12 mutual solubility (25 °C)
a
no.
component
Tb (°C)
XH2O,az (wt %)
Tb,az (°C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
diethyl ether methylene chloride methyl tert-butyl ether methyl acetate chloroform hexaneb diisopropyl ether vinyl acetate tetra chloromethaneb ethyl acetate methyl ethyl ketone benzene cyclohexaneb propyl formate ethylene dichloride tri chloroethylene isopropyl acetate heptaneb dioxane n-propyl acetate methyl propyl ketone diethyl ketone tolueneb methyl isobutyl ketone butyl alcohol n-butyl acetate octaneb 4-methyl,2-pentanol ethyl benzeneb isoamyl acetate butyl ether
34.5 43.5 55 57.1 61 68.7 69 72.7 76.75 77.15 79.6 80 80.8 80.9 82.3 86.2 90.8 98.4 101.3 101.6 101.7 102 110.7 115.8 117.4 125 125.75 131.8 136.2 142 142.6
1.26 1.5 4 2.6 2.8 5.6 4.5 7.3 4.1 8.47 11.3 8.83 8.4 2.3 8.2 7.02 10.6 12.9 17.6 14 19.5 14 13.5 24.3 42.5 28.7 25.5 43.3 33 36.3 33
34.15 38.1 52.6 55.8 56.1 61.6 62.2 66 66 70.38 73.4 69.25 69.5 71.6 71.6 73.4 76.6 79.2 89.8 82.4 83.3 82.9 84.1 87.9 92.7 90.2 89.6 94.3 92 93.8 92.9
H2O in organic (% mol)
organic in H2O (% mol)
5.22 0.784 6.7a 34.6 0.586 0.06 4.37a 4.96a 0.0801 13a 34.2 0.3 0.058 5.8 0.784 11.3 7.03
1.55 0.417 1.05 7.65 0.119 2.78 × 10-4 0.16 0.16 0.0092 1.7 7.63 0.045 1.2 × 10-3 0.553 0.159 0.055 0.395 5 × 10-3
8.35 14.7 7.59 0.237 10 51.2 10 0.081 23.5 0.252
0.33 1.2 1.05 00106 0.343 1.92 0.1 1.1 × 10-5 0.286 2.8 × 10-3
At 20 °C. b Forms azeotrope with acetic acid.
acid to yield the diacetate of ethylene glycol. Ethylene glycol acetates are widely used as solvents for paints, coatings, adhesives, cellulose, plastics, fats, and wood stains. Ethylene glycol monoacetate can be mixed with diesel to prepare oxygenated diesel fuel. A detailed kinetic study of esterification of ethylene glycol using cation exchange resin (Amberlyst 36) has been reported in the literature.7 Esterification of ethylene glycol in RD with entrainer has not been explored to date. The choice of entrainer that forms azeotrope with water depends on the reaction temperature,
azeotrope temperature, and mutual solubility of heterogeneous azeotrope components. Table 2 provides the list of the possible entrainers which form an azeotrope with water. The decision on the amount of entrainer is mainly dictated by the reaction temperature. Relatively, less entrainer is required if the boiling point of the entrainer is close to the boiling point of the reaction mixture. The difference between azeotrope temperature and reaction temperature dictates the efficacy of separation of water from acetic acid which is next to water in terms of its volatility. The mutual solubility of the components in the two phases of
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Figure 2. (a) Composition profile of reboiler. (b) Temperature profile of reboiler. (c) Conversion of EG and AA. (d) Selectivity of DAEG with respect to time for reboiler duty ) 0.7 kW; nonreactive stages ) 12; catalyst loading ) 5% w/w of reactants; mole ratio (acetic acid/ethylene glycol) ) 2; TIR ) 100 ( 1 °C. Table 3. UNIQUAC Binary Interaction Parameters7 (T in K) component (i/j)
DEG
AA
MAEG
DAEG
water
DEG AA MAEG DAEG water
0 288.43 236.84 360.94 722.44
-214.98 0 224.50 1101.82 -180.16
-12.65 -219.36 0 -156.03 -344.69
141.62 -521.77 278.52 0 -113.52
-606.48 300.68 567.78 812.11 0
Table 4. Kinetic Parameters for Esterification of EG7 parameter
value
k10 (mol/(g s)) k20 (mol/(g s)) k30 (mol/(g s)) k40 (mol/(g s)) E1 (kJ/mol) E2 (kJ/mol) E3 (kJ/mol) E4 (kJ/mol)
539.14 114.41 156.43 95.68 39.84 39.85 39.9 39.86
heterogeneous azeotrope is also an important factor, and low mutual solubility of an entrainer in water leads to its minimal loss.6 The relatively large difference between the azeotrope (1,2dichloro ethane (EDC)-water) temperature (71.6 °C) and reaction temperature (∼100 °C) requires less stages for separation of water. Further, EDC is almost immiscible in water and hence is our choice in the present case. We first perform semibatch reactive distillation (SBRD) studies in reactive
reboiler mode to demonstrate the potential of using entrainer in reactive distillation. The effect of various operating parameters such as catalyst loading, mole ratio, entrainer amount, and reboiler duty on the selectivity of mono/diacetate of ethylene glycol in RD is studied. The role of entrainer is not only important from the point of view of complete conversion of acetic acid but it is also crucial in obtaining desired selectivity toward mono or diacetate. The experimental results are compared with simulations performed using Aspen Custom Modeler.13 Further, the steady state simulations are performed using Aspen Plus13 to propose various continuous RD configurations that offer the desired performance. Experimental Work Materials and Catalyst. Ethylene glycol (99.9 wt %) and acetic acid (99.9 wt %) were obtained from SDFineChem. India. Isopropyl alcohol (AR grade, moisture
