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Ind. Eng. Chem. Res. 2010, 49, 9058–9067
Entrainer-Based Reactive Distillation for Esterification of Glycerol with Acetic Acid Amit Hasabnis and Sanjay Mahajani* Department of Chemical Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, India
The applicability of reactive distillation for esterification of glycerol with acetic acid in the presence of Amberlyst-15 as catalyst and ethylene dichloride as an entrainer is evaluated through experiments and simulation. The reaction is studied in both semibatch and continuous reactive distillation systems. The effect of different parameters such as entrainer amount, catalyst loading, and reboiler duty is studied. The results indicate that entrainer-based semibatch reactive distillation can enhance the selectivity toward triacetin to about 100%, which is much greater than that offered by any conventional reactor with stoichiometric mole ratio of reactants. Simulations for both semibatch and continuous reactive distillation are performed, and results agree reasonably well with those obtained by experiments. The best possible design and operating parameters are obtained through detailed simulation using an experimentally validated model. A column configuration is recommended for a continuous process. Introduction Glycerol has recently emerged as an important raw material. It is obtained as a byproduct in the synthesis of biodiesel, which is by now a popular renewable energy source. The economics of the biodiesel production can be improved considerably by developing efficient processes for glycerol-based value-added chemicals.1 Various syntheses reactions are being explored to make valuable products from glycerol. Esterification of glycerol with acetic acid (AA) giving mono-, di-, and triglycerides is one such reaction. Triglyceride, that is, 1,2,3-triacetoxypropane, generally known as triacetin, is used as a plasticizer in various applications such as filters for cigarettes, as food additive (solvent for flavoring), and as an antiknocking agent for fuel. A series-parallel scheme given by eqs 1-3, shows formation of triacetin along with other intermediate esters. k1
glycerol + acetic acid {\} monoacetin + water
(1)
k4 k2
monoacetin + acetic acid {\} diacetin + water
(2)
k5 k3
diacetin + acetic acid {\} triacetin + water
(3)
k6
The conventional process suffers from limited selectivity and conversion because all the three reactions are reversible in nature. Hence, the use of newer technology such as reactive distillation (RD) may be explored to obtain the desired product, that is, triacetin, in a large proportion. RD is the combination of chemical reaction and distillation within a single unit, and it has a wide range of applications.2 RD has been proven to be extremely efficient in obtaining almost quantitative conversion in the case of reactions which are otherwise seriously affected by equilibrium limitations. It shifts the chemical equilibrium toward the desired product through continuous removal of one or more products. In the case of * To whom correspondence should be addressed. Tel.: (022) 2576 7246. Fax: (022) 2572 6895. E-mail:
[email protected]. Address: Dept. of Chemical Engineering, I.I.T. Bombay, Powai, Mumbai, 400076, India.
high boiling components (e.g., glycerol and its derivatives) reactive distillation needs to be applied with caution due to the existence of a high temperature zone in the column, which may adversely affect the catalyst activity and life. In such a case, the use of entrainer keeps the reactive zone temperature below the thermal stability limit of the catalyst and increases the catalyst life. Moreover, in some cases, the use of an entrainer also helps in increasing the efficiency of water removal.3 As regards to the literature on entrainer-based reactive distillation, a detailed study of a similar reaction, that is, esterification of ethylene glycol with AA producing mono- and diacetate of ethylene glycol, is reported by Thotla et al.3 They used Amberlyst-36, a cation-exchange resin as a catalyst, and 1,2-dichloroethane, that is, ethylene dichloride (EDC) as an entrainer. The effect of different parameters on the selectivity of diacetate of ethylene glycol is studied by experiments and simulations in semibatch reactive distillation mode (SBRD). Simulations were performed to propose a column configuration that offers the desired performance. A list of other possible entrainers is also provided. Esterification of alkanolamines with carboxylic acids like acrylic acid and methacrylic acid in RD is carried out using an appropriate solvent by Kimura et al.4 and Teijin.5 RD for the esterification of fatty acids (C2-C4) with various entrainers is studied by Dimian et al.6 They also discussed the conceptual design of a process for the esterification of lauric acid with 1-propanol using sulfated zirconia as catalyst. Literature on esterification of glycerol with AA suggests that various catalysts have been investigated in the past. Table 1 provides the summary of the results obtained. It is evident that in most of the cases, since water is not removed during the course of the reaction, the temperature, pressure, and mole ratios used are unacceptable from a process engineering point of view. Out of all the catalysts, ion exchange resins (Amberlyst-15 and 35) are promising candidates because of their versatility, that is, their high activity in both aqueous and nonaqueous media, and their proven performance in a reactive distillation column. Ion exchange resin shows reasonably good yield toward triacetin at lower temperature and pressure than other catalysts such as para-toluene sulfonic acid (pTSA), hetropoly acids, MgSO4, etc. Reaction may also be carried out in the absence of catalyst at high temperature and pressure.7,8 Use of catalysts such as
10.1021/ie100937p 2010 American Chemical Society Published on Web 09/01/2010
Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010
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Table 1. Summary of the Work Reported on Esterification of Glycerol with AA Sr no. 1
catalyst
performance
conditions
without catalyst
glycerol conversion 96% glycerol conversion 97%
3
PTSA/H2SO4/ trifluoroacetic acid/ potassium hydrogen sulfate/ aluminum sulfate/zinc sulfate ion exchange resin (Amberlyst-15 and 35), vMgSO4 catalyst without catalyst
4
phosphotungstic acid
triacetin yield 84.6%
5
Amberlyst-15
triacetin yield above 95%
6
Fe2O3/S2O2-8/La3+ solid superacid
triacetin yield 93%
7 8 9
heteropoly acid SO42-/ZrO2-/TiO2 H2SO4, ion-exchange resin (Amberlyst-15 and 36)
triacetin yield 97.2% triacetin yield 93.6% ∼100%
10
sulfonic acid
combined diacetin and triacetin yield 80%
11
without catalyst
triacetin yield 35%
12
dodecamolybdophosph-oric acid (PMo) encaged in USY zeolite Amberlyst-15
triacetin yield very low (4%)
2
13
triacetin yield 87% triacetin yield 83%
triacetin yield 41%
aminosulfonic acid, stannic chloride, tungestophosphoric acid, etc. is also reported.9–11 The continuous process for the production of triacetin from glycerol, AA, and acetic anhydride without using any entrainer is reported by Bremus et al.12 This process consists of a continuous tray column reactor wherein, glycerol and excess AA flow in a countercurrent mode. They also use small amounts of acetic anhydride as water scavenger. A wide range of temperature (100-250 °C) and pressure (0.2-30 bar) is covered. The maximum conversion obtained for glycerol is 96% at 7.5 bar pressure and 160 °C using 32 plates in the column. In this process 20-40 wt % AA and 1-5 wt % acetic anhydride remain unreacted. In the present work, we explore the possibility of EDC as an entrainer for the esterification of glycerol with AA for the production of triacetin in reactive distillation. It is expected that high pressure and temperature used by Bremus et al.12 may be avoided in the proposed alternative. The choice of entrainer depends mainly on the reaction temperature. Moreover, the azeotropic temperature and mutual solubilities of entrainer and water in respective phases also influence this decision. The difference in azeotropic temperature and reaction temperature decides the efficacy of separation of AA from water that is to be removed through distillate. Even though AA and water do not form an azeotrope, they are difficult to separate because of the presence of tangent pinch in the vapor liquid equilibrium. As explained later, EDC as an entrainer fulfills all the abovementioned requirements. A detailed study on the kinetics of esterification of glycerol with AA using cation exchange resin (Amberlyst-15) has been reported in the literature.13 We have carried out few experiments to verify this proposed kinetic model. Further, we perform semibatch reactive distillation (SBRD) to confirm the applicability of entrainer in RD. The effect of various parameters such as entrainer amount, catalyst loading, and reboiler duty, in this case, is studied in detail. Simulations are performed using Aspen
temperature, 180-250 °C; pressure, 3-30 bar temperature, 100-180 °C; pressure, 0.2-3 bar; catalyst/reactant ) 0.01-0.5% temperature, ∼30 °C; reaction time, 72 h temperature, 100-130 °C; pressure, 20-50 mmHg catalyst/reactant, 3.8 wt %; temperature, 136-155 °C; reaction, time 7 h temperature, 80 °C; reactive chromatography glycerol/AA ) 1:6; catalyst/ reactant ) 3.3 wt % not given temperature, 130 °C temperature, 100-140 °C; excess AA and acetic anhydride used reaction time, 4 h; glycerol/AA ) 1:9; temperature, 125 °C pressure, 1070 kPa; temperature, 120-160 °C; glycerol/AA ) 1:4 different PMo loading (0.6-5.4 wt %) checked in supercritical CO2; pressure, 65-300 bar, temperature, 100-150 °C.
reference Bremus et al.12
Lu and Ma14 Kharmov8 Zhang and Yuan10 Gelosa et al.13 Li et al.15 Ronghua16 Wu et al.17 Herseczki and Morton18 Meloro et al.19 Galan et al.7 Ferreira et al.20 Marzieh and Hassan21
Custom Modeler, and the results are compared with the experimental results. In the latter part of this paper, we present the results on applicability of continuous reactive distillation (CRD) for this reaction. Experimental runs are performed in continuous mode to validate the predictions by an equilibriumstage model from Aspen Plus. Further, the best possible design and operating conditions are recommended for continuous reactive distillation based on the parametric studies performed using the experimentally validated simulator. Finally, a suitable RD column configuration is suggested that gives substantial yield of triacetin. Experimental Work Materials. Glycerol (99%) was obtained from Qualigens fine chemicals, India. Glacial AA and isopropyl alcohol (AR grade, moisture < 0.02%) were 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 moisture, it was kept under vacuum for 10 h at 70 °C. Analysis. A gas chromatograph (Chemito, GC 8610) equipped with flame ionization detector (FID) was used to analyze the reaction mixture of glycerol, AA, and esters of glycerol. The analysis was carried out using a 30 m long Stabilwax column (ID ) 0.53 mm) supplied by Restek Corporation. Nitrogen was used as a carrier gas with a flow rate of 0.5 mL/min. The oven temperature was varied over a range of 60-240 °C. A gas chromatograph (GC-911; Mak Analytica India, Ltd.) equipped with thermal conductivity detector (TCD) was used to analyze water separately. For this analysis, Porapack-Q column was used with hydrogen as a carrier gas at a flow rate of 30 mL/min. Kinetics of Glycerol Esterification. A concentration-based kinetic model (eq 4-6) proposed by Gelosa et al.13 was used
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Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010
Table 2. Parameters of the Multicomponent Langmuir Adsorption Isotherm13 component
adsorption equilibrium constant K, (cm3/mol)
pure component saturation constant Γi∞ (mol/g)
glycerol AA water monoacetin diacetin triacetin
714 564 965 512 310 107
0.00532 0.00422 0.0221 0.00395 0.00257 0.0012
Table 3. Values of the Kinetic Parameters13
Table 4. Activation Energy and Pre-exponential Factor activation energy (J/mol)
pre-exponential factor gm/(mol · sec)
gly + AA f mono + water
1.46 × 105
3.14 × 1025
mono + AA f di + water
4.07 × 104
1.06 × 109
di + AA f tri + water
4.24 × 104
6.85 × 108
mono + water f gly + AA
1.4 × 105
1.02 × 1026
reaction
parameter
value
di + water f mono + AA
5.29 × 104
1.93 × 1011
k1 (gm/(mol · sec)) k2 (gm/(mol · sec)) k3 (gm/(mol · sec)) Γ K EQ,1 Γ K EQ,2 Γ K EQ,3
2740 775 462 23.5 2.68 0.11
tri + water f Di + AA
7.52 × 104
6.3 × 1012
in the simulations performed to predict the performance of the reactive distillation column. N
∏ (Γ
r1 ) k1ΓglycerolΓacetic acid[1 -
EQ,i)
γi,1
Γ ] /KEQ,1
(4)
i)1
N
r2 ) k2ΓmonoacetinΓacetic acid[1 -
∏ (Γ
γi,2 Γ EQ,i) /KEQ,2]
(5)
i)1
N
r3 ) k3ΓdiacetinΓacetic acid[1 -
∏ (Γ
Γ /KEQ,3 ]
γi,3
EQ,i)
(6)
i)1
Γi is the concentration of ith component in the adsorbed phase calculated by using the Langmuir adsorption isotherm given by eq 7. Kk is the kinetic constant of kth reaction; γi.k is the stoichiometric coefficient of the kth reaction. It should be noted that the concentration appearing in the rate expression refers to the adsorbed phase concentrations. Γi )
KiΓ∞i CLi N
1+
∑
(7)
KjCLi
i)1
CLi is the concentration of ith component in liquid phase; Ki corresponds to the adsorption equilibrium constant and Γi∞ is saturation constant for a pure component. The values of the parameters of the multicomponent adsorption Langmuir isotherm, the kinetic parameters, activation energy, and preexponential factor are given in Tables 2, 3, and 4, respectively. We performed independent batch experiments on reaction kinetics to validate the reported model and its parameter values. The description of the experimental apparatus and procedure used for measurement of batch kinetics can be found elsewhere.22 Parity plot for the time-dependent glycerol and AA conversions from different runs performed at different mole ratios is shown in Figure 1 for different feed ratios of glycerol and AA. It shows that the model predictions agree well with the experimental results. Semibatch Reactive Distillation (SBRD). SBRD runs are less time-consuming and require relatively less quantity of chemicals. Hence, they may be performed as the first step in the process development studies to quickly evaluate the feasibility of RD for a given reaction. Entrainer is used to remove water from the reaction mixture as it helps to achieve close to 100%
yield toward the desired product, triacetin, even at stoichiometric mole ratio. Further, it helps in maintaining the reactive zone temperature below the thermal stability limit of the catalyst (
