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Ind. Eng. Chem. Res. 2009, 48, 1816–1823
Catalytic Synthesis of Glycerol Monoacetate Using a Continuous Expanded Bed Column Reactor Packed with Cation-Exchange Resin Takuya Fukumura Department of Chemical Engineering, Ichinoseki National College of Technology, Hagisyo-Takanashi, Ichinoseki 021-8511, Japan
Takuji Toda, Yuichiro Seki, Masaki Kubo, Naomi Shibasaki-Kitakawa, and Toshikuni Yonemoto* Department of Chemical Engineering, Tohoku UniVersity, Aoba-yama 6-6-07, Sendai 980-8579, Japan
The selective and continuous production of monoacetin using an expanded-bed column reactor, in which the reactive fluid flows vertically upward and the catalyst is hydrodynamically floated, was studied. Acetic acid and glycerol were used as the substrates and the proton-type cation-exchange resin Amberlyst 16 was used as the catalyst. Batch reaction experiments under various conditions were first conducted to investigate the details of the heterogeneous catalytic reaction mechanism. The esterification of acetic acid with glycerol to produce the target product monoacetin was proved to proceed predominantly over the undesired consecutive esterification reactions to diacetin and triacetin. The intraparticle and extraparticle mass-transfer resistances for the catalytic resin were negligible. The Eley-Rideal-type catalytic reaction model was successfully applied to the experimental data for the various conditions. Next, the hydrodynamic correlation between the flow rate of the reactive fluid and the corresponding expanded-bed height on the continuous reactor was determined experimentally. Furthermore, the reaction and transport model for a continuous expanded-bed column reactor was constructed based on the kinetic model for the batch system. The numerical calculation simulated the experimental data well, and the optimum operating conditions were successfully determined by the theoretical analysis. 1. Introduction Acetic acid esters are widely used as flavors, solvents, and food additives. Until now, the esters have been produced via the esterification reaction of acetic acid with alcohols, mainly using a homogeneous sulfuric acid catalyst. For the homogeneous catalytic process, however, there are several problems such as (i) corrosion by the acid catalysts, and (ii) the requirement of an extra process to remove the acid catalyst after the reaction process. Recently, much attention has been given to the cation-exchange resin as a heterogeneous solid catalyst, to overcome the problems of using the homogeneous sulfuric acid catalyst. Many studies using proton-type cation-exchange resins for the production of acetic acid esters such as methyl acetate,1,2 ethyl acetate,3,4 iso-amyl acetate,5 phenyl acetate,6 and glycerol triacetate7 have been conducted. Glycerol monoacetate (which is also called “monoacetin”) is produced industrially via the reaction of acetic acid with glycerol and utilized as a solvent or a food additive. Furthermore, it can be the potential precursor of the acetylated monoglyceride (ester of acetic acid and fatty acid and glycerol), which has been recently expected for use as a safety plasticizer. Therefore, it is industrially important to design a continuous production system for monoacetin using the proton-type cationexchange resin as the heterogeneous catalyst. In the present study, the batch esterification experiments for the production of monoacetin using acetic acid and glycerol substrates with a proton-type cation-exchange resin catalyst were conducted to elucidate the effects of various operating factors on the reaction behavior. Next, the reaction processes were theoretically analyzed by applying the Eley-Rideal (ER)-type kinetic model to the experimental data. Based on the experimental and theoretical results for the batch reaction system, the expanded-bed column reactor (EBCR) system was designed and * To whom correspondence should be addressed. Tel.: +81-22-7957255. Fax: +81-22-795-7258. E-mail address: toshiy@ rpel.che.tohoku.ac.jp.
operated to confirm the effectiveness for the selective and continuous production of monoacetin. 2. Experimental Section 2.1. Batch Esterification Experiment. Special-grade acetic acid and glycerol were used as substrates, and both were purchased from Wako Pure Chemical Industries (Tokyo, Japan). Amberlyst 16 (Rohm & Haas, Japan), which has a nominal ionexchange capacity of 4.9 mol/(kg dry resin) and a nominal particle size of 0.60-0.80 mm (supplied water-wet) in the proton form, was used as the cation-exchange resin catalyst. The fresh resin initially contained color impurities; therefore, it was rinsed with pure water, followed by ethanol. The resin was dried in the oven at 351.15 K for 24 h, to remove the ethanol and water that remained in the resin. A 100 cm3 solvent-free substrate solution with the required molar ratio of acetic acid to glycerol (1:1, 1:2, or 3:1), contained in a glass bottle (inner volume of 500 cm3) with a cap, was placed in a thermal bath at a temperature of 323.15 K. After the solution temperature reached that of the bath, the esterification reaction was initiated by adding an adequate amount of catalyst resin to the bottle. The experimental conditions are listed in Table 1. The bold values in the table represent those of the basic condition. Reaction samples (1 cm3) were collected at a specific time interval during the reaction. The concentrations of acetic acid and formed esters (monoacetin, diacetin, triacetin) were determined using an high-performance liquid chromatography (HPLC) system that was equipped with an ultraviolet (UV) detector (Model L4200, Hitachi Co. Ltd., Tokyo, Japan) and a reversephase polymer column (Model Rspak DE 413, Showa Denko Co. Ltd., Tokyo, Japan). An acetonitrile/water mixture (20:80 v/v) was used as the mobile phase, and the flow rate was 0.6 cm3/min. The sample injection volume was 10 µL, and the wavelength of the UV detector was 230 nm. 2.2. Continuous Esterification Experiment. The continuous esterification experiments were conducted using an expanded-
10.1021/ie800625g CCC: $40.75 2009 American Chemical Society Published on Web 01/21/2009
Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 1817 a
Table 1. Experimental Conditions parameter
value
temperature working volume shaking speed catalyst concentration initial acetic acid concentration initial glycerol concentration resin particle size
323.15 K 100 cm3 80 spm, 160 spm 75 g/dm3, 150 g/dm3 7.67 mol/dm3, 4.92 mol/dm3, 12.3 mol/dm3 7.67 mol/dm3, 9.83 mol/dm3, 4.09 mol/dm3 0.39-0.85 mm, ∼0.39 mm
a
Bold values represent those of the basic condition.
Figure 3. Effect of shaking speed on the esterification rate, using Amberlyst 16 cation exchange resin as a catalyst (particle size ) 0.39-0.85 mm).
Figure 1. Schematic diagram of the expanded-bed column reactor. 8
bed column reactor (EBCR). A schematic illustration of the reactor system is shown in Figure 1. Five grams of wet resin was loaded into a glass tube (reactor) with an inner diameter of 11 mm and length of 300 mm. The reactor was vertically placed in a thermal bath at 323.15 K. The feed solution was continuously supplied to the bottom of the reactor at constant flow rates, using a peristaltic pump (Model Micro Tube Pump MP-3, Tokyo Rikakikai Co, Ltd.). As the reactive fluid flows upward, the catalyst resin floats upward and the expanded bed forms. A solvent-free equimolar mixture of acetic acid and glycerol (7.67 mol/dm3) was used as the feed solution. The residence time of the fluid in the expanded bed was regulated by changing the flow rate of the feed solution in the range of 0.088-0.81 cm3/
Figure 2. Reaction scheme of the consecutive esterification.
Figure 4. Effect of catalyst (Amberlyst) particle size on the esterification rate, under a shaking speed of 160 spm.
min. After the steady state was attained, the effluent solution from the top of the reactor was collected and the concentrations of the respective solutes were determined using the previously described HPLC method. 3. Formulation of Kinetic Model for Esterification Using a Cation-Exchange Resin as a Heterogeneous Catalyst 3.1. Effects of Intraparticle and Extraparticle MassTransfer Resistances. The protons of the sulfo groups in the resin solid matrix catalyze the consecutive esterification reactions, as shown in Figure 2.
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To derive the model kinetic equations, the effects of the intraparticle and extraparticle mass-transfer resistances should be clarified. Figure 3 shows the batchwise experimental time courses of the acetic acid concentration for shaking speeds of 160 spm (basic condition) and 80 spm. Both plots matched each other well, so that the extraparticle mass-transfer resistance (i.e., film mass-transfer resistance) was negligible. Figure 4 shows the experimental time courses of the acetic acid concentration, using two types of dry resin particles, with diameters in the range of 0.39-0.85 mm (basic condition) and
