Article pubs.acs.org/IECR
Catalysts, Kinetics, and Reactive Distillation for Methyl Acetate Synthesis Cuncun Zuo,† Langsheng Pan,*,† Shasha Cao,‡ Chunshan Li,‡ and Suojiang Zhang*,‡ †
School of Chemical Engineering, Xiangtan University, Yuhu District, Xiangtan 411105, People’s Republic of China Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Haidian District, Beijing 100190, People’s Republic of China
‡
S Supporting Information *
ABSTRACT: Two novel acidic cation-exchange resins, namely, NKC-9 and D072, were first developed to catalyze the esterification of acetic acid with methanol. Kinetics of methyl acetate synthesis are determined by the experiments in a batch reactor at temperatures from 45 to 60 °C under atmospheric pressure, with molar ratio of reactants (θBo = nmethanol:nacetic acid) from 0.8 to 1.3. The kinetic data are correlated with the Q-H, E-R, and LHHW and pseudohomogeneous (P-H) models. Then, the reactive distillation process for methyl acetate synthesis is simulated by the Aspen Plus process simulator. Lifetime evaluation of NKC-9 was carried out to study the conversion rate of acetic acid and selectivity of methyl acetate during the long-period running in the fixed-bed reactor. Finally, a series of characterizations such as Fourier transform infrared spectroscopy, X-ray diffraction, and polarizing microscopy were performed to investigate the structure and properties of NKC-9.
1. INTRODUCTION Methyl acetate is an important organic chemical raw material, and high-purity methyl acetate can be used to synthesize acetic anhydride, methyl acrylate, vinyl acetate, and ethyl amide. Moreover, it plays an important role in the synthesis of coating materials, nitro-cellulose, cellulose acetate, cellulose ethers, and celluloid. Methyl acetate also has wide applications in the various industries, which mainly produce essences, plasticizers, and certain fats.1−6 The distillation column reactor packed with ion-exchange resin offers significant superiorities over the routine method of conducting the sequential reaction and separation sequentially.7−9 Kinetics of the esterification of propionic acid with nbutanol over Amberlyst 35 was investigated under different experimental conditions.10 Lee et al.11 investigated the kinetic behaviors of the catalytic esterification of amyl alcohol acetate in a fixed-bed reactor using Dowex 50Wx8-100 (an acidic cation-exchange resin) as the catalyst. Xu et al.12 successfully obtained glycolic acid by hydrolysis of methyl glycolate over ion-exchange resin. Chang et al.13 used the Newton homotopy arc length differential method to simulate the synthesis of ethyl acetate by catalytic reactive distillation. Song et al.14 studied the effect of heterogeneous catalysis on the transition from the nonreactive stage to the reaction equilibrium, and developed a heterogeneous Langmuir−Hinshelwood−Hougen −Watson (LHHW) type reaction rate model for the synthesis of methyl acetate. The nonrandom two-liquid (NRTL) model was used to correct the nonideality of the liquid phase in the reaction system of n-butyl alcohol propionate synthesis.15 In the present study, two strongly acidic cation-exchange resins (NKC-9 and D072) were developed for the catalytic esterification to synthesize methyl acetate. The kinetics experiments were conducted in a batch reactor under different conditions. Given that there was great deviation of the reaction system from the ideal solution, the NRTL model was chosen to © 2014 American Chemical Society
correct the nonideality of the liquid phase. The kinetic data were correlated with the Q-H, E-R, LHHW, and P-H models. Then, simulation and calculation of the catalytic esterification to synthesize methyl acetate in the catalytic distillation column were carried out.
2. EXPERIMENTAL SECTION 2.1. Materials. Acetic acid and methanol were purchased from Sinopharm Chemical Reagent Co., Ltd., both of which were of analytical grade. Hydrochloric acid and sodium chloride were also obtained from the same chemical reagent factory. Distilled water was used for the preparation of all solutions. D072 and NKC-9 were manufactured by Chemical Plant of Nankai University (China). Before D072 and NKC-9 were used for the catalysts for the esterification reaction, the purchased resins were soaked in the saturated NaCl solution for 8 h to remove certain impurities. To ensure that the pH of two cationexchange resins was 7, the resins were washed with HCl solution and distilled water sequentially. 2.2. Appratus and Procedure. The kinetics experiments of the esterification of methyl acetate were performed in a 250 mL three-necked flask equipped with a reflux condensing tube, a sampling device, and a thermometer. The reaction temperature was controlled to within ±0.1 K by Super thermostat water bath. First, acetic acid and the catalyst were placed the reactor and heated to the desired reaction temperature. Methanol preheated to the reaction temperature was then added into the reactor through a constant pressure drop funnel to start the esterification reaction. The course of the reaction was monitored by determining of the composition of small Received: Revised: Accepted: Published: 10540
January 26, 2014 June 2, 2014 June 12, 2014 June 23, 2014 dx.doi.org/10.1021/ie500371c | Ind. Eng. Chem. Res. 2014, 53, 10540−10548
Industrial & Engineering Chemistry Research
Article
samples which were withdrawn from the liquid mixture at regular intervals. After a steady state was achieved, the product was collected in a sampling flask. Product samples were analyzed by gas chromatography (GC6890, made in China) with a flame ionization detector and a chromatographic capillary column PEG-20M (30 m × 0.20 mm × 0.25μm). The column temperature rose to 160 °C by a programmed temperature method. The vaporizing chamber and detector temperatures were set at 180 °C. High purity nitrogen gas (>99.999%) was used as carrier. The conversion rate of acetate acid is defined as the ratio between the amount of acetate acid transformed and the total amount of acetate acid. Pretreated NKC-9 beads with an average particle size of 35 meshes and glass wool were packed in a fixed-bed reactor. The catalyst load (W) was 0.025 g catalyst/g liquid mixture for the base case. With different space-times, the experiment was repeated to investigate the effect of space-time on the conversion rate of acetic acid. 2.3. Characterizations. The surface morphology of NKC-9 was observed by scanning electron microscopy (SEM; Quanta250; EDAX Genesis-SiLi). Pore size diameter distribution curve of the fresh NKC-9 was recorded on a Automatic mercury analyzer (AutoPore IV 9510), which includes two low voltage stations and a high voltage station. Pores ranging in size from 50 Å to 1000 μm can be measured by this Automatic mercury analyzer. Elements content of the fresh NKC-9 was determined by an elemental analyzer (Vario EL cube, Germany), which is provided with high measurement accuracy. The Fourier transform infrared spectroscopy (FT-IR) spectra of the fresh and regenerative NKC-9 were recorded on a NICOLET 380 spectrometer (Thermo Nicolet Corporation, USA) in the optical range of 500−4000 cm−1 by averaging 32 scans at a resolution of 4 cm−1. XRD patterns of the fresh and regenerative NKC-9 catalyst were obtained using a power X-ray diffractometer (Rigaku Dmax-C) with Cu Kα radiation (k = 1.5405 Å). The XRD patterns were obtained from 2θ = 10° to 90°. The differences of the fresh and regenerative NKC-9 were assessed visually using a polarizing microscope (Olympus BX51, Olympus, Melville, NY) at 10× to 50× magnifications.
Figure 1. Evaluation of catalytic activity of NKC-9 and D072.
amount of catalyst load. The catalyst load ranged from 0.020 g catalyst/g liquid mixture to 0.030 g/g liquid mixture, and blank experiment was also used. As shown in Figure 2, the required
Figure 2. Effect of catalyst load (NKC-9) on the conversion rate of acetic acid (45 °C, θBo = 0.8:1).
3. RESULTS AND DISCUSSION 3.1. Selection of Strongly Acidic Cation-Exchange Resin Catalysts. To investigate the catalytic activity of the strongly acidic cation-exchange resins (D072 and NKC-9), a preliminary comparison was performed. As is clearly shown in Figure 1, before reaching the reaction equilibrium, the conversion rate of acetic acid in the presence of NKC-9 was higher than that in the presence of D072. In addition, the required time to reach the reaction equilibrium was relatively shorter with NKC-9 as the catalyst. Moreover, no dimethyl ether was detected in the experiments with NKC-9 as the catalyst, which indicated the high selectivity of NKC-9. However, a small quantity of dimethyl ether was found when D072 was used. Therefore, NKC-9 was selected for further study of the catalytic esterrification. 3.2. Kinetics Experiments and Optimization of Esterification Conditions. 3.2.1. Effect of Catalyst Load. The same experiment was repeated at 45 °C with a molar ratio of reactants of 0.8:1. The effect of catalyst load on the conversion rate of acetic acid was determined by changing the
time to reach the reaction equilibrium was shortened significantly with the presence of catalyst compared with that of the blank experiment. When the catalyst load was increased from 0.020 g catalyst/g liquid mixture to 0.025 g catalyst/g liquid mixture, the conversion rate of acetic acid measured significantly increased under the same reaction time, which was attributed to the increase in available active sites caused by the change in catalyst load. However, when the catalyst load was increased from 0.025 g catalyst/g liquid mixture to 0.030 g catalyst/g liquid mixture, the conversion rate of acetic acid measured was nearly identical under the same reaction time. These results demonstrated that 0.025 g catalyst/g liquid mixture was the optimum catalyst load. 3.2.2. Effect of Reaction Temperature. To determine the effect of reaction temperature on the catalytic esterification, the same experiment was repeated at different reaction temperatures, i.e., 45, 50, 55, and 60 °C, with a catalyst load of 0.025 g catalyst/g liquid mixture and a molar ratio of reactants of 0.8:1. As shown in Figure 3, before the reaction equilibrium was 10541
dx.doi.org/10.1021/ie500371c | Ind. Eng. Chem. Res. 2014, 53, 10540−10548
Industrial & Engineering Chemistry Research
Article
Figure 3. Effect of reaction temperature on the conversion rate of acetic acid using NKC-9 as the catalyst (θBo = 0.8:1, and the catalyst load was 0.025 g catalyst/g liquid mixture).
Figure 4. Effect of molar ratio of reactants on the conversion rate of acetic acid using NKC-9 as catalyst (60 °C, and the catalyst load was 0.025 g catalyst/g liquid mixture).
reached, the conversion rate of acetic acid significantly increased temperature under the same reaction time, which indicated that high temperature facilitates the forward reaction. This result is attributed to the partial vaporization of the aqueous mixtures at high temperatures and long contact times. However, the disparity among the equilibrium conversion rate of acetic acid is very small in the temperature range studied. Sanz et al.16 have also obtained similar results in methyl lactate hydrolysis. 3.2.3. Effect of Molar Ratio of Reactants. To determine the optimum molar ratio of reactants (θBo), the same experiment was repeated with different reactant molar ratios (θBo) containing 0.8:1, 1.0:1, 1.1:1, 1.2:1, and 1.3:1 at 60 °C, and the catalyst load was 0.025 g catalyst/g liquid mixture. According to the theories of chemical equilibrium, increasing the reactant ratio can enhance the conversion rate of methyl acetate. However, acetic acid has a high boiling point, and it is provided at a smaller proportion in the vapor phase. Hence, the yield of methyl acetate is calculated on the basis of the conversion rate of acetic acid. In the present study, when the molar ratio of reactants was increased from 0.8:1 to 1.3:1 in the primary stage of the reaction, a significant increase in the conversion rate of acetic acid was found (Figure 4). When the molar ratio of reactants was increased from 0.8:1 to 1.2:1, the equilibrium conversion rate of acetic acid increased as well. Nevertheless, when the reactant molar ratio was increased from 1.2:1 to 1.3:1, the equilibrium conversion rate of acetic acid changed insignificantly. This phenomenon was attributed to the difficulties in the separation of methyl acetate at higher molar ratio of reactants when the reaction is close to equilibrium. 3.2.4. Elimination of External Diffusion. To study the reaction kinetics of the system, the effect of external diffusion must be excluded. External diffusion effects could be eliminated by increasing the stirring speed at the optimum catalyst load, reaction time, and molar ratio of reactants aforementioned. To determine a suitable stirring speed, the same experiment was repeated under different stirring speeds including 400, 500, and 600 rpm. The experimental results are shown in Figure 5. When the stirring speed was increased to from 350 to 450 rpm, a significant increase in the conversion rate of acetic acid was found at the same reaction time. However, when the stirring speed was increased from 450 to 550 rpm, the conversion rate
Figure 5. Effect of stirring speed on the conversion rate of acetic acid using NKC-9 as catalyst (60 °C; θBo = 1.2:1; the catalyst load was 0.025 g catalyst/g liquid mixture).
of acetic acid was almost constant. Therefore, the effect of external diffusion can be eliminated when the stirring speed is >450 rpm. Given the serious abrasion on equipment and large energy consumption caused by high stirring speed, 450 rpm was used. 3.2.5. Elimination of Internal Diffusion. To investigate the effect of internal diffusion on catalytic esterification, commercial NKC-9 was screened into several different particle sizes, including 24, 28, 32, and 35 meshes. The same experiment was repeated with different catalyst particle sizes at the optimal catalyst load, reaction time, and molar ratio of reactants aforementioned. As shown in Figure 6, when the catalyst particle size is >28 meshes, the conversion rate of acetic acid was unaffected by the catalyst particle size. However, when the catalyst particle size is
