Research Note pubs.acs.org/IECR
Modeling of Monolithic Catalyst Washcoated with TS‑1 and Reactor for Continuous Cyclohexanone Ammoximation Reaction Rui Sun,†,‡ Feng Xin,*,†,‡ and Libin Yang†,§ †
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China § Tianjin Key Laboratory of Marin Resources and Chemistry, Tianjin University of Science and Technology, Tianjin 300457, China ‡
ABSTRACT: The cyclohexanone ammoximation reaction in a monolithic reactor over titanium silicalite-1 (TS-1) was simulated based on our previous experiments. The catalyst powders of TS-1 were wash-coated on the internal surface of each channel in a honeycomb cordierite support, and external circulation was utilized to vary the flow rate of liquids passing through the channels. A one-dimensional model was employed to investigate the influences of temperature from 318.15 K to 343.15 K and external circulation flow rate of 265−765 mL min−1 on the conversion of reactants. The simulation was from startup to steady-state operation after enough iterative circulations, where the outcomes of simulation agreed favorably with experimental data, and the concentration changes in different computing circulation times are helpful to reveal the reaction process, determine the initial concentrations of reactants in the reactor for shortening duration of startup, and illustrate that the microchannels in the monolithic catalyst effectively provide a good mass-transfer process for the reaction.
1. INTRODUCTION Monolith catalysts or reactor were first developed in the 1970s for treating automotive exhaust gas. In recent years, monoliths, as multiphase reactors to replace conventional reactors such as trickle-bed and slurry reactors, have received more and more attention.1,2 The monolith, which is fabricated on ceramic supports in a honeycomb arrangement, often consists of a matrix of uniformly parallel channels with inside dimensions of roughly 1−2 mm. The surface of each channel is usually covered with a porous washcoat containing the catalyst. Detailed modeling and simulation of monolith reactors can help investigate the complex processes that occur in the channels. In addition, these models can be classified as onedimensional, two-dimensional, or three-dimensional models, according to the spatial dimensions. The most widely used model is the one-dimensional (1D) two-phase model, which generally neglect the mass-transfer resistance in the washcoat, especially when the catalytic washcoat is sufficiently thin (335 K, and the conversion decreases sharply at low temperature. Generally, the model matches the experimental data well; therefore, it goes a step further to prove that the calculated results obtained in this work are reliable. 5.2. Influence of Circulation Flow Rate (Q) on Cyclohexanone Conversion. Figure 3 clearly illustrates
(4)
in which cnew is the concentration of species i in the raw i materials, q is the volume flow rate of the raw materials, Q means the circulation flow rate of liquid phase, and cend i,f is the concentration of species i at the outlet of the channel (as the same in the circulation flow). In this study, we set cnew cyc is 2.99 mol/L, q = 0.3 mL as the experiment value, and the cend i,f = 0 at the start of the calculation. Based on of the kinetic study above, the computer program for the solution is prepared by using Matlab. The ODEs have been solved by the Runge−Kutta method. The nonlinear equations are solved by the subroutine program “fsolve” of Matlab. The convergence condition after several circulation calculation times is ′
−5 end |ciend , f − ci , f | < 10
(5)
which means the absolute value of the difference between the values before and after a circulation calculation is 0.99. Considering the low weight of catalyst is insufficient to cover the entire channel and achieve a high conversion, we should load the TS-1 catalyst to >1 g.
Figure 2. Influence of reaction temperature on the cyclohexanone conversion. Experiment conditions: mTS‑1 = 2 g, Q = 265 mL min−1.
Figure 4. Effect of mTS‑1 on the cyclohexanone conversion. Experiment conditions: T = 343.15 K, Q = 265 mL min−1. C
DOI: 10.1021/ie505043u Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Research Note
Industrial & Engineering Chemistry Research 5.4. Changes of c0i,f and cend along the Computing i,f Circulation Times. With the occurrence of the external circulation, reactants accumulate from the reaction startup, and finally tend to the steady state as described in Figure 5. It noted
reaction. The steady concentration of H2O2 is ∼0.134 mol/L. Considering the computed c0i,f on the experiment condition above, we can feed 0.364 mol/L NH3·H2O and 0.134 mol/L H2O2 into the reactor when startup to shorten the reaction startup time. Besides, we can also use the model to ascertain the c0i,f under other conditions. 5.5. Comparison of the Model with Ideal Plug Flow Model. In order to investigate the effect of mass-transfer process in the microchannels of monolith catalyst on the reaction, we compared the 1D model with the ideal plug-flow model which used the intrinsic kinetics directly. Figure 7 shows
Figure 5. Changes of c0i,f /cend i,f along the computing circulation times. Experiment conditions: T = 343.15 K, mTS−1 = 2 g, Q = 265 mL min−1.
that, at the beginning of the reaction, the concentration of reactant continues to increase and then reaches a peak value. During this stage, the reaction occurs very slowly leading to that the difference between the c0i,f and cend i,f increases slightly. After the peak concentration, the difference continues increasing more significantly, demonstrating that the reaction starts to react faster until the cyclohexanone reacts more than the addition amount, and then reaches steady-state with a high conversion. The excessive ammonia and hydrogen peroxide can accumulate to a high concentration value and promote the reaction to occur. The concentrations of reactants along the computing circulation times are shown in Figure 6, from which we can see that the convergence of concentration of NH3·H2O is the most time-consuming, and reach to a high steady value up to ∼0.364 mol/L. In addition to the main cyclohexanone ammoximation reaction, H2O2 also has a self-decomposition
Figure 7. Difference between the ci,f in ideal plug-flow model and ci,f/ci,s in the 1D model along monolith channel length. Experiment conditions: external circulation flow rate Q = 265 mL min−1, mTS‑1 = 2 g.
that there is little difference between the ci,f along monolith channel length of the two models, and ci,s in the 1D model is just a little smaller than ci,f. It can be concluded that the micro size of the channels effectively weakens the influence of external mass transfer on the reaction, and the reaction rate is slow, which leads the ci,f decreasing in an approximately linear form.
6. CONCLUSION A 1D model was performed to investigate the influence of key experiment conditions on the performance of the monolith reactor washcoated with TS-1 catalyst for continuous cyclohexanone ammoximation reaction. The calculated results as shown were used to predict the behavior of the monolith reactor by comparing with experimental data. It was concluded that when the temperature of the reaction is higher than 335 K with external circulation flow rate Q under 500 mL min−1 and mTS‑1 more than 1 g, the outlet conversion can achieve up to 99%. Under the experiment conditions of temperature = 343.15K, mTS‑1 = 2 g, Q = 265 mL min−1 and cpsi = 250, we can feed 0.364 mol/L NH3·H2O and 0.134 mol/L H2O2 into the reactor before startup to shorten the reaction time to reach steady-state. It also indicated that the microchannels of the monolith catalyst effectively provide a good mass-transfer process for the reaction. Generally, the model established in this work with the favorable agreement with experiment data can make a contribution to the study of this reaction process.
Figure 6. Concentrations of reactants along the computing circulation times. Experiment conditions: T = 343.15 K, external circulation flow rate Q = 265 mL min−1, mTS‑1 = 2 g. D
DOI: 10.1021/ie505043u Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Research Note
Industrial & Engineering Chemistry Research
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(9) Taramasso, M.; Perego, G.; Notari, B. Preparation of Porous Crystalline Synthetic Material Comprised of Silicon and Titanium Oxides. U.S. Patent 4,410,501, 1983. (10) Liu, Y.; Zong, L.; Xin, F. Hydrothermal Synthesis of TS-1 Film with Mixed Templates on a Cordierite Support. Mater. Res. Innov. 2008, 12 (2), 84−89. (11) Yang, L. B.; Xin, F.; Lin, J. Z. Continuous Heterogeneous Cyclohexanone Ammoximation Reaction Using a Monolithic TS-1/ Cordierite Catalyst. RSC Adv. 2014, 4 (52), 27259−27266. (12) Li, Y. X.; Wu, W.; Min, E. Z. Kinetics of Cyclohexanone Ammoximation over Titanium Silicate Molecular Sieves. Chin. J. Chem. Eng. 2005, 13 (1), 32−36. (13) Yang, L. B.; Xin, F.; Jin, Y. Deactivation Kinetics of Ammoximation over TS-1 Catalyst. Chin. J. Appl. Chem. Ind. 2007, 36 (12), 1166−1168. (14) Groppi, G.; Tronconi, E. Theoretical Analysis of Mass and Heat Transfer in Monolith Catalysts with Triangular Channels. Chem. Eng. Sci. 1997, 52 (20), 3521−3526.
AUTHOR INFORMATION
Corresponding Author
* Tel.:+86 (022) 27409533. Fax: +86 (022) 27892359. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge National Natural Science Foundation of China for the financial support (Project No. 21276180).
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NOMENCLATURE a = half of the triangular channel side length, m c0 = concentration at the inlet of the channel, mol/L cnew = concentration in the raw materials, mol/L cend = concentration at the outlet of the channel (in the circulation flow), mol/L Dm = molecular diffusivity of the reactant in the fluid phase, m2 min−1 km = mass transfer coefficient between the liquid and catalyst washcoat, m min−1 L = length of the monolith catalyst, m mTS‑1 = mass of TS-1 catalyst in each channel, g P = interfacial perimeter, m Q = circulation flow rate of liquid phase, mL min−1 q = volume flow rate of the raw materials, mL min−1 r = reaction rate calculated per unit catalyst phase mass, mol g−1 min−1 Sh∞ = constant Sherwood number T = temperature, K w = mass fraction X = conversion of the reactants z = coordinate along the length of the channel, m
Subscripts and Superscripts
i = species i f = fluid phase s = surface between two phases cyc = reactant cyclohexanone
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REFERENCES
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DOI: 10.1021/ie505043u Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
