Ind. Eng. Chem. Res. 2006, 45, 8807-8814
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Hydrogenation of Dicyclopentadiene into endo-Tetrahydrodicyclopentadiene in Trickle-Bed Reactor: Experiments and Modeling Guozhu Liu,†,‡ Zhentao Mi,*,† Li Wang,† Xiangwen Zhang,† and Shuting Zhang‡ State Key Laboratory of Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China, and School of EnVironment Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China
Hydrogenation of dicylcopentadiene (DCPD) into endo-tetrahydrodicylcopentadiene (endo-THDCPD), in the presence of a Pd/Al2O3 catalyst, was experimentally and theoretically studied in a quasi-adiabatic trickle-bed reactor (TBR) with a diameter of 24 mm and a length of 850 mm. Effects of several operation parameters, including the liquid hourly space velocity (LHSV) (5.86-14.65 h-1), hydrogen pressure (1.0-2.0 MPa), inlet liquid concentration (0.52-1.35 mol/L), and inlet temperature (319.15-379.15 K) on the TBR performance were investigated systematically in terms of DCPD conversions, THDCPD yields, global hydrogenation rates, and axial temperature profiles. A plug-flow model for TBR incorporating partial wetting, mass and enthalpy balance, and phase equilibia behavior was developed to simulate the experimental results, based on a phenomenological pellet-scale model suggested by Rajashekharam et al. in a previous publication [Chem. Eng. Sci. 1998, 53 (4), 787-805]. The comparisons of experimental and simulated results indicated that the developed model reliably predicted the performance and axial temperature profiles. Introduction Trickle bed reactors (TBRs), in which gas and liquid reactants flow cocurrently downward through a fixed bed of catalyst particles, are used extensively for hydrodesulfurization, hydrodenitrogenation, and hydrocracking reactions in the petroleum refining industry, and hydrogenation, oxidation, and hydration reactions in the petrochemical processing, and waste treatment in the field of biochemical engineering, as well as other applications.1-3 Generally, TBRs have been designed and operated under steady-state mode in the industrial applications previously described. However, over the past 15 years, a novel and promising operating mode, known as unsteady-state operation, has generated growing interest and broad attention, in view of the many advantages it brings, such as increasing the masstransfer rate of limiting reactant, improving the performance of TBR (both the conversion and selectivity), simultaneous prevention of flow maldistribution and hot spot formation.4,5 It is well-known that most of the industrial important reactions represent complex reaction kinetics and are very often highly exothermic reactions. Therefore, from a practical point of view, it is important to study the effects of the unsteady-state operation on both reactor performance and bed temperature profiles for the complex reaction with significant heat effect, which is a crucial issue in the design and scaleup of TBRs for the potential industrial applications. Regardless of the numerous literature dealing with the performance of TBR under the periodic operation for several reactions,4-10 little information has been reported on the thermal wave in the reactor under different unstable-state operation strategies and parameters, such as ONOFF or PEAK-BASE, cycle period and split. Recently, Silveston and Hanika4 indicated that the hydrogenation of dicylcopentadiene (DCPD) is a good model reaction to study * To whom correspondence should be addressed. Tel./Fax: 86-2227402604. E-mail address:
[email protected]. † State Key Laboratory of Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University. ‡ School of Environment Engineering and Technology, Tianjin University.
the reactor performance (including conversion and selectivity) and heat behavior of the periodically operated TBR for the highly exothermic complex reactions for the following reasons: the reaction proceeds under relatively mild conditions and presents both conversion and selectivity issues, and significant temperature rise can be observed. To the best of our knowledge, in the literature, only Chou et al.11 investigated DCPD hydrogenation into dihydrodicylcopentadiene (DHDCPD) over Ni/γ-Al2O3 in an adiabatically operated TBR, and they proposed a differential equations model and an approximate model, assuming a number of mixing cells in series to predict the reactor performance and the temperature rise. Consequently, it is still necessary to further investigate the steady-state operation of TBR for the hydrogenation of DCPD into endotetrahydrodicylcopentadiene (endo-THDCPD) to obtain further understanding on the TBR performances under steady-state operation and to provide more knowledge for the selection of the practical modulation strategies and parameters for the unsteady-state operating mode. The objective of this work is to investigate hydrogenation of DCPD into endo-THDCPD in the presence of a Pd/Al2O3 catalyst in a quasi-adiabatic TBR. Effects of several operation parameters, such as liquid hourly space velocity (LHSV), operation pressure, inlet liquid concentration. and inlet temperature, on DCPD conversions, THDCPD yields, global hydrogenation rates, and axial temperature profiles were investigated experimentally. Attempts were also made to develop a mathematic model that incorporated phase equilibia as well as mass and enthalpy balance to simulate the experimental results, based on a phenomenological pellet-scale model that was suggested by Rajashekharam et al.12 The developed TBR model provides a reliable tool for the DCPD hydrogenation reactor simulation and design. Experimental Studies Reaction System. In this work, the catalytic hydrogenation of DCPD dissolved in n-hexane, DHDCPD, and then endoTHDCPD over Pd/Al2O3 is used as a model reaction, as shown in Scheme 1.
10.1021/ie060660y CCC: $33.50 © 2006 American Chemical Society Published on Web 11/08/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006
Figure 1. Schematic diagram of the experimental apparatus. Legend: 1, liquid feed tank; 2, pump; 3, three-way valve; 4, data acquisition system; 5, trickle-bed reactor (TBR); 6, gas-liquid separator; 7, condenser; 8, rotameter; and 9, mass-flow controller.
Scheme 1. Dicyclopentadiene (DCPD) Hydrogenation to Tetrahydrodicyclopentadiene (endo-THDCPD)
The reaction is very important for the synthesis of an organic intermediate adamantine and a high-energy jet fuel, JP-10, which is called the isomerization of endo-THDCPD. In our previous paper, the intrinsic kinetics of DCPD hydrogenation into endoTHDCPD over a Pd/Al2O3 catalyst was determined over a wide range of temperature, reactant concentration and hydrogen pressure using stirred semi-batch reactors in the absence of transport limitations and a kinetics expression was proposed to fit the experimental data, which was used in the current work.13 Materials. DCPD with a purity of >98.5% was purchased form Yangli Petrochemical, Inc. (Hangzhou, PRC). Hydrogen (purity of >99.99%) from a cylinder was purchased from Chenxi Idurstrial Gas Co., Ltd (Tianjin, PRC). Analytic-reagent-grade n-hexane was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, PRC) and used as the solvents for DCPD. The catalyst used was in the form of 2.0 mm-diameter pellets, 0.3 wt % Pd/Al2O3 with had an eggshell distribution (0.05-mm thickness) to minimize intraparticle diffusion resistance. Equipment and Procedures. Figure 1 shows a schematic diagram of the experimental apparatus. The trickle-bed reactor (TBR) is composed of a stainless steel tube with 850 mm length and 24 mm ID. At the top of reactor, there is a liquid distributor that consists of 20 pipes with an outside diameter (OD) of 2 mm and a legnth of 20 mm. Outside of the reactor, three furnaces that are surrounded by aluminosilicate fiber with a thickness of 100 mm are provided and controlled independently by three Al-508T temperature controllers (Yudian Automation Technology Company, Xiamen, PRC). The catalyst bed (210 mm in length) is filled with the Pd/ Al2O3 spherical pellets, which are diluted by inert Al2O3 spherical pellets, of the same size, with a volume ratio of 1:3. The pre- and post-packing of the catalyst bed, using the inert Al2O3 spherical pellets, has bed lengths of 350 and 290 mm, respectively, which ensures uniform distribution of the fluid that is flowing over a cross section and efficient heat transfer from the heat wall to the fluid. More details about the reactor properties are summarized in Table 1. For the purpose of measuring the axial temperature profile
of the catalyst bed, a 3-mm-OD stainless steel pipe is placed axially along the reactor and used as the thermal well, in which eight 0.35-mm-diameter thermocouples (type K, Institute of Thermoscopic Materials, Shenyang, PRC) are located, from the top to the bottom of the catalyst bed. The temperatures at the different positions are monitored and recorded using a data acquisition system that was implemented in a computer. Before each run, the catalyst is reduced in situ with hydrogen at 540.15 K for 2 h to ensure the reproducibility of the catalyst activity. After the catalyst pellets are pre-wetted with n-hexane using a high-pressure-metering pump, pure hydrogen from a gas cylinder and the DCPD solutions are cocurrently fed into the top of the reactor. The gas-liquid mixture leaving the reactor is sent to a gas-liquid separator, in which the liquid-gas mixture is completely cooled. The gas was vented through a back-pressure valve to ensure a constant pressure in the reactor, and the liquid was drained at the same time. The liquid samples were taken and analyzed using gas chromatography (GC) after the operation stabilized. Analysis. The liquid samples of the reaction mixtures are analyzed using Series HP4890 GC equipment (HewlettPackard, Palo Alto, CA) with a flame ionization detector (FID) and a Hewlett-Packard HP-5 capillary column (30 m × 0.32 mm × 0.25 µm). Nitrogen is used as the carrier gas. The temperature of the column is maintained at 403.15 K, whereas that of the injector and detector is kept at 453.15 and 523.15 K, respectively. The area normalization method is used to determine the concentrations of the components quantitatively. The reproducibility of the results is checked, and the error in the most unfavorable cases is
