- Email: [email protected]
Design of novel Pt-structured catalyst on anodic aluminum support for VOC’s catalytic combustion
Applied Catalysis A: General 350 (2008) 150–156
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Design of novel Pt-structured catalyst on anodic aluminum support for VOC’s catalytic combustion Lifeng Wang *, Thanh Phong Tran, Dong Vien Vo, Makoto Sakurai, Hideo Kameyama Department of Chemical Engineering, Faculty of Engineering, Tokyo University of Agriculture and Technology, 24-16 Nakacho 2, Koganei-Shi, Tokyo 184-8588, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 April 2008 Received in revised form 22 July 2008 Accepted 25 July 2008 Available online 3 August 2008
Emission control of volatile organic compounds (VOCs) is one of the priorities for environmental catalysis. Catalytic combustion is one of promising technologies for elimination of VOCs. A novel platinum-structured catalyst was designed and is presented in this work. Aluminum was used as support for the catalyst after anodic oxidation treatment and in a shifted arrangement bent, short channels are applied in the design of the structured catalyst. The simulation results show that the novel design of the structured catalyst brings in the higher catalytic activity, compared with the traditional structured catalyst which has long, straight channels. Lastly, the novel structured catalyst was manufactured and its catalytic activity was evaluated with the catalytic combustion of VOC. The results show that the conversion obtained from the reactor with the structured catalyst is almost identical to that obtained from the tube reactor packed with the plate catalyst in which the reaction rate is controlled by surface reactions. Hence, it can be concluded that the mass transfer can be improved by the novel design of the structured catalyst and that the mass transfer in gas phase is not the rate-determining step for the catalytic combustion of VOC. ß 2008 Elsevier B.V. All rights reserved.
Keywords: VOC Structured catalyst Anodic oxidation treatment Electro-deposition technology Mass transfer
1. Introduction Volatile organic compounds (VOCs) are defined as organic compounds with high vapor pressure that are easily vaporized under an ambient temperature and pressure. Their emission from industrial processes and vehicle exhausts, such as aliphatic, aromatic, benzene, toluene, is considered as a severe air pollutant. The progressive increase of VOC emissions, their hazardous nature, and the increasingly restrictive environmental regulations in industrialized countries have encouraged the development of different methods for the elimination of VOCs. Catalytic oxidation is an effective way for reducing the emissions of VOCs. In this way, two kinds of catalysts are usually applied: (i) noble metal-based solids which possess high activity but are expensive and (ii) metal oxide-based samples which are cheaper but less active. Supported precious metals as Pt and Pd are well established as efficient catalysts for the oxidation of different VOC [1–5]. Reactor design for the catalytic combustion process of VOC is very important especially when the catalyst with high activity is applied. The classic reactors with dumped beds of catalyst particles are one of the possible solutions for VOC combustion processes. In
* Corresponding author. E-mail address: [email protected] (L. Wang). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.07.033
fact, the reactors with packed beds are easy to construct, but they cause a high-pressure drop and do not allow reducing particle size to obtain high specific surface area. For this reason, the structured catalysts with high surface and low pressure-drop have been adopted in reactor designs. Especially, honeycomb monoliths have become the standard catalyst shape in most applications of environmental catalysis, after their successful commercial application to the control of automotive exhausts and to the reduction of nitrogen oxides. However, two findings have long discouraged the extensive use of monolithic catalysts in the catalytic combustion of VOC [6–10]: (a) Conventional parallel channel monoliths are virtually adiabatic: it is known that the catalytic combustion of VOC is an exothermic reaction and that the adiabatic temperature rise varies with the concentration of VOC but always exceeds 100 K. Hence, the adiabatic of the conventional monoliths would severely limit the control of the temperature in the catalytic combustion reaction of VOC. (b) Fluid flow in a small and long channel of a typical monolith (0.8–4 mm in diameter) is undoubtedly laminar, which will result in poor mass transfer and heat transfer. This drawback also may leads to serious malfunctions especially when confronted with VOCs, since catalytic combustion of VOCs always runs in diffusion regime, where the reaction rate is
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
Nomenclature A C D Ea k N r S T u
y y r
pre-exponential factor (m s1) concentration (mol m3) diffusion coefficient (m2 s1) activation energy (J mol) reaction rate constant (m s1) flow rate of VOC (m3 s1) specific reaction rate (mol m2 s1) catalyst surface (m2) reaction temperature (K) velocity (m s1) catalyst volume (m3) kinematical viscosity (kg m1 s1) density (kg m3)
limited by the mass transfer between catalyst and flowing gas mixture. This, in turn, results in an undesirable increase in the reactor length (raising investment costs) and flow resistance (raising pumping costs).
151
developed plate catalyst. The design of the novel structured catalyst was carried out based on the obtained reaction kinetics parameters by simulation. Lastly, the novel structured catalyst was manufactured and its catalytic activity was evaluated with the catalytic combustion of VOC. The simulation results indicate that the catalytic activity of the structured catalyst can be enhanced when the long, straight channels are replaced by bent, short channels. The activity experiments demonstrate that the conversion obtained from the reactor with the structured catalyst is almost the same as that obtained from the tube reactor packed with the plate catalyst. In our research, the mass transfer between catalyst and flowing gas mixture was modified so as to ensure that the reaction will be controlled by the surface reaction in the packed tube reactor. Moreover, it has been proved that the catalytic combustion reaction of VOC will be controlled by the mass transfer in the structured catalyst where the traditional long, straight channels were adopted, such as in honeycomb monoliths [9]. Hence, it can be concluded that the concentration distribution on the channels can be improved by the design of the novel structured catalyst and that the mass transfer is not the rate-determining step for the VOC’s catalytic combustion reaction. 2. Experimental
To overcome at least part of the above-listed problems connected with the catalytic combustion of VOC, we propose a kind of platinum-structured catalyst supported on an anodic oxidized aluminum support. Aluminum was used as support in our group because it has fine thermal conductivity and can be formed into any shape such as a mesh, fin, plate or serrate. It is known that coating technology is always used to support metal or Al2O3, SiO2 on metal carriers to produce catalyst or support [11–13]. However, adhesion of the coating layer to the support is not strong and the layer can be damaged easily [14]. In our group, anodic oxidation technology was applied to produce anodic alumite film on aluminum surfaces and the film can be used as the support for catalysts preparation, since anodization leads to formation of unbranched, regular and nearly concentric pores, as shown in Fig. 1, which are advantageous for catalytic reactions. Cu, Pt, Pd, Ni catalysts supported on the anodized aluminum plate have already been developed successfully in our group [15–20], we named them anodic alumite catalysts. In this work, the Pt catalyst was prepared on the anodic alumite plate with an electro-deposition technology [15–17]. Then, the reaction kinetics parameters of the catalytic combustion of VOC were calculated based on the activity experiments results of the
2.1. Catalyst preparation Plate catalyst supports were prepared by anodisation of 4 cm 8 cm of 0.3 mm thick aluminum plates (JIS A1050). Each plate was first rinsed with acetone and degreased in an aqueous solution of 20 wt% NaOH for 3 min at room temperature, then washed again with deionised water and immersed in a 30 wt% HNO3 solution for 1 min at room temperature. Each plate was then washed with deionised water and dried. The treated plate was oxidized as the anode in an oxalic acid solution against a catholically polarized graphite plate counter-electrode. The solution was kept at a constant temperature of 293 K, while the oxalic acid concentration was set at 4 wt%. The distance between the two electrodes was set to be 8 cm. Under a direct current density of 50 A m2, anodic oxidation was carried out for 10 h. Each plate was then dried at ambient temperature for 12 h and calcined in air at 623 K for 1 h, in order to decompose the residual oxalic acid in the film. Finally, the dried supports were stored in a desiccator. In the electro-deposition process, the support was soaked in 0.1 g L1 (based on platinum) chloroplatinic acid hexahydrate
Fig. 1. SEM micrographs of anodic alumite film.
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
152
solution at ambient temperature (precursor salt, H2PtCl66H2O, Wako Pure Chemical Industries, Ltd.) as one electrode against the counter electrodes of the stainless steel plates at a distance of 2 cm. The solution pH was about 3.4 without the use of any pHconditioning agent. Ordinary power (50 Hz, 110 V) was used in the experiment, with a voltage regulator to adjust the experimental voltage to a desired voltage. After the supporting, the plate was washed with deionised water and then dried for 4 h at ambient temperature. Finally, the plate was calcined in air at 773 K for 3 h and then stored in a desiccator. The structured catalyst (250 mm 40 mm 100 mm) was also prepared by the same process in the work. 2.2. Catalyst characterization In this study, the anodic alumite film thickness was measured with an optical microscope. The specific surface area and pore distributions could be obtained via a nitrogen adsorption method (SA3100, Beckman Coulter Inc.), where the outgas conditions were set at 573 K for 1 h. An inductively coupled plasma spectrometer (ICPS-7510, Shimadzu Corp.) was used to measure the platinum loading after the catalysts were dissolved in nitrohydrochloric acid. Surface micrographs of the catalysts were taken by a FE-SEM (S-4800, Hitachi, Ltd.). 2.3. Activity experiment In this work, catalytic combustion of VOC was used to evaluate the catalytic activity of the developed catalyst. Acetaldehyde and cyclohexanone (Kokusan Chemical Co., Ltd.) were chosen to represent VOC. Air from a compressor flowed into a saturator that was filled with liquid hydrocarbon and then mixed with the VOC stream that was evaporated by heating. The mixed air–VOC stream was fed into a reactor. Part of the plate catalyst (1 cm 2 cm) was cut into pieces 2–4 mm2, diluted with 4 g quartz sand (30–50 mesh, Kishida Chemical Co., Ltd.) and this material was packed into the reactor with an i.d. of 10 mm. In the reactor, the mass transfer can be moderated with the packed quartz sand and a suitable flow rate and one can ensure that the reaction in the reactor will be controlled by the surface reaction. The catalytic activity of the developed structured catalyst was also evaluated in this work. Part of the catalyst (50 mm 40 mm 100 mm) was packed into a reactor as shown in Fig. 2. Reactant gas was heated with an electrical heater before entering into the catalyst bed. The temperature in the middle of the
Fig. 2. Reactor packed with the structured catalyst.
Table 1 Properties of the structured catalyst Size (mm) Weight of the catalyst (g per monolith) Volume (m3 per monolith) Surface area (m2 per monolith) Volume fraction of voids Wall thickness (mm) Anodic alumite film thickness (mm) SBET (m2 per monolith) Average pore size (nm) Pt loading (g/m2)
250 40 100 341.4 0.001 1.08 0.83 0.3 0.1 2 2.04 104 42.3 1.0
catalyst bed was controlled. In this work, acetaldehyde and cyclohexanone were both used. The flow rate was set at 50 or 100 L min1 to keep the identical F/S with that in the tube reactor. VOCs were measured using a gas chromatograph with an FID detector (GC-390B, GL Sciences Inc.). 3. Results and discussion 3.1. Catalyst properties The properties of the catalyst used in the research are expressed in Table 1. The thickness of the aluminum is about 0.3 mm; after the anodic oxidation, 0.1 mm anodic alumite films can be formed on the surface. Table 1 shows that the BET-surface area is about 20,000 m2 per monolith. The average pore size of the anodic oxidation film is about 42.3 nm. The pores also can be found on the micro-photo of the cross-section of the anodic oxidation film, which was taken by FE-SEM. The platinum loading on the structured catalyst is 1.0 g m2, which means that the supported platinum on the structured catalyst is about 1.0 g. 3.2. Reaction kinetics of VOC The activity of the developed plate catalyst was evaluated with the catalytic combustion reaction of acetaldehyde and cyclohexanone under different inlet concentration and flow rate conditions. The results are presented in Figs. 3 and 4. The conversion of acetaldehyde decreases with the increase of flow rate, as seen from Fig. 3. Acetaldehyde can be decomposed completely when the reaction temperature was increased to 613 K. Fig. 4 shows that
Fig. 3. Acetaldehyde ignition curves. (~, SV 50,000 h1; inlet concentration: 86 ppm. &, SV: 40,000 h1; inlet concentration: 100 ppm. &, SV: 30,000 h1; inlet concentration: 64 ppm.)
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
Fig. 4. Cyclohexanone ignition curves. (~, SV: 60,000 h1; inlet concentration: 80 ppm. &, SV: 30,000 h1; inlet concentration: 150 ppm. &, SV: 30,000 h1; inlet concentration: 80 ppm.)
cyclohexanone can be combusted more easily; however, it cannot be combusted completely at lower temperature. It is known that catalytic combustion of VOC performed in the excess of oxygen can be simplified into a simple reaction pattern described by a first-order kinetic equation [21,22]: Ea dN r ¼ kC ¼ A exp C¼ dS RT
(1)
Here r is a specific reaction rate; k is the rate constant and C is the concentration of VOC in the gas phase; A is a pre-exponential factor and Ea is the activation energy. k can be calculated based on the experimental results. Based on the calculation results, the Arrhenius plots are plotted and expressed in Figs. 5 and 6. From Fig. 5, the reaction rate parameters of the catalytic combustion of acetaldehyde can be calculated and the result is expressed below: r ¼ 46:18 e36374=RT C
(2)
Fig. 6 shows that the reaction rate is determined by the surface reaction within the low reaction temperature regime and that the reaction rate is controlled by the pore diffusion rate when the reaction temperature exceeds 513 K. Hence, it can be concluded
153
Fig. 6. Arrhenius plot of cyclohexanone reaction.
that the catalytic combustion of cyclohexanone has a high reaction rate and the reaction rate is limited by the gas diffusion rate at high reaction temperature. The kinetics parameters of the surface reaction were calculated with the data obtained at low temperature and are expressed below: r ¼ 432 e41960=RT C
(3)
3.3. Channel design and simulation of structured reactor It has been reported [9,23] that the fluid flow in a small and long channel of a typical monolith (0.8–4 mm in diameter) is undoubtedly laminar. Generally, transport coefficients are rather poor for laminar flow, as the flowing fluid moves parallel to the walls, without vortices or perpendicular streams characteristic of a turbulent regime. When so-called fully developed laminar flow is considered, the profiles of velocity, concentration and temperature are parabolic and molecular diffusion (or thermal conduction) throughout the channel is therefore a predominant mass (or heat) transport mechanism. In such conditions, Sherwood and Nusselt numbers are nearly constant, dependent on the channel shape, but almost not influenced by the Reynolds number. This implies that mass transfer coefficients achieve low values, also dependent on the channel shape and hydraulic diameter, which cannot be enhanced for example by increasing fluid velocity. This situation finally brings an undesirable increase in the reactor length. Hence the design of the channel shape is very important. Kołodziej and Łojewska [9] have proved that the mass transfer in structured catalyst can be enhanced with the design of the catalyst structure. A short-channel structure is preferred over a long-channel structure, especially when the short channels are in an arranged shifted pattern. Several short-channel structures of various cross-sectional shapes are presented in the literature. A kind of novel platinum-structured catalyst was developed in our group based on the following design ideas to enhance the mass transfer and to improve its catalytic activity: (1) Cut the long-channel unit into several short-channel units and arrange the units in a shifted pattern, as shown in Fig. 7
Fig. 5. Arrhenius plot of acetaldehyde reaction.
While mass transport is rather poor for laminar flow in long fluid ducts, it is well known that within the entrance region of the dust, transfer coefficients are significantly higher. Thus, it seems reasonable to enhance mass transport by replacing a single long channel by several short units. However, it is very difficult to prepare the short-channel structure with the ceramics which is always used in the commercial monolith.
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
154
Fig. 7. Channel design of the structured catalyst.
Aluminum is very feasible and can be formed into any shape; hence, it is adopted in our research. In this work, the simulation for the VOC’s catalytic combustion on a 10 cm long-channel unit and several short-channel units (total length is 10 cm) was carried out. In order to simplify the simulation, several assumptions are made as follows: (1) The whole system is considered as a steady state for reactor operation. (2) The gases are assumed to be ideal gases and suitable for the ideal gas law. (3) The chemical reaction would take place only on the catalyst surface and is an isothermal reaction. (4) In the simulation, the pressure drop is neglected in all runs. (5) The temperature distribution in the reactor is homogenous. The partial differential equations are given in Table 2, where the catalyst surfaces are regarded as the interior boundaries in this model. The PDEs were solved by using FEMLAB, a PDE solver tool from COMSOL Inc. The simulation result of acetaldehyde reaction is expressed in Fig. 8 and that of cyclohexanone reaction is expressed in Fig. 9. Fig. 8 shows that the conversion of acetaldehyde increases from 58.7% to 62.8% at 573 K, when the 10 cm channel was cut to two parts. However, the increase of conversion is not obviously when the channel was cut from 8 parts to 16 parts. Fig. 9 also exhibits the same trend. The concentration distribution of acetaldehyde in the channel is expressed in Fig. 10, which shows that the gas flow in the middle of the catalyst channel has a high possibility to flow near the catalyst layer in the following channel when the channel was cut into several parts, which will enhance the diffusion reflux because a high concentration gradient will appear near the catalyst surface. As a result, the reaction performance is improved. In addition, it is known that shorter catalyst layers will cause better velocity distribution [24]. Hence, short channels are introduced in the design of the
Fig. 8. Conversion of acetaldehyde changes with the number of short channel stacks at different reaction temperatures. (Total channel length is 5 10 cm. The conversions were calculated when the channel was cut into 1, 2, 4, 8 and 16 parts, as shown in Fig. 7.)
Fig. 9. Conversion of cyclohexanone changes with the number of short channel stacks at different reaction temperatures. (Total channel length is 5 10 cm. The conversions were calculated when the channel was cut into 1, 2, 4, 8 and 16 parts, as shown in Fig. 7.)
Table 2 Partial differential equations and parameters of model Partial differential equations Gas phase Mass transfer @c þ @ D @c u @@yc ¼ @@y D dy @x @x Navier–Stokes velocity equation 2 2 r u @@uxx þ @@uyy ¼ rg þ y @@xu2x þ @@yu2y Boundaries y¼0 ui ¼ c0;i u0 ; u ¼ u0 y ¼ catalyst surface ui ¼ r; u ¼ 0 Parameters of model Amount of catalyst: 50 cm Thickness of catalyst: 0.03 cm Distance between catalyst surfaces: 0.15 cm Flow rate: 2.16 m/s Inlet concentration: 100 ppm
Fig. 10. Concentration distribution of acetaldehyde on the structured catalyst channel.
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
Fig. 12. Activity difference between the bent channel and straight channel. : straight channel. bent channel;
155
:
Fig. 11. Single channel design of structured catalyst.
structured catalyst in the work. Considering the manufacturing cost, a 1.0–1.3 cm channel will be preferred in our design. (2) Each channel is designed as bend shape instead of straight line It has been stated [23] that the fluid flow in a small, straight and long channel of a typical monolith is laminar; hence, the straight line shape is abandoned in our design. As shown in Fig. 11, the bent shape is adopted in our design. The comparison between the straight channel and the bent channel was carried out with simulations and the result is expressed in Fig. 12. The simulation results show that the conversion of acetaldehyde increases from 16.4% to 19.9% at 573 K when the bent channel was adopted instead of the straight channel and the conversion of cyclohexanone increases from 20.2% to 26.7%. Hence, it can be found that the bent channel is more suitable in the design of the structured catalyst than the straight channel. 3.4. Evaluation of the developed structured catalyst Based on the simulation results, the novel structured catalyst was designed and prepared in our group, as shown in Fig. 13. The bent, short channels are introduced in the structured catalyst. Moreover, the channels are arranged in a shifted pattern. The
length of the channel is about 1.0 cm and the distance between the catalyst surfaces is 0.15 cm. The catalytic activity of the developed structured catalyst was also evaluated with the catalytic combustion reaction of acetaldehyde and cyclohexanone under different inlet concentration and flow rate conditions, which were identical with those for the packed tube reactor. The conversions of acetaldehyde are expressed in Fig. 14, in which the conversion result obtained from the tube reactor packed with the plate catalyst is along the X-axis and the conversion result obtained from the reactor packed with the structured catalyst is along the Y-axis. The results show that the conversion in the reactor with the structured catalyst is almost the same as the conversion in the tube reactor packed with the plate catalyst. As stated above, in the packed tube reactor the mass transfer can be moderated with the packed quartz sand and it has been proved that the reaction rate is not determined by the laminar diffusion. The conversion of acetaldehyde obtained from the reactor packed with the structured catalyst is the same as that obtained from the packed tube reactor, which indicates that the reaction rates of acetaldehyde in the two reactors are identical. Hence, it can be deduced that the mass transfer in the reactor packed with the structured catalyst is also moderated with the design of the
Fig. 13. Photos of the structured catalyst developed in the work.
156
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
4. Conclusions
Fig. 14. Conversion of acetaldehyde in the reactors (SV: 10,000–30,000 h1, inlet concentration: 50–100 ppm, reaction temperature: 400–600 K).
The applications of structured catalysts are limited by some disadvantages, such as low thermal conductivity and poor mass transfer, when the traditional honeycomb-structured catalyst was adopted. In our research, the shifted arrangement of bent, short channels were adopted in the design of the structured catalyst. Aluminum has fine thermal conductivity and is very feasible. It can be formed into any shape, hence it was selected as the support material for the structured catalyst after the anodic oxidation treatment. The simulation results show that the novel design of the structured catalyst brings in higher activity, compared to the traditional structured catalyst which has straight, long channels. The developed structured catalyst was packed into a reactor and the catalytic combustion reactions of VOC were carried out to evaluate the catalytic activity. The reactor almost has the same catalytic activity as a packed tube reactor, in which the reaction is controlled by the surface reaction. It means that the reaction on the structured catalyst developed in our group is also controlled with the surface reaction. It has been proved that the catalytic combustion of VOC on the traditional honeycomb-structured catalyst is controlled by the interphase mass transfer because the fluid flow in the catalyst is laminar. Therefore, it can be concluded that the fluid flow was improved greatly and that the laminar diffusion is not the rate-determining step in the reaction when the structured catalyst developed in our group was applied. Moreover, the material of the support of the catalyst is aluminum, hence the structured catalyst has fine thermal conductivity, compared with the traditional ceramics structured catalyst. That some drawbacks of structured catalyst can be overcome when the novel structured catalyst made of aluminum was adopted can be concluded from the research results. References
Fig. 15. Conversion of cyclohexanone in the reactors (SV: 10,000–30,000 h1, inlet concentration: 50–100 ppm; reaction temperature: 400–600 K).
structured catalyst and is not the determining-step for the reaction. The catalytic combustion reaction of cyclohexanone was also carried out in the research and the result is expressed in Fig. 15. The conversion result obtained from the packed tube reactor is also shown on the X-axis and the conversion result obtained from the reactor packed with the structured catalyst is along the Y-axis. The result also shows that there is almost no difference between the conversion between the packed tube reactor and the reactor packed with the structured catalyst. Based on the research results, we can conclude that the reactions are controlled by the surface reactions in the structured catalyst reactor, when the short, shifted arrangement channel design is applied, the mass transfer can be enhanced; and the laminar diffusion is not the rate-determining step in the catalytic combustion of VOC.
[1] R.W. van den Brink, R. Louw, P. Mulder, Appl. Catal. B: Environ. 16 (1998) 219– 226. [2] K. Okumura, T. Kobayashi, H. Tanaka, M. Niwa, Appl. Catal. B: Environ. 44 (2003) 325–331. [3] S. Scire`, S. Minico`, C. Crisafulli, Appl. Catal. B: Environ 45 (2003) 117–125. [4] K. Everaert, J. Baeyens, J. Hazard. Mater. B 109 (2004) 113–139. [5] J.M. Padilla, G. Del Angel, J. Navarrete, Catal. Today 133–135 (2008) 541–547. [6] G. Groppi, E. Tronconi, Chem. Eng. Sci. 55 (2000) 2161–2171. [7] G. Groppi, E. Tronconi, Catal. Today 105 (2005) 297–304. [8] Z. Jiang, K. Chung, G. Kim, J. Chung, Chem. Eng. Sci. 58 (2003) 1103–1111. [9] A. Kołodziej, J. Łojewska, Catal. Today 105 (2005) 378–384. [10] P. Avila, M. Montes, E.E. Miro´, Chem. Eng. J. 109 (2005) 11–36. [11] V. Meille, Appl. Catal. A: Gen. 315 (2006) 1–17. [12] M.A. Ulla, R. Mallada, L.B. Gutierrez, L. Casado, J.P. Bortolozzi, E.E. Miro´, J. Santamarı´a, Catal. Today 133–135 (2008) 42–48. [13] O. Sanz, L.C. Almeida, J.M. Zamaro, M.A. Ulla, E.E. Miro´, M. Montes, Appl. Catal. B: Environ. 78 (2008) 166–175. [14] S. Lee, A. Gavriilidis, Catal. Commun. 3 (2002) 425–428. [15] L. Wang, V.D. Vien, K. Suzuki, M. Sakurai, H. Kameyama, J. Chem. Eng. Jpn. 38 (2005) 106–112. [16] L. Wang, V.D. Vien, M. Higuchi, M. Sakurai, H. Kameyama, J. Chem. Eng. Jpn. 39 (2006) 889–895. [17] L. Wang, Y. Guo, V.D. Vien, M. Sakurai, H. Kameyama, J. Chem. Eng. Jpn. 39 (2006) 1165–1171. [18] L. Wang, Q. Zhang, M. Sakurai, H. Kameyama, Catal. Commun. 8 (2007) 2171– 2175. [19] Q. Zhang, L. Wang, M. Sakurai, T. Kitajima, H. Takahashi, M. Nakaya, T. Ootani, H. Kameyama, J. Chem. Eng. Jpn. 39 (2006) 1172–1181. [20] Y. Iwasaki, Y. Suzuki, T. Kitajima, M. Sakurai, H. Kameyama, J. Chem. Eng. Jpn. 40 (2007) 178–185. [21] T.F. Garetto, E. Rinco´n, C.R. Apesteguı´a, Appl. Catal. B: Environ. 48 (2004) 167–174. [22] C.T. Wong, A.Z. Abdullah, S. Bhatia, J. Hazard. Mater 157 (2008) 480–489. [23] J. Łojewska, A. Kołodziej, P. Dynarowicz-Łatka, A. Wesełucha-Birczyn´ska, Catal. Today 101 (2005) 81–91. [24] C. Fukuhara, A. Igarashi, J. Chem. Eng. Jpn. 37 (2004) 415–421.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Design of novel Pt-structured catalyst on anodic aluminum support for VOC’s catalytic combustion Lifeng Wang *, Thanh Phong Tran, Dong Vien Vo, Makoto Sakurai, Hideo Kameyama Department of Chemical Engineering, Faculty of Engineering, Tokyo University of Agriculture and Technology, 24-16 Nakacho 2, Koganei-Shi, Tokyo 184-8588, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 April 2008 Received in revised form 22 July 2008 Accepted 25 July 2008 Available online 3 August 2008
Emission control of volatile organic compounds (VOCs) is one of the priorities for environmental catalysis. Catalytic combustion is one of promising technologies for elimination of VOCs. A novel platinum-structured catalyst was designed and is presented in this work. Aluminum was used as support for the catalyst after anodic oxidation treatment and in a shifted arrangement bent, short channels are applied in the design of the structured catalyst. The simulation results show that the novel design of the structured catalyst brings in the higher catalytic activity, compared with the traditional structured catalyst which has long, straight channels. Lastly, the novel structured catalyst was manufactured and its catalytic activity was evaluated with the catalytic combustion of VOC. The results show that the conversion obtained from the reactor with the structured catalyst is almost identical to that obtained from the tube reactor packed with the plate catalyst in which the reaction rate is controlled by surface reactions. Hence, it can be concluded that the mass transfer can be improved by the novel design of the structured catalyst and that the mass transfer in gas phase is not the rate-determining step for the catalytic combustion of VOC. ß 2008 Elsevier B.V. All rights reserved.
Keywords: VOC Structured catalyst Anodic oxidation treatment Electro-deposition technology Mass transfer
1. Introduction Volatile organic compounds (VOCs) are defined as organic compounds with high vapor pressure that are easily vaporized under an ambient temperature and pressure. Their emission from industrial processes and vehicle exhausts, such as aliphatic, aromatic, benzene, toluene, is considered as a severe air pollutant. The progressive increase of VOC emissions, their hazardous nature, and the increasingly restrictive environmental regulations in industrialized countries have encouraged the development of different methods for the elimination of VOCs. Catalytic oxidation is an effective way for reducing the emissions of VOCs. In this way, two kinds of catalysts are usually applied: (i) noble metal-based solids which possess high activity but are expensive and (ii) metal oxide-based samples which are cheaper but less active. Supported precious metals as Pt and Pd are well established as efficient catalysts for the oxidation of different VOC [1–5]. Reactor design for the catalytic combustion process of VOC is very important especially when the catalyst with high activity is applied. The classic reactors with dumped beds of catalyst particles are one of the possible solutions for VOC combustion processes. In
* Corresponding author. E-mail address: [email protected] (L. Wang). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.07.033
fact, the reactors with packed beds are easy to construct, but they cause a high-pressure drop and do not allow reducing particle size to obtain high specific surface area. For this reason, the structured catalysts with high surface and low pressure-drop have been adopted in reactor designs. Especially, honeycomb monoliths have become the standard catalyst shape in most applications of environmental catalysis, after their successful commercial application to the control of automotive exhausts and to the reduction of nitrogen oxides. However, two findings have long discouraged the extensive use of monolithic catalysts in the catalytic combustion of VOC [6–10]: (a) Conventional parallel channel monoliths are virtually adiabatic: it is known that the catalytic combustion of VOC is an exothermic reaction and that the adiabatic temperature rise varies with the concentration of VOC but always exceeds 100 K. Hence, the adiabatic of the conventional monoliths would severely limit the control of the temperature in the catalytic combustion reaction of VOC. (b) Fluid flow in a small and long channel of a typical monolith (0.8–4 mm in diameter) is undoubtedly laminar, which will result in poor mass transfer and heat transfer. This drawback also may leads to serious malfunctions especially when confronted with VOCs, since catalytic combustion of VOCs always runs in diffusion regime, where the reaction rate is
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
Nomenclature A C D Ea k N r S T u
y y r
pre-exponential factor (m s1) concentration (mol m3) diffusion coefficient (m2 s1) activation energy (J mol) reaction rate constant (m s1) flow rate of VOC (m3 s1) specific reaction rate (mol m2 s1) catalyst surface (m2) reaction temperature (K) velocity (m s1) catalyst volume (m3) kinematical viscosity (kg m1 s1) density (kg m3)
limited by the mass transfer between catalyst and flowing gas mixture. This, in turn, results in an undesirable increase in the reactor length (raising investment costs) and flow resistance (raising pumping costs).
151
developed plate catalyst. The design of the novel structured catalyst was carried out based on the obtained reaction kinetics parameters by simulation. Lastly, the novel structured catalyst was manufactured and its catalytic activity was evaluated with the catalytic combustion of VOC. The simulation results indicate that the catalytic activity of the structured catalyst can be enhanced when the long, straight channels are replaced by bent, short channels. The activity experiments demonstrate that the conversion obtained from the reactor with the structured catalyst is almost the same as that obtained from the tube reactor packed with the plate catalyst. In our research, the mass transfer between catalyst and flowing gas mixture was modified so as to ensure that the reaction will be controlled by the surface reaction in the packed tube reactor. Moreover, it has been proved that the catalytic combustion reaction of VOC will be controlled by the mass transfer in the structured catalyst where the traditional long, straight channels were adopted, such as in honeycomb monoliths [9]. Hence, it can be concluded that the concentration distribution on the channels can be improved by the design of the novel structured catalyst and that the mass transfer is not the rate-determining step for the VOC’s catalytic combustion reaction. 2. Experimental
To overcome at least part of the above-listed problems connected with the catalytic combustion of VOC, we propose a kind of platinum-structured catalyst supported on an anodic oxidized aluminum support. Aluminum was used as support in our group because it has fine thermal conductivity and can be formed into any shape such as a mesh, fin, plate or serrate. It is known that coating technology is always used to support metal or Al2O3, SiO2 on metal carriers to produce catalyst or support [11–13]. However, adhesion of the coating layer to the support is not strong and the layer can be damaged easily [14]. In our group, anodic oxidation technology was applied to produce anodic alumite film on aluminum surfaces and the film can be used as the support for catalysts preparation, since anodization leads to formation of unbranched, regular and nearly concentric pores, as shown in Fig. 1, which are advantageous for catalytic reactions. Cu, Pt, Pd, Ni catalysts supported on the anodized aluminum plate have already been developed successfully in our group [15–20], we named them anodic alumite catalysts. In this work, the Pt catalyst was prepared on the anodic alumite plate with an electro-deposition technology [15–17]. Then, the reaction kinetics parameters of the catalytic combustion of VOC were calculated based on the activity experiments results of the
2.1. Catalyst preparation Plate catalyst supports were prepared by anodisation of 4 cm 8 cm of 0.3 mm thick aluminum plates (JIS A1050). Each plate was first rinsed with acetone and degreased in an aqueous solution of 20 wt% NaOH for 3 min at room temperature, then washed again with deionised water and immersed in a 30 wt% HNO3 solution for 1 min at room temperature. Each plate was then washed with deionised water and dried. The treated plate was oxidized as the anode in an oxalic acid solution against a catholically polarized graphite plate counter-electrode. The solution was kept at a constant temperature of 293 K, while the oxalic acid concentration was set at 4 wt%. The distance between the two electrodes was set to be 8 cm. Under a direct current density of 50 A m2, anodic oxidation was carried out for 10 h. Each plate was then dried at ambient temperature for 12 h and calcined in air at 623 K for 1 h, in order to decompose the residual oxalic acid in the film. Finally, the dried supports were stored in a desiccator. In the electro-deposition process, the support was soaked in 0.1 g L1 (based on platinum) chloroplatinic acid hexahydrate
Fig. 1. SEM micrographs of anodic alumite film.
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
152
solution at ambient temperature (precursor salt, H2PtCl66H2O, Wako Pure Chemical Industries, Ltd.) as one electrode against the counter electrodes of the stainless steel plates at a distance of 2 cm. The solution pH was about 3.4 without the use of any pHconditioning agent. Ordinary power (50 Hz, 110 V) was used in the experiment, with a voltage regulator to adjust the experimental voltage to a desired voltage. After the supporting, the plate was washed with deionised water and then dried for 4 h at ambient temperature. Finally, the plate was calcined in air at 773 K for 3 h and then stored in a desiccator. The structured catalyst (250 mm 40 mm 100 mm) was also prepared by the same process in the work. 2.2. Catalyst characterization In this study, the anodic alumite film thickness was measured with an optical microscope. The specific surface area and pore distributions could be obtained via a nitrogen adsorption method (SA3100, Beckman Coulter Inc.), where the outgas conditions were set at 573 K for 1 h. An inductively coupled plasma spectrometer (ICPS-7510, Shimadzu Corp.) was used to measure the platinum loading after the catalysts were dissolved in nitrohydrochloric acid. Surface micrographs of the catalysts were taken by a FE-SEM (S-4800, Hitachi, Ltd.). 2.3. Activity experiment In this work, catalytic combustion of VOC was used to evaluate the catalytic activity of the developed catalyst. Acetaldehyde and cyclohexanone (Kokusan Chemical Co., Ltd.) were chosen to represent VOC. Air from a compressor flowed into a saturator that was filled with liquid hydrocarbon and then mixed with the VOC stream that was evaporated by heating. The mixed air–VOC stream was fed into a reactor. Part of the plate catalyst (1 cm 2 cm) was cut into pieces 2–4 mm2, diluted with 4 g quartz sand (30–50 mesh, Kishida Chemical Co., Ltd.) and this material was packed into the reactor with an i.d. of 10 mm. In the reactor, the mass transfer can be moderated with the packed quartz sand and a suitable flow rate and one can ensure that the reaction in the reactor will be controlled by the surface reaction. The catalytic activity of the developed structured catalyst was also evaluated in this work. Part of the catalyst (50 mm 40 mm 100 mm) was packed into a reactor as shown in Fig. 2. Reactant gas was heated with an electrical heater before entering into the catalyst bed. The temperature in the middle of the
Fig. 2. Reactor packed with the structured catalyst.
Table 1 Properties of the structured catalyst Size (mm) Weight of the catalyst (g per monolith) Volume (m3 per monolith) Surface area (m2 per monolith) Volume fraction of voids Wall thickness (mm) Anodic alumite film thickness (mm) SBET (m2 per monolith) Average pore size (nm) Pt loading (g/m2)
250 40 100 341.4 0.001 1.08 0.83 0.3 0.1 2 2.04 104 42.3 1.0
catalyst bed was controlled. In this work, acetaldehyde and cyclohexanone were both used. The flow rate was set at 50 or 100 L min1 to keep the identical F/S with that in the tube reactor. VOCs were measured using a gas chromatograph with an FID detector (GC-390B, GL Sciences Inc.). 3. Results and discussion 3.1. Catalyst properties The properties of the catalyst used in the research are expressed in Table 1. The thickness of the aluminum is about 0.3 mm; after the anodic oxidation, 0.1 mm anodic alumite films can be formed on the surface. Table 1 shows that the BET-surface area is about 20,000 m2 per monolith. The average pore size of the anodic oxidation film is about 42.3 nm. The pores also can be found on the micro-photo of the cross-section of the anodic oxidation film, which was taken by FE-SEM. The platinum loading on the structured catalyst is 1.0 g m2, which means that the supported platinum on the structured catalyst is about 1.0 g. 3.2. Reaction kinetics of VOC The activity of the developed plate catalyst was evaluated with the catalytic combustion reaction of acetaldehyde and cyclohexanone under different inlet concentration and flow rate conditions. The results are presented in Figs. 3 and 4. The conversion of acetaldehyde decreases with the increase of flow rate, as seen from Fig. 3. Acetaldehyde can be decomposed completely when the reaction temperature was increased to 613 K. Fig. 4 shows that
Fig. 3. Acetaldehyde ignition curves. (~, SV 50,000 h1; inlet concentration: 86 ppm. &, SV: 40,000 h1; inlet concentration: 100 ppm. &, SV: 30,000 h1; inlet concentration: 64 ppm.)
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
Fig. 4. Cyclohexanone ignition curves. (~, SV: 60,000 h1; inlet concentration: 80 ppm. &, SV: 30,000 h1; inlet concentration: 150 ppm. &, SV: 30,000 h1; inlet concentration: 80 ppm.)
cyclohexanone can be combusted more easily; however, it cannot be combusted completely at lower temperature. It is known that catalytic combustion of VOC performed in the excess of oxygen can be simplified into a simple reaction pattern described by a first-order kinetic equation [21,22]: Ea dN r ¼ kC ¼ A exp C¼ dS RT
(1)
Here r is a specific reaction rate; k is the rate constant and C is the concentration of VOC in the gas phase; A is a pre-exponential factor and Ea is the activation energy. k can be calculated based on the experimental results. Based on the calculation results, the Arrhenius plots are plotted and expressed in Figs. 5 and 6. From Fig. 5, the reaction rate parameters of the catalytic combustion of acetaldehyde can be calculated and the result is expressed below: r ¼ 46:18 e36374=RT C
(2)
Fig. 6 shows that the reaction rate is determined by the surface reaction within the low reaction temperature regime and that the reaction rate is controlled by the pore diffusion rate when the reaction temperature exceeds 513 K. Hence, it can be concluded
153
Fig. 6. Arrhenius plot of cyclohexanone reaction.
that the catalytic combustion of cyclohexanone has a high reaction rate and the reaction rate is limited by the gas diffusion rate at high reaction temperature. The kinetics parameters of the surface reaction were calculated with the data obtained at low temperature and are expressed below: r ¼ 432 e41960=RT C
(3)
3.3. Channel design and simulation of structured reactor It has been reported [9,23] that the fluid flow in a small and long channel of a typical monolith (0.8–4 mm in diameter) is undoubtedly laminar. Generally, transport coefficients are rather poor for laminar flow, as the flowing fluid moves parallel to the walls, without vortices or perpendicular streams characteristic of a turbulent regime. When so-called fully developed laminar flow is considered, the profiles of velocity, concentration and temperature are parabolic and molecular diffusion (or thermal conduction) throughout the channel is therefore a predominant mass (or heat) transport mechanism. In such conditions, Sherwood and Nusselt numbers are nearly constant, dependent on the channel shape, but almost not influenced by the Reynolds number. This implies that mass transfer coefficients achieve low values, also dependent on the channel shape and hydraulic diameter, which cannot be enhanced for example by increasing fluid velocity. This situation finally brings an undesirable increase in the reactor length. Hence the design of the channel shape is very important. Kołodziej and Łojewska [9] have proved that the mass transfer in structured catalyst can be enhanced with the design of the catalyst structure. A short-channel structure is preferred over a long-channel structure, especially when the short channels are in an arranged shifted pattern. Several short-channel structures of various cross-sectional shapes are presented in the literature. A kind of novel platinum-structured catalyst was developed in our group based on the following design ideas to enhance the mass transfer and to improve its catalytic activity: (1) Cut the long-channel unit into several short-channel units and arrange the units in a shifted pattern, as shown in Fig. 7
Fig. 5. Arrhenius plot of acetaldehyde reaction.
While mass transport is rather poor for laminar flow in long fluid ducts, it is well known that within the entrance region of the dust, transfer coefficients are significantly higher. Thus, it seems reasonable to enhance mass transport by replacing a single long channel by several short units. However, it is very difficult to prepare the short-channel structure with the ceramics which is always used in the commercial monolith.
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
154
Fig. 7. Channel design of the structured catalyst.
Aluminum is very feasible and can be formed into any shape; hence, it is adopted in our research. In this work, the simulation for the VOC’s catalytic combustion on a 10 cm long-channel unit and several short-channel units (total length is 10 cm) was carried out. In order to simplify the simulation, several assumptions are made as follows: (1) The whole system is considered as a steady state for reactor operation. (2) The gases are assumed to be ideal gases and suitable for the ideal gas law. (3) The chemical reaction would take place only on the catalyst surface and is an isothermal reaction. (4) In the simulation, the pressure drop is neglected in all runs. (5) The temperature distribution in the reactor is homogenous. The partial differential equations are given in Table 2, where the catalyst surfaces are regarded as the interior boundaries in this model. The PDEs were solved by using FEMLAB, a PDE solver tool from COMSOL Inc. The simulation result of acetaldehyde reaction is expressed in Fig. 8 and that of cyclohexanone reaction is expressed in Fig. 9. Fig. 8 shows that the conversion of acetaldehyde increases from 58.7% to 62.8% at 573 K, when the 10 cm channel was cut to two parts. However, the increase of conversion is not obviously when the channel was cut from 8 parts to 16 parts. Fig. 9 also exhibits the same trend. The concentration distribution of acetaldehyde in the channel is expressed in Fig. 10, which shows that the gas flow in the middle of the catalyst channel has a high possibility to flow near the catalyst layer in the following channel when the channel was cut into several parts, which will enhance the diffusion reflux because a high concentration gradient will appear near the catalyst surface. As a result, the reaction performance is improved. In addition, it is known that shorter catalyst layers will cause better velocity distribution [24]. Hence, short channels are introduced in the design of the
Fig. 8. Conversion of acetaldehyde changes with the number of short channel stacks at different reaction temperatures. (Total channel length is 5 10 cm. The conversions were calculated when the channel was cut into 1, 2, 4, 8 and 16 parts, as shown in Fig. 7.)
Fig. 9. Conversion of cyclohexanone changes with the number of short channel stacks at different reaction temperatures. (Total channel length is 5 10 cm. The conversions were calculated when the channel was cut into 1, 2, 4, 8 and 16 parts, as shown in Fig. 7.)
Table 2 Partial differential equations and parameters of model Partial differential equations Gas phase Mass transfer @c þ @ D @c u @@yc ¼ @@y D dy @x @x Navier–Stokes velocity equation 2 2 r u @@uxx þ @@uyy ¼ rg þ y @@xu2x þ @@yu2y Boundaries y¼0 ui ¼ c0;i u0 ; u ¼ u0 y ¼ catalyst surface ui ¼ r; u ¼ 0 Parameters of model Amount of catalyst: 50 cm Thickness of catalyst: 0.03 cm Distance between catalyst surfaces: 0.15 cm Flow rate: 2.16 m/s Inlet concentration: 100 ppm
Fig. 10. Concentration distribution of acetaldehyde on the structured catalyst channel.
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
Fig. 12. Activity difference between the bent channel and straight channel. : straight channel. bent channel;
155
:
Fig. 11. Single channel design of structured catalyst.
structured catalyst in the work. Considering the manufacturing cost, a 1.0–1.3 cm channel will be preferred in our design. (2) Each channel is designed as bend shape instead of straight line It has been stated [23] that the fluid flow in a small, straight and long channel of a typical monolith is laminar; hence, the straight line shape is abandoned in our design. As shown in Fig. 11, the bent shape is adopted in our design. The comparison between the straight channel and the bent channel was carried out with simulations and the result is expressed in Fig. 12. The simulation results show that the conversion of acetaldehyde increases from 16.4% to 19.9% at 573 K when the bent channel was adopted instead of the straight channel and the conversion of cyclohexanone increases from 20.2% to 26.7%. Hence, it can be found that the bent channel is more suitable in the design of the structured catalyst than the straight channel. 3.4. Evaluation of the developed structured catalyst Based on the simulation results, the novel structured catalyst was designed and prepared in our group, as shown in Fig. 13. The bent, short channels are introduced in the structured catalyst. Moreover, the channels are arranged in a shifted pattern. The
length of the channel is about 1.0 cm and the distance between the catalyst surfaces is 0.15 cm. The catalytic activity of the developed structured catalyst was also evaluated with the catalytic combustion reaction of acetaldehyde and cyclohexanone under different inlet concentration and flow rate conditions, which were identical with those for the packed tube reactor. The conversions of acetaldehyde are expressed in Fig. 14, in which the conversion result obtained from the tube reactor packed with the plate catalyst is along the X-axis and the conversion result obtained from the reactor packed with the structured catalyst is along the Y-axis. The results show that the conversion in the reactor with the structured catalyst is almost the same as the conversion in the tube reactor packed with the plate catalyst. As stated above, in the packed tube reactor the mass transfer can be moderated with the packed quartz sand and it has been proved that the reaction rate is not determined by the laminar diffusion. The conversion of acetaldehyde obtained from the reactor packed with the structured catalyst is the same as that obtained from the packed tube reactor, which indicates that the reaction rates of acetaldehyde in the two reactors are identical. Hence, it can be deduced that the mass transfer in the reactor packed with the structured catalyst is also moderated with the design of the
Fig. 13. Photos of the structured catalyst developed in the work.
156
L. Wang et al. / Applied Catalysis A: General 350 (2008) 150–156
4. Conclusions
Fig. 14. Conversion of acetaldehyde in the reactors (SV: 10,000–30,000 h1, inlet concentration: 50–100 ppm, reaction temperature: 400–600 K).
The applications of structured catalysts are limited by some disadvantages, such as low thermal conductivity and poor mass transfer, when the traditional honeycomb-structured catalyst was adopted. In our research, the shifted arrangement of bent, short channels were adopted in the design of the structured catalyst. Aluminum has fine thermal conductivity and is very feasible. It can be formed into any shape, hence it was selected as the support material for the structured catalyst after the anodic oxidation treatment. The simulation results show that the novel design of the structured catalyst brings in higher activity, compared to the traditional structured catalyst which has straight, long channels. The developed structured catalyst was packed into a reactor and the catalytic combustion reactions of VOC were carried out to evaluate the catalytic activity. The reactor almost has the same catalytic activity as a packed tube reactor, in which the reaction is controlled by the surface reaction. It means that the reaction on the structured catalyst developed in our group is also controlled with the surface reaction. It has been proved that the catalytic combustion of VOC on the traditional honeycomb-structured catalyst is controlled by the interphase mass transfer because the fluid flow in the catalyst is laminar. Therefore, it can be concluded that the fluid flow was improved greatly and that the laminar diffusion is not the rate-determining step in the reaction when the structured catalyst developed in our group was applied. Moreover, the material of the support of the catalyst is aluminum, hence the structured catalyst has fine thermal conductivity, compared with the traditional ceramics structured catalyst. That some drawbacks of structured catalyst can be overcome when the novel structured catalyst made of aluminum was adopted can be concluded from the research results. References
Fig. 15. Conversion of cyclohexanone in the reactors (SV: 10,000–30,000 h1, inlet concentration: 50–100 ppm; reaction temperature: 400–600 K).
structured catalyst and is not the determining-step for the reaction. The catalytic combustion reaction of cyclohexanone was also carried out in the research and the result is expressed in Fig. 15. The conversion result obtained from the packed tube reactor is also shown on the X-axis and the conversion result obtained from the reactor packed with the structured catalyst is along the Y-axis. The result also shows that there is almost no difference between the conversion between the packed tube reactor and the reactor packed with the structured catalyst. Based on the research results, we can conclude that the reactions are controlled by the surface reactions in the structured catalyst reactor, when the short, shifted arrangement channel design is applied, the mass transfer can be enhanced; and the laminar diffusion is not the rate-determining step in the catalytic combustion of VOC.
[1] R.W. van den Brink, R. Louw, P. Mulder, Appl. Catal. B: Environ. 16 (1998) 219– 226. [2] K. Okumura, T. Kobayashi, H. Tanaka, M. Niwa, Appl. Catal. B: Environ. 44 (2003) 325–331. [3] S. Scire`, S. Minico`, C. Crisafulli, Appl. Catal. B: Environ 45 (2003) 117–125. [4] K. Everaert, J. Baeyens, J. Hazard. Mater. B 109 (2004) 113–139. [5] J.M. Padilla, G. Del Angel, J. Navarrete, Catal. Today 133–135 (2008) 541–547. [6] G. Groppi, E. Tronconi, Chem. Eng. Sci. 55 (2000) 2161–2171. [7] G. Groppi, E. Tronconi, Catal. Today 105 (2005) 297–304. [8] Z. Jiang, K. Chung, G. Kim, J. Chung, Chem. Eng. Sci. 58 (2003) 1103–1111. [9] A. Kołodziej, J. Łojewska, Catal. Today 105 (2005) 378–384. [10] P. Avila, M. Montes, E.E. Miro´, Chem. Eng. J. 109 (2005) 11–36. [11] V. Meille, Appl. Catal. A: Gen. 315 (2006) 1–17. [12] M.A. Ulla, R. Mallada, L.B. Gutierrez, L. Casado, J.P. Bortolozzi, E.E. Miro´, J. Santamarı´a, Catal. Today 133–135 (2008) 42–48. [13] O. Sanz, L.C. Almeida, J.M. Zamaro, M.A. Ulla, E.E. Miro´, M. Montes, Appl. Catal. B: Environ. 78 (2008) 166–175. [14] S. Lee, A. Gavriilidis, Catal. Commun. 3 (2002) 425–428. [15] L. Wang, V.D. Vien, K. Suzuki, M. Sakurai, H. Kameyama, J. Chem. Eng. Jpn. 38 (2005) 106–112. [16] L. Wang, V.D. Vien, M. Higuchi, M. Sakurai, H. Kameyama, J. Chem. Eng. Jpn. 39 (2006) 889–895. [17] L. Wang, Y. Guo, V.D. Vien, M. Sakurai, H. Kameyama, J. Chem. Eng. Jpn. 39 (2006) 1165–1171. [18] L. Wang, Q. Zhang, M. Sakurai, H. Kameyama, Catal. Commun. 8 (2007) 2171– 2175. [19] Q. Zhang, L. Wang, M. Sakurai, T. Kitajima, H. Takahashi, M. Nakaya, T. Ootani, H. Kameyama, J. Chem. Eng. Jpn. 39 (2006) 1172–1181. [20] Y. Iwasaki, Y. Suzuki, T. Kitajima, M. Sakurai, H. Kameyama, J. Chem. Eng. Jpn. 40 (2007) 178–185. [21] T.F. Garetto, E. Rinco´n, C.R. Apesteguı´a, Appl. Catal. B: Environ. 48 (2004) 167–174. [22] C.T. Wong, A.Z. Abdullah, S. Bhatia, J. Hazard. Mater 157 (2008) 480–489. [23] J. Łojewska, A. Kołodziej, P. Dynarowicz-Łatka, A. Wesełucha-Birczyn´ska, Catal. Today 101 (2005) 81–91. [24] C. Fukuhara, A. Igarashi, J. Chem. Eng. Jpn. 37 (2004) 415–421.