Counteracting Catalyst Deactivation in Methane Aromatization with a

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Ind. Eng. Chem. Res. 2010, 49, 996–1000

Counteracting Catalyst Deactivation in Methane Aromatization with a Two Zone Fluidized Bed Reactor Marıá Pilar Gimeno,† Jaime Soler,‡ Javier Herguido,† and Miguel Meneńdez*,† Aragoń Institute of Engineering Research (I3A), UniVersity of Zaragoza, c/Marıá de Luna 3 50018 Zaragoza, Spain, and Institute of Nanoscience of Aragoń (INA), UniVersity of Zaragoza, c/Pedro Cerbuna 12, 50009 Zaragoza, Spain

Methane aromatization is a promising alternative for the transformation of natural gas to liquid products but suffers the problem of a fast catalyst deactivation. The use of a two zone fluidized bed reactor (TZFBR) has been studied as a method to counteract this problem. In this reactor, two zones are created in a fluid bed by feeding the hydrocarbon at an intermediate point and a regenerating gas to the bottom of the reactor. A suitable catalyst for operation in the fluid bed has been developed. The results show that the TZFBR provides stable operation, thanks to the in situ regeneration of the catalyst in the lower part of the reactor. Three regenerating agents have been tested: oxygen, water, and carbon dioxide. Steady state operation has been achieved with all of them, but the best selectivity to aromatic products (benzene, toluene, and xylenes) was obtained with carbon dioxide. A significant effect of the reactor shape was also observed. The conversion and selectivity achieved with this reactor appear among the best values reported in the literature, with the advantage of being obtained at a steady state. 1. Introduction Natural gas (mostly methane) reserves are abundant throughout the world and are becoming a promising energy source. The wider chemical utilization of natural gas both as fuel and as an alternative feedstock for the petrochemical industry can help to gradually replace diminishing crude oil resources.1 However, the transportation of large amounts of gas suffers from a major limitation due to high transport prices.2 In order to overcome this problem, gas-to-liquid (GTL) processes have been proposed as an alternative.3 The conversion of natural gas to liquids is usually carried out through syngas as an intermediate step (in a single stage using Fischer-Tropsch or in several stages with methanol as an intermediate product). The necessity of using several steps and high pressure reactors makes the process expensive: 60% or more of the capital cost of GTL plants is associated with the reforming of methane to synthesis gas.4 So, a large incentive exists for the development of processes for direct conversion of methane without going through synthesis gas as an intermediate. Dehydrogenation and aromatization under nonoxidizing conditions using Mo/HZSM-5 catalysts had already been proposed in 1993 in a seminal work,5 and since then, this reaction has been widely studied,6-9 becoming an alternative for natural gas conversion and a method to manufacture aromatic products. However, the industrial development of this process has been stopped by the deactivation of the catalyst, which reduces drastically the conversion in a few hours,10 making necessary a catalyst regeneration step. Coke formation is also responsible for the failure of alternative reactors such as the multifunctional membrane fixed bed reactor.11 In this case, although hydrogen withdrawal is beneficial for shifting the equilibrium to higher methane conversion, the low hydrogen partial pressure also favors the competitive formation of hydrogen-deficient species, leading to coke deposition on the catalyst. * To whom correspondence should be addressed. Tel.: +34 976761152. Fax: +34 976 762142. E-mail: [email protected]. † Aragoń Institute of Engineering Research (I3A). ‡ Institute of Nanoscience of Aragoń (INA).

A suitable device for achieving steady state operation in catalytic reactions prone to deactivation by coke is the two zone fluidized bed reactor (TZFBR). In this multifunctional

Figure 1. Scheme of a two zone fluidized bed reactor (TZFBR). Here, u1 and u2 denote the different gas velocities in different parts of the bed.

10.1021/ie900682y  2010 American Chemical Society Published on Web 09/18/2009

Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010

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Figure 2. Scheme of the different reactor shapes and configurations employed: (a) cylindrical TZFBR, (b) conical ZFBR, (c) two section TZFBR, and (d) conventional fluidized bed reactor (FBR).

Figure 3. Evolution of methane conversion and BTX/HC selectivity with time. Operation conditions: T ) 700 °C; ur ) 1.5; Wcat ) 12 g; fed molar ratio 19% N2/80% CH4/1% CO2. Cylindrical TZFBR (Figure 2a): W/F ) 0.15 g · min/mmol. Conical TZFBR (Figure 2b) and Two section TZFBR (Figure 2c): W/F ) 0.36 g · min/mmol.

Figure 4. Evolution of methane conversion and BTX/HC selectivity with time for different oxidants with the conical TZFBR. Operation conditions: T ) 700 °C; ur ) 1.5; Wcat ) 12 g; W/F ) 0.36 g · min/mmol; fed molar ratio 19% N2/80% CH4/1% oxidant.

reactor, two reactions, the desired reaction and the catalyst regeneration, are performed in a single vessel. To achieve this objective, two zones are created in a fluidized bed by the simple procedure of feeding in the hydrocarbon stream at an intermediate point of the bed and an oxidizing stream

at the bottom of the reactor (Figure 1). This distribution of the feed at two points creates two zones in the bed with different environments: an oxidizing atmosphere in the lower part and a reducing atmosphere in the upper part.

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Figure 5. Evolution of methane conversion and BTX/HC selectivity with temperature. Conical TZFBR. Operation conditions: ur ) 1.5; Wcat ) 12 g; W/F ) 0.36 g · min/mmol; fed molar ratio 80% CH4, different percentages of CO2 and nitrogen to balance.

In those reactions where the catalyst is deactivated by coke deposition, it can be regenerated in the lower part of the reactor. The regenerated catalyst is transported to the upper part of the bed in the wake of the bubbles, and the deactivated catalyst falls in the emulsion phase to the oxidizing zone, according to the well-established mechanism of solid mixing in fluidized beds. A second advantage of the TZFBR is that heat can be produced in the lower zone by the combustion of coke and the solid transports this heat to the upper zone, where the reaction (often endothermic, as methane aromatization is) occurs. The heat transport associated with the solid movement provides a good isothermicity, one of the well-known advantages of fluidized bed reactors. 2. Experimental Section 2.1. Catalyst Preparation. The first stage in developing a TZFBR for methane aromatization was to obtain a catalyst suitable for operation in a fluidized bed. Mo/ZSM-5 was chosen as the active material, since it provides good selectivity to aromatics and does not include noble metals. However, the particle size of the zeolite was too small for operation in a fluidized bed and an agglomerant was needed.12 The preparation procedure included the following steps: (i) mixing of H-ZSM-5 zeolite (provided by Su¨d-Chemie, SiO2/Al2O3 ratio ) 27) with bentonite (Aldrich), (ii) calcination and sieving of the powder to the desired particle size (100-250 µm), (iii) acidification of the powder (ion exchange in HCl solution), and (iv) ion exchange with a Mo salt. The amount of Mo, measured by ICP, was 6 wt %. The resulting catalyst provides smooth fluidization and has been stable for several hundreds of hours of operation in laboratory reactors. Scanning Electron Microscopy-Electron

Figure 6. Evolution of methane conversion (A) and BTX/HC selectivity (B) with time. Conical TZFBR. Operation conditions: T ) 700 °C; ur ) 1.5; Wcat ) 12 g; fed molar ratio 19% N2/80% CH4/1% CO2, W/F ) 0.36 g · min/mmol. The results for the conventional FBR are shown at the same conditions.

Probe Micro Analysis (SEM-EPMA) showed that Mo was present in the final catalyst at the same points as the zeolite crystals, proving that its deposition was a true ion exchange. The Mo content, measured by ICP, was 6 wt %. A good fluidization was observed with this catalyst, and a minimum fluidization velocity (umf ) 39 cm3 (STP) · cm-2 · min-1) was measured in N2 at 700 °C. 2.2. Experimental System. The TZFBR described in the introduction has been employed in this work. However, it was found that since the volumetric flow of oxidant in the lower part of the reactor and the total flow in the upper part were very different, the use of a TZFBR with a variable section13 can provide more flexibility, since there is a limited range of relative velocity, ur, defined as u/umf at each point of the reactor. This variable section allows the lower part of the reactor to remain fluidized (u > umf) but avoids a too high relative gas velocity in the upper part (where the methane stream is introduced at the bed), which could lead to excessive losses of solid. Therefore reactors with three different shapes (as shown in Figure 2) have been tested: (i) cylindrical having the same diameter in all the bed (2.8 cm), (ii) conical the diameter varying continuously from the bottom (1.8 cm) to the top (2.8 cm), and (iii) two sections smaller diameter in the lower section (1.8 cm) than in the upper section (2.8 cm). All the reactors were made of quartz, with a porous sintered quartz plate as the gas distributor. The oxidant (oxygen, water, or carbon dioxide) mixed with a small amount of inert gas was fed at the bottom of the reactor, and methane was fed in at an intermediate point through a movable quartz tube inserted in the top. The gases were fed by means of mass flow meters, except water, that was supplied by means of a water saturator in the gas stream fed to the reactor. The products were analyzed


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Figure 7. Comparison of the results with those from the literature: benzene selectivity vs conversion in a conical TZFBR. Operation conditions: T ) 700 °C; ur ) 1.5; Wcat ) 12 g; W/F ) 0.36 g · min/mmol; fed molar ratio 19% N2/80% CH4/1% CO2.

by gas chromatography (Varian CP3800). Carbon balance closure had always an error smaller than 3% when the reactor was operating under steady state. 3. Results and Discussion 3.1. Effect of Reactor Shape. The results obtained with the TZFBR are shown in Figure 3. It may be seen that in all the cases a stable operation is achieved after about 1 h, thanks to the dynamic equilibrium between coke formation in the reduction zone and catalyst regeneration by coke gasification in the lower zone. In these experiments CO2 was used as an oxidizing agent. A second remarkable fact is the high selectivity to aromatic products: benzene, toluene, and xylene (BTX) with respect to the hydrocarbons obtained (HC), which is among the best of the values reported in literature for this level of conversion (as will be discussed in section 3.5). A third remarkable finding is the strong effect of the reactor. Even with the same W/F, the conversion and selectivity vary strongly, depending on the shape of the TZFBR. Under the operating conditions shown in Figure 3, the cylindrical TZFBR provides the highest conversion, but the conical reactor achieves the best selectivity to BTX. The reactor with two sections shows the lowest conversion and selectivity, probably because of fluid dynamic problems associated with the sudden change of section. All the experiments suggest that a deep knowledge of the fluid dynamics, including solid and gas flow, will be a key issue for the development of this new kind of reactor. 3.2. Comparison of Oxidants. There are several reactants that can be used for catalyst regeneration. Oxygen is usually employed for the regeneration of coked catalyst, but if the temperature is high enough, water or carbon dioxide can also react with the deposited coke. Figure 4 shows the results with the three regenerating agents. It may be seen that the best conversion and a similar selectivity are achieved when CO2 is employed. The lower conversion with water can be due to the formation of hydrogen during the gasification of coke, which affects methane aromatization according to the Le Chatelier’s principle. The low conversion with oxygen may be due to an excessive regeneration that affects MoC, which was supposed to be the active species. In the case of selectivity, it is similar with all the regenerating compounds. 3.3. Effect of Temperature. Since methane aromatization is endothermic and the equilibrium conversion is limited by the thermodynamic equilibrium, there is an obvious interest in the use of higher temperatures. However, in conventional reactors, the increase in temperature results in a faster deactivation,11 and therefore, the results are usually limited to a narrow

temperature interval. In the TZFBR, the catalyst deactivation is compensated by the continuous regeneration, allowing a steady operation over a wide temperature interval. As is shown in Figure 5, it is possible to achieve higher conversion at 800 °C than at 700 °C using a TZFBR, but only if the faster coke formation is compensated by a larger concentration of regenerating agent. In these experiments, the conversion at 800 °C is clearly higher than at 700 °C, but only if a feed with 3% CO2 is employed. The changes in selectivity with temperature also depend on the CO2 concentration. At 700 °C, the use of 3% CO2 results in a much lower selectivity to BTX, in accordance with previous results. In contrast, at 800 °C, where the conversion is the highest when 3% CO2 is employed, the selectivity is similar (in the range between 85 and 95%) for the three values of CO2 concentration. 3.4. Long-Term Stability. Although previously shown experiments have quite a constant performance after the first hour, an experiment with a larger time on stream was needed to ensure the stability of the system and to compare with the conventional fluidized bed reactor in the absence of oxidant. This experiment is shown in Figure 6, where the excellent stability of the TZFBR during several hours is shown in comparison with the same process without regeneration. This experiment proves that one of the main problems for the industrial application of methane aromatization, the catalyst deactivation, can be solved by the use of this multifunctional reactor. 3.5. Comparison with Literature Results. A comparison with previously published results on methane aromatization using Mo-ZSM-5 catalysts is shown in Figure 7. It may be seen that a much higher yield to benzene is obtained with the TZFBR. In addition, it is worth remembering that the literature values shown in Figure 7 are instantaneous results in experiments with catalyst deactivation whereas the values obtained with the TZFBR reactor correspond to the steady state when the coke formation is counteracted continuously by the regeneration in the lower section of the bed. 4. Conclusions The experimental results presented in this work show that the use of TZFBR compensates the catalyst deactivation by regenerating the catalyst “in situ”. It implies that the reactor can be operated for a long time at steady state, as was demonstrated by comparing the same operation in a conventional fluidized bed and in the TZFBR. Moreover, a very high selectivity to aromatics (95% of the hydrocarbons produced are BTX), among the best values reported in literature, was achieved at a conversion close to the thermodynamic equilibrium. The

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continuous regeneration has the consequence of the possibility of operating at high temperatures (800 °C) where the thermodynamics is favorable for endothermic reactions such as methane aromatization. The TZFBR with a variable section constitutes a valuable tool, very useful for this reaction, since the flow rate of oxidizing product must be much smaller than the total flow. With a suitable adjustment of the variable section, the performance of the TZFBR may be greatly improved. This innovation provides a new parameter for tailoring the TZFBR because it allows the reactor section to be adapted to the needs of the reaction/ regeneration process. This advantage improves the possibilities for using methane aromatization as an industrially competitive alternative for the transformation of natural gas to liquids and for the production of aromatics. Acknowledgment The authors thank the Ministry of Science and Technology (Spain) for financial support through project CTQ 2007-63420. Literature Cited (1) Liu, J.-F.; Jin, L.; Liu, Y. Methane Aromatization over Cobalt and Gallium-Impregnated HZSM-5 Catalyst. Catal. Lett. 2008, 125, 352. (2) Skutil, K.; Taniewski, M. Some technological aspects of methane aromatization (direct and via oxidative coupling. Fuel Process. Technol. 2006, 87, 511. (3) Keshav, T. R.; Basu, S. Gas-to-liquid technologies: Indian’s perspective. Fuel Process. Technol. 2007, 88, 493. (4) Holmen, A. Direct conversion of methane to fuels and chemicals. Catal. Today 2009, 142, 2. (5) Wang, L.; Tao, L.; Xie, M.; Xu, G. Dehydrogenation and aromatization of methane under non-oxidizing conditions. Catal. Lett. 1993, 21, 35. (6) Xu, Y.; Liu, S.; Wang, L.; Xie, M.; Guo, X. Methane activation without using oxidants over Mo/HZSM-5 zeolite catalysts. Catal. Lett. 1995, 30, 135. (7) Pinglian, T.; Zhusheng, X.; Tao, Z.; Laiyuan, Ch.; Liwu, L. Aromatization of methane over different Mo-supported catalysts in the absence of oxygen. React. Kinet. Catal. Lett. 1997, 61 (2), 391.

(8) Su, L.; Liu, L.; Zhuang, J.; Wang, H.; Li, Y.; Shen, W.; Xu, Y.; Bao, X. Creating mesopores in ZSM-5 zeolite by alkali treatment: a new way to enhance the catalytic performance of methane dehydroaromatization on Mo/HZSM-5 catalysts. Catal. Lett. 2003, 91 (3-4), 155. (9) Liu, H.; Shen, W.; Bao, X.; Xu, Y. Identification of Mo active species for methane dehydro-aromatization over Mo/HZSM-5 catalysts in the absence of oxygen: 1H MAS NMR and EPR investigations. J. Mol. Catal. A: Chem 2006, 244, 229. (10) Shu, J.; Adnot, A.; Grandjean, B. P. A. Bifunctional behaviour of Mo/HZSM-5 catalysts in methane aromatization. Ind. Eng. Chem. Res. 1999, 38, 3860. (11) Rival, O.; Grandjean, B. P. A.; Guy, C.; Sayari, A.; Karachi, F. Oxygen-free methane aromatization in a catalytic membrane reactor. Ind. Eng. Chem. Res. 2001, 40, 2212. (12) Meneńdez, M.; Herguido, J.; Te´llez, C.; Soler, J.; Gimeno, M. P. Two zone fluidized bed reactor. PCT Application ES2009/070241, 2008. (13) Meneńdez, M.; Herguido, J.; Te´llez, C.; Soler, J.; Gimeno, M. P. Procedure to obtain aromatic hydrocarbons from methane. PCT Application ES2009/070240, 2008. (14) Ohnishi, R.; Liu, S.; Dong, Q.; Wang, L.; Ichikawa, M. Catalytic Dehydrocondensation of methane with CO and CO2 toward benzene and naphtalene on Mo/HZSM-5 and Fe/Co-modified Mo/HZSM-5. J. Catal. 1999, 182, 92. (15) Ha, V. T. T.; Tiep, L. V.; Meriaudeau, P.; Naccache, C. Aromatization of methane over zeolite supported molybdenum: active sites and reaction mechanism. J. Mol. Catal. A: Chem. 2002, 181, 283. (16) Shu, Y.; Ichikawa, M. Catalytic dehydrocondensation of methane towards benzene and naphthalene on transition metal supported zeolite catalysts: templating role of zeolite micropores and characterization of active metallic sites. Catal. Today 2001, 71, 55. (17) Li, B.; Shijie, L.; Li, N.; Chen, H.; Zhang, W.; Bao, X.; Lin, B. Structure and acidity of Mo/ZSM-5 synthesized by solid state reaction for methane dehydrogenation and aromatization. Microporous Mesoporous Mater. 2006, 88, 244. (18) Hassan, A.; Sayari, A. Highly active, selective and stable Mo/RuHZSM-5 catalysts for oxygen-free methane aromatization. Appl. Catal. A: Gen. 2006, 297, 159.

ReceiVed for reView April 29, 2009 ReVised manuscript receiVed August 10, 2009 Accepted August 28, 2009 IE900682Y

Counteracting Catalyst Deactivation in Methane Aromatization with a

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