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Energy & Fuels 2000, 14, 490-494
Methane Pyrolysis in a Hot Filament Reactor Qi Sun,† Yongchun Tang,‡ and George R Gavalas*,† Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, and Chevron Research and Technology Co., Richmond, California 94802 Received September 13, 1999. Revised Manuscript Received January 19, 2000
An electrically heated platinum filament mounted inside a quartz tube was used as a high temperature flow reactor for methane pyrolysis. The reaction products included carbon and a mixture of hydrocarbons, mainly C1-C5 alkanes, alkenes, and benzene. The hydrocarbons were measured by gas chromatography and the carbon gravimetrically. At 1275 °C filament temperature and 100ms residence time in the hot region of the reactor, methane conversion was 19.7% and selectivity of hydrocarbon products 68%. Lowering the flow rate increased the conversion but lowered sharply the selectivity of hydrocarbon products. Coking of the filament surface gradually lowered the filament temperature and the conversion of methane. This deactivation process could be slowed by adding a few percent of oxygen to the methane feed.
Introduction Conversion of methane to liquid fuels or chemicals is currently practiced by the indirect route involving steam reforming or partial oxidation to synthesis gas followed by Fischer-Tropsch or methanol synthesis. The incentive, however, to exploit remote natural gas fields is currently motivating research on a number of direct conversion routes by oxidative or dehydrogenative coupling to higher hydrocarbons. Oxidative coupling has been studied for a number of different catalysts and comprehensive reviews have been presented by Lunsford.1,2 The best catalysts provide about 20% CH4 conversion with combined C2H4 and C2H6 selectivity of 80%. The resulting C2 yield is about 16%. Dehydrogenative oligomerization has been investigated with and without catalysts and under a variety of conditions. A review of work prior to 1995 can be found in ref 3. In more recent work, noble metals and certain oxides dispersed in ZSM-5 have been used as catalysts for methane aromatization at 600-700 °C. For example, reaction over a packed bed of MnOx-Na2O followed by a bed of HZSM-5 at 600 °C gave aromatic yields up to 6.5%.4 Dehydrogenative oligomerization on MoOx/ZSM-5 catalysts at 700 °C gave benzene selectivities 80-90% (the remaining products were ethylene and ethane) at 6-7% CH4 conversion.5-7 The catalytic activity gradually declined with time on stream due to carbon formation. * Author to whom correspondence should be addressed at Caltech 210-41, Pasadena, CA 91125. Tel: 626-395-4152. Fax: 626-568-8743. E-mail:
[email protected]. † California Institute of Technology. ‡ Chevron Research and Technology Co. (1) Lunsford, J. H. Angew. Chem. 1995, 34, 970-980. (2) Lunsford, J. H. Stud. Surf Sci. Catal. 1994, 81, 1-12. (3) Holmen, A.; Olsvik, O.; Rokstad, O. A. Fuel Process. Technol. 1995, 249-267. (4) Marczewski, M.; Marczewska, H. React. Kinet. Catal. Lett. 1994, 53, 33-38.
Dehydrogenative oligomerization of methane has also been tested with carbon catalysts including fullerene black,8,9 activated carbon,10 carbon fibers,11 and soot doped with various metals.12 In the above referenced dehydrogenative reactions, high selectivities to C2 and higher hydrocarbons could be obtained at low conversions. For example, at 1000 °C, a carbon fiber catalyst at 20% conversion gave 82% selectivity to hydrocarbons11 while a Mn-doped fullerene soot gave 80% selectivity to C2+ hydrocarbons at 7-8% methane conversion.12 At high conversions, carbon (with variable content of hydrogen) becomes the main product. The limitation to low conversion is understandable in terms of the consecutive nature of the reaction which can be crudely represented as
CH4 f C2+ f carbon where C2+ denotes all hydrocarbons above methane. The products C2+ are more reactive than methane, therefore, at any given temperature their yield goes through a maximum at some intermediate residence time that becomes shorter with increasing temperature. The first reaction step in all cases is the purely thermal or catalyzed methane activation
CH4 f CH3 + H (5) Wang, L.; Tao, L.; Xie, M.; Xu, G. Catal. Lett. 1993, 21, 35-41. (6) Chen, L.; Lin, L.; Xu, Z.; Li, X.; Zhang, T. J. Catal. 1995, 157, 190-200. (7) Wong, S.-T.; Xu, Y.; Liu, W.; Wang, L.; Guo, X. Appl. Catal. 1996, 136, 7-17. (8) Kushch, S. D.; Moravskii, A. P.; Muradyan, V. Ye.; Fursokov, P. V. Pet. Chem. 1997, 37, 112-118. (9) Hirschon, A. S.; Wu, H.-J.; Wilson, R. B.; Malhotra, R. J. Phys. Chem. 1995, 99, 17483-17486. (10) Yagita, H.; Ogata, A.; Obuchi, A.; Mizuno, K.; Maeda, T.; Fujimoto, K. Catal. Today 1996, 27, 433-436. (11) Mochida, I.; Aoyagi, Y.; Fujitsu, H. Chem. Lett. 1990, 15251526. (12) Hirschon, A. S.; Du, Y.; Wu, H.-J.; Wilson, R. B.; Malhotra, R. Res. Chem. Intermed. 1997, 23, 675-683.
10.1021/ef9901995 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/15/2000
Methane Pyrolysis in a Hot Filament Reactor
Figure 1. Schematic of the hot filament reactor. Detail shows a one-layer filament.
This step has usually higher activation energy than the subsequent reactions of the intermediate products, therefore, the yield of intermediate products could be increased by increasing the temperature, provided that the reaction time could be suitably controlled. At temperatures above 1100 °C, controlling the temperaturetime profile is difficult using conventional furnaceheated reactors. Electrical discharge technology has been used to bypass these limitations. Conversion of methane on NaY zeolite catalyst immersed in a low temperature, atmospheric pressure DC plasma gave 30-40% selectivity of C2 hydrocarbons at 38% conversion.13 To maintain carbon-free electrodes and enhance selectivity, the feed gas was 10% CH4, 10% H2, 1% O2, the balance He. Another type of DC plasma was employed for noncatalytic conversion of methane to acetylene.14 A carbon-free, stable discharge was maintained by diluting with Ar gas and including 10% O2. Under these conditions about 56% selectivity to acetylene was obtained at methane conversion 84%. These are impressive product yields but industrial application is problematic because of the high power consumption and the relatively low throughput in the electrical discharge reactors. In an unusual setup, methane was converted to gases and liquids in a thermal diffusion column reactor with a Pt-impregnated carbon rod serving as the heating element and the catalyst.15 At methane conversions of 53-68%, the products were 25-43% gases, 53-68% liquids, and 3-15% solid carbon. The gaseous products were mostly ethane with smaller quantities of ethylene and acetylene. In this paper we report experiments with a Pt hot filament reactor designed for attaining high temperatures and short contact times. Experimental Section Figure 1 is a schematic of the reactor. The hot filament consists of a 0.2032 mm diameter Pt wire bent into planar shape in a single or double layer and placed inside a quartz tube of 1.1 cm i.d. The filament was held perpendicular to the direction of flow by two copper electrodes connected to a voltage transformer (Variac). The individual folds of the filament were spaced 1 mm from each other. Operating at a voltage below 50 V and current below 20 A the filament could be heated to 1300 °C at which most of the filament power is dissipated by radiation. A wire mesh could not be used for the same purpose (13) Liu, C.-J.; Mallinson, R.; Lobban, L. J. Catal. 1998, 179, 326334. (14) Kado, S.; Sekine, Y.; Fujimoto, K. Chem. Commun. 1999, 24852486. (15) Suzuki, K.; Takahashi, R.; Onoe, K.; Yamaguchi, T. Energy Fuels 1999, 13, 482-484.
Energy & Fuels, Vol. 14, No. 2, 2000 491 because its low resistance would require current in the hundreds of amperes at very low voltage. The temperature of the filament was measured by an optical pyrometer while a movable Pt/Pt-10%Rh thermocouple was used to measure the temperature of the gas downstream of the filament. Prior to each reaction run the temperature of the filament was set by adjusting the voltage. Methane flow was then initiated and minor adjustments were made to the voltage to maintain the desired temperature. After the initial adjustment of the temperature the voltage was not changed for the duration of the experiment. The feed gas was introduced in the reactor from the side of the electrodes, without preheating. The feed flow rate was controlled by a mass flow rate controller, and the pressure was close to atmospheric. The product gas passed through a 2 µm filter and was analyzed by an on-line gas chromatograph with flame ionization detector (HP 5890 Series II) and a 0.19 picric acid/Graphpack column. The column could not separate acetylene from ethylene, and only the sum of the three C2 hydrocarbons is given in the results. Likewise, all C3 hydrocarbons are included under C3 and all C4 hydrocarbons under C4. The lines leading from the reactor to the GC were heated to prevent condensation of benzene and heavier products. The volumetric flow rate of the product gas was measured by an electronic flowmeter (J & W Model ADM 1000). Methane conversion and selectivities of hydrocarbon products were calculated from the feed and product flow rates and the product composition. The carbon produced was calculated by weighing the reactor before and after the reaction.
Results and Discussion Effect of Temperature and Contact Time. Figure 2a,b gives methane conversion and product selectivities versus filament temperature at fixed feed flow rate for reaction over a two-layer filament. At the highest temperature of 1310 °C, methane conversion was 19.7% with products 32% carbon, 51% C2 hydrocarbons, and 17% higher hydrocarbons, mainly benzene and propylene. At this particular flow rate the maximum yield of hydrocarbon products was 13.4% and was obtained at the highest temperature. Figure 2 also shows that the selectivity of C2-C4 hydrocarbons declines monotonically with temperature but the selectivity of aromatics (C6H6 and C10H8) goes through a shallow maximum at 1080-1100 °C. Measurement of the gas temperature along the reactor at 1275 °C filament temperature gave a profile where the temperature remained above 800 °C in a 2 mm region downstream of the filament (Figure 3). The contact time (nominal) defined as the ratio of the segment of reactor volume maintained above 800 °C to the feed flow rate at feed temperature 30 °C and atmospheric pressure was 430 ms. Taking into account the temperature in the reaction region but ignoring expansion due to the change of total gaseous products, the residence time was estimated to be about 100 ms. The temperature decline downstream of the filament was due to cooling of the quartz tube by free convection. Since the highest yield was attained at the highest temperature, further increase of the temperature with suitable reduction of the contact time might have improved the hydrocarbon yield. However, the filament broke when its temperature was raised to 1350 °C. The variation of methane conversion and selectivity with flow rate at 1275 °C is shown in Figure 4a,b. As expected, decreasing the flow rate raises the yield but
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Figure 3. Temperature profile along the hot filament reactor at filament temperature 1275 °C and 40 mL/min flow rate.
Figure 2. (a) Methane conversion and selectivity to C2 hydrocarbons and carbon versus filament temperature in a two-layer hot filament reactor. The methane feed was undiluted at 40 mL/min flow rate. (b) Selectivity to C3+ hydrocarbons versus filament temperature for the experiment specified in (a).
lowers the hydrocarbon product selectivity. The maximum yield of hydrocarbon products is 15% and is attained at the intermediate flow rate of 20 mL/min. The results shown in Figures 2-4 were all obtained using a two-layer filament. Experiments were also carried out using a single-layer filament and are compared in Figure 5 with the results from the two-layer filament for the same flow rate and temperature. As expected, the single layer filament had somewhat lower conversion and higher selectivity. Experiments were also performed with a single layer filament having 0.7 mm rather than 1 mm spacing between the individual folds, but there was little change in conversion and selectivity. As shown in the foregoing figures, carbon formation is substantial at temperatures above 1000 °C. Most of this carbon is formed in the gas phase as evidenced by its deposition on the surface of the quartz tube down-
stream of the filament. Some carbon is also formed on the filament causing gradual decline of conversion to approximately one-half of its initial value within 1 h of reaction. The conversions and selectivities listed in Figures 2, 4, and 5 were the initial values measured within the first 10 min of methane flow. The activity of the filament could be fully restored by burning the carbon off the filament with air. To moderate deactivation 1-3% of oxygen was introduced in the methane feed enabling operation for 1.5-3 h without appreciable decline of activity. Figure 6 compares conversion and selectivity for 0, 1%, and 3% oxygen addition. Oxygen addition has a minor effect on conversion but improves modestly the selectivity to hydrocarbon products with carbon deposition correspondingly lower. Figure 6 also includes the results of addition of 1% water to the feed. Addition of water also decreased carbon deposition but greatly lowered conversion. As mentioned earlier, addition of oxygen was also essential in plasma reactors to maintain carbon-free electrodes and a stable discharge.13,14 To obtain a better understanding of deactivation by carbon deposition it is recalled that the voltage across the filament is maintained constant during the reaction, hence, as carbon is deposited the filament surface changes to a carbon surface and the filament diameter increases by several hundreds of microns. The expansion of filament surface and the increase of emissivity cause higher radiative losses and a corresponding drop in the temperature. In one run, for example, after 1 h of operation, the filament temperature declined from 1275 to 1150 °C. Part of the activity decline, therefore, was caused by the filament temperature decline. The question is whether the change of the filament surface from Pt to carbon also contributed to the activity decline. To explore this issue, the reaction was run for 1 h at 1275 °C starting with a clean filament obtaining the conversion and selectivities listed in Table 1 under step 1. After 1 h of operation carbon deposition caused the filament temperature to decline to 1150 °C. The reaction was then run for another 1-h period giving the results
Methane Pyrolysis in a Hot Filament Reactor
Energy & Fuels, Vol. 14, No. 2, 2000 493
Figure 5. Comparison of methane conversion and product selectivity between a one-layer and a two-layer filament held at 1275 °C. The methane flow rate was 40 mL/min.
Figure 4. (a) Methane conversion and selectivity to C2 hydrocarbons and carbon versus flow rate in a two-layer filament reactor at filament temperature 1275 °C. (b) Selectivity to C3+ hydrocarbons versus flow rate for the experiment specified in (a).
listed in Table 1 under step 2. After carbon burnout the reaction was resumed at 1150 °C for 1 h. During this last step methane conversion rose to 6.8%, 70% higher than obtained with the carbon-coated filament. Carbon production also increased to 17.5% of the products. It follows that the Pt-catalyzed component of the reaction is faster than the carbon-catalyzed component and that deactivation takes place by masking of the Pt surface as well as by the temperature decline. Figure 7 shows the change of surface morphology of a fresh Pt filament before reaction, after 1 h of reaction, and after carbon burnout. After 1 h of reaction a carbon coating of more than 0.3 mm had formed on the filament and after burnout of this carbon the surface of Pt filament became rough and faceted. Despite its increased roughness, the regenerated Pt surface had the same activity for the reaction as the original Pt surface.
Figure 6. Methane conversion and product selectivities in a one-layer filament reactor versus percent oxygen or steam added to the methane feed. The other conditions were as specified in Figure 5.
Conclusions The Pt filament reactor is very convenient for rapidly heating the reactants to temperatures up to about 1300 °C. Measurement of the gas temperature indicates that high temperatures persist for a few mm downstream of the filament, depending on filament temperature and gas flow rate. Methane decomposition in the hotfilament reactor proceeds by a combination of catalytic and gas-phase reaction steps. At 1275 °C filament temperature the maximum yield of hydrocarbon products was 15% and was attained at about 22% methane conversion with 68% of the converted methane going to hydrocarbons and the remainder to carbon. At higher
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Table 1. Conversion of Methane and Product Selectivities over Clean and Carbon-Covered Single-Layer Platinum Filament (flow rate: 40 mL/min pure methane) selectivity to product step
temp. °C
filament status
conversion %
C2
C3-C4
C6H6
C10H8
carbon
1 2 3
1275 1150 1150
fresh after 1 h of reaction after burning carbon out
11.0 3.7 6.4
55.1 55.4 53.2
15.3 24.6 20.0
7.5 7.8 7.2
1.2 2.1 1.6
20.8 10.2 17.5
Figure 7. SEM micrographs of the surface of a Pt filament (a) before reaction, (b) after 1 h reaction, and (c) after carbon burnout.
conversions carbon production rose sharply. The reaction rate declined with time on stream because the filament surface changed from platinum to carbon and because the filament temperature declined. Addition of 1-3% oxygen in the methane feed slowed deactivation and moderately improved hydrocarbon selectivity. This simple reactor may be useful for kinetic studies of reactions consisting of a catalytic first step followed by homogeneous steps. However, control of the temperature profile and residence time remains difficult. Aside
from issues of selectivity, scale-up of the hot filament reactor to industrial size appears impractical because of the high electrical energy consumption engendered by the radiative losses.
Acknowledgment. Funding of this work was provided by Chevron Research and Technology Company. EF9901995
