Chapter 32
Synthesis Gas Formation by Direct Oxidation of Methane over Monoliths 1
Downloaded by UNIV OF ARIZONA on December 17, 2012 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch032
D. A. Hickman andL.D. Schmidt Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455
The production of H and CO by catalytic partial oxidation ofCH in air and O at atmospheric pressure has been examined over Pt- and Rh-coated monoliths at residence times of less than 10 sec, with a Rh-coated foam monolith giving H and CO selectivities as high as 0.90 and 0.96 respectively. Studies of several catalyst configurations including Pt-10% Rh woven gauzes and Pt- and Rh-coated ceramic foam and extruded monoliths show that better selectivities are obtained by operating at higher temperatures, maintaining high rates of mass transfer through the boundary layer at the catalyst surface, and using high metal loadings. For both metals, the high selectivities to H and CO strongly suggest that the primary surface reaction is methane pyrolysis,CH -> C + 4H, and Rh catalysts are superior to Pt because the formation ofH Ovia OH is energetically less favorable on Rh. 2
4
2
-2
2
2
4
2
Much recent research (1-5) has been devoted to converting methane to products that are more easily transported and more valuable. Such more valuable products include higher hydrocarbons and the partial oxidation products of methane which are formed by either direct routes such as oxidative coupling reactions or indirect methods via synthesis gas as an intermediate. The topic of syngas formation by oxidation of CH has been considered primarily from an engineering perspective (1-5). Most fundamental studies of the direct oxidation ofCH have dealt with theCH + O reaction system in excess O2 and at lower temperatures (6-10). Synthesis gas is usually produced from methane by steam reforming:
4
4
CH + H 0->CO + 3H , 4
2
2
4
ΔΗ =+49.2 kcal/mole
2
(1)
This endothermic reaction is driven by heating the reactor externally or by adding oxygen to the feed to provide the necessary energy by highly exothermic combustion reactions. A typical steam reformer operates at 15 to 30 atm and 850 to 900°C with a N1/AI2O3 catalyst and a superficial contacttime(based on the feed gases at STP, Current address: The Dow Chemical Company, Midland, MI 48674 0097-6156/93/0523-0416$06.00/0 © 1993 American Chemical Society In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
32. HICKMAN AND SCHMIDT
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Synthesis Gas Formation
standard temperature and pressure) of 0.5 to 1.5 seconds (77), which corresponds to a residence time of several seconds. The CO/H2 ratio of the reformer product gases is often modified by the water-gas shift reaction: CO + H 0C0 + H , 2
2
ΔΗ =-9.8 kcal/mole
2
(2)
A high temperature water-gas shift reactor (~ 400°C) typically uses an iron oxide/chromia catalyst, while a low temperature shift reactor (~ 200°C) uses a copper-based catalyst. Both low and high temperature shift reactors have superficial contact times (based on the feed gases at STP) greater than 1 second (72). The direct oxidation of C H is an alternate route for synthesis gas production: Downloaded by UNIV OF ARIZONA on December 17, 2012 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch032
4
CH + ^0 ->CO + 2H , 4
2
ΔΗ =-8.5 kcal/mole.
2
(3)
While this reaction is exothermic, the oxidation reactions which produce H2O from methane are much more exothermic, CH + | 0 4
-> CO + 2 H 0 ,
2
2
C H + 2 0 -> C 0 + 2 H 0 , 4
2
2
2
ΔΗ = -124.1 kcal/mole,
(4)
ΔΗ = -191.8 kcal/mole.
(5)
Oxidation reactions are much faster than reforming reactions, suggesting that a single stage process for syngas generation would be a viable alternative to steam reforming. The objective of the research described here is to explore synthesis gas generation by direct oxidation of CH4 (reaction 3). A reactor giving complete conversion to a 2/1 mixture of H2 and CO would be the ideal upstream process for the production of CH3OH or for the Fischer-Tropsch process. As discussed above, currently implemented or proposed processes utilize a combination of oxidation and reforming reactions to generate synthesis gas from CH4 and O2. In this work, we seek a faster, more efficient route of syngas generation in which H and CO are the primary products of CH4 oxidation. It is expected that this may be difficult because the reactions H2 + O2 -> H2O and CO + ^ O2 -> CO2 are extremely fast (13-15), either heterogeneously or homogeneously, while CH4 activation is quite slow except at high temperatures. In this paper, we summarize results from a small scale methane direct oxidation reactor for residence times between 10" and 10" seconds. For this work, methane oxidation (using air or oxygen) was studied over Pt-10% Rh gauze catalysts and Pt- and Rh-coated foam and extruded monoliths at atmospheric pressure, and the reactor was operated autothermally rather than at thermostatically controlled catalyst temperatures. By comparing the steady-state performance of these different catalysts at such short contact times, the direct oxidation of methane to synthesis gas can be examined independent of the slower reforming reactions. 2
4
2
Apparatus The details of the experimental apparatus and procedures are outlined in another paper (16). The reactor consisted of a quartz tube with an inside diameter of 18 mm which held the monolith or gauze pack. The reactor was operated at a steady state temperature which is a function of the heat generated by the exothermic reactions and
In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
Downloaded by UNIV OF ARIZONA on December 17, 2012 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch032
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CATALYTIC SELECTIVE OXIDATION
the heat lossesfromthe reactor. Thus, the temperatures reported in these experiments are autothermal steady-state temperatures, which are a function of the heat generated by the exothermic oxidation reactions. To better approximate adiabatic operation, the catalyst was immediately preceded and followed by inert alumina monoliths which acted as radiation shields, and the outside of the tube at the reaction zone was usually insulated. When higher autothermal temperatures were desired, the feed gases were preheated by heating the upstream section of the reaction tube externally. Bypass of the reactant gases around the annular space between the catalyst sample and the quartz tube was rninimized by sealing the catalyst sample with a high temperature alumina-silica cloth. The feed gas flow rate was monitored and controlled by mass flow controllers. Product gases were fed through heated stainless steel lines to a sample loop in an automated gas chromatograph. The GC analysis was performed using two isothermal columns (80°C) in series, a Porapak Τ and a Molecular Sieve 5A column. When necessary, a second GC analysis using a temperature programmed Hayesep R column was used to separate and detect small hydrocarbons (such as ethylene and ethane) and H2O. Although direct oxidation of methane to synthesis gas occurs at feed compositions outside the flammability limits, all of these experiments were conducted in a reactor inside a fume hood. The Tygon feed gas lines were not clamped too tightly so that a sudden pressureriseof a few psig would disconnect the tubing and stop die flow of gases to the reactor. Furthermore, the mixing point of the feed gases was inside the hood to prevent propagation of a flame through tubing outside the fume hood. In addition, all of the product gases were burned in an incinerator so that any toxic and otherwise dangerous products would be converted to CO2 and H2O before venting to the atmosphere. Monoliths Three basic types of catalysts were studied in these experiments: Pt-10% Rh gauzes, foam monoliths, and extruded monoliths. The gauze catalysts were 40 mesh (40 wires per inch) or 80 mesh Pt-10% Rh woven wire samples which were cut into 18 mm diameter circles and stacked together to form a single gauze pack 1 to 10 layers thick. Gauze catalysts are used industrially in the oxidation of NH3 to NO for HNO3 production and in the synthesis of HCNfromNH3, CH4, and air. The foam monoliths were OC-AI2O3 samples with an open cellular, sponge like structure. We used samples with nominally 30 to 80 pores per inch (ppi) which were cut into 17 mm diameter cylinders 2 to 20 mm long. A 12 to 20 wt.% coating of Pt or Rh was then applied directly to the alumina by an organometallic deposition. The cordierite extruded monoliths, having 400 square cellsAn , were similar to those used in automobile catalytic converters. However, instead of using an alumina washcoat as in the catalytic converter, these catalyst supports were loaded directly with 12 to 14 wt.% Pt in the same manner as the foam monoliths. Because these extruded monoliths consist of several straight, parallel channels, theflowin these monoliths is laminar (with entrance effects) at the flow rates studied. To a good approximation, we believe that all of the supported catalysts behave as Pt or Rh metal surfaces. The loadings are sufficient to deposit uniform films ~1 μιη thick on the AI2O3 or cordierite ceramics. Scanning electron microscopy (SEM) micrographs of these monoliths before use revealed that the catalyst formed large crystallites on the support with the metal completely covering the support surface. The absence of significant changes in reaction selectivity over several hours of use and the similar performance of all the monoliths confirms that the performance of these catalysts is not strongly influenced by the support, even 2
In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
32. HICKMAN AND SCHMIDT
Synthesis Gas Formation
419
though S E M showed that the metal formed large crystalline grains during reaction, thus exposing ceramic surfaces. For all types of monoliths, experiments were run on many samples. We shall show results from only a few of these, but the results were nominally reproducible for all samples. Reaction Stoichiometry and Equilibrium Before presenting the results of our experiments, we briefly describe the essential roles of stoichiometry and thermodynamics in this system. The performance is governed by the conversions of CH4 and O2 and the selectivities in producing H2 and CO Downloaded by UNIV OF ARIZONA on December 17, 2012 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch032
:
0.5 F SH
2
Έ
=
F
H
=
Έ
^CH in~ ^CH out 4
and
S 0=
^H
4
Έ
C
^CH
H
,
F
+ 2
(6)
v
^H 0 2
= ϋ - % 7 — 4 i n
- *CH
?
( )
+±
