Partial oxidation of methane to syngas at high space velocities over Rh-coated spheres

Applied Catalysis A: General 211 (2001) 53–68

Partial oxidation of methane to syngas at high spacevelocities over Rh-coated spheres

K.L. Hohna,∗, L.D. Schmidtba Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506-5102, USA

b Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA

Received 24 May 2000; received in revised form 23 October 2000; accepted 27 October 2000

Abstract

The effect of space velocity on the partial oxidation of methane using different support geometries has been studied.While on a foam alumina monolith syngas selectivity drops as space velocity is increased above 4× 105 h−1, the use ofnon-porous alumina spheres as the support allows high reactant conversions and syngas selectivities even at space velocitiesof 1.8 × 106 h−1. The differences between monoliths and spheres are discussed in order to understand why spheres givesuperior results. It is suggested that differences in heat transfer within the two support geometries may play a major role in thedifferent results between spheres and monoliths. A convective heat transfer model suggests that higher rates of convection ina monolith will lead to lower front temperatures than in a sphere bed, a trend that becomes important at high space velocitiesin leading to blowout and lower syngas selectivities. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Methane oxidation (partial); Syngas; Rhodium; Millisecond contact times

1. Introduction

The production of syngas is a very importantprocess to the petrochemical industry because syngasis the building block for many chemicals includ-ing methanol, higher hydrocarbons through Fischer–Tropsch chemistry, and acetic acid. Industrially,syngas is produced by steam reforming:

CH4 + H2O → 3H2 + CO, 1Hr = 205.9 kJ/mol

This reaction is highly endothermic, so it is performedin large furnaces to supply the necessary energy. Steamreforming operates at temperatures of around 900◦C ata pressure between 15 and 30 atm over a Ni/a-Al2O3

∗ Corresponding author. Fax:+1-785-532-7372.E-mail address:[email protected] (K.L. Hohn).

catalyst with a residence time on the order of seconds[1].

Much research has been devoted to producing syn-gas through alternate processes. The partial oxidationof methane to syngas has been extensively studied:

CH4 + 12O2 → CO+ 2H2, 1Hr = −35.6 kJ/mol

This reaction has been investigated in various reac-tors, and high syngas yields have been reported [2–7].It can be carried out either in conjunction with steamreforming by autothermal reforming [2] or in the ab-sence of steam by catalytic partial oxidation [3–7].

An important issue in catalytic partial oxidation hasbeen the space velocity at which the reactors operate.Gas hourly space velocity (GHSV) is defined as theratio of the volumetric flow of reactants at standardconditions (25◦C and 1 atm) to the total catalyst vol-ume, giving a quantity with units of inverse time. This

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0926-860X(00)00835-8

54 K.L. Hohn, L.D. Schmidt / Applied Catalysis A: General 211 (2001) 53–68

quantity is not equal to the inverse of the residencetime because inlet temperatures and pressures, ratherthan reactor conditions, are considered.

Many studies have reported high selectivities at aspecific space velocity, but find that an increase inthe space velocity leads to a loss in conversion andselectivity. Ashcroft and workers reported 90% CH4conversion and a syngas selectivity in excess of 94%on a Rh catalyst at a space velocities under 104 h−1

[3]. However, at space velocities of 105 h−1, low con-versions and selectivities resulted. Dissenayake et al.saw a similar trend in their experiments on Ni cata-lysts [4]. They found a significant decrease in conver-sion and selectivity as the space velocity was raisedfrom near 105 to around 106 h−1. The autothermalcatalytic partial oxidation of methane to syngas firstreported by Hickman and Schmidt [5] which uses aRh-coated monolith has also been found to be limitedat high space velocities. Witt and Schmidt reporteda decrease in methane conversion and syngas selec-tivity as space velocity was increased from near 105

to 4× 105 h−1 [8]. The decrease in syngas selectivitywith space velocity in these processes presents a sig-nificant limitation to these processes, since it wouldbe advantageous to run as high a space velocity aspossible to maximize syngas production.

Recent results suggest that the catalytic partial oxi-dation over Rh in an autothermal reactor can be run atspace velocities of at least 1.8× 106 h−1 if a differentsupport geometry is used. The use of alumina spheresrather than alumina monoliths prevents blowout of thefront of the bed as space velocity is increased andallows high conversions and selectivities to be main-tained at space velocities in excess of 106 h−1.

2. Experimental

The experimental setup used was similar to that pre-viously described [9]. Reactant gasses were controlledby Brooks mass flow controllers. Gases were premixedbefore flowing into the quartz reactor tube (18 mm i.d.,16 in. long). Pressure in the reactor was controlled bya downstream valve, and was always held at 2 psig.Products were sent to a gas chromatograph using twoHaysep D columns to separate the products. Thermalconductivity (TCD) and flame ionization (FID) detec-tors were used to analyze products.

Nitrogen was used as a diluent in all experimentsand was used as a chromatograph standard. Chro-matograph areas were used to find the number ofmoles for all products except H2O which was cal-culated by closing the oxygen mole balance. Bothcarbon and hydrogen atom balances closed for allexperiments within 3%.

Monolith and sphere samples were prepared bynearly identical procedures. Before loading, thespheres were sintered to remove internal surface areaby the following procedure. The sphere sample washeated in a furnace from 120 to 1000◦C at a rateof 20◦C/min. Then the temperature was raised from1000 to 1200◦C at 5◦C/min and held at that tem-perature for 6 h. Finally, the sample was cooled to120◦C at a rate of 20◦C/min. It was found that the useof either pre-sintered spheres or unsintered spheres(change of surface area from∼200 m2/g to less than10 m2/g) gave the same results, though all the resultsdescribed here are for pre-sintered spheres.

Rhodium was deposited on both spheres and mono-liths by the incipient wetness technique, using anaqueous solution of rhodium nitrate. Catalysts werecalcined in air for 6 h at 600◦C after the metal saltwas added. Nearly equivalent mass loadings were pre-pared on sphere and monolith samples that were to becompared to give a roughly equal number of catalystsites. In addition, SEM images were taken to ensurethat the catalyst was deposited on both supports inthe same way.

Monoliths were loaded in 18 mm i.d. quartz tubesbetween two blank monoliths that acted as radiationshields. Sphere beds were loaded into a 14 mm i.d.quartz tube onto a transparent quartz frit that hadbeen sintered into the tube and held the sphere bedin place. Enough spheres to make up a bed of 0.5 or1 cm depth (shorter depths were used to give the high-est space velocities) were placed in the tube, and analumino-silicate cloth was placed on top of the spheresto prevent axial heat losses from radiation. Insulationwas wrapped around both reactors to keep operationclose to adiabatic. For all runs, the CH4/O2 ratio was2 and 20% N2 was added.

Monolith supports were obtained from Hi-TechCeramics (92% Al2O3, 8% SiO2). Spherical sup-ports were obtained from UOP and Alcoa (92.1%Al2O3, 0.02 SiO2, 0.02 Fe2O3, 0.3 Na2O, 7.5% LOI(250–1200◦C)). Sphere sizes quoted in this paper are

K.L. Hohn, L.D. Schmidt / Applied Catalysis A: General 211 (2001) 53–68 55

the measured average size of the spheres. Sievingof the spheres suggested a nearly monodisperse sizedistribution with a mean of the value quoted.

In general, experiments were run up to the high-est space velocity possible. Space velocity was limitedby the upper flow rate of the mass flow controllers.Space velocity could be increased by using a smallertube diameter, and this was done for the sphere beds(using a 14 mm i.d. quartz tube rather than an 18 mmi.d. tube) because no difference in results was notedbetween the two sizes. However, in the case of mono-liths a radial dependence was noted previously [8]when smaller diameter tubes were used, so only 18 mmtubes were used in this work. Given the range of flowcontrollers, this meant a maximum space velocity of5×105 h−1 for the monoliths and 1.8×106 h−1 for thespheres.

For the monoliths, flow rates corresponded to a su-perficial velocity (i.e. velocity of flow upstream fromthe catalyst) in the range of 20–98 cm/s, with residencetimes at reaction conditions (1000◦C) ranging between1.2 and 6 ms. For the sphere beds, higher linear ve-locities and lower residence times were investigated.Superficial velocity ranged from 32 to 357 cm/s, withresidence times between 160 and 4 ms.

3. Results

3.1. Effect of space velocity on rhodium monoliths

At space velocities near 105 h−1 the catalytic par-tial oxidation of syngas gives high conversions andselectivities. However, the performance of Rh-coatedmonoliths suffers at high space velocities. Fig. 1 showsthe conversions, selectivities and front and back tem-peratures on a∼5 wt.% Rh monolith as GHSV ischanged. The top panel plots the CH4 and O2 conver-sions as functions of space velocity. Oxygen conver-sion was complete and CH4 conversion was around78% at 105 h−1. Both conversions dropped as spacevelocity was increased. The CH4 conversion fell toaround 35% at 4× 105 h−1, while the O2 conversionfell to approximately 76%. The middle panel showsthe CO and H2 selectivities versus GHSV. CO se-lectivity fell from around 95% at 105 h−1 to 72% at4 × 105 h−1. The H2 selectivity dropped from 88 toaround 40% over the same range of space velocities.

These decreases in conversion and selectivity cor-responded to a change in temperature. While at spacevelocities near 105 h−1 the front and back temperatureswere roughly equal, at space velocities of 4×105 h−1

there was a steep temperature gradient. As shown inthe bottom panel of Fig. 1, the front of the mono-lith cooled off to∼180◦C while the back heated upto around 1200◦C. This steep temperature profile waslikely responsible for the low conversions and selec-tivities. The low temperature at the front likely led tothe formation of CO2 and H2O. The high back tem-perature was high enough that it may have allowed gasphase reactions to play a role, leading to the observedproduction of around 3% C2 hydrocarbons [8].

The space velocity at which blow out occurs is afunction of the weight loading of the monolith. Fig. 2shows the effect of space velocity on three 45 ppimonoliths loaded with 0.102 g Rh (5.3 wt.%), 0.149 gRh (7.8 wt.%), and 0.21 g Rh (10.3 wt.%). The topleft panel shows the CH4 conversion as a function ofGHSV. The top right panel shows the O2 conversionversus GHSV. The bottom left panel shows the COselectivity as a function of GHSV. The bottom rightpanel shows H2 selectivity versus GHSV.

As seen in these figures, the higher the weightloading, the higher the space velocity before the per-formance decreases significantly. The results for thecatalyst with 0.149 and 0.21 g Rh were nearly identi-cal. Those two displayed only slight decrease in con-version and selectivity with increasing space velocity.CH4 conversion fell from around 78% at 105 h−1 to75% at 4× 105 h−1. O2 conversion was completefor the entire range of space velocities investigated.The CO selectivity for those two catalysts stayednearly constant at around 94%. The H2 selectivitydecreased slightly from 88% at 105 h−1 to under 80%at 4× 105 h−1.

The monolith loaded with only 0.102 g Rh showeda much steeper loss in conversion and selectivity. TheCH4 conversion dropped from 78 to 35% between 105

and 4× 105 h−1. The O2 conversion dropped from100% at 105 h−1 to 76% at 4×105 h−1. The CO selec-tivity fell from around 95 to 72% for the same range ofGHSV. The H2 selectivity fell from 88 to around 40%.

The higher loadings of Rh allowed the monolith tomaintain activity for a wider range of GHSV. Eventhough the higher loadings prevented complete blowout at a GHSV of 4× 105 h−1, the slope of all the


Fig. 1. Typical results for the partial oxidation of methane on a 5% rhodium/alumina monolith as a function of space velocity (CH4/O2 = 2).Conversion and selectivity fell as GHSV was increased. The front of the monolith cooled, while the back reached higher temperatures.


Fig. 2. Partial oxidation of methane on rhodium monoliths with different weight loadings as a function of space velocity. Higher weightloadings prevented complete blowout at 4× 105 h−1, but all trends decreased, even with higher Rh loading.

curves was down, suggesting that conversions andselectivities would decrease further as GHSV is in-creased toward 106 h−1.

3.2. Effect of space velocity on rhodium spheres

While Rh-coated monoliths appear to be limited inthe range of space velocities they can operate, it is pos-sible that changing the geometry of the support willallow the process to be run at higher space velocities.Experiments were conducted to study the effect of us-ing spherical alumina particles as the support in thepartial oxidation of methane. A packed bed of sphereswill give very different mass and heat transfer char-acteristics than a monolith, and may prevent blow outand loss in selectivity at high space velocities.

The flow rate was varied from 3 to 33 SLPM (GHSVof 105–1.8×106 h−1) over a 0.5 cm bed of Rh-coatedspheres (12.6 wt.%, 0.09 g Rh total weight) to deter-mine the effect of flow rate. The methane and oxygenconversions are shown as a function of GHSV in the

top panel of Fig. 3. CO and H2 selectivities are shownversus GHSV in the bottom panel.

Conversions and selectivities remained high evenat space velocities just under 2× 106 h−1. Blow outwas prevented in the sphere bed. Under no experimen-tal conditions was blow out observed on a sphere bedwith spheres of this or similar size. Oxygen conver-sion was complete at all GHSV. Methane conversionremained relatively constant at around 86%. CO se-lectivity increased steadily as space velocity was in-creased, reaching a value of 94.5% at 1.8 × 106 h−1.H2 selectivity was between 94 and 96% for all spacevelocities. No higher hydrocarbons were detected.

The spheres gave comparable results to monolithsat low space velocities, giving higher CH4 conversion(86 versus 78%), higher H2 selectivities (94 versus86%), and slightly lower CO selectivity (93 versus95%). However, the main advantage of spheres wasat high space velocity, where a sphere bed maintainedhigh selectivities and conversions at the same spacevelocities at which a monolith blew out.


Fig. 3. Partial oxidation of methane on 12.6% rhodium on 800mm alumina spheres at space velocities in excess of 106 h−1. Conversionand selectivity remained high at all space velocities.


3.3. Comparison of monoliths and spheres

Spheres appear to give superior results, despitethe fact that the support material and catalyst are thesame. This suggests that factors other than chem-istry, such as mass and heat transfer within the cat-alyst region, are important, especially at high flowrates.

In order to understand why spheres are better thanmonoliths, several properties of the monolith and thesphere bed were tabulated and compared. Those prop-erties are shown for each of several samples of spheresand monoliths in Table 1.

In this table, all values for the monoliths wereobtained from the manufacturer [10]. Values for thespheres were either calculated based on geometricalconsiderations, or measured. The void fraction in thesphere bed was based on calculations by Reyes andIglesia [11] using Monte Carlo simulations to calcu-late structural properties of packed beds. The porediameter, defined for spheres as the diameter of thespace between spheres through which reactant gassespass, was calculated according to the relationship:

mean pore size= 2ε

3(1 − ε)Γs

whereε is the void fraction of the bed andΓ s is thesphere radius [12]. This formula gives the mean poreradius. The surface areas for the spheres were mea-sured by the BET method. The first number given isthe surface area before sintering, where there is a highdegree of microporosity. The second number in paren-theses is the surface area after the spheres have beensintered according to the procedure already detailed.The thermal conductivity was calculated according toa model developed to simulate the transfer of heatthrough a packed bed of spherical particles. More de-tails about this calculation will be given later.

In addition, a sample of 800mm spheres was ex-amined using SEM and compared to previously takenSEM micrographs of Rh-coated monoliths. The SEMmicrographs suggest that Rh coats both samples in asimilar way. Before use, the Rh metal appears as pud-dles on the catalyst surface. After use, the form ofthe Rh has changed for both supports, with particlesof Rh dispersed on the surface of both supports witha diameter of 0.5–1mm. The similarities between theappearance of Rh on both supports suggests that the

differences in syngas results are not caused by differ-ences in metal dispersion on the support.

3.4. Effect of bed structure

It has previously been shown that 80 ppi (80 poresper linear inch) monoliths give better syngas yieldsthan 45 ppi monoliths near 105 h−1 [13]. One of theprimary differences between these two monolithsis the diameter of the pores. The pore diameter inan 80 ppi monolith is half that in a 45 ppi mono-lith. It was suggested that the smaller pore diameterwas advantageous for syngas production because ofhigher mass transfer rates of reactants to the surface[13].

Because pore diameter in the monoliths appearsto be a significant factor in syngas selectivity, anycomparison of spheres to monoliths must take thisvariable into account. Pore diameter is important be-cause it influences mass transfer within the catalystbeds. Smaller pore diameters mean a shorter diffusiondistance for reactants until they contact the surface.This would be critical if gas phase reactions are im-portant because smaller pores would more efficientlyquench gas phase radicals and suppress homogeneousreaction.

In an attempt to compare monoliths and sphereswith similar mass transfer properties, the pore diam-eters for both geometries were calculated and com-pared as shown in Table 1. The pore diameters ofthe 45 ppi monolith and the 800mm sphere bed areroughly equal, as are the pore diameters of the 80 ppimonolith and 400mm sphere bed.

Fig. 4 shows conversions and selectivities for 45 ppiand 800mm spheres as functions of GHSV. These twosamples had approximately the same amount of Rh onthem (about 0.1 g Rh on each). The top panel showsthe CH4 and O2 conversions versus GHSV for themonolith and the spheres. The middle panel shows COselectivity versus GHSV for the same two catalysts.The bottom panel shows H2 selectivity as a functionof GHSV.

All three panels demonstrate that the Rh-coatedspheres maintained their activity at all GHSV’s whilethe 45 ppi monolith experienced a sharp decline in ac-tivity as the GHSV was increased to 4× 105 h−1. At105 h−1, the 45 ppi gave 78% CH4 conversion, 95%CO selectivity, and 88% H2 selectivity. At the same


Fig. 4. Comparison of 45 ppi monolith vs. 800mm spheres (nearly equal mass loadings). At the same space velocities spheres gave bettersyngas yields for GHSV >2×105 h−1.


Table 1Properties of sphere beds with different size spheres and monoliths with varying pores per linear inch

Sample Void frac-tion (%)

Pore diameter(mm)

Surface area(m2/g)

Thermal conductivityat 1000◦C (W/mK)

45 ppi foam monolith, 92% Al2O3, 8% SiO2 84 420 ∼1 1.2880 ppi foam monolith, 92% Al2O3, 8% SiO2 83 210 ∼1 N/A400mm Al2O3 spheres ∼42 192 ∼180 (∼6) 1.71800mm Al2O3 spheres ∼44 420 181 (6.5) 1.551200mm Al2O3 spheres ∼45 654 200 (6) 1.483200mm Al2O3 spheres ∼59 3044 350 0.78

space velocity, the 800mm spheres gave 86% CH4conversion, 93% CO selectivity, and 93% H2 selectiv-ity. The results were nearly the same, with the spheresgiving better CH4 conversion and H2 selectivity whilethe monolith gave a better CO selectivity. However,at 4× 105 h−1 the results on the sphere were nearlythe same, while the results on the monolith were 33%CH4 conversion, 72% CO selectivity, and 38% H2 se-lectivity. In addition, the O2 conversion was no longercomplete, having dropped to 76%. Even though the45 ppi monolith and 800mm spheres have a similarpore diameter in the bed, the Rh-coated spheres gavesuperior results at high space velocities.

The top panel of Fig. 5 shows methane and oxy-gen conversions for 80 ppi (13.6% Rh, 0.25 g Rh) and400mm spheres (15.2% Rh, 0.276 g Rh) as functionsof GHSV. The bottom panel shows H2 and CO selec-tivities versus GHSV for the same two samples. It canbe seen that the 400mm spheres maintained high con-versions and selectivities at all GHSV while the 80 ppimonolith gave much lower conversions and selectivi-ties as the space velocity was increased. The conver-sions and selectivities were about the same for eithersample at space velocities of around 2× 105 h−1 orless. At 105 h−1, the 80 ppi monolith gave 82% CH4conversion, 95% CO selectivity, and 90% H2 selectiv-ity. At just over 2×105 h−1, the 400mm spheres gavearound 85% CH4 conversion, 92% CO selectivity, and94% H2 selectivity. At 4× 105 h−1, the results wereabout the same on the 400mm spheres while they haddecreased to 54% CH4 conversion, 83% CO selectiv-ity, and 57% H2 selectivity on the 80 ppi monolith.Oxygen conversion was complete for either sample atall space velocities. Again the spheres gave better re-sults at high space velocities even though the monolithand the spheres have a similar pore diameter.

3.5. Temperature profile as a function of spacevelocity

Heat transfer may be the reason that spheres are bet-ter than monoliths at high space velocities. As previ-ously shown, a steep temperature profile forms in themonolith as space velocity is increased, with the frontof the monolith at too low temperatures to catalyzemethane oxidation. This is not the case in the spheres,as the front of the spheres was always >500◦C. Thefront and back temperatures of a sphere bed made upof 800mm spheres were measured as space velocitywas increased. The front and back temperatures forthe sphere bed and for a 45 ppi monolith are plottedas a function of GHSV in Fig. 6.

Fig. 6 shows that at space velocities around 105 h−1

the front temperature was actually hotter than the backtemperature on the sphere bed. This could be seenvisually, as the front few layers of the spheres werebright orange, while the back spheres appeared darker.As space velocity was raised, the front temperaturecooled off to around 600◦C at a space velocity ofnear 5× 105 h−1. The back temperature remainedconstant at all space velocities at a temperature of750◦C.

The comparison between the sphere bed and themonolith showed the important difference betweenthe two support geometries. Increasing space velocityled to a dramatic drop in the front temperature ofthe monolith, as it decreased from 950 to 100◦C asGHSV changed from 105 to 4× 105 h−1. At the sametime the exit temperature got very hot, reaching atemperature of 1200◦C at 4×105 h−1. On the spheres,however, it is likely that different heat transfer char-acteristics in the bed allowed the front temperatureto remain higher over the same ranges of GHSV. The


Fig. 5. Comparison of 80 ppi monolith vs. 400mm spheres (nearly equal mass loadings). At the same space velocities spheres gave bettersyngas yields for GHSV >2×105 h−1.


Fig. 6. Comparison of experimental temperature profiles for a rhodium-coated monolith and sphere bed. As space velocity was increased,a sharp temperature profile developed in the monolith, while the effect was much less for the sphere bed.

steep temperature profile developed in the monolithled to products other than syngas. The sphere bedeffectively prevented such a temperature profile andmaintained high selectivity to syngas.

3.6. Pressure drop considerations

Pressure drop is a significant concern in the spherebed. The lower void fraction of the sphere bed willresult in a higher pressure drop than a monolith. Thiscould be a significant concern for any industrial ap-plication.

The pressure drop within a 45 ppi monolith andseveral sphere beds were measured using a pressuremanometer connected to ports in the reactor tube up-stream and downstream of the catalyst zone. As ex-pected, the pressure drop for the monolith was lowerthan for the sphere beds. As the flow rate was in-creased from 3 to 10 SLPM (GHSV increasing from105 to 4× 105 h−1) the pressure drop in the monolithincreased from around 0.004 atm to almost 0.02 atm.The 800mm spheres, on the other hand, developedhigh pressure drops as the flow rate was increased.The pressure drop increased from around 0.03 atm at3 SLPM to almost 0.1 atm at 7 SLPM. This pressurewas at the limit of the manometer’s scale. Pressuredrop was less on larger spheres. The 1200mm spheresreached a pressure drop of around 0.05 atm at 8 SLPM.The largest spheres used, 3200mm in diameter, had

very similar pressure drop characteristics to the foammonolith, giving roughly the same pressure drop val-ues at the same flow rates.

The pressure drop is a function of the sphere size.Larger spheres give lower pressure drops at all flowrates. This is due to the tightness of packing. Thesmaller spheres pack tighter than the larger spheres be-cause the larger spheres do not pack well at the walls,leading to larger void fractions along the tube walls.This is reflected in an overall higher void fraction forthe larger spheres.

Although pressure drop is higher for a sphere bedthan for a monolith, this may not be as significant aproblem as it appears. Pressure drop is frequently pre-sented as pressure drop per unit length. For the 800mmspheres, the pressure drop would be 0.1 atm/cm at 10SLPM. This is a high pressure drop per cm, but theencouraging thing is that the 1 cm depth appears to beall that is needed for conversion of methane to syngasby catalytic partial oxidation. Scale-up of this reactorwould make the reactor wider, not deeper.

3.7. Effect of sphere size

It has been shown that pressure drop is a function ofsphere size. Larger spheres pack looser than smallerspheres, resulting in a larger void fraction and lowerpressure drop. It may be advantageous to use largerspheres to minimize pressure drop, as long as the large


spheres perform as well as the smaller spheres. For thisreason, it is important to evaluate the effect of spheresize on the catalytic partial oxidation of methane.

It is also important to test this effect to get an ideafor the heat transfer effects in the bed. Conductionoccurs in the sphere bed through both intraparticle(within the sphere) and interparticle (between adjacentspheres). Larger sphere beds will conduct heat almostexclusively through intraparticle heat transfer becausethere are few contacts between neighboring spheres.Smaller spheres, on the other hand, will conduct heatmostly by conduction near the sphere contact point(interparticle conduction) because there are many con-tact points between spheres. It is interesting to look atdifferent sphere sizes and how they perform at differ-ent flow rates to get an idea of how the different modesof heat transfer affect the sphere bed performance.

To answer these questions, experiments were per-formed to determine the effect of sphere size. Foursphere sizes were used: 400, 800, 1200, and 3200mm.All spheres had been pre-sintered to remove micro-porosity before being loaded with approximately 0.2 gRh. The 400mm spheres were loaded with 0.28 g Rh(15.2% Rh), the 800mm spheres were loaded with0.15 g Rh (8.3% Rh), the 1200mm spheres wereloaded with 0.23 g Rh (11.1% Rh), and the 3200mmspheres were loaded with 0.26 g Rh (12% Rh).

The methane and oxygen conversions are shown inthe top panel of Fig. 7. CO selectivity for the spheresamples are shown in the middle panel of Fig. 7. H2selectivity is plotted versus GHSV for all of the spheresizes in the bottom panel.

As seen in these figures, spheres in the range of400–1200mm gave similar results. Methane conver-sions at GHSV above 2× 105 h−1 were all around 84or 85%. CO selectivities for those sphere sizes werebetween 92 and 93%. H2 selectivity varied between94 and 96% with the 400mm spheres giving the bestresults. Experimental error was estimated at 1% forconversion and CO selectivity and 2% for H2 selec-tivity at a maximum, so it is possible that all of theseresults were the same.

The 3200mm spheres performed far worse than theother sphere sizes as the space velocity was increased.At 105 h−1 the 3200mm spheres gave 81% conver-sion, 92% CO selectivity, and 95% H2 selectivity. AsGHSV increased, CH4 conversion decreased to 71%,H2 selectivity decreased to 84%, and the CO selec-

tivity slightly decreased to around 90.5%. Two thingsmay be happening with increasing GHSV. It is pos-sible that the sphere bed is blowing out, as occurredon the foam monoliths. Because the spheres are solarge, there are few contact points between neighbor-ing spheres. This means that the effective thermal con-ductivity is low (calculated to be 0.8 W/mK). This mayallow blow out to occur and lead to lower conversionsand selectivities. Another issue is the packing of thespheres. The ratio of the tube diameter to the spherediameter is only around six. This results in a largevoid fraction of approximately 59%. The void frac-tion is very large at the walls, where the spheres donot pack well. It is possible that the reactant gassesare bypassing the bed as the flow rate is increased andthis bypass is what leads to the lower CH4 conversionand CO and H2 selectivities.

Sphere size seems relatively unimportant based onthese experiments. As long as the spheres pack well,giving a low void fraction, the conversions and se-lectivities are about the same. It is possible that thereis an optimal sphere size, though that has not beenshown here.

4. Discussion

The experimental results suggest that the use ofspheres as the catalytic support for catalytic partialoxidation of methane allows syngas to be producedwith high selectivity at all space velocities up to 1.8×106 h−1. This is in contrast to performing this reactionon a monolith-supported catalyst, which gives poorsyngas yields at space velocities above 4× 105 h−1.The question is why spheres are a better support thanmonoliths at high space velocities.

The answer appears to lie in behavior near blowout.The performance on a monolith decreases dramaticallyin parallel with the development of a steep temperatureprofile, while blowout was not seen on Rh-spheres.The front temperature remained fairly hot even at thehighest space velocities used.

It was shown that increasing the amount of rhodiumon a monolith could prevent blow out at space veloci-ties up to 5×105 h−1. This suggests that higher rates ofmass transfer and reaction rate within the catalyst bedcan keep activity high as space velocity is increased.However, the slight downward trend observed for the


Fig. 7. Partial oxidation of methane on rhodium-coated spheres of different sizes. Results were nearly identical except for the largestsphere size.


high loadings of Rh suggest that eventually their per-formance will suffer. In addition, because of the highcost of rhodium, it is preferable to use lower loadings.

The use of alumina spheres as the catalyst supportallowed high reactant conversions and syngas selec-tivities to be maintained at very high space velocities.Because the surface areas of sphere bed and mono-lith were similar (∼5–6 m2/g for spheres,∼1 m2/g formonoliths), the pore size within the beds was similardepending on the sphere size and monolith ppi, andthe Rh loadings were similar, the mass transfer and re-action rates are expected to be nearly the same usingeither support. This suggests that some other factormust explain the difference between the supports. Heattransfer appears to be a likely explanation because ofthe very different observed temperature profiles thatdeveloped as space velocity was increased.

4.1. Comparison of heat transfer in a sphere bedversus a monolith

Axial heat transfer is of primary importance in thiswork. Heat can be transferred axially in four differ-ent modes.Convectionoccurs axially in the directionof flow, acting to transfer heat from the surface tothe cooler gasses. Heat is generated byreaction onthe catalyst surface. This heat generation is a functionof temperature because of its dependence on reactionrate. Conductionoccurs within the alumina surface,and can transfer heat axially either in the direction offlow or opposite of the flow direction depending on thetemperature profile.Radiationcan efficiently transferheat from a portion of the catalyst bed at sufficientlyhigh temperature to permit radiation (>480◦C [14]).

At low flow rates conduction, convection and radi-ation transfer heat primarily in the direction of flowbecause much of the reaction will occur on the ini-tial part of the catalyst where reaction compositionsare high. However, at higher flow rates when the cat-alyst bed begins to blow out, heat is likely conductedand radiated in a direction opposite of flow since thetemperature is higher at the exit of the bed.

When heat is removed from the surface faster thanit is generated by reaction, the temperature will fall.When the temperature falls below the ignition tem-perature of methane oxidation, reaction no longer oc-curs on that portion of the monolith. This behavior isknown as blowout. Blowout will occur easier on a cat-

alyst geometry that has a high convective heat transfercoefficient, low thermal conductivity and low radiativeheat transfer.

Qualitative heat transfer calculations have been per-formed to evaluate the relative axial heat transfer char-acteristics of a monolith versus a sphere bed. In thesecalculations, a constant value for the heat generationrate minus conductive heat losses was assumed. Thisassumption was done to simplify the problem and tofocus on differences in the convective properties of thetwo supports. The heat generation from reaction minusconductive heat losses was taken to be 4×103 W/m2.By equating the convective heat loss with this heatgeneration rate, the surface temperature could be cal-culated.

Heat lost by convection was calculated accordingto:

Qconv = hc(Tsurf − Tgas)

The gas temperature was assumed to be 25◦C. Theconvective heat transfer coefficient was calculatedfrom the Nusselt number, found for the sphere bedfrom the correlation of Whitaker [15] and for themonolith by the correlation of Younis and Viskanta[16]. For the monolith, the heat transfer coefficientcalculated from the correlation was a volumetric heattransfer coefficient, so it had to be multiplied by thearea to volume ratio of the bed, assumed to be thegeometric ratio (4/d).

Fig. 8 shows the calculated inlet surface tempera-tures for both geometries at flow rates between 4 and30 SLPM. This is only a qualitative calculation, so thevalues found for temperature are not expected to repre-sent those measured experimentally. However, qualita-tively the calculations can suggest why the sphere bedgives high selectivities at the same conditions wherea monolith is blown out.

The calculations show that the inlet surface tem-perature of the sphere bed should be higher thanthe monolith, as is observed experimentally, becauseconvective heat transfer is much more efficient at re-moving heat from the monolith. The convective heattransfer coefficients for the monolith were calculatedto be about double that of the sphere bed, most likelybecause of the tortuous flow passages in the foammonolith [15]. As flow rate is increased, the calculatedinlet temperatures fall for either support geometrydue to higher rates of convection, as expected.


Fig. 8. Simulated inlet temperatures for a monolith vs. a spherebed with varying flow rate. Inlet temperature was calculated byequating heat generation and convection of heat away from thesurface. The sphere bed temperatures were predicted to be higherthan the monolith at the same flow rate due to higher rates ofconvection in the monolith.

Due to the simple, qualitative nature of this cal-culation, there are two significant factors that are notaccounted for. First, the rate of heat generation wasnot calculated as a function of temperature, althoughtemperature should have a significant effect on thereaction rate. The consequence of this is that the heatgeneration at lower temperatures is likely overesti-mated. When the front temperature drops below thecatalytic ignition temperature of methane, reactionshould cease and the front temperature will reflectheat conduction from portions of the catalytic bedwhere reaction is still occurring.

A second limitation is the absence of conductionand radiation in the calculation. Higher rates of ra-diative and conductive heat transfer may help preventblowout. For this reason, the effective axial thermalconductivities for a monolith and a sphere bed wereestimated and compared.

Experimental data obtained by Sweeting et al. [10]suggests an effective thermal conductivitiy of a 45 ppi,92% Al2O3 monolith of 1.28 W/mK at 1000◦C, in-cluding the effects of both conduction and radiation.The stagnant thermal conductivity of a sphere bed wascalculated from the correlation developed by Kuniiand Smith [14] and modified by Specchia et al. [17].A value of 1.55 W/m K was calculated assuming avoid fraction in the 800mm sphere bed of 45%. Even

without accounting for radiation in the sphere bed, thethermal conductivity in a sphere bed was calculated tobe 30% higher than the effective thermal conductivityin the monolith. This will mean that the sphere bed islikely better able to conduct heat from the back to thefront of the bed at high flow rates and prevent blowout.

These calculations support the idea that heat trans-fer differences between the two support geometries areresponsible for the different temperature profiles ob-served. By less efficient convection and more efficientconduction, the sphere bed was able to maintain hightemperatures throughout the bed at all experimentalspace velocities while the monolith was not.

The difference in temperature profile is thought tobe the most important factor to explain the differ-ences in catalytic results. When the monolith blowsout, there is a shorter reaction zone. This leads to adecrease in methane and oxygen conversions with in-creasing space velocity. The selectivity is affected aswell. Oxygen conversion decreases less steeply thanmethane conversion, with oxygen increasingly beingconverted to CO2 and H2O when the front of themonolith is cooling off. The role of gas phase reactionsmay become important at the highest flow rates whenthe exit of the catalyst is at temperatures in excess of1200◦C, as suggested in a previous paper on the par-tial oxidation of methane on Pt monoliths at high flowrates [9].

These calculations support the idea that heat trans-fer differences between the two support geometries areresponsible for the different temperature profiles ob-served. By less efficient convection and more efficientconduction, the sphere bed was able to maintain hightemperatures throughout the bed at all experimentalspace velocities while the monolith was not.

The difference in temperature profile is thought tobe the most important factor to explain the differ-ences in catalytic results. When the monolith blowsout, there is a shorter reaction zone. This leads to adecrease in methane and oxygen conversions with in-creasing space velocity. The selectivity is affected aswell. Oxygen conversion decreases less steeply thanmethane conversion, with oxygen increasingly beingconverted to CO2 and H2O when the front of themonolith is cooling off. The role of gas phase reac-tions may become important at the highest flow rateswhen the exit of the catalyst is at temperatures in ex-cess of 1200◦C, as suggested in a previous paper on the


partial oxidation of methane on Pt monoliths at highflow rates [9].

5. Conclusions

Rh-coated spheres give superior syngas perfor-mance than Rh-coated monoliths. The conversionof methane and selectivity to syngas both decreasedramatically on monoliths at space velocities in ex-cess of 5× 105 h−1. This occurs with any weightloading of Rh or pore size, though higher weightloadings prevent as significant a loss in syngas yieldat 5 × 105 h−1. In contrast, Rh-coated spheres inthe size range of 400–1200mm show no evidence ofblow out or reduced activity even at space velocitiesof 1.8 × 106 h−1.

The performance of Rh-coated spheres is relativelyindependent of sphere size, although the largest spheresize (3200mm) gave significantly worse results. In-ternal surface area has no effect on performance, assintered and unsintered spheres gave nearly identicalresults. Pressure drop is higher in the spheres than ina monolith due to smaller void fraction in the bed, butmay not be particularly detrimental to a possible in-dustrial application since a bed length of 0.5–1 cm isall that is required for high syngas yields.

Rh spheres are probably better than monoliths be-cause of heat transfer. A foam monolith support hashigher convective heat transfer, but lower conductivetransfer, meaning that a monolith will blow out atconditions where a sphere bed will not. The low fronttemperature in the monolith and shorter reaction zonethat exists at blowout probably leads to poor syngas se-lectivity. The sphere bed, on the other hand, maintainshigh temperatures throughout the bed, giving highsyngas selectivity at all experimental space velocities.

Acknowledgements

Support of this work by Exxon is gratefullyacknowledged. Special thanks are expressed toSebastian Reyes and Jennifer Feeley for many helpfuldiscussions.

References

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Partial oxidation of methane to syngas at high space velocities over Rh-coated spheres

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