Published: December 19, 2011

r 2011 American Chemical Society 7573 dx.doi.org/10.1021/ie202098q | Ind. Eng. Chem. Res. 2012, 51, 7573–7583

ARTICLE

pubs.acs.org/IECR

Experimental and Modeling Analysis of the Thermal Behavior of anAutothermal C3H8 Catalytic Partial Oxidation ReformerDario Livio, Alessandro Donazzi, Alessandra Beretta, Gianpiero Groppi,* and Pio Forzatti

Laboratory of Catalysis and Catalytic Processes, Dipartimento di Energia, Politecnico di Milano, Piazza Leonardo da Vinci 32,20133 Milano, Italy

ABSTRACT: In this work, a spatially resolved sampling technique is applied to characterize the performance of a C3H8 CPOreformer and to compare it with that of a CH4 reformer. The case of Rh-coated honeycomb catalysts is examined. The axial profilesshow that higher temperatures are reached in C3H8 CPO, especially at the reactor inlet. Surface hot-spot temperatures around950 �C lead the catalyst to rapid loss of activity. A detailed model analysis is also applied to better understand the reasons for theobserved differences of the thermal behavior. On one hand, the heat release via oxidation reactions is controlled by O2 mass transferrate and thus proportional toO2 inlet concentration, which is∼20% higher in the C3H8/air mixture at equal C/O ratio. On the otherhand, while CH4 steam reforming is partly chemically controlled, C3H8 steam reforming is mainly limited by gas�solid diffusion.Thus, a less efficient balance between exo- and endothermic reactions occurs in the case of C3H8 CPO, and this results in muchhigher hot-spot temperatures. As a consequence, specific strategies for the optimization of the thermal behavior are requireddepending on the fuel. Modeling of the C3H8 CPO results shows that an increased catalyst load or a suitable aspect ratio of thereactor, combined with a decrease of the flow rate, produces a beneficial moderation of the hot-spot temperature of the catalytic wall.

1. INTRODUCTION

In the last two decades the catalytic partial oxidation (CPO) ofmethane has been extensively studied both experimentally andtheoretically. A significant amount of data is available in theliterature concerning the catalytic materials, the catalyst stability,the reaction mechanism, the impact of diffusive limitations andthe reactor design.1�13 Recently, the focus of research has shiftedtoward the study of the CPO of heavier fuels, such as LPG orlogistic fuels (gasoline, kerosene, diesel)14�16 with interest inpotential commercial applications such as the on-board anddistributed production of H2 and syngas.17 For these fuels, it isgenerally understood that complete conversion of reactants andhigh syngas selectivity can be obtained by using Rh-basedcatalysts supported on honeycomb or foam monoliths, and thatmajor challenges concern the catalyst deactivation, in terms ofcoke production, and the thermal behavior of the reactor, interms of temperature of the surface hot spot.15

Concerning the thermal behavior of the reformer, in aprevious study on CH4 CPO

18 we have shown that at high flowrates (20 NL/min) and at 350 �C preheat temperature themaximum surface temperature rises well above 900 �C, lead-ing the catalyst to rapid deactivation, likely due to rhodiumsintering.19 Such deactivation is an autocatalytic process thatstarts in the first part of the monolith: the loss of activity pro-motes the temperature rise, which in turn causes a further loss ofactivity, which spreads across the whole reactor.

In the case of CH4, in order to minimize the hot-spottemperature, prevent catalyst sintering, and extend the stableoperation of the reactor, criteria for the reactor design have beenproposed. These are based on the modification of the balancebetween the rate of the exothermic reactions (kinetically limitedby external mass transfer) and the rate of endothermic steamreforming (kinetically controlled by the surface chemistry).For honeycomb-supported catalysts, we have shown that the

sensitive design parameters are the channel opening, the reactoraspect ratio, the catalyst load, and the positioning of the frontheat shield.18,20,21

In the case of C2+ fuels, it may be expected that the thermalstability of the catalyst becomes even more critical. On a purelythermodynamic basis an important increase of adiabatic tem-perature rise is associated with the use of a fuel heavier thanmethane (e.g., ΔTad,C3H8

= 783 �C vs ΔTad,CH4= 650 �C, for

fuel/air mixture at C/O = 0.9, TIN = 25 �C, P = 1 atm), mainlybecause of the increasing concentration of O2 in the feed mixturewith increasing number of C atoms, for a given C/O ratio.

Thanks to its wide availability and possibility of being stored asa liquid, propane can be regarded as a case molecule for the studyof the CPO of light hydrocarbons. Investigating Rh-basedcatalyst supported over FeCrAlloy and Al2O3 foams, Holmenand co-workers22,23 reported temperatures exceeding 900 �Cboth for CPO and OSR (oxidative steam reforming) experi-ments. At these temperatures, the authors found that loss ofmetal dispersion occurred. A small fraction of CH4 and C2H4

were detected in the reaction products and were associated withthe activation of gas phase chemistry. Coherently with theseresults, we have recently reported the formation of coke pre-cursors, such as C2H4 and C3H6, in the CPO of C3H8 over Rh-based catalysts supported on a honeycomb: by application of thespatially resolved sampling technique, peaks of C2+ hydro-carbons have been observed in the first millimeters of thechannel, that is, in correspondence of the hot spot (950 �C).24

Special Issue: Russo Issue

Received: September 13, 2011Accepted: December 19, 2011Revised: December 16, 2011

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Thus, given the known consequences of high temperatures,which influence both the catalyst morphology and the chemicalmechanism, it is important to characterize and fully rationalizethe thermal behavior of C3H8 CPO. This has been little treated inthe literature. In fact, the experimental investigations concerningthe CPO of C2+ hydrocarbons have mostly focused on theintegral performances of the reactors, in terms of temperatureand composition of the syngas.14�16,25�29 Some works have alsomeasured the evolution of the temperature profile along the axisof the catalyst by means of sliding thermocouples.30,31 Only a fewexperimental works have focused on the temperature and con-centration profiles within the catalyst,8,32,33 but none, to ourknowledge, has addressed the CPO of C3H8.

The present paper addresses an experimental and numericalstudy of C3H8 CPO over Rh-coated honeycomb monoliths. Theaim of this work is the analysis and the rationalization of thethermal behavior of the reactor by means of a spatially resolvedsampling technique. Also, potential optimization strategies forminimizing the hot spot temperatures are proposed.

2. EXPERIMENTAL AND MODELING

2.1. Catalytic Materials and Adiabatic Lab-Scale Reactor.C3H8 CPO experiments were performed over 2 wt % Rh/α-Al2O3 catalysts, supported onto 400 cpsi cordierite honeycombmonoliths (diameter = 24 mm, length = 40 mm). The catalystwas prepared by incipient wetness of α-Al2O3 with an aqueoussolution of Rh(NO)3 and by dipping the honeycomb into a slurryof the powders, followed by blowing the excess of the slurry withair. The catalyst was deposited over a ∼25 mm length of thesupport (Figure 1). The remaining inert part (∼15 mm) was leftuncoated and acted as a continuous front heat shield. As shown in arecent study,21 this heat shield allowed the preservation of theadiabaticity of the system and minimization of axial heat dispersionby radiation at the front face of the catalyst. A catalyst load of600�650 mg was estimated by weight difference before and aftercoating the monolith, which results in∼3.9 g/L Rh load referred tomonolith volume. The thickness of the layer (∼14 μm) wascalculated assuming a washcoat density of 1.38 g/cm3, indepen-dently derived from dedicated measurements of weight and thick-ness on flat FeCrAlloy slabs, washcoated with the same procedure.The experiments were carried out in a lab-scale adiabatic

reactor. The thermal insulation was realized by wrapping thereactor with a very thick layer of quartz wool. The catalyticmonolith was placed in between a FeCrAlloy foam monolith anda cordierite honeycomb, which act as thermal shields and flowmixers. To avoid C formation, the catalyst and the heat shieldswere inserted in a quartz tube. The reactor was equipped withseparated electric heaters for preheating the reactants. Once thelight-off of the reaction occurred, the preheating system wasturned off in order to perform autothermal tests without any

external heat input; that is, the inlet temperature of the feedmixture was equal to room temperature (TIN = 25 �C).The spatial sampling technique was applied to collect tem-

perature and concentration profiles along the axis of the reactor.The setup is described in detail elsewhere.20,34 For the purpose ofthis work it is worthy to note that, to realize the measurement, afused silica capillary was inserted in the central channel of thecatalytic monolith. The capillary was moved along the channelwith a linear actuator. A submillimetric K-type thermocouple andan optical fiber (45� polished tip) connected to a narrow band IRpyrometer (Impac Infrared, IGA 5-LO) were used to collect thetemperature profiles. The thermocouple measurements weretaken as representative of the temperature of the gas phase,while the pyrometer measurements were representative of thetemperature of the catalyst surface. To measure the compositionof the gas phase, the capillary was connected to a micro-GC(Agilent 3000A).All the CPO experiments were carried out at atmospheric

pressure, with 10 NL/min flow rate and C/O ratio of 0.9. Themeasured thermal efficiency (estimated as the ratio of theexperimental and the theoretical adiabatic temperature rise)was always higher than 0.98.2.2. Mathematical Model of the Reactor. The experimental

results were quantitatively analyzed by a 1D, dynamic, hetero-geneous, fixed-bed, single-channel model of the adiabatic reactor.The model is described elsewhere35 and consists of mass,enthalpy and momentum balances for the gas and solid phase.Heat and mass transfer coefficients were estimated according tospecific correlations for square channels.36 The model includedboth homogeneous and heterogeneous kinetic schemes. Gasphase reactions for C1�C3 species were taken into accountaccording to the detailed scheme by Ranzi and co-workers.37 Amolecular kinetic scheme was adopted to describe the hetero-geneous chemistry of C3H8 CPO. This scheme was indepen-dently derived on the basis of a study performed in an isothermalannular reactor, which represents an extension of previous workson CH4 CPO38,39 and will be subject of a dedicated paper.Experiments of CPO and steam reforming of C3H8 were carriedout within the temperature range 300�850 �C, at varying spacevelocity (GHSV = 7� 105 to 9� 106 h�1), C/O ratio (0.5�0.9),reactants dilution (C3H8 = 0.25�4%), and cofeed of products(H2O = 1�2%, H2, CO = 0.5�1%). The kinetic scheme wasderived by analyzing the experimental data with a 1D mathematicalmodel of the annular reactor,38 and it consisted of the whole set ofCH4 CPO reactions (CH4 total oxidation, CH4 steam reforming,direct and reverse water gas shift, H2 and CO oxidation) plus severaladditional reaction steps, namely C3H8 total oxidation, C3H8 steamreforming, CO methanation and the steam reforming of someC2�C3 intermediates (C2H6, C2H4, C3H6). The complete set ofrate expressions and kinetic parameters is reported in Table 1. In linewith the rate expressions for CH4 conversion, C3H8 oxidation andsteam reforming were found to be first order dependent on C3H8

partial pressure, and independent of the concentration of thecoreactant (O2 or H2O). It was also assessed that the rate constantsof the oxidation and the steam reforming of C3H8 are about 2.5 timesgreater than those of CH4, with comparable activation energy. Thekinetic parameters of C2�C3 intermediates were set equal to those ofC3H8both in total oxidation and steam reforming.The importance ofincluding heterogeneous conversion steps for the hydrocarbonsspecies such as propylene, ethylene, and ethane produced by gas-phase reactions has been discussed in a previous paper.24 Thesimplifying character of the kinetics herein adopted for such steps

Figure 1. Photo of the catalytic honeycombmonolith with a continuousinert front heat shield at the beginning.

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(equaled to the rate of steam reforming of propane) is due onone side to the absence at this stage of the work of specificpieces of evidence, and on the other side to the satisfactoryresponse of the model.The numerical analysis herein reported is fully predictive, with

no parameter adjustment. The only input data of the calculationswere the catalyst amount, the Rh load and dispersion (20%, asestimated experimentally by H2 chemisorption measurements),the geometrical parameters, and the physical properties of thehoneycomb support. A detailed list of the relevant input param-eters of the simulations is reported in Table 2.

3. RESULTS AND DISCUSSION

3.1. Thermal Behavior and Stability in C3H8 CPO. Figure 2ashows the axial temperature profiles measured by the pyrometerand the thermocouple in a series of autothermal C3H8 CPOexperiments performed at increasing reactants concentration.The tests were carried out at 10 NL/min flow rate by maintain-ing the C/O ratio at 0.9 and progressively increasing the concen-tration of C3H8 from 4 to 11% v/v. The temperature profiles hadthe typical features of a CPO experiment: the temperaturerecorded by the pyrometer showed a hot spot in the first partof the catalyst (0�10 mm), associated with the occurrence of theexothermic oxidative chemistry, which was followed by a de-crease toward the exit section, due to the prevailing role of theendothermic chemistry. The temperature measured by thethermocouple had a sharp rise, passed through a maximum andfinally matched the pyrometer temperature, in line with thethermodynamic equilibrium. Extensive discussions on the mea-surement and the analysis of the temperature profiles can befound in the literature.20,34

Table 3 reports the conversion of C3H8, the selectivity ofsyngas and the temperaturemeasured by the thermocouple at theoutlet of the catalyst (∼26 mm in Figure 2a) compared with thethermodynamic equilibrium values and with the model predic-tions. The outlet performances were very close to the adiabaticequilibrium, except for the experiment at 4% C3H8. In this lattercase, the reaction was controlled by kinetics due to the lowtemperatures. In line with the temperature rise, the conversion ofC3H8 increased, accompanied by an increase in the selectivity ofsyngas. Importantly, at increasing C3H8 concentration, the hotspot measured by the pyrometer became sharper and reached950 �C with the stoichiometric mixture. Such a high localtemperature was detrimental for the catalyst activity, as revealedby reference CH4 CPO experiments (Figure 2b); such experimentswere performed after each C3H8 CPO run at increasing concentra-tion under conditions (27% vol CH4, 0.9 C/O, 10 NL/min flowrate) that guarantee stable operations and are suitable to follow theoccurrence of the deactivation. Noteworthy, as reported by Berettaet al.18 for CH4CPO, the catalyst deactivation is expected to cause amarked increase of the hot-spot temperature, while the outlettemperature and reactor performance in terms of fuel conversionand syngas selectivity maintain almost constant. The results ofFigure 2b show that no deactivation was evident after the experi-ments at 4% and 6% C3H8, whereas, after the experiment at 9%C3H8, the hot spot in CH4 CPO was 40 �C higher. Additionally, adramatic increase of 100 �C of the hot spot in CH4 CPO wasapparent after the run under stoichiometric C3H8 conditions,accompanied by a 3 mm shift of the peak inside the channel, whichstrongly suggests a loss of activity in the front part of the catalyst. Inline with the results of Beretta et al.,18 no change of CH4 conversionand syngas selectivity was observed at the outlet of the reactor(Table 4).

Table 1. Rate Equations and Kinetic Parameters in C3H8 CPO over 2% Rh/α-Al2O3 Catalyst

reaction ratei [mol gcat�1s�1] ki

873K [mol atm�1 gcat�1s�1] EACT [kJ mol�1]

C3H8 total oxidation C3H8 + 5O2 f 3CO2 + 4H2O ROxC3H8 ¼ kOxC3H8 PC3H81 þ KadsH2OPH2O

σO2 2.500 � 10�1 80.00

C3H8 steam reforming C3H8 + 3H2O T 3CO + 7H2 RSRC3H8 ¼ kSRC3H8 PC3H8 ð1 � ηSRC3H8Þ1 þ KadsCOPCO þ KadsO2 PO2

σH2O 2.486 � 10�1 84.63

CH4 total oxidation CH4 + 2O2 f CO2 + 2H2O ROxCH4 ¼ kOxCH4 PCH41 þ KadsH2OPH2O

σO2 1.030 � 10�1 91.96

CH4 steam reforming CH4 + H2O T CO + 3H2RSRCH4 ¼ kSRCH4PCH4 ð1� ηSRCH4

Þ1 þ KadsCOPCO þ KadsO2PO2

σH2O

ηSRCH4< 1

1.027 � 10�1 91.80

C2H4 steam reforming C2H4 + 2H2O T 2CO + 4H2 RSRC2H4 ¼ kSRC2H4 PC2H4 ð1 � ηSRC2H4Þ1 þ KadsCOPCO þ KadsO2 PO2

σH2O 2.486 � 10�1 84.63

C2H6 steam reforming C2H6 + 2H2O T 2CO + 5H2 RSRC2H6 ¼ kSRC2H6 PC2H6 ð1 � ηSRC2H6Þ1 þ KadsCOPCO þ KadsO2 PO2

σH2O 2.486 � 10�1 84.63

C3H6 steam reforming C3H6 + 3H2O T 3CO + 6H2 RSRC3H6 ¼ kSRC3H6 PC3H6 ð1 � ηSRC3H6Þ1 þ KadsCOPCO þ KadsO2 PO2

σH2O 2.486 � 10�1 84.63

Water gas shift CO+H2O T CO2 + H2RWGS ¼ kWGSPH2Oð1� ηWGSÞσCO

ηWGS < 16.831 � 10�3 74.83

Reverse water gas shift CO2 + H2 T CO + H2ORRWGS ¼ kRWGSPCO2 ð1� ηRWGSÞσH2

ηRWGS < 11.277 � 10�2 62.37

H2 oxidation H2 + (1)/(2)O2 f H2O ROxH2 ¼ kOxH2PH2σO2 2.666 � 103 61.65

CO oxidation CO + (1)/(2)O2 f CO2 ROxCO ¼ kOxCOPCOσO2 1.937 � 101 76.07

CO methanation CO + 3H2 T CH4 + H2ORMetCO ¼ kMetCOPH2 ð1� ηMetCOÞσCO

ηMetCO < 11.200 � 10�3 88.02

surface adsorption Ki873K [atm�1] ΔHADS [kJ mol

�1]

O2 5.461 � 100 �72.83CO 2.114 � 102 �37.15H2O 8.974 � 100 �57.48

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The comparison of Figure 2 clearly shows that, with stoichio-metric fuel/air mixtures, the C3H8 CPO experiment is char-acterized by much higher temperatures than the CH4 CPOexperiment and this is the cause of the rapid loss of activity. In pr3.2. Species and Temperature Profiles in C3H8 CPO and

CH4 CPO. The kinetic factors can be well evidenced by focusingon the axial profiles of temperature and concentration measuredin CPO experiments with stoichiometric mixtures. Figure 3compares the results of a C3H8 CPO experiment (panels a and c)with the results of a CH4 CPO experiment (panels b and d). A

common trend was evident: in the first 7 mm of the channel, thereactants were consumed with the production of syngas, H2O,and CO2. Once O2 was completely depleted, the fuel was furtherconverted at the expenses of H2O by steam reforming, with theadditional production of syngas. In the case of C3H8, a smallfraction of CH4 and of other intermediates (Figure 3c, inset) wasalso formed, as a consequence of a side gas phase-reaction pathway,as extensively discussed elsewhere.24 Concerning the tempera-ture profile, a difference of nearly 100 �C between the two testswas maintained along the axis of the monolith, which grew to200 �C at the hot spot. Overall, the catalysts were extremelyactive: the profiles showed a flat trend (corresponding to thereaching of the thermodynamic equilibrium) within a few mmfrom the entrance, and a strong superposition of the oxidativeand the reforming reactions was apparent. The model simula-tions (solid lines) were satisfactory, especially with respect to thetemperature profiles. A good match was indeed obtained, whichclearly showed that the thermocouple provided a close estimateof the gas phase temperature, while the pyrometer provided anaccurate description of the catalyst surface temperature. Thedeviation observed outside the catalyst (between �0.5 and0 mm) was due to a measurement artifact, as discussed in a previouswork dedicated to the application of the optical fiber pyrometer inCPO.34 Concerning the concentration profiles, a good match wasfound for all the major species. We observed some deviationsbetween measured and calculated profiles of the hydrocarbon gas-phase intermediates at the reactor inlet, which however have anegligible impact on the consumption of the reactants and on thethermal behavior of the reactor. A source of inaccuracy could be theadoption of a lumped description of cross sectional concentrationand temperature profiles in the honeycomb channel.Given the accordance with the experimental data, the numer-

ical analysis can be used to provide a reliable picture of thereaction mechanism, in terms of axial evolution of the local ratesof reactants consumption and production of H2 and CO. Therates per m3 of reactor (Figure 4) are calculated at the surfacetemperature and composition. In line with previous results of

Figure 2. Axial temperature profile for autothermal CPO experiments. (a) C3H8 CPO at increasing concentration of the reactants: C/O = 0.9, C3H8 =4�11% v/v, flow rate = 10NLmin�1,TIN = 25 �C. (b) CH4CPO at reference conditions: C/O= 0.9, CH4 = 27% v/v, flow rate = 10 nLmin�1,TIN = 25 �C.The experiments are performed after each run in C3H8 at increasing concentration.

Table 2. Relevant Geometrical and Physical Properties of theHoneycomb Monolith

Honeycomb

diameter (mm) 24

length (mm) 40

channel opening (mm) 1.092

void fraction (-) 0.74

cordierite density (g cm�3) 2.3

cordierite thermal conductivity (W m�1 K�1) 2.5

Catalyst

density (g cm�3) 1.38

Rh load (% w/w) 2

Rh dispersion (%) 20

Catalyst A

length (mm) 25

weight (mg) 600

thickness (μm) 14

Catalyst B

length (mm) 27

weight (mg) 650

thickness (μm) 14

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CH4CPO,40,41 also in the case of C3H8 CPO, once the reactor lit

off, oxygen is almost completely consumed by H2 oxidation,while the consumption of the fuel exclusively occurs via steamreforming, which is also responsible for the production of COand H2. WGS plays a minor role and leads to the formation ofCO2 in the first part of the catalyst. Such numerical analysisreveals that under the operating conditions herein tested, theC3H8 CPO process can be very well approximated to thecoupling of H2 oxidation, C3H8 steam reforming, and WGSequilibrium.3.3. Role of Mass Transfer Limitations in C3H8 CPO and

CH4 CPO. A deeper insight into the evolution of the reactants isobtained by analyzing the concentration profiles at the catalystwall predicted by the model (Figure 5). In both the experiments,the consumption of O2 was governed by external mass transfer,as indicated by the zero concentration of O2 at the catalyst wall.Coherently, the length for complete O2 consumption keptalmost unchanged, being independent of the O2 concentrationand very weakly dependent on the gas temperature. A differentsituation emerged when focusing on the profiles of the fuelconsumption. The wall concentration of CH4 was initially lowerthan the concentration in the gas bulk and slowly decreased untilreaching the equilibrium value at the outlet of the monolith.Instead, the C3H8 consumption showed a trend similar to that of

O2: the wall concentration dropped to zero immediately after thecatalyst entrance, while the consumption of C3H8 in the gasphase spread over 10 mm. To better rationalize this difference,which suggests the presence of different controlling regimes, theCarberry number for each reactant can be introduced (eq 1).

Cai ¼ CBi � Cw

i

CBi � Ceq

ið1Þ

In the equation,CiB is the concentration of the ith species in the

bulk of the gas phase, Ciw is the concentration at the catalyst wall

and Cieq is the concentration calculated assuming local equilib-

rium at the composition and temperature of the catalyst surface.According to this definition, the external mass transfer regimecorresponds to Ca f 1 and the chemical regime is representedby Ca f 0. The axial evolution of the Ca numbers is plotted inFigure 5c. As expected, this analysis confirmed that O2 con-sumption was totally limited by external mass transfer. Instead, ahigher Ca number was found for C3H8 compared with CH4.Specifically, the curves showed that CH4 consumption wascontrolled by a mixed chemical-diffusive regime that approachedthe chemical regime by the end of the channel, while C3H8

consumption was more strongly hindered by external masstransfer along the entire axis of the catalyst. This is due to thelow diffusion coefficient of C3H8 in the gas mixture, which isnearly half of the diffusion coefficient of CH4 (DC3H8,N2

/DCH4,N2=

0.53 according to the Fuller correlation42).The thermal behavior observed in C3H8 CPO and CH4 CPO

is strictly related to the occurrence of the different regimes thatgovern the fuel consumption. This can be shown by analyzing therates of consumption of the reactants via total oxidation andsteam reforming, as well as the rates of heat removal and heatrelease. As previously discussed, the O2 consumption is mainlydue to the oxidation of H2 and was limited by the diffusion of O2

from the gas bulk to the surface. For C3H8 and CH4, the rate ofconsumption was estimated as the rate of steam reforming. Therates of heat release and removal were taken as the product of therate of consumption ri and the reaction enthalpy ΔHR

0, andtherefore had the dimension of a power density. In line with thefull control by external mass transfer limitations, the rate ofoxygen consumption was about 20% higher in the case of C3H8

(Figure 6a,c), namely due to the different inlet concentration ofO2 (18.7% in C3H8 CPO vs 15.3% in CH4 CPO). Thus, the heatreleased by the two fuels followed the same ratio, given that inboth processes H2 combustion is the O2 consuming reaction(ΔHR

0 = �483 kJ/molO2).

On opposite, despite of a higher intrinsic kinetic rate of steam re-forming (consider that kC3H8,SR is about 10-fold higher than kCH4,SR

at the solid hot spot temperature), the ratio of the local rates of fuelconsumption was nearly 0.5, exclusively due to the occurrence oflarger mass transfer limitations for C3H8 (Figure 6b,c). Since thereaction enthalpy of steam reforming per mol of C3H8 is more thantwice that of CH4 (+497 kJ/molC3H8

vs +206 kJ/molCH4), the

power density locally removed in C3H8 CPO was slightly higher.Figure 6d reports the total power density, which was cal-

culated as ∑i = 1NR ri(�ΔR,i), where NR is the number of the reactions

of the kineticmechanism.As a consequenceof the coupling betweenoxidation and steam reforming, the reactor can be divided in twozones: in the first part (0�4 mm) of the catalyst, where the hotspot was located, the rate of heat release was larger than that ofheat removal and the total power density of C3H8 CPO was

Table 3. Autothermal C3H8 CPO at Increasing Concentra-tion of the Reactants. C/O = 0.9, C3H8 = 4�11% v/v, FlowRate = 10 NL min�1, TIN = 25 �C. C3H8 Conversion, SyngasSelectivity, and Temperature at the Outlet of the Catalysta

χ C3H8 [%] σ CO [%] σ H2 [%] TOUT [�C]

4% C3H8 exp 89 60 67 542

mod 81 63 74 554

eq 100 55 60 564

6% C3H8 exp 98 78 80 650

mod 97 81 84 657

eq 100 80 80 659

9% C3H8 exp 100 88 89 712

mod 99 89 90 724

eq 100 89 89 724

11% C3H8 exp 100 92 94 787

mod 100 94 94 788

eq 100 94 94 822aComparison of experimental (exp), calculated (mod), and thermo-dynamic equilibrium (eq) results.

Table 4. Autothermal CH4 CPO at Reference Conditionsa

χ CH4 [%] σ CO [%] σ H2 [%] TOUT [�C]

reference 84.48 86.50 91.60 665

after 4% C3H8 84.41 86.45 91.61 667

after 6% C3H8 84.19 86.61 91.68 665

after 9% C3H8 84.73 86.85 91.78 665

after 11% C3H8 84.89 87.28 91.60 666

equilibrium 85.79 86.03 91.71 678a Flow rate = 10 NL min�1, C/O = 0.9, CH4 = 27% v/v, TIN = 25�C.CH4 conversion, syngas selectivity, and temperature at the outlet of thecatalyst. Comparison of experimental and thermodynamic equilibriumresults. The experiments are performed after each run in C3H8 atincreasing concentration.

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higher than that of CH4 CPO. In the second zone (4�20 mm),endothermic steam reforming became the prevailing reaction,and the total power density of C3H8 CPO reached slightlynegative values, but comparable to those of CH4 CPO. Thesepieces of evidence fully explain why under stoichiometric condi-tions higher temperatures are observed in C3H8 CPO comparedwith CH4 CPO. On one hand, the rate of heat released by theoxidation reactions is slightly faster for C3H8, given that the rateof oxygen consumption is controlled by external mass transferand O2 concentration is lower for CH4. On the other hand, therate of heat removal by C3H8 steam reforming was slowed downby the diffusive limitations.To better appreciate the impact of diffusive limitations on

temperature profiles, a simulation of C3H8 CPO was performedby considering a diffusion coefficient of C3H8 equal to that ofCH4 (red lines). As shown in Figure 7, by increasing the C3H8

diffusivity, the hot spot temperatures of both the solid and thegas phase became less sharp, and the maximum temperaturesdecreased by ∼90 �C and ∼80 �C, respectively. The shapes ofthe temperature profiles were very similar to those of a CH4CPOsimulation, indicating that the occurrence of mass transferlimitations plays a major role in determining the catalyst over-heating in the first portion of the monolith.3.4. Effect of Design Parameters on Temperature Profiles.

The interplay between surface kinetics and mass transfer largelycontrols the thermal behavior of CPO reactors and needs to beconsidered when optimization strategies of the temperature

profile are proposed. In the case of CH4 CPO, catalyst loadand channel opening are effective design parameters and can betuned to favorably change the local balance between the exo- andthe endothermic reactions and realize an optimal temperaturedistribution along the axis of the catalyst. As experimentally andtheoretically shown in a previous investigation,20 the mixedchemical-diffusive regime that controls the reforming activityin CH4 CPO is such that the temperature profile can be smooth-ened by increasing the catalyst load. In fact, an increase of thecatalyst load, whichmeans an increase of the active Rhmetal area,promotes the rate of steam reforming reaction and the heat removal,without affecting the oxidation rate. As well, a considerable modera-tion of the hot spot can be achieved by enlarging the channelopening (that is, by decreasing the honeycomb cpsi): in this way, thereduction of the externalmass transfer coefficient locally reduces therate of heat release by oxidation, while it influences to a much lesserextent the rate of steam reforming and of heat removal, causing a netflattening of the whole temperature profile.Herein it is interesting to understand to what extent the same

design parameters can be exploited in C3H8 CPO. Figure 8ashows the simulations of C3H8 CPO experiments at varying thecatalyst load from 250 to 750 mg (i.e., from 1.6 to 4.5 g/L of Rh).The calculations were performed considering autothermal con-ditions, stoichiometric C3H8/air feed, and constant honeycombvolume (diameter = 24 mm, length = 20 mm). According to thischoice, the thickness of the catalyst layer was different in eachsimulation. The simulations show that there is still a moderate,

Figure 3. Spatially resolved profiles of temperature and composition for autothermal CPO experiments: (a and c) C3H8CPO, catalyst A. (b and d) CH4

CPO, catalyst B. Symbols and thin lines are experimental data. Thick lines are model predictions.

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beneficial effect of the catalyst load on the temperature profile,with a 55 �C reduction of the hot spot. This means that thecontrol by external diffusion is not complete, in line with thevalue of the C3H8 Carberry number (Figure 5c), which does notreach the unity. However, in the calculations, the impact ofinternal diffusive limitations was assumed negligible and, there-fore, the effect of the catalyst load on the temperature profile canbe even smaller.

Figure 8b reports the simulations of C3H8 CPO experimentsat decreasing the honeycomb cpsi from 400 to 90 (∼1 to 2.5 mmchannel opening) and constant catalyst load (250 mg). Differ-ently from CH4 CPO, the increase of the channel opening has anegative impact: even if smoother temperature gradients areobserved, especially in the gas phase, the surface hot spot isunaltered and the temperature progressively grows to higherlevels. At the lowest cpsi, the reaction does not even reach the

Figure 4. Axial evolution of the rate of consumption of reactants and production of H2 and CO, calculated at the surface composition and temperature.Conditions as in Figure 2.

Figure 5. Consumption profiles of the reactants for the experiments of Figure 2. Filled symbols are experimental data. Solid lines are model predictionsof themolar fraction in the gas bulk. Dashed lines are model predictions of themolar fraction at the catalyst wall: (a) C3H8 CPO; (b) CH4CPO; (c) axialevolution of Carberry numbers for O2, CH4, and C3H8.

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thermodynamic equilibrium, with loss of C3H8 conversion andsyngas selectivity. This result is again due to the diffusivelimitations that affect C3H8 steam reforming: in this case, slowingdown the mass transfer rate not only reduces the local heatrelease, but also hinders the rate of heat consumption, thus

counterbalancing the positive effect on the oxidation rate. Over-all, the results of Figure 8 suggest that, in the case of C3H8 CPO,different strategies must be adopted to moderate the hot spot.A possible solution is to reduce the heat release by decreasing

the total flow rate. In this respect, the simulations of Figure 9ashow the effect that a reduction of the flow rate from 15 to5NL/min has on the temperature profiles of the gas and the solidphase. The operating conditions are the same as in Figure 8a. Themain effect of lower flow rates is a moderation of the surface hotspot of about 100 �C, while the maximum of the gas phasedecreases of 25 �C with a 3 mm shift toward the catalyst inlet.However, it is crucial to note that the reduction of the flow ratealso lowers the syngas productivity of the reactor. An optimiza-tion strategy that does not affect the syngas productivity is thenpreferable and can be accomplished by decreasing the aspectratio of the reactor (i.e., the ratio between length and diameter,L/D). The results are reported in Figure 9b. The reference case(black lines) is characterized by an aspect ratio of 0.8 (diameter =24 mm, length = 20 mm), while the optimal ratio (red lines,L/D = 0.1) is obtained with a 2-fold diameter (48 mm) and one-fourth of the length (5 mm). In the calculations, the catalystweight was maintained constant. With the disk shape configura-tion, a marked decrease of the temperature is obtained alongthe entire length of the catalytic monolith and a reduction of∼130 �C of the surface hot spot is apparent. This effect happensbecause upon decreasing the aspect ratio, the linear velocity of thegas decreases and the heat conducted from the catalytic monolithto the front heat shield (back dispersion) becomes increasingly

Figure 7. Simulated effect of an increase of C3H8 diffusion coefficienton axial temperature profiles: C/O = 0.9, C3H8 = 11% v/v, flow rate =10 NL min�1, TIN = 25 �C.

Figure 6. (a) Axial evolution of the rate of O2 consumption via H2 oxidation; (b) axial evolution of the rate of fuel consumption via steam reforming; (c)axial evolution of the ratio of the rates of oxidation and steam reforming. The ratio is calculated only in the first 8 mm of the catalyst, before completeconversion of the reactants or thermodynamic equilibrium conditions are reached; (d) axial evolution of the ratio of the total power density. Conditionsas in Figure 2.

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important compared to the heat removed by convection, thusresulting in a moderation of the surface temperatures.Finally, we recall a preliminary discussion in a previous

study:21 another promising choice is the optimization of theinternal layout of the reactor in terms of distance between thefront heat shield and the catalyst entrance, which causes areduction of the hot spot via heat loss by radiation toward thewalls of the reactor.

4. CONCLUSIONS

Overheating is a critical issue for a Rh-based catalyst whenperforming the CPO of hydrocarbon fuels. In this work, we

address an experimental and theoretical investigation of thethermal behavior of a C3H8 CPO reformer with referenceto the case of Rh-supported honeycomb catalysts. Spatiallyresolved CPO experiments with stoichiometric C3H8/air mix-tures showed that temperatures as high as 950 �C were reachedon the catalyst surface, which caused rapid deactivation. Thecomparison with reference CPO experiments carried out withstoichiometric CH4/air mixtures revealed that much highertemperatures were reached and sharper gradients were presentin the case of C3H8. The reasons for these differences wererationalized, and catalyst design criteria for moderating the hotspot were analyzed. The spatially resolvedmeasurements and themodel analysis of the axial concentration profiles confirm that the

Figure 8. Simulated axial temperature profiles for autothermal C3H8 CPO experiments. C/O = 0.9, C3H8 = 11% v/v, flow rate =10 NL min�1,TIN = 25 �C: (a) effect of the catalyst weight (400 cpsi honeycomb); (b) effect of the channel opening (constant catalyst weight 250 mg).

Figure 9. Simulated axial temperature profiles for autothermal C3H8 CPO experiments. C/O = 0.9, C3H8 = 11% v/v, catalyst weight = 250 mg,TIN = 25 �C: (a) effect of the total flow rate; (b) effect of the reactor aspect ratio (flow rate = 10 NL min�1).

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balance between the rate of the exothermic oxidation (fullylimited by O2 mass transfer) and the rate of endothermic steamreforming plays a pivotal role in determining the final tempera-ture profile. In fact, in the case of C3H8 CPO the rate of heatrelease is larger due to the higher O2 concentration in the inletmixture, whereas the rate of heat removal is slowed down becauseof the higher external diffusive resistances that affect the steamreforming reaction. In light of these results, strategies differentfrom CH4 CPO are required to minimize the hot spot. Indeed,C3H8 CPO simulations show that enlarging the channel openingof the honeycomb does not produce beneficial effects, whileonly a moderate temperature decrease (55 �C) is obtained byincreasing the catalyst load. The moderation of the hot spot caninstead be accomplished by reducing either the total flow rate orthe aspect ratio of the reactor. Combined with these solutions,the adoption of a more dissipative reactor configuration is alsosuggested to optimize the axial temperature profile, for instanceby separating the front heat shield from the catalytic monolith inorder to enhance heat dissipation by radiation.

’AUTHOR INFORMATION

Corresponding Author*E-mail: gianpiero.groppi@polimi.it.

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