comparative study between fluidized bed and fixed bed reactors

Applied Catalysis A: General 223 (2002) 225238

Comparative study between fluidized bed and fixed bed reactors inmethane reforming combined with methane combustion for the

internal heat supply under pressurized conditionKeiichi Tomishige, Yuichi Matsuo, Yusuke Yoshinaga, Yasushi Sekine,

Mohammad Asadullah, Kaoru FujimotoDepartment of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan

Received 1 June 2001; received in revised form 21 July 2001; accepted 23 July 2001

Abstract

The effect of catalyst fluidization on the conversion of methane to syngas in methane reforming with CO2 and H2O in thepresence of O2 under pressurized conditions was investigated over Ni and Pt catalysts. Methane and CO2 conversion in thefluidized bed reactor was higher than those in the fixed bed reactor over Ni0.15Mg0.85O catalyst under 1.0 MPa. This reactoreffect was dependent on the catalyst properties. Conversion levels in the fluidized and fixed bed reactor were almost the sameover MgO-supported Ni and Pt catalysts. It is suggested that this phenomenon is related to the catalyst reducibility. On acatalyst with suitable reducibility, the oxidized catalyst can be reduced with the produced syngas and the reforming activityregenerates in the fluidized bed reactor. Although serious carbon deposition was observed on Ni0.15Mg0.85O in the fixed bedreactor, it was inhibited in the fluidized bed reactor. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Methane reforming; Methane combustion; Fluidized bed reactor; Carbon deposition; NiO-MgO solid solution catalysts

1. Introduction

Reforming of methane with H2O and/or CO2 isone of the promising methods for producing syn-gas. Although a reforming reaction is a highly en-dothermic reaction, partial oxidation of methane isexothermic. In many cases, it has been reported thatthe partial oxidation of methane consists of methanecombustion and successive reforming with the pro-duced water and CO2 [14]. This is because the rateof combustion is much higher than that of reform-

Corresponding author. Present address: Institute of MaterialsScience, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki305-8573, Japan. Tel.: +81-298-53-5030; fax: +81-298-55-7440.E-mail address: [email protected] (K. Tomishige).

ing. Therefore, a serious temperature gradient in thecatalyst bed is caused and it creates hot spots [5]. Inaddition, the carbon deposition on the catalyst surfaceis another problem in this system. The carbon is de-posited on the catalyst in the oxygen-free reformingzone [6]. This is a common problem in the field ofsyngas production. Especially, the carbon depositionis most significant in the CO2 reforming of methane[7]. So catalysts with high resistance to carbon de-position have been demanded. Our research grouphas reported that NiO-MgO solid solution catalystshave excellent stability in CO2 and steam reformingof methane, and that the rate of carbon depositionwas much lower than that on supported Ni catalystsand commercial catalyst for the steam reforming[810].

0926-860X/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0 9 2 6 -860X(01 )00757 -8

226 K. Tomishige et al. / Applied Catalysis A: General 223 (2002) 225238

Some research studies about the reforming ofmethane with H2O, CO2 and O2 using the fixed bedreactor have been carried out [11]. The fluidized bedreactor is an attractive option in the syngas productiondue to the combination of combustion and reforming.This is because the fluidized bed reactor provides ahigh rate of heat transfer in order to maintain theisothermal operation [12] and also because the per-manent circulation of the catalyst particles favors theburning of carbon on it in the oxygen-rich zone of thecatalyst bed [6].

CO-rich syngas can be utilized for FischerTropsch,methanol and dimethyl ether syntheses. Pressurizedsyngas is more favorable because most synthesisreactions have been carried out under pressurizedconditions [13]. The problem of carbon deposition ismuch more serious under pressurized reaction con-dition. While high conversions can be obtained atatmospheric pressure, the economics of the processseem to be favorable at high pressure [14].

In this article, the reforming of methane with CO2,H2O and O2 was investigated under pressurized condi-tion (2.0 MPa) over NiO-MgO and MgO-supportedcatalysts. Especially, this study focused on the com-parison between the fluidized bed and the fixed bedreactors.

2. Experimental

2.1. Preparation of catalysts

NixMg1xO (x = 0.03, 0.07, 0.15) catalysts wereprepared by the coprecipitation method from an aque-ous solution of Ni(CH3COO)24H2O (Kanto Chem-ical Co. Inc., >98.0%) and Mg(NO3)26H2O (KantoChemical Co. Inc., >99.0%) using K2CO3 (KantoChemical Co. Inc., >99.5%) as the precipitant. Afterbeing filtered and washed with hot water, the precipi-tate was dried at 393 K for 12 h, and then pre-calcinedin air at 773 K for 3 h. Furthermore, they were pressedinto disks at 600 kg/cm2, and then calcined at 1423 Kfor 20 h. Pt/Ni0.03Mg0.97O, Pt/MgO, Ni/MgO cata-lysts were prepared by impregnating the support withan acetone solution of Pt(C5H7O2)2H2O (SoekawaChemicals, >99%) or Ni(C5H7O2)2H2O (SoekawaChemicals, >99%). In the case of Pt/MgO, the load-ing is Pt/(Pt + Mg) = 3, or 0.009 mol%. In the case

of Ni/MgO, the loading is Ni/(Ni + Mg) = 3 mol%.In the case of Pt/Ni0.03Mg0.97O, the loading isPt/(Ni +Mg) = 0.009 mol%. The catalyst was driedat 393 K in air for 12 h. MgO support was preparedby the same precipitation method as NixMg1xO.Pre-calcination, pressing and calcination conditionswere the same as those of NixMg1xO. The catalystswere crushed and sieved to particles with 80150mdiameter.

2.2. Catalytic reaction

Methane reforming with CO2, H2O and O2 wascarried out in a fluidized bed reactor and a fixed bedreactor under pressurized conditions. The illustrationof the fluidized bed reactor is shown in Fig. 1. Thereactor had its quartz tube (6 mm i.d.) inside a stain-less steel tube (10 mm i.d.). A sintered quartz meshwas used as a distributor in the fluidized bed reac-tor. In the fixed bed reactor, quartz wool was put onthe catalyst bed so as to prevent catalyst particles

Fig. 1. Schematic representation of fluidized bed reactor.

K. Tomishige et al. / Applied Catalysis A: General 223 (2002) 225238 227

from moving. Pretreatment of catalysts was H2 re-duction at 1173 K for 0.5 h under hydrogen flow atatmospheric pressure. CH4 was introduced to the re-actor through the thin quartz tube, whose outlet waslocated just before the distributor. CO2, O2 and H2Owere introduced into the reactor outside the CH4 feedtube. A microfeeder was used for the introduction ofH2O to the high-pressure reaction system. The to-tal pressure was 1.0 and 2.0 MPa. The reaction tem-perature was monitored inside (TC1) and outside thereactor (TC2). The reaction temperature was usuallycontrolled by TC1 as described in Fig. 1. The reac-tion temperature was 6731173 K, and 0.2 g catalystwas used for each experiment. A space velocity wasGHSV = 19,000110,000 cm3/g h and this apparentlycorresponds to 1.48.6 cm/s at 1073 K and 1.0 MPa.The effluent gas was analyzed with an FID gas chro-matograph (GC) (column packing: Gaskuropack 54)equipped with a methanator for CH4, CO, CO2 anda TCD (column packing: Molecular Sieve 13X) wasused for H2 analysis. An ice bath was set betweenthe reactor exit and a sampling port for GC analy-sis in order to remove water in the effluent gas. CH4(99.9%), O2 (99%), CO2 (99.9%), and H2 (99%) werepurchased from Takachiho Co. Ltd., and they wereused without further purification. In all reaction tests,the oxygen conversion reached 100%. Methane con-version of 30% is due to combustion in the case ofCH4/CO2/O2 = 50/20/30. Methane conversion be-yond this 30% can be assigned to reforming.

In order to obtain heating and cooling profiles ofmethane conversion, the reactant gas was fed to thecatalyst bed without H2 pretreatment on the catalystsother than reduced Ni0.03Mg0.97O. At first, reactantgas was fed to the reactor and the pressure becomehigher, up to 1.0 MPa at room temperature. The cata-lyst was heated to 673 K. Then, the catalyst was heatedstepwise up to 1173 K from 673 K by 50 K. Further-more, it was cooled stepwise from 1173 to 673 K. Ateach temperature, the effluent gas was analyzed. Inthe case of reduced Ni0.03Mg0.97O, only the coolingprofile was investigated.

2.3. Characterization of catalysts

Chemisorption experiments were carried out inhigh-vacuum system by volumetric methods. Researchgrade gases (H2: 99.9995%, O2: 99.99%, Takachiho

Trading Co. Ltd.) were used without further purifica-tion. Before H2 and O2 adsorption measurement, thecatalysts, which had been reduced in a fixed-bed flowreactor, were treated again in H2 at 1123 K for 30 min.H2 adsorption was performed at room temperature,and the amount of O2 consumption was obtained at873 K. Gas pressure at adsorption equilibrium wasabout 26.3 kPa. The sample weight was about 0.5 g.The dead volume of the apparatus was about 30 cm3.The surface area of catalyst was measured by BETmethod with Gemini (Micromeritics).

Scanning electron microscope (SEM) observationwas carried out using S-3000N (HITACHI). The appa-ratus for energy dispersive X-ray (EDX) analysis wasequipped with SEM and EDX apparatus was obtainedfrom EDAX.

Transmission electron microscope (TEM) imageswere taken by means of JEM-2020F (JEOL) op-erated at 200 kV. Samples were dispersed in tetra-chloromethane by supersonic waves and put on Cugrids for the TEM observation in the air.

Thermogravimetric analysis (TGA) for the estima-tion of carbon amount was carried out by the magneticsuspension balances (Rubotherm). After the catalyticreaction, a part of the catalyst (ca. 50 mg) was tookout from the catalyst bed. TGA profile was measuredunder flowing air (20 ml/min) at the heating rate of30 K/min. The weight loss was observed in the tem-perature range between about 800 and 1000 K. Thiscan be assigned to the combustion of deposited car-bon. It is possible to estimate the amount of carbondeposition on the basis of this weight loss.

3. Results and discussion

3.1. Performance of Ni0.15Mg0.85O in the fluidizedand fixed bed reactors

Fig. 2 shows the comparison of CH4 conversionand the reactor temperature as a function of space ve-locity in CH4/CO2/O2 and CH4/CO2/H2O reactionsover Ni0.15Mg0.85O. The purpose of this experimentis the evaluation of the internal heat-supplying ef-fect. The reaction condition of CH4/CO2/H2O =35/35/30 corresponds to the gas composition whenmethane combustion proceeds in the reactant gasesof CH4/CO2/O2 = 50/20/30. In this experiment, the


Fig. 2. Comparison of CH4 conversion (, ) and the re-actor temperature (, ) as a function of space velocity inCH4/CO2/O2 (, ) and CH4/CO2/H2O (, ) reactions overNi0.15Mg0.85O. Partial pressure CH4/CO2/O2 = 50/20/30 andCH4/CO2/H2O = 35/35/30. Reaction conditions: total pressure1.0 MPa; H2 pretreatment 1173 K; catalyst weight 0.2 g; parti-cle size 80150m. In this experiment, the output power ofthermo-controller was kept constant during the reaction. The re-actor temperature was monitored with TC1.

output power of thermo-controller was kept constantin order to make the external heat supply constant.Methane conversion in CH4/CO2/H2O reaction is cal-culated on the basis of CH4/CO2/O2 reaction beforethe apparent combustion proceeds. In CH4/CO2/O2reaction, the temperature increased with high spacevelocity, and high methane conversion was kept. Incontrast, methane conversion and temperature de-creased with the space velocity in CH4/CO2/H2O.The difference in methane conversion and the reactortemperature becomes larger and larger with higherspace velocity. These results show that the heat wassupplied effectively into the reactor and that the con-version of methane was enhanced.

Fig. 3 shows methane conversion and H2/CO inmethane reforming with CO2 and O2 as a functionof oxygen partial pressure over Ni0.15Mg0.85O cata-lyst using the fluidized bed and fixed bed reactors.Methane conversion increased with higher oxygen par-tial pressure in both the fluidized bed and the fixedbed reactors. This space velocity is thought to be highenough to fluidize the catalyst bed judging from the re-sults shown later. In the case that oxygen partial pres-sure was zero, which corresponds to CO2 reformingof methane, the methane conversion and H2/CO ratio

Fig. 3. Methane conversion (, ) and H2/CO ratio (H17009,) in methane reforming with CO2 and O2 as a function ofoxygen partial pressure over Ni0.15Mg0.85O catalyst using thefluidized bed (, H17009) and fixed bed reactors (, ). Re-action conditions: reaction temperature 1073 K; total pressure1.0 MPa; CH4/CO2/O2 = 50/50 x/x (x = 0, 10, 20, 30);SV = 75,000 cm3/g h; H2 pretreatment 1173 K; catalyst weight0.2 g; particle size 80150m. Dotted line in methane conversioncan be due to methane combustion.

were almost the same as the values in the fluidizedand fixed bed reactors. In contrast, the fluidization en-hanced the methane conversion in the case of oxygenaddition.

Fig. 4 shows the dependence of the conversionand H2/CO ratio on the space velocity in methanereforming with CO2 and O2 using the fluidized bedand fixed bed reactor over Ni0.15Mg0.85O. At SV =19,000 cm3/g h, the conversion in the fluidized bedreactor was almost the same as that in the fixed bedreactor. This gas composition corresponds to the re-action equilibrium from the experiment under lowerspace velocity. At higher SV > 19,000 cm3/g h, CH4and CO2 conversions in fluidized bed reactor werealways higher than those in fixed bed reactor. Thisdifference indicated that the catalyst particles werefluidized enough at 37,000 cm3/g h. Therefore, it issuggested that the minimum fluidization velocity islower than 37,000 cm3/g h.

Fig. 5 shows the conversion and H2/CO ra-tio in methane reforming with CO2 and O2 overNi0.15Mg0.85O catalyst using a fluidized bed reac-tor in the reaction temperature range of 8731073 K(TC1). Methane and CO2 conversion increased withthe higher reaction temperature. At 1073973 K,


Fig. 4. Dependence of CH4 (, ) and CO2 conversion (H17009,), and H2/CO ratio (, ) on the space velocity in methanereforming with CO2 and O2 using fluidized bed (, H17009, )and fixed bed (, , ) reactor over Ni0.15Mg0.85O. Reactionconditions: reaction temperature 1073 K; total pressure 1.0 MPa;CH4/CO2/O2 = 50/20/30; H2 pretreatment 1173 K; catalyst weight0.2 g; particle size 80150m. Dotted line in methane conversioncan be due to methane combustion.

Fig. 5. Reaction temperature dependence of CH4 conversion inmethane reforming with CO2 and O2 over Ni0.15Mg0.85O cat-alyst using fluidized bed reactor. Reaction temperature: 1073 K(); 1023 K (); 973 K (H17009); 873 K (). Reaction conditions:total pressure 1.0 MPa; CH4/CO2/O2 = 50/20/30; H2 pretreatment1173 K; catalyst weight 0.2 g; particle size 80150m. Reactiontemperature was monitor with TC1. Dotted line in methane con-version can be due to methane combustion.

methane conversion decreased with higher SV andshorter contact time. On the other hand, methane andCO2 conversion increased slightly with SV at 873 K.In this case, at higher space velocity, larger amount ofoxygen is supplied together with methane and the re-action heat of methane combustion is supplied to thereactor. Therefore, the catalyst bed is heated and highconversion is obtained even at short contact time. Thetemperature of TC1 was almost constant, but that ofTC2 became a little higher with higher SV. TC1 con-tains the exothermic factor by the combustion and theendothermic factor by the reforming. Even at 873 K,the fluidization is effective in methane reforming withCO2 and O2.

The effect of the H2O addition to the reactant gasesis listed in Table 1. Methane conversion was almostconstant at various H2O/CO2 ratios. The H2/CO ratiowas dependent on the partial pressure of H2O and wasin the range between 1.4 and 2.5. This indicated thatthe composition of produced syngas can be controlledby using a mixture of CO2 and H2O.

3.2. Catalyst characterization of Ni0.15Mg0.85Obefore and after the reaction

Fig. 6 shows CH4 and CO2 conversion and H2/COratio in the product as a function of time-on-streamin methane reforming with CO2 and O2 overNi0.15Mg0.85O under 2.0 MPa at 1023 K. Methaneand CO2 conversion in the fixed bed reactor wasslightly higher than those in fluidized bed reactor.This is because the higher total pressure causes morecontribution of methane oxidation in the gas phase,and less contribution of methane combustion on thecatalyst surface. The difference in methane conversionwas so small. This is because the amount of reducedNi0.15Mg0.85O catalyst, which has the reforming ac-tivity, was in similar for each of the two reactors asdiscussed later.

Fig. 7 shows the TGA results of Ni0.15Mg0.85O cat-alyst after the reaction (Fig. 6) in the fluidized andfixed bed reactor. No weight loss was observed onthe catalyst in fluidized bed reactor. In contrast, theweight loss due to carbon combustion (8501050 K,130 mg/gcat carbon) was clearly observed on the cat-alyst in the fixed bed reactor. This indicates that thefluidized bed reactor inhibited the carbon depositionduring the reaction.


Table 1Partial pressure dependence of CO2 and H2O in methane reforming with CO2, H2O and O2 in fluidized bed reactora

Partial pressure (MPa) CH4 conversion (%)b Formation rate (mol/s) H2/COCH4 O2 CO2 H2O CO H2

0.5 0.3 0.2 0.0 78 111 156 1.40.5 0.3 0.1 0.1 79 93 182 2.00.5 0.3 0.0 0.2 76 74 184 2.5

a Reaction conditions: Ni0.15Mg0.85O 0.2 g; particle size 80150m; total pressure 1.0 MPa; SV = 56,000 cm3/g h; H2 reduction at1173 K.

b 30% of methane conversion is assigned to methane combustion.

Fig. 8 shows the SEM image of Ni0.15Mg0.85O af-ter the reaction (Fig. 6). On Ni0.15Mg0.85O after thereaction in fixed bed reactor, a lot of whisker carbonwas observed. EDX analysis detected the large signalassigned to carbon. On several particles, whisker car-bon and the signal of carbon by EDX were observed.In contrast, there were some particles on which neitherwhisker carbon nor EDX signal assigned to carbonwere observed. TGA result supports that large amountof carbon was totally deposited on the catalyst. On theother hand, from SEM and EDX analysis, there weretwo kinds of catalyst particles in the fixed bed reac-tor. One is the catalyst particles with carbon and theother is the catalyst particles without carbon. This is

Fig. 6. Change of CH4 (, ) and CO2 (, ) conversion andH2/CO (H17009, ) with time-on-stream in methane reforming withCO2 and O2 over Ni0.15Mg0.85O. Fluidized bed reactor (,H17009,);fixed bed reactor (, , ). Catalyst weight 0.2 g; particle size80150m; temperature 1023 K; total pressure 2.0 MPa; reactantCH4/CO2/O2 = 50/20/30; GHSV = 75,000 cm3/g h. Dotted linein methane conversion can be due to methane combustion.

Fig. 7. Thermogravimetric analysis of Ni0.15Mg0.85O after the re-action shown in Fig. 6 using fluidized bed (solid line) and fixedbed reactor (dashed line). Reaction temperature 1023 K; total pres-sure 2.0 MPa; CH4/CO2/O2 = 50/20/30; H2 pretreatment 1173 K;catalyst weight 0.2 g; particle size 80150m. TGA condition:heating rate 30 K/min; air flowing; sample weight 50 mg.

probably because the catalyst bed was divided to twoparts. One part is in the reducing atmosphere, and theother is in the oxidizing atmosphere.

On Ni0.15Mg0.85O after the reaction in fluidized bedreactor, whisker carbon was not observed and the EDXsignal due to carbon was not detected. This indicatedthat fluidized bed reactor inhibited the carbon depo-sition in methane reforming with CO2 and O2 evenunder highly pressurized condition. According to theSEM observation, the image of Ni0.15Mg0.85O afterthe reaction in fluidized bed reactor was almost thesame as that of Ni0.15Mg0.85O after H2 reduction.

On the catalyst after the reaction under 1.0 MPa at1073 K, serious carbon deposition was not observedeven in either reactor. This is because the rate of carbon


formation is lower at higher reaction temperature andlower total pressure.

Fig. 9 shows the TEM image of Ni0.15Mg0.85Obefore and after the reaction (Fig. 6). Ni metal par-ticles located on NixMg1xO were observed. Theaverage diameter of Ni particles on Ni0.15Mg0.85Oafter H2 reduction was estimated to be 333 nm. Thedistribution of the particle size is quite sharp. Thisvalue agrees with that estimated from the results ofhydrogen chemisorption. On Ni0.15Mg0.85O, after thereaction in the fixed bed reactor, a lot of whisker

Fig. 8. SEM image of Ni0.15Mg0.85O catalyst and the results of EDX analysis: (a) Ni0.15Mg0.85O after the reaction in fixed bed reactor;(b) Ni0.15Mg0.85O after the reaction in fluidized bed reactor; (c) EDX analysis of (a); (d) EDX analysis of (b). Reaction conditions arethe same as described in Fig. 6. The field of EDX analysis is just the same as SEM image.

carbon was observed and the whiskers had variousdiameters. In contrast, on Ni0.15Mg0.85O after thereaction in fluidized bed reactor, no whisker carbonwas observed. The distribution of Ni particle size israther broad (45100 nm). The average particle sizewas estimated to be 68 5 nm. This indicates thatthe aggregation of Ni metal particles proceeded dur-ing the reaction. This phenomenon can be causedby going through the region where methane com-bustion proceeds and the temperature is very high[5].


Fig. 8. (Continued).


Fig. 9. TEM image of Ni0.15Mg0.85O catalyst: (a) Ni0.15Mg0.85O after H2 reduction at 1173 K for 0.5 h; (b) Ni0.15Mg0.85O after thereaction (Fig. 8) in fixed bed reactor; (c) Ni0.15Mg0.85O after the reaction (Fig. 8) in fluidized bed reactor. Reaction conditions are thesame as described in Fig. 6.

3.3. Performance of other catalysts in the fluidizedand fixed bed reactors

Fig. 10 shows the conversions and H2/CO ratioin the product as a function of the space velocityin methane reforming in the fluidized bed and fixedbed reactors over MgO-supported Ni and Pt catalysts.Methane conversions on 3 mol% Ni/MgO and 3 mol%Pt/MgO were almost the same in the fluidized andfixed bed reactors. The difference between these tworeactors in the conversion on these supported catalystswas much smaller than that on Ni0.15Mg0.85O catalyst.Generally speaking, the difference between the flu-

idized bed and the fixed bed reactors is caused mainlyby heat transfer effects. However, in our case, the re-actor size is small enough to transfer the heat to all theparts of the catalyst bed in both reactors. If the heattransfers in the fluidized bed reactor more effectivelythan in the fixed bed reactor, the conversion must beenhanced by catalyst fluidization on 3 mol% Ni/MgOand 3 mol% Pt/MgO. However, on both catalysts, theeffect was quite small. In addition, the temperatureof TC1 and TC2 were not so different in each exper-iment with various catalysts in both reactors. Theseresults support that the effect of the heat transfer didnot cause any different behavior in the conversion,


Fig. 10. Dependence of CH4 (, ) and CO2 conversion (H17009,), and H2/CO ratio (, ) on the space velocity in methanereforming with CO2 and O2 using fluidized bed (, H17009, )and fixed bed (, , ) reactor MgO-supported Ni and Ptcatalysts: (a) 3 mol% Ni/MgO; (b) 3 mol% Pt/MgO. Reactionconditions: reaction temperature 1073 K; total pressure 1.0 MPa;CH4/CO2/O2 = 50/20/30; H2 pretreatment 1173 K; catalyst weight0.2 g; particle size 80150m. Dotted line in methane conversioncan be due to methane combustion.

and the conversion can be dependent on the catalystproperties.

Catalytic performances in methane reforming withCO2 and O2 using fluidized bed reactor and that inCO2 reforming of methane using fixed bed reactor arelisted in Tables 2 and 3, respectively. Methane conver-sion in these reactions did not reach the equilibriumconversion. On NixMg1xO catalysts, the conversionincreased with higher Ni content in methane reform-ing with CO2 and O2. In contrast, the difference in theconversion among these catalysts was not so signif-

Table 2Catalytic performance in methane reforming with CO2 and O2using fluidized bed reactora

Catalyst Conversion (%) H2/COCH4b CO2

Ni0.15Mg0.85O 78 3 1.4Ni0.07Mg0.93O 67 14 1.4Ni0.03Mg0.97O 59 20 1.3Pt/Ni0.03Mg0.97Oc 66 11 1.43 mol% Ni/MgO 64 19 1.43 mol% Pt/MgO 57 26 1.3Pt/MgOd 39 45 0.7MgO 38 57 1.1

a Reaction conditions: reaction temperature 1073 K; total pres-sure 1.0 MPa; CH4/CO2/O2 = 50/20/30; SV = 75,000 cm3/g h;H2 pretreatment 1173 K; catalyst weight 0.2 g; particle size80150m.

b 30% of methane conversion is assigned to methane combus-tion. The loading of Pt.

c Pt/(Ni+Mg) = 0.009 mol%.d Pt/Mg = 0.009 mol%.

icant in CO2 reforming of methane. The addition ofPt to Ni0.03Mg0.97O promoted both reactions, whilethe small amount of Pt supported on MgO exhibitedlittle catalytic activity for methane reforming. This issupported by our previous results in the reforming un-der atmospheric pressure [15,16]. On MgO-supportedNi and Pt catalysts, the conversion was lower thanNi0.15Mg0.85O and Ni0.07Mg0.93O in both reactions.This indicates that Ni0.15Mg0.85O is an effective cat-alyst for methane reforming with CO2 and O2.

Table 3Catalytic performance in CO2 reforming of methane using fixedbed reactora

Catalyst Conversion (%) H2/COCH4 CO2

Ni0.15Mg0.85O 57 75 0.75Ni0.07Mg0.93O 58 75 0.74Ni0.03Mg0.97O 51 69 0.69Pt/Ni0.03Mg0.97Ob 55 65 0.843 mol% Ni/MgO 50 62 0.793 mol% Pt/MgO 40 50 0.78Equilibrium 67 83 0.88

a Reaction conditions: reaction temperature 1123 K; total pres-sure 1.0 MPa; CH4/CO2 = 50/50; SV = 56,000 cm3/g h; H2 pre-treatment 1173 K; catalyst weight 0.2 g; particle size 80150m.

b The loading of Pt: Pt/(Ni+Mg) = 0.009 mol%.


Table 4Properties of NixMg1xO solid solution and MgO-supported catalysts

Catalyst Ni/(Ni + Mg)(%)

Pt/(Ni + Mg)(%)

BET(m2/g)

H2a(mol/g)

O2b Dredc(%)

Ddispd(%)

Particle size (nm)Adsorptione TEMf

Ni0.15Mg0.85O 15 0 3 1.1 38.4 2.3 2.8 34.6 33 3Ni0.07Mg0.93O 7 0 3 0.5 13.7 1.7 3.5 27.7 Ni0.03Mg0.97O 3 0 4 0.3 6.1 1.7 4.6 21.1 Pt/Ni0.03Mg0.97O 3 0.009 4 0.3 6.5 1.8 4.0 24.3 3 mol% Ni/MgO 3 0 4 1.1 118.1 31.7 0.9 107.8 3 mol% Pt/MgO 0 3 4 1.7 100f 0.2

a Chemisorption experiment: H2 consumption at 298 K.b Chemisorption experiment: O2 consumption at 873 K.c Reduction degree: 2 (O2 consumption)/(total Ni), assuming that Ni0 + (1/2)O2 NiO.d Dispersion of reduced Ni particles: amount ratio of H2 consumption to O2 consumption, assuming H/Nis = 1 and total reduced

Ni = 2 (O2 consumption).e Calculated by adsorption/dispersion = 971/10 [20].f It is assumed that all Pt atoms are reduced.

Catalyst properties of NixMg1xO and MgO-supported catalysts are listed in Table 4. BET surfacearea of the catalysts was almost the same. The amountof H2 consumption corresponds to the amount of sur-face Ni metal atoms (Nis) on the basis of H/Nis = 1.The amount of O2 consumption was measured at873 K, where Ni metal can be oxidized to Ni2+, andthis corresponds to the amount of Ni reduced on thebasis of O/Ni0 = 1. These data give the reduction de-gree and the dispersion of Ni metal particles of the cat-alysts. NixMg1xO catalysts with higher Ni contentexhibited the higher reduction degree and lower Nidispersion. This tendency is also true for NixMg1xOcatalysts with higher BET surface area [17]. In termsof the reduction degree, supported Ni catalysts hadmuch higher values than NiO-MgO solid solutioncatalysts, and they had much lower dispersion.

Fig. 11 shows the response for heating and cool-ing in methane reforming with CO2 and O2 us-ing the fluidized bed reactor. On Ni0.15Mg0.85Oand Ni0.03Mg0.97O catalysts, methane combustionstarted at 823 K and methane reforming started at1023 K in the heating process. However, the con-version on Ni0.15Mg0.85O increased with the re-action temperature more significantly than that onNi0.03Mg0.97O. On the other hand, Ni0.03Mg0.97Oreduced with hydrogen at 1173 K obtained the con-version as high as Ni0.15Mg0.85O. As reported pre-viously, Ni0.03Mg0.97O exhibited low reducibility,and H2 reduction at high temperature (>1073 K)

generated the catalytic activity for steam reform-ing of methane [10]. Therefore, low conversion onNi0.03Mg0.97O must be due to low reducibility ofthe catalyst. Ni0.15Mg0.85O shows higher reducibilitythan Ni0.03Mg0.97O [17]. In addition the conver-sion on Ni0.15Mg0.85O in the heating process wasmuch lower than that in the cooling process. Thisis due to the difference in the reduction degree ofthe catalysts. While the catalyst is reduced graduallywith methane and/or produced syngas in the heat-ing process, the catalyst in the cooling process canhave higher reduction degree. The conversion dif-ference between the heating and cooling processesindicate that the reducibility of Ni0.15Mg0.85O isnot so high. In the profiles on 3 mol% Ni/MgO,methane combustion started at 823 K, and this tem-perature was the same as that on Ni0.03Mg0.97O andNi0.15Mg0.85O. However, the reforming started at973 K, and this temperature was 100 K lower. Thisindicates that 3 mol% Ni/MgO has higher reducibil-ity. Furthermore, the similar conversion was observedin the heating and cooling processes. This also in-dicates the high reducibility of 3 mol% Ni/MgO. Inthe profiles on 3 mol% Pt/MgO, methane combus-tion and reforming has started even at 673 K. Thisindicated that Pt has much higher activity of combus-tion than Ni, and Pt also has higher reducibility. Asimilar conversion was also observed in the heatingand cooling profiles on 3 mol% Pt/MgO, and theseindicate very high reducibility of the catalyst. We


Fig. 11. The heating and cooling response of methane conversion in methane reforming with CO2 and O2 over various catalysts: (a)Ni0.15Mg0.85O (); (b) Ni0.03Mg0.97O (), Ni0.03Mg0.97O with H2 pretreatment at 1173 K for 30 min (); (c) 3 mol% Ni/MgO (),3 mol% Pt/MgO (); (d) Pt/Ni0.03Mg0.97O (Pt/(Ni + Mg) = 0.009 mol%) (). Reaction conditions: catalyst weight 0.2 g; particle size80150m; total pressure 1.0 MPa; GHSV = 56,000 cm3/g h; CH4/CO2/O2 = 50/20/30; fluidized bed reactor. Catalysts were used withoutthe hydrogen pretreatment, except Ni0.03Mg0.97O with H2 pretreatment at 1173 K. Dotted line in methane conversion can be due to methanecombustion.

investigated the additive effect of Pt to Ni0.03Mg0.97O.The addition of a small amount of Pt (molar ratioPt/Ni = 3 103) to Ni0.03Mg0.97O changed theprofiles drastically. Methane combustion started at773 K on Pt/Ni0.03Mg0.97O and this temperature was50 K lower than that on Ni0.03MgO0.97O, and thereforming also started at 773 K on Pt/Ni0.03Mg0.97O.Furthermore, the conversion in the heating and cool-ing profiles was almost the same. This indicated thatthe catalyst reducibility was drastically enhanced bythe addition of Pt. This agrees with our previous re-port [16]. On Pt/Ni0.03Mg0.97O, Pt is already reducedat 773 K, and reduced Pt promotes the reduction of Nivia hydrogen spillover. This is supported by the basicdata reported in [16]. This bimetallic PtNi system

is the effective catalyst for the methane reformingwith CO2 and O2 as listed in Table 2. It has beenreported that the synergistic effects of Pt and NiOwere observed in methane oxidation using fluidizedbed reactor under atmospheric pressure [18]. In addi-tion, it has been reported that PtNi alloy was formedon Pt/Ni0.03Mg0.97O [16]. Considering a comparisonbetween these profiles and the fluidization effect, weconcluded that the conversion in methane reformingwith CO2 and O2 was not enhanced by the fluidiza-tion on a catalyst which showed similar behavior inheating and cooling profiles. In contrast, the conver-sion was enhanced by the fluidization on the catalystwhich showed the higher conversion in the coolingprofile than that in the heating one. This indicates that


the fluidization effect is closely related to the catalystreducibility.

3.4. The effect of fluidization

It has been reported that the fixed bed consistsof two different regions in the partial oxidation ofmethane [11]. In one region, where oxygen is present,the catalyst is in the oxidized state. In the otheroxygen-free region, the catalyst is in the reduced state.This agrees with the results that there are two kinds ofcatalyst particles suggested by SEM and EDX analy-ses. The catalyst particles with no deposited carbonwould be in the region where oxygen is present, andthe catalyst particles with a lot of carbon would bein the oxygen-free region. The reforming proceedson the reduced Ni and the carbon is deposited on thereduced catalyst. There are also oxygen-free zone andoxygen-containing zone in the fluidized bed reactor.When methane conversion using the fluidized bedand fixed bed reactors is almost the same, one mayassume that the amount of catalyst in each zone isalso the same in both reactors.

Such results indicate that the conversion wasalmost the same in fixed bed and fluidized bed reactor

Fig. 12. A model of fluidized bed reactor in methane reforming with CO2 and O2.

on Ni/MgO and Pt/MgO. These catalysts exhibitedmuch higher reducibility than NixMg1xO catalysts.Since the catalyst has high reducibility and methanecan reduce the catalyst, it seems that the catalystin oxygen-containing zone can be present in the re-duced state to some extent. It is known that Pt metalis very stable under oxygen atmosphere [19], andthis can explain that the catalyst fluidization doesnot enhance the methane conversion. Furthermore,since the reduction degree of catalyst is considerablyhigh on Ni/MgO, it is suggested that the catalyst inoxygen-containing zone can be in the reduced statelike Pt/MgO. On the catalysts with high reducibility,the amount ratio of the catalyst in the reduced stateto that in the oxidized state is high. In contrast, onthe catalysts with lower reducibility, the amount ratiobecomes lower. In this case, more catalyst is oxidizedby the fluidization. On the catalysts with mediumreducibility, the amount ratio is at the medium level.

The fluidization causes the re-reduction of oxidizedcatalyst and regenerates the active site for the methanereforming. This is because the circulation of the cat-alyst particles makes it possible to carry the oxidizedcatalyst to the reduction zone (upper part of the bed)and the reverse. A model of fluidized bed reactor is


depicted in Fig. 12. In fixed bed reactor, oxidized cat-alyst cannot be re-reduced because of no movement.

In terms of carbon deposition, the effect of catalystfluidization is significant. Fluidization inhibited thecarbon deposition in methane reforming as shown inFig. 7. The deposited carbon is formed on the cat-alyst in oxygen-free reforming zone, and it movesto oxygen-rich zone by catalyst fluidization. The de-posited carbon also reacts with oxygen and is gasified.The TGA results show that the carbon can react withoxygen at the reaction temperature and the highertemperature zone easily. When the rate of carbondeposition is higher than the rate of carbon removal,carbon is accumulated even in this reactor. There-fore, one should use the catalyst on which carbonformation rate is low.

4. Conclusions

The effect of catalyst fluidization is found to be de-pendent on the catalyst reducibility. The enhancementof conversion in methane reforming with CO2, H2Oand O2 is caused by the fluidization on the catalystwhose reducibility is at a suitable level. Fluidizationhas quite a favorable effect on the inhibition of car-bon deposition. This is probably because the catalystparticles are circulated between the oxidizing and thereducing zone and the carbon gasification proceeds inthe oxidizing zone. When the carbon deposition rateon the catalyst is not high, carbon-free operation ispossible under pressurized condition in the fluidizedbed reactor.

Acknowledgements

A part of this research has been supportedby the Future Program of Japan Society for the

Promotion of Science under the Project Synthesisof Ecological High Quality of Transportation Fuels(JSPS-RFTF98P01001).

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Comparative study between fluidized bed and fixed bed reactors in methane reforming combined with methane combustion for the internal heat supply under pressurized conditionIntroductionExperimentalPreparation of catalystsCatalytic reactionCharacterization of catalysts

Results and discussionPerformance of Ni0.15Mg0.85O in the fluidized and fixed bed reactorsCatalyst characterization of Ni0.15Mg0.85O before and after the reactionPerformance of other catalysts in the fluidized and fixed bed reactorsThe effect of fluidization

ConclusionsAcknowledgementsReferences

comparative study between fluidized bed and fixed bed reactors

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