International Journal of Hydrogen Energy 32 (2007) 3315–3319www.elsevier.com/locate/ijhydene

Semi-continuous hydrogen production from catalytic methanedecomposition using a fluidized-bed reactor

Naresh Shah∗, Shankang Ma, Yuguo Wang, Gerald P. HuffmanUniversity of Kentucky, 533 S. Limestone Street, Suite 107, Lexington, KY 40508, USA

Received 25 August 2006; received in revised form 18 April 2007; accepted 22 April 2007Available online 20 June 2007

Abstract

Non-oxidative, catalytic decomposition of hydrocarbons is an alternative, one-step process to produce pure hydrogen with no production ofcarbon oxides or higher hydrocarbons. Carbon produced from the decomposition reaction, in the form of potentially valuable carbon nanotubes,remains anchored to the active catalyst sites in a fixed bed. To facilitate periodical removal of this carbon from the reactor and to make hydrogenproduction continuous, a fluidized-bed reactor was envisioned. The hypothesis that the tumbling and inter-particle collisions of bed materialwould efficiently separate nanotubes anchored to the active catalyst sites of the bed particles was tested and shown to be invalid. However, aswitching mode reaction system for the semi-continuous production of hydrogen and carbon nanotubes by periodic removal and replenishmentof the catalytic bed material has been successfully demonstrated.� 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Hydrogen production; Non-oxidative; Catalytic; Methane decomposition; Carbon nanotubes; Switching mode; Fluidized-bed reactor

1. Introduction

Previously, we have developed alumina supported binary cat-alysts, (M–Fe)/Al2O3 (M=Pd, Mo, or Ni), that are very activefor non-oxidative, catalytic dehydrogenation of lower alkanesto produce pure hydrogen and carbon nanotubes (CNTs) [1–3].Nano-sized, alumina supported, binary catalysts are capable ofhigh conversion of gaseous lower alkanes to hydrogen and car-bon at temperatures substantially lower than the equilibriumthermal decomposition temperature. One major setback in suchlaboratory scale experiments has been accumulation of CNTs,a potentially valuable byproduct, within the reactor. In a fixed-bed mode of operation, this fouling due to CNT tangling andaccumulation starts hindering the flow of the reactant and prod-uct gas streams, builds back pressure and reduces activity.

Before the multi-walled CNTs can be used as a valuablebyproduct, it is essential that they are of high purity. Typically,the nanotubes produced by both the arc method and chemi-cal vapor deposition (CVD) method (used for this research)

∗ Corresponding author. Tel.: +1 859 257 4027; fax: +1 859 257 7215.E-mail address: naresh@uky.edu (N. Shah).

0360-3199/$ - see front matter � 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2007.04.040

contain substantial amounts of catalyst particles and non-nanotubes (amorphous) carbon. To purify such as-producednanotubes, usually one or more chemical purification steps,such as acid wash, alkali wash, and air/oxygen/CO2 exposure,are necessary. These treatments are performed at completelydifferent conditions than the reaction conditions used to pro-duce nanotubes and hence have to be carried out in a separateex situ batch process. One possible way to dislodge nanotubesanchored to the catalyst particles on the surface of the aluminasupport in situ is by mechanically breaking the nanotubes atthese linkages by physical agitation. Fluidization of the bedshould induce turbulent flow of the catalytic bed media andrepeated inter-particle collisions that could shear nanotubesfrom the active metallic catalysts. Once freed, fine nanotubesshould be carried away by the fluidizing gas stream to be exter-nally collected and the catalyst should continue to decomposemethane, producing additional hydrogen and CNTs. In thispaper, we report results from the tests carried out to validatethe aforementioned hypothesis.

Most of the hydrogen production work using fluidized-bedreported in the literature concern with the production of syngas[4–7]. When used for the production of CNTs, fluidized-bedreactors are typically fed with higher carbon content gases and

3316 N. Shah et al. / International Journal of Hydrogen Energy 32 (2007) 3315–3319

not methane [8–11]. Qian et al. [12] claim better performancein a system which employs combined catalyst reduction andmethane decomposition in a fluidized-bed reactor over a sep-arated process. Qian et al. [13] have also employed two-stagefluidized-bed reactor system operating at two different temper-atures for production of hydrogen and CNTs from methanedecomposition. More recently, Lee et al. [14], Muradov et al.[15], and Dunker et al. [16] have used carbon based fluidized-bed material for thermal decomposition of methane. The carbonproduced from methane decomposition is allowed to accumu-late within the pores as well as on and around the particles ofbed material with net increase in the bed material and dilutionof the active catalytic sites.

2. Experimental

Fig. 1 is a schematic diagram of the fluidized-bed reactor.Gas handling and gas analysis system have been described pre-viously [1]. It has a large diameter reactor tubing (for highmethane flow rate required for fluidization) and a quartz fritfused to the quartz wall of the reactor (for catalyst support).Because there is no thermal methane decomposition at or belowreactor temperature of 700 ◦C, the pores in the quartz frit werenever plugged with carbon. By increasing the methane flow ratefrom 10 ml/min to > 100 ml/min, the reactor mode can be var-ied from fixed-bed to fluidized-bed. At high methane flow rate(∼15 l/min), the catalyst bed can be fluidized very vigorouslyand expanded completely out of the length of the tube to empty

22 mm

Three-zone furnace

& temperature

control system

14 inches

14 inches

Metering

Value1.0 g catalyst powder

(75–150 µm)

2 feet

Quartz frit

Fluidized–bed reactor

Two–way valve

Gas outGC

Methane

Fig. 1. Schematic diagram of the new fluidized-bed reactor.

the reactor of the catalyst and attached CNTs. Using a two-wayvalve, new catalyst can then be added to the reactor from thetop at low or zero methane flow rate.

For all fluidized-bed reactor experiments, 1 g of as prepared(0.5%Mo–4.5%Fe)/�-Al2O3 catalyst [1] was used at the reactortemperature of 650.700 ◦C. To achieve good fluidization, thecatalyst fluidized-bed particle size was kept between 75 and150 �m.

3. Results

Fig. 2 shows volume percent hydrogen in the reactor exitstream as a function of time on stream for different methanefeed flow rates. With increasing methane flow rate, hydro-gen production efficiency decreases as methane-catalyst con-tact time decreases.

To compare efficiency of the hydrogen production, we haveto compare areas under individual curves in Fig. 2. As shown inTable 1, the reactor is most efficient in producing hydrogen atthe lowest flow rate and highest reactant-catalyst contact time.As the flow rate increases, some of the methane just passesthrough the voids in the catalyst bed without coming in contactwith any active metallic catalyst sites. At even higher flow rates,more methane acts simply as a carrier or fluidizing medium andnot a reactant. As mentioned earlier, methane decompositionunder these reaction conditions is purely a catalytic reactionand if a methane molecule does not come in direct contact withan active catalytic site, it will not undergo decomposition.

N. Shah et al. / International Journal of Hydrogen Energy 32 (2007) 3315–3319 3317

0

10

20

30

40

50

60

70

0 100 200 300 400 500 600 700 800

Time (minute)

Hydro

gen V

olu

me %

15 ml/min

35 ml/min

55 ml/min

75 ml/min

95 ml/min

115 ml/min

200 ml/min

Fig. 2. Time-on-stream hydrogen production at different methane flow rates.

Table 1Hydrogen production efficiency at different methane flow rates and differentreactor modes

Methane flowrate (ml/min)

Linearvelocity(m/s)

Total methaneinput (ml)in 800 min

Total hydrogenproduced (ml)in 800 min

Efficiency(H2/CH4) (%)

Fixed bed mode15 2.37 12 000 9210 7735 5.52 28 000 4830 1755 8.68 44 000 3350 875 11.84 60 000 3110 5

Fluidized bed mode95 14.99 76 000 2830 4

115 18.15 92 000 2000 2200 31.57 160 000 1790 1

Based on these observations, there is no real advantage ofoperating the reactor in the fluidized-bed mode. Usually, thefluidized-bed mode is used to improve mass and heat transferin a reactor. In our small bench-scale reactor, there is essen-tially no mass or heat transfer limitation in the fixed-bed modeand by fluidizing the bed, more methane just passes throughthe reactor without reacting to produce hydrogen. However, itis worth reflecting that the main reason for operating the re-actor in a fluidized-bed mode was not to improve the catalystactivity. Rather, the tumbling and collisions of bed particles ina fluidized-bed was hypothesized to cause separation of nan-otubes from the anchoring metal catalyst sites. The loss in ac-tivity on hydrogen production is due to the encapsulation of thebinary Mo–Fe particles by graphitic layers. So, if the bed ma-terial can be agitated sufficiently to cause detachment of CNTsfrom the binary Mo–Fe catalyst particles, additional methanecan come in contact with the active catalyst sites and the cata-lyst can continue to participate in methane decomposition. Toachieve both high hydrogen production efficiency and separa-tion of nanotubes, a periodic mode switching experiment wasperformed.

In this periodic mode switching experiment, the reactor wasmostly operated in the high efficiency, low flow rate, fixed-bedmode for generation of hydrogen. At regular intervals, the flow

0

10

20

30

40

50

60

0 50 100 150 200 250 300

Time (minute)

Hyd

rog

en

Vo

lum

e % Switching mode

Fixed-bed mode

Fig. 3. Comparison of switching mode and fixed-bed mode hydrogen pro-duction by catalytic methane decomposition.

Fig. 4. TEM image of typical carbon product of catalytic methane decom-position reactions.

rate was increased for short durations, at the penalty of low hy-drogen production efficiency, to fluidize the bed and thereby toseparate nanotubes from the bed material to open up the activecatalyst sites and to remove these separated nanotubes fromthe reactor. Fig. 3 compares hydrogen production for a fixed-bed mode operation and a periodic mode switching operation.It indicates that after short fluidization, the active catalyst sitesdo not regain their catalytic activity because of separation andculling of the CNTs.

A typical TEM image of the final CNTs product in theperiodic mode switching methane decomposition reaction(50 ml/min, 700 ◦C) shows that the catalyst (support and metalparticles) and CNTs remain bound together (Fig. 4). CNT

3318 N. Shah et al. / International Journal of Hydrogen Energy 32 (2007) 3315–3319

0

5

10

15

20

25

30

35

40

45

0 200 400 600

Time (minute)

Hyd

rog

en

Vo

lum

e %

Fig. 5. Semi-continuous hydrogen production by cyclic blowing out the cata-lyst bed with attached nanotubes and replenishing the reactor with completelynew catalytic bed material at 650 ◦C.

agglomerates were formed because CNTs were mechanicallyintertwining during the growth of CNTs, which were more like“spaghetti”. It is worth noting that classic inter-particle ag-glomeration due to sintering did not happen in the reactor. Thelonger and straighter the CNTs get, the more perfect CNTs areformed with increased reaction time. This leads to the volumeexpansion of the CNT agglomerates. The hypothesis of thetumbling and collisions of bed particles in the fluidized-bedreactor to cause separation of nanotubes from the anchoringmetal catalyst sites by using fluidization therefore appears to beinvalid. It also indicates that there is a very strong interactionbetween the CNTs and catalysts.

During initial shakedown experiments of the new reactor set-up at different methane flow rates, we had noticed that at thevery high methane flow rate, the catalyst bed can be fluidizedvery vigorously and expanded completely out of the length ofthe tube. Since regeneration of the catalysts by agitation clean-ing of CNTs during fluidization was found to be unpractical,the option of using a high fluidization flow rate (∼15 l/min)to blow CNTs and spent catalysts out of the reactor was ex-plored. As a result, all the catalyst bed material can be ejectedout of the reactor almost instantaneously at the operating tem-perature just by increasing the flow rate. Fig. 5 shows the hy-drogen production for four such replenishing/complete blowoutcycles. Blowout of the catalyst and attached nanotubes at highmethane flow rate and replenishing of the reactor with freshas-prepared (unreduced) catalyst at zero methane flow rate wasdone at the reaction temperature in less than 2 min of time. Thecatalyst gets reduced to metallic state and is activated in situin 20–30 min at 50 ml/min methane flow rate before startingto decompose methane to produce hydrogen and CNTs. Whenthere is a noticeable decrease in hydrogen production from thepeak hydrogen production, the methane flow rate is increasedto eject out still active catalyst and attached CNTs and the cy-cle is repeated. When the catalyst is blown out, it is still active.In these experiments, no attempt was made to recycle this cata-lyst. If recycled, such catalyst will possibly have much shorter

induction time before it again starts to produce hydrogen andCNTs by methane decomposition as it has already been reducedto metallic state.

4. Conclusion

To overcome the fouling of the fixed-bed reactor by CNTsin methane dehydrogenation and to make hydrogen and nan-otube production continuous, fluidized-bed mode and periodicmode switching catalytic decomposition experiments were per-formed. These experiments showed that the hydrogen produc-tion is most efficient only in a fixed-bed mode at low methaneflow rates. The results also negated the validity of the hypoth-esis that the tumbling and inter-particle collisions of bed mate-rial would efficiently separate nanotubes anchored to the cata-lyst bed particles. A periodic mode switching reaction systemfor semi-continuous production of hydrogen and CNTs by cat-alytic decomposition of methane has been developed. In the“production cycle” of the switching mode reaction, methanewas decomposed to hydrogen and CNTs over the catalyst ata low flow rate for periods of approximately 100 min. In the“maintenance cycle”, the catalyst and CNT are blown out ofthe reactor with 15 l/min of methane flow rate and new catalystis injected at zero methane flow rate and is reduced and acti-vated in situ at a low methane flow rate in about 25–30 min.Semi-continuous hydrogen production was achieved over sev-eral cycles by changing methane flow rates while maintainingreactor at the constant operating temperature.

Acknowledgments

This work was supported by the U.S. Department ofEnergy through the Office of Fossil Energy (FE), NationalEnergy Technology Laboratory (NETL), under Contract No.DE-FC26-02FT41594.

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