ELSEVIER Catalysis Today 21 ( 1994) 48 1488

catalysis today

Partial oxidation of methane to carbon monoxide and hydrogen in a fluidized bed reactor

A. Santos, M. Menkndez, J. Santamaria * Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain

Abstract

A fluidized bed reactor has been used to investigate the partial oxidation of methane to syngas over a NiO/NiAlzO, catalyst. The axial concentration profiles of the different species along the bed have been determined and related to the observed temperature profiles. The amount of carbon deposited on the catalyst under different operating conditions has also been determined. The results show that under suitable conditions, the bubbling fluidization regime gives a stable reactor performance with

high conversion and selectivity, and a limited amount of carbon formation.

1. Introduction

The partial oxidation of methane to syngas likely involves the combustion of part of the methane to CO and CO2 followed by steam and CO2 reforming of the remaining methane. Unlike the conventional steam reforming process, the partial oxidation to syngas is mildly exothermic as a whole, and therefore does not require an expensive heat input.

In spite of the mild exothermicity of the process, large temperature gradients can be generated in fixed bed reactors because of the high reaction rate [ 1,2]. The issue of temperature gradients within the bed has given rise to considerable experimental difficulties when trying to report kinetic data. Thus, for instance, Choudhary et al. [ 3-51 reported the formation of syngas with good yields at low temperature on a variety of catalysts. However, as Dissanayake et al. [ 61, and Chang and Heinemann [7] later reported, the results obtained by Choudhary seem to be the result of undetected hot spots within the bed.

Another problem, which can be especially important if Ni-based catalysts are used in the process, is the deactivation of the catalyst by carbon formation. Thus,

* Corresponding author.

SDI0920-5861(94)00113-8

482 A. Santos et al. /Catalysis Today 21 (1994) 481488

Claridge et al. [ 81 found both encapsulate and whisker carbon on commercial Nickel catalysts during syngas production. Vernon et al. [ 91 also reported a fast decline in the methane conversion and syngas selectivities as carbon (up to 40%) was deposited on Ni/A1,03 catalysts.

The existence of hot spots as a result of a very fast reaction rate obviously poses important problems for the operation of industrial reactors. Because of its heat transfer characteristics, the fluidized bed reactor is in principle a suitable device to handle this process. The use of the solid catalyst as an internal heat carrier in the bed should provide a more homogeneous reactor temperature. In addition, the movement of particles within the fluidized bed reactor can help to limit the amount of carbon formation by recirculation of the carbon-fouled catalyst to the oxygen- rich zones of the bed. So far, the works published on the use of fluidized beds for the conversion of methane to syngas are scarce, although the results obtained are encouraging. Thus, Olsbye et al. [ lo] reported an almost flat temperature profile within the bubbling phase of the fluidized bed, with conversions and selectivities close to equilibrium values. However, the temperature drop in the freeboard gave rise to a decrease in the conversion and selectivity, due to back reactions taking place on the catalyst drawn into this region. The performance could be improved by fitting a quartz sinter to prevent the catalyst from entering the freeboard. Bhar- adwaj and Schmidt [ 1 l] presented results in which a reasonable approach to isothermality was also attained, with temperature variations smaller than 50°C between two given positions in the bed. These authors investigated the behaviour of the reactor in the autothermal mode and with external heating. Although the concentration profiles were not determined, good conversions and selectivities could also be obtained in this case by using a Pyrex frit and an expanded section to enable catalyst disengagement. In view of the previous fluidized bed studies, special attention was devoted in this work to the investigation of the influence of the temperature along the reactor on the observed conversions and selectivities, and to the discussion of procedures to attain a favourable temperature profile.

2. Experimental

A 30 mm i.d., 235 mm long quartz reactor has been used as a fluidized bed in all the experiments carried out. The reactor is equipped with a sintered quartz distrib- utor plate and a quartz thermowell in the bubbling zone, which was used for the control thermocouple. In addition, an opening at the top of the reactor could be used to insert a movable quartz probe to take gas samples at different bed heights. In this case, the duct leading to the on-line gas chromatograph was evacuated before each sample was taken, and then allowed to equilibrate. This produced a high initial velocity in the quartz probe which was useful to minimize the reaction of the gases sampled. Alternatively, the quartz probe could be replaced by a quartz thermowell in order to monitor temperature profiles. The position of the reactor relative to the

A. Santos et al. /Catalysis Today 21 (1994) 481-488 483

external electrical furnace could be modified in order to create different temperature profiles in the freeboard region of the reactor. Two different relative positions were used, namely position 1, in which approximately 85% of the reactor length was heated, and position 2, in which the reactor was partly slid out of the furnace and only 66% of its length was heated.

A NiO/NiAlZ04 catalyst was prepared following the method used by Al-Ubaid and Wolf [ 121 with some minor variations. The minimum fluidization velocity was determined using nitrogen at the reaction temperature, and then corrected to account for the different density and viscosity of the reaction mixture. The value of the minimum fluidization velocity was 0.5 cm/s (standard conditions). The typical catalyst loading was 25 g, although experiments with a lower catalyst mass were also carried out. With a 25 g loading and a reduced velocity of 2.5, the height of the bubbling zone was typically about 8 cm, and the length of the freeboard was 15.5 cm. Attrition and agglomeration problems were avoided by optimization of the catalyst preparation procedure. The loss of catalyst fines by elutriation was lower than 0.8% in 12 h of experiment.

CH,/O,/N, mixtures were fed to the reactor by using mass flow controllers, at reaction temperatures between 750 and 900°C. Different W/F ratios were investi- gated, but the CH4/02/N2 feed ratio was kept at 2/l / 1 unless otherwise indicated. The product gases were analyzed by on-line gas chromatography. The carbon mass balance closures were always better than + 5%. The amount of carbon deposited on the catalyst after an experiment was determined by burning off the deposits with air and measuring the amounts of CO and COZ formed.

3. Results

Fig. 1 shows the temperature profiles in the reactor at four different temperature set-points, with a total flow rate of 500 cm3 (STP) /min. It can be seen that

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750°C

800-Z

850°C

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. l

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Distance from the distributor (cm)

Fig. 1. Temperature profiles in the fluidized bed at four different temperature set points. Flow rate: 500 cm3 (STP)/min.

484 A. Santos et al. /Catalysis Today 21 (1994) 481-W

900

800

G 700

e 600

_kI 500

6 400

B 300

g 200

100

Position 1

Position 2 :-‘;;:-:::-:,: 0 5 10 15 20 25

Distance from the distributor (cm)

Fig. 2. Temperature profiles in the bed at two different reactor-furnace positions.

essentially isothermal fluidized bed operation was achieved within the bubbling zone, which corresponds to approximately the first 8 cm of the bed. The maximum temperature difference that could be measured axially with a moving thermocouple between any two positions was of only 10-l l”C, even though the conversions were higher than 84% in all cases. As the gases left the bubbling zone, they underwent rapid cooling at a rate of about 20”C/cm in the initial 5 cm, and a faster cooling thereafter.

As stated in the Introduction, the rapid drop of temperature in the vicinity of the fluidized bed is likely to have a negative effect on the conversions and selectivities reached, because the reactions leading to the unfavourable equilibrium conditions at lower temperatures are accelerated by the catalyst entrained into this region [ lo]. To assess the extent of this effect, several experiments were carried out at two different relative positions of the reactor and the external furnace. Figs. 2 and 3 show the results obtained at 850°C. It can be seen that the temperature of the bubbling zone is approximately homogeneous at any of the two positions; however, considerably different temperature profiles can be observed in the freeboard.

Fig. 3 shows that the conversion profile within the fluidized bed is approximately flat, and very close to the equilibrium value (approximately 96.5%) at both reactor

Y”

0 20 DisLce fr:m the ktributor (cmf5

Fig. 3. Methane conversion profiles along the reactor. Total flow rate: 500 cm3 (STP) /min. T, = 850°C.

A. Santos et al. /Catalysis Today 21 (1994) 481-488 485

100 4

e ? 98

3

.zz 96

zi 22

8 94

3

8

0 92 1 @.

u

90 0 5 10 15 20 25 O

Distance from the distributor (cm)

Fig. 4. Selectivity to CO and HZ/CO ratio along the bed. Conditions as for Fig. 3.

positions. This seems to indicate that the equilibrium is achieved rapidly, in agree- ment with earlier results [ lo]. In our case, the observed behaviour is also consistent with the fluid dynamics of the system employed: At relatively low values of the reduced velocity a significant proportion of the gas is in the emulsion phase, where a fast equilibrium is achieved. Fig. 3 also shows that, once the gas leaves the dense phase and enters the freeboard, the conversion of methane decreases, following the tendency towards the higher methane concentrations in the equilibrium at lower temperatures (the equilibrium conversions are approximately 93.5% at 800°C and 89% at 750°C). The decrease is more pronounced for position 2, where the tem- peratures are lower.

In a fixed bed reactor operated at very short contact times this problem may be overcome by a rapid cooling of the product gas once it leaves the catalyst bed. In a bubbling fluidized bed, however, the catalyst is not static. Each of the bubbles draws a wake of catalyst which is projected out of the bed, into the first few centimeters of the freeboard (termed the disengagement zone), which as already shown, has a lower temperature. The catalyst there can catalyze the back reactions that approach the new equilibrium conditions, and as a result the methane concen- tration increases. The selectivity to carbon monoxide follows the opposite trend, as shown in Fig. 4, decreasing as the distance to the distributor plate increases, while

the HZ/CO ratio stays around 2. The effect of the operating conditions was investigated with the reactor in position

1. Fig. 5 shows the effect of the set point temperature. It can be seen that both the methane conversion and the CO selectivity increase with temperature ( T,) , which is a consequence not only of the higher temperature in the bubbling zone, but also of higher temperatures in the disengagement zone as the temperature set point is raised. The variation of these parameters with the total gas flow rate is shown in Fig. 6. In fixed bed reactor experiments, an increase in the total gas flow rate often results in an increase in the total heat released, which in turn increases the auto- thermal reaction temperature and the observed conversion. However, in this case Fig. 6 shows a small decrease in the methane conversion when the gas flow rate is increased. Since the bubbling bed temperature was kept at 800°C the results

486 A. Santos et al. /Catalysis Today 21 (1994) 481488

2

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700 750 800 850

Temperature (“C)

Fig. 5. CH, conversion, CO selectivity and Hz/CO ratio as a function of T,. Other conditions as for Fig. 3.

observed are most likely a consequence of an enlarged disengagement zone at higher flow rates, due to the existence of faster and more numerous bubbles in the bed.

Given the influence of the temperature in the disengaging zone, an experiment was also carried out using a lower mass of catalyst in the bed ( 18 g) . This gave a lower fluidized bed height (about 6.5 cm instead of 8)) thus moving the disengaging zone towards higher temperatures. The results (not shown), yielded a slightly lower methane conversion profile within the bubbling zone (93.9 instead of 94.6%, obtained with the 25 g bed). However, the decrease in methane conversion in the disengaging zone was considerably less pronounced. As a result, the conversion at the reactor exit was 1.4% higher in the bed with 18 g of catalyst, which further confirmed the influence of the reactions in the freeboard.

Finally, an important feature of the oxidation of methane to syngas over Ni- based catalysts is that, at least under certain conditions, it can lead to high levels of carbon deposition. Table 1 shows the results of carbon deposition obtained with a CH4/02/N2 ratio of 2/1/l. It can be seen that, under these conditions, carbon

d

8 85J I I 0 300 400 500 600 700 800 900 1000

Gas flow rate ( cm3(STP)/min)

Fig. 6. Variation of the CH, conversion, CO selectivity and HJCO ratio with gas flow rate. T, = 800°C.

A. Santos et al. /Catalysis Today 21 (1994) 481-488 487

Table 1

Carbon formation on a NiO/NiAl,O, catalyst, CHJ02/N2 =2/ I/ 1

Temperature set point Reactor position Time Grams of carbon per

(“C) (tin) 25 grams of catalyst

800 1 755 0.145 800 2 300 0.083 850 1 585 0.226 850 2 500 0.088 800 1 515 0.112 800 2 710 0.117

formation on the catalyst is almost negligible. Thus, for instance, during the first 5 h at 800°C only 0.083 g of carbon were formed (position 2), and a further period of almost 7 h only gave 0.03 extra grams of carbon. This was confirmed by the mass balances of carbon when the reaction was started with a carbon-free catalyst. The initial mass balance closures were around 85% and increased until values close to the 100% level were reached, in about 2 h of experiment.

The reason of the low rates of carbon formation probably lies in the intrinsic fluid dynamics of the bed. In a fluidized bed reactor the catalyst is continuously recirculated, and therefore it is exposed to a changing reaction atmosphere. Among the species present in the reactor are 02, HZ, CO2 and H20. All of them are known carbon gasifying agents under appropriate reaction conditions. Thus, the carbon formed in a given region of the bed (e.g., an oxygen-deficient region) is transported along with the catalyst to a different region (e.g., the oxygen-rich bed entrance), where it is gasified. In this way, an operational equilibrium can be achieved between carbon formation and gasification, which may explain the very low or negligible carbon formation obtained in this and other works [ 111 using fluidized bed reactors. However, the carbon gasification capacity is limited and when the CH,/O* ratio was increased from 2/ 1 to 5 / 1 a high carbon formation rate was observed, giving 1.4 g of carbon in 15 min and 2.6 g in 30 min.

4. Conclusions

The fluidized bed reactor is a suitable device to carry out the partial oxidation of methane to synthesis gas. It can provide stable, isothermal operation and low rates of carbon formation. The conversion and selectivity can be maximized by mini- mizing the amount of catalyst entering the colder zones of the reactor. This can be achieved by choosing appropriate fluidization conditions and by generating an adequate temperature profile in the disengagement section of the reactor.

Acknowledgements

This work was carried out with financial support from DGICYT (PB90-0920 and PB93-031) and DGA (P-IT-4/91).

488 A. Santos et al. /Catalysis Today 21 (1994) 481-488

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