MgO C - American Chemical Society

Chapter 18

A Comparative Study on CH -CO Reforming over Ni/SiO -MgO Catalyst Using Fluidizedand Fixed-Bed Reactors 4

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Downloaded by UNIV OF MICHIGAN ANN ARBOR on September 19, 2015 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch018

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A. Effendi, Z.-G. Zhang, and T. Yoshida Resource and Energy Division, Hokkaido National Industrial Research Institute, 2-17 Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan (email: [email protected])

CH -CO reforming to syngas was studied by using micro-fluidized-and fixed-bed reactors over Ni/SiO -MgO at 700 °C, 1 atm with CO /CH =1. The catalytic activity in the fluidized-bedreforming showed higher CH and CO conversions and larger H /CO ratio compared to those in the fixed-bed reforming. An efficient contact between reactants and catalyst in the fluidization was suggested to be partly responsible for the enhancements. Moreover, the fluidization minimized a temperature gradient of the catalyst bed and a concentration gradient of the reactant gases. Although only a limited amount of carbon, which consisted of amorphous and crystallized forms, was deposited on spent catalysts, a relatively higher XPS intensity of crystallized carbon was observed on the fixed-bed catalyst than that on the fluidized-bed one. This suggested a faster deactivation of the catalyst in the fixed-bed reactor. In addition, an uneven coke deposit was observed alongfixed-bedpositions. 4

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© 2002 American Chemical Society In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Introduction Catalytic C 0 reforming of C H (Eq. 1 ) has been renewed as an attractive route for direct production of synthesis gas. Under the given conditions, however, a competitive coke formation occurs as results of C H decomposition (Eq. 2) and CO disproportionation (Eq. 3), causing a catalyst deactivation. In addition, a reverse water-gas-shift (RWGS) reaction (Eq. 4) occurs simultaneously during the reforming. 2

4


4

CH

4

+C0

2

CH

4

->

C + 2H

2CO ~>

C0

2

+H

->2CO + 2 H

2

Δ Η Γ

Δ Η

2

C+C0

2

Γ

A H

2

r

->CO+H 0 2

2 9 8

2 9 8

2 9 8

Δ Η Γ

= + 247.9kJ/mol (1)

= + 75.0 kJ/mol (2)

=-172.0 kJ/mol (3)

= +41.0 kJ/mol (4)

2 9 8

Ni-based material has been considered as potential catalysts for industrial applications, and their modifications to high resistant coke ones by adding basic oxides such as MgO have been widely investigated. A loading of 5 wt % N i exhibited a high activity and much less magnitude of coke deactivation (1-4). Beside the massive coke deposition, performing C H - C 0 reaction by using a fixed-bed reactor has limitations due to its high endothermicity. Poor heattransfer can cause a temperature gradient across a fixed-bed, creating a cool-spot. To minimize these difficulties, the use of a fluidized-bed reactor has been approached (1,5,6). A fluidized reforming allowed catalyst-circulations, resulting in almost isothermal reactor operation, less concentration gradient of reactant gases and lower deactivation of catalysts (1,5,6). Lab-scale experiments showed that thermodynamic equilibrium could be achieved (5). On the other hand, a mechanical loss due to catalyst attrition during the fluidization might take place. In the present work, the catalytic C H - C 0 reforming has been examined by using a micro fluidized-bed reactor over Ni/Si0 -MgO at 700 °C, 1 atm with C0 /CH =1. For a comparison, fixed-bed experiments on the reforming were carried out under the same conditions. In both reactions, catalytic parameters such as C 0 and C H conversions as well as H /CO ratio were investigated. Coke deposits on spent catalysts were also characterized. 4

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In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

277

Experimental

Catalyst Preparation and Characterization A catalyst with a loading of 4.5 wt % NiO on Si0 -MgO (Nikki Chemicals Co. Inc., 75.6 wt % S i 0 , 21.3 wt % MgO and particle size of 45-88 μπί) was prepared by a multiple impregnation using an aqueous solution of nickel nitrate precursor. The support containing the active metal was dried at 70 °C between impregnation steps, then at 110 °C overnight and finally calcined in air at 550 °C for 4 hrs. Bulk compositions were determined by using ICP-AES (Shimadzu GV-1000P), whereas the surface by XPS analysis ( V G E S C A L A B 2201 X L ) . Surface area (70.8 m /g) and average pore volume (16.2 cm /g) were measured by N adsorption according to the BET method (Belsorb 28 apparatus, Bel Japan, Inc.). Prior to the analysis, samples were degassed at 200°C for 4 hrs. Phase analysis of powder catalysts was done by X R D (Rigaku Ltd.) using C u K a radiation (35 k V , 20 mA). Samples for T E M (JEM2010, JEOL Ltd.) were powdered and dispersed in ethanol to prepare suspensions, then applied to sample grids for this analysis. Crystallites of NiO with average particle sizes of 100-130 Â were observed from latter analyses. 2


2

2

3

2

Apparatus and Conditions of Reforming A quartz micro-reactor (id of 9 mm) was applied for fluidized- and fixedbed experiments. A thermocouple inserted inside a thermowell was used for measuring the catalyst-bed temperature and the furnace temperature was controlled by another thermocouple placed outside the wall of the reactor. A power supply (Chino SU KP3000) equipped to a tubular gold-coated glass furnace was employed. This furnace allowed an even heat distribution through the reactor-wall. Gases (CH , C 0 , and Ar) were purified by passage through appropriate adsorbents, and delivered to the reactor through mass flow controllers (Tokyo Keiso Co. Ltd.). Both reactions were performed under same conditions, namely at a furnace temperature of 700 °C, 1 atm with a C0 /CH =1, and equal contact time (88 ml/min, STP, 150 mg catalyst, so W/F= 0.64 g.h/mol). In the case of the fluidized-bed experiments, the feed was supplied up-flow through a porous quartz disc. A down-flow gas stream was employed for fixedbed experiments. Initially, the catalyst was purged with Ar (40 ml/min, STP) up to 700 °C, and then C 0 / C H gases were switched to the reactor. Gases were analyzed on-line by a G C (Yanaco G3800 GC) using a 2m χ 3mm id activated carbon column at isothermally 110 °C and 50 Nml/min Ar carrier. Reactants and products (CO, H 0 and H ) from the reactor effluent were passed through a condenser and anhydrous Mg(C10 ) sorbent to trap residual moisture. Amount of condensed water as well as the total flow rate of effluent-gas was quantified. 4

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4

278 A catalytic test using the support alone resulted in below 0.1 % of CO and H yields, showing a negligible activity.

2


Coke Determination Coke amount on spent catalysts after reaction tests was determined by using a TG analyzer (Shimadzu DT40). Approximately 15 mg sample was pre-dried up to 110°C under 10% 0 /Ar (30 ml/min) for 10 min and a weight-loss profile was recordedfromthis temperature to 825°C with a heating rate of 10°C/min. Then, the profile was corrected with a baseline measuredfroma blank test. XPS surface analysis with C l s spectra was used to examine the nature of carbonaceous species. The AlKcc was used as an energy source (1486.6 eV), operated at 10 kV, 15 mA and mainly an energy pass of 30 eV. As a reference line, the Au4f spectrum with a binding energy (BE) of 84.0 eV was used for all measurements of pelletized samples. Moreover, changes in surface compositions of metal atoms were also examined. 2

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Results and Discussion

Conditions for Fluidized- and Fixed-Bed Experiments Choice of the present experimental conditions was based on results given in Fig. 1. The figure shows conversions of C0 , C H and H /CO ratio as a function of space velocity (W/F) using a fixed-bed reactor. Both conversions increased with the increase in W/F, then unchanged beyond 1.30 g.h/mol. Similar tendency was also obtained for the H /CO ratio. The suppression of their conversions in a range beyond 1.30 g.h/mol suggested that the reforming was strongly influenced by the diffusion of the reactant gases at the interphase of catalyst particles. Thus, W/F=0.64 g.h/mol was chosen for both experiments, in which the diffusion effect was considered to be less significant and the requirement for a minimum fluidization was met. Consequently, the fluidizedbed of 150 mg catalyst achieved a minimum height of H «4 mm, velocity of U m f ^ ml/min, STP, as visually measured and the u/u of > 1.5 was employed. 2

4

2

2

mf

mf

Catalytic Activity using Fluidized- and Fixed-Bed Reactors All catalytic activities on CH -C0 reforming were tested at 700 °C, with C 0 / C H = 1.0, 1 atm for 30 hrs time-on-stream (tos) and the results are shown in Fig. 2 and Table I. Catalytic activity obtained from the fluidized-bed 4

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279


280 experiment showed superiority on conversions as well as H /CO ratio compared to thatfromthe fixed-bed one. The conversions of C H and C 0 were higher by 16 and 19 %, whereas the H /CO indicated 0.69 and 0.54 for the fluidized- and fixed-bed reactions, respectively. For all cases, C 0 conversions were always higher than the C H ones, moreover, the resulted H /CO and C 0 / C H ratios were differed from the stoichiometric values due to the occurrence of accompanied reactions (Eq. 2-4). The higher H /CO ratio obtained from the fluidized-bed reaction was attributed to relatively less selectivity towards the RWGS reaction, as the water yield (3.4 mol %) was lower than that from the fixed-bed one (3.8 mol %). To elucidate the catalytic activity difference using both type reactors, particular parameters such as catalyst-bed temperatures, pressure-drops over the reactors, and coke deposits were further discussed. 2

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2

2

4

2

2

4


2

Effects of Catalyst-bed Temperature Due to the highly endothermic reforming, the catalyst-bed temperature gradient and its effect on the catalytic activity were a major importance to investigate. Under Ar alone at 700 °C, no temperature difference was observed between the catalyst-bed and furnace for both cases. However, when the supply of a C 0 / C H mixture started, the bed temperature was gradually decreased and remained constant after 1 hr tos. 2

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Table I. Results of C H - C 0 reforming over Ni/Si0 -lVlgO catalyst 4

Bed reactor

Tb

2

2

C0 /CH 2

4

H /CO 2

(°Q

Fluidized Fixed Fluidized at 684 °C Fixed-fluidized 2

1

695 684 684 696

Conversions (%) CH co 37.7 52.7 21.5 34.0 51.9 37.5 23.2 37.9 4

0.84 0.77 0.78 0.81

0.69 0.54 0.68 0.63

2

Coke wt% 1.5 1.6 nd nd ;

2

not determined for 1.5 hrs in fixed-bed, then fluidized mode for 4.0 hrs tos

Although an axial temperature gradient over catalyst-beds was not determined, measuring temperatures at the center of the catalyst-bed might reflect a thermal reaction behavior in fluidized- and fixed-bed reactors. As shown in Fig. 3, the fluidized-bed temperature increased to 695 °C, then attained a steady state, whereas the fixed-bed temperature showed a slight increase, and remained unchanged at 684 °C. As the temperature difference was related to the thermal behavior, this might also reflect a measure of the overall change in enthalpy (7). According to this approach, the bed temperatures showed different reaction-heats, so a lower catalyst temperature of the fixed-bed indicated a larger reaction-heat absorbed for this catalytic reforming. The fixed-bed disfavored the thermal efficiency due to an inferior heat-transfer, as consequence lowering the


281 70

60

co V ° ° O o

2

0

O o O

o

o

0

o

0

o

0

o

o

0

o

o

0

o

0

50 Downloaded by UNIV OF MICHIGAN ANN ARBOR on September 19, 2015 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch018

CH (Λ C

o

0

o

o

/

fluidized bed

4

40

Ο Φ >

ο ο

30

-

1

/

fixed bed

'! CH

4

20

10 500

1000

2000

1500

time on stream (min.)

Figure. 2. Conversions of CH and C0 over Ni/Si0 -MgO resulted from the reforming usingfixedandfluidized-bedreactors 4

2

2

700 fluidized-bed :

X

X X

X

X X

X

X

X X

X

X

X

X

X

X

X

X X

X

X

X X

X

X

X

X

X

690 fixed-bed

ο J3

680

670 500

-+-

4-

1000

1500

2000

time on stream (min.)

Figure. 3. Catalyst-bed temperature profiles of fluidized- andfixedreactors during CH -C0 reforming over Ni/SiO MgO 4

2

r



282 overall bed temperature. In the case of the fluidized-bed, the catalyst circulation in gases led to less heat-transfer resistance, and the absorbed-heat for the endothermic reforming was favored. Moreover, an even-heating bed was possibly achieved assuring an isothermal region, inhibiting a temperature gradient. The above temperature difference of 11 °C might represent their variance in the catalytic activities. To examine the influence of the bed-temperatures, the fluidized-bed reforming at 684 °C was also conducted. Table I shows that lowering the catalyst-bed temperature to 684°C, which was the same bedtemperature as that of the fixed-bed applied, did not decrease significantly the catalytic activity, and the conversion levels remained close to those resulted at 695 °C. Hence, this temperature variation did not cause the difference in the catalytic activity between the fluidized- and fixed-bed reactions.

Profiles of Pressure-drop Pressure-drop over both reactors was monitored for 30 hrs tos in which its alteration might reflect coke amount. At both initial activity tests, the pressuredrops increased slightly, and then remained significantly unchanged for 30 hrs tos. This was possibly associated to the limited amount of coke formed, «1.5 wt %, analyzed by the TG as shown in Table I.

Deposited Coke Evidence of a Rapid Catalyst Deactivation in Fixed-bed. The lower catalytic activity in the fixed-bed might be related to a rapid coke formation at initial stages. To understand coke poisoned-levels on the catalyst deactivation in the fixed-bed, a short run of a catalytic test was carried out firstly in a fixed-bed for 1.5 hrs, then immediately switched to a fluidized-mode for 4 hrs. As shown in Fig. 4, the catalytic activity was lowered in the fixed-bed reaction, and then was recovered when switched to a fluidized-mode. However, the final C H and C 0 conversions appeared to be slightly higher than the result obtained from the fixed-bed mode, and much smaller than their conversions obtained by the fluidized-bed experiment alone (see Table I). It suggests that the formation of coke at the initial stage of the fixed-bed reaction gave decisive effects on lowering the catalytic activity. 4

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Characterization on the nature of coke. Since the coke amount of both spent catalysts indicated insignificant difference, the XPS analysis was applied to distinguish the nature of the surface carbonaceous species. Figure 5 shows the C l s spectra for the fresh and spent catalysts. C l s line of the fresh catalyst


283


70 (a) fixed 60 (b) fluidized

_

50

(b)

(a)

CO

c ο

S2 ο >

C0 40

-

2

o o o .

37.9 %

ο

Ο

c ο

Θ

ο

Ο

30

CH *

_

20

•

4

• ·

23.2%

•

10 150

450

300

time on stream (min.) Figure. 4. Conversions of CH and COj over Ni/Si0 -MgO obtained from the fluidized reforming after experiencing thefixed-bedmode 4

2



284

294

292

290

2S8

286

284

282

Binding Energy - eV

Figure. 5. XPS Cl s spectra offresh Ni/SiOr-MgO fromfluidized-andfixed-bedreforming

and spent catalysts resulted


285 demonstrated a main peak with a B E of 290 eV assigned to carbonate, C 0 " species possibly due to the exposure to air containing C 0 . Whereas, C l s lines of both spent catalysts presented peak-maxima at 287.0 and 284.7 eV, indicating the presence of amorphous (-C-CO-) and crystallized (-C-C-) structures, respectively. In the case of the fluidized-bed sample, the crystallized one was relatively a minor species. On the other hand, the average fixed-bed sample showed that the crystallized carbon became a major species and the amorphous form was less pronounced. Table II shows the relative intensity of the crystallized to amorphous forms. According to the temperature-programmed oxidation (TPO) studies (24,6,9,10), it is well known that crystallized form is a less reactive carbon and usually oxidized at higher temperatures, whereas the highly reactive amorphous carbon is readily oxidized at lower temperatures. This kind of reactive species would be formed at initial stages of catalytic runs, then could be removed by gasifying with C 0 . At the same time, a possible remaining carbon would be gradually converted to a crystallized form. It became a major species on N i catalyst after 1 hr (2-5). 3


2

2

Table II. XPS relative intensities of crystallized to amorphous forms and surface elemental compositions (atomic %) of Ni/Si0 -MgO catalysts 2

Catalyst

Lc-c-fi-c-co- 0

Ni

Si

Mg

C

Fresh

-

63.8

2.5

19.5

4.5

9.7

Spent (fluidized-bed)

0.67

58.6

0.7

16.4

3.0

21.3

Spent (fixed-bed)

1.93

55.4

1.1

16.0

2.8

24.7

In the catalyst fluidization, a concentration gradient of the reactant gases and an accumulation of less reactive carbon leading to the formation of crystallized carbon could be prevented. During the C H - C 0 reforming, the formation of carbon by C H cracking and its gasification with C 0 were in balance and these occurred simultaneously. When the carbon was formed, it would be immediately gasified by C 0 , so this process inhibited a conversion of less reactive to crystallized forms. Table II shows that on both spent catalysts, the surface concentrations of metals and oxygen decreased as carbon was accumulated. As more crystallized species was observed on the fixed-bed spent catalyst, which suggested to be a primary cause for the catalyst deactivation due to a poisoned encapsulation of Ni particles (3,4,9,10). Therefore, the catalyst deactivation was relatively faster in the fixed-bed reaction. 4

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2

2

Carbonaceous species along thefixed-bed.As the color intensity of the spent catalyst resulted from the fixed reactor increased from dark gray to black with bed-positions, this might reflect its coke distribution. Accordingly, a separate experiment on a longer fixed-bed (10 mm) was carried out by using a


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-C-CDownloaded by UNIV OF MICHIGAN ANN ARBOR on September 19, 2015 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch018

284.7

-J 294

I

!

I

L

I

292

290

288

286

284

L_ 282

Binding E n e r g y - e V

Figure. 6. XPS Cl s spectra of spent catalysts sampled at top and bottom of the fixed-bed


287 smaller reactor (id of 4 mm) under the same condition. Figure 6 demonstrates C l s spectra of the top and bottom samples recovered after 5 hrs tos. The carbonaceous species from the top sample was dominated by a species with a B E of 287.0 eV, whereas the crystallized carbon positioned at 284.7 eV was a significant species from the bottom sample. Hence, various carbonaceous species was distributed as a function of bed positions. It was previously suggested (2-4,9,10) that the formation of amorphous species was associated with C H decomposition. In the bottom of the fixed-bed reactor, the C H concentration was higher than that of C 0 , as a result, the C 0 / C H ratio was less than unity. The concentration gradient of C 0 was possibly caused by the progress of the reforming with fixed-bed positions. At the bottom of the catalyst bed, the C 0 concentration would be minimum, leading to the suppression of the carbon gasification with C 0 . Thus, the carbon concentration increased continually with fixed-bed positions and was relatively higher on catalysts in the bottom of the bed. Moreover, since the presence of C 0 was insufficient to remove all carbon formed, CO disproportionate (Eq.3) would possibly take place. 4


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Conclusions A comparative study on the C H - C 0 reforming using two different reactor types leads to the following conclusions. Conducting of the C H - C 0 reforming in a fluidized-bed reactor increased the performance of the catalytic activity and H /CO ratio, compared to that in the fixed-bed one. Amorphous and crystallized forms were observed on the spent catalysts in the fluidized- and fixed-bed reactions. The presence of the crystallized form was a major carbon species in the fixed-bed sample, as the result, the catalyst deactivation in the fixed-bed reforming was relatively faster. In addition, a concentration gradient of the reactant gases over the fixed-bed resulted in uneven carbon distribution. 4

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Acknowledgements We acknowledge the financial support by the Japan International Science and Technology Exchange Corporation under the Science and Technology Fellowship Program. We also thank N . Imai for TPR, T G measurements, and Mr. O. Nishimura (HNIR1, Sapporo) for XPS analysis.

References 1. 2.

Effendi, Α., Z-G. Zhang, M. Sahibzada, T. Yoshida, Am. Chem. Soc. Div. Petrol. Chem. Prepr., 45(135-137) (2000) Schmitz, A . D. and T. Yoshida, "Greenhouse Gas Control Technologies", Pergamon (1999) 355


288 3. 4. 5.

Wang, S. and G. Q. Lu, Ind. Eng. Chem. Res., 38 ( 1 9 9 9 ) , 2615-2625 Lu, G. Q. and S. Wang, Chem. Tech., January ( 1 9 9 9 ) 37-43 Mlecko, L., S. Malcus and T. Wurzel, Ind. Eng. Chem. Res., 36 ( 1 9 9 7 ) 4459-4465 6. Olbye, O., L. Mlecko and T. Wurzel, Ind. Eng. Chem. Res., 36 ( 1 9 9 7 ) 5180-5188 7. Gadalla, A. M. and B. Bower, Chem. Eng. Sci. 42(1988), 3049-3062 8. Kunii, D. and O. Levenspiel, Fluidization Engineering 2 edition, Butterworth-Heinemann ( 1 9 9 1 ) 9. Himeno, Y., K. Tomoshige, K. Fujimoto, Sekiyu Gakkashi, 42(4) ( 1 9 9 9 ) , 252-257 10. Wang, S., G. Q. Lu, G. J. Millar, Energy and Fuels, 10 ( 1 9 9 6 ) , 896-904


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MgO C - American Chemical Society

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