Ind. Eng. Chem. Res. 1987,26, 1704-1707
1704 Subscripts
A-D = defined in Figure 4 E = defined in Figure 5 F = feed, defined in Figure 4 I, J, L = defined in Figure 5 N = diffuser inlet 0 = output, defined in Figures 4 and 5 T = defined in Figure 5 VI = pumping chamber 1, defined in Figure 4 Vz = pumping chamber 2, defined in Figure 4 1, 2 = defined in Figure 4 31 = flow in flow junction exiting through alternate leg 32 = flow in flow junction exiting through output leg
Literature Cited Crane Company “Flow of Fluids Through Valves, Fittings, and Pipe”, Technical Paper 410,Chicago, 1976. Miller, D. S.Internal Flow Systems; British Hydromechanics Research Association Fluid Engineering Series; BHRA: Branfield, Bedford, England, 1978;Vol. V.
Priestman, G. H.; Tippetts, J. R. “Development and Potential of Power Fluidics for Process Flow Control”, Trans. Inst. Chem. Eng. 1984,62(2),57-80. Priestman, G. H.; Tippetts, J. R. “Characteristicsof a Double-Acting Fluidic Pump with Hot and Cold Water”, J.Fluid Control 1986, 16(4), 19-39. Robinson, S. M. “Developmentof a Continuous-Flow Fluidic Pump”, Ph.D. Thesis, Department of Chemical Engineering, University of Tennessee, Knoxville, 1985. Smith, G. V.; Counce, R. M. “Performance Characteristics of Axisymmetric Venturi-Like Reverse-Flow-Diverters”,J . Fluid Control 1986,16(4), 19-39. Tippetts, J. R., et al. “Flow Control Circuits for Toxic Fluids”, Proceedings Fluidics State-of-the Art Symposium, Harry Diamond Laboratories, Washington, D.C., 1974,Vol. 111. Tippets, J. R., et al. “Developments in Power Fluidics for Applications in Nuclear Plant”, J . Dyn. Syst., Meas., Control 1981,103(4), 342-351. White, F. M. Fluid Mechanics, McGraw-Hill: New York, 1979. Received for review August 1, 1985 Revised manuscript received December 22, 1986 Accepted May 10, 1987
Catalytic Deactivation of Methane Steam Reforming Catalysts. 1. Activation Miriam E. Agnelli,? Mario C. Demicheli,? and Esther N. Ponzi*? Centro de Investigaci6n y Desarrollo en Procesos Catallticos (CZNDECA), Facultad de Ciencias Exactas, U.N.L.P.-CONICET, 1900 La Plata, Argentina
An alumina-supported catalyst was studied both in its original state and after activation and sintering. Chemical composition and textural properties were determined, and crystalline compounds were identified. Active-phase and support transformations occurring during activation were determined by differential thermoanalysis (DTA), temperature-programmed reduction (TPR), and X-ray diffraction. T h e catalyst activated by means of various procedures was characterized by measuring crystallite size.
The first stage in the steam reforming process is activation of the catalyst, which consists in reducing the nickel compound. This matter has been widely investigated as regards both structure and oxide reduction. Holm and Clark (1968) observed that the difficulty in reduction increased as the heat treatment temperatures were increased from 300 to 700 “C. Lo Jacono et al. (1971) investigated the interaction which takes place when nickel oxide is supported on q- and y-aluminas. They found that the surface spinel was affected by the atmosphere of firing and by the nature of the supports. Roman and Delmon (1973) investigated the influence of promoters and carrier on catalyst reduction, whereas Bartholomew and Farrauto (1976) studied the effect of calcination and heating rate during activation. Zielidsky (1982) observed that nickel oxide appeared in the catalysts in two forms, as “free” and “fixed” oxide. During the temperature-programmed reduction, the free nickel oxide reacts first and the surface area of the metal formed is small. As a result of the reduction of fixed oxide, the metal surface area in the catalyst increases considerably. The procedure used for activation may modify the catalyst activity. Bartholomew et al. (1980), in a study on CO hydrogenation, ascribed Members of CONICET’s Scientific and Technological Research staff. 0888-5885/87/2626-1704$01.50/0
activity variation to the higher or lower interaction of the metal with the support. Rostrup-Nielsen (1984) observed that partial poisoning with sulfur retarded whisker carbon formation rate to a greater degree than the steam reforming reaction and attributed this fact to blocking of ensembles. The ensembles required for carbon nucleation would apparently be greater than those needed for the steam reforming reaction to take place. The purpose of our investigation was to study the steam reforming reaction with deactivation caused by carbon and/or coke deposits. In order to use the concept of separable kinetics (Butt et al., 1978), it is necessary to determine the kinetics of the main reaction (steam reforming) without deactivation. A way of achieving this is to perform the kinetic study avoiding any kind of deactivation (coking, poisoning, sintering). Part 1of this paper covers the study of the catalyst and its activation, and part 2 deals with the kinetic study with no deactivation. The conclusions reached on coke deposition will be disclosed in a further paper. Experimental Section Chemical Analysis. Of the sample catalyst 0.2 g was weighed in a platinum dish, and 5 mL of HF, 5 mL of HC1, and 2 mL of HCIOl were added. The mixture was evaporated to heavy fumes on the hot plate, and 15 drops of 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1705 catalyst
... A
inert
t
L
----.
PI-PI-RB
10%
@.-0
C
Figure 1. Flow chart of experimental equipment.
HC1 and 10 mL of water were added. The mixture was then cooled and transferred to a volumetric flask following filtering to eliminate graphite. It was made up to volume and mixed. Ni, Al, and Ca contents were determined by atomic absorption spectroscopy using an Instrumental Laboratory Model IL 357 and 457 AA/AE spectrophotometer. Specific and Metallic Areas. Total surface area was measured by Nz adsorption using a Micromeritics physical adsorption analyzer Model Accusorb 2100 E. The data were interpreted by using the BET equation and an effective cross-sectional area of 16.2 A2 for N,. Measurement of the nickel surface area was carried out by using a conventional Pyrex glass volumetric adsorption apparatus capable of lV-torr vacuum. The hydrogen was purified in successive columns packed with copper metal and molecular sieve cooled with a dry iceacetone mixture. Following reduction of catalyst sample in hydrogen at 450 "C (2 h) and evacuation at 450 "C (5 h), nickel surface area was measured by using hydrogen chemisorption at room temperature. The amount of hydrogen adsorbed by the catalyst was measured after allowing 60 min for the adsorption to approach equilibrium. Hydrogen uptake was found by extrapolating the straight-line portion of the isotherm to zero pressure. Catalyst surface was calculated assuming that 1atom of adsorbed hydrogen occurs per 1 surface nickel atom having a surface area of 6.5 A2. X-ray Diffraction. Identification of crystalline compounds and determination of average crystallite size were carried out by X-ray diffraction (XRD) using a Philips diffractometer with Ni filter and Cu Ka radiation. The mean crystallite size (D) was related to the pure X-ray broadening (0) by the Scherrer formula D = KX/@cos 0 or
K X / ( B - b)'/'
COS
0
where X is the wavelength of the X-rays and 0 the angle between incident and diffracted beams. The observed width of the X-ray pattern line at half-peak height (Bo) and the instrumental line broadening determined from the half-maximum linewidth of Si crystal (bo)were corrected by Ka doublet broadening (Klug and Alexander, 1973), which yielded corrected values of B and b, respectively. TemperatureBrogrammed Reduction (TPR)and Differential Thermal Analysis (DTA). A flow chart of the experimental equipment is shown in Figure 1. The total flow rate and the hydrogen concentration can be varied independently. The hydrogen-nitrogen mixture (10% H,) was passed through a column packed with molecular sieve 5A. The gas stream leaving the reactor was
L
65
60
55
50
LS
LO
35
30
25
15
20
28
Figure 2. X-ray diffraction pattern with (a) fresh catalyst, (b) calcined catalyst (in Nzstream up to 550 "C), and (c) sintered catA = CalzA114033, B = boehmite, C = calcite, alyst (20 h at 800 T): G = graphite, N = nickel, 0 = nickel oxide, OL = a-alumina, and y = y-alumina.
passed through a cold trap cooled with liquid air before it was allowed into the hot wire detector (Gow-Mac). A temperature controller (Netzsch Model 406) was used to vary the temperature between 25 and 1100 OC with linear heating rate. The reactor (A in Figure 1) was a DTA Netzsch Model 404. The mixture leaving the hot wire detector was analyzed with a Carlo Erba Fractovap chromatograph using a Porapack Q column of 3.2 m and He as gas carrier. The experimental operating variables used in TPR were chosen according to the criteria suggested by Monti and Baiker (1983). They define a characteristic number K* K* = So/V*Co The number relates the amount of reducible sample (So), total flow rate (V*),and hydrogen concentration (Co). For heating rates between 6.0 and 18.0 OC/min, the limiting values are 55 < K* < 140 8. Catalyst samples of NiO (500 pmol of NiO) were heated in H,-N2 mixture. The total flow rate was 80 cm3/min (10% HJ, and the rate of temperature rise was 10 "C/min. Temperature, hydrogen consumption rate, and difference in temperature between the catalyst and the standard sample were simultaneouslyrecorded as a function of time. Gas samples were taken periodically for chromatographic analysis.
Results and Discussion Catalyst Characterization. The X-ray diffraction pattern of fresh catalyst is shown in Figure 2a. It can be seen from the figure that the crystalline compounds are boehmite, a-alumina, nickel oxide, calcite, and graphite. The data for fresh catalyst are shown in Table I. This table exhibits data for composition, crystalline compounds,
1706 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 Table I. Catalyst Properties sintered catalyst
fresh catalyst composition,
53.0 CaO 8.0 NiO 31.8 Alz03, HzO boehmite a-Alz03 a-alumina NiO nickel oxide calcite Ca C 0 3 C graphite 50
wt%
crystalline compds
specific area, m2/g of catalyst Ni surface area, m2/g of catalyst
y-Alz03 a-A1203 Ni CaizA11& C 36
2.2
Table 11. Feed Influence on Crystal Size sample heating activation 1 Hz-Nz Hz-N, 2 N2 Hz-Nz 3 HZ H2 4 H2-Nz Hz-N2" a
0
-> h-
a
endothermic
1
I
exothermic
415 530 350 490
Sample 1 after sintering for 20 h.
appeared. Curve b of Figure 2 shows the X-ray diffraction diagram for the sample of catalyst calcined in nitrogen stream. As water is a strong reduction reaction inhibitor (Bandrowski et al., 1962) NiO
400
crystal size, A
+ Hz e Ni + H 2 0
and support transformation temperature coincided with the decrease in the TPR signal, it was assumed that the species of NiO reduced in the 400-490 "C range was only one and that the origin of both signals was inhibition by water. In order to confirm this assumption, a sample was calcined in nitrogen stream from room temperature to 830 "C. Heating rate was 10 "C/min (curve d, Figure 3). The sample was cooled and reduction was carried out (curve e, Figure 3). There was only one peak in the 400-490 "C range. This result supported the assumption that only one species was reduced in that temperature range. In this case, it has been considered that the species is free NiO since the temperature coincides with that recorded for unsupported NiO reduction. The fresh catalyst TPR signal appearing at 660 "C may be originated in hydrogen consumption as well as in the production of gaseous species such as C02 (from calcite decomposition) and CHI (originated in gasification of carbon with hydrogen). Assuming the H2-Nz mixture had an ideal gas behavior, mixture conductivity was calculated on the basis of mole fractions of the components lz = X X i k ,
I
I
1
300 LOO so0
I
1
I
600
700
eo0
I
1 *C
Figure 3. Temperature-programmed reduction and differential thermal analysis with (a) nickel oxide (TPR), (b) fresh catalyst (TPR), (c) catalyst reduction (DTA), (d) catalyst calcination (DTA), and (e) sintered catalyst (TPR).
and overall BET surface area. The reduction profiles of bulk NiO and catalyst are shown in Figure 3. The reduction profile of pure nickel oxide powder (curve a) has a single peak of hydrogen consumption with a maximum a t 395 "C. For fresh catalyst (curve b), t h e e hydrogen consumption peaks appear on the reduction profile (400,490, and 660 "C) and two endothermic peaks (460 and 660 "C) appear in the DTA profile (curve c). Experiences with fresh catalyst show that, at 460 "C, when endothermic transformation takes place, catalyst reduction rate decreases. In order to identify that transformation, the following experiments were carried out: three 600-mg samples were heated in air, nitrogen, and hydrogen streams, respectively. Heating rate was 10 "C/min, and final temperature was 550 "C. In all three cases the DTA signal appeared at the same temperature. Each catalyst sample was analyzed by X-ray diffraction, and in all three cases the characteristic boehmite peaks disappeared, whereas those of y-alumina
When hydrogen was consumed, the mixture conductivity decreased. The same occurred if reactor exhaust gases contained COz and/or CH4. In order to confirm whether the peak in the fresh catalyst TPR diagram at 660 "C corresponded to C02and/or CH4, periodical samples were taken during the run. Neither of those gases were present. Therefore, the signal appearing at 660 "C was attributed to a nickel species which had interacted with the support. This interaction increased when the catalyst was heated in nitrogen stream prior to reduction. This was made evident by a temperature increase from 660 to 680 OC where the last peak appeared and also by an increase from 700 to 780 OC in the temperature necessary to complete the reduction. Activation. Catalyst samples were activated by varying the atmosphere during heating and activation. Nickel crystallite size was determined. Heating was carried out at 10 "C/min up to 800 "C, and activation was performed at this temperature during 20 min. Results are shown in Table 11. The crystallite smaller size was obtained with pure hydrogen and the larger size when heating was achieved with nitrogen stream, and hydrogen was added to activate at high temperature. The sample activated in the Hz-Nz mixture was sintered for 20 h. The crystal size increased by sintering from 415 to 490 A. As a consequence of activation and sintering, boehmite was transformed into y-alumina, graphite content decreased, calcite disappeared, and a calcium and aluminium compound formed. The X-ray diffraction diagram (curve
Ind. Eng. Chem. Res. 1987,26, 1707-1713
c, Figure 2) shows small peaks corresponding to the compound Ca12A114033.
Conclusions Programmed thermal reduction of fresh catalyst shows three signals in the 250-950 OC range. The first two (400 and 490 "C) correspond to only one species and the origin of both signals has been attributed to the water freed by the support retarding the reaction rate. When support water is eliminated before carrying out the reduction, there appears only one signal at 400 "C which coincides with that of unsupported NiO. The last signal in the TPR diagram (660 "C) has been attributed to NiO having interacted with the support. When the catalyst is heated previously in nitrogen stream, the reduction peak appears at 680 OC. From the DTA and X-ray diffraction experiments, it has been concluded that transformation of boehmite into yalumina occurs at 460 "C. Catalyst activation by varying the gas mixture evidences that the smallest crystallite size is obtained by using hydrogen in the heating and activation stages. Sintering for 20 h at 800 "C leads to an increase of NiO crystallite size from 415 to 490 A and to formation of a calcium and aluminium compound whose stoichiometry is Ca12A114033. Acknowledgment We gratefully acknowledge technical assistance by Lic Norbert0 Firpo for chemical analysis and Ing. Marcos Garazi for metallic area. We express thanks to Nestor Bernava for TPR measurements. We acknowledge the financial aid received from CONICET.
Nomenclature B = corrected sample line broadening
1707
b = corrected reference line broadening Co = inlet hydrogen concentration, ~ m o l / c m ~ D = average crystallite size K* = Monti's characteristic number ki = thermal conductivity of component i k = thermal conductivity of the gaseous mixture K = constant of the Scherrer formula So = amount of reducible species, pmol V* = total flow rate, cm3 (NTP)/s X i= molar fraction of component i Greek Symbols
0 = breadth of pure diffraction profile X = wavelength of X-rays B = angle between incident and diffracted beams Registry No. CH,, 74-82-8; AlzO3, 1344-28-1; NiO, 1313-99-1; Ni, 7440-02-0; Ca12A114033, 12005-57-1.
Literature Cited Bandrowski, J.; Bickling, C. R.; Yang, K. H.; Hougen, 0. A. Chem. Eng. Sei. 1962, 17, 379. Bartholomew, C. H.; Farrauto, R. J. Catal. 1976, 45, 41. Bartholomew, C. H.; Pannell, R. B. J. Catal. 1980, 65, 390. Bartholomew, C. H.; Pannell, R. B.; Butler, J. L. J. Catal. 1980,65, 335-347. Butt, J. B.; Wachter, C. K.; Billimoria, R. M. Chem. Eng. Sci. 1978, 33, 1321. Holm, V. C. F.; Clark, A. J. Catal. 1968, 11, 305. Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed.; Wiley: New York, 1973. Lo Jacono, M.; Schiavello, M.; Cimino, A. J. Phys. Chem. 1971, 75(8), 1044. Monti, D. A. M.; Baiker, A. J. Catal. 1983,83, 323. Roman, A.; Delmon, B. J. Catal. 1973, 30, 333. Rostrup-Nielsen, J. R. J. Catal. 1984, 85, 31. Zielifisky, J. J. Catal. 1982, 76, 157. Received f o r review September 30, 1985 Revised manuscript received February 19, 1987 Accepted March 30, 1987
Catalytic Deactivation on Methane Steam Reforming Cata1ysts. 2. Kinetic Study Miriam E. Agnelli,t Esther N. Ponzi,*+and Avedis A. Yeramiant Centro de Investigacidn y Desarrollo en Procesos Cataliticos (CINDECA), Facultad de Ciencias Exactas, U.N.L.P.-CONICET, 1900 La Plata, Argentina
T h e kinetics of methane steam reforming reaction over an alumina-supported nickel catalyst was investigated at a temperature range of 640-740 O C in a flow reactor at atmospheric pressure. The experiments were performed varying the inlet concentration of methane, hydrogen, and water. A kinetic scheme of the Houghen-Watson type was satisfactorily proposed assuming the dissociative adsorption of CH4 as the rate-limiting step, but this kinetic scheme can be easily replaced by a first-order kinetics (rCH4 = k p c H a ) for engineering purposes. Catalyst activation with H2 and N2 mixtures or with the reactant mixture results in the same extent of reaction. Catalyst deactivation due to carbon deposition is a serious problem in the steam reforming process. This deposition may act in three ways: (1) fouling the metal surface, (2) blocking the catalyst pores and voids, or (3) disintegrating the catalyst support. The various ways of deactivation depend on the type of deposited carbon: whisker-like carbon produces loss of the catalytic activity blocking the pores, whereas encapsulating carbon produces deactivation by fouling the metal surface. Whisker-like Members of CONICET's Scientific and Technological Research staff. 0888-5885/87/2626-1707$01.50/0
carbon also breaks the structure of the catalyst support, which causes pressure increase in the reformer furnace tubes (Rostrup-Nielsen, 1975). To our knowledge, no studies have so far attempted to postulate an adequate model for deactivation in the steam reforming conditions. Such a system can be modeled by using the separable kinetic concept (Butt et al., 1978) which allows us to express the instantaneous rate of the reaction equation as the product of terms containing concentration (C), temand activity ( a ) dependences: perature (T), r = r l ( C , T)r&) 0 1987 American Chemical Society
