I n d . Eng. Chem. Res. 1992,31, 1030-1035
1030
Raask, E. Sintering characteristics of coal ashes by simultaneous dilatometry-electrical conductance measurements. J . Therm. Anal. 1979,16,91-102. Reid, W . T. The relation of mineral composition to slagging fouling and erosion during and after combustion. Prog. Energy Combust. Sci. 1984, 10, 159-175. Skrifvars, B.-J. Sinteringof fuel aph, a laboratory testing method (in Swedish). Licentiate thesis, Abo, Akademi University, Turku, Finland, 1990. Skrifvars, B.-J.; Hupa, M.; Hyoty, P. Superheater fouling due to limestone injection in coal-fired boilers. J. Znst. Energy 1991a, 64 (Dec), 196-201.
Skrifvars, B.-J.;Hupa, M.; Hyoty, P. Composition of recovery-boiler dust and its effect on sintering. Tappi J. 1991b, 74 (No. 6, June), 185-189. Smith, E. J. D. The Sintering of Flv-ash. J. Znst. Fuel 1956, XXZX (185, June), 253-260. Sondreal.E. A.: Tufte. P. H.: Beckerinn. W. Ash fouline in the combustion of low rank wes&rn U.S. ccks. Combust. Sci. Technol. 1977, 16, 95-110.
Received for review May 23, 1991 Reuised manuscript received November 18, 1991 Accepted December 4, 1991
Mechanistic Effects of Arsenic Oxide on the Catalytic Components of DeNO, Catalysts Erich Hums Power Generation Group (KWU),Siemens AG, P.O. Box 3220,D-8520 Erlangen, Germany
The interaction of arsenic oxide as a DeNO, catalyst poison was studied with a TiOz-Mo-V composite oxide catalyst (11). It showed differences in catalytic and other properties from a TiO2-MoO3-VzO5 catalyst (I) obtained by monomolecular dispersion of Moo3 and Vz05 on TiOz. Since the arsenic poisoning mechanism on Moo3 is obviously identical in both catalyst systems, the vanadium-molybdenum composite oxide phase used here is responsible for the differences in behavior from TiO2-MoO3-VzO5 catalyst (I). As a poisoned intermediate, an As4Mo3OI5phase was identified by X-ray diffraction. This phase is formed initially and subsequently transformed to the more stable MoAsz07phase. The latter phase was analyzed by structural analysis and possesses mixed oxidation numbers, namely, As3+and As5+. Mo6+,however, retains its valence. Replacing Moo3 with Mooz suppresses formation of a composite oxide phase containing arsenic.
Introduction It is a well-known fact that DeNO, catalysts installed on the high-dust side of wet-bottom furnaces are deactivated far more rapidly than those in dry-bottom furnaces. This rapid deactivation is, on the basis of current knowledge, essentially caused by the arsenic oxide content of the flue gas. By comparison with dry-bottom furnaces in which the catalysts are merely exposed to the catalyst poison burden entrained in the coal, a concentration of catalyst poisons takes place in wet-bottom furnace plants, which varies with the amount of ash returned to the furnace (Kautz et al., 1975; Gerhard et al., 1985; Cramer, 1986). Since arsenic oxide has been under discussion as a catalyst poison for DeNO, catalysts containing TiO,, numerous attempts have been made to find plausible models and explanations of the poisoning mechanism (Hums, 1991). A property specific to catalysts containing tungsten oxide which are exposed to flue gas containing arsenic oxide is that they deactivate at much faster rates than catalysts containing molybdenum oxide; reference is also made to this fact in the patent literature on this subject. Comparison of the performance of three catalysta after 1800 h of exposure in the flue gas of a wet-bottom furnace shows this very impreasively, but focusea attention in particular on the TiO,-Mo-V composite oxide catalyst (11)and on the Ti02-Mo03-V,0s catalyst (1)-a catalyst obtained by formation of a monomolecular dispersion of Moo3and V20s(monolayer) on TiOP Discrepancies in the catalysts occur in the distribution of pore radii (Hums, 1991) and the deactivation rates (Figure 1). The material compositions also do not lead to SO, SO3 oxidation rates that correspond to their vanadium content (Figure 2; Table I). Due to the fact that the two catalyst systems are made up of the same elements, this behavior can only be at-
-
Table I. Catalyst Composition component
TiW
88.50
VZO, additives
catalysts, wt % I 80.58
79.93
10.42 0.41 8.59
10.41 1.16 8.50
0.60 10.90
I1
tributed to differences in the structure of active sites. These differences in the structure would in particular be evident in the reactivity with arsenic oxide and affect the respective catalytic components of the catalysts. In those chemical reactions in which solids are involved, surface layers are by nature the starting point of reactions which may propagate into the solid. In this case we have investigated the influence of As203on the phase composition of V205-M003mixtures free of TiOzby X-ray diffraction (XRD). We established that As203triggers the same phase transformation (V9Mo6OU V6M0402s)in V205-Mo03systems both with and without TiOz (Hums and Gabel, 1991). A crystalline composite oxide phase containing arsenic is not involved. It cannot be denied that this phase transformation occurs on the Ti02-Mo03-V20s catalyst (I) which we endeavor to prevent in the case of the TiOpMo-V composite oxide catalyst (11). But it has not yet been possible to find direct proof of this with low surface dispersion as with monomolecular layer. In the following, this topic is expanded to include the Moo3 component and a check made to establish whether A s 2 0 3 reacts with molybdenum oxide to form composite oxides at temperatures approaching those encountered in power plants. In this way it was thus possible to synthesize the A L ~ ~ M phase, O ~ Owhich ~ ~ is as yet unknown in the literature,
0888-588519212631-1030$03.00/00 1992 American Chemical Society
-
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 1031 Relative activity
tWk0
>
5.000
9.000
13.00
17.00
21.00
25.00
29.00
33.00
37.00
41.00
45.00
5700
61 00
65.00
6900
7300
7700
81 00
8500
I TiW
I
II *
4500
Figure 1. Relative activity after 1800 h of exposure in a high-dust wet-bottom furnace for the catalyst TiW, the TiO2-MoO3-V2O5 catalyst (I), and the Ti02-Mo-V composite oxide catalyst (11). *, arsenic oxide reduced. Oxidation rate
Reduction rate
T".-
I- '
112
40
c
83
17
0
0 TiW
I
Figure 2. Comparison of the oxidation rate, kmz,and the reduction rate, kNO , for the catalyst TiW, the Ti02-Mo03-V205 catalyst (I), and the 'ri02-Mo-V composite oxide catalyst (11) at 350 OC. *, hydrocarbon reduced.
and to index it using X-ray powder diffraction (Hums and G6bel,1990). The existence of this compound rouses the justifiied hope of obtaining fundamental insights into the poisoning mechanism by defining its structure.
Experimental Section This paper does not elaborate on catalyst preparation. In all cases, however, the production process involves blending essentially the components Ti02 (anatase), ammonium heptamolybdate ( T ~ ~ , - M o ~ ~ catalyst - V ~ O (I)), ~ or Ti02 (anatase) coprecipitated with W03 (TiW) in each case with soluble vanadium salts, or blending Ti02 (anatase) and Moo3with a vanadium-molybdenum composite oxide (TiO,-Mo-V composite oxide catalyst (11))to produce a spreadable mass which is then rolled onto expanded
4900
5300
diffraction angle (2 Theta) __*
Figure 3. X-ray powder diffraction pattern of the As4M~OI6 phase (I) and the MOO, Phase (2) (JCPDS No. 35-0609) and the residual pattern as a result of the phase transformation of (1) MoAs,O,.
-
metal. The samples were then dried and calcined at 500 "C. Analytical data on the specimen catalysts are described elsewhere (Hums, 1991). The samples were analyzed in a test reactor before and after exposure in a power plant as to their catalytic activity (gas composition 400 mL/m3 NO, 400 mL/m3 NH3, 500 mL/m3 SO2, 4 vol % 02,10 vol % H20, remainder N,; reaction temperature 350 "C; linear velocity (LV) 4 m/s; volumetric flow rate 4.35 m3(NTP)/h). The ksoz values were analyzed before exposure (gas composition: 500 mL/m3 SO2, 4 vol % 02,10 vol % H20, remainder N,; reaction temperature 350 "C; linear velocity (LV) 2 m/s; volumetric flow rate 2.18 m3(NTP)/h). IR spectra of the catalysts (Ti02-Mo03-V206catalyst (I) and Ti02-Mo-V composite oxide catalyst (11))tested in the power plant and their reference samples were recorded in diffuse reflection in the range from 4000 to 500 cm-' using a Bruker FTIR spectrometer (Type JFS 88). The Spectra Tech diffuse reflectance cell was flushed with argon at 100 cm3/h. Spectra were recorded at T = 50 "C. As203was mixed with Moo3 or Moo2,melted in quartz ampules, and heat-treated at 430 "C for 430 h. The powder samples were then measured using a transmission powder X-ray diffractometer (Siemens Type F) with Guinier focusing (Figures 3 and 4). Identification was performed using the DIFRAC AT analysis system (Siemens) and the JCPDS data file (up to 1988). The X-ray diffraction pattern for the M o b 0 , phase was calculated from single crystal data using a Guinier simulationprogram corresponding to the relations
I(h) = P L G H P v(h)l2
(1)
1 + cos2 28 4 sin2 8 cos e I(h) is the intensity of the reflex referred to the Bragg reflex (h),Pis the polarization factor (P= (1+ cos228)/2), L is the Lorentz factor (L = l/(sin e cos e)), G is the geometry factor (G= cos O/sin 28), H is the area frequency factor (the number of symetrically equivalent h vectors which, due to their vectors of equal length, belong to the same diffractioncone), Tis the Debye-Waller factor, F(h)
PLG =
1032 Ind. Eng. Chem. Res., Vol. 31, No. 4,1992
5.000
9.000
13.00
17.00
21.00
25.00
4900
53 00
57 00
61 00
6500
I 45 00
diffrawon angle (2 Theta)
29.00
33.00
37.00
41.00
45.00
-
7700
81 00
8500
6900
7300
1'
Figure 4. X-ray powder diffraction pattern of the Mooz phase (1) (JCPDS No. 32-067) after reaction with As203(2) (JCPDS No.361490).
is the structural factor referred to the Bragg reflex (h),and 0 is the diffraction angle. The X-ray diffraction data thus obtained are compiled in Table I1 for all patterns with intensities >4%.
Results and Discussion Differences in Catalytic Behavior and Catalyst Appearance. Ti02-Mo03-V205 catalyst (I) and Ti0,M e V composite oxide catalyst (11) exposed to flue gas in a wet-bottom furnace show pronounced differences in catalytic behavior and in pore radii distribution characteristics, which are determined exclusively by catalyst preparation process variables. They also differ insofar as the material compositionsof the catalysts investigated do not lead to SO2 SOsoxidation rates that correspond to their vanadium content. This could be taken as an indication that the active species of the two catalyst systems differ structurally. A demarcation to the Ti02-MeV composite oxide catalyst (II) is given by KnBzinger (19901, Hausinger et al. (1988), and Hilbrig (1989),who confirmed for the Ti02-Mo03-V206 catalyst (I) that the spreading of MOO, and V205 had reached molecular dispersion (monolayer). This concept of monolayers in oxidic catalysts is required, according to Russell and Stokes (1946), who observed that molybdenum-alumina catalysts show maximum activity when the carrier was completely covered by an Moo3 monolayer. But this activity limit is greatly exceeded in reference to the monolayer in Ti02-Mo03V206catalyst (I),when V206and MOO, were replaced by the composite oxide phase V6M04025. The spreading mechanism seems to be suppressed in this form in the case of the composite oxide phase. Based on the above dispersion concept, coordinatively unsaturated M05 sites (M = V, Mo, W)are formed for V206, Moo3, and W 0 3 supported on the Ti02 carrier. These represent Lewis acid centers for H20,NH,,or other Lewis basea. Because of the saturation of the coordination vacancy, these centers assume a pseudooctahedral structure (Hilbrig, 1989). In the event of adsorption the stretching overtone of the terminal oxo group of the metal atom M is perturbated. As the vanadium minority phases in the Ti02-Mo03-V206catalyst (I) and the Ti02-Mo-V
-
Table 11. X-ray Powder Diffraction Pattern Calculated by Guinier Simulation Program 28, deg d, A P' h k l 11.12 %95 8.55 2 0 0 11.88 7.44 -1 0 1 18.37 13.64 1 6.49 53.58 0 1 18.59 4.77 37.40 0 1 -3 1 22.68 3.92 23.21 1 0 0 23.15 0 2 3.84 17.39 -2 1 1 24.40 42.73 3.64 1 100.00 26.22 2 1 3.40 27.49 13.56 2 0 2 3.24 42.79 0 1 28.42 3.14 -5 0 7.31 29.36 3.04 -4 2 1 30.43 2 6.68 0 2.94 -4 1 1 7.61 30.55 2.92 1 2 32.82 2.67 62.98 -3 1 1 13.45 33.49 2.67 4 0 10.76 33.79 2.65 0 6 1 19.95 0 5 34.36 2.61 13.91 34.48 -1 0 3 2.60 35.19 4 0 2 6.77 2.55 1 2 15.12 36.90 2.43 3 1 5.27 39.30 2.29 6 0 2 39.54 0 0 7.60 2.28 1 40.25 -2 3 6.00 2.24 2 1 -2 11.84 42.34 2.13 1 0 6.22 44.53 2.03 7 1 5.72 45.96 1.97 3 -5 7.24 47.32 1.91 -8 0 2 2 2 2 8.50 48.83 1.86 1 2 10.43 49.40 1.84 -5 1 4 5.25 49.78 1.83 3 51.74 1 1.76 4 5.39 -3 2 1.73 7.59 52.97 0 6 0 4 1.70 5.73 53.81 -6 2 7.16 55.61 8 0 1.65 2 56.98 -7 1 1.61 6.56 57.13 1 3 1.61 6.70 -8 58.42 2 1.58 6.33 3 -5 2 4 1.36 5.58 68.80 -6
composite oxide catalyst (11) were not detectable spectroscopically in either catalyst system, there is no other way available than to characterize the remainder of the phase composition in terms of spectroscopic differences. This approach was used to indirectly localize the cause of catalytic differences between the two catalyst systems. Determination of Mo-O-As Phase Composition by XRD. The reaction of Moo3 with As20s yields As4MoO3OI5which, when subjected to extended heat treatment, is transformed into M0As207. It is to be observed that nucleus formation dominates over grain growth for the As4Mo3OI5phase. Replacing Moo3 with Moo2 and thus changing the coordination sphere suppresses the formation of a compound with As203. Under comparable test conditions thisreaction triggered by b o 3dom not take place (Figure 4). Determination of Formal Oxidation Number of Arsenic by XAS and XPS. X-ray absorption spectroscopy (XAS) using synchrotron radiation was the method chosen by Hilbrig et al. (1989) for determining the formal oxidation number of arsenic. Characterization of the arsenic on the catalyst surface in terms of its structure and the nature of interaction with the active components of the catalysts offers in any case the key to defining the differences in the deactivation behavior of the two catalyst systems. The location of the arsenic absorption edge of the Ti02-Mo0,-V205 catalyst (I) used in the power plant was assigned primarily to arsenic with a valence of 5+ on the basis of reference substances (As,As2O3, Ag3AsO!, KAsF6, and As205).The chemical shift of the araenic absorption edge is between that of Asb+in Ag,AsOl and As205. Using X-ray photoelectron spectroscopy (XPS),
Ind. Eng. Chem. Reg.,VoL 31,No.4,1992 1033
000 0
-3.3
As
Mo
+3.3
Figure 5. Projection of a unit cell dong axen b for MoAs,O?.
Rademacher et aL (19BZ)found both Ass+ and As6+in theae samples. This apparent contradiction calls for a more precise defiiition of the question as to whether the As6+ measured in transmission using XAS is primarily in the TiOz bulk, while XPS can only cover the near-surface areas of the catalyst. Structural Proposal of the Poisoned MOO,by ExAFS and Structure Analysis. A contradiction is taken into consideration when one questions the structural vicinity of As3+and Asw in the catalyst surface and the As-0 distances (averaged over the entire solid) of crystallographically measured model substances are used for reference purposes. Hilbrig (1989)assigned the A s 4 radial distances for the arsenic in the TiOz-Mo03-Vz05 system on the basis of the reference substances Ag&04 and &05 in the EXAFS (extended X-ray adsorption fine structure) range. This author indicated the resultant A s 4 radial distances to be 1.67 A for the three oxygen atoms of the first shell and 1.94 A for the only oxygen atom of the second shell. This 3 + 1 oxygen coordination around the As6+ center and the A s 4 spacing are typical of isolated orthoarsenate structures as present, for example, in N e HAs0,.7Hz0 (Ferraris and Chiari, 1969; Baur and Khan, 1970). According to Hilbrig et al. (1991)this orthoarsenate structure is bound directly to the TiOz (anatase) carrier. This structure could also be expected to interfere with the stretching overtone of the molybdenum oxo group. A
mechanistic uroIMBIII for the I imical attaq of the vanadium specik k analogy to the molybdenum oxo group was still not referenced. Hibrig (1989)postulates coordination to the MOO5odahedralvacancy. By contrast with reversible NH, adsorption, a chemical bond with the central atom incorporating orthoarsenate is expected. The existence of an M+As6+ bond was not, however, demonstrated. The necessary increase in the coordination number of the molybdenum cawed by arsenic oxide cannot be confiied by measurements at the molybdenum absorption edge either (Habrig, 1989). We recently demonstrated interaction between arsenic and molybdenum in the oxygen lattice of powder samples, which we prepared by reacting MOO, with AszO, (Humsand G6bell,1990). In ow efforts to grow single crystals to defiie the structure, we were surprised to find that the As4Mo3OI5phase mentioned in the above publication converts by phase transformation into MoAs,O, (Figure 3). The structure of this phase was determined on the basis of measurements on single crystals (Humset al., 1991). The diffraction peak locations show g o d agreement with those obtained by powder diffraction (Table 111)and confirm that the reflexes can be traced exclusively to MoAszO, and no other phase is to be considered. Indexing based entirely on powder data is currently in prcgrem (Humsand Giibel, 1992). The fact that the MoAs,O, phase possesses arsenic of mixed valences again draws attention to As3+ and AsS+analyzed
1034 Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 Table 111. X-ray Powder Diffraction of t h e Residual Reflexes 28, deg d, A 11.012 8.0279 10.14 11.792 13.58 7.4985 13.553 22.65 6.5278 18.497 29.17 4.7926 22.582 15.86 3.9341 23.120 30.38 3.8437 24.292 3.6608 30.79 100.00 26.114 3.4094 27.245 3.2704 72.30 3.1499 34.36 28.309 45.31 29.471 3.0282 22.63 30.484 2.9298 4.55 31.209 2.8634 32.709 2.7354 31.80 9.21 33.364 2.6833 24.50 33.665 2.6600 34.260 2.6151 13.60 34.360 2.6077 9.90 34.751 2.5793 6.65 5.72 35.130 2.5523 10.61 36.800 2.4402 38.885 2.3140 16.30 11.54 39.439 2.2828 4.99 40.137 2.2447 12.55 42.195 2.1399 4.28 44.398 2.0387 9.64 46.220 1.9625 4.16 47.188 1.9244 11.33 47.549 1.9106 48.705 1.8680 5.96 11.33 49.654 1.8345 52.796 1.7324 9.88 53.696 5.72 1.7055 55.574 6.65 1.6523 56.964 1.6152 8.34 58.273 5.01 1.5820 68.590 1.3670 4.93 Table IV. Comparison of As-0 Distances in Arsenic Compounds compound As-0 distance, A literature As5+in Ti0,- 1.6700 1.6700 1.6700 1.9400 Hilbrig (1989) Mo03-V206 LiMoOzAsO, As5+ (3.95)' 1.6810 1.6880 1.6890 1.7200 Linnros (1970) As5+ (4.41)' 1.7020 1.6890 1.6650 1.7170 MOAS~O, As5+ (3.7)' 1.6708 1.6720 1.6835 1.7011 Hums et al. As3+ (3.7)' 1.7782 1.7802 1.8507 (1991) As203 (4.0)' 1.7800 1.7800 1.7800 Pertlik (1978)
" R , value for refinement.
by Rademacher et al. (1992) using XPS. The orthoarsenate in this compound is located tetragonally between the edge-linkedmolybdenum octahedrons, with which it shares three oxygen atoms. (Figure 5 ) . In contrast to this agreement of data, the As-0 distances in the MoAs207phase differ from those measured by EXAFS for the Ti02-Mo03-V206system (Table IV). Because the MoAs20, structure forms isolated layers which are linked two-dimensionally, a monomolecular dispersion on Ti02 (anatase) is conceivable. Calculations are now in progress (Hums and Spitznagel, 1992). FTIR Spectroscopy. The preliminary stage could be a coordinatively unsaturated Moo6 unit, as proposed by Hilbrig et al. (1991) as a reactive species for arsenic oxide poisoning. Differences are not observed in the shift of the M d stretching overtones in the catalysts after exposure to flue gas containing arsenic oxide. The shift to lower wavenumbers is the same in both catalyst systems from 1968.51 to 1951.16 cm-'. This indicates that the poisoning
mechanism acting on MOO, is obviously identical in the two catalysts.
Conclusion On the basis of crystallographic analyses of the catalyst component Moo3, which was transformed with As203a t temperatures approaching those encountered in power plants, it was possible to elucidate the structural attack site of arsenic on molybdenum. An As4Mo3OI5 M0&07 phase transformation occurs as a function of the exposure period. Molybdenum apparently does not change its formal valence of 6+. Replacing Moo3 with Mooz suppresses formation of a composite oxide phase containing arsenic. At temperatures approaching those encountered in power plants, the V6M04026phase mentioned in our earlier publication (Hums and Gobel, 1991) and the molybdenum component when chosen as Moo2are therefore not exposed to chemical attack by arsenic oxide via compound formation. The only feature left for mechanistic elucidation of the differences in chemical deactivation of the two catalysts for the Ti02-Mo-V composite oxide catalysts (11)therefore seems to be the specific property of the V6Mo4OZs composite oxide phase. But there is still insufficient knowledge as to how it interacts with the carrier. In the future our interest will focus on this phase, which we use preferably as a precursor for DeNO, catalysts exposed to flue gas containing arsenic oxide. Registry No. TiW, 71673-37-5; Ti02, 13463-67-7; Moo3,
-
1313-27-5; V2O5, 1314-62-1; As203, 1327-53-3; As4Mo3OI5, 133357-12-7; MoAs~O~, 136295-62-0; NO,, 11104-93-1.
Literature Cited Baur, W. H.; Khan, A. A. On the Crystal Chemistry of Salt Hydrates. VI. The Crystal Structures of Disodium Hydrogen Orthoarsenate Heptahydrate and of Disodium Hydrogen Orthophosphate Heptahydrate. Acta Crystallogr. 1970, B26, 1584-96. Cramer, H. Grundlegende Untersuchungen tiber die Emission von Spurenstoffen bei der Verbrennung von Kohle. VGB Kraftwerkstech. 1986,66(8), 750-3. Ferraris, G.; Chiari, G. The Crystal Structure of NazHAs04-7Hz0. Acta Crystallogr. 1970,824,1574-83. Gerhard, L.; Kautz, K.; Pickardt, W.; Scholz, A.; Zimmermeyer, G. Untersuchung der Spurenelementverteilung bei der verbrennung von Steinkohle in drei Kraftwerken. VGB Kraftwerkstech. 1985, 65 (lo), 753-63. Hausinger, G.; Schmelz, H.; Knozinger, H. Effects of the Method of Preparation on the Properties of Titania-Supported Vanadia Catalysts. Appl. Catal. 1988,39,267-83. Hilbrig, F. Beitriige zur Natur von Titandioxid getragenem Wolframoxid. Dissertation Munich, 1989. Hilbrig, F.; Gobel, H. E.; Knozinger, H.; Schmelz, H.; Lengeler, B. Interaction of Arsenious Oxide with DeNO,-Cataiyats. An X-Ray Absorption and Diffuse Reflectance Infrared SpectroscopyStudy. J. Catal. 1991,129,168-76. Hums, E. Entwicklung von Siemens/KWU DeN0,-Kataiysatoren der zweiten Generation fur den Einsatz hinter Schmelzkammerfeuerungen bei hohen Arsenoxidkonzentrationen. Chem. Ztg. 1991,NO.2,33-7. Hums, E.; Gobel, H. E. X-Ray Powder Diffraction Data of a New Compound As4Mo3VIS.Powder Diffr. 1990,5 (3), 170-2. Hums, E.; Gobel, H. E. Effecta of As203 on the Phase Composition of V,05-Mo01-Ti0, (Anatase)DeN0.-Catalvsta. Znd. EM. " Chem. Res.-1991,30; 18l.r-18. Hums. E.: Gobel, H. E. ComDarison of X-Rav Powder Diffraction Pattern with Diffraction Data by Structural" Analysis of Mo&07 Using Different Indexing Programs. To be published, 1992. Hums, E.; Spitznagel, G. W. M o h o 7 Monomolecular Dispersion on TiOz Carrier-A Quantum Chemical Calculation. To be published, 1992. Hums, E.; Burzlaff, H.; Rothammel, W. Evidence of the Chemical Interaction between Arsenious Oxide and Molybdenum in DeNO*-Catalysts by Structure Analysis of MoAsz07.-Appl.Catal. 1991,73,L19-L24.
Ind. Eng. Chem. Res. 1992,31, 1035-1040 Kautz, K.; Kirsch, H.; Laufhiitte, D. W. Spurenelementgehalte in Steinkohlen und den daraus entstehenden Reingassauben. VGB Kraftwerkstech. 1975,55 (lo), 672-6. Knbzinger, H. Benetzung im festen Zustand-Ein neuer Weg zur Herstellung uon oxidischen TrEigerkatalysatoren; Dechema: Frankfurt, June 1,1990. Linnros, B. The Crystal Structure of LiMo02Asz0,. Acta Chem. Scand. 1970,24, 3711-22. Pertlik, F. Structure Refinement of Cubic Asz03(Arsenolithe) with Single-Crystal Data. Czech. J. Phys. 1978, B B , 170-6.
1035
Rademacher, J.; Borgmann, D.; Hopfengiirtner, D.; Wedler, G.; Hums, E.; Spitznagel, G. W. X-Ray Photoelectron Spectroscopic (XPS) Study of DeNO, Catalysts after Exposure to Slag Tap Furnace Flue Gas. Appl. Catal. 1992, in press. Russell, A. S.; Stokes, Jr., J. J. Surface Area in Dehydrocyclization Catalysis. Ind. Eng. Chem. 1946, 38, 1071-4.
Received for review May 13, 1991 Revised manuscript received August 1, 1991 Accepted October 14,1991
Intrinsic and Global Reaction Rate of Methanol Dehydration over 7-A1203Pellets Gorazd BerEiEt and Janez Levec*J Department of Catalysis and Chemical Reaction Engineering, Boris KidriE Institute of Chemistry, and Department of Chemical Engineering, University of Ljubljana, 61 000 Ljubljana, Slovenia, Yugoslavia
Dehydration of methanol on y-Al,O, was studied in a differential fixed-bed reactor at a pressure of 146 kPa in a temperature range of 290-360 "C. A kinetic equation which describes a Langmuir-Hinshelwood surface controlled reaction with dissociative adsorption of methanol was found to fit the experimental results quite well. Coefficients in the equation follow the Arrhenius and the van't Hoff relation. The calculated value for the activation energy was found to be 143.7 kJ/mol, while calculated values for the heat of adsorption of methanol and water were 70.5 and 42.1 kJ/mol, respectively. The measured global reaction rates for 3-mm catalyst particles were compared to those calculated by means of intrinsic kinetics and transport processes within the particles. A reasonable agreement was found when the effective diffusion coefficients for reaction components were calculated using a parallel-pore model assuming that only Knudsen diffusion is important.
Introduction Catalyticdehydration of methanol over an acidic catalyst (e.g. y-A1203)offers a potential process for dimethyl ether (DME)production, which is used as an alternative to freon spray propellants. In the MTG process, as has been described by Chang et al. (1978),the first reactor performs such a reaction. The open literature provides no information on kinetic equations which can be used successfully in designing a commercial reactor. From the patent literature (Woodhouse, 1935; Brake, 1986) it can be concluded that reaction takes place on pure y-alumina and on y-alumina slightly modified with phosphates or titanates, in a temperature range of 250-400 "C and pressures up to 1043 kPa. The kinetics of methanol dehydration on acidic catalysts has been studied extensively resulting in different kinetic equations. A summary of the published equations is presented in Table I. Most of the equations, i.e. eqs 4-9, have been derived from the experiments conducted in conditions not found in an industrial reactor. The experiments were mainly performed with mixtures of methanol, water, and nitrogen at low vapor pressures. Since water produced during the reaction considerably retards the reaction rate, the derived rate equations have, more or less, a semiempiricalcharacter and are not suitable for the industrial reactor design, where reaction takes place at high conversion levels. The outlet component concentrations correspond to the equilibrium values. However, the rate equations (1)-(3) in Table I, which were derived for an acidic ion exchange resin as a catalyst and are based on the Langmuir-Hinshelwood (L-H) or the Eley-Rideal (E-R) mechanism, can be used for design purposes after a reversible term is introduced into the driving-forceterm. The aim of this work was to determine an intrinsic rate equation which can be used to model the global reaction t Boris
KidriE Institute of Chemistry.
* University of Ljubljana.
Table I. Summary of the Published Rate Equations ref eauation
Kallo and Knozinger, 1967
-rM
=k
CM1I2
+ k2Cw
Sinicyna et al., 1986; kKM2CM2 Gates and -rM = (1 + KMCM K w C W ) ~ Johanson, 1971 Figueras et al., 1971 kKMCM'I2 -rM = 1 + KMCM1/' KwCw
+
(5)'Vb
+
~KMCM (7)' (1 + K M C M ) ~ Schmitz, 1978 -rM kl + kzCM Wb Rubio et al., 1980 -rM = klCM1lz- kzCwl/z (9Y 'Acidic ion exchange resin as catalyst. bAlumina or silica-alumina as catalyst. Than et al., 1972
-rM
=
rates in a pilot-plant reactor where 3-mm catalyst particles were used. In order to calculate the global reaction rate the effectivenessfactor must be known. Since the intrinsic kinetic equation is highly nonlinear the effectivenessfactor can be calculated only numerically.
Experimental Section Catalyst. A Bayer SAS 350 -pAl,O, catalyst support in the form of 3-mm spheres was employed as a catalyst. In order to avoid the intraparticleresistances,spheres were
o a a a - ~ a s ~ ~ ~ ~ ~ ~ s ~ ~ -0i 1992 o ~ ~American $ o ~ . oChemical o / o Society
