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.
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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 Catalytic dehydration 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 semiempirical character 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-force term. 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 effectiveness factor must be known. Since the intrinsic kinetic equation is highly nonlinear the effectiveness factor 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 intraparticle resistances, spheres were
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1036 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992
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