Ind. Eng. Chem. Res. 1992,31, 1040-1045
1040
A , B, D = constants in eq 11, dimensionless c = specific heat of component i, kJ/(mol K) = concentration of component i, kmol/m3 dp = particle diameter, m dT = reactor diameter (Figure 6), m Di = diffusivity of component i, m2/s Deff = effective diffusivity of component i, mz/s E , = activation energy, kJ/mol AH = reaction enthalpy, kJ/mol ks = rate constant of surface reaction, mol/(g,,,/h) K = equilibrium constant, dimensionless Ki = adsorption constant of component i, m3/kmol P = pressure, bar r = radial coordinate, m R = particle radius, m -rM = reaction rate, mol/(g,,/h) -RM = global reaction rate, mol/(g,./h) Hi= heat of adsorption of component i , kJ/mol T = temperature, K TI= reactor inlet temperature (Figure 6), K V = particle volume, m3 x i = mole fraction of component i X = conversion, dimensionless
4
Greek Letters c = catalyst particle porosity, dimensionless CB = catalyst bed porosity (Figure 6), dimensionless 9 = effectiveness factor, dimensionless
A r = ~ effective heat conductivity of catalyst particle, kJ/(s
m K)
ui = stoichiometric coefficient of pp = particle density, g/cm3 T
component i, dimensionless
= tortuosity, dimensionless methanol mass flow rate (Figure 6), kg/h
@JM =
Subscripts
D = dimethyl ether i = component i (H20,(CH3)20,CH30H, N,) M = methanol S = conditions at catalysts surface v = per volume W = water 0 = initial conditions Registry No. MeOH, 67-56-1;-pAlz03, 1344-28-1;DME, 115-10-6.
Literature Cited Bakshi, K. R.; Gavalas, G. R. Effects of Nonseparable Kinetics in Alcohol Dehydration over Poisoned Silica-Alumina. AIChE J. 1975,21,494-500.
BerEiE, G. Dehydration of Methanol over yA1208.Kinetics of Reaction and Mathematical Model of an Industrial Reactor. Ph.D. Dissertation, The University of Ljubljana, 1990. BerEiE, G.; Levec, J. Reactor Model for the Catalytic Gas-Phase Dehydration of Methanol to Dimethyl Ether (DME). Vestn. Slou. Kem. Drus. 1991,38,253-270. Brake, L. D. U.S. Patent 4,595,785,1986. Chang, C. D.; Kuo, J. C. W.; Lang, W. H.; Jacob, S. M. Process Studies on the Conversion of Methanol to Gasoline. Ind. Eng. Chem. Process Des. Deu. 1978,17,255-260. Duggleby, R. G. Regression Analysis of Nonlinear Arrhenius Plots: An Empirical Model and a Computer Program. Comput. Biol. Med. 1984,14,447-455. Figueras, F.; Nohl, A.; Mourgues, L.; Trambouze, Y. Dehydration of Methanol and tert-Butyl Alcohol on Silica-Alumina. Trans. Faraday SOC.1971,67,1155-1163. Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design; John Wiley & Sons: New York, 1979;p 185. Gates, B. C.; Johanson, L. N. Lungmuir-Hinshelwood Kinetics of the Dehydration of Methanol Catalyzed by Cation Exchange Resin. AIChE J . 1971,17,981-983. Hindmarsh, A. C. Solving Ordinary Differential Equations on an IBM-PC Using LSODE. LLNL Tentacle Magazine; LLNL Livermore, CA, April 1986;Vol. 6,No.4. Kallo, D.; Knozinger, H. Zur Dehydratisierung von Alkoholen an Aliminiumoksid. Chem.-Ing.-Tech. 1967,39,676-680. KlusaEek, K.; Schneider, P. Stationary Catalytic Kinetics via Surface Concentrations from Transient Data. Methanol Dehydration. Chem. Eng. Sci. 1982,37,1523-1528. Massaldi, H. A.; Maymo, J. A. Error in Handling Finite Conversion Reactor Data by the Differential Method. J. Catal. 1969,14, 61-68. Riggs, J. M. An Introduction to Numerical Methods for Chemical Engineers; Texas Tech University Press: Lubbock, TX, 1988;p 406. Rousseeuw, P. J.; Leroy, A. M. Robust Regression and Outlier Detection; John Wiley & Sons: New York, 1987. Rubio, F. C.; Diaz, S. D.; Castillo, D. D.; Trujillo, J. D.; Alvarez, R. A. Deshidratacion Catalitica de Metanol en Fase Vapor. Ing. Quim. (Madrid) 1980,12,113-119. Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis; M.I.T. Press: Cambridge, MA, 1970; p 42. Schmitz, G. Deshydration du Methanol Sur Silice-Alumine. Chim. Phys. 1978,746504355, Sinicyna, 0. A.; Cumakova, V. N.; Moskovskaja, I. F. Kinetika Degidratacii Metanola do Dimetilovogo Efira na SVK Ceolite. Kinet. Katal. 1986,27,1160-1162. Than, L. N.; Setinek, K.; Beranek, L. Kinetics and Adsorption on Acid Catalysts. IV. Kinetics of Gas-Phase Dehydration of Methanol on a Sulphonated Ion Exchanger. Collect. Czech. Commun. 1972,37,3878-3884. Woodhouse, J. C. U.S. Patent 2,014,408,1935. Yang, K. H.; Hougen, 0. H. Determination of Mechanism of Catalyzed Gaseous Reactions. Chem. Eng. B o g . 1960,46,146-157.
Received for review May 7, 1991 Accepted November 1, 1991
Kinetics of the Catalytic Hydrochlorination of Methanol to Methyl Chloride Albert0 M. Becerra, Adolfo E. Castro Luna, Daniel E. Ardissone, and Marta I. Ponzi* Facultad de Ingenieria y Administracion, Uniuersidad Nacional de San Luis, INTEQUI-CONICET, Au. 25 de Mayo 384, 5730 Villa Mercedes, San Luis, Argentina
The intrinsic kinetics of the catalytic hydrochlorination of methanol to methyl chloride on yAl,O, was determined from experiments in a tubular reactor in the temperature range of 513-593 K and at atmospheric pressure, after a catalyst screening and a study of operative conditions. A large number of detailed reaction mechanisms was considered. A strategy of model discrimination and parameter estimation led to a Hougen-Watson type model with statistically significant and thermodynamically consistent parameters. The two main technologies used commercially for obtaining methyl chloride are hydrochlorination of methanol
and chlorination of methane. Methyl chloride is used as an intermediate in the obtainment of chlorinated bypro-
0888-5885/92/2631-1040$03.00/00 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 1041 ducts and in the manufacture of silicons, synthetic rubber, and methyl cellulose and as general methylating agent. The catalytic reactor system operates commercially at about 573-623 K, at an equimolal feeding ratio, and at a space velocity of 300 volumes of STP gas per hour per volume of the catalyst employed. Svetlanov et al. (1966) mentioned as catalysts ZnC12and CdC12on various supports such as aluminum oxide, silica gel, aluminum silicate, and carbon. Impregnation of alumina with ZnC1, has several disadvantages, like pore blockage with its consequent area lowering, and the low melting point of ZnC1, (558 K) and its high vapor pressure limit the working temperature. Schlosser et al. (1970) used aluminum oxide as a catalyst because of its relatively high activity and comparatively low price. Also, Thyagarajan et al. (1966) used alumina-silica gel and alumina. Different zeolites have also been mentioned as catalysts for this reaction (Magoren, 1983). The kinetics of the catalytic reaction in the gaseous phase between methanol (Me) and hydrogen chloride (CL) for obtaining methyl chloride (MC) is of great importance for the design and accurate modeling of the industrial reactor. In spite of the industrial importance of the product, there are not many studies on the subject, and only first-order models have been investigated so far. On the basis of previous information, a commercial alumina was chosen as catalyst after the catalytic behaviors of five commercial samples were experimentally compared. Finally, a kinetic study of the Me hydrochlorination reaction free of diffusional limitations was carried out. The reaction can be represented as follows: CH8OH + HC1- CHBCl+ H20 AHo298 = -34.7 kJ/mol (1) In the presence of a catalyst, reaction 1can be accompanied by the following reactions: 2CH30H e (CH3)20+ H 2 0 m0298 = -23.8 kJ/mol (2) (CH,),O
+ 2HC1-
2CH3C1 + H 2 0
m0298 = -45.8 kJ/mol (3)
The MC total rate of formation, rt, is rt = rl + r3
(4)
rl and r3 being the reaction r a t a of (1)and (3), respectively. The reactions are highly exothermic. Reactions 1and 3 are practically irreversible, while reaction 2 is limited by equilibrium.
Experimental Section Experimental Setup. A conventional experimental setup was used. CL is fed from a cylinder through a pressure regulator and a capillary flow meter, using sulfuric acid as the manometer fluid. The CL flow is then measured more accurately in a bubble meter using an acid aqueous solution of cetyltrimethylammonium chloride in order to obtain a stable bubble in the buret. Me is fed by a nitrogen current that becomes saturated as it passes through a thermostated bubbler set containing this reagent. The gas flow is adjusted with a mass flow controller and measured with a bubble meter. Both currents are mixed when they enter the reactor in an ascending flow. The current of unchanged reactants and producta goes through a train of four washing flasks. The first one contains water and the second 4 % NaOH at room temperature so that Me and CL are retained. The
Table I. Physical Properties of Alumina Pellets Catalyst: 7-A1203 F-. PIA
source S,/(m2/g) 6/(g/cm3) c N2 Hg Cimvap 100 0.40 0.85 35 Alcoa 187 1.25 0.50 18-30 21-30 Ketjen 195 0.81 0.69 37 37 Kaiser 258 0.96 0.63 20 20 Rhone Poulenc 90 0.77 0.68 40 50
third and fourth flasks contain H2S04of 60 OB8 in order to retain the water and ether eventually produced during reaction 2. The tubular reactor (Pyrex, 200-mm length, 20-mm i.d.) was operated in the integral mode. A thermocouple is sliding in a central tube of 6-mm od. The reaction section containing 0.5 g of catalyst has a length of 20 mm. The original catalyst was crushed to 0.25 f 0.05 mm to eliminate internal limitations to mass transfer and diluted with ground glass to ensure isothermicity. MC contained in the gaseous product was determined by absorption in glacial acetic acid through a method specially developed for quickly and accurately determining methyl chloride (Allison and Meighan, 1919). In this method a sample of the reactor outlet is measured in a gas buret and passes several times into a Hempel pipet containing glacial acetic acid. The volume of the remainder gas is measured again. The difference between the last and the former volumes indicates the methyl chloride present in the sample. The maximum error in the determination was 5%. This method avoided the corrosion problems found through analysis by gas chromatography. The temperature control was, in all runs, better than f l K, and the gas flow control was better than 5%. Catalysts. Commercial alumina samples from Cimvap, Alcoa, Ketjen, Rhone Poulenc, and Kaiser were used. Physical properties of these aluminae are shown in Table I. Preliminary saturation of the surface with hydrogen chloride was found to promote a faster catalyst operation stability.
Results and Discussion Although the reaction system is complex, the total rate of MC formation may be attributed to reaction 1on the following basis. (a) Independent research on reactions 1 and 3 carried out by Svetlanov and Flid (1966) states that r3
