Ind. Eng. Chem. Res. 1993,32, 2478-2484
2478
Catalytic Dehydration of Methanol to Dimethyl Ether. Kinetic Investigation and Reactor Simulation Gorazd Ber6iEt and Janez Levee'** Laboratory of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, and Department of Chemical Engineering, University of Ljubljana, P.O.Box 537, 61001 Ljubljana, Slovenia
One-dimensional heterogeneous and pseudohomogeneous plug flow models were employed to model an adiabatic fixed bed reactor for the catalytic dehydration of methanol to dimethyl ether. Longitudinal temperature and methanol conversion profiles predicted by these models were compared to those experimentally measured in a pilot reactor. The reactor was packed with 3-mm yAlzOs pellets and operated in a temperature range of 29O-36OoCand a t a pressure of 2.1 bar. Intraparticle mass transport was found to be the rate-controlling step. Introduction
Catalyticdehydrationof methanol over an acidic catalyst offers a potential method for production of dimethyl ether (DME),a new spray propellant. 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 250-40O0C and pressures up to 10 bar. The kinetics of methanol dehydration on an acidic catalyst has been studied extensively,resulting in different kinetic equations. Most of the rate equations have been derived from the experiments conducted in conditionsnot found in an industrial reactor. The experimenta were mainly performed with mixtures of methanol, water, and nitrogen at low water vapor pressures. Since water produced during the dehydration considerably retards the reaction rate, the derived rate equations are not suitable for the commercial reactor design,where the reaction takes place at highconversions. BerEiE and Levec (1992) recently reported an intrinsic rate equation in the form
which represents the kinetic behavior of the dehydration reaction over 7-alumina more realistically than equations published earlier. Equation 1thus offers a powerful tool that can be used to simulate the dehydration reactor performance. In the open literature, no information on the reactor simulation is given in spite of the fact that the methanol dehydration reaction is considered one of the basic reactions in the so-called C-1chemistry (e. g., MTG Process; Chang et al.,1983). The aim of this work is to present the experimental and simulated results of an adiabatic fixed bed dehydration reactor operated at conditions typically employed in a large-scale reactor. The global rates as well as the temperature and concentration profiles predicted by different models are compared with the experimental results obtained in a pilot reactor having the production capacity of about 150 kg of DMElday. Experimental Section The pilot-scale reactor system employed in this study is illustrated in Figure 1. Operating conditions as well as
* Author to whom correspondence should be addressed. +
Laboratory of Catalysisand Chemical ReactionEngineering.
* Department of Chemical Engineering.
some properties of the reactor and catalyst are listed in Table I. The reactor consisted of four stainless steel segments connected by flanges. The radial temperature gradient in the isolation shell was measured by thermocouples located at different radial positions in the shell and connected to the microprocessor-based thermoregulator. By means of two independently powered heating tapes in each element, one placed on the outside surface of the reactor wall and the other placedwithin the isolation shell, it was possible to maintain almost zero temperature gradient in the shell, and therefore adiabatic conditione existed within the catalyst bed. The radial and axial temperatures in the catalyst bed were measured using a HP 3421A data acquisitionlcontrolunit and a HP 150PC. Thermocouples were located in the bed axis at different axial positions, namely, at z = 0, 5, 10, 15, 25,36,45,55, and 65 cm, respectively. At the axial positions of 10,25, and 45 cm, thermocouples were also placed radial at F = 0, i = 0.5R, and i = 0.95R. Sampling tubes were located within the catalyst bed at 7; = 0. Gas samples, taken at different axial positions (z = 10,25, and 45 cm) and from the reactor outlet, were led through a four-way Valco SD flow path valve to a GC sampling valve. Analysis of the gas samples was performed by means of a HP 5890 GC equipped with a TCD. The SS column (230cm X 118)waspackedwithPorapakT (100/120mesh), while helium was used as the carrier gas. The oven temperature was kept at llO°C for the first 17 min and then continuously increased to 130 OC (at a rate of 4OoC/ min). A gas sample (0.25 mL) was injected by means of the sampling valve kept at 110OC. A calibration curve prepared for each component (methanol,water, and DME) was used to determine the compositionof each gas sample. Duration of the analysis was about 25 min. Experiments with 3-mm particles were also carried out in a differential fixed bed reactor in a temperature range of 290-360 OC and the constant pressure of 146 kPa. The reactor was operated free of the external mass- and heattransfer resistances. The inlet concentrationsof reactants were varied between 15 and 90 mol 7% for methanol and between 0 and 50 mol % for water. Even though DME was not present in the feed,we simulatedan integral reactor that is made up of series of differential reactors. The objective was to determine the rate in these differential reactors (BerEiE and Levec, 1992). Reactor Model
A compromise between an excessive complexity (e.g., a two-dimensional heterogeneous model) and oversimpli-
Q888-5885/93/2632-2478$~4.00/0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2479
Figure 1. Schematic drawing of the experimental apparatus for catalytic dehydration of methanol: (1)gas cylinder (nitrogen), (2)methanol reservoir, (3)mass flow controller, (4)metering pump, (5)evaporator, (6)ball valve, (7) preheater, (8) manometer, (9) needle valves, (10)tube and shell heat exchanger, (11) phase separator, (12)coolers, (13)thermocouples, (14)calcium silicate insulation block, (15)glass wool, (16) catalyst bed, (17)inert bed, (18)sampling valve. Table I. Properties of Catalyst and Reactor and Operating Conditions catalyst diameter of catalyst particles (mm) catalyst bed height (mm) catalyst particle density (kp/m3) catalyst bed porosity (/) reactor diameter (mm) reactor height (mm) preheater packing height (mm) inlet temp (K) pressure (bar) methanol feed rate (L/h)
Bayer SAS y-AhOs 3 lo0 1.47 0.40 78 loo0 250 551.15-56.15 2.1 4.34-6.74
Table 11. One-Dimensional Plug Flow Models of the Fixed Bed Reactor Used in This Studv interfacial intraparticle model t w e of model eradients zradients desimation heterogeneous yes yes I heterogeneous no Yes I1 heterogeneous yes nd I11 pseudohomogeneous no nd IV a Intraparticle gradients are accounted for in the apparent rate coefficients (eq 12).
'10
fication (e.g., a one-dimensional pseudohomogeneous model) is usually made when one wishes to reduce the computational time without an appreciable loss of the accuracy of model prediction. An adiabatic reactor can be well described by one-dimensional models. In this study, we adopted heterogeneous models that account for the intraparticle and/or interfacial gradients as well as a pseudohomogeneous model. The models used are summarized in Table 11. In the heterogeneous model, the mass and heat balances in the fluid phase with isobaric plug flow are governed by the following equations:
where the overall effectivenessfactor, 70, accounts for the intraparticle as well as interparticle mass- and heattransport limitations (model I). The overall effectiveness factor is defined as a ratio of the actual (global)rate to the rate based on bulk conditions, thus
=
V'JJM(T,C;) dV (4)
rM(P,CP) where the reaction rateat the catalystactivesite, rM(T,Ci), is given by eq 1. Equations 2 and 3 are subject to the initial conditions that specify the feed composition and temperature:
c,"=ck0
atr =O
P=T$ atz=o
(5)
The objective is to solve eqs 2 and 3 for the temperature and concentration profiles in the gas phase along the reactor longitudinal coordinate. As indicated in eq 4, the overall effectiveness factor can be obtained by integrating the rate of reaction over the volume of the whole particle. The concentration and temperature profiles within the catalyst particle are obtained by the simultaneous solution of the governing equations which result by writing a mass and heat balance within the particle
2480 Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 0.20.
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