Experimental Study and Modeling of an Adiabatic Fixed-bed Reactor for Methanol Dehydration to Dimethyl Ether

M. Fazlollahnejad, M. Taghizadeh,*, A. Eliassi and G. Bakeri

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Experimental Study and Modeling of an Adiabatic Fixed-bed Reactor for Methanol Dehydration to Dimethyl Ether

M. Fazlollahnejad1, M. Taghizadeh1,*, A. Eliassi2and G. Bakeri3

1Department of Chemical Engineering, Babol University of Technology, 4714871167 Babol, Iran2Chemical Industries Research Department, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran3Catalyst Research Group, Petrochemical Research and Technology Company, National Petrochemical Company, Tehran, Iran

One-dimensional heterogeneous plug flow model was employed to model an adiabatic fixed-bed reactor for the catalytic dehydration of methanol to dimethyl ether. Longitudinal temperature and conversion profiles predicted by this model were compared to those experimentally measured in a bench scale reactor. The reactor was packed with 1.5 mm γ-Al2O3pellets as dehydration catalyst and operated in a temperature range of 543-603 K at an atmospheric pressure. Also, the effects of weight hourly space velocity (WHSV) and temperature on methanol conversion were investigated. According to the results, the maximum conversion is obtained at 603.15 K with WHSV of 72.87 h－1.

methanol, dimethyl ether, modeling, fixed-bed reactor

1 INTRODUCTION

Two processes are used for DME production, indirect [3-8] and direct processes [9-11]. In indirect process, methanol is converted to DME in a catalytic dehydration reactor over a solid-acid catalyst by the following reaction:

In the second process (direct process), a synthesis gas (a mixture of H2and CO gases) is used as the feed of the process. In this process, the synthesis gas is primarily converted to methanol and then it is followed by methanol dehydration to DME. The net reaction is as follows:

In our previous work, the effects of temperature and feed composition on catalytic dehydration of methanol to dimethyl ether over gamma-alumina were studied and the results showed that the conversion of methanol strongly depended on the operating temperature in the reactor. Also, conversion of pure methanol and mixture of methanol and watertime were studied and the effect of water on deactivation of the catalyst was investigated [12]. In this work, acidic gamma-alumina has been used as the catalyst for the dehydration of methanol to DME. Also, we reached the optimum WHSV that gives us maximum conversion of methanol in three inlet temperatures of methanol gas.

2 EXPERIMENTAL

2.1 Apparatus

The schematic diagram of the used set up is shown in Fig. 1. Pure methanol was pumped from methanol storage tank at different rates from 44 g·h－1up to 1021 g·h－1to an evaporator and then to a supper heater before entering the reactor. The superheated methanol was sent to an adiabatic fixed-bed reactor. The axial reactor temperature at any point of the catalyst bed was measurablea thermo-well using a thermocouple. The reactor outlet products were passed through an air cooler and a double pipe heat exchanger to cool down to the ambient temperature. The cooled products were sent to a gas-liquid separator. A back pressure regulator (BP-LF690, pressure Tech2000, England) was placed on this separator to regulate the system pressure. Reaction products were analyzed by a gas chromatograph (Varian CP-3800) equipped with thermal conductivity detector (TCD) and flame ionization detector (FID). Also, the remaining methanol in the exit products was measured and the methanol conversion was estimated with comparison to the entrance methanol.

Figure 1 A schematic diagram of the experimental apparatus for catalytic production of DME from methanol 1—nitrogen cylinder; 2—methanol feed tank; 3—dosing pump; 4—flow meter; 5—mixer; 6—evaporator; 7—preheater; 8—adiabatic fixed-bed reactor; 9—air cooler; 10—condenser; 11—liquid-gas separator; 12—back pressure regulator

2.2 Chemicals

Acidic gamma-alumina as dehydration catalyst was prepared from Engelhard (Netherland). Methanol was obtained from Iran Petrochemical Company (IPC). Characterization of the used catalyst and methanol and the operational conditions are reported in Table 1.

3 DEVELOPMENT OF REACTOR MODEL

3.1 Model assumptions

The mathematical model for adiabatic fixed-bed reactor is based on the following assumptions: (1) The feed current in reactor is plug flow and the gas phase is assumed to behave ideally; (2) The reactor is operated at steady state conditions; (3) Heat transfer from reactor to environment is negligible; (4) The reactor is in isobaric operation; (5) Diffusion limitation in catalyst pores is negligible; (6) Radial gradients of concentration and temperature are absent.

3.2 Mathematical model

Where,0(effectiveness factor) that is expressed by Eq. (7) is supposed to be 1, because catalyst particles are so small that concentration and temperature variations in particles can be neglected. Thus, this factor does not have any important effect on temperature of reactor and conversion of methanol.

Equations (1) and (2) are subject to the initial conditions that specify the feed composition and temperature:

3.3 Numerical method

The numerical integration of the reactor mass and heat balances [Eqs. (1) and (2)] was performed by using a fourth-order Runge Kutta while the surface conditions (S,S) in the heterogeneous model were determined by a Newton iteration method. In these calculations,, Δr,, andcwere considered as a function of inlet temperature.

Table 1 Properties of catalyst and operating conditions

4 RESULTS AND DISCUSSION

4.1 Model validation, prediction and experiments

Model results and the obtained experimental data are shown in Figs. 2-7. In Figs. 2-4 variations of temperaturereactor length are shown. When the inlet temperature of the feed is increased from 543 to 603 K, the reaction rate is increased; therefore, the reaction is reached to its equilibrium state faster. According to Figs. 2-4 the experimental data are well predicted by the model. In Figs. 5-7 conversionreactor length are shown. It can be observed that conversion is increased by increasing the inlet feed temperature.

According to the obtained results, the best inlet feed temperature is 573 K. The maximum conversion at this condition was 95% with WHSV of 48.85 h－1. Our experiments showed that at higher temperatures, methanol conversion increases negligibly, but the selectivity respect to DME decreases.

4.2 Effect of WHSV on methanol conversion

The influence of WHSV on the conversion of methanol at three different inlet feed temperatures (543-603 K) is shown in Fig. 8. At a constant inlet temperature, methanol conversion increases with decreasing WHSV, however, when the WHSV is less than 36.4 h－1, the methanol conversion does not increase with decreasing WHSV. For instance,MeOHincreases from 48.9% to 93.00% at 543 K when WHSV decreases from 100.4 h－1to 36.4 h－1. Increasing the WHSV will cause an increase in the gas velocity, which promotes mass transfer but leads to a decrease in the contact time of reactant species [14]. As a result, the conversion of methanol increases with decreasing WHSV until it reaches a maximum value. For the lower WHSV, although the decrease in the flow rate causes to increase the contact time, but in this case the mass of catalyst is too low to increase the conversion.

Also, in higher WHSV the operative temperature should be increased to have better conversion of methanol (Table 2).

As it is reported in Table 2, 7 runs are performed. A constant mass flow rate of methanol as a feed is used at three inlet temperatures for each run. The maximum conversion of methanol is 93% at 543.15 K with WHSV of 36.44 h－1, 95% at 573.15 K with WHSV of 48.58 h－1, and 95.8% at 603.15 K with WHSV of 72.87 h－1.

Table 2 Experimental data for conversion of methanol at different inlet temperatures and atmospheric pressure

Figure 8 Experimental data of methanol conversionWHSV at three different inlet feed temperatures (Dashed lines are trend of changes)inlet/K:●?543;■?573;▲?603.15

5 CONCLUSIONS

A one-dimensional heterogeneous model is developed to simulate the adiabatic fixed-bed reactor. The axial temperature and conversion profiles in an adiabatic fixed-bed reactor for DME production are well predicted by the proposed model. Also, the effect of WHSV on methanol conversion in different inlet feed temperatures are considered. The results show that the maximum conversion of methanol is 93% at 543.15 K with WHSV of 36.44 h－1, 95% at 573.15 K with WHSV of 48.58 h－1, and 95.8% at 603.15 K with WHSV of 72.87 h－1. Therefore, according to the obtained results, the maximum conversion is obtained at 603.15 K with WHSV of 72.87 h－1.

AcknowledgEmentS

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NOMENCLATURE

Cconcentration in fluid phase, kmol·m－3

cspecific heat of fluid, kJ·kg－1·K－1

Δrheat of reaction, kJ·kmol－1

thermodynamic equilibrium constant

Kadsorption constant, m3·kmol－1

sreaction rate constant, kmol·kg－1·h－1

Mmolecular weight, kg·kmol－1

pressure, Pa

gas constant, Pa·m3·kmol－1·K－1

Mrate of methanol disappearance, kmol·kg－1·h－1

temperature, K

superficial velocity, m·h－1

particle volume, m3

reactor longitudinal coordinate, m

effectiveness factor

νstoichiometric coefficient

gas-phase density, kg·m－3

Bcatalyst bed density, kg·m－3

Subscripts

b bulk conditions

th component (methanol, DME, water)

M methanol

s surface conditions

W water

0 inlet conditions

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* To whom correspondence should be addressed. E-mail: taghizadehfr@yahoo.com