KINETICS STUDY OF ONE-STEP SYNTHESIS OF DME FROM SYNGAS

Chengyuan Cheng1, Dasheng Chen

2, Haitao Zhang

1, Weiyong Ying

1,*, Dingye Fang

1

1. Engineering Research Center of Large Scale Reactor Engineering and Technology, Ministry of

Education, State Key Laboratory of Chemical Engineering, East China University of Science and

Technology, Shanghai China 200237

2. Shanghai Wujing Chemical Co. Ltd., Shanghai China 200241

Abstract: The intrinsic kinetics of the dimethyl ether (DME) synthesis process over a by-functional

catalyst have been investigated at 220~260℃, 2~6MPa and 1000~2500mL/(g-cal h) using isothermal

integral reactor. The bi-functional catalyst was prepared by physical mixing of commercial

CuO/ZnO/Al2O3 as methanol synthesis catalyst and γ-alumina as methanol dehydration catalyst. The

three reactions including methanol synthesis from CO and H2, CO2 and H2, and methanol dehydration

were chosen as the independent reaction. The L-H kinetic model was presented for dimethyl ether

synthesis and the parameters of the model were obtained by using Levenberg-Marquardt method. The

model is reliable according to statistical analysis and residual error analysis. Also the effects of

different parameters on the reactor performance have been investigated based on the presented kinetic

model.

Key words: Dimethyl ether; Syngas; One-step; Fixed-bed; Kinetics model

1. INTRODUCTION

Dimethyl ether (DME) is a liquefied gas with handling characteristics similar to those of liquefied

petroleum gas (LPG) (Semelsberger et al., 2005). It can be produced from a variety of feed-stock such

as natural gas, crude oil, residual oil, coal, waste products and bio-mass. Many investigations have

been carried out on DME to determine its suitability for use as a fuel in diesel-cycle engines

(Arcoumanis et al., 2008). Compared to some of the other leading alternative fuel candidates (i.e.,

methane, methanol, ethanol, and Fischer-Tropsch fuels), dimethyl ether appears to have the largest

potential impact on society, and should be considered as the fuel of choice for eliminating the

dependency on petroleum.

At the present time, DME is commercially prepared by dehydration of methanol using acidic

porous catalysts such as zeolites, silica-alumina, alumina, etc. Recently, an original technique named

STD (synthesis gas to dimethyl ether) process was developed for the direct synthesis of DME from

synthesis gas (syngas) in a single reactor on bi-functional catalysts composed of copper-based

* Correspondence to: Professor Wei-yong Ying, E-mail: wying@ecust.edu.cn

methanol synthesis catalysts and dehydration catalyst.

The main reactions in the STD process can be shown as follow:

Methanol synthesis:

CO+2H2→CH3OH (1)

CO2+3H2→CH3OH+H2O (2)

Methanol dehydration:

2CH3OH→CH3OCH3+H2O (3)

Water gas shift:

CO+H2O→H2+CO2 (4)

Methanol is converted to DME by dehydration of methanol (Eq(3)) on the acid catalyst. Then, the

equilibrium conversion shifts toward the right-hand side of reaction (1) (2). The combination of these

reactions results in a synergistic effect relieving the unfavorable thermodynamic for methanol synthesis:

methanol, product in the first and second step, is consumed for reaction to DME and water. The water

is shifted by the WGSR reaction (4) forming carbon dioxide and hydrogen, the latter being a reactant

for the methanol synthesis. Thus, one of the products of each step is a reactant for another. This creates

a strong driving force for the overall reaction allowing very high syngas conversion in one single pass.

The research of direct DME synthesis is focused on the catalyst and the process at present, but in

order to provide basic data for designing the reactor for plant or industry, the kinetics study of direct

DME synthesis is necessary. And there is no industry-scale slurry reactor has been built so far because

the experience of scaling up is scarce for slurry reactor. However, there is much information available

about the design and the operation on the fixed-bed reactor, so the fixed-bed reactor could be easily

rapidly built on industry-scale. In order to better understand the process especially the relationship of

CO and CO2 synthesis to methanol, we do the intrinsic kinetic research and choose Eq(1) (2) and (4) as

independent reaction.

The present study, investigate the intrinsic kinetics of one-step DME synthesis on the

CuO-ZnO-Al2O3/γ-alumina catalyst in fixed-bed reactor. The kinetics models for this process based on

Longmuir-Hinshelwood mechanism. Moreover, the influence of different process parameters such as

pressure, temperature, space velocity is simulated by the proposed kinetic models.

2. EXPERIMENT

2.1 Catalyst

Bi-functional catalyst was prepared by admixing of the two catalysts, commercial methanol

synthesis catalyst (CuO-ZnO-Al2O3) and methanol synthesis catalyst (γ-alumina). Two commercial

catalyst were finely milled and sieved to size 150~180μm, and well mixed at mass ratio 1:1. This mass

ratio obtained in previous study (Liu et al., 2002).

2.2 Experimental set-up

The STD reaction kinetic study was carried out in an isothermal integral reactor, in which the

6.005g catalyst was filled in. The catalyst is well mixed with quartz sand in the same size. Top and

bottom of the catalyst bed is filled with quartz sand too, ensure that L/R>8.

Before each kinetic test, the catalyst was activated in situation by reduction using a flow of 5% H2

in N2 at 240℃ approached at 1℃/min from room temperature for 12 h.

2.3 Experimental condition

The kinetic experiments were always carried out under steady-state condition. This state was achieved

within 20h from start up. Mass and heat transfer limitations were negligible during the experimental

conditions chosen. Both internal and external particle diffusion resistance were confirmed absent.

In order to carry out kinetic modeling, a broad range of experimental conditions have been carried

out under the following reaction conditions: 220–260℃, 2–6MPa, 1000-2500mL/(g-cat h) which was

sufficiently far from equilibrium conditions. The hydrogen rich CO lean syngas is best suit the

fixed-bed reactor, so the H2/CO molar ratio = 2:1 to 4:1. For each experiment the carbon balances over

the reactor were calculated. The deviations were very small, usually less than 5% and in 90%

experiments the balances chose to within 2.5%.

3. SIMULATION AND PARAMETER ESTIMATE

The simulation of DME synthesis reactions was based on a PFR model. Several kinetic models for

methanol synthesis and methanol dehydration reactions were tested (Ng et al., 1999; Moradi et al.,

2008) under both independent and combined synthesis conditions. From the initial screening, The

model for methanol synthesis proposed by Vanden Bussche and Froment (1996) based on a strictly

sequential reaction mechanism of CO to CO2 to CH3OH via surface carbonate, and the dehydration

model proposed by Bercic and Levec(1992) based on reaction of dissociative adsorbed methanol, were

selected for analysis and simulation of the combined process. The intrinsic kinetics rate equations are

blow:

3

22

1

,)1(

)/1(

2222

212

HHCOCOCOCO

HCOfMHCO

ionhydrogenatCOfKfKfK

ffKfffkr (5)

4

33

2

,)1(

)/1(

2222

22222

2

HHCOCOCOCO

HCOfOHMHCO

ionhydrogenatCOfKfKfK

ffKffffkr (6)

2

2

3

)1(

)/1(32

MM

MfOHDMEM

ationMeOHdehydrfK

fKfffkr (7)

The fugacity of each component was calculated by SHBWR equation; Kf is the equilibrium of

reaction i

Parameter estimation was based on the minimization of the objective function:

I

i

K

k

calikiki yywS1 1

2

,exp,

2 )( (8)

Where yi is the mole fraction of key component and wi stands for the weight factor for response i.

Choose CO, CO2, DME as three key component.

To establish the kinetic parameters as a function of temperature, the following equation was

used:RT

BATk i

ii exp)(

Where ki denotes a model parameter, Ai is the pre-exponential factor, and Bi is the activation energy for

rate constant or heat of adsorption for adsorption equilibrium constant. The parameters of the proposed

kinetics models are given in table 1.

Where M is number of the experiments, Mp is number of parameters in the equation of model.

Table 1 Kinetic parameter for DME synthesis

Parameters Ai Bi

k1 2.41×103 -20,963

k2 0.167×103 -13,522

k3 1.676×103 -44,235

KCO 0.72124 -8,732

KCO2 0.00004 47,842

KH2 0.18417 11,232

KM 0.31585 16,083

4. RESULT AND DISCUSSION

It is of interest to explore the dependence of kinetics of DME synthesis on the operating condition

such as temperature, pressure and space velocity. For the purposes of quantitative comparison with

experimental result, the conversion of the feed carbon monoxide and yield of DME were used. These

were defined as follows:

inCOin

outCOoutinCOin

COyF

yFyFX

,

,, (9)

inCOin

outDMEout

DMEyF

yFY

,

, (10)

4.1 Effect of temperature

Fig.1 shows the influence of temperature on the conversion of CO with results of kinetics model.

As is observed, when temperature is increased in the range from 220 to 260℃ the conversion of CO

increases, because the reactions rate is kinetically controlled in this region. The reaction is controlled

by thermodynamic equilibrium at high temperature or low space velocity, and the thermodynamic

influence becomes dominant in that case. Since methanol synthesis and methanol dehydration are both

exothermic reactions, higher temperature lead to an unfavorable effect on the equilibrium conversion of

synthesis gas.

Fig.1 Effect of temperature on experimental result: P=5Mpa, SV=1500mL/(g-cat h), (□ )CO

conversion, (△)DME yield

4.2 Effect of pressure

Fig.2 shows the CO conversion as a function of reaction pressure in the kinetics model. As is

observed, when pressure is increased in the range from 2 to 6Mpa the CO conversion increases, which

is the logical consequence whereby methanol synthesis is the limiting step of the whole reaction. Also,

methanol synthesis is a mole-number-reducing reaction so that pressure enhancement favors the

conversion of CO and yield of DME. Since the number of moles in the both sides of methanol

dehydration and water-gas shift reactions are the same, so the pressure has no effect on these reactions.

This implies that the reaction of DME production may be carried out under a similar pressure as in the

conversional synthesis of methanol.

Fig.1 Effect of pressure on experimental result: T=250℃, SV=1500mL/(g-cat h), (□)CO conversion,

(△)DME yield

4.3 Effect of space velocity

Fig.3 shows the effect of space velocity on the performance of STD. With increasing SV, the CO

conversion and DME yield keep decreasing. Because higher space velocity means shorter residence

time of reactants in contact with the catalyst, and short residence time is unfavorable for the

consecutive reactions. The DME yield is decreasing less than CO conversion.

Fig.3 Effect of space velocity on experimental result: T=240℃, P=5MPa, (□)CO conversion,

(△)DME yield

5. CONCLUSIONS

A intrinsic kinetic model for the STD process over a CuO-ZnO-Al2O3/γ-alumina catalyst based on

Longmuir-Hinshelwood mechanism are established. The simulation data was found to agree well with

the experiment results over a wide range of experimental conditions. This accurate kinetic model can

be used in future reactor modeling and scaling-up. The STD process was favorable at high temperature,

pressure and low space velocity.

Acknowledgements

The authors gratefully acknowledge the financial support by National Basic Research Program of

China (973 program) (No.2005CB221205) and by 11th Five-year plan of China.

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