Study On Reaction Process Of Methanol Dehydration To Dimethyl Ether

The shortage of oil and gas brings development opportunities to the new coal chemical industry in our country. With the improvement of the industrial technology and maturity of coal chemical industry poly-generation technology, dimethyl ether can replace liquefied petroleum gas and diesel oil as an environmental friendly chemical raw material. It is an important downstream product which has been hailed as "clean fuel of the21century" of the coal chemical industry chain.Gas phase methanol dehydration to dimethyl ether is an easy operating and continuous process which is suitable to large-scale production. The basic principle is that the heterogeneous reaction of methanol steam takes place in fixed bed catalytic reactor over solid acid catalyst and methanol steam is dehydrated to dimethyl ether. The kinetics of methanol dehydration to dimethyl ether reaction and mathematical model of reactor can be used to optimize the operation and guidance the design and magnifying of the reactor.The catalytic performance of two kinds of methanol dehydration catalysts was compared and MD-2was selected as the better catalyst. An intrinsic kinetic equation was developed over catalyst MD-2. The isothermal diffusion-reaction model was established based on previous kinetic model, the methanol concentration distribution in the catalyst particle under certain conditions was discussed. According to industrial dimethyl ether reactor of220,000t/y dimethyl ether and a new tube and shell reactor of400,000t/y dimethyl ether, reactor models were developed, the influence of different operation conditions and catalyst particle sizes on the performance of reactor was investigated.Dehydration of methanol to dimethyl ether over a commercial γ-AI2O3catalyst was studied using an isothermal integral reactor at the temperature interval240～340℃, liquid hourly space velocity (LHSV) of0.9～6.0h-1, pressures between0.1and1.0MPa. An intrinsic kinetics equation based on the mechanism of Langmuir-Hinshelwood dissociative adsorption was developed for the dehydration reaction. The parameters of the kinetic model were obtained by the levenberg-marquardt method. The residual error distribution and statistic test showed that this intrinsic kinetic model was reliable and acceptable.A two-dimensional isothermal diffusion-reaction model was established for cylindrical shaped industrial catalyst based on previous kinetic model. The internal effectiveness factor and the concentration distribution of methanol in the catalyst were obtained by the finite element method. The reaction-diffusion model was verified by the global kinetics data. The calculation data agreed well with the experimental data and the average absolute value of the comparative error is7.72%, so the model can be used to calculate the internal effectiveness factor of the cylindrical shaped methanol dehydration catalyst. The range of the internal effectiveness factor of methanol is0.57-0.83under experimental conditions, which means that the reaction was influenced to some extent by internal diffusion. The methanol concentration distribution in catalyst can be obtained by diffusion-reaction model.According to the reactor of220,000t/y dimethyl ether, one-dimensional heterogeneous model for the staged adiabatic fixed bed reactor was derived to simulate and discuss the influence of different operation condition. The comparison of simulated bed temperatures with the real bed temperatures tested from the commercial methanol dehydration reactor shows good agreement. This study reveals that the heterogeneous one dimensional reactor model is suitable for simulating this industrial reactor. The influence of different operation conditions such as inlet methanol temperature, inlet methanol flow rates and catalyst particle sizes on the performance of reactor and the axial temperature profile of catalyst bed was investigated. The distribution of the internal effectiveness factor for catalyst particle along the catalytic bed height was also obtained to optimize the operation of this industrial reactor.A tube-shell fixed-bed reactor of400,000t/y dimethyl ether was proposed, the mathematical model for tube and shell reactor was established based on previous intrinsic kinetics and diffusion-reaction model. The concentration distribution of the methanol and dimethyl ether, axial temperature profile of the catalyst bed and the distribution of the internal effectiveness factor for catalyst particle along the catalytic bed height can be caluculated by this reactor model.The reactor performance was simulated at the inlet temperature interval250-290℃, methanol flow rate of2000～3600kmol/h, boiling water temperature interval270～310℃, pressures between0.7and1.5MPa based on the tube-shell reactor model. The inlet temperature was proved to have little effect on the reactor performance. The hot temperature of catalyst bed, outlet mole fraction of dimethyl ether and methanol conversion were all decreased slightly with the increase of the methanol flow rate, but the daily capacity of dimethyl ether was increased obviously. The increase of the inlet pressure had little effect on the hot temperature of catalyst bed, outlet mole fraction of dimethyl ether, methanol conversion and the daily capacity of dimethyl ether. The boiling water temperature had significant influence on the methanol conversion and the hot temperature of the catalyst bed. With the increase of the boiling water temperature, the outlet mole fraction of dimethyl ether, methanol conversion and the daily capacity of dimethyl ether were all increased. The hot temperature of catalyst bed is on the decline with the increase of the catalyst particle size. The outlet mole fraction of DME, methanol conversion and the daily capacity of dimethyl ether all decreased due to the increase of the catalyst particle size.