Multi-scale Modeling And Optimization Of MTO Fixed Bed Reactor In Reaction Performance

Olefins such as ethylene and propylene, as the most basic chemical raw materials, occupy an extremely important role in modern oil and chemical industries. With the rising demand for light olefins and rising oil prices, methanol replace oil as a raw material preparing olefins becomes the most promising route. MTO reaction is a highly exothermic catalytic reaction, and the olefins are the intermediates. In order to get more light olefins, the development of high activity, high selectivity, stability and good performance catalyst becomes a key research to MTO process.In order to observe practical optimization of catalyst, the catalyst design must be considered from both the scale of catalyst itself and the scale of industrial reactors. In this paper, the numerical simulation method is used to conduct multi-scale research of MTO process, which are single particle scale, particles scale and reactor scale.Firstly, we establish a reaction-diffusion model of single catalyst particles in order to optimize the reaction performance of methanol to olefins (MTO) at the particle scale. The intraparticle component mass fraction and temperature distribution of eight kinds of shapes of catalysts were simulated and based on simulation, and then use effectiveness factor and the selectivity of the desired products were compared. The results show that smaller particle size and porosity are conducive to mass and heat transfer, and hence the effectiveness factor of the catalyst is high; as the radius of the spherical particles is reduced from5mm to0.5mm, the selectivity of ethylene decreases by2%, the selectivity of propylene and butane increases by5%; the changes of porosity have little effect on the selectivity of olefins, and the changes were all under3%; the effectiveness factor of the hollow tube is the greatest, and it has maximum selectivity of C3H6, which is53.6%, and the sum of the selectivity of C3H6and C2H4is71%.Secondly, we establish a model of the cutting120°reaction tube filled with12particles, the flow and reaction properties of the packed beds filled with four particle morphology are respectively simulated, and then observing the following conclusions:due to the larger surface area,the shortest diffusion path and the lower bed voidage, the average velocity of fixed bed filled with hollow tubular is the largest, the average temperature is minimum, species mass fraction gradient are minimum, average reaction rate is maximum, so that it has the highest efficiency factor, to sum up, hollow tube is the optimal catalyst morphology.Finally, we establish industrial-scale fixed bed reactor model, investigate the influence of particle shape and the reactor operating conditions on the catalytic properties and the transform performance, the conclusions are as following:under specified operating conditions, CH3OH rapidly reacts completely, the reactor hotspot is near the entrance of catalyst bed layer, the mass fraction of ethylene along the reactor axis is increasing, while the mass fraction of propylene and butylene are increased at first, and then decreased after the reaction of CH3OH is complete, the total selectivity of light olefins can be up to96%; spherical, hollow tube,3-lobe and dentiform are the better shape because of the lower hotspot of the catalyst bed and the higher selectivity of ethylene; within the range of operational conditions, we should choose a higher feed temperature, a higher ratio of feed water to methanol, a lower WHSV, so that the lower hotspot of the reactor and the higher selectivity of ethylene and total olefin can be observed.