Study On The Characteristics Of Gas-solid Residence Time In Methanol To Olefin Reactor

The gas-solid residence time in the methanol-to-olefin reactor is closely related to the reactor performance,and an appropriate residence time can ensure good product distribution and product selectivity.In the actual industrial process,the reaction of methanol to olefins is a reaction process in which the molecular weight increases and the gas velocity changes.The change in gas velocity will have a significant impact on the gas-solid residence time characteristics in the reactor.Therefore,this study independently designed and built a two-dimensional turbulent fluidized bed experimental device,using a variety of measurement methods to study residence time characteristics of the turbulent fluidized bed.The experimental operating conditions were designed according to the gas velocity changes in the actual methanol-to-olefin process,the gas-solid residence time characteristics in the turbulent fluidized bed with variable gas velocity were studied in detail.First,using high-speed camera measurement methods,systematically study the distribution characteristics of the macroscopic flow characteristics of solids in a two-dimensional turbulent fluidized bed under variable gas velocity conditions.The results show that the axial distribution of solids concentration and particle velocity under different conditions presents an"S-shaped"or"inverted C-shaped"distribution characteristic,the lateral distribution of solids concentration presents a distribution characteristic of thin center and thick side walls,and the lateral distribution of particle velocity is characterized by high center and low side walls.Changes in superficial gas velocity and static bed height have a significant effect on the macroscopic flow characteristics of the solids.The solids concentration value range under variable gas velocity operating conditions is 0.033～0.214,and the particle velocity value range is0.06～0.83 m/s.Compared with the macroscopic flow characteristics of solids under constant gas velocity operating conditions,the solids concentration and particle velocity are distributed more uniformly under variable gas velocity.Based on the images obtained by the high-speed camera method,the OTSU method was used to identify the cluster and the characteristics of the cluster were studied.The results show that the change of static bed height has a significant effect on the average concentration of cluster and the frequency of cluster,the change of superficial gas velocity has an effect on the average velocity of cluster,the existence time of cluster,and the average size of cluster.Compared with the characteristic parameters of cluster under constant gas velocity operating conditions,the average size distribution of cluster under variable gas velocity operating conditions is narrow,only 4～9 mm;the frequency of cluster is relatively high,the highest is up to 196 Hz;the cluster exist time is relatively short,the shortest time is as low as 37.3 ms;the average velocity of the cluster is relatively small,the minimum is as low as-0.80 m/s.Combined with the above-mentioned images,the cross-correlation principle and the theory of random walk were used to study the characteristics of the residence time of solids in the turbulent fluid bed with variable gas velocity.The solids axial diffusion coefficient is between 0.013-0.027 m~2/s and the solids lateral diffusion coefficient is between 0.0012-0.0049 m~2/s under the condition of variable gas velocity.The axial diffusion of solids is mainly in the reactor,the increase of the static bed height or the superficial gas velocity results in a significant increase in the solids diffusion coefficient.Compared with the solids diffusion coefficient of the turbulent fluidized bed with constant gas velocity,the solids diffusion coefficient is relatively large under the condition of variable gas velocity,and the effect of the change of the static bed height or the superficial gas velocity on the solids diffusion coefficient is more obvious.Finally,the pulse gas tracing technology was used to study the gas residence time distribution curve in the turbulent fluidized bed with variable gas velocity.Combining the one-dimensional axial diffusion model and open-close boundary conditions,the gas Pelect number and gas diffusion coefficient under different operating conditions are obtained.The gas Pelect number is between 8.14 and 10.24,and the gas diffusion coefficient is between 0.03 and 0.20 m~2/s.Compared with the gas residence time characteristics of the turbulent fluidized bed with constant gas velocity,with the increase of the gas diffusion distance,the changes of gas backmixing increase small under the condition of variable gas velocity,the effect of the change on the gas residence time characteristic parameter is more obvious.