Multiscale Simulation And Optimization Of Reactor Performance For Methanol Synthesis From Synthesis Gas

Here, models at catalyst particle scale, particle group scale, and reactor bed scale were set up for methanol synthesis from synthesis gas. Based on computational fluid dynamics (CFD) software Fluent, mass transfer, heat transfer, momentum transfer and the influence of different operating conditions were studied for the reaction system.In single particle catalyst scale respect, the performance of partiles with internal and/or external diffusion was studied. The effect of particle shape on effectiveness factors and selectivities was analyzed. Moreover, effectiveness factors, selectivities, yields, temperature distributions and component mole fraction distributions of catalyst particles with flow regions were obtained. The results show that,(1) the order of effectiveness factors of catalyst particles with different shapes follows an order of3-Lobe>4-Lobe> hollow tube>2-Lobe> tooth> cylinder (Lp=2dp)> cylinder (Lp=dp)> sphere, and the selectivity of the eight particles is0.88-0.93;(2) temperature rises fast at the entrance of the catalyst particles with flow regions, and then tend to be gentle;(3) external diffusion plays an important role in determining reaction performance.In particle group scale respect, models of eight particle groups were set up based on body center cubic (BCC) packing form. Mass transfer, energy transfer and momentum transfer characteristics were studied and the effectiveness factors of target particle, average effectiveness factors, selectivities, conversions and yields of particle groups were investigated. The results show that,(1) temperature of the fluid has no change in the entrance section, but temperature of the particles continues to rise along the axial height. The range of temperature continues to decrease in the outlet section. The total difference of temperature between inlet and outlet is less than2K;(2) velocity increases when the fluid flows through the gaps between the up and down adjacent particles, but decreases between the left and right neighboring particles. Velocity decreases and no reflux exists largely behind the center paticle in spherical particle group. Flow direction changes behind the center particle of other shapes, and forms the reflux and vortex;(3) the largest mole fraction of CH3OH is inside the particles, and the maximum value is between3.28%-4.38%;(4) the effectiveness factor of CH3OH of the center particle is between0.32-0.35, and the effectiveness factor of RWGS reaction is between0.32-0.39;(5) the average effectiveness factors of CH3OH is between0.29-0.33, and the average effectiveness factor of RWGS reaction is between0.29-0.35;(6) the selectivity is between0.87-0.88and smaller than the single particle catalyst;(7) the conversion and yield of3-lobe particle group is the best, and their values are1.45%and1.26%, respectively.In reactor scale respect, effects of the catalyst particle shape, inlet temperature, operating pressure, inlet mass flow rate, wall temperature and feed composition on the temperature distribution and composition distribution of the reactor tube at axial position were studied. The results show that, reactor tube I:temperature and mole fraction of compositions rise fast in the entrance section, and the reaction reach their equilibrium at40mm into the tube; reactor tube II:(1) the change of axial temperature, mole fraction of CH3OH and CO of the tube with different particle shapes are basically the same. The selectivity of catalyst beds is between3.21-3.24, the conversion is between1.16-1.18, and the yield is between0.36-0.37. The order of the selectivity, conversion and yield of the reactor is the same with the effectiveness factor of the single catalyst particles;(2) the most appropriate inlet temperature is between473.15-513.15K, operating pressure is about5.0MPa, inlet mass flow rate is between2.8×10-2-5.6×10-2kg·s-1, wall temperature is between503.15-523.15K, inlet mole fraction of CO2is3%.