Research And Optimization Of Phase Dispersion Structure Of Fischer-Tropsch Synthesis Stirred Tank

A stirred tank reactor is used as the laboratory device to test the Fisc her-Tropsch synthesis(FTS)catalyst,and in the tank the flow field is very complex.The apparent gas velocity in the stirred tank is very small,so the solid holdup at the bottom of the stirred tank is relatively high due to the restriction of the structure of the tank,and the dispersion is not uniform.The phase dispersion at the bottom of the stirred tank could not be greatly improved by optimizing the internal components and operating conditions.It makes a problem that the parallelism and repeatability of the stirred tank reactor is not satisfactory.In order to solve the problem of phase dispersion in the stirred tank,the influence of different internal structure,apparent gas velocity and stirring speed on the phase distribution in stirred tank were investigated by high-speed photography method and optical fiber probe method,and the cold model experiment mediums were FTS light diesel oil and air.At the same time,the flow field of the stirred tank was simulated by computational fluid dynamics(CFD).And the new types of phase dispersion structure were designed and the reaction performance of Fe-based catalyst for FTS was investigated.The primary works and results are as follows:(1)The cold model was designed according to the hot experimental device,and the effects of internal components such as baffle,inlet gas distributor,agitator blade and stirring speed on gas holdup and bubble size were studied in detail by high-speed photography method.The results show that under the action of high speed stirring,the fluid was swiftly thrown to the side wall,and under the action of baffle disturbance,the fluid flowed up and down along the baffle,forming two dispersion centers.After adding the catalyst,there was no catalyst deposition in front of and behind the baffle,but there was a certain amount of catalyst deposition at the bottom,and the amount of deposition increased with the amount of catalyst.The form of inlet gas distributor had great influence on phase dispersion,and the dispersing effect of sintered metal distributor was the best.The problem of gas distribution and catalyst dispersion could be well solved by reforming the mode of inlet distributor at the bottom.(2)The optical fiber probe was prepared by chemical etching.It was used to study the local gas holdup of cold model experiment.And the influence of stirring speed,apparent gas velocity and inlet gas distributor on local gas holdup were studied.The local gas holdup in the stirred tank at different axial and radial directions was investigated.The experimental results show that the local gas holdup would increase with the apparent gas velocity and the stirred speed.When the stirred speed reached some value,the local gas holdup would not increase all the time.And the change of inlet gas distributor would obviously improve the uniform distribution of the gas.(3)The flow field in the stirred tank was simulated by CFD,which using the Euler-Euler two fluid model along with standard k-? turbulence model.The relative motion between rotating impeller and stationary baffles was modeled with the multiple reference frame(MRF)approach.The simulation results show that changing the structure of the gas-liquid stirred tank could effectively improve phase dispersion of the tank bottom.(4)The new types of phase dispersion structure were designed to investigate the FTS reaction performance over Fe-based catalyst.The results show that the FTS reaction parallelism of the new phase dispersion structure was basically consistent.The stability of the catalyst was improved effectively,and the selectivity of products was relatively small.