| The carbonation method has the advantages of green and simple process and low cost,and has become one of the most economical technologies for producing γ-Al2O3.The traditional carbonation process is to introduce CO2 into the stirred tank reactor with NaAlO2 solution in the form of bubbling.Due to the fact that CO2 gas cannot be uniformly distributed in the liquid phase,the synthesized γ-Al2O3 has the disadvantages of low specific surface area and uneven pore distribution.With the aggravation of the trend of heavy and poor quality of crude oil resources,the γ-Al2O3 synthesized by traditional equipment can no longer meet the actual production requirements.The microreactor has the advantages of small equipment volume and high mass transfer efficiency.Using it as a reactor for carbonation experiments is a novel method for the preparation of γ-Al2O3.Therefore,this thesis selects microreactor for carbonation experiments,which not only solves the problems of wide pore distribution,large crystalline particles and more impurities in traditional carbonation process,but also promotes the absorption and utilization of CO2,which is beneficial to the realization of dual carbon strategy goals.The main research results of this thesis are as follows.(1)CFD software was used to simulate the gas-liquid flow,mass transfer and chemical reaction in the microreactor.Firstly,it was determined that a stable Taylor flow would be formed when the liquid phase apparent flow rate and the gas-liquid ratio were in the range of 0.1-0.5 m/s and 1～8,respectively.According to the data of the flow process,a cell model was established and the velocity field was calculated.The maximum velocity of the axial position of the liquid plug center was about 1.5 times of the Taylor bubble velocity ub,and 3 times of the average velocity inside the liquid plug is close to ub.The vortex structure formed in the liquid plug unit has a great influence on the concentration distribution of the reactants and products in the liquid phase,and the concentration cloud and velocity vector plots show a strong synergy.(2)The variations of CO2 absorption rate X,liquid-phase individual mass transfer coefficient kL and enhancement factor E were simulated and analyzed at different NaAlO2 solution concentrations,gas-liquid apparent flow rates and times.The results showed that X,kL and E tended to increase with the increase of gas-liquid apparent flow rate and NaAlO2 concentration.The kL of chemically absorbed CO2 ranged from 1×10-4 to 18×10-4 m/s,which was 4-16 times higher than that of physical absorption,proving that the chemical reaction had a significant enhancement of CO2 absorption.The correlation equation for the liquid-side volume mass transfer coefficient kLa under the CO2-NaAlO2 system was fitted and predicted,and the relative average deviation of the predicted kLa from the kLa calculated by the simulation was within 10.50%.(3)The feasibility of the preparation of nano-γ-Al2O3 by continuous carbonation in microreactor was analyzed theoretically based on numerical simulations,and the effects of various factors during the gelation reaction on the properties of the precursor pseudoboehmite and γ-Al2O3 were investigated by combining the microchannel technology with the continuous carbonation process.The structural properties were analyzed by X-ray diffractometer,scanning electron microscope and specific surface area analyzer.The optimal preparation conditions were determined as follows:final pH of 10.3,gelation temperature of 40℃,NaAlO2 solution concentration of 0.1 mol/L,liquid phase apparent flow rate of 0.3 m/s,gas-liquid flow rate ratio of 3:1,and microchannel tube diameter of 1 mm.The specific surface area and pore volume of γ-Al2O3 synthesized based on this condition were 368 m2/g and 0.4 cm3/g,respectively.It was demonstrated that nano-γ-Al2O3 with fibrous pore channels could be produced by carbonization method in the microchannel reactor.(4)The pore volume and pore size of γ-Al2O3 with high specific surface area synthesized by carbonization method in the microchannel were modified.The aging process,filtration process,and roasting temperature were firstly adjusted during the preparation process,and then polyethylene glycol 400 and cetyltrimethylammonium bromide(CTAB)were introduced to modify γ-Al2O3.After characterization,it was found that the pore channels of γ-Al2O3 produced by using CTAB and ethanol filtration were abundant and homogeneous.The final nanoscale fibrous γ-Al2O3 with specific surface area,pore volume and average pore size of 347.7 m2/g,1.127 cm3/g and 13nm,respectively,were prepared.This paper proposes a process combining microreactor and carbonization method to prepare nano-γ-Al2O3,which not only creates the microreactor technology of continuous carbonization method,improves the carbonization process,but also further improves the quality of γ-Al2O3 products and promotes the further industrialization development of γ-Al2O3. |
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