CFD Simulation On Supercritical Synthesis Of 4-hydroxy-2-butanone

4-hydroxy-2-butanone,as an important intermediate for fragrances and vitamin synthesis,is usually produced by aldol condensation of acetone and formaldehyde.Compared with traditional synthetic methods at room temperature and pressure,supercritical synthesis of 4-hydroxy-2-butanone has the advantages of no additional catalyst,less waste,fast reaction rate and high production yield,which has great long-term competitiveness from the viewpoint of green chemistry.Supercritical synthesis of 4-hydroxy-2-butanone is usually conducted in a pipe reactor with Tee junction where supercritical acetone mixes and reacts with formaldehyde aqueous solution.In this thesis,computational fluid dynamics simulation is used to study the supercritical synthesis of 4-hydroxy-2butanone from the perspectives of mixing and reaction.Firstly,the mixing of acetone and water was studied as the acetone-water system was similar to the reaction system of supercritical synthesis of 4-hydroxy-2-butanone.Therefore,physical properties of the acetone-water system were required.The critical temperatures of acetone-water solution were determined by measuring the temperatures where the gas-liquid interfaces disappeared in fused quartz capillary tubes with different filling ratios.The critical pressure of the acetone-water solution was measured by heating the solution of the same composition and filling ratio to its critical temperature in an autoclave.The standard uncertainties of critical temperatures and pressures were 0.55 K and 0.029 MPa respectively.The P-T-p-X data of the acetonewater solution were obtained by heating acetone-water solutions with known compositions and weights to 15 MPa and 20 MPa.The standard uncertainty of measured densities of the acetone-water solutions was 0.3 kg/m3.A volume-translation Peng-Robinson equation of state was used to fit the P-T-p-X data,and the average absolute relative deviation between the calculated and experimental results was 1.19%.Secondly,the process of supercritical acetone mixing with room-temperature water at 17 MPa in a T-junction pipe was studied by computational fluid dynamics simulation.Realizable k-ε turbulence model and wall enhanced treatment were employed.The distributions of pressures,velocity magnitudes,temperatures and components were provided and explained.Meanwhile,density difference was found to play an important role in mixing,and wall heating promoted mixing by decreasing density difference.The results of single factor study of supercritical and subcritical mixing showed that fluids in T-junction pipe with different branch pipe positions,flow rates of main pipe and branch pipe diameter had various flow patterns that resulted in various mixing efficiencies.In most cases,subcritical mixing had better performance than supercritical mixing.In addition,mixing in Venturi-junction pipe was also simulated.The results indicated that the mixing performance of Venturi junction was better than T junction due to a vortex with large circulation quantity close to the outlet of the throat pipe of Venturi junction.Thirdly,the kinetics of supercritical synthesis of 4-hydroxy-2-butanone was studied by experiment,which provided chemical reaction models for simulation.Different from previous studies,side reactions involving formaldehyde were studied in detail.Formaldehyde was found to react with methyl vinyl ketone,formic acid and itself.Formic acid was found to be mainly consumed by the reaction between formaldehyde and itself.A new reaction network containing side reactions involving formaldehyde was proposed.In addition,the effects of initial formaldehyde mass fraction,initial formic acid mass fraction and temperature on the reaction were studied by single factor experiment.Based on the experimental data at 523 K-563 K and 17 MPa,the kinetic parameters of reactions containing side reactions were fitted.The absolute average relative deviation between calculated and experimental results was less than 10%.Fourthly,based on the simulation of mixing between supercritical acetone and roomtemperature water and the kinetics study of supercritical synthesis of 4-hydroxy-2butanone,effects of mixing and feeding methods on reaction were studied by CFD simulation which was verified by experimental data.By comparing mixing and reaction in pipes with upper T junction,horizontal T junction and Venturi junction,the results showed that good mixing had little effect on reaction,but bad mixing significantly reduced the selectivity of main product.After confirming the effects of mixing,pipes with upper T junction were selected to study the effects of feeding branches numbers on reaction which indicated that the increase of branch number promoted main product selectivity.Moreover,effects of operating conditions(branch spacing,reaction temperature and initial formic acid mass fraction)on reaction were studied by single factor simulation.The results showed that the increase of initial formic acid concentration,branch spaces and temperatures helped improve main product selectivity.