Studies on Intensification of Liquid-Liquid Heterogeneous Reaction of Fatty Acid Methyl Esters Epoxidation by Hydrodynamic Cavitation

Author:Wu Zhen Yu

Supervisor:zhang lian zhong nie yong

Database:Doctor

Degree Year:2019

Download:9

Pages:137

Size:11916K

Keyword:

Epoxidation of fatty acid methyl esters(FAMEs)is an important liquid-liquid heterogeneous reaction in oil chemical industry.The obtained epoxidized fatty acid methyl esters(EFAMEs)is one kind of green,non-toxic and environmentally friendly plasticizer,and is used widely in PVC processing industry;meanwhile,it is also used as intermediate to pruduce high value-added products.Generally,in industry,the FAMEs epoxidation is carried out by using performic acid catalytic method in mechanically stirred reactors(MSR).This process is generally limited by mass and heat transfer,especially at high reaction temperatures,which becomes the bottleneck of the process.It is generally known that hydrodynamic cavitation technique can efficiently intensify the transport processes of heterogeneous systems,and have been applied in many liquid-liquid systems.However,few studies focused on characteristics of“fluid flow-transport-reaction”and their relationships in hydrodynamic cavitation multiphase apparatus,which limits the application and popularization of hydrodynamic cavitation technique in reaction systems.Based on the background,a hydrodynamic cavitation multiphase reactor(HCMR)was designed to intensify FAMEs epoxidation,in order to eliminate the limitation of mass and heat transfer.The HCMR was studied from droplet scale to reactor scale,exploring the mesoscale mechanism and regulation of the HCMR.The characteristics of“fluid flow-transport-reaction”in the HCMR were clarified,which can be the basis of designation and scaling up of the HCMR.The study of this dissertation was divided into five parts.(i)macroscopic kinetic of FAMEs epoxidation,confirming the effect of droplet size on reaction.(ii)based on the kinetic study,designing HCMR to intensify the FAMEs epoxidation.The HCMR mainly includes hydrodynamic cavitation device and reaction tank.(iii)to optimize the hydrodynamic cavitation device,characteristics of oil-water two-phase flow was studied,and correlations of Sauter mean diameter(d32)and droplet size distribution(DSD)were obtained.(iv)Reaction tank is the main region of reaction.To optimize reaction tank,droplet dynamic in reaction tank was simulated by combination of computational fluid dynamic(CFD)and population balance model(PBM)(v)modeling of the HCMR applied in FAMEs epoxidation to understand the“fluid flow-transfer-reaction”characteristics in the HCMR.In addition,industrial scale trail was carried out.The main conclusions of each part were as follows:(I)Macroscopic kinetics of FAMEs epoxidation was studied by performic acid catalytic method in a MSR.Droplet size of water in the MSR was measured by a high-speed camera,and a d32 correlation was established to determine mass transfer area.The ring-opening reaction,epoxidation reaction with mass transfer limitation and epoxidation reaction without mass transfer limitation were studied independently,and the kinetic parameters were estimated by non-linear regression method.The activation energies of the ring-opening reaction induced by formic acid,water and performic acid were 21.5,38.3 and 91.3 kJ/mol,and the activation energies of double bond epoxidation were 78.4kJ/mol.The mass transfer coefficients of formic acid and performic acid were 1.45-3.52m/min at 50-80℃.According to the model calculation,it needs d32<400μm to eliminate the mass transfer limitation of FAMEs epoxidation at 80℃,while d32 was about 550-1600μm when the stirring speed was 150-350 rpm.The stirring power was proportional to the third power of the stirring speed,which means that eliminating mass transfer limitation by increasing stirring speed requires high energy consumption.(II)HCMR was designed to intensify transfer process of FAMEs epoxidation.Comparing mass transfer performance between HCMR and MSR,adding surfactants to stabilize the system,d32 in the HCMR was measured at inlet pressure of 2.5 bar,which was about 1/8 of that of MSR at 400 rpm,which means that mass transfer area of the HCMR was 1-2 orders of magnitude higher than that of the MSR.Comparing heat transfer performance between HCMR and MSR,heat transfer coefficient of the HCMR was 2-3 times that of MSR.When hydrogen peroxide was added to HCMR at once,the temperature fluctuation range of the system was±1℃;when hydrogen peroxide was added to MSR within 30 minutes,the temperature fluctuation range was±10℃.The reaction time of FAMEs epoxidation by HCMR was 2-4 h shorter than that by MSR,and the selectivity was improved.In addition,to achieve similar product quality,the energy consumption of HCMR is about 55%of MSR.(III)With FAMEs as continuous phase and water as disperse phase,the dispersion law of oil-water two-phase flow through hydrodynamic cavitation device was studied.A method to stabilize the droplets downstream the hydrodynamic cavitation device was established.The droplets stabilized was measured under a microscope to measure the droplet size.The effects of inlet pressure,aperture ratio and dispersed phase holdup on droplet size were investigated.The results showed that good oil-water mixing can be achieved by controlling the cavitation number between 0.33 and 0.66.When the dispersed phase holdup was 0.25,the corresponding d32 downstream the hydrodynamic cavitation device was 30-40 um;the increase of aperture ratio was beneficial to oil-water mixing;the increasing trend of d32 was in proportion to the dispersed phase holdup.In addition,the DSD of relative droplet size(d/d32)distribution under different conditions was strongly self-similarity,when aperture ratio of the cavitation device was fixed.Based on the above experimental results,the Hinze-Kolmogoroff theory,i.e.the maximum stable droplet diameter theory,was applied to the correlation of local d32downstream the hydrodynamic cavitation device,the average deviation between calculated and experimental values was 8.02%,and a DSD prediction model was given.(IV)With FAMEs as continuous phase and water as dispersed phase,the droplet dynamics(such as droplet breakup and coalescence)in a 10 m3 reactor was simulated by CFD-PBM method in Fluent.In HCMR,fluid circulated between the hydrodynamic cavitation device and the reaction tank.The boundary condition for simulating droplet dynamics in the reaction tank was the DSD at the inlet of the reaction tank,which can be calculated by the DSD prediction model of the hydrodynamic cavitation device.The effects of inlet pressure,dispersed phase holdup and circulating flow rate on droplet size in the reaction tank were investigated.The results showed that when dispersed phase holdup was 0.3,the cavitation number was 0.33-0.66,the circulating flow rate was 40-60 m3/h,the average d32 was about 170μm;meanwhile,the dispersed phase concentration distribution in the reaction tank was relatively uniform.(V)In order to achieve the application of HCMR to the FAMEs epoxidation,the link between“fluid flow-transfer-reaction”needs to be developed.The process involves droplet and reactor scales,and the relationship between them was established by mesoscale coupling of HCMR.The coupling method was:calculating the boundary conditions by using the d32 and DSD correlations downstream the hydrodynamic cavitation device;then,simulating droplet dynamics in the reaction tank by using CFD-PBM method;and substituting the obtained d32 in the reaction tank into the macroscopic kinetic model of FAMEs epoxidation,to simulate the reaction process in the HCMR.According to the simulation results,a HCMR of about 10 m3 was designed and applied in EFAMEs production.By comparing the reaction information predicted by the model with the operation data of the industrial plant,it was found that the predicted results of the model were in good agreement with the actual process.The designed HCMR can eliminate the limitation of the transfer process and minimize the reactor size.