| The gas-liquid mass transfer process widely exists in industrial fields such as chemical industry,oil refining and environmental protection.Enhancing mass transfer has always been a focus for many researchers.Double film theory shows that the mass transfer resistance mainly exists in the boundary layer,where the mass transfer carries out by molecular diffusion.The Rayleigh-Benard effect,an interfacial turbulence phenomenon,was found in the gas absorption into some pure organic solvents.The interfacical convection can effectively enhance the mass transfer in the boundary layer.However,current researches on the Rayleigh-Benard effect has been focus on the physical absorption system without water,and which in the chemical absorption process containing water is rarely reported.Inducing and effectively regulating interfacial convection in the chemical absorption process is the key to realize the Rayleigh-Benard effect as a means of enhancing mass transfer and applying it to the actual absorption process.Aiming at the process of chemical absorption,this work proposed a method for inducing interfacial turbulence by adding suitable organic solvents to the absorbents to achieve the transformation of molecular diffusion into turbulent diffusion in the boundary layer,thereby greatly enhancing mass transfer.This thesis mainly conducts research from three aspects:feasibility,operability(adjustable)and effectiveness.In terms of feasibility,we first comprehensively investigated the effect of organic solvents on CO2 absorption-desorption performance.It was found that the addition of acetonitrile and isopropanol can increase the absorption rate by 319%and 33%,respectively.In addition,adding organic solvents also had a positive effect on other absorption-desorption performances,such as desorption rate and cyclic loading.As the only index that can be directly measured in absorption kinetic researches,absorption rate is a reflection of the coupling of multiple processes including dissolution,reaction,and mass transfer.In this work,we explored the mechanism of promoting the absorption rate of organic solvents via a series of decoupling experiments.The results showed that the interface turbulence caused by the Rayleigh-Benard effect was the major contributor to the organic solvent increasing the absorption rate,which showed that it is feasible to induce the interface turbulence by introducing an organic solvent.At the same time,visual experiments were used to directly observe the interface turbulence.In different absorption systems,"inverted mushroom","waterfall","boiling"and other forms of turbulent micelles were generated at the interface.These turbulent micelles can flow into the bulk liquid in a very short time,causing the violent disturbances and greatly improving the mass transfer process.At this time,the absorption rate of MEA,NaOH,K2CO3 aqueous and other systems can be increased by 3 to 7 times.Through model calculations,it was found that the eddy current diffusion coefficient is several times to dozens of times higher than the molecular diffusion coefficient.Based on the above researches,the intensity of turbulence generated by chemical absorption and physical absorption was compared by calculation.The results showed that the turbulence intensity in the chemical absorption system was two orders of magnitude greater than that in the physical absorption system.The interfacial turbulence cannot be generated in mixed solution of organic solvent and water.In the chemical absorption system,by regulating the reaction rate and mass transfer rate,the influence of turbulence intensity was systematically studied and the internal mechanism of interface turbulence was discussed.The results showed that breaking the hydrogen bond between water molecules in aqueous solution and then increasing the density density gradient between the interface solution and the liquid phase is the key to inducing turbulence.The research also showed that the types and concentrations of reactants,the types and concentrations of organic solvents,the viscosity of the solution,and the surface tension were all effective means to adjust the turbulence intensity Ra.Theoretically,selecting the appropriate organic solvent according to the absorption system and adjusting the turbulence intensity Ra can make the Rayleigh-Benard effect became an effective means of mass transfer enhancement,transforming the molecular diffusion in the laminar boundary layer of the vertical interface into convection diffusion.Finally,applied the method of mass transfer enhancement to the common process of MDEA/PZ carbon capture to verify the effectiveness.First,the essential role of mass transfer on the absorption process of MDEA/PZ was discovered and proved.The higher mass transfer resistance in the absorption process will lead to the limitation of PZ activation and the lower absorption rate.To solve the problem,the effective utilization rate of PZ was increased by enhancing mass transfer.The effects of organic solvents such as isopropanol,acetonitrile and n-heptane on absorption performance and PZ activation were investigated.The results showed that the addition of isopropanol,acetonitrile and n-heptane to the MDEA/PZ and MDEA solutions can obviously enhance the absorption rate and the activation of PZ.The activation effect of the three on PZ can be increased by 36%-420%.The absorption rate of the MDEA/PZ/acetonitrile composite absorbent was the largest,which was 12 times higher than that MDEA aqueous solution under the same CO2 loading.The results proved the effectiveness of the mass transfer enhancement method proposed in this work. |
