Study On Gas-liquid Flow And Mass Transfer Of CO₂ Absorption Into Alkanolamine/Ionic Liquid Aqueous Solution In Microchannels

In recent years,microchemical technology has attracted much attention in many industry fields,such as biochemistry,medicine,chemical engineering.The study on hydrodynamics and mass transfer characteristics of gas-liquid two-phase flow in microchannels is a necessary foundation for the design,optimization,manipulation and scale-up of microchemical devices.In this paper,the gas-liquid two-phase dispersion,flow and mass transfer of CO₂ absorption into alkanolamine/ionic liquid aqueous solution in the microchannel were visually investigated by the high-speed camera.Futhermore,the mechanisms of mass transfer and enhancement in the baffled microchannel were explored.The main research contents include:Densities and viscosities of binary and ternary mixtures of monoethanolamine or methyldiethanolamine(MEA or MDEA),1-butyl-3-methylimidazolium tetrafluoroborate([Bmim][BF₄])and water(H₂O)were measured at atmospheric pressure and T=(293.15 to 333.15)K.The intermolecular interactions in mixtures were discussed through volumetric and viscometric properties deduced from experimental results and FT-IR spectroscopic analysis.The excess molar volumes of binary and ternary systems were fitted by Redlich-Kister equation and Singh equation respectively,and the viscosities were correlated by Jouyban-Acree equation and its extended equation respectively,which showed good prediction performance.The flow pattern,void fraction and pressure drop of gas-liquid two-phase flow of CO₂ absorption into MEA/[Bmim][BF₄]mixed aqueous solution in microchannels were studied systematically.It was found that the higher chemical reaction rate could result in a shift of the flow pattern transition lines between slug flow and slug-annular flow towards higher gas velocity,and a reduction in void fraction and pressure drop.Taking the effect of chemical absorption into ccount,two correlations were proposed with the Hatta number for predicting the void fraction and Chisholm parameter C_p.Furthermore,the evolution of size and velocity of CO₂ bubbles and mass transfer coefficient,and the total mass transfer coefficient in the flow process were studied.The results show that chemical absorption could cause a rapid decrease in the size and flow velocity of CO₂ bubbles.The absorptivity of bubbles is linear with the relative velocity(the ratio of bubble average velocity to two-phase superficial velocity).The evolution of mass transfer coefficient is mainly related to the liquid flow rate and chemical reaction rate.Besides,the total mass transfer coefficient in the flow stage is also affected by the residence time.An empirical correlation of overall liquid-side volumetric mass transfer coefficient was proposed by the gas Reynolds number,liquid Reynolds number and Damk?hler number.The liquid-side mass transfer coefficient(k _L),specific surface area(a),liquid-side volumetric mass transfer coefficient(k _La)and CO₂ loading ratio for four alkanolamine/ionic liquid aqueous solutions-CO₂ in the microchannels were investigated and compared under different concentration ratios of alkanolamine to ionic liquid and gas and liquid flow rates.The k _L,k _La and a in the experimental microchannels were compared with those in the traditional equipments.In order to enhance the gas-liquid mass transfer in the microchannel,the staggered rectangular baffles were embedded on both sides of the microchannel.The volumetric mass transfer coefficient,pressure drop and effective energy efficiency of water and mixed MEA/[Bmim][BF₄]aqueous solution absorbing CO₂ in microchannels with or without baffles were studied.The effects of operating conditions,characteristic parameters of baffles and channel width on mass transfer enhancement in the baffled microchannel were investigated systematically.For the physical and chemical absorption process,the mass transfer enhancement factors in the baffled microchaneels are up to 2.8 and 1.6 with slight increases in pressure drop less than 0.5kPa(23%)and 0.37kPa(30%),respectively.