Study On The Hydrodynamics And Mass Transfer Properties In Gas-ionic Liquid Systems

As a new class of attractive solvents, ionic liquids (ILs) are very promising for gas separation, considering their unique characteristics, such as negligible vapor pressure, tunable property and high gas solubility. Although using ILs in gas separation system is a research hotspot, there are very few successful applications in industry. One of the important reasons is that the research related to the hydrodynamics and mass transfer properties in gas-IL system is very limited, which become the main bottleneck for industrial application of ILs. ILs are entirely composed of ions, which have very different inner environment with conventional molecular solvents. Present hydrodynamic models which are suitable for molecular solvents can't be directly applied into ILs. Thus, it is crucial to establish the transport models for ILs. Therefore, single bubble behavior and multi-bubble behavior in ILs were investigated in this work. Based on the bubble behavior work, the mass transfer properties in ILs were further studied. Combining thermodynamics and mass transfer properties in gas separation process, the "absorption degree" method was proposed. The main innovative work and achievements are as follows:(1) Study on the single bubble behavior in different IL systems by using a high speed image pick-up system. The influence of water content, gas velocity, liquid temperature and orifice diameter on bubble behavior was investigated, and found that a small amount of water has a significant impact on bubble behavior. In comparison with surface tension, viscosity plays a more leading role in determining the bubble behavior in ILs. With the increase of water content in ILs, the bubble velocity increases and bubble diameter decreases. Present models can't be directly applied to predict the bubble behavior in IL systems. Based on plenty of experimental data, two new empirical correlations were proposed to predict bubble velocity and bubble diameter in IL systems, indicating relative errors within± 10% and±7%, respectively.(2) Study on the multi-bubble behavior in different gas-IL systems. The effects of gas superficial velocity, liquid temperature and axial location on bubble mean diameter, gas holdup and gas-liquid interfacial area were studied. Bubbles in the bottom part of the column are relatively uniform, while bubbles in the middle region are very close to the bubbles in the top region because dynamic equilibrium between coalescence and break-up has nearly reached in the middle region. Gas-liquid interfacial area is mainly influenced by gas superficial velocity and liquid temperature. Increasing temperature or gas superficial velocity will be conducive to obtain a high interfacial area. A new correlation used for predicting bubble mean diameter in gas-IL system is proposed with a relative error less than±6%.(3) Study on the mass transfer in CO2 absorption processes with ILs. The method of calculating the liquid-side mass transfer coefficient in IL systems was established by considering the unique characteristics of ILs into mass transfer basic models. The influence of temperature, stirred speed and IL concentrations on gas-IL mass transfer properties was investigated. The results demonstrate that the mass transfer process in ILs is different from that in conventional solvents. The liquid-side mass transfer coefficient in IL systems is influenced not only by the viscosity but also the molecular structures of ILs. Based on the data of liquid-side mass transfer coefficients in physical absorption process, the kinetic parameters, namely, the enhancement factor, reaction rate constant and activation energy in CO2 chemical absorption process by MEA-IL-H2O system were obtained. Considering both the mass-transfer rates and the stability of IL, the new IL-based system MEA+[bmim][NO2]+H2O is recommended for CO2 absorption.(4) A new criterion "absorption degree", which combines both thermodynamics and mass transfer properties, was proposed to comprehensively evaluate the absorption performance. The absorption degree method was applied to analyze the CO2/CH4 and NH3/CO2 separation performance by different IL systems, and effects of temperature and IL concentration on thermodynamics and mass transfer properties were studied. Based on the absorption degree analysis,50 wt%[bmim][NO3]+50 wt% NHD system is recommended for CO2/CH4 separation and [bmim][DCA] is suitable for NH3/CO2 separation.(5) A set of equipment for investigating the effect of external electric field on liquid interface in ILs was established. The liquid interface behavior in ILs under external electric field was preliminarily analyzed. The liquid-liquid interface between IL and n-hexane has an obvious instability under a high electric field. With the increase of electric field, fluctuation of interface appeared. Then, the charged small IL droplets were formed. Finally, the liquid fog turned up, indicating a continuous discharge phenomenon of IL interface.