(720e) Gas Holdup and Mass Transfer Coefficient of Carbon Dioxide-Ionic Liquids System in Bubble Column

  With the deterioration of climate change in recent years, carbon dioxide (CO₂) capture becomes an urgent affair focused on by many governments and scientific researchers. Ionic liquids (ILs), as salts with lower melting points than traditional salts (< 373.15 K), have been reported as potential solvents to capture CO₂. They have low vapor pressure, high thermodynamics stabilities, low solvents losses, low energy consumption and recycling properties in CO₂ capture, therefore research on the industrialization of CO₂ capture by ILs is significant. However, CO₂ capture dynamics properties in ILs, such as gas holdup, volumetric mass transfer coefficient, liquid-side mass transfer coefficient and interfacial area, is limited in current literatures.

  In this work, gas holdup and mass transfer coefficient of carbon dioxide-ILs system in bubble column have been investigated by analysis the bubble behaviors and the CO₂ concentration in ILs. The bubble behaviors were monitored by high speed camera system and the CO₂ concentration could be measured by gas chromatography. The experiments were carried out in ILs of 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF₆), 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF₄) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF₄). Gas holdup and mass transfer coefficient were measured at temperature ranging from 313.15 K to 355.15 K (at 10 K intervals) and pressure ranging from 0.2 MPa to 2.0 MPa. In addition, the effect of gas velocity of CO₂ (from 0.1 mL/min to 100 mL/min) was also investigated in this work. Gas holdup was measured using the volume expansion method by comparing the volume change of ILs in the process of CO₂ capture. The volumetric mass transfer coefficient, k_La, was measured by the equation:

Whereis the initial CO₂ concentration in ILs and is saturated concentration of CO₂ in ILs. The average bubble diameter was measured by the photographic method. The pictures of bubble could be obtained by high speed camera system and then the average diameter of bubbles was calculated by software. Liquid-side mass transfer coefficient, k_L,was calculated from:

Where a is he specific interfacial area of CO₂ and ILs, which is obtained from the Sauter mean diameter and the gas holdup:

  It was shown that the average diameter of bubble, interfacial area and gas holdup will change in the process of CO₂ capture with the change of temperature, gas velocity and diameter of the distributor. The law of these changes was investigated in this work, which will steer the future researches to push the industrialization of CO₂ capture by ILs.