374291 To Dramatically Improve the Ability of CO2 Capture By Ionic Liquids - Is It a Critical Criterion to Focus in Nano-Scale?
Ionic liquids (ILs) have been regarded as promising materials in post-combustion CO2 capture. Nevertheless, due to the low gas−liquid mass transfer rate of CO2 in ILs caused by their high viscosity, direct use of ILs as sorbents is not feasible for large-scale application. How to dramatically improve the CO2 capture ability of ILs is still a pending problem.
Recently the applications of immobilizing ILs into porous solid supports and the supported IL membranes (SILMs) have shown much potential in enhancing the mass transfer rate of CO2, which was mainly attributed to the decrease of film thickness, the shortened diffusion path and the enlarged gas-liquid contact area after immobilization. However, for ILs immobilization as sorbents, the mass transfer rate of CO2 decreases with the IL loading on the porous supports. Therefore, the sorbents with rapid mass transfer rate are not the best in the capacity due to the restricted IL loading. In short, there exists a trade-off between the mass transfer rate and the CO2 capacity. On the other hand, for SILMs, the reported CO2 permeability with membrane thickness in micro-scale still has not met the requirements to be potentially industrially viable. Estimation of CO2 permeability of membrane with IL thickness in nano-scale is needed for further investigation. From these points of view, a mechanism covers the applications of ILs both in sorbent and membrane, and reveals the mass transfer rate of CO2 in IL is very important to understand the whole picture.
In this initial proof-of-concept work, two task-specific ILs (TSILs) were immobilized into titanium dioxide and kinetics of the as-prepared sorbents was analyzed based on the methodology of non-equilibrium thermodynamics. Experimental results showed that the apparent rate constant of mass transfer of the whole absorption process was in three different orders of magnitudes, indicating three distinct regions in CO2 absorption process by IL in form of film thickness in ~ 2 μm or ~2 mm, ~100 nm and ~10 nm. Such results had certain universality. Judging from the scale of film thickness of IL, the three regimes showed cover the IL application of being direct used as sorbent (larger than micro-scale), being used in SILM (mostly micro-scale), and being used as sorbent by immobilization on porous support (micro-scale and nano-scale). Further analysis showed that the rapid mass transfer rate in nano-scale cannot be made full use of in supported IL sorbents. As for SILMs, the film thicknesses of IL in most state-of-art SILMs are in micro-scale. Although the mass transfer rate has been enhanced and is faster than direct use of IL, a rough estimation of the order of magnitudes conducted in this study showed clearly that the CO2 flux would be enhanced by hundred times when the membrane thickness of IL was in nano-scale. Thus only thousands of hollow fiber (HF) membrane tubes and little amount of IL would be needed for a recovery capacity of 0.5 ton/h CO2. Further confirmation is needed regarding the selectivity and stability of such SILMs.
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