Carbon capture and storage has been a trending topic ever since the call to control and curb CO2 emission was made global. Attempts and researches to improve the efficiency of existing capture technologies have flourished, with recent studies on carbon removal with reactive solvents venturing into the discipline of microfluidics to benefit from the small and precise.
Generally this research takes on a microfluidic approach to synthesise microencapsulated solvents (MECS), which are chemical solvents encapsulated in microscopic polymeric shells with diameters of less than 1000 µm. This research stems from the notion of using microcapsules with high surface area and permeability to capture CO2 effectively, which was first presented by researchers at the Lawrence Livermore National Laboratory in the U.S. This research comprises studies of the following three sections: (a) design of a microfluidic platform and droplet synthesis with control over droplet size, monodispersity and inner structure; (b) encapsulation of carbon solvents and evaluation of the performance of microcapsules on CO2 absorption and (c) enhancement of CO2 uptake with nanoadditives.
The huge potential of droplet-based microfluidics has stimulated rapid development of flexible platforms to generate highly monodispersed droplets with diverse structures. A facile approach using off-the-shelf microdevice was introduced and employed to synthesise capsules containing liquid solvents. The needle-based drop maker can be easily assembled from commercially available components without requiring laborious effort, and at an affordable price as low as 3 USD per drop maker. The device is also flexible to be reconfigured for controlled production of droplets with desired formation and structures. Monodispersed MECS containing aqueous ethanolamine (MEA) and potassium carbonate (K2CO3) have been successfully synthesised via the off-the-shelf device at a controlled rate, yielding capsules with coefficient variance of less than 2%.
Current researches on encapsulated sorbents have paved a way for the development of advanced carbon solvents such as anhydrous solvents the like of ionic liquids. Such solvents are reported to have promising traits such as having high reaction kinetics and low heat of absorption, yet they also suffer from inherent issues such as having high viscosities. Immobilising the solvents within polymeric shells provides solutions to the aforementioned issues, aside from enhancing mass transfer through the extended surface area that microcapsules have to offer. While other works have been directed towards encapsulating advanced solvents known to have characteristic properties which otherwise would not work in traditional gas absorber, in this work, attention is driven towards two conventional aqueous solvents that have been vastly used in carbon removal, namely aqueous potassium carbonate and ethanolamine, with each having different reaction kinetics. Carbonate solutions have transpired to be the benchmark for MECS as they were the first to be encapsulated, and subsequent studies have revolved around them. However, no experimental work has been carried out on encapsulating MEA thus far, which is a benchmark solvent for post-combustion capture technologies. The main difference between these two solvents is, MEA is known to have inherently fast reaction kinetics while carbonate solutions have relatively slow kinetics. However it has been postulated that enhancement of mass transfer of MECS with chemical solvents with fast kinetics such as MEA would not be as significant as that obtainable from chemical solvents with slow kinetics. This is because the encapsulating shell layer adds to another resistance for mass transfer, and the mass transfer process is shifted from liquid-side controlled to shell-side controlled, such effect is more apparent for chemical solvents with faster kinetics.
Following successful encapsulation of both solvents, MEA and K2CO3 MECS were subjected to CO2 uptake tests at 3 different temperatures. Despite the enhancement was not as significant as that attained by K2CO3 capsules which demonstrated up to 5-6 fold increment in CO2 uptake, encapsulated MEA has also shown higher absorption rate when compared to its neat solvent. The inherently faster absorption kinetics of MEA yields a general higher absorption rate and an overall mass transfer coefficient, indicating the liquid-side mass transfer is more dominant and highly dependent on solvent type.
An attempt was then made to improve the CO2 uptake of the encapsulated solvents by incorporating nanoparticles into the carbon solvent. A macro scale experiment was first carried out to examine the effect of nanoadditives dispersion on mass transfer enhancement. The results have shown slight improvement in CO2 uptake. Nanofluids made from base MEA solvents and acid treated graphene nanoplatelets were encapsulated and tested. Notably encapsulated nanofluids have shown higher CO2 uptake than base MEA.
|Date of Award
|8 Jul 2021
- Univerisity of Nottingham
|Yong Ren (Supervisor), Chenggong Sun (Supervisor) & Colin E. Snape (Supervisor)
- Carbon capture