Absorption of Greenhouse Gases in Liquids: A Molecular Approach

Abstract

The increase in concentrations of greenhouse gases is responsible for global warming over the past few years. A major portion of the emitted greenhouse gases contains carbon dioxide (CO2). The capture of carbon dioxide from the effluent sources, its transport, and storage has been identified as the most promising method to mitigate global warming by reducing the carbon footprint in the atmosphere. Post-combustion CO2 capture processes mainly use chemical solvents like monoethanolamine (MEA) to capture CO2 from flue gas streams. The regeneration of CO2 from the rich solvents is an energy intensive process which decreases the overall efficiency. Other issues like solvent volatility/corrosiveness, toxicity, and solvent costs are critical for choosing the best solvent for post-combustion capture. There is a need for designing new solvents which are less energy intensive, less toxic and corrosive than the ones existing in the industry. Solvent design is extremely challenging, since there are potentially millions of molecules that can be used as a solvent for post-combustion CO2 capture and to find the best solvent is extremely difficult from an experimental point of view. Understanding the thermodynamic processes taking place during chemisorption help in studying the effects of solvents on the absorption/desorption of CO2 in the solvents and their constituent reactions. Ultimately, these aid in developing new solvents that have high capacity to absorb CO2 and low energy penalties to regenerate the CO2 rich solvents. Some of the thermodynamic processes that take place are absorption/desorption from the gas phase, the reaction of CO2 with the aqueous solvent and the diffusion of CO2 molecules into the aqueous solvent mixture. Understanding these processes can help in developing a screening tool that can predict the solubilities, equilibrium constants and conversions, diffusivities of CO2 in different solvents. Molecular simulations can be used in this regard to great effect. Monte Carlo simulations can be used to study equilibrium conversions of the different species in the reacting mixture with great efficiency and accuracy. They can also measure the absorption/desorption of CO2 from the gas phase into the solvent. Molecular Dynamics simulations can help calculate diffusion coefficients of CO2 in the reacting mixture. In this thesis, we develop advanced techniques and methods that can potentially be used to screen large number of solvent molecules and to potentially select the most promising ones. Existing molecular methods are insufficient to efficiently describe the chemisorption and diffusion of CO2 in liquid solvents. We have developed new methods to study the different thermodynamic processes taking place between the CO2 and solvent molecules. The high accuracies and efficiencies of molecular simulations make it an attractive tool in designing new solvents which are more efficient, less toxic and less corrosive than the ones that are available in the market.