Solvent Characterization for Post Combustion CO2 capture using Molecular Modeling

Abstract

The increase in greenhouse gases in the atmosphere presents has led to an unprecedented warming of the earth and thereby poses serious threats to the environmental and human health. Mitigation of this climate change has thus become a necessity in the present world. Post combustion CO2 capture (PCC) based on reactive absorption is one of the most mature technologies studied for reducing global warming. One of the main challenges this technology faces is a reduction in its energy requirements to make it a globally accepted and widely used technology. Optimum solvent screening and solvent development for PCC is one of the areas that promises significant potential for achieving low energy demands. This thesis focuses mainly on solvent development for PCC and how using computational chemistry tools can improve our basic understanding of the chemistry of the various solvent systems.

Performance of PCC solvents is, to a large extent, governed by two equilibrium constants viz. pKa of the solvent and its carbamate stability. Both of the equilibrium constants have been studied in great detail in this work for potential amines, polyamines and amino acid solvents. These equilibrium constants have been modeled with gas phase quantum mechanical calculations combined with different solvation models with the help of various thermodynamic cycles. Continuum solvation models (PCM, SM8T, DivCon) and the explicit solvation shell model have been studied. The temperature dependency of the pKa and carbamate stability constants plays a very important part in understanding the solvent behavior in the system. Temperature effects on these equilibrium constants are studied by using the PCM and SM8T continuum solvation models.

The temperature dependency of solvent properties using these models also helps in understanding the basic parameterization of these models. The temperature dependency of the PCM model is formulated in form of a simple equation in this work. Temperature dependent correlations for enthalpy of deprotonation, carbamate formation and heat of absorption of the overall reaction between a solvent and gaseous CO2 are calculated based on computational chemistry based ln K values input to the Gibbs Helmholtz equation. These correlated results are used in an e-UNIQUAC thermodynamic model for better understanding of Post Combustion CO2 capture solvent chemistry.

To summarize, the present work is aimed at employing theoretical chemistry methods to gain better understanding of PCC solvent chemistry. Models and methodologies presented in this work can be used for predicting properties of other solvents in future. The work has shown that integration of computational chemistry with thermodynamic models from chemical engineering can provide useful and interesting results. Finally, this thesis provides useful and predictive tools for estimating basic solvent parameters of the solvent viz. pKa, carbamate stability constants, temperature dependency of pKa and Kc and enthalpy. More advanced tools can be used in future for obtaining better quantitative accuracy for different properties.