Semiclathrate-based post- and pre-combustion CO2 capture: thermodynamic and spectroscopic analyses

Abstract

The accumulation of CO2 and other greenhouse gases in the atmosphere has led to global warming. These climate changes are threatening the earth and its inhabitants. So, the clathrate hydrates formation application is interesting to many researchers and experts.This research is focused on minimizing CO2 capture using clathrate hydrates formation because CO2 is attributed to a large portion of global warming. Clathrate hydrates are are non-stoichiometric crystalline compounds formed by a physical reaction between small guest molecules and host water molecules at low temperature and high pressure conditions. Water molecules form a framework while the guest molecules are trapped. These two different molecules are mechanically intermingled but not chemically. Clathrate hydrates have many technological applications, such as separation processes, natural gas storage/transportation and carbon dioxide sequestration. These clathrate hydrate-based technologies are expected to be an innovative method for solving energy and environmental issues. However, the major drawback of the clathrate hydrate-based technologies is that they require the maintenance of a specific temperature and pressure for storing or capturing gas molecules in the hydrate cages. Clathrate hydrates can be divided into true and semi clathrate hydrates. These hydrates have many similar physical and chemical properties, but the main difference between the two is that there exists a chemical interaction between the host and guest molecules. The chemical interaction between semiclathrate hydrates is not yet fully understood. Recently, semi-clathrate hydrates have been reported to be formed by the existence of a hydrate promotor such as quaternary ammonium salts (QASs), amines, and alcohols. The presence of a hydrate promotor forms semiclathrate hydrates under a higher temperature and lower pressure conditions when compared with the pure hydrate systems. Semi-clathrate hydrates have unoccupied cages which can be applied to gas separation/sequestration for capturing CO2. Quaternary ammonium salts (QASs) form semiclathrate hydrates under higher temperature and lower pressure conditions when compared with the pure hydrate systems. These semi-clathrate hydrates have vacant small cages which can be used for capturing small-sized gas molecules, while the large cages are occupied by the TBA cation. The main purpose of this study is to develop an innovative energy-efficient and eco-friendly CO2 separation method using semiclathrate hydrates formation, formed by quaternary ammonium salts (QASs). This research is based on selective partitioning of the CO2 and N2 (or H2) gases during hydrates formation. CO2 has a higher occupancy in the hydrate phase, even though both gases can form hydrates, since CO2 has a larger molecular diameter and more thermodynamic stability than N2 (or H2). The feasibility of the semiclathrate hydrates based CO2 capture method will be examined with a focus on the macroscopic phase behavior and microscopic analytical methods such as NMR, and Raman spectroscopy to investigate the guest gas enclathration behavior. In addition, thermal properties will also be measured using a high pressure micro differential scanning calorimeter (HP μ-DSC) in order to provide heat of formation and dissociation values of semiclathrate hydrates.  In this study, clathrate-based CO2 capture from flue and fuel gas was investigated in the presence of quaternary ammonium salts (QASs) as a semiclathrate former (tetra-n-butyl ammonium bromide (TBAB), chloride (TBAC), and fluoride (TBAF)), tetrahydrofuran (THF) as a water-soluble sII hydrate former, and cyclopentane (CP) as a water-insoluble sII hydrate former. The clathrate stabilities of the CO2 (20%) + N2 (80%) + promoter systems and the CO2 (40%) + H2 (60%) + promoter systems were measured using an isochoric method. The clathrate equilibrium pressures at a specified temperature were significantly reduced in the presence of these thermodynamic promoters. Gas storage capacity and CO2 composition analysis in both vapor and clathrate phases were conducted using gas chromatography. The CO2 in flue gas mixtures was found to be enriched approximately 61% in semiclathrate phase. In addition, the 5.6 mol% THF solution showed the largest gas storage capacity during the clathrate formation, but it demonstrated the lowest CO2 concentration (35%) in the clathrate phase after the completion of clathrate formation. In addition, the CO2 in fuel gas mixtures was found to be enriched approximately 95% in semiclathrate phase after the completion of semiclathrate formation. The inclusion of CO2 in the clathrate phase was also confirmed via Raman spectroscopy. The overall experimental results are useful for the clathrate-based CO2 capture process from flue and fuel gas.