The Development of Physical Solvents for CO2 Separation in CCUS and Hydrogen Production

Climate change mitigation and the global target of Net-Zero emissions by 2100 requires the development of new technology which can drastically limit global cumulative emissions. CO2 capture using physical solvents shows promise in decarbonising hard to abate sectors, particularly where elevated CO2 pressures are present, such as in hydrogen production. Dimethylated polyethylene glycols are a key commercial physical CO2 capture solvent, however their reproductive toxicity has led to a decline in their use in recent decades. The tripropylene glycol (TPG) structure was investigated as a non-toxic replacement, while hopefully providing the same favourable performance as a CO2 capture solvent. The regioisomeric distribution of the TPG ethers was analysed and elucidated through diagnostic modification of the TPG ethers and NMR analysis. This showed a potentially interesting isomeric distribution that potentially impacted its reactivity and toxicity. The TPG core was first optimised through the addition of varying alkyl chains on the terminal positions resulting in a library of dialkylated TPGs from which optimal structural features could be determined. For the increased production of a key ether, a flow system was designed and optimised using principles of DoE, leading to an increase of output from the batch process of 25 times. The solubility of the ether library was measured using a VLE apparatus designed by C-Capture Ltd., allowing the solubility performance of the library to be compared. Surface tension was also measured to help to rationalise the structural effects of the modified ethers on solubility. The TPG library was shown to outperform commercial glymes and other physical solvents, with the ethylated ethers especially showing excellent performance. A 13C DOSY NMR technique was developed, which was shown to accurately determine the tracer diffusivity of 13CO2 through the TPG library and known solvents, with the ethylated ethers again showing the best performance. The fundamental aspects of physical CO2 capture were investigated by rationalising the structural effects on solvent performance with respect to diffusion and solubility.