(554c) Solubility of CO2 in CO2-Philic Oligomers, Including PPGDME, PDMS and PFPE

Coal gasification plants employ the water-gas shift reaction to produce a high pressure gas-phase mixture containing substantial amounts of CO2 and H2. It is desirable to separate this mixture into two streams; a CO2-rich stream for sequestration and a hydrogen stream for energy. Current commercial physical-absorption technologies are designed to selectively absorb one compound (e.g. CO2) at high pressure, and then release it at low pressure during regeneration, thereby yielding a high pressure stream of (non-absorbed) H2 and a low pressure stream of CO2. For example, a polydisperse mixture of poly(ethylene glycol) di-methylethers, PEGDME, can be used as a solvent for the absorption of CO2.

The objective of our research is to compare the CO2 solvent strength of PEGDME at conditions represented by the solvent-rich end of the Px diagram with other oligomers that are known to be even more ?CO2-philic? (based on their ability to dissolve in CO2 at conditions represented by the CO2-rich end of the Px diagram). These oligomeric solvent candidates include poly(propylene glycol) di-methyl ether (PPGDME), poly(propylene glycol) di-acetate (PPGDAc), poly(di-methyl siloxane) (PDMS), and perfluoropolyether (PFPE). Pressure-composition phase diagrams are presented for the pseudo-binary systems of CO2 with PEGDME Mw 250, PPGDME Mw 230, Krytox® PFPE GPL 100 Mw ~1000, or PDMS Mw 237, each at 25 oC. Henry's law constants were also calculated for each pseudo-binary system in the high polymer concentration region. Although the performance of PPGDME, PEGDME and PDMS are comparable when compared on a wt% of CO2 absorbed basis, PPGDME appears to be the best CO2 solvent based on the ability to absorb CO2. The solubility of CO2 in PPGDME and PPGDAc has also been determined at several other temperature-pressure conditions, and the results will be compared with the solvent strength of PEGDME oligomers with a comparable number of repeat units. Further, the viscosity of the PPGDME and PPGDAc will be compared with the viscosity of PEGDME oligomers with a comparable number of repeat units.

PPGDME is less hydrophilic and more oleophilic than PEGDME, and the impacts of these characteristics on the use of PPGDME for CO2 absorption from gas mixtures will be discussed.