(494b) CO2-Philic Oligomers as Alternatives for PEGDME in CO2 Absorption

Advanced gasification power plants will employ the water-gas shift reaction producing a high pressure gas-phase mixture containing CO2, H2 and water. A polydisperse mixture of poly(ethylene glycol) di-methylethers, PEGDME, can be used as a solvent for the absorption of CO2. The objective of this study was to identify alternative physical solvents that would selectively dissolve only CO2 from this mixture, while exhibiting low viscosity.

The first phase of our research involved the comparison of the CO2 solvent strength of PEGDME with that of other low volatility oligomers that are known to be ?CO2-philic?. These oligomeric solvent candidates include poly(propylene glycol) di-methyl ether (PPGDME), poly(propylene glycol) di-acetate (PPGDAc), poly(butylene glycol) di-acetate (PBGDAc) with linear C4 monomers, poly(dimethyl siloxane) (PDMS), perfluoropolyether (PFPE), and glyceryl tri-acetate (GTA). Pressure-composition phase diagrams are presented for the pseudo-binary systems of CO2 with PEGDME n=6, PPGDME n=6, PDMS n=6, PBGDAc n=3.2, PPGDAc n=6.7, and GTA (which is analogous to a trimer of polyvinyl acetate), each at 25C and 40C. 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 on a weight basis, PPGDME and PDMS appear to be the best CO2 solvents based on the ability to absorb CO2. Further, the viscosity of these compounds at 22C and 40C indicates that PDMS is significantly less viscous than all of the others, including PEGDME. Further, PDMS and PFPE are essentially immiscible with water, and water is only slightly soluble in PPG- and PBG-based oligomers and GTA, whereas water and PEGDME are completely miscible.

Small volatile CO2 solvents were also examined in the second phase of this work. Phase behavior results in the form of pressure-composition diagrams are presented for the binary systems of CO2 and N-formylmopholine, methanol propylene carbonate, acetone, 1,4 dioxane, isooctane, methyl acetate, 2-butoxy ethoxy acetate, and 2-(2-butoxyethoxy) ethyl acetate. Acetone and 1,4-dioxane are the most CO2-philic compounds however, their high vapor pressures prevent them from being viable solvents.