Physical Solvents That Are 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. This gas mixture at elevated pressures provides ample driving force in order to use a physical solvent that will selectively absorb, not chemically bind, to the CO₂. Potential physical absorbers include (a) liquid mixtures of CO₂-philic polydisperse oligomers, (b) small, volatile, liquid CO₂-philic solvents, and (c) CO₂-philic solids capable of melting in the presence of CO₂. The objective of this study was to identify alternative physical solvents that would selectively dissolve only CO₂ from this mixture.

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 or branched C4 monomers, poly(dimethyl siloxane) (PDMS), perfluoropolyether (PFPE), and glycerol 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 25 ^oC and 40 ^oC. 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, an property of interest upon commercialization, of these compounds at 22 ^oC and 40 ^oC 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 PPGDME 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 acetone, methyl acetate, 1,4-dioxane, 2-methoxyethyl acetate, methanol, 2-nitropropane, n,n-dimethylacetamide, acetylacetone, 1-nitropropane, isooctane, 2-(2-butoxyethoxy)ethyl acetate, n-formylmorpholine, propylene carbonate, 2-butoxyethyl acetate, and n-tert-butylformamide. Acetone, methyl acetate and 1,4-dioxane are the most CO₂-philic compounds on a weight basis, however, their boiling points are relatively low. 2-methoxyethyl acetate, the next best solvent, has a significantly higher boiling point, however. (The best performing solvents on a molar basis are 2-(2-butoxyethoxy)ethyl acetate, methyl acetate, and 2-methoxyethyl acetate.) Hydrocarbon solvents that are highly oxygenated tend to be CO₂-philic so long as the oxygens are contained in carbonyl group, ether, or acetate groups. The hydroxyl group, however, is a CO₂-phobic moiety.

Lastly, solid CO₂-philes have been investigated as a potential CO₂ solvent. These solids are typically sugar acetates, or benzene rings having tert-butyl groups and acetate groups attached to the aromatic ring the benzene, or ether oxygens within the ring. The solid solvents that were investigated are β-D-ribofuranose 1,2,3,5-tetraacetate, 2,6-di-tert-butyl-4-methylphenol, 1,2,4-triacetoxybenzene, 2,4-di-tert-butylphenol, sucrose octaacetate, and 1,3,5-trioxane. The unique property of solid solvents is that the CO₂ can be desorbed at an elevated pressure (the three-phase SLV pressure), as opposed to a typical pressure swing desorption pressure that is usually around or a little above 1 atmosphere. The ability of these solvents to melt in the presence of substantial amounts of hydrogen in the gas mixture and selectively absorb CO₂ will be presented.