Solubility of CO2 and H2 In PDMS and PEGDME At 25 – 120 Oc

Advanced gasification power plants will employ the water-gas shift reaction to produce a high pressure gas-phase mixture containing CO₂, H₂ and water. This shifted syngas mixture at elevated pressures provides ample driving force for the use of physical solvents that will selectively absorb, but do not chemically bind, CO₂. Potential physical absorbents 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 investigate the use of poly(dimethyl siloxane) (PDMS) as an alternative physical solvent to poly(ethylene glycol) dimethyl ether (PEGDME) that would selectively dissolve only CO₂ at elevated temperatures.

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” at 25 ^oC and 40 ^oC. These oligomeric solvent candidates included poly(propylene glycol) dimethyl ether (PPGDME), poly(propylene glycol) diacetate (PPGDAc), poly(butylene glycol) diacetate (PBGDAc) with linear or branched C₄ monomers, PDMS, perfluoropolyether (PFPE), and glycerol tri-acetate (GTA) (which is analogous to a trimer of polyvinyl acetate). Although the performance of PPGDME, PEGDME and PDMS are comparable on a weight basis, PDMS appears to be the best CO₂ solvent based on ability to absorb CO₂. Further, at 22 ^oC and 40 ^oC, PDMS is significantly less viscous than all of the other solvents and is essentially immiscible with water at 22 ^oC and 40 ^oC.

Due to this past success, high molecular weight PDMS has been investigated at elevated temperatures and compared to PEGDME. Three different molecular weights of PDMS, having viscosities of 10 cSt, 20 cSt, and 50 cSt, have been examined at 80 ^oC, 100 ^oC, and 120 ^oC, respectively. The PDMS was examined in two binary systems, first with CO₂ and then with H₂ in an effort to quantify selectivity. If this solvent is capable of operating at these conditions, there would not need to be a cooling step prior to the absorption of CO₂, and water vapor would remain in the fuel gas and be expanded in the turbine to produce additional power.