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.
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