279148 Hydrophobic Polymeric Solvents for the Selective Absorption of CO2 from Warm Gas Streams that also Contain H2 and H2O
Hydrophobic polymers may provide an alternative to physical solvents based on the hydrophilic solvent polyethyleneglycol dimethylether (PEGDME) for the pre-combustion capture of CO2 from a warm, high pressure, post-WGS reactor gas stream that also contains H2O and H2. The candidate polymers are designed to absorb as much CO2 as possible, and as little H2 and water as possible. There are two promising candidates: polydimethyl siloxane (PDMS or silicone oil), and polypropyleneglycol dimethylether (PPGDME) with branched repeat units. PDMS is commercially available over an extremely wide MW range (up to 500,000+), while PPGDME (H3C(CH2CH3CHO)xCH3) can be made from commercially available PPG diols with molecular weights below 10,000.
In this work, the solubility of hydrogen, CO2 and water in PEGDME, PDMS and PPGDME is presented for pressures up to ~2000 psi and temperatures to 120oC. The results indicate that PDMS is completely hydrophobic, PPGDME absorbs several weight percent water, and PEGDME is completely miscible with water in all proportions. All three solvents absorb comparable amounts of CO2 (on a wt% basis). The solubility of H2 in the solvents at ~25oC is roughly three orders of magnitude less than the solubility of CO2, with PEGDME absorbing the least H2 and PDMS absorbing the most H2 (about 2.5 times more than PPGDME). Unfortunately, because the solubility of CO2 decreases with increasing temperature as the solubility of H2 increases, the CO2-selectivity of the solvents (as estimated by the ratio of CO2 solubility:H2 solubility at the same conditions) decreases with increasing temperature.
Doe to its thermal stability, hydrophobicity, commercial availability, low price, low viscosity, benign health and safety attributes, and favorable solvent attributes for CO2 and H2, PDMS appear to be the most promising hydrophobic solvent for the selective removal of CO2 from a high pressure, warm, post-WGS gas stream.