(484e) Hydrophobic Polymeric Solvents for the Selective Absorption of CO2 From Warm Gas Streams That Also Contain H2 and H2O

Hydrophobic Polymeric Solvents for the Selective Absorption of CO₂ from Warm Gas Streams that also Contain H₂ and H₂O

Robert Enick, David Luebke, Hunaid Nulwala, Samantha Warman, Craig Stevenson, Peter Koronaios

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 hydrophobic polymers are designed to absorb as much CO₂ as possible, and as little H₂ and water as possible. There are three promising candidates: polydimethyl siloxane (PDMS or silicone oil), PDMS-PEGDME hydrid solvent developed by our team,  and a commercial PDMS/Fe compound with the addition of iron as an anti-oxidant to enhance its high-temperature performance.  PDMS is commercially available over an extremely wide MW range (up to 500,000+). The PDMS-PEGDME hybrid solvent is also easy to synthesize from commercially available starting materials.  Two silicone oils containing both methyl and phenyl groups were also tested and found to be much less promising candidates.

In this work, the solubility of hydrogen, CO₂ and water in PEGDME, PDMS, PDMS-PEGDME and PDMS/Fe is presented for pressures up to ~2000 psi and temperatures to 120^oC.  The results indicate that PDMS is completely hydrophobic over broad ranges of temperature and pressure, and PEGDME is completely miscible with water in all proportions.  PDMS-PEGDME and PDMS/Fe are hydrophobic at ambient temperature but the hydrophobicity of both have not yet been measured over a broad temperature range. These three hydrophobic solvents absorb comparable amounts of CO₂ (on a wt% basis).  The solubility of H₂ in the solvents at ~25^oC 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.