Molecular Dynamics Study of Novel Cryoprotectants and of CO2 Capture by sI Clathrate Hydrates

• Main Author: Nohra, Michael
• Other Authors: Woo, Tom
• Language: en
• Published: Université d'Ottawa / University of Ottawa 2012
• Subjects: Molecular dynamics; cryoprotectants; AFGP; CO2; capture; sI clathrate hydrates; Thermodynamic integration; Gibbs Free energy
• Online Access: http://hdl.handle.net/10393/23084; http://dx.doi.org/10.20381/ruor-5876

The first project in this work used classical molecular dynamics to study the ice recrystallization inhibition potential of a series of carbohydrates and alcochols, using the hydration index, partial molar volumes and isothermal compressibilities as parameters for measuring their cryogenic efficacy. Unfortunately, after 8 months of testing, this work demonstrates that the accuracy and precision of the density extracted from simulations is not sufficient in providing accurate partial molar volumes. As a result, this work clearly demonstrates that current classical molecular dynamics technology cannot probe the volumetric properties of interest with sufficient accuracy to aid in the research and development of novel cryoprotectants.The second project in this work used molecular dynamics simulations to evaluate the Gibbs free energy change of substituting CO2 in sI clathrate hydrates by N2,CH4, SO2 and H2S flue gas impurities under conditions proposed for CO2 capture (273 K, 10 bar). Our results demonstrate that CO2 substitutions by N2 in the small sI cages were thermodynamically favored. This substitution is problematic in terms of efficient CO2 capture, since the small cages make up 25% of the sI clathrate cages, therefore a significant amount of energy could be spent on removing N2 from the flue gas rather than CO2. The thermodynamics of CO2 substitution by CH4, SO2 and H2S in sI clathrate hydrates was also examined. The substitution of CO2 by these gases in both the small and large cages were determined to be favorable. This suggests that these gases may also disrupt the CO2 capture by sI clathrate hydrates if they are present in large concentrations in the combustion flue stream. Similar substitution thermodynamics at 200 K and 10 bar were also studied. With one exception, we found that the substitution free energies do not significantly change and do not alter the sign of thermodynamics. Thus, using a lower capture temperature does not significantly change the substitution free energies and their implications for CO2 capture by sI clathrate hydrates.