Phase Equilibrium and Mass Transfer in Hydrate Forming CO2-Water Systems
Understanding the behavior and fate of CO2 in aqueous systems is important both for developing potential CO2 sequestration options and for understanding the impacts of seepage or leakage of the stored CO2 into aqueous environments.
Two-phase equilibrium between CO2 hydrate (H) and a water-rich liquid (L) are experimentally measured and theoretically described between 273 K and 280 K and at pressures up to 30 MPa. Concentrations of CO2 in the water phase ranging between 0.0163 and 0.0242 mole fractions were studied. The theoretical and experimental results indicate that the equilibrium pressure is very sensitive to concentration at all temperatures. These equilibria represent the solubility of CO2 hydrate in a water phase. The effect of salinity on the hydrate formation was also studied. A modified model which was based upon the variable chemical potential model of Lee and Holder (Lee and Holder, 2002) was introduced. There was a good agreement between the calculated and the experimental results, which further verified the theory. A simplified version of the model was also proposed and it can provide quick and reasonable estimations of the equilibrium conditions of hydrates at low concentrations and medium to low pressures.
For the first time, the effect of thermal expansion of the occupied hydrate lattice is incorporated into the model. Accurate prediction of hydrate equilibria for several gases (methane, carbon dioxide and xenon) was obtained.
The third part of this work modeled dissolution rates of CO2 droplets have been obtained under a range of conditions that include those that exist in the deep ocean down to 3000 m. A model was developed based on the dissolution rates obtained at different background concentrations of CO2 that allows calculation of mass transfer coefficients at different temperatures and pressures. The impact of different background concentrations on the mass transfer coefficient was also investigated. The model also accounts for the impact of a hydrate coating on the drop. Utilization of our data for modeling may be desired to predict the fate of CO2 released into aqueous environments like the deep ocean, since they were obtained under more realistic conditions.
Advisor:Gerald D. Holder; Kenneth Jordan; Robert P. Warzinski; J. Karl Johnson,; Robert M. Enick,
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:06/12/2007