Semester

Fall

Date of Graduation

2009

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Chemical and Biomedical Engineering

Committee Chair

Brian J. Anderson.

Abstract

Natural gas hydrates are likely to contain more carbon than in all other fossil fuel reserves combined worldwide. Most of the natural gas hydrate deposits contain CH4 along with other hydrocarbon gases like C2H 6, C3H8 and non-hydrocarbon gases like CO 2 and H2S. Thus, if CH4 stored in natural gas hydrates can be recovered, the hydrates would potentially become a clean energy resource for the next 10,000 years. The production of CH4 from natural gas hydrate reservoirs has been predicted by reservoir simulators that implement phase equilibria data to predict various production scenarios. Therefore, it is very important to predict accurately phase equilibria of mixed hydrates. In this work an empirical correlation of dissociation pressure with respect to temperature and gas phase composition for CH4-C 2H6 mixed hydrate system is developed by fitting to available experimental data. It is a simple method with limited accuracy. Statistical thermodynamics approach developed by van der Waals and Platteeuw in 1959 provides best approximation to predict the phase equilibrium data. They assumed that there are no lattice distortions due to the guest molecules, hence constant reference parameters are used for different guest molecules. Later, Hwang et al. by his molecular dynamics found that there are lattice distortions due to the guest molecules and Holder et al. proposed that the reference chemical potential difference Dm0w and reference enthalpy difference Dh0w varies with the guest molecule. In this work, a correlation of Dm0w and Dh0w with respect to guest molecular size is developed to estimate the values of Dm0w and Dh0w . The cell potential method developed by Anderson et al. is modified for variable reference parameters. The method is validated by reproducing the phase equilibria of simple hydrates and the structural transitions that are known to occur. Three-dimensional phase equilibria and structural transitions occurring in the mixed hydrates like CH4-C2H6, CH4-N2 and N2-CO2 are predicted accurately without fitting to experimental data. The phase equilibria of CH 4-CO2 and CH4-N2-CO 2 hydrates are predicted to assess the production of CH4 from the reservoirs by replacing CH4 in the hydrate by pure CO 2 and N2+CO2 mixture which serves dual purpose of CH4 recovery and CO2 sequestration.

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