(330g) Predictions of Mixed Hydrate Phase Equilibria and the Swapping of CH4 Hydrate with CO2 and CO2+N2 Mixtures

Natural
gas hydrates are likely to contain more carbon than in all other fossil fuel
reserves combined. Thus, if CH₄ stored in the natural gas hydrates
can be recovered, the hydrates will become a potential clean energy resource
for next 1000 years. Methods currently being employed for recovery of CH₄
from hydrate deposits include depressurization, thermal stimulation and
inhibitor injection, which all involve the dissociation of hydrate and the release
of significant volumes of water. These processes may cause geomechanical stress
on the reservoir leading to subsidence. Another method of production of CH₄
gas from the gas hydrate is through the injection of CO₂. CO₂
sequestration in natural-gas-hydrate reservoirs by replacing the CH₄
is potentially attractive as it serves the dual purpose of long-term storage of
a greenhouse gas (CO₂) and the production of natural gas (CH₄).
Recent experimental studies (Park et al., PNAS, 103(34), 12690, 2006) have
shown that the fraction of recovered methane can be improved by using a mixture
of N₂ and CO₂ as opposed to pure CO₂.

Based
on an analytical solution to the Lennard-Jones Devonshire approximation to the
van der Waals-Platteeuw statistical mechanics model for hydrate equilibrium,
the cell potential method is used to predict the phase equilibrium
data of the mixed hydrate.  The reference parameters and cell potential
parameters are obtained by fitting to the experimental simple hydrate data and
are used to predict the mixed hydrate equilibria without fitting to any
experimental data. Reference parameters vary with the guest molecules, depending
on the size of the guest molecule. The method is validated by testing its
predictive ability against available experimental data and has been shown to
predict hydrate structural transitions accurately. It is verified by predicting
structural transitions for cyclopropane to occur at 256.3 K and 274.5 K. The
reliability of the method is demonstrated by its ability to predict cage
occupancies accurately.

Natural
gas hydrates can
be formed either by bacterial activity at shallow depths or by thermal
pyrolysis of fossil organic matter which contains methane and significant
amounts of other higher hydrocarbons (C₂-C₅) and other
non-hydrocarbon gases.  To understand the replacement
of CH₄ from the hydrate it is important to know the phase
equilibrium data of the mixed hydrates. In this work, the phase equilibria of many
different binary mixed hydrates such as CH₄-C₂H₆,
CH₄-CO₂, CH₄-N₂ and N₂-CO₂
have been predicted so as to assess the production of the CH₄
from natural gas hydrate reservoirs formed by thermogenic gases. Interestingly,
both CH_4, and C₂H₆ form a structure I hydrate when
the gas phase contains only one component but the CH₄-C₂H₆
mixed hydrate forms structure-II at various gas mixture compositions. Likewise,
N₂ as a simple hydrate forms structure II, but along with other
guest molecules it can form structure I. These structural transitions are
predicted for these mixed hydrates. N₂-CH₄ and N₂-CO₂
predictions are confirmed using experimental results. Additionally, phase
equilibria for ternary hydrates, including N₂-CO₂-CH₄
are calculated. Using this thermodynamic framework, the replacement of CH₄
in hydrate with N₂-CO₂ mixture is studied and is
verified using the experimental data published by Park et al. (2006). In this
system, 23% of CH₄ in the hydrate is replaced by N₂
whereas 62% of CH₄ is replaced by CO₂ i.e.; approximately
85% of CH₄ encaged in hydrate is recovered by using N₂-CO₂
(80:20) mixture.  An optimization of the N₂-CO₂ ratio
was performed using our thermodynamic model, thus predicting the maximum recovery
of CH₄ from a natural gas hydrate reservoir while simultaneously
sequestering CO₂.