(728g) Phase Equilibrium Prediction for Gas Hydrate: Comparing Equation of State Models

Phase
equilibrium prediction for gas hydrate: Comparing equation of state models

Harshal
J. Dongre and Amiya K. Jana

Energy and
Process Engineering Laboratory, Department of Chemical Engineering, Indian
Institute of Technology-Kharagpur, India - 721302

Abstract

In
mankind’s push towards cleaner and greener sources of energy, methane gas is
the most sought fuel for its least carbon dioxide production during combustion.
Natural deposits of methane clathrates on deep ocean floors present itself as
an abundant supply for the same. Yet, extraction of this gas remains a major
challenge as direct disruption to the reserves may irreversibly affect the
marine life at large. Fortuitously, carbon dioxide hydrate forms at the
temperature and pressure in proximity to that of methane hydrate. Thus,
simultaneous extraction of methane and sequestration of carbon dioxide can be
aspired to systematically regulate climate change. Since, flue gases from most
sources contain nitrogen in addition to carbon dioxide, selective extraction of
carbon dioxide may be expensive. Thus, the stability of double hydrates of nitrogen
and carbon dioxide becomes crucial for its applicability in ocean bed
stabilization. The thermodynamic study of such a system can help us identify this
stability region. The co-existence of multiphase is therefore addressed by the corresponding
model to best describe each phase. The activity coefficient model estimates the
deviation from ideality in the bulk liquid phase, while the equation of state
(EoS) model helps to predict properties of vapor phase under consideration. Though
the selection of EoS is random in most studies, a performance analysis between
the ones most used can be instrumental in equilibrium predictions. In this
study, a comparison between Soave-Redlich-Kwong (SRK), Patel-Teja (PT),
Peng-Robinson-Stryjek-Vera (PRSV) EoS is made for hydrate equilibrium predictions.
The UNIQUAC activity coefficient model is used for liquid phase nonidealities,
whereas the model of van der Waals and Platteeuw is employed to describe the
hydrate phase. Moreover, Kihara spherical shell potential model is used to
account for the host-guest interactions of hydrate lattice. In the preliminary
study, the hydrate equilibrium prediction for methane guest is carried out. All
the values used in the model are reported in the literature ^[1] and
no additional parameter optimization is done. Performance between the EoS
models for hydrate equilibrium studies, is quantified based on the percent average
absolute deviation (%AARD) from the experimental data available in literature ^[2].
Amongst the concerned EoS models, PT results in 8.10% AARD followed by SRK (13.60%)
and PRSV (17.19%) as depicted in Figure 1. Thus, the PT EoS is selected in
estimating vapor phase fugacity for mixture of nitrogen and carbon dioxide
gases. For this mixed hydrate case, the AARD obtained is 14.67%. Thus, the
better performance of the PT EoS is attributed to its use of acentric factor in
describing fluid phase interactions, which becomes crucial for polar
substances. The phase equilibrium predictions for single and mixed hydrates can
be useful in mimicking the gas hydrate thermodynamics in various hydrate based applications.

Figure
1. Comparison of the equation of state models for methane hydrate phase
equilibria predictions

References

[1] El Meragawi, S., Diamantonis, N. I.,
Tsimpanogiannis, I. N., & Economou, I. G. (2016). Hydrate–fluid phase
equilibria modeling using PC-SAFT and Peng–Robinson equations of state. Fluid
Phase Equilibria, 413, 209-219.

[2] Sloan Jr, E. D., & Koh, C. A.
(2007). Clathrate hydrates of natural gases. CRC press.