Consistent Thermodynamic Calculations for Hydrate Properties and Hydrate Phase Transitions
Bjørn Kvamme
Abstract
Natural gas hydrates in natural sediments are dominated by methane. It is a clean source of hydrocarbon energy compared to conventional oil and gas. Most of these hydrates are created from biogenic degradation of organic material in the upper crust and are almost pure methane hydrates. Production of methane from hydrates can also be combined with steam cracking of CH4 to H2 and CO2. The produced CO2 can be used for the continued production of CH4 in a zero-emission cycle for producing hydrogen from CH4 hydrates. One of the challenges in theoretical evaluation of various concepts for producing hydrates has traditionally been related to limitations in theoretical descriptions and associated thermodynamic calculations. Any theoretical concept for phase transition dynamics based on physical principles is based on the implicit couplings between thermodynamic control, mass transport dynamics, and associated heat transport dynamics. In this work, I apply a simple theory based on extensions of the classical nucleation theory (CNT). This is used as a basis for illustrating the need for a consistent thermodynamic model for free-energy-related quantities and energy quantities. The model is also utilized to address the thermodynamic control of hydrate phase transitions in various hydrate schemes, as well as in associated energy changes. Specifically, it is argued that the pressure reduction method is limited by low temperature. One of the kinetic bottlenecks in hydrate production is the slow transport of molecules across an interface of strong hydrogen bonds between liquid water and hydrate. Free-energy changes are also limited in the pressure reduction method. Using CO2 with limited amounts of N2 is thermodynamically feasible, in terms of both free-energy changes and released enthalpy during the formation of hydrate from the injected CO2/N2 mixture. Addition of limited amounts of low-molecular-weight surfactants is needed to keep the water/CO2 mixture interface open and not blocked by hydrate films.