Plenary Lecture

Multiscale modeling of phase transitions dynamics

Professor Bjorn Kvamme
Department of Physics and Technology
University of Bergen, Allegaten 55
N-5007 Bergen, Norway
E-mail: bjorn.kvamme@ift.uib.no

Abstract: The possible scale (time and space) for molecular dynamics simulations is very often a limitation in studies of phase transitions. Nucleation phenomena, thermodynamic properties and interface properties are examples of properties that may be sampled very well from molecular dynamics simulations. In this work we demonstrate how molecular dynamics simulations can be combined with Phase Field Theory simulations in studies of fluid/solid phase transitions. Several examples are discussed but the main focus is devoted to phase field simulations for estimation of the conversion rate of CH₄ hydrate to CO₂ hydrate in the presence of liquid CO₂. The simulations are conducted under conditions typical for underwater gas hydrate reservoirs. In the computations, all model parameters are evaluated from physical properties taken from experiment or molecular dynamics simulations. The simulations are conducted in the absence of solid walls. But the available space for hydrate has been deducted from the real measured porosity of the sandstone used in the experiments. It has been found that hydrate conversion is a diffusion controlled process, as after a short transient, the displacement of the conversion front scales with t^1/2. Assuming a diffusion coefficient of D_s = 1.1 x 10^-11 m²/s in the hydrate phase. Formation of methane hydrate in porous media was also monitored and quantified experimentally using Magnetic resonance imaging (MRI) and experimental conditions of 83 bar and 4° C . In contrast to hydrate formation in the absence of porous media no significant induction times are observed. As expected the conversion rate diminishes with time, proportional to dilution of the CO₂ phase with released CH₄, and also reduced CH₄/CO₂ contact surfaces. Replacing the diluted CO₂ phase with pure CO₂ increases the kinetic rate again back to a rate similar to the initial rate and a similar progress. Within the resolution of the experiment (~100μm) there was no detectable dissociation of the hydrate during the exchange process. The predicted time dependent conversion rate is in a reasonable agreement with results from Magnetic Resonance Imaging experiments. This value of the diffusion coefficient is higher (roughly 5 times) than expected for the bulk hydrate phase. One reason for this might be the presence of liquid like channels separating the hydrate from the solid mineral. This is in accordance with molecular simulation studies which demonstrate that hydrate is not stable close to solid mineral walls dues to the chemical potential of water molecules adsorbed on the mineral walls.

Brief Biography of the Speaker:
Professor Bjorn Kvamme obtained his M.Sc. in Chemical Engineering and Ph.D. in Chemical Engineering from the Norwegian University of Technology and Natural Sciences. He has been with SINTEF in Trondheim. He was also a pioneer in starting up education of M.Sc. as well as a doctoral program in Process Engineering in Telemark, now implemented as a part of Telemark University College. He has been affiliated with UoB, Department of Chemistry, since 1998 and entered a position as Professor in Gas Processing at Department of Physics March 2000. He is the author/coauthor of more than hundred publications plus several book chapters and proceedings. His main research effort has been in thermodynamics and statistical thermodynamics. Topics include natural gas hydrates, equilibrium, stability and kinetics, polar solutions and electrolyte solutions, kinetics of phase transitions, oil in water emulsions, separation, gas cleaning, and seawater chemistry.