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in Wiley InterScience (www.interscience.wiley.com).Dissociation processes of methane hydrate under water flow conditions were investigated by a combination of experimental observations and numerical simulations using computational fluid dynamics (CFD). In Part I of this study, the dissociation process induced by water flow at pressures above the three-phase [hydrate (H)-liquid water (L w )-vapor (V)] boundary in an isothermal x-P phase diagram is discussed. Dissociation experiments were carried out with a methane hydrate ball (diameter % 10 mm) suspended in a flow cell, and the overall dissociation rate of methane hydrate without bubble formation was measured under various conditions of pressure, temperature, and volumetric flow rate of water. A linear phenomenological rate equation in the form of the product of the dissociation rate constant k bl and the molar Gibbs free energy difference DG, between the hydrate phase and the ambient aqueous phase, was derived by considering the Gibbs free energy difference as the driving force for the dissociation. The molar Gibbs free energy difference was expressed by the logarithm of the ratio of the concentration of methane dissolved in water at the hydrate surface to the solubility of methane in the aqueous solution in equilibrium with the hydrate. The dissociation rate constant k bl was determined from the experimental results of the overall dissociation rate combined with the numerical simulation results of the concentration profile of methane by the CFD method. The obtained dissociation rate constant was found to be independent of the ambient water flow rate, indicating that the rate constant is intrinsic for the hydrate dissociation within the conditions examined in this study. The rate constant was independent of the pressure, whereas the temperature dependency was described by an Arrhenius-type equation with the apparent activation energy of 98.3 kJ/mol.

A new separation method using gas hydrate formation is proposed for separating HFC-134a from gas mixtures containing N2 and HFC-134a. The feasibility of this separation method was investigated from various points of view. First, to determine the mixed hydrate stability region, three-phase equilibria of hydrate (H), liquid water (Lw), and vapor (V) for HFC-134a + N2 + water mixtures with various HFC-134a vapor compositions were closely examined in the temperature and pressure ranges of 275-285 K and 0.1-2.7 MPa, respectively. Second, the compositions of the hydrate and vapor phases at a three-phase equilibrium state were analyzed for identical mixtures at 278.15 and 282.15 K to confirm the actual separation efficiency. Third, kinetic experiments were performed to monitor the composition change behavior of the vapor phase and to determine the time required for an equilibrium state to be reached. Furthermore, X-ray diffraction confirmed that the mixed HFC-134a + N2 hydrates were structure II. Through an overall investigation of the experimental results, it was verified that more than 99 mol % HFC-134a could be obtained from gas mixtures after hydrate formation and subsequent dissociation processes. Separation of HFC-134a using hydrate formation can be carried out at mild temperature and low-pressure ranges. No additive is needed to lower the hydrate formation pressure.

Formation of CO2 hydrate using a Kenics-type static mixer was studied experimentally. The flows of liquid CO2 and water were mixed in the static mixer, and CO2 hydrate was formed continuously from the two-phase flow. The patterns of hydrate formation were found to be dependent on the flow velocities of liquid CO2 and water. The flow of agglomerated hydrate chunks in water occurred under relatively CO2-rich conditions, while dispersed flow of tiny particles of CO2 hydrate with small liquid CO2 drops was observed under relatively water-rich conditions. These effects could be explained by two mechanisms occurring in the static mixer, namely, continuous shedding of hydrate films from the interface between liquid CO2 and water induced by the shearing force and breakup of the CO2 drops. The energy consumption by the static mixer for the hydrate formation process was estimated, and it was significantly less than that for a stirring vessel type reactor. A continuous hydrate formation process could be achieved using the static mixer.

Gas hydrate formation kinetics were investigated in a Kenics static mixer. When operated under thermodynamic conditions (pressure or temperature), the hydrate formation rate increased and the HFC-134a gas bubble was covered with a hydrate “shell”. This hydrate shell inhibited hydrate growth because of resistance to mass transfer. Water recycling in the hydrate reactor accelerated hydrate formation by increasing the gas−water interface during water−gas cocurrent flow and causing the continued presence of a fresh interface in the counterflow. The kinetic data indicated that the hydrate formation rate would be equal to the mass transfer rate including the rate of hydrate shedding from the gas bubble. Enriched HFC-134a gas could be continuously recovered from an HFC-134a-nitrogen mixture in a continuous hydrate formation system. The hydrate formation rate constant for the mixed gas depended on the feed gas components.