Formation and dissociation of clathrate hydrate in stoichiometric tetrahydrofuran-water mixture subjected to one-dimensional cooling or heating

Formation and growth, or melting, of a polycrystalline layer of tetrahydrofuran (THF) hydrate from, or into, a liquid solution having the same composition as that of the hydrate have been observed in a macroscopically one-dimensional heat-transfer system under atmospheric pressure. Experiments were performed with either the liquid THF-water solution or the THF hydrate initially filling a 260 cm(3) cell which was sealed top and bottom by temperature-controlled copper plates and by glass plates on the side. In one group of experiments, the cell filled with the solution was initially adjusted at the hydrate-solution equilibrium temperature, T-eq(4.4 degreesC). The top copper plate was then cooled quasi-stepwise, while the temperature at the bottom copper plate was either unchanged or increased quasi-stepwise, resulting in the growth of a planar polycrystalline hydrate layer down from the surface of the top plate. In another group of experiments, the cell was initially filled with a polycrystalline THF-hydrate phase at a temperature slightly lower than T-eq. Successively, the temperature at the bottom plate was increased quasi-stepwise to exceed T-eq, resulting in the melting of the hydrate phase from the bottom. The behavior of such growth and melting of the hydrate layers observed in the experiments is in general agreement with that predicted by relevant theoretical/numerical analyses of transient conductive and/or free-convective heat transfer from/to the hydrate-solution interface, where the temperature is assumed to be fixed at T-eq. Also described in this paper is an unexpected finding in a particular experimental condition-the formation of column-like hydrate crystals extending almost across the 20-mm spacing between the top and bottom plates, which precedes the growth of a planar polycrystalline layer. (C) 2001 Elsevier Science Ltd. All rights reserved.