The effect of pressure on methane hydration in pure water and sodium chloride solutions

Traditionally, the effect of pressure on hydrate formation has been described using the Clausius-Clapeyron equation, which assumes that only vapor phase volume changes are significant. According to this assumption, equilibrium data for any hydrate former will fit to straight lines with equal slope on a plot of logarithmic pressure versus temperature. But both data and theory shows that this is not true at higher pressures. In this research, equilibrium data of methane hydrate dissociation were measured up to pressures of 70 Mpa and at 4 different concentrations of NaCl. These measurements were done using a new experimental technique, Raman spectroscopy, to detect the presence and determine the structure of hydrates. The data show significant deviations from linear Clausius-Clapeyron behavior. Calculations have been done on literature data for methane hydrate and new data. Both the Clapeyron relation and van der Waals and Platteeuw hydrate theory has been used. Both models represent the data well, but show interesting, minor deviations at high salt concentration. (C) 2001 Elsevier Science B.V. All rights reserved.