The difference between aspired and acquired hydrate volumes – A laboratory study of THF hydrate formation in dependence on initial THF:H2O ratios

Abstract

The study reports on the differences between theoretically expected and effectively obtained volume fractions of THF hydrate depending on the THF-H2O ratio in the initial solution against the background of using it as a substitute for natural hydrate in laboratory simulations. Besides the stoichiometric solution, initial solutions with either H2O or THF as excess phase were prepared to define the wanted volume of hydrate in advance. In order to achieve a chemical equilibrium a complete conversion of H2O and THF into THF hydrate and the presence of a pure excess phase is impossible. Based on the specific enthalpy of hydrate- and ice melting gained from calorimetric measurements, considerably lower than expected hydrate volumes are concluded. For the stoichiometric solution, containing 19.1 Wt% THF, enthalpy recalculations and the occurrence of an ice melting endotherm indicate incomplete conversion with a residual of 4.3 Vol% unconverted THF-H2O solution. The deviations from expectations increase with decreasing amount of aspired THF hydrate saturation and are stronger when formed from H2O excess solutions with up to 25 Vol% less hydrate than projected for full conversion. THF-rich solutions form hydrate with melting enthalpies that recalculate for up to 15 Vol% hydrate less than theoretical assumptions. In samples with initial THF concentrations below 5 Wt% and above 82.7 Wt% no hydrate formation was evident. Based on the results we propose corrections to the initial solutions when defined THF hydrate volumes are required. Furthermore, THF excess and temperatures below zero assure stable conditions for hydrate-liquid setting at atmospheric pressure.