Influence of fluoroalkanes on the phase behaviour of methane gas hydrate systems

In this study, the influence of tetrafluoromethane and fluoroform on the phase behaviour of methane gas hydrates has been investigated. The pressure and temperature conditions for the phase boundary hydrate–liquid water–vapour (H–Lw–V) are determined experimentally for vapour phase compositions varying from 0 to 1 in the ternary systems water + methane + tetrafluoromethane, water + methane + fluoroform, and water + carbon dioxide + fluoroform. Additionally the phase boundary hydrate–liquid water–liquid fluoroform (H–Lw–Lg) has been determined for the ternary systems containing fluoroform in a limited range of vapour phase compositions. For the experiments Cailletet equipment has been used, that allows studying the phase behaviour in a pressure range from 1 to 14 MPa, and the temperature range considered was 273–298 K approximately. The experimental data were modelled using the van der Waals and Platteeuw model, which included a description of the fluid phases with the Soave–Redlich–Kwong equation of state and correction for the solubility of gaseous components in the liquid phase with Henry's law according to the Krichevsky–Kasarnovsky relation. The system water + methane + tetrafluoromethane shows azeotropic behaviour and forms structure I gas hydrate in the investigated vapour phase composition range. The pressures for the phase boundary H–Lw–V of this system can be predicted within 6%. The modelling work of the water + methane + fluoroform system shows that at the H–Lw–V phase boundary, structure I is formed, while there are indications that at the H–Lw–Lg phase boundary a change from structures I to II occurs. Equilibrium pressures of this system can be predicted within 15%.