Calorimetric investigation of hydrates of pure isobutane and iso- and normal butane mixtures

This paper shows the accurate calorimetric measurements of the phase equilibrium of pure isobutane, normal butane and their mixture in water-saturated quartz powder and in bulk. The heat capacity, temperature derivative of pressure at constant volume, enthalpy and pressure, as a function of temperature, for binary and ternary systems (water-isobutane, water-normal butane, water-isobutane-normal butane) have been measured in the hydrate's forming range (the temperature range of 234-280 K and pressure up to 10 MPa). Based on experimental data of heat capacity and temperature derivative of pressure at constant volume, the phase diagrams for hydrates of isobutane and mixture of isobutane and normal butane have been constructed. The isobutane-normal butane ratio of gaseous phase has been defined. The lower and upper quadruple points of isobutane hydrate and the quintuple point of isobutane hydrate formed by mixture of butanes have been detected. Isobutane is a hydrate former. Normal butane has an effect on the phase equilibrium of hydrates. An upper quadruple point of the pure isobutane hydrate is transformed into a line of upper quadruple points of the isobutane hydrate, which is in equilibrium with the binary mixture isobutane - normal butane. A line of upper quadruple points starts at the upper quadruple point of pure isobutane hydrate and stretches up to the quintuple point of hydrate formed by the mixture of butanes. The content of normal butane at the quintuple point is 29.9 ± 0.5% mol and does not depend on its concentration in the original mixture (before hydrate formation). The stability of isobutane and normal butane concentration in mixture is provided due to partial dissociation or formation of hydrate. A three-component system (water - normal butane - isobutane) of five phases (water - ice-hydrate - liquid butanes - gaseous butanes) has zero degree of freedom at the quintuple point. There is only one value of pressure, temperature and content of butanes in which five phases can exist together in equilibrium.