THERMODYNAMIC STABILITY AND SELF-PRESERVATION PROPERTIES OF SEMI-CLATHRATES IN THE METHANE+TETRA-n-BUTYL AMMONIUM BROMIDE+WATER SYSTEM

Unlike normal gas hydrates, research on semi-clathrate hydrates is still at its early stages. Experimental data are needed both to characterize these hydrates and to further develop appropriate predictive models. In this work, the thermodynamic phase boundary of sem i-clathrate hydrates of methane–tetra-n-butyl ammonium bromide– water at TBAB concentration of 40.7 wt% (3.70 mol%) was determined and compared to the data for other concentrations. The measurements were carried out in a laboratory–scale stirred tank reactor using an isochoric step-heating method at temperature range of 291.05 to 297.25 K and pressure range of 3.65 to 16.73 MPa. The comparison of these results with the literature data at other TBAB concentrations showed that the chosen concentration of 40.7 wt% yields the most stable semiclathrates with the highest shift in the phase boundary. In addition, dissociation rate of semi-clathrates at atmospheric conditions was measured at 274.65 and 270.15 K showing considerable stability for this system. The test results showed that 5 vol% of the total methane stored is released at 270.15 after 240 hours. This means a very low dissociation rate of only 0.5 vol% per day on average. The results of this study can be useful in up-scaling the process of gas storage and transportation by hydrates.