Pseudo-retrograde behavior of the hydrate phase in systems of carbon dioxide, water, tetrahydrofuran and sodium chloride

Similar to retrograde hydrate behavior at high pressure, the low pressure pseudo-retrograde hydrate behavior is characterized as a behavior wherein, upon the reduction of pressure at a constant temperature, hydrate phase will be formed in the system. However, the number and types of phases coexisting before and after a sequence of phase transition are different in the case of pseudo-retrograde hydrate behavior whereas the number and types of phases coexisting remain the same in the case of retrograde behavior. This pseudo-retrograde hydrate behavior has been predicted based on modeling work and observed experimentally for hydrate systems consisting of ethane and propane as guest components. Ballard et al. hypothesized that pseudo-retrograde behavior will occur, but not constrain to any pseudo-binary systems in which sI and sII hydrate formers that have fairly low vapor pressures are present. Earlier experimental measurements from our laboratory have shown that pseudo-retrograde behavior does occur in mixed carbon dioxide and tertrahydrofuran systems and the effects of carbon dioxide concentration on the occurrence of the behavior have been discussed. The aim of the present work is to study the effects of tetrahydrofuran concentration and the presence of electrolyte (sodium chloride) on the occurrence of pseudo-retrograde behavior in mixed hydrate systems. All experimental measurements are carried out based on a synthetic method in a Cailletet apparatus. In this apparatus, the phase behavior measurement is based on the observation of the number and types of phases present at known temperature and pressure conditions. In carbon dioxide + tetrahydrofuran + water system, experimental results indicate that the occurrence of the pseudo-retrograde hydrate behavior is depending on the concentration of tetrahydrofuran in the systems. Pseudo-retrograde behavior is detected for mixtures having low concentration of tetrahydrofuran (1.2 and 3.0 mol% overall concentration). As the concentration of tetrahydrofuran is increased to higher value, no evidence of pseudo-retrograde behavior is observed. If the occurrence of the pseudo-retrograde hydrate behavior is attributed to the structural transition of sII hydrate to sI hydrate, the increase of tertrahydrofuran molecules reduces the probability of carbon dioxide molecules to occupy the large cavities of sII hydrate and blocking hydrate structural transition to occur. Experimental results also indicate that no pseudo-retrograde behavior is occurring in carbon dioxide + tetrahydrofuran + water + sodium chloride system.