Improved Experimental Determinations of Phase Equilibria and Structural Transitions of Mixed Gas Hydrates under Isothermal Conditions

Abstract

Investigations on the intrinsic properties of gas hydrates with multiple guests are essential to scientific and technological fields. In particular, even though evaluating and designing a hydrate phase process require isothermal phase equilibria, it is difficult to obtain extensive data with various components and compositions in a short period of time due to the static-analytic method. The present study introduces a new experimental determination on hydrate phase equilibria using continuous dissociation induced by extremely slow vapor volume expansion at a constant temperature. When a syringe pump is automatically operated at the microliter level during the dissociation process, the endothermic dissociation can be traced from the temperature readings. The validity and stability of the proposed technique were evaluated using pure CH 4 hydrates, and repeated measurements of three-phase (L W −H−V) equilibrium conditions are used to optimize the volumetric expansion rates. Then, an experimental approach is applied to incipient CH 4 + C 2 H 6 hydrates and identifies the structural transition behavior. This method is thought to provide extensive data and further improvements in terms of hydrate phase equilibria with multiple gas components.

Temperature-Dependent Structural Transitions in Methane–Ethane Mixed Gas Hydrates

“…11 In that report, we demonstrated that our newly developed method enables an approximate determination of structural transition points with an accuracy comparable to Raman spectroscopy. In addition, the lower transition point of incipient CH 4 + C 2 H 6 hydrates at 283.10 K was determined in advance of the present work.…”

“…31 Table 3 lists all the results of isothermal phase equilibrium measurements in our previous and present studies. 11 These transition points can be ascribed to the change in structural behavior from sII inhibition to sII promotion by connecting the coordinates. However, a more detailed analysis of structural transitions needs to be carried out to ensure the accuracy of the experimental data and fitting results.…”

“…Isothermal three–phase equilibrium T – y 1 diagram for incipient CH 4 + C 2 H 6 hydrates in the lower transition region (Solid symbol, this work; open symbol, literature data).…”

“…The experimental apparatus and procedure of this work are based on our previous work. 11 An amount of 10 g of liquid water was initially charged into the equilibrium cell. A mechanical agitator was assembled with a pressure vessel to maintain the stirring rate at 150 rpm.…”

“…The experimental apparatus and procedure of this work are based on our previous work . An amount of 10 g of liquid water was initially charged into the equilibrium cell.…”