"Continuous" Method for the Fast Screening of Thermodynamic Promoters of Gas Hydrates Using a Quartz Crystal Microbalance

The selection of thermodynamic promoters is an essential issue in determining the economy of a hydrate application. It is highly desirable to devise a fast, accurate, and relatively inexpensive measurement method to screen the best promoter from among many candidates. Although new equipment based on the quartz crystal microbalance (QCM) was suggested years ago, it has been regarded as unfeasible because of the contact problem between the surface of a quartz crystal and the hydrate. In this study, we report a solution to this problem by introducing a small QCM (d < 1 cm) on which a 2 mu L droplet is placed. A proper heating rate for the cell is important in determining hydrate dissociation points. We found that the maximum heating rate for our system with the hydrophilic quartz crystal surface was 3 K/h, which is approximately 30 times faster than the conventional method. Our QCM data exhibited excellent agreement with the previously reported values for the phase equilibria of CO2 hydrate with soluble thermodynamic promoters, such as tetrahydrofuran (THF) or 1,4-dioxane, as well as pure CO2 hydrate. In addition, phase equilibrium data for H-2 hydrate with THF were also obtained, and the agreement with the reported data was satisfactory. We were able to acquire multiple hydrate dissociation points continuously and quickly at the rate of 1 h per point. The maximum error between two sets of data was approximately +/- 0.16 K. Because the uncertainty of the thermocouple was approximately +/- 0.1 K, our results confirm that the "continuous" QCM method is acceptable as an efficient tool for the fast screening of the best promoter from among many soluble candidates.