Microscopic Insights into the Effect of the Initial Gas–Liquid Interface on Hydrate Formation by In-Situ Raman in the System of Coalbed Methane and Tetrahydrofuran

Abstract

The serious issues of energy shortage and greenhouse gas emission have led to the development of coalbed methane (CBM) with new commercial ramifications. A hydrate-based gas separation technology is introduced to recover methane from CBM. However, the mechanism of hydrate nucleation needs to be clear for enhancing the hydrate formation rate and gas recovery efficiency. In this work, we studied, by means of in-situ Raman spectroscopy, the microscopic characterizations of hydrates forming in/around the initial gas–liquid interface in the case of CBM and tetrahydrofuran (THF). It is found that the hydrates accumulate as a film with horizontal crevices in the initial gas–liquid interface. These crevices prevent the hydrate film from hindering gas–liquid contact and limiting hydrate formation. Raman spectroscopy results illustrate that the initial gas–liquid interface shows a positive impact on water aggregation, and that the holding gas molecules stay stably with the water molecules. Nitrogen molecules encage into the cavities of THF hydrates along with methane molecules. For the interface and hydrate layer, water aggregation is evaluated by the Raman intensity ratio of hydrogen-bonded water (BW) and free water (FW) without any hydrogen bonds, abbreviated as I BW / I FW . A value of I BW / I FW higher than 0.85 can symbolize the occurrence of hydrate nucleation in the interface and help assess the hydrate formation.

Coupling Amino Acid with THF for the Synergistic Promotion of CO2 Hydrate Micro Kinetics: Implication for Hydrate-Based CO2 Sequestration

“…As CO 2 gas dissolves in water, the peak of 1382.0 cm –1 of CO 2 in the solution gradually increases during the dissolution and cooling stage (see red and blue colored lines in Figure a). In addition, THF is identified in the solution, which corresponds to the Raman peak of THF C–C–C–C stretching (ring breathing) at 886.5 and 917.0 cm –1 . , …”

“…The aqueous solution separates into a thin layer (THF-rich phase) estimated to be ≤0.25 mm (see Table S3 and Figure S11) at the top of the aqueous solution and a thick layer (water-rich phase) at the bottom due to the difference of CO 2 solubility in H 2 O and THF 33 (see Figure 7b). Upon cooling, binary CO 2 -THF sII hydrate forms in the aqueous phase first consuming both THF and CO 2 in the solution 62 (see Figure 7c). As THF molecules are fast consumed for sII hydrate formation, the thickness of the top THF-rich layer is reduced rapidly resulting in the exposure of the CO 2 gas phase with the aqueous solution (see Figure 7d).…”

“…In addition, THF is identified in the solution, which corresponds to the Raman peak of THF C− C−C−C stretching (ring breathing) at 886.5 and 917.0 cm −1 . 62,63 Since the first nucleation events, it is observed that the two distinct peaks for THF aqueous solution at 886.5 and 917.0 cm −1 merge into one peak at 917.0 cm −1 (see Figure 6c). The disappearance of the Raman peak at 886.5 cm −1 and the increasing intensity of the Raman peak at 917.0 cm −1 suggests that THF molecules are consumed in the solution and participated in the THF and CO 2 -THF sII hydrate formation occupying the 5 12 6 4 cages of those.…”