Analysis of the Dissolution of CH4/CO2-Mixtures into Liquid Water and the Subsequent Hydrate Formation via In Situ Raman Spectroscopy

Abstract

We report an experimental study for the investigation into the suitability of hydrate formation processes for the purification of methane (CH 4 ) from carbon dioxide (CO 2 ) at a sub-cooling temperature of 6 K and a pressure of 4 MPa. The experiments were conducted in a stirred batch reactor. Three different initial CH 4 /CO 2 mixtures with methane fractions of 70.1 mol%, 50.3 mol%, and 28.5 mol% were tested. The separation efficiency was quantified by measuring in situ via Raman spectroscopy the ratios of CH 4 /CO 2 in the gas mixture, the liquid water-rich phase before hydrate formation, and the solid hydrate phase after the onset of the hydrate formation. The results indicated that the main separation effect is obtained due to the preferential dissolution of CO 2 into the liquid water-rich phase before the onset of the hydrate formation.Energies 2020, 13, 793 2 of 17 hydrate phase is prone to error, especially if the solubility of the gas species in the liquid water-rich phase, which coexists with the hydrate phase, is high. To the knowledge of the authors, there has been no study that correlates the composition of the CH 4 /CO 2 gas mixture, with the dissolution of CH 4 and CO 2 into the liquid water-rich phase before the hydrate formation and the subsequent incorporation of CH 4 and CO 2 into the solid gas hydrate phase.Raman spectroscopy is one possible method for the remote and in situ measurement of the composition of fluid and solid mixtures and has been applied to the analysis of gas hydrates and fluid mixtures in the context of gas hydrates [26,27].Therefore, the object of this work is to put into context the composition of the gaseous CH 4 /CO 2 -mixture with the dissolution of CH 4 and CO 2 into the liquid water-rich phase before hydrate formation and the incorporation of CH 4 and CO 2 into the solid pure hydrate phase using Raman spectroscopy. Three different initial gas compositions were employed to evaluate the effect of the initial gas composition on the fractions of CH 4 and CO 2 in the vapor phase, the liquid water-rich phase, and the hydrate phase. Finally, CO 2 selectivities relevant for gas separation or gas purification processes are discussed. Materials and Methods MaterialsThe experiments were conducted with deionized water with a conductivity of less than 10 µS/cm and with gaseous premixed binary CH 4 /CO 2 -mixtures (Linde, molar purity 99.5%) with three certificated CH 4 molar fractions of 0.701, 0.503, and 0.285. ApparatusA schematic diagram of the experimental setup is given in Figure 1, which consists of a high-pressure view cell (maximum pressure 25 MPa, internal and non-adjustable volume 26 mL) and a Raman probe.Energies 2020, 13, x FOR PEER REVIEW 2 of 17

Mini Review on Application and Outlook of In Situ Raman Spectrometry in Gas Hydrate Research

“…For gas separation, Li et al studied the separation of CH 4 from CH 4 /CO 2 gas mixtures via forming hydrate using in situ Raman spectroscopy and obtained the ratios of CH 4 /CO 2 in the gas mixtures based on the Raman data to express the separation efficiency. Cai et al studied the CO 2 separation from fuel gas via hydrate and monitored the structure of the compounds at different positions and with different methods of gas supply using in situ Raman spectroscopy.…”

“…For instance, Ota et al 71 obtained the composition of CH 4 /CO 2 gas mixtures from a calibration curve and further calculated the composition of CO 2 /CH 4 hydrate. Li et al 72 also applied the ratio calibration method to determine the molar ratios of gas mixtures and mixed hydrate.…”

“…81 The formation process of CO 2 /N 2 /TBAC semiclathrate 49 showed that two guest gas molecules both enclathrate in the small cages of semiclathrate hydrate according to the enclathrated CO 2 molecule peaks at 1273 and 1380 cm −1 and the enclathrated N 2 molecule peaks at 2324 cm −1 . For the CO 2 /H 2 /TBAB/nano-Al 2 O 3 system, Li et al 82 discovered that CO 2 molecules were captured in the small cages of the semiclathrate hydrate according to the peaks at 1274 and 1380 cm For gas separation, Li et al 72 studied the separation of CH 4 from CH 4 /CO 2 gas mixtures via forming hydrate using in situ Raman spectroscopy and obtained the ratios of CH 4 /CO 2 in the gas mixtures based on the Raman data to express the separation efficiency. Cai et al 84 studied the CO 2 separation from fuel gas via hydrate and monitored the structure of the compounds at different positions and with different methods of gas supply using in situ Raman spectroscopy.…”