The plateau effects and crystal transition study in Tetrahydrofuran (THF)/CO2/H2 hydrate formation processes

Hydrate-based carbon dioxide (CO2) capture and hydrogen (H2) purification is a promising technology in clean energy fields. In this work, in order to reveal the effect and mechanism of tetrahydrofuran (THF) on the hydrate-based CO2 separation from Integrated Gasification Combined Cycle (IGCC) syngas, the CO2/H2/THF hydrates formation processes were studied with and without memory effect. According to the pressure drop curves, there appear two pressure plateaus in the CO2/H2/THF hydrate formation processes. Furthermore, with the usage frequency of the THF solution increasing, the plateau effects are more ambiguous and difficult to be observed. It is interesting that the Raman spectra for CO2 and H2 molecules also reveal slim Raman shifts between the two different hydrate plateaus. According to the powder X-ray diffraction (PXRD) patterns, indeed, the detail Miller indices indicates that CO2/H2/THF hydrate mainly forms THF•16.8 H2O structure in the first plateau, while mainly forms THF•17 H2O structure in the second plateau. The reason for this phenomenon is mainly the influence of CO2, its large molecular size and the localized tension it causes in the water network of the small cages which can enhance the storage capability for the large cages of THF hydrate. The experimental results illustrate that the highest Split fraction (S.Fr) is 69.02% obtained at 6 MPa/284.85 K (memory effect), and this work highlights that the memory solution are more suitable for industrial application.