High‐Pressure Optical Cell for Hydrate Measurements Using Raman Spectroscopy

A high‐pressure optical cell was designed and fabricated for the measurement of hydrate phase equilibria. These measurements were carried out using Raman spectroscopy, which provided molecular microscopic evidence of hydrate dissociation, along with macroscopic temperature and pressure evidence. Initial measurements were made of hydrates formed from methane (CH4) dissolved in deionized water. Distinct signatures of CH4 in different phases (aqueous and sI hydrate) were the basis for this study. Time resolved Raman spectra monitored the dissociation of hydrate as cell temperature was increased. The spectra depicted the transition of CH4 in the two cavities of sI hydrate to CH4 dissolved in water. A pressure trace of the experiment indicated a slight jump in pressure of about 30 psi (0.2 MPa) at the hydrate dissociation point, which agreed with the Raman spectroscopic data. Experimental data obtained using this method agreed well with literature data for the simple methane system. The technique demonstrated that the optical cell is suitable for Raman measurements of hydrate phase equilibria at high pressures. Ultimately, this procedure will be used to measure thermodynamic conditions for more complex systems.