Unveiling the CO2 Hydrate Phase Diagram from Computer Simulation: Locating the Hydrate-Liquid-Vapor Coexistence and its Upper Quadruple Point

TL;DR

This work tackles the challenge of predicting the CO$_2$ hydrate phase diagram, focusing on the hydrate–water–vapor three-phase line and the upper quadruple point $Q_2$. By extending a solubility-based method to systems with vapor phases and combining it with previous DC simulations for condensed phases, the authors determine the $H$–$L_{H_2O}$–$V$ coexistence and locate $Q_2$. The computed $Q_2=(283.5~ ext{K}, 44~ ext{bar})$ agrees closely with experimental values, validating the approach and enabling a more complete, low-pressure CO$_2$ hydrate phase diagram, including regions previously inaccessible to purely simulation-based methods. This methodology promises broader applicability to other hydrates exhibiting gas-vapor transitions and contributes to understanding hydrate behavior under conditions relevant to CCS and storage applications.

Abstract

Carbon dioxide (CO2) hydrates hold promising applications in capturing and separating CO2 for climate change mitigation. Understanding their behavior at the molecular level is therefore essential, and computer simulations have become powerful tools for exploring their formation and stability, providing valuable insights into their underlying mechanisms. In this work, we perform molecular dynamics simulations to compute the three-phase coexistence line involving the stability region where CO2 is in the vapor phase: CO2 hydrate - liquid water - vapor. This computation was previously inaccessible using the traditional three-phase direct coexistence technique. To achieve this, we employ a novel solubility-based method, which allows us to accurately evaluate the coexistence line. Finally, we have determined the upper quadruple point (Q2) where the four phases, namely hydrate, liquid water, liquid CO2, and vapor, coexist. Our pioneering result for the Q2 value shows remarkable agreement with experimental observations, validating the accuracy of our findings and representing a significant milestone in the field of gas hydrate research.

Figures (4)

• Figure 1: Top left: Snapshot of the hydrate-liquid coexistence. Top right: Snapshot of the liquid-vapor coexistence. Middle: Schematic solubility of a gas as a function of temperature of an aqueous phase when in contact via a planar interface with a hydrate phase (left), vapor phase (right), and in a triple coexistence (center). Bottom: Snapshot of the triple coexistence hydrate-liquid-vapor.
• Figure 2: Solubility of CO$_2$, as a function of pressure, for the four isotherms studied in this work in the aqueous phase when in contact via a planar interface with the vapor phase. The meaning of the symbols is explained in the legend. Magenta vertical lines represent the pressures chosen to interpolate the solubilities in this work. The continuous black, red, green, and blue lines have been obtained by linearly fitting the solubility results at each temperature. The cross points between the magenta lines and each fitting line correspond to the solubility values used for the $T_3$ determination. Green open diamonds represent the experimental solubility of CO$_{2}$ taken from the literature Servio2001aWang2025a at $280\operatorname{K}$ and two different pressures.
• Figure 3: Solubility of CO$_2$, as a function of temperature, at five pressures (10, 15, 20, 30, and $40\,\text{bar}$) of an aqueous phase when in contact via a planar interface with the vapor phase or the hydrate phase. The meaning of the symbols and lines in the main plot is represented in the legend. The red and green dashed curves shown in the inset correspond to correlations based on experimental data reported in the literature Servio2001aWang2025a for the solubilities of CO$_2$ in the aqueous solution with in contact with the vapor and hydrate phase along the $20$ and $30,\text{bar}$ isobars.
• Figure 4: Phase diagram of the CO$_{2}$ + water mixture assuming an excess of CO$_2$. The open black symbols represent the experimental three-phase lines for the H-L$_{\text{H}_2\text{O}}$-L$_{\text{CO}_{2}}$ (diamonds), Sloan2008a H-L$_{\text{H}_2\text{O}}$-V (squares), Sloan2008a H-L$_{\text{CO}_{2}}$-V (left triangles), Sloan2008a L$_{\text{H}_2\text{O}}$-L$_{\text{CO}_{2}}$-V (right triangles), Sloan2008a and H-Ih-V (down triangles) Sloan2008a equilibria. The filled-black square and circle are the experimental quadruple points Q$_{1}$ and Q$_{2}$, Sloan2008a respectively. Cyan-filled circles correspond to simulation results obtained by some of us in a previous work Miguez2015a for the H-L$_{\text{H}_2\text{O}}$-L$_{\text{CO}_{2}}$ three-phase line. Red-filled circles correspond to the three-phase line for the H-L$_{\text{H}_2\text{O}}$-V equilibrium obtained in this work. The filled violet circle is the simulation quadruple point Q$_2$ obtained in this work.