Carbon dioxide hydrates for carbon capture, utilization, and storage

Abstract

With global temperatures continuing to rise, actions like carbon capture, utilization, and storage (CCUS) must be taken to halt and even reverse this trend. In all stages of CCUS, carbon dioxide properties must be well characterized for any of these schemes to work. With the high pressures and potential for low temperatures (subsea or via JT cooling), gas hydrate formation must be considered. Carbon dioxide fluid properties at hydrate forming conditions will be important moving forward with CCUS. In order to better characterize nearly pure carbon dioxide streams for CCUS, the water content of carbon dioxide was evaluated across a wide range of temperatures and pressures, including hydrate forming conditions, to develop a drying specification for carbon dioxide prior to transportation to help ensure the safe transportation of carbon dioxide. A bulk water phase needs to be avoided in carbon dioxide transportation to minimize hydrate formation and avoid pipeline corrosion. The saturation water content of carbon dioxide at hydrate forming conditions was measured to aid in this design specification. While it has been measured before, literature data disagree across more than an order of magnitude. This work characterized the solid phase to ensure that it was hydrate and not metastable liquid or ice via X-ray CT, for the first time, in hydrate phase equilibria studies. Since liquid water, ice, and hydrate all have different vapor pressures, it was found that the identity of the water phase has a significant impact on the measured carbon dioxide water content (changes up to 50%), with proper equilibrium values taking up to 48 hours to measure. A high pressure microfluidic system was developed for use with carbon dioxide hydrate, in an effort to shorten the experimental time for carbon dioxide water content measurements. Water finished converting to hydrate in 1 hour in the 200 micron microfluidic channels (vs. 48 hours for the 0.5 inch large-scale apparatus), significantly increasing the experimental throughput for these water content experiments. As in the large-scale setup with X-ray CT, in-situ Raman spectroscopy was coupled with the microfluidic device to ensure the correct identity of the water phase and that stable equilibrium phases were reached. Raman also allowed for the carbon dioxide phase to be closely monitored, as some literature data appear to have a two phase mixture of gas and liquid carbon dioxide. Ultimately the carbon dioxide water content was measured across a wide range of conditions (-30 to 20 °C and 200 to 1,500 psi) and found to be a very weak function of pressure in the liquid carbon dioxide phase. Additionally, carbon dioxide hydrate growth rates were collected at low subcoolings (<2 K) in the same microfluidic device. Finally, gas hydrates were evaluated for their potential to separate a syngas mixture stream. A model methane and nitrogen (carbon monoxide substitute) was first evaluated, highlighting the modest separation ability of hydrates as they were able to take a 50/50 (mol) feed and enrich the methane to 75% in the hydrate phase; however, several stages are required for high purity products. A syngas alcohol synthesis products mixture was also used, with hydrate (carbon dioxide, methane, and ethane) being able to enrich reactant (hydrogen and nitrogen; as a carbon monoxide substitute) concentrations in the recycle stream, enhancing the overall reaction kinetics. Hydrates are quite adept at this separation, as an ultra high purity product is not required, the stream was already at high pressure, and the compounds to be separated have very different hydrate formation conditions. As the field of CCUS continues to mature, studying carbon dioxide (and its hydrates) properties will continue to be of importance.