(645a) Experiment and Theory for the Dew Point of Water in Compressed CO2

When compressing and transporting CO₂ for carbon capture and sequestration, it is important to know the water dew point as a function of temperature, pressure, and water content; this determines the degree to which it is necessary to dry the CO₂ in order to avoid condensation that can lead to corrosion in pipelines.  This dew-point relationship is sensitive to the vapor-phase nonideality, on which there is limited theoretical information and only a few scattered experimental data at conditions relevant for pipeline transport.  Dew-point data can also inform thermodynamic modeling of other parts of the process where H₂O and CO₂ are present.

To provide needed data, we have constructed an apparatus to measure the water mole fraction at saturation in a compressed gas.  The apparatus consists of a saturator and the NIST gravimetric hygrometer (an instrument originally designed for work in humidity standards).  The saturator humidifies the gas by bringing it into equilibrium with liquid water at a precisely controlled temperature and pressure.  Then, the gravimetric hygrometer measures the water mole fraction of the humid gas with a relative uncertainty better than 0.1%.  For CO₂, the apparatus can operate at pressures up to 5 MPa and at temperatures from approximately 1 °C to 85 °C.

We have measured dew-point data for several isotherms both above and below room temperature.  Preliminary analysis indicates that these data are consistent with what would be expected based on recent theoretical predictions by Wheatley and Harvey, based on an ab initio potential-energy surface, of the interaction second virial coefficient between water and carbon dioxide.