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Dew-point measurements for water in compressed CO2

Summary:

The NIST Thermodynamic Metrology Group (TMG) is using its standard gravimetric hygrometer as part of a facility for making high-accuracy measurements of the dew point temperature for water in compressed CO2 as a function of mole fraction and pressure. Knowledge of this relation is important for the effort to perform Carbon capture and sequestration (CCS) in power plants.

Description:

CCS is a new technology designed to decrease global warming by minimizing the release of CO2 from coal-burning power plants into the atmosphere.  In CCS, the flue gases from the power plants are captured, processed, compressed, and transported to storage locations (e.g. geological formations such as oil and gas reservoirs).  Certain power-generation processes (e.g. oxy-fuel combustion and chemical looping combustion) emit flue gases composed of CO2/H2O mixtures with very small amounts of impurities. These processes are particularly attractive for use with CCS because it is considerably easier to segregate the CO2 in their flue gases.

In CCS, a sufficient amount of water must be removed from the captured CO2/H2O mixture before it is ready for compression, transport, and storage. Water condensation causes corrosion in the pipelines and other conventional carbon-steel materials that it contacts. Also, compression and transport of CO2 require more energy when it is moist. The simplest method for drying the captured CO2/H2O mixture is to cool it until sufficient water condensation occurs.  Knowledge of the dew point temperature TDP for water in CO2 as a function of water mole fraction (x) and pressure (p) is essential for designing this cooling process. To relate TDP to x and p, a thermodynamic quantity called the water-vapor enhancement factor (fw) must be known.  This quantity varies with both TDP and p.

For determining fw(TDP,p) for CO2, the TMG has built a new humidity generator that is capable of saturating the gas with water at pressures up to 5 MPa and temperatures from 1 °C to 85 °C. The generator features two high-pressure saturators. In these vessels, CO2 flows over water inside a long channel, mixing CO2 with water vapor at its saturated vapor pressure. Measurements of the temperature and pressure in the saturator yield TDP and p. The gas can be fully saturated at flow rates up to 6 L/min. 



 Schematic of high-pressure humidity generator  Top and bottom saturator pieces
Schematic of high-pressure humidity generator
Top and bottom saturator pieces


The gravimetric hygrometer measures the water mole fraction in the gas exiting the generator. It separates the water from the gas using desiccants and subsequently determines the masses of the water and the gas separately. These masses are then used to calculate the water mole fraction in the gas. The NIST gravimetric hygrometer is the only working gravimetric hygrometer in a National Measurement Institute today.  It is capable of determining water mole fractions with an expanded uncertainty (k = 2) of 0.1%.  The facility consisting of the humidity generator and gravimetric hygrometer is capable of determining TDP(x,p) to 0.01 °C.



Collection tubes for gravimetric hygrometers 
Gas (left) and water (right) collection tubes for the gravimetric hygrometer


The facility has been validated using air as the working gas.  Its measurements of fw(TDP,p) for air agree to within the measurement uncertainties with the measurements made at NIST by Dick Hyland and Arnold Wexler in the 1970s, which are still considered the standard.  

Measurements of fw(TDP,p) using CO2 are currently underway.  Plans have been made to measure fw(TDP,p) at five temperatures over the range 20 °C to 85 °C.Measurements are being made at ten pressures over the range 1 MPa to 5 MPa at each of these temperatures. These measurements will be used by NIST theorists in the Applied Chemicals and Materials Division to construct the interaction second virial coefficient for CO2-water mixtures; this will enable predictions of fw(TDP,p) over values TDP and p where measurements have not been made.

Contact: Christopher Meyer
301-975-4825