Database for real CO₂

Real pure CO₂ can be characterised by a thermodynamic database to define a Gas or a Solvent by the EOS GAS or EOS SOLVENT data block.

The CO₂ database can be included with two different keywords. The first option requires to state explicitly the CO₂ formula weight of 44 g/mol. An example is given below:

EOS GAS
  SURFACE_DENSITY 1.87 kg/m^3
  FORMULA_WEIGHT 44 g/mol
  DATABASE co2_db.dat
END

The alternative option uses the CO2_DATABASE keyword, and does not require to state the CO₂ formula weight, which is assigned to 44.00098, internally, by the simulator. An example is given below:

EOS GAS
  SURFACE_DENSITY 1.87 kg/m^3
  CO2_DATABASE co2_db.dat
END

In both cases described above, the external database must have the format of an EOS properties database, and contains all the properties needed do define the CO₂ (density, viscosity, enthalpy, fugacity).

A database for pure CO₂ is provided with the software, and it is made available in the database folder found in the installation directory. The data file is called ‘co2_db.dat’ and it is valid in the pressure and temperature ranges reported below:

\[\begin{split}\begin{split} &P &= [0.1 – 2500.1] \mbox{ Bar} \\ &T &= [ 0 – 375 ] \mbox{ °C} \end{split}\end{split}\]

The data is sampled using a discretisation step of dP = 5 Bars for the pressure and dT = 2.5 °C for the temperature.

The reference state used for the enthalpy is

\[H(P=3.4851\mbox{ MPa}, T=0 °C) = 200 \mbox{ kj/kg},\]

where \(P=3.4851 \mbox{MPa}\) and \(T = 0 °C\) correspond to a state of saturated liquid at \(0 °C\).

The CO₂ database can be generated using the CO2_SPAN_WAGNER_DB data block. See example below:

EOS GAS
  CO2_SPAN_WAGNER_DB
    PRESSURE_MIN 5 Bar
    PRESSURE_MAX 100 Bar
    PRESSURE_DELTA 1 Bar
    TEMPERATURE_MIN 38 C
    TEMPERATURE_MAX 150 C
    TEMPERATURE_DELTA 2 C
    DATABASE_FILE_NAME co2_db.dat
  END
END

Where:

PRESSURE_MIN and PRESSURE_MAX are the minimum and maximum pressure values defining the database pressure range. For the validity of the model available, the pressure range must fall within the following limits: \(0.1 \mbox{ Bar} < P < 2500 \mbox{ Bar}\). The minimum and maximum pressures are optional, if not entered they are defaulted to 0.1 and 2500 Bar respectively.

TEMPERATURE_MIN and TEMPERATURE_MAX are the minimum and maximum temperature values defining the database temperature range. For the validity of the model available, the temperature range must fall within the following limits: \(−55°C <T<375°C\). The minimum and maximum temperatures are optional, if not entered they are defaulted to \(0\) and \(375 °C\) respectively.

TEMPERATURE_DELTA is the temperature step used to discretise the temperature. It is an optional parameter, if not entered is defaulted to \(2.5 °C\).

Temperatures can be entered only in degree Celsius.

DATABASE_FILE_NAME is used to specify the name of file where the CO₂ database will be printed. This is an optional paramter, if not entered will be defaulted to “co2datbase_eos.dat”.

The properties in the CO₂ database are computed using the Span-Wagner model, see: [SW96].

As none of the flow models implemented can handle multi-thermodynamic CO₂ states in the same simulation, care must be taken so that pressure and temperature conditions do not span the CO₂ phase transition region, even if the range of validity of the CO₂ database can cover these cases.