# 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). The fugacity is only required for the GAS_WATER mode.

A database for pure CO2 is provided with the software, and it is made available in the data folder (data/databases). 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 a 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.