Selecting the Right Thermodynamic Model

There are methods to choose the appropriate thermodynamic models; see Ref. 78. This choice depends on the nature of the property, composition of the mixture, operational pressure and temperature, and availability of model’s parameters for the simulation; see Ref. 79. Below is a decision tree that can be used as a guide for choosing the thermodynamic model:

Figure 6-33: Decision tree to select thermodynamics model.

It is possible to modify the database species properties such as parameters and temperature-dependent function for a thermodynamic system thermodynamic system. In the same manner, you can add new species that do not exist in the database. To do this, right-click on the Property package in the Model builder and select Export thermodynamic system. Save the thermodynamic system as an xml file.

All the parameters and temperature-dependent properties are saved inside <Compound></Compound> command phrase. For example, to edit the acentric factor of methane from 0.01141 to 0.2, open the thermodynamic system file, and under compound methane, change the value for acentric factor as <AcentricFactor >0.2</AcentricFactor>.

For temperature-dependent functions (for example, liquid Density), the section appears as

<Phase>Liquid</Phase>

</Density>

where Tlb and Tub are lower and upper bound for temperature, respectively. The a1 to a4 are coefficients for a cubic polynomial as f(T) = a0 + a1*T+ a2*T2+ a3*T3 fitted to the experimental data. The unit for temperature is K; see Table 6-3 for other units:

Table 6-3: Unit for temperature-dependent properties.
Property	Unit
liquid density	mol/m3
ideal heat capacity	J/mol/K
ln vapor pressure	1
heat of vaporization	J/mol
vapor viscosity	Pa.s
ln liquid viscosity	1
vapor thermal conductivity	W/m/K
liquid thermal conductivity	W/m/K
surface tension	N/m

The database includes all the references for constant and temperature-dependent properties. In the saved thermodynamic system, it is located between <Comment></Comment>. Alternatively, under the node, you can right-click on the created species function created and select .