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 6-36: Decision tree to select thermodynamics model.
It is possible to manually modify the species properties, such as parameters and temperature-dependent functions, in the database file. In the same manner, you can manually add new species that do not exist in the database. To do this, right-click a system node under Thermodynamics and select Export thermodynamic system. Save the thermodynamic system as an xml file.
All the parameters and temperature-dependent properties are saved inside a <Compound CompID="name"></Compound> block. For example, to edit the acentric factor of methane from 0.01141 to 0.2 perform the following steps
<Compound CompID="methane">
<AcentricFactor >0.2</AcentricFactor>.
To use the system with the updated species, right-click the Thermodynamics node and select Import System.
For temperature-dependent functions, for example for the (saturated) liquid density, the database input is of the form:
<Density>
<Phase>Liquid</Phase>
<Coefficients>Tlb;a0;a1;a2;a3;Tub</Coefficients>
<Data>Tlb;f(Tlb);Tub;f(Tub)</Data>
<Comment></Comment>
</Density>
Here Tlb and Tub defines the lower and upper bound for a temperature range, and a1 to a4 are the corresponding coefficients for a cubic polynomial as f(T) = a0 + a1T+ a2T2+ a3T3 fitted for that range. Multiple sequential ranges can be added by appending temperature ranges and coefficient sets. The <Data></Data> block can be used to input data points directly.
The unit for temperature is K. The unit used for the temperature dependent properties (fitted polynomials functions) are defined in the table below: