Changes to the enthalpy basis and reference state for components

For compositional Fluid Types that use one of the liquid activity coefficient (LACT) methods, in the Methods area, in the Enthalpy Route list, you can select Custom to turn on the following controls in the Component List section:

• In the Enthalpy Basis list, you can change the enthalpy calculation route for the component. You can choose from one of the following options:

  • System: AVEVA Process Simulation uses the system reference state and the system calculation route for the enthalpy calculations.

  • Ideal Gas: AVEVA Process Simulation uses a user-specified reference state and the ideal gas calculation route for the enthalpy calculations.

  • Sat. Liquid: AVEVA Process Simulation uses a user-specified reference state and the saturated liquid calculation route for the enthalpy calculations.

    You should use the Sat. Liquid option only when the critical temperature for the component is well below the expected maximum process temperature.

    See Enthalpy calculations for liquid activity methods for more information on the differences in the enthalpy calculation routes.

• In the Ref T box, you can enter the reference temperature that you want to use for the enthalpy calculations.

• In the Ref H box, you can enter the reference enthalpy that you want to use for the component. This value should be the change between the system reference enthalpy and the enthalpy at the specified enthalpy basis and reference temperature. The system reference enthalpy is the ideal gas formation at 25°C. Therefore, if you select Sat. Liquid for the Enthalpy Basis or set the Ref T value to any value other than 298.15 K (25°C), you must update the Ref H value to account for the change in the reference enthalpy.

  You can use the following equation to calculate the Ref H value when you select Sat. Liquid for the Enthalpy Basis:

  

  where

  Tref is the Ref T value.

  For example, if you select Sat. Liquid for the Enthalpy Basis and set the Ref T value to 298.15 K (25°C), you should set the Ref H value to the difference between the ideal gas heat of formation at 25°C and the latent heat of vaporization at 25°C.

  

  If you select Sat. Liquid for the Enthalpy Basis and set the Ref T value to 273.15 K (0°C), you should set the Ref H value to the difference between the ideal gas heat of formation at 25°C and the latent heat of vaporization at 25°C minus the enthalpy change between the enthalpy of the liquid at 0°C and the enthalpy of the liquid at 25°C.

  

  You can use the following equation to calculate the Ref H value when you select Ideal Gas for the Enthalpy Basis:

  

  where

  DdepHiV is the departure function that computes the difference between the enthalpy of the real gas and the enthalpy in the ideal gas state. See Departure functionthe AVEVA Process Simulation Thermodynamics Reference Guide for more information.

  For example, if you select Ideal Gas for the Enthalpy Basis and set the Ref T value to 273.15 K (0°C), you should set the Ref H value to the difference between the ideal gas heat of formation at 25°C and the enthalpy change between the vapor at 0°C and the vapor at 25°C.

  

  If you select Ideal Gas for the Enthalpy Basis and set the Ref T value to 298.15 K (25°C), there is no change from the system reference enthalpy and you should set the Ref H value to the ideal gas heat of formation at 25°C.

  

• In the Phase Ch T box, you can enter the temperature at which the phases transition during the enthalpy calculations. In this case, AVEVA Process Simulation modifies its enthalpy calculations so that it uses only the latent heat of vaporization at the phase-change temperature when it needs to calculate the enthalpy of a phase transition. Typically, you specify a phase-change temperature if you have low confidence in your correlation for the latent heat of vaporization, but a high confidence in the value of that correlation at the phase-change temperature.

  If you leave this value blank or set it to zero, the software does not use the specified phase-change temperature.

  See Enthalpy calculations for liquid activity methods for more information on how the calculations change when you specify a phase-change temperature.

You can obtain the values for the heat of formation, latent heat of vaporization, and departure function from AVEVA Thermodynamic Data Manager if you do not have data readily available for these values.