ASPEN PLUS TUTORIALS: INTRODUCTION COURSE - THERMODYNAMIC METHODS #4
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ASPEN TUTORIALS: • ASPEN PLUS
Aspen Tutorial #4: Thermodynamic Methods
Outline:
• Available Thermodynamic Property Methods
• Recommended Methods for Selected Applications
• Influence of Thermodynamic Method on Our Problem
In our previous tutorials, I have been telling you which thermodynamic methods to choose. In this video we will be covering the many thermodynamic methods that are available in Aspen and examining their influence on the results of our simulation. This tutorial is a little shorter than the previous ones, but the information presented here is one of the most important concepts to understand when using simulation programs.
Available Thermodynamic Property Methods:
Aspen has four main types of Property Methods: Ideal, Equation of State, Activity Coefficient, and Special Systems. In addition, an advanced user can modify any of these available methods or create a new property method on their own.
Open up your Aspen simulation. Select the Help Topics under Help on the Menu Bar. This will open up the Aspen Plus Help window. On the left hand side of the screen, select the Index tab and type in Property Methods. Select Property Methods in the list on the left hand side and then select the Available Property Methods option.
These methods use the various equations of state that are learned about in chemical engineering thermodynamics, to calculate the equilibrium distribution ratio. The two most familiar methods from this section are Peng-Robinson and Redlich-Kwong-Soave. You will also notice that Aspen provides many of the minor variations to the most common methods (i.e. a modified Peng-Robinson equation).
The next group of available property methods is the Activity Coefficient group. This group uses various relationships to calculate the liquid phase activity coefficient and then calculate the vapor fugacity using a second relationship. Some of the most common methods for this group are NRTL UNIFAC VANLAAR WILSON. As before, there are many modifications to the basic set of choices, which are useful for specific applications.
You will notice in the stream tables above that both the IDEAL and WILSON thermodynamic methods do not predict any separation of our two liquid streams in the Flash separator (indicated by the zero flow in stream W-A1). However, the NRTL thermodynamic method predicts a separation that is less efficient than that predicted by the SRK method from last week. the results with the SRK thermodynamics were better than what really occurs and this is supported by these results.
Thanks to the Prof. Rene Overney for the material for these tutorials.
courses.washington.edu/overney