Physical Property Methods

The observant reader may have noticed in Figure 1.3 that the physical property method specified for the VLE calculations in the benzene/toluene example was "Chao-Seader." This method works well for most hydrocarbon systems.

One of the most important issues involved in distillation calculations is the selection of an appropriate physical property method that will accurately describe the phase equilibrium of the chemical component system. The Aspen Plus library has a large number of alternative methods. Some of the most commonly used methods are Chao-Seader, van Laar, Wilson, Unifac, and NRTL.

In most design situations there is some type of data that can be used to select the most appropriate physical property method. Often VLE data can be found in the literature. The multivolume DECHEMA data books1 provide an extensive source of data.

If operating data from a laboratory, pilot plant, or plant column are available, they can be used to determine what physical property method fits the column data. There could be a problem in using column data in that the tray efficiency is also unknown and the VLE parameters cannot be decoupled from the efficiency.

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