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Graphical EOS: Compressibility Charts

The Compressibility Factor

This is where Z comes into the problem.

Z is a dimensionless quantity from which you can calculate the value of your unknown property, P, , or T, using the known values of the other two properties.

Recall that the defining equation for Z is :

Values of Z have been determined experimentally for a very large number of gases at a wide variety of temperatures and pressures.

All of this data has been used to create a series of three charts called the

Generalized Compressibility Charts

These charts have Z on the vertical axis and Pr on the horizontal axis. Curves of constant Tr and different curves of constant appear on the charts.

You can see one of the three

Generalized Compressibility Charts

on the next page and you can see all three in the Chapter 2 First Aid Kit.

Lesson 2E: page 10 of 13

You may be wondering what all these reduced properties have to do with the compressibility factor, Z.
Well, this graphical EOS boils down to three plots that let you determine Z given any two of the 3 reduced properties.
Then, from the equation shown here that defines Z, you can calculate the unknown property, whether it’s temperature pressure or molar volume that is unknown.
So what exactly is plotted in these Generalized Compressibility Charts ?
The charts plot Z as a function of reduced pressure.
There are many curves on the charts and they can get pretty confusing. It takes some practice to learn to read them accurately. We’ll work an example problem soon and let you try your hand at reading the charts when you get to the quiz at the end of the lesson.
What are all these curves ?
There is one family of curves in which each curve represents on constant reduced temperature.
There is another family of curves in which each curve represents on constant value of the ideal reduced molar volume.
Why are there 3 different charts if they all plot exactly the same thing ?
There is one chart for reduced pressures between 0 and 10.
There is another chart that expands the region for reduced pressures between 0 and 1.
Finally, there is the high pressure chart for reduced pressures between 0 and 40.
Remember that the reduced properties, like reduced pressure, are dimensionless.
OK, flip the page and we’ll take a look at one of the generalized compressibility charts.

Graphical EOS: Compressibility Charts

The Compressibility Factor

Generalized Compressibility Charts

Generalized Compressibility Charts

Ch2, Lesson E, Page 10 - Graphical EOS: Compressibility Charts