Chemical and Biomolecular Engineering

Official blog of the Lehigh University Chemical Engineers

The Art of Charts

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The main focus of system problems evaluated in ChE 31, and in chemical systems in general, is how the substances react to changes in the surrounding parameters. That is, changes in pressure, temperature, and new products being added to the system. For a simple example, if a solution needs to be heated to evaporate water from a desired solid product, a chemical engineer can determine what temperature and pressure is most effective for the system to accomplish the task. It is essential to understand how materials will behave, given certain parameters, in order to determine the balance on a chemical system.

There is nearly infinite combinations of materials that can be studied at different parameters. It is impossible to have a simple list of values to quantify all the factors of a chemical system. With the ability to alter pressure, temperature and many other properties of a single system, chemical engineers need a method to simply present information about the system. In order to find a solution to a problem, an engineer must be able to easily find values at different parameters. For this reason, chemical engineers rely heavily on charts, graphs and tables. Charts effectively present information about substance(s) and make it easy to determine the value one needs. In ChE 31, I am constantly using charts in the textbook to find constants and numbers to solve material balances. For example, there is a table in my textbook that presents the vapor pressure of water based on the temperature and mass fraction of water. Here are three different charts that I have used so far:

MgSO4 Phase Diagram

MgSO4 Phase Diagram

This is an MgSO4 Phase Diagram. It’s difficult to explain in text, but at different temperatures MgSO4 exists as a saturated solution (in water) and as crystals (hydrated). Given a temperature, one can determine the mass percentage of MgSO4 in the solution and in the crystals using the diagram. The diagram can also be used to determine the phase (solid/liquid) and concentration of MgSO4 at different temperatures.

Pxy Txy DiagramThis is a Pxy or Txy diagram. The difference between the two different diagrams is one has a constant pressure and one is at a constant temperature. These diagrams are used when you have a closed volume with two different substances that are in the liquid and gas phases. Different materials condense/evaporate at different temperatures and are based on the temperature and pressure of the system. Basically, you use either a given temperature or pressure to determine the molar percentage (x-axis) of each substance in the solution. For example, if the liquid composition of benzene is 30%, the composition of Toluene is 70%. The compositions of vapor would be different.

Ternary Phase Diagram

Ternary Phase Diagram

Lastly is a ternary phase diagram. We haven’t used these much so far, but they are utilized when you have a solution of three different materials. Usually, one of the substances (A) dissolves in the other two. The diagram is used to determine how much of substance A dissolves in each of the other two solvents, based on the amount of each solvent in the system.

As a whole, charts are extremely helpful in efficiently presenting useful information. It is imperative to be able to understand and use these diagrams to solve problems and be a successful engineer.


Author: Ben Dunmire

I am a sophomore Chemical Engineering major at Lehigh. On campus, I am the president of Club Baseball and a member of AIChE and NSCS. Outside of school, I enjoy the outdoors, fishing, and any type of athletic activity. You can reach me at

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