Question

For thermodynamics, how can I decide if should I use Lee/Keslar or virial correlations to find VR, HR, SR and Z?

Answer 1

Virial Equation of State (EOS) is applicable at following conditions:

• They are only applicable for gases
• The virial EOS of state does not take into account the shape and size of the molecule. They are valid only spherical molecules. If the collision of the molecules depends strongly on the shape and size of the molecule (which is what happens most of the time) virial equations cannot be used
• It cannot be used when there are different phases in the system Hence if there is situations like Vapor-liquid, liquid-liquid equilibrium and so on, the state of the system such as volume of vapor, liquid, etc cannot be estimated using virial equation of state.
• Hence, they can be only used for gases or vapor phase alone. They are valid at moderately low pressure and high temperatures

Lee/Keslar EOS:

• In order to overcome the limitations of virial EOS, Lee/Keslar developed a EOS
• This is valid for both vapor and liquid phases of the system
• They are valid for wide range of temperatures and pressure including high pressure and low temeperatures unlike the virial equations of state
• They take into account the size and shape of the molecule based on the accentric factor and works for molecules which are not spherical as well
• They can be used to estimate system properties at avpor-liquid, liquid-liquid equilibrium and so on

So depending on the system and molecules of interest, you can choose the EOS based on the above discussons.