Dear ChE 212 students,

I thought everyone may benefit from this answer to a question about doing the energy balance on the dryer.

Even though the equilibrium phase of each (E,F,G) is a liquid at 30 C, it is possible for them to be in the vapor phase at a lower mole fraction than 1. (Real life examples: you have probably smelled the solvent coming off a paint, or the ethanol in a strong alcoholic beverage)

For a compound that enters at T1 as a liquid and exits at T2 as a vapor, the best path for computing specific enthalpy is

1) choose your standard state where H=0 for the compound (or elements, if that is what you'd like to choose)

2) Calc change in enthalpy from that standard state to the liquid state at T1. This gives you H_1 in kJ/mol or kJ/kg.

3) Calc change in enthalpy from that standard state to the vapor state at T2. This may include a path such as

a) heat liquid from Tstandard to Tboil b) evaporate a Tboil, with change in enthalpy = heat of vaporization c) heat (or cool) vapor from Tboil to T2

This gives you H_2 in kJ/mol or kJ/kg.

If you'd like to test this out, a quick way to do it is using liquid water and water vapor. Pick a T less than 100C for which the steam tables in the book provide H for liquid and vapor. Then use the path in step 3 to calculate the change in enthalpy between the two. You can compare your answer to the actual difference in enthalpy. You probably won't come up with the same answer, but as long as you're close then you're probably doing it right.

Prof. Greenfield

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