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08.07 - Implementation of Departure Functions Click here. 20 2

Helmholtz Example - Modified vdW EOS (, 13min) A sample derivation of the Helmholtz departure implicit in the Gibbs departure given Z = 1 + abρ/(1+)^3 - (9.5aρ/RT)/(1+aρ/RT). Note that the limits of integration matter for this EOS. The audio is inferior for this live video, but it responds to typical questions and confusion from students in the audience. Some students might find it helpful to hear the kinds of questions that students ask. The responses slow the derivation down so that no steps are skipped and key steps are reiterated multiple times. Just turn the volume up!
Comprehension questions:
1. Which part of this EOS is non-zero at the zero density limit of integration?
2. Is there a sign error on one of the terms in this video? Check the derivation independently.
3. Derive the Helmholtz departure given Z = 1 + 4bρ/(1-)2 - (9.5aρ/RT)/{1-a/bRT[1-4bρ+4(bρ)2]}.
4. Derive the Helmholtz departure given Z = 1 + 4bρ/(1-2) - (9.5aρ/RT){1+4/bRT[1-2(bρ)2]}/{1-a/bRT[1-4bρ+4(bρ)2]})/{1-a/bRT[1-4bρ+4(bρ)2]}

07.08 Matching The Critical Point Click here. 20 1

Visualizing the vdW EOS (, 16min) Building on solving for density, describes plotting dimensionless isotherms of the vdW EOS for methane at 5 temperatures, two subcritical, two supercritical, and one at the critical condition. From these isotherms in dimensionless form, it is possible to identify the critical point as the location of the inflection point where the temperature first exits the 3-root region. This method can be adapted to any equation of state, whether it is cubic or not. The illustration was adapted from a sample test problem. This screencast also addresses the meaning of the region where the pressure goes negative, with a (possibly disturbing) story about a blood-sucking octopus.

Comprehension Questions:

1. What are the dimensions of the quantity (bP/RT)?
2. Starting with the expression for Z(ρ,T), rewrite the vdW EOS to solve for the quantity (bP/RT) in terms of () and (a/bRT).
3. Consider the following EOS: Z = 1 + 2/(1-2) - (a/bRT) /(1-)2. Estimate the value of bPc/(RTc) for this EOS.
4. Consider the following EOS: Z = 1 + 2/(1-2) - (a/bRT) /(1-)2. Estimate the value of (a/bRTc) for this EOS.
5. Compute the values of a(J-cm3/mol2) and b(cm3/mol) for methane according to this new EOS.

08.07 - Implementation of Departure Functions Click here. 20 1

Helmholtz Example - vdW EOS (, 18min) This video begins with a brief review of the connection of the Helmholtz departure with all other departures then shows four sample derivations assuming that Z is given by the vdW EOS: (1) the Helmholtz departure , (2) the internal energy departure from the Helmholtz departure. (3) the Helmholtz energy from the internal energy (4) the Z factor from the Helmholtz departure. The procedures illustrated here can be applied to any EOS starting with any part (U, A, or Z) as given to derive any other departure: ZUHAGS.
Comprehension Questions: The virial EOS for SW fluids can be written as: Z = 1 + Bρ/RT where B = 4b+[4b(λ^3-1)] [exp(βε)-1], b = πNAσ^3/6.
1. Derive an expression for the Helmholtz departure.
2. Use the result of #1 to derive the internal energy departure.
3. Use the result of #2 to derive the Helmholtz departure. What is the integration constant in this case?

07.06 Solving The Cubic EOS for Z Click here. 20 2

Using a macro to create an isotherm (Excel) (, 14:31) The tabular Excel display is convenient for viewing all the intermediate values, but no so good for building a table such as for an isotherm. This screencast shows how to write/edit a macro to build a table by copying/pasting values. The screencast creates an isotherm on a Z vs. Pr plot over 0.01 < Pr < 10.