Dear ChE 313 students,
Many of you received your first project back this morning at the end of Materials class CHE 333. If you aren't in that class, or if you missed it this morning, please stop by my office to pick it up.
I graded the project out of 45 points. I allocated these as follows:
* questions 1 through 5: 8 points each * overall writing, organization, etc.: 10 points
Those of you adding in your head (at least some of you do, right??) may be confused that this leads to 50 points. I decided afterwards that I was grading on a difficult scale, so I adjusted the total possible points downwards by 5.
Some broad comments about the project are below. Please read these.
Prof. Greenfield
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The projects shared some common good features:
* Everyone found reasonable estimates of the per capita CO2 emissions.
I personally view the UN Intergovernmental Panel on Climate Change http://www.ipcc.ch/ as the gold standard in this regard, since this group comprises scientists working together from around the world.
* Everyone agreed that storing CO2 as a gas wouldn't work, because it would take up too much volume. Good conclusion.
* Many people included an appendix with detailed calculations.
I found this to be very useful. It showed me exactly how you were going about doing your calculations, what assumptions you were making, etc. I encourage everyone to include some example calculations in an appendix for project 2. If you have questions about what such an appendix would look like, feel free to stop by my office and I can help out.
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There were also some very common errors:
* Work was calculated using ideal gas law.
The project specifically asked you to use the van der Waals equation of state. An important aspect was practicing how to do calculations such as the integral of P dV when P is more complicated than RT/V. The ideal gas law is a particularly poor choice as pressure increases beyond a few bar, and near the experimental critical point it is very inaccurate. (In particular, an ideal gas doesn't show a critical point because it can't liquefy.)
* The suggested paths usually were unrealizable in practice.
A very common suggestion was an isothermal compression of the CO2 up towards its critical pressure. As we had been learning during the thermodynamic cycle classes (e.g. 3/2,7,9), a compressor is usually adiabatic and ideally isentropic, with a resulting temperature rise as the pressure increases. Phase change usually then occurs by removing or adding heat (e.g. condenser in an air conditioning system).
* The CO2 comparison (sequestration work vs chemical energy) didn't satisfy a material balance.
Many of you compared a fuel source such as gasoline or propane to CO2, essentially comparing the work obtained from a mole of fuel to the work required to sequester a mole of CO2. A problem, though, is that almost all hydrocarbon fuels lead to more than 1 mole of CO2. (Methane from natural gas is the only exception.) The heat released from a mole of propane, for example, needs to be compared with the work required to sequester 3 moles of CO2.
An additional concern is that not all of the heat of combustion can be used for useful work. A power cycle is limited by a thermal efficiency, and even the Carnot efficiency is often only 1/3, so a lot of the available chemical energy is wasted.
* The report didn't include an executive summary.
The project page asked you to begin the report with an executive summary. The idea is to present your key results (what you found, not just what you did) in a concise, short paragraph that is appropriate for a manager to scan quickly to learn the overall results. You should all definitely include an executive summary as part of the second project.
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