Methanol (Gibbs Study)

[nrgeng]

I am continuing to work with the Gibbs Minimization Reactor Unit. I am back to the Reaction Type: Specify Compounds. When this Reaction Type originally failed to meet my expectations, I moved to the Reaction Type: Reaction Package which would not handle three reactions until Revision ..14. Thank you.

The problem(s) with Reaction Type: Specify Compounds is that the heat duty is positive for a known set of exothermic reactions. Please compare +194 kW (Specify Compounds) with -219 kW ( Reaction Package). I am attaching two flowsheets, one for each Reaction Type. You will easily see other indications of problems (I think). You already have the fsd file—the last one that I e-mailed you. If you want another copy please request it.

I thought, when I started this Unit study, that the results would be similar for both Reaction Types. I was definitely wrong. I hope that you can help. Is my mindset the problem here?

[jasper]

The solution in which the Gibbs energy goes up rather than down is obviously wrong. But if I try both approaches lead to approximately the same solution, that of about 74.6% CO conversion.

Can you send me the fsd that produces the wrong solution and describe the steps how I make it produce the wrong solution? If I just open the document you sent earlier and hit solve I get the proper solution.

[nrgeng]

I sent you the fsd file that you requested by e-mail. I am attaching a file to this posting that shows the second problem that I have encountered with this simulation, which was previously unreported.

The actions taken to repeat my experience with this simulation: (1) load the fsd file, reset the flowsheet, then solve it [looks good]; (2) reset the flowsheet, change to Reaction Type: Specify Compounds, then solve it [same error(s) as reported]; (3) reset the flowsheet, change to Reaction Type: Reaction Package, then solve it [error dialog, see attached file]. At this point, the flowsheet is ruined because you cannot switch back to the previous flowsheet which solved. A puzzlement to me.

I hope this information helps you in resolving these issues.

COCO Version 2.5.0.14 (x86, 32-bit)

[jasper]

First case, ok

Second case: make sure to also select Methanol as component, or you will get only the equivalent to the CO2 hydrogenation reaction. Note that the reactions that are created are reported in the output report, along with their reaction extents. If you include Methanol you will get pretty much the same answer as the first case. The answer without Methanol as reactive compound also appears ok within convergence tolerance. My statement in a previous mail that the Gibbs energy cannot go up was wrong: the one in the report does not include formation terms. What cannot go up is G = H - T*S where both H and S include the formation terms (EnthalpyF and EntropyF).

Third case: I can reproduce the convergence failure. I will look into it.

[jasper]

The three reactions you specify are

(A) CO + 2 H2 -> CH3OH
(B) CO2 + 3 H2 -> CH3OH + H2O
(C) CO + H2O -> CO2 + H2

these are not independent: (B) + (C) = (A)

so the system is over specified. Such a system may or may not converge, actually I would expect it won't converge, not sure why it sometimes does. Lucky I guess. Specify 2 out of 3 of your reactions and it should converge ok.

Also note that in case the reactive compounds are specified, the elemental analysis results 2, not 3, reactions. So the system has only 2 degrees of freedom. The approach using the elemental analysis results a system for which the degrees of freedom are orthogonal. The reactions are

0.510754 Carbon dioxide + 0.340503 Hydrogen + 0.0851257 Methanol = 0.59588 Carbon monoxide + 0.510754 Water
0.408248 Carbon monoxide + 0.816497 Hydrogen = 0.408248 Methanol

so for [CO, CO2, H2, H2O, CH3OH], the stoichoimetry vectors are

[0.59588, -0.510754, -0.340503, 0.510754, -0.0851257]
[-0.408248, 0, -0.816497, 0, 0.408248]

the inner product of these two vectors is zero. This is a pleasant property of the system for the solver.

[jasper]