GreenGroup/RMG-Java — comment on commit 2947e7b8d5b1e3e19543f2489990fa42e43ecad2
File:  line 0
By gmagoon:

OK, with CHEMKIN MFC 5.0, it seems that there is nothing special about the PLOG Ea units...they are interpreted in whatever units specified elsewhere in the file (e.g. kcal/mol) and don't need to be cal/mol. So, although it is possible there is a different/earlier version of CHEMKIN that handled this differently or had a bug, it seems that the the PLOG format rates should use the same units as the other Arrhenius expressions. I will revert this edit in addition to making the fix for SeedMechanism.parsePLOGline()

Here are the details of how I reached this conclusion:
I constructed the following dummy test file, which has a PLOG rate expression and an Arrhenius rate expression with the same values, one going from A->B, one from A->C:

ELEMENTS H C O N Ne Ar He Si S Cl END
SPECIES
CH(28)
Ar
N2
JP10(1)
MA110(12)
MA111
END
THERMO ALL
300.000 1000.000 5000.000
! The first four sets of polynomial coefficients (Ar, N2, Ne, He) are from
! THIRD MILLENIUM IDEAL GAS AND CONDENSED PHASE THERMOCHEMICAL DATABASE FOR
! COMBUSTION WITH UPDATES FROM ACTIVE THERMOCHENICAL TABLES
! Authors: Alexander Burcat and Branko Ruscic
!
! The rest of the species are estimated by RMG (http://rmg.mit.edu/)
!Primary Thermo Library: GRI-Mech3.0 (Species ID: s00009010)
! [_ SMILES="InChI=1/CH/h1H/mult4" _]
CH(28) C 1H 1 G 300.000 5000.000 995.043 1
2.60390342E+00 1.65740283E-03-4.08551381E-07 4.41529946E-11-1.74774104E-15 2
7.10753424E+04 6.90310860E+00 3.46420815E+00 4.09864469E-04-1.86067896E-06 3
3.24996854E-09-1.36820234E-12 7.07946864E+04 2.20694472E+00 4
! [_ SMILES="InChI=1/Ar" _]
Ar L 6/88Ar 1 G 200.000 6000.000 1000. 1
0.25000000E+01 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 2
-0.74537500E+03 0.43796749E+01 0.25000000E+01 0.00000000E+00 0.00000000E+00 3
0.00000000E+00 0.00000000E+00-0.74537500E+03 0.43796749E+01 4
! [_ SMILES="InChI=1/N2/c1-2" _]
N2 G 8/02N 2 G 200.000 6000.000 1000. 1
2.95257637E+00 1.39690040E-03-4.92631603E-07 7.86010195E-11-4.60755204E-15 2
-9.23948688E+02 5.87188762E+00 3.53100528E+00-1.23660988E-04-5.02999433E-07 3
2.43530612E-09-1.40881235E-12-1.04697628E+03 2.96747038E+00 4
! [_ SMILES="InChI=1/Ne" _]
Ne L10/90Ne 1 G 200.0 6000.00 1000.0 1
0.25000000E+01 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 2
-0.74537500E+03 0.33553227E+01 0.25000000E+01 0.00000000E+00 0.00000000E+00 3
0.00000000E+00 0.00000000E+00-0.74537498E+03 0.33553227E+01 4
! [_ SMILES="InChI=1/He" _]
He REF ELEMENT He 1 G 200.000 6000.000 1000. 1
2.50000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 2
-7.45375000E+02 9.28723974E-01 2.50000000E+00 0.00000000E+00 0.00000000E+00 3
0.00000000E+00 0.00000000E+00-7.45375000E+02 9.28723974E-01 0.00000000E+00 4

!Primary Thermo Library: JP10 (Species ID: JP10)
! [_ SMILES="InChI=1/C10H16/c1-2-9-7-4-5-8(6-7)10(9)3-1/h7-10H,1-6H2" _]
JP10(1) C 10H 16 G 250.000 5000.000 995.043 1
2.11276455E+01 4.69745784E-02-1.61707340E-05 2.53908066E-09-1.49243686E-13 2
-2.17805308E+04-9.93673803E+01 2.64666089E+00 2.93234923E-03 2.28607707E-04 3
-2.80975977E-07 1.01110601E-10-1.22444499E+04 1.91378052E+01 4

!Primary Thermo Library: JP10 (Species ID: MA110)
! [_ SMILES="InChI=1/C10H16/c1-2-6-9(5-1)10-7-3-4-8-10/h1,5,9-10H,2-4,6-8H2" _]
MA110(12) C 10H 16 G 250.000 5000.000 995.043 1
2.09242524E+01 4.88046776E-02-1.70023261E-05 2.68646192E-09-1.58595908E-13 2
-1.18942022E+04-8.89601537E+01 2.72073421E+00 2.08405559E-02 1.77620065E-04 3
-2.29859404E-07 8.39325316E-11-3.26451248E+03 2.39307279E+01 4

!Primary Thermo Library: JP10 (Species ID: MA110)
! [_ SMILES="dummy" _]
MA111 C 10H 16 G 250.000 5000.000 995.043 1
2.09242524E+01 4.88046776E-02-1.70023261E-05 2.68646192E-09-1.58595908E-13 2
-1.18942022E+04-8.89601537E+01 2.72073421E+00 2.08405559E-02 1.77620065E-04 3
-2.29859404E-07 8.39325316E-11-3.26451248E+03 2.39307279E+01 4
END

REACTIONS	KCAL/MOL	MOLES
JP10(1)=MA110(12)	1.000e+14	0.00	100.00	!Seed Mechanism: JP10seedmech

JP10(1)=MA111	1.000e+00	0.00	0.00	!Seed Mechanism: JP10seedmech
PLOG / 0.03947 1.0E+14 0.0 100.0 /
PLOG / 10.0 1.0E+14 0.0 100.0 /

END

If the PLOG value is interpreted using the overall units of kcal/mol, the PLOG reaction will have the same rate coefficient as the Arrhenius with a barrier of Ea = 100 kcal/mol. If however, the PLOG value is interpreted in units of cal/mol, the PLOG reaction will have a significantly higher rate coefficient with a barrier of only Ea = 100 cal/mol = 0.1 kcal/mol.

The MFC mechanism viewer allows you to plot the rate coefficient as a function of temperature. The rate plots for both of these reactions appeared identical.

To make sure that these are the rates that the solver is actually using, I also tried running this mechanism in a closed homogeneous batch reactor at a few different residence times/ temperatures. In all cases, the final concentrations of B and C were identical (note also that B and C have the same thermo, so the reverse rates should be the same if the forward rates are the same)