Hi,
I was wondering what is the unit of reaction rate.
I checked the user-guide 19.2, and it seems a little bit of confusing for me.
What does the denominator “mol CH4” means?
By the way, I also checked the 8.4.9 of user guide, it explains that all the unit of amount of substance
is kmol rather than “mol”.
Be careful with the units for reaction rates.
For TFM (usr_rates.f): CGS: mol/s/cm3, SI: kmol/s/m3
For DEM/PIC (usr_rates_des.f): CGS：mol/s， SI: kmol/s
The “rate” and “molCH4” is indeed confusing!
The real SI unit for the amount of substance should be mol. While MFIX SI unit is kmol. So in MFiX SI, the unit for:
Molecular weight(MW) is kg/kmol,
Heat of reaction (DH) is J/kmol,
Gas constant: J/K/kmol.
Thank you.
But why the user guide express like that（ I mean the fraction “kmol CH4/（m3 s) over mol CH4”)?
The section you reference is there to indicate that the rate should not be specified with respect to any given species.
For example, consider the reaction
CH4 + 2O2 --> CO2 + 2H2O
Let’s assume you are provided a reaction rate (for this reaction scheme) in terms of kmoles of oxygen (O2). This rate needs to be divided by two when implemented in MFIX, because MFIX multiplies the RATE calculated in usr_rates.f by the stoichiometric coefficient of each species when calculating how much of each species was produced or consumed by a particular reaction. (Note that if the rate had been specified with respect to methane or carbon dioxide, then the rate would not need to be modified).
This also underscores why you have to be careful when specifying stoichiometric coefficients. Using the previous example, the provided rate can be used “as-is” as long as you enter the chemical equation as
0.5CH4 + O2 --> 0.5CO2 + H2O
such that the coefficient of oxygen is one. This is why it is very important to understand what you are implementing because you could accidently scale your reaction and never realize it.
A best practice is to compare implemented rates against basic experimental data (like TGA results). Below is an example of testing MFIX rates vs. published data that we did early this earlier this year.
Thank you for your explanation.
But I mean “why the denominator is ‘mol CH4’instead of ‘kmol CH4’?”
Just a clarification: The reaction rate unit for for DEM/PIC (usr_rates_des.f) is
mol/s in CGS
kmol/s in SI
It is strongly recommended to always use MFiX SI units. The GUI only supports SI units. It will convert old mfix.dat that were written in CGS units, but it cannot convert udfs and it is the user’s responsibility to make sure udfs use the correct unit system.
In the MFiX 19.2 online user guide, the unit of the reaction rate for DEM/PIC should be specified more clearly. “…usr_rates_des.f must have units of reacted moles per time (i.e., moles/sec)”, this sentence is not clear, should be rephrased to " …usr_rates_des.f must have units of reacted moles per time (i.e., moles/sec for CGS units and kmoles/sec for SI units)" to keep consistent with the description of rate units for TFM.
4.13.3.4.1. Reaction Rates
Define chemical reaction rates in user defined function (UDF) files.
A reaction rate should be given in either usr_rates.f or usr_rates_des.f for each reaction listed in the data file.
All TFM gas and solids phase reactions as well as homogeneous gas phase reactions for DEM simulations are to be included in usr_rates.f. Reaction rates defined in usr_rates.f must have units of reacted moles per time per volume (i.e., moles/sec/cm3 for CGS units and kmoles/sec/m3 for SI units).
All discrete phase heterogeneous (particle/gas) reactions are to be included in usr_rates_des.f located in the des subfolder. Reaction rates defined in usr_rates_des.f must have units of reacted moles per time (i.e., moles/sec).
