Unit of DEFINE_VR_RATE in Porous Media
 

Hi folks,
I´m simulating a small lab-scale watergas-shift reactor, which is basically a tube with a catalyst bulk. The catalyst is ground to a size, where no mass transfer limitations inside the catalyst occur. I have an intrinsic reaction rate (Unit: mol/g_catalyst/s), which I want to use in my model via UDF/DEFINE_VR_RATE
I´m simulating the reaction zone as a porous medium.
The question:
Which unit is used in this DEFINE_VR_RATE UDF? I know, it´s supposed to be kmol/m^3/s for a homogeneous gas-phase reaction (which means for the Volume the cell volume m^3_Cell is used), but I don´t know how to calculate it the right way in context of a porous zone.
Is the proper Volume m^3_Fluid (i.e. Cell Volume Minus Catalyst inside the cell) or as well m^3_Cell?
I hope someone has experience regarding that topic and can help me.
Regarding this I already had contact with fluent support twice last year but unfortunately (and strangely enough) each time they gave me a different answer... :(
Thank you!