# Determining mol/L at STP from volume and mol fractions in a CFD program

I'm using some CFD software and the outputs are a bit unusual so I thought I would double check my understanding on how to do these conversions.

The data available per cell is: cell volume, particle volume fraction, average fluid density, mol fraction of gas species, temperature, pressure

I am trying to come up with the mol/L at STP as a way of normalizing for density effects to compare concentration of a gas species throughout the height of a reactor. Temperature and pressure vary wildly, partially because this is a fluidized bed and solids distribution plays a big role in pressure.

So far my process has been convert from mol fraction to mass fraction.

And then multiply (cell volume-cell volume*particle volume fraction) by average fluid density to get gaseous mass in the cell. Then I can use the mass fraction to determine the amount of mass for a given species. Obviously at this point I have the mols of each species.

The problem is then I need the density of that specific species at a given temperature and pressure (currently working at temperatures around 950 C and 110kPa but they very cell by cell obviously) to get back to volume. Is the only way to do this to estimate the density?

The other problem is that let's say I do get the volume by estimating the density. Then I convert to volume at STP by multiplying by 273 K/T and P/101325 Pa or whatever it is. Are the mol fractions I had at non-STP still usable? Or do I just take these volumes and divide by 22.4 and that's the mols I should be using.

Can someone please straighten out my thinking on this. Also are there any good sources for density of a gas species as a function of pressure and temperature. Everything I look at online seems really bad.

• Re: "density of a gas species as a function of pressure and temperature" --PV = nRT is pretty much as far as theory goes. However there are formulas with data fitted constants that given better results see Wikipedia article Real Gas. I'm not sure how well the models would work for mixtures. – MaxW Apr 19 at 21:53
• @MaxW I'm pretty sure these do not count as ideal gases, unfortunately. Really I was looking for empirical data but you're right, I didn't consider that the fact that it's a mixture might make this tough. I'm a bit disappointed this got downvoted, I feel it's a fundamental question though maybe I wrote too much and should've framed it in a simpler way. – user8151560 Apr 20 at 1:33
• Though I just realized that I think I don't need the individual volume contributions and I was being dumb. That simplifies things greatly. – user8151560 Apr 20 at 1:45
• Please avoid acronyms like CFD which not everyone (such as myself) will be familiar with. – Buck Thorn Apr 20 at 10:08