What is this strange density I see being used?

I am in an internship right now and going through a bunch of the old documents of chemical flow rates. They involve flows in pound moles, moles, and SCF (standard cubic feet) all per minute. When going through I noticed most gas streams were multiplied by a mysterious number (usually in the high 300's) to find the SCFM flowrate from the lbmol/min flowrate. No one here seems to know what this number is, not even those who have been here a long time. They only know that it works.

Does anyone know where this number is derived from?

Answer:
Given the conversion, there MUST be a "constant" in there.

You're converting a "pound" of material based upon its molecular weight to cubic feet (one assumes at Standard Temperature and Pressure, else the results will be all over the map). An lbmol of O2 would weigh 32 pounds (molecular mass of O2 being a pair of O at 16 which get you to 32).

Remember Boyle's law where PV = nrT? The "r" is the constant in that expression. Converting your "lbmol" of gas (which represents, in different units, the "n"umber of moles in that equation) to a specific "V"olume will require the use of a constant. The constant in the Boyle's equation will differ depending upon the units being used. Dollars to donuts that's the number you are seeing. Having never used lbmol (even as an America-jin, I learned it all in metric), I don't have the number off the top of my head, but I'll bet somewhere here does.

If you assume that the other factors in the equation (pressure and temperature) remain the same, you will always be able to multiply the mass of a gas to get cubic feet. That's the number you're seeing.
Maybe they were using the conversion factor:
1 lb mol/min = 385.4 SCFM

This conversion factor is used at EPA standard conditions, so the factor will vary depending on the environmental conditions, which is probably why the number isn't always the same.

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