Mixing Two Gases: How to Calculate Combined Volumetric Flow Rate?

[Red_CCF]

Hi

I'm working on a project where I have a two flow controller controlling the volumetric flow rate (in the order of 0.01-0.1ml/min) of two different gases. The gases are mixed together and I'm wondering if there's a way to figure out the exiting volumetric flow rate of the combined flow and what kind of information is required? Is just adding the two volumetric flow rate (as if the gases are a liquid) a good approximation?

Thanks

 

[pa5tabear]

What are the gases? Assuming ideality, then yes, that's exactly what you would want to do.

However, in reality, there might be a small change in volume from mixing.

 

[Red_CCF]

What are the gases? Assuming ideality, then yes, that's exactly what you would want to do.

However, in reality, there might be a small change in volume from mixing.

I'm doing N2 and air for now but CO2 or H2 may also be involved instead of one of N2 or air. So ideally, one can add the volume flow rates together? I'm a bit confused on why since gases are compressible.

Thanks

 

[pa5tabear]

I'm doing N2 and air for now but CO2 or H2 may also be involved instead of one of N2 or air. So ideally, one can add the volume flow rates together? I'm a bit confused on why since gases are compressible.

Thanks

Okay good point. I wasn't thinking very well. You will have to take into account the conditions at each point (temp, pressure). I was thinking that you could treat them ideally, and if the conditions are the same, then you could expect the volumes to be additive.

I'd expect all those gases mentioned to be near ideal, so using the ideal gas law should be fairly accurate.

 

[Red_CCF]

Okay good point. I wasn't thinking very well. You will have to take into account the conditions at each point (temp, pressure). I was thinking that you could treat them ideally, and if the conditions are the same, then you could expect the volumes to be additive.

I'd expect all those gases mentioned to be near ideal, so using the ideal gas law should be fairly accurate.

I'm still puzzled with this approximation. By conservation of mass:
mdot1 + mdot2 = mdottotal and if the volume flow rates are additive and ignoring compressibility effects (as my flow rates are quite low)

I get ρ1*Vdot1 + ρ2*Vdot2 = ρtot*Vtot and if Vtot = Vdot1+Vdot2 this implies that all the density values are equal? I'm not sure how even assuming that the gases are ideal and have constant temperature and pressure will allow this approximation to be valid?

Any help is appreciated
Thanks