#### dRic2

Gold Member

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I have a reaction ## aA → bB ## in a PFR reactor. The simplest equation to write is:

## \frac {dF_A} {dV} = -R ## where ##F## is the Molar flow rate. ##F = C_A*Q## where ##Q## is the volumetric flow rate and ##C_A## is the molar concentration of A.

My textbooks says that if ##a=b## (the total number of moles doesn't change) ##Q## is constant. I don't understand why, can someone explain me? Moreover, sometimes, in the exercise ##Q## is considered constant even if ##a≠b##...

So my question is: "When ##Q## can be considered constant?

Thank you

Ric