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Volumetric flow rate in a PFR reactor

  1. Jan 28, 2018 #1
    Hi pf,

    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
     
  2. jcsd
  3. Jan 29, 2018 #2

    BvU

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    Do they mention that this is for a gas phase reaction ? And the ideal gas law applies ? And the temperature is constant ?
     
  4. Jan 29, 2018 #3
    If temperature (and pressure) are constant and the total number of moles doesn't change, using ##PV=nRT##, it's clear to me that the volumetric flow is constant.
    If temperature (and pressure) are constant but the total number of moles does change I can still consider the volumetric flow as constant if the change in the total number of moles is very small (eg working with ppm).

    Now let's a reaction like ##A + E → 2B + C##

    In gas phase under which conditions (if possible) ##Q## can be considered constant?
    In liquid phase phase under which conditions (if possible) ##Q## can be considered constant?

    Thank you
    Ric

    Ps: I hope my english is good enough
     
  5. Jan 30, 2018 #4
    Yes, if the actual change in the number of moles is small (e.g., if only a small amount of A is available to react)
    Yes again, if the actual change in the specific volume of the mixture is small.
     
  6. Jan 30, 2018 #5
    Thank you.
     
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