Volumetric flow rate in a PFR reactor

  • Thread starter dRic2
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dRic2

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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
 

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 ?
 

dRic2

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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
 
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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?
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)
In liquid phase phase under which conditions (if possible) ##Q## can be considered constant?
Yes again, if the actual change in the specific volume of the mixture is small.
 

dRic2

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Thank you.
 

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