Volumetric flow rate in a PFR reactor

In summary, the volumetric flow rate, Q, can be considered constant in a gas phase reaction when the temperature and pressure are constant and the total number of moles does not change significantly. In a liquid phase reaction, Q can also be considered constant if the change in the total number of moles or specific volume is small.
  • #1
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
 
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  • #2
Do they mention that this is for a gas phase reaction ? And the ideal gas law applies ? And the temperature is constant ?
 
  • #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
 
  • #4
dRic2 said:
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.
 
  • #5
Thank you.
 

1. What is the definition of volumetric flow rate in a PFR reactor?

Volumetric flow rate in a PFR (plug flow reactor) is the volume of fluid that passes through a reactor per unit time. It is a measure of the amount of fluid that is flowing through the reactor and is typically expressed in units of volume per time (e.g. m^3/s).

2. How is volumetric flow rate calculated in a PFR reactor?

The volumetric flow rate in a PFR reactor is calculated by dividing the total volume of fluid that exits the reactor by the total time it takes for the fluid to pass through the reactor. This can be expressed as Q = V/t, where Q is the volumetric flow rate, V is the volume of fluid, and t is the time.

3. What factors affect the volumetric flow rate in a PFR reactor?

The volumetric flow rate in a PFR reactor can be affected by various factors such as the size and design of the reactor, the pressure and temperature of the fluid, and the viscosity and density of the fluid. It can also be influenced by the type and concentration of reactants, as well as any catalysts or inhibitors present in the reactor.

4. How does the volumetric flow rate impact the performance of a PFR reactor?

The volumetric flow rate plays a crucial role in the performance of a PFR reactor. It affects the residence time of the fluid in the reactor, which in turn, determines the extent of reaction and the conversion of reactants. A higher flow rate can reduce the residence time and lead to a lower conversion, while a lower flow rate can increase the residence time and improve the conversion.

5. How can the volumetric flow rate be controlled in a PFR reactor?

There are several ways to control the volumetric flow rate in a PFR reactor. These include adjusting the reactor design and dimensions, manipulating the inlet pressure and temperature, and using pumps or valves to regulate the flow rate. In some cases, the use of baffles or flow restrictors can also be effective in controlling the volumetric flow rate in a PFR reactor.

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