PFR Reaction Rate: Understanding CSTR vs. PFR

[gfd43tg]

Hello,

In class, my professor said that in a CSTR, the reaction rate is constant throughout a well mixed CSTR. He said in most cases, a PFR will react faster than a CSTR.

I can see how the PFR will have a fast reaction rate at the entrance to the tube, and slowest at the exit. However, what he said that I haven't been able to wrap my head around/convinced of is when he said that the reaction rate at the exit to the tube is equal to the CSTR for the same conditions.

This means that the reaction rate in a PFR is always greater than or equal than the rate in a CSTR. Why is this? Why couldn't the exit have a reaction rate slower than a CSTR?

 

[Chestermiller]

I think what he was saying was that, for equal exit reactant concentrations, the reaction rate at all other locations in a PFR will be higher than it is throughout the CSTR.

Chet

 

[gfd43tg]

It was hard to understand due to his accent, but I know for sure there was no assumption of equal exit reaction concentrations. The notion I got was that the lowest possible reaction rate in a PFR is equal to the reaction rate of a CSTR. Maybe it's just better I ask him directly, since now I'm just speculating what he said.

 

[Chestermiller]

Maybe he meant to say it, and just forgot.

Chet

 

[alphy]

Thank you for sharing your thoughts on this topic. I can understand your confusion and desire for clarification on this matter.

Firstly, it is important to understand that the reaction rate is not necessarily constant throughout a CSTR. In a well-mixed CSTR, the reactants are constantly being stirred and mixed, leading to a relatively uniform concentration of reactants at any given time. This means that the reaction rate will remain relatively constant as long as the concentration of reactants remains the same.

On the other hand, in a PFR, the reactants are not constantly being mixed. Instead, they flow through the reactor at a constant rate, with the concentration of reactants changing as they move through the reactor. This leads to a varying reaction rate along the length of the reactor, with the highest rate at the entrance and the lowest rate at the exit.

Now, why is the reaction rate at the exit of a PFR equal to that of a CSTR for the same conditions? This is because at the exit of the PFR, the concentration of reactants has reached a steady state, meaning that the rate of reaction is equal to the rate of consumption. In other words, the rate of reaction at the exit of the PFR is equal to the rate of reaction in the CSTR, where the concentration of reactants is also at a steady state.

It is possible for the reaction rate at the exit of a PFR to be slower than that of a CSTR, but this would require different conditions, such as a lower flow rate or a higher initial concentration of reactants. However, for the same conditions, the reaction rate at the exit of a PFR will always be equal to or greater than that of a CSTR, as the PFR allows for a longer reaction time and a more thorough mixing of reactants.

I hope this explanation helps to clarify the difference between CSTR and PFR and how their respective reaction rates are related. Keep questioning and seeking understanding in your studies, it will only make you a better scientist.