Question

A 10-L adiabatic PFR was used for a reaction that converts A and B to form C and D. The inlet temperature was originally 100°C and the outlet temperature was 240°C. If the flow rate to the reactor is cut in half, the outlet temperature increases to 242°C. Why? Explain possible reasons leading to such a small change of temperature upon a significant change of flow rate?

Answer 1

Assuming that the flow is steady and complete conversion is taking place

Applying the energy balance equation:

FAo, FBo, FC, FD are flow rates

Cpa, Cpb, Cpc, Cpd are specific heats of components

T1 is the inlet temperature and T2 is the outlet temperature.

ΔH is the heat of reaction.

Now, we can assume that Cpa= Cpb= Cpc=Cpd = C.

And FAo + FBo = FC + FD (Mass is conserved)

Therefore the equation becomes:

So,

(Since the reaction is exothermic)

Therefore,

We can see that there is a term of flow rate on the numerator as well as the denominator.

Hence, the outlet temperature depends very slightly on the flow rate.

We can also say this theoretically because the change in temperature during a reaction depends mainly on the Heat of reaction which is not dependent on the flow rate.