Why does PFTR usually have higher conversion rate than CSTR?

Question

please provide a qualitative explanation.
abbreviation:
PFTR-plug flow tubular reactor
CSTR-continuous stirred tank reactor

Answer 1

Here is a qualatative explination. You can prove in a few different ways, including by integrating the reaction rate curve for the different reactors, for a given conversion. The description below will be valid for most commonly occuring reactions.

The conversion in a reactor depends on the average reaction rate as well as residence time.

A CSTR is well mixed, and the average reaction rate will be that of the conditions of the bulk mixture. The composition of the reacor product is also the same as that in the reactor. For most reactions (especially equilibrium reactions) the rate of reaction decreases with increasing concentrations of final product (and decreasing concentrations of reagent). This leads to a catch 22 situation. You generally want high concentrations of final product from the reactor, but in order to get that you will end up with a low average reaction rate in the reactor (and hence a very large reactor).

In a plug flow reactor, the rate is not constant. In the first section of the reactor, the rates are high (high concentration of feed and low concentration of product). As the material goes through the reactor the rates drop. The average rate is still higher and hence the conversion for a given reactor volume is also better.

With CSTR's the conversion in 2 CSTR's each with volume = x will be higher than a single reactor of volume 2x. Adding more and more CSTR's in series improves the average rate and yields. A plug flow reactor can be approximated by a large number of CSTR's in series.

Answer 2

This is because in the CSTR the raw materials are diluted more by the products. and yes dilution from the products affects the PFR aswell but not to the same extent (e.g. the products at the outlet don't dilute the raw materials at the inlet.)

Also I believe there are cases where the CSTR can get higher conversion, for example where you have an auto-catalytic reaction(catalysed by the products).

(this is assuming the cstr and pfr have similar volume+flowrate)

Answer 3

Cstr And Pfr