Chemical Reaction Engineering: CSTR vs PFR

To carry out reaction, there are two kinds of continuous reactors used: CSTR and PFR. In CSTR the concentration remains constant throughout the reactor which is equal to outlet concentration, while in PFR the concentration changes along the length of reactor. The rate of reaction depends on the concentration of the reactant for a non zero order reaction. Therefore,  the rate of reaction in CSTR remains constant corresponding to out concentration of the reactant, but the rate of reaction decreases along the length of reactor in PFR.

The shaded area corresponds to the volume of reactor required to carry out reaction. So, if the rate of reaction decreases as the concentration decreases, that means (1/-rA) increases, the rate of reaction in PFR is high intially and decreases along the length of reactor. So it can be seen that volume in case of PFR is less than that required for CSTR.
The volume required to achieve a specified conversion for a positive order reaction is less when we connect CSTRs in series than a single CSTR of volume equal to the sum of the volume of CSTRs connected in series. this is because of the increased rate of reaction in small CSTR. while in PFR the volume reamains same whether we use a single PFR or PFRs connected in series.