Why are CSTRs often used in series and how does this affect performance?

CSTRs in series approach PFR behavior as the number of tanks increases - concentration decreases stepwise rather than being immediately diluted to exit value. For n equal-volume CSTRs in series with first-order kinetics: CA/CA0 = 1/(1+k*tau_each)^n. For same total volume, multiple CSTRs achieve higher conversion than single CSTR. Practical limit is 3-5 tanks due to diminishing returns. Series arrangement also allows interstage heating/cooling, feeding at different stages, and staged catalyst ages.