Reactor Selection Rules

Quick-reference decision logic for choosing reactor types, comparing CSTR vs PFR, and ordering series configurations.

Batch vs. Flow Reactors

Choose Flow Reactors (PFR preferred) when:

  • Fast reactions: characteristic time 0.01 – 100 s
  • Large-scale continuous production
  • Steady-state operation is desired
  • Heat removal is easier in tubular geometry

Choose Batch Reactors when:

  • Slow reactions: characteristic time 1,000 – 100,000 s
  • Small-scale or specialty chemical production
  • Multiple products from the same equipment
  • Flexibility in reaction time is needed

CSTR vs. PFR (Same Chemistry)

For positive-order reactions (most common):

  • PFR always requires less volume than CSTR for the same conversion
  • CSTR instantly dilutes feed to exit concentration → operates at the LOWEST concentration → SLOWEST rate
  • PFR maintains high concentration near the inlet → faster rate → more efficient use of volume

Levenspiel plot insight: PFR volume = area under the 1/r curve; CSTR volume = rectangle at the exit (1/r) value. The rectangle is always larger than the area under the curve.

Exceptions — prefer CSTR when:

  • Negative-order reactions (higher concentration → slower rate, CSTR wins)
  • Autocatalytic reactions (some product needed to catalyze the reaction)
  • Reactions where mixing is needed to dilute hazardous concentrations
  • Heat management is critical (CSTR maintains uniform temperature)

Reactors in Series

PFRs in series:

  • n PFRs in series = 1 PFR of the same total volume
  • Order of PFRs does NOT matter (just add volumes)

CSTRs in series:

  • n CSTRs in series ALWAYS outperforms 1 CSTR of the same total volume
  • As n → ∞, performance of n CSTRs → PFR performance
  • Even splitting into 2 CSTRs gives significant improvement

Mixed CSTR + PFR in series (for positive-order):

  • For maximizing conversion at fixed total volume: CSTR first, then PFR
  • CSTR handles the high-concentration (high-rate) region quickly
  • PFR efficiently drives conversion further where rate is slower

Levenspiel Plot Decision Tool

How to use the Levenspiel plot (−F_{A0}/r_A vs X):

  • Plot −F_{A0}/r_A on the y-axis vs conversion X on the x-axis
  • PFR volume = area under the curve from 0 to X_f
  • CSTR volume = rectangle: height = (−F_{A0}/r_A) at exit × width = X_f
  • Compare areas to decide which reactor type is more volume-efficient

What the shape tells you:

  • Monotonically increasing curve (normal): PFR is always better
  • Curve with a minimum: CSTR is better up to the minimum, then PFR
  • At the minimum of −1/r_A, a CSTR operating at that conversion is optimal

Non-Isothermal Reactor Selection

Exothermic reactions (ΔH_R < 0):

  • Temperature rises with conversion — rate increases initially
  • CSTR: single operating point; may have multiple steady states
  • PFR: temperature profile rises along the tube; risk of hot spots
  • Adiabatic PFR can reach very high temperatures — check equilibrium constraints

Endothermic reactions (ΔH_R > 0):

  • Temperature drops with conversion — rate slows down
  • Supply external heat to maintain temperature
  • PFR with staged heating is often practical

Adiabatic temperature change (quick check):

  • ΔT_max = (−ΔH_R · C_{A0}) / (ρ · Ĉ_p)
  • This is the maximum temperature change if 100% of A reacts
  • Actual ΔT = ΔT_max × X_A

Interactive: Series CSTRs vs PFR

Explore how increasing the number of CSTRs in series approaches PFR performance for a 1st-order reaction.