Comparison

CSTR vs PFR: Which Reactor Fits the Process?

A CSTR is a well-mixed tank. A PFR is a tube with no axial mixing. Compare kinetics, heat control, mixing, pressure drop and flexibility.

Chemical & Process · Last reviewed October 17, 2026

Short answer: for a positive-order reaction a PFR gives more conversion than a CSTR of the same volume, so it is more compact for high conversion. A CSTR is chosen for good temperature control, easy handling of solids and viscous fluids, and flexible operation, and a series of CSTRs can approach PFR performance.

	CSTR	PFR
Mixing	Perfectly mixed, with the same composition everywhere and in the exit stream	No mixing along the flow. Composition changes along the length
Rate	Runs at the low outlet concentration, so a lower average rate for positive-order reactions	Runs at higher concentrations early in the tube, so a higher average rate
Volume for high conversion	Larger	Smaller
Heat management	Easy: uniform temperature, jackets and coils, and a stable operating point	Harder: temperature varies along the tube, and hot spots occur in exothermic reactions
Pressure drop	Low	Can be significant in long tubes or packed beds
Solids and viscous fluids	Handle well	Prone to fouling and blocking
Operating flexibility	Flexible in flow and residence time, and good for start-up	Narrower: depends on the tube length and flow
Maintenance	Agitator seals and bearings, and cleaning of the vessel	Fouling of tubes, catalyst replacement and tube cleaning
Typical use	Liquid-phase reactions, polymerization, fermentation, slow reactions	Gas-phase reactions, fast reactions, large-scale continuous production

A conversion comparison

For a first-order reaction with k = 0.5 min⁻¹ and a 4-minute residence time, the Damköhler number kτ is 2. The CSTR conversion is kτ ÷ (1 + kτ) = 66.7%, and the PFR conversion is 1 − e^(−kτ) = 86.5%. To reach 86.5% in a CSTR you would need a volume 3.2 times larger.

How to choose

Look at the kinetics: order, rate constant and the conversion you need.
Look at the heat of reaction and how the heat will be removed or supplied.
Consider the phase and the fluid: gas, liquid, solids, viscosity.
Compare the volumes and the cost, including a CSTR series as an option.
Plan the operation, the start-up flexibility and the maintenance.

Common mistakes

Using the PFR equation for a stirred tank, or the CSTR equation for a tube.
Ignoring heat effects: an exothermic reaction in a PFR can run away.
Comparing reactors of different volumes without using space time.
Assuming that mixing is perfect in a large, viscous tank.

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Questions

Does a CSTR ever beat a PFR?

For reactions where a low concentration improves selectivity, or where the rate falls with conversion in some way, a CSTR can do better. It also handles heat and solids better.

What is a series of CSTRs?

Several stirred tanks in a row. As the number rises, the behavior approaches that of a PFR, while keeping the CSTR advantages.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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