Continuous Stirred-Tank Reactor
Definition and meaning of Continuous Stirred-Tank Reactor in chemistry.
A continuous stirred-tank reactor (CSTR) is a vessel in which reactants are fed in and products are withdrawn continuously while an impeller keeps the contents well mixed, so the composition and temperature inside the tank are uniform and equal to the outlet stream.
In more detail
Because mixing is assumed to be instantaneous and perfect, a fluid element entering the CSTR can leave immediately or stay indefinitely, giving a broad distribution of residence times rather than the single fixed time seen in a plug-flow reactor. This uniform, low reactant concentration throughout the tank means a CSTR generally requires a larger volume than a plug-flow reactor to reach the same conversion for the same reaction, but its constant conditions make temperature control and scale-up straightforward. CSTRs are widely used in industrial polymerization, fermentation, and wastewater treatment, and are often connected in series to approximate plug-flow performance while retaining easy mixing and cooling.
Key facts
| Field | Physical Chemistry |
|---|---|
| Design equation | V = F·(C0 − C)/(−r), evaluated at outlet conditions |
| Mixing assumption | Ideal, instantaneous mixing (uniform composition and temperature) |
| Common use | Series (CSTR cascades) for polymerization, fermentation, wastewater treatment |
In continuous ethanol fermentation, a CSTR is fed a steady stream of sugar solution and yeast while an equal volume of fermented broth is withdrawn, keeping sugar and ethanol concentrations, pH, and temperature constant inside the tank at steady state.
Frequently asked questions
How is a CSTR different from a plug-flow reactor (PFR)?
In a CSTR, composition is uniform throughout the tank and equal to the exit stream, whereas in a PFR composition changes continuously along the length of the reactor, so a PFR usually needs less volume for the same conversion.
Why do industrial plants use CSTRs in series?
A cascade of CSTRs approaches the conversion efficiency of a single PFR of the same total volume while keeping the simpler mixing, temperature control, and mechanical design of stirred tanks.