Continuous Stirred Tank Reactors

What is a Continuous Stirred Tank Reactor?

A Continuous Stirred Tank Reactor (CSTR) is an agitated vessel with a continuous feed of reactants and a continuous discharge of the reaction mixture (product). The feed and discharge rates are controlled to maintain constant reaction conditions (concentration, temperature and reaction rate) ensuring a consistent product stream is produced.


The behaviour of a CSTR is often approximated or modelled by that of an ‘ideal CSTR’, which assumes perfect mixing. In a perfectly mixed reactor, the reagent is instantaneously and uniformly mixed throughout the reactor upon entry. Consequently, the output composition is identical to the composition of the material inside the reactor, which is a function of residence time and reaction rate. The CSTR is the ideal limit of complete mixing in reactor design, which is the complete opposite of a plug flow reactor (PFR). In practice, no reactors behave ideally but instead fall somewhere between the mixing limits of an ideal CSTR and a plug flow reactor.

Characteristics of a CSTR 

There are well-defined key advantages to using flow technologies as compared to standard batch chemistry methods:​

  • Run at a steady state with a continuous flow of reactants and products

  • Feed assumes a uniform composition throughout the reactor

  • The exit stream has the same composition as in the tank

When CSTRs are in series, the previous reactor operates at a higher concentration than the one after, therefore the rate and the conversion are greater than the subsequent reactor in series, which then builds on the conversion in the previous reactor.

The continuous stirred tanks in series model is used to determine the number of theoretical stages, or 'plugs', within a flow reactor. The greater the number of theoretical stages, the nearer the flow reactor is to true plug flow reactors.

It should be noted that true plug flow is a theoretical ideal that is never practically achieved, and is defined as an infinite number of theoretical stages or 'plugs'. AM Technology defines real-world plug flow behaviour within a flow reactor as ten theoretical stages or more.

Related Pages...

Relationship between reactants and product within a CSTR

CSTR and plug flow reactor series

Relationship between reactants and product within CSTRs in series

Continuous Stirred Tanks in Series Model

Plug flow reactors

Useful External Links

Flow Chemistry (

“One of today's most important tools for modernizing the pharmaceutical industry is a process known as continuous manufacturing”

Director of the FDA’s Center for Drug Evaluation and Research, Janet Woodcock M.D.