A continuous stirred tank reactor (CSTR) is a batch reactor equipped with an impeller or other mixing device to provide efficient mixing. In chemical engineering the name CSTR is often used to refer to an idealised agitated tank reactor used to model operation variables required to attain a specified output.
In flow chemistry, a continuous stirred tank reactor (CSTR) equipped with features to continuously feed and exhaust reactants is an example of a mechanically mixed flow reactor.
Flow Chemistry and the CSTR
For practical use in flow chemistry a single CSTR suffers from severe back mixing. It also gives extremely poor control residence time. A wide range of residence times is often referred to as a broad residence time distribution (RTD, see below). Each molecule of reagent does not necessarily flow through the reactor in the same time. The back mixing and broad RTD combine to limit the performance of a single CSTR. For most reactions the back mixing and variable residence time have a negative impact on product yield, selectivity and space-yield for the reactor.
For flow chemistry continuous stirred tank reactors are often used in cascades of 3 or 4 CSTRs. By adopting this configuration of multiple CSTRs the residence time control can be much improved and the back mixing significantly reduced.
Residence Time Distribution in CSTR
The following figures compare the RTD for continuous stirred tank reactors. The RTD of single and multiple cascades of CSTRs are compared with the RTD for a plug flow reactor (PFR).
- RTD for typical plug flow reactor (PFR)
- RTD for a typical single CSTR
- RTD for a typical cascade of four CSTRs
Examples of continuous stirred tank reactors
References: Schmidt, Lanny D. (1998). The Engineering of Chemical Reactions. New York: Oxford University Press. ISBN 0-19-510588-5.