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The continuous flow stirred-tank reactor (CSTR), also known as vat- or backmix reactor, is a common ideal reactor type in chemical engineering. A CSTR often refers to a model used to estimate the key unit operation variables when using a continuous agitated-tank reactor to reach a specified output. (See Chemical reactors.) The mathematical model works for all fluids: liquids, gases, and slurries.
The behavior of a CSTR is often approximated or modeled by that of a Continuous Ideally Stirred-Tank Reactor (CISTR). All calculations performed with CISTRs assume perfect mixing. In a perfectly mixed reactor, the output composition is identical to composition of the material inside the reactor, which is a function of residence time and rate of reaction. If the residence time is 5-10 times the mixing time, this approximation is valid for engineering purposes. The CISTR model is often used to simplify engineering calculations and can be used to describe research reactors. In practice it can only be approached, in particular in industrial size reactors.
Assume:
Integral mass balance on number of moles Ni of species i in a reactor of volume V.
1.
where Fio is the molar flow rate inlet of species i, Fi the molar flow rate outlet, and
Assume:
A → products
NA = CA V (where CA is the concentration of species A, V is the volume of the reactor, NA is the number of moles of species A)
2.
The values of the variables, outlet concentration and residence time, in Equation 2 are major design criteria.
To model systems that do not obey the assumptions of constant temperature and a single reaction, additional dependent variables must be considered. If the system is considered to be in unsteady-state, a differential equation or a system of coupled differential equations must be solved.
CSTR's are known to be one of the systems which exhibit complex behavior such as steady-state multiplicity, limit cycles and chaos.
Application
Continuous flow stirred-tank reactor is applied in waste water treatment process. CSTR has rapid dilution rate which makes it resistant to both high pH waste and low pH volatile fatty acid wastes. On the other hand, CSTR is less efficient because it require larger reactor volume to achieve the same removal rate as other reactor model.