Continuous Stirred-Tank Reactor Definition in Physical Chemistry

Continuous Stirred-Tank Reactor

Definition and meaning of Continuous Stirred-Tank Reactor in chemistry.

A continuous stirred-tank reactor (CSTR) is a large tank used to mix and react chemicals endlessly. New chemicals are pumped in at the exact same time as finished products are pumped out. A large blade inside the tank constantly stirs the liquid so everything stays perfectly mixed.

In more detail

Chemists assume the mixing inside this reactor happens instantly and perfectly. Because of this, the temperature and chemical makeup are exactly the same everywhere inside the tank. The liquid leaving the tank is also identical to the liquid inside the tank.

This perfect mixing creates a very interesting situation for the chemical reaction. A single drop of liquid entering the tank might leave right away. Alternatively, it might stay inside the tank for a very long time.

This means the incoming chemicals spend vastly different amounts of time reacting. The constant mixing keeps the starting chemical concentration very low, so reactions happen slower. Therefore, a CSTR needs to be much larger than a plug-flow reactor to finish the same amount of chemical work.

However, this reactor design is still extremely popular in big chemical factories. The uniform temperature makes the entire process very safe and easy to control. Engineers also find it very easy to build larger versions of these tanks.

Chemical factories often connect several of these massive reactors in a long row. This setup helps them get the best features of both reactor types.

Key facts

Field

Physical Chemistry

Design equation

V = F·(C₀ − C)/(−r)

Mixing assumption

Ideal, instantaneous mixing

Concentration

Uniform throughout the entire tank

Common use

Fermentation and wastewater treatment

Example

Large industrial factories use these continuous reactors to make ethanol fuel from sugar. A steady stream of sugary water and live yeast flows into a giant mixing tank. Inside the warm tank, the active yeast eats the sugar and slowly makes ethanol. At the exact same time, an equal amount of the fermented liquid flows out. The giant stirring blade keeps the temperature, sugar levels, and ethanol levels perfectly stable. The factory can run this tank day and night without ever stopping to clean it.

Frequently asked questions

How is a CSTR different from a plug-flow reactor?

A CSTR is perfectly mixed, so the temperature and chemical levels are identical everywhere inside. A plug-flow reactor is an unmixed pipe where chemicals change continuously as they travel.

Why are CSTRs so popular in large factories?

They are very easy to build at massive scales. The perfect mixing also makes it very easy to control the temperature and prevent dangerous heat buildup.

Why do plants connect multiple CSTRs in a row?

Connecting them in a series gives factories the high efficiency of a plug-flow reactor while keeping the easy temperature control of a stirred tank.