Continuous Stirred Tank Reactors

A continuous stirred tank reactor (CSTR) is a continuous reactor that runs at a steady state. They are used most frequently with liquid phases, but can handle gas or solid reactions as well. No matter what the reactants are or the product is, all CSTR’s have the same components. There is an inlet stream(s) that bring all of the reactants in at a particular rate. This stream(s) dumps into a large container; there is a shaft with a blade attached (stirrer) in the reactor that rotates around to mix the reactants. Finally there is an outlet stream, which the solution will exit from the reactor. The rates of the inlet and outlet streams must be carefully calculated in order to keep the volume inside the tank to stay the same. If this calculation is wrong, there is a great danger that the reactor will overflow, or not have enough solution in it.

This picture was taken from a website generated by the University of Michigan, and is a very simple representation of what a CSTR looks like. The arrows represent the stream(s) responsible for bringing in reactants and removing products. The line in the middle represents the stirring unit. Note that this piece may not always be the same. There are different blades that attach to the shaft which cause different flow patterns within the tank. Tanks can be by themselves, or they can be joined together. The outlet stream leading into another CSTR allows this, and it helps ensure the contents of the solution are even more homogeneous. In a chemical reaction, it is very important that the ingredients are mixed well to avoid parts going through without being reacted.

The purpose of continuously stirred reactor is to keep reactants colliding steadily with each other during a reaction. For an easy example I will use Kool-Aid. The ingredients to make the Kool-Aid are: water, sugar, and Kool-Aid mix. When I make Kool-Aid for myself, I put the amount of sugar and Kool-Aid mix in a pitcher, add water, stir, and that is all it takes. However if I wanted to make Kool-Aid for two million people, I would probably want to be making it in large batches, and preferably without stopping. I could build a machine to help me do this. Something would take the sugar, mix, and water to a large container. Inside the container would be a spoon attached to a motor that spins it to mix up the ingredients, and it could have a drain to remove the mixed Kool-Aid at the same rate as the water going in. If the CSTR did not mix the reactants very well, the Kool-Aid would not turn out very successful. There would be spots that were so sweet the consumer’s lips would pucker and they would spit it out. This is an example of why everything must be mixed well in a reaction.

There are many advantages to having this type of reactor. Temperature, which can play a huge part in a chemical process, is easier to control with this kind of reactor. They are cheaper to make, have a higher heat capacity, and cleaning the reactor is easy because the inside of the reactor is more accessible than many other reactors. Also because of the simplicity of the components involved in the reactor, they are very easy to maintain and do not require much work to keep running. The only disadvantage of these reactors is that the ratio of products and reactants to the size of reactors is low. In other words, it takes more space to mix the components in comparison to other reactors.

There are many other types of reactors out there. For example, in a batch reactor there is reactants put in a container and the container is closed for a period until the reaction is done. A semi-batch reactor uses the same principle as CSTR except the reactions do not run at a steady state. A CSTR cannot be used for all types of reactions; there could be specific needs of a reaction that another type of reactor would work better. There is a right kind of reactor for any process, and the necessities of the process are what will determine the type of reactor used.

References

http://www.engin.umich.edu/~cre/

The Virtual Encyclopedia

Aris, Rutherford 1989 Elementary Chemical Reactor Analysis

Winterbottom, JM 1999 Reactor Design for Chemical Engineers

Nauman, EB 1992 Chemical Reactor Design

Froment, Gilbert 1990 Chemical Reactor Analysis and Design