Static mixers, also called motionless mixers, or inliners, have the effect of reducing the velocity difference across a pipe, hence the plug flow type.
This type of flow regime can be extremely advantageous when dealing with certain types of reactions, dosing and sampling systems, etc.
An ideal plug-flow reactor assumes ideal mixing (red line), where no back mixing occurs. Backmixing is more common in a stirred tank reactor utilizing a dynamic mixer.
