Vocabulary on Mixing
The term ‘mixing’ is used in various ways in the field of chemical engineering. Here are some useful definitions. Common mis-understandings of terms are also described.
Mixing
Mixing 1: The intimate contact of components to obtain one homogenous fluid [1].
Mixing 2: In chemical engineering process concept design “mixing” means a process function, or a process building block, or a unit operation, that performs mixing. Also, the term “mixer” is used with the same meaning as mixing [1].
Mixing is also used for dispersing a phase in a different phase such as dispersing a gas as bubbles into a liquid. In general, this use does not lead to misunderstanding, as it is obvious what is meant.
Micro-mixing
If the mixing is down to the uniform molecular level, then it is called micro-mixing. This term is mainly used in chemical reaction engineering, when fast reactions are involved, and the rate of micro-mixing affects the reactor performance in terms of conversion and selectivity [2].
Micro-mixing is however also relevant in other unit operations such as in mixing (mixers).
Back-mixing
The term “back-mixing” is used in chemical reaction engineering for a continuous flow reactor concept with uniform concentrations inside the reactor. This concept reactor type is called Mixed Flow Reactor (MFR) by Levenspiel in his textbook Chemical Reaction Engineering[3]. It is also called Continuously operated Stirred Tank Reactor (CSTR) [2].
If the reactor is also micro-mixed, then the reactor concept is called Continuously operated ideally Stirred Tank Reactor (CiSTR). [4].
Back-mixing is relevant in designing reactors and in other unit operations such as distillation, extraction, and mixing.
The term CSTR is sometimes mis-used for a mechanically stirred tank reactor. If the reactor in case is operated batch-wise, then this mis-use can have two consequences: a) that others interpret it as a continuously operated reactor and b) that others interpret it as a back-mixed reactor.
References
[1] Schaschke C. A dictionary of chemical engineering. OUP Oxford; 2014.
[2] Harmsen J, Bos R. Multiphase Reactors: Reaction Engineering Concepts, Selection, and Industrial Applications. De Gruyter; 2023.
[3] Levenspiel O. Chemical reaction engineering. John Wiley & sons; 1998.
[4] Westerterp KR, van Swaaij WP, Beenackers AA, Kramers H. Chemical reactor design and operation. John Wiley & sons; 1984.