The simultaneous motion of two or more immiscible fluids is of considerable importance in chemical engineering, and there is an abundance of examples, such as:
1. Mixing of coal particles with water and pumping the resulting slurry through a pipeline.
2. Simultaneous production of oil and natural gas upward through a vertical well.
3. Vaporization of water or other liquids inside the heated tubes of a vertical-tube evaporator.
4. Contacting a reacting fluid with catalyst particles in a fluidized bed.
5. Absorption of a component from a gas stream that is rising through a packed bed in a tower, down through which an absorbing liquid is flowing.
6. Aerated bioreactors, ...