Mass transfer is defined as the movement of a component from one location to another, where this component is present in a concentration different from the equilibrium one.
Mass transfer occurs whenever a disequilibrium condition exists, which is then the driving force for the movement of the component. Mass transfer can take place within only one phase from one point to another or between two different phases.
Mass transfer occurs by two basic mechanisms:
- molecular diffusion
- turbulent diffusion
The first one is a random and spontaneous microscopic movement of individual molecules within a phase, whether gas, liquid or solid, while the second one is a random macroscopic fluid motion. The phenomenon of the molecular diffusion is pretty slow respecting to the turbulent diffusion.
In this section and for the purposes of the Hyper-TVT lecture, only molecular diffusion due to the concentration gradient is considered. The treatment of this topic will focus on binary systems, since the molecular diffusion theory is simpler to be applied on them than on multicomponent systems.
Different driving forces like pressure, temperature and forced diffusion, e.g. by applying an electrical field, can cause molecular movement but this will not be taken in account within the treatment of the TVT topics.
Most often in large-scale separation processes, like the ones treated in the Hyper-TVT lecture, mass transfer takes place between two phase across their interface. For this reason, most care must be given to the right choice of equipment size and design to properly mix the fluids and to maximize the interface between the two exchanging phases.