ABSTRACT
The transfer of a particular chemical species from one phase
to another and themovement of such species along gradients
within phases are fundamentally important processes in any
system involving more than one chemical component.
Forces that can promote such transfer include: free energy,
pressure, electrical charge, temperature, and concentration.
These driving forces are physical quantities of importance
not only in biological systems but also in other systems
encountered in engineering practice.[1] The transport of
chemical species as a result of such driving forces is com-
monly called mass transfer. Mass transfer subdivides into
molecular mass transfer and convective mass transfer. The
term diffusion is often used for the former. Diffusion deals
with random molecular migration of matter through a med-
ium, whereas convective mass transfer involves the migra-
tion of matter into a moving fluid or a stream of gas. For
example, if an open container of acetic acid is placed in one
corner of a room, the odor will soon be detected throughout
the entire room. In this case, there is a combination of
molecular diffusion and convection. If a lump of sugar is
placed in a glass of water, the sugar will eventually dissolve
and spread in all parts of the volume without stirring. In this
case convection will play aminor role. In solids, there can be
no convection and all movements are by molecular diffu-
sion. The general equation for all types of mass transfer is
