(i) Mass Transfer by Diffusion –
The transport of mass by random molecular motion in quiescent or laminar flowing fluids is known as mass transfer by ‘diffusion’ and is analogous to heat transfer by conduction. Mass transfer by diffusion occurs due to
(a) concentration gradient,
(b) temperature gradient
(c) hydrostatic pressure difference.
(ii) Convective Mass Transfer –
The rate of molecular diffusion of mass can be accelerated by the bulk motion of the fluid. Mass can be transported between the boundary of a surface and a moving fluid (drying of clothes, molecular diffusion of a sugar cube in a cup of coffee by stirring, moist air flowing over the surface of an ocean and precipitation on a dry land etc.), or between two moving fluids which are relatively immiscible (formation of clouds, vapourisation of water in a tea kettle). This mechanism of mass transfer is called ‘convectIve mass transfer’ and is analogous to heat transfer by convection (free or forced).