Mass transfer is the net movement of mass from one location, usually meaning stream, phase, fraction or component, to another. Mass transfer occurs in many processes, such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. Mass transfer is used by different scientific disciplines for different processes and mechanisms. The phrase is commonly used in engineering for physical processes that involve diffusive and convective transport of chemical species within physical systems.
Mass Transfer is a Unit Operation which itself is sub-divided into several operation like
3)Distillation-Temp. Difference and Conc. Difference
The common examples of mass transfer in our everyday life and in many industries are:
-diffusion of smoke discharged by tall chimney into the atmosphere,
-a drop of ink diffusing in a glass of still water,
-evaporation of a drop of perfume in a room,
-humidification of air flowing over a spray pond or cooling tower,
-mixing of diesel or petrol with air inside an internal combustion engine,
-diffusion welding of metals,
-diffusion of neutron in a nuclear reactor.
There are basically two modes of mass transfer:
(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).