MASS TRANSFER AND MASS TRANSFER OPERATIONS

Mass transfer

• Mass transfer means the transfer of substance through another on a molecule scale.
• The net movement of mass from one location usually by a stream, phase, fraction, or component, to another.
• This mass transfer occurs due to the concentration difference or gradient.

Hence The process of transfer of mass as a result of the concentration difference of a component in a mixture or two-phase which are in contact is called mass transfer

• Example:- evaporation of water from a pool of water into a stream of air flowing over the water surface

Mass transfer operations

The operation carried out for separating the component of the mixture, involving the transfer of (mass) material from one homogeneous phase to another, due to the difference in vapor pressure, solubility or diffusivity and in which driving force for transfer is a concentration difference is called mass transfer operations

• In the mass transfer operations, neither equilibrium phase consists of only one component. 
• Hence when two-phase is initially contacted, they will not be of equilibrium compositions. 
• The system then attempts to reach equilibrium by a relatively slow diffusive movement of the constituents, which transfer in part between the phases in the process. 
• Separations are therefore never complete, although they can be brought as near completion as desired. 
• Mass transfer occurs in many processes such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. 

In mass transfer operations, the Mass transfer may occur

• In one direction (gas absorption)
• In both direction (distillation)
• With simultaneous heat transfer( drying and crystallization)
• With simultaneous chemical reaction (Absorption of CO2 in an aqueous solution of KOH)
• With the exchange of one or more components
• Isothermally and non-isothermally