Mass Transfer: Meaning and Process | Physics

In this article we will discuss about the meaning and process of mass transfer.

Meaning of Mass Transfer:

When a system contains two or more components whose concentrations vary from point to point, there is a natural tendency for mass to be transferred, minimising the concentration differences within the system. The transport of one constituent from a region of higher concentration to that of lower concentration is called mass transfer.

Consider the process of water evaporation from a water pool into an air stream flowing over the water surface. The concentration of water molecules would be higher just above the liquid surface compared to that in the main portion of air stream. Higher concentration means more molecules per unit volume.

This concentration difference or gradient would result in the transport of water molecules from the place where its concentration is more towards the main air stream. Apparently the diffusing substance moves from a position of higher concentration to that of lower concentration so that the concentration differences within the system are reduced to minimum.

Mass transfer appears in many practical applications of chemical and mechanical engineering.

Some examples of industrial importance of mass transfer are:

i. Humidification of air in a cooling tower

ii. Evaporation of petrol in the carburetor of a petrol engine

iii. Evaporation of liquid ammonia in the atmosphere of hydrogen in Electrolux refrigerator

iv. Dispersion of oxides of sulphur (pollutants) from a power plant discharge of neutron in a nuclear reactor

v. estimation of depth to which carbon will penetrate in a mild steel specimen during the act of carburising

Some experiences of mass transfer in our day-to-day life are:

a. Initial dissolution and subsequent uniform spreading of a lump of sugar added to a cup of tea

b. Initial diffusion (to the surface) and subsequent evaporation (into the atmosphere) of water retained into the wet clothes or a log of wood

c. Diffusion of smoke (from a tall chimney) into the surrounding atmosphere

d. Pleasant fragrance presented by a perfume which is imparted throughout the surrounding atmosphere

Mass Transfer Processes – Classification:

The mass transfer operations can be classified into the following categories:

(i) Diffusion Mass Transfer; Molecular or Eddy Diffusion:

The molecular diffusion is the transport of matter on a microscopic level as a result of diffusion from a region of high concentration to a region of low concentration in a system of a mixture of liquids or gases. The diffusion mass transfer occurs when a substance diffuses through a layer of stagnant fluid. It is independent of any convection within the system.

The molecular diffusion is further categorised into;

a. Ordinary diffusion resulting from concentration gradient; the diffusing substance moves from a position of high concentration to one of low concentration

b. Thermal diffusion which may occur by virtue of temperature gradient

c. Pressure diffusion resulting from hydrostatic pressure differences that provides the driving potential

d. Forced diffusion which results from the action of external forces

The eddy diffusion occurs when one of the diffusing fluids is in turbulent motion. The eddying motion greatly increases the speed of mass transfer as it is in addition to molecular diffusion.

(ii) Convective Mass Transfer: Free or Forced:

Mass transfer due to convection involves transfer between a moving fluid and a surface, or between two relatively immiscible moving fluids. The convective mass transfer depends both on the transport properties and on the dynamic (laminar or turbulent) characteristics of the flowing fluid.

(iii) Mass Transfer by Change of Phase:

Mass transfer also takes place whenever there is a change from one phase to another. Quite often, the mass transfer is by the simultaneous action of diffusion and convective mass transfer.

For example:

i. Hot gases escaping from a chimney rise by convection and then diffuse into air above the chimney

ii. Mixing of water vapour with air during evaporation of water from the lake surface is partly due to convection and partly due to diffusion.

Again, when water boils in an open air, there is first transfer of mass from liquid to vapour state (change in phase). The vapour mass from the liquid interface is next transferred to the open air both by convection as well as by diffusion.

We generally do not use the term mass transfer to describe gross or bulk fluid motion due to mechanical work such as motion of air induced by a fan, or motion of water being forced through a pipe line.