EXAMPLES OF MASS TRANSFER

Example of mass transfer

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 the difference is called mass transfer operations

Example:- Distillation, Gas absorption, Liquid Extraction, Drying

Example of mass transfer operations

• Depending upon phase contact between Gas, liquid and solid phase following are the classification of the mass transfer operations

Gas-gas phase contact

• The fragrance of incense sticks after lightning diffuse from one room to another room. 

Gas-liquid contact

• A cup of water left in a room eventually evaporate as a result water molecules diffusing into the air. This is liquid to gas diffusion.
• Gas absorption:- If a mixture of ammonia and air is contacted with liquid water, a large portion of ammonia will dissolve in the liquid, and in this way air ammonia mixture can be separated.
• Desorption or stripping:- If air is brought into contact with an ammonia-water solution, some of the ammonia leaves the liquid and enters the gas phase.
• If the liquid phase is a pure liquid, containing one component while the gas contains two or more, the operation is humidification or dehumidification depending upon the direction of transfer
• Humidification of the air: - When dry air contacted with liquid water result in evaporation of some water into the air
• Dehumidification: - When very moist air contacted with pure liquid water may result in condensation of part of moist air in the air. 

Gas-liquid contact

• A piece of solid CO2 ( dry ice ) will also get smaller and smaller in time as the CO2 molecules diffuse into the air. This is a solid to gas transfer example.
• Fractional sublimation: - If a solid solution is partially vaporized without the appearance of the liquid phase, the newly formed vapor phase and residual solid each contains all the original components, but in different propositions
• Drying or Desorption:- If solid which is moistened with a volatile liquid is exposed relatively dry gas, the liquid leaves the solid and diffuses into the gas
• Adsorption: - If diffusion takes place from solid to gas phase is called adsorption.
• For example:- If a mixture of water vapor and the air is brought into contact with activated silica gel, the water vapors diffuse to solid, which retains it strongly, and the air is thus dried.

Liquid-liquid contact

• Liquid extraction operations:- This separation involving contact of two insoluble liquid phases.
• for example - Acetone water solution is shaken in separating funnel with carbon-tetra chloride and the liquid allowed to settle, a large portion of acetone will be found in the carbon chloride-rich phase and will thus have been separated from the water.
• Fractional extraction:- a solution of acetic acid and acetone can be separated by adding it to the insoluble mixture of water and carbon tetrachloride. After shaking and setting, both acetone and acetic acid will be found in both liquid phases, but in different proportions.

Liquid-solid contact

• If a spoon of sugar is added in a cup of coffee will eventually sweeten the coffee because sugar molecule diffuses into the coffee.
• Leaching:- In this process diffusion is from solid to liquid phase takes place.
• For example- Leaching of gold from its ore by cyanide solutions. Another example is the leaching of cottonseed oil from the seeds by hexane.
• Adsorption:- In this process diffusion is from liquid to solid phase takes place
• For example:- The colored material which contaminates impure cane sugar solutions can be removed by contacting liquid solutions with activated carbon.

Solid-solid contact

• The diffusion rate is very slow
• If metallic gold and metallic are placed in intimate contact, over a period of time molecules of lead will diffuse into the gold and vise versa. but the rate of diffusion is very slow.