Mass Transfer

Mass Transfer:

   The transfer of material from the region of higher concentration to the region of lower concentration is termed as mass transfer.

Why mass transfer is needed:

   In chemical process industries we are concerned with the transformation of raw material into useful products. The products formed in industries are countless such as many varieties of petrochemicals, organic and inorganic chemicals, dyes, polymers, detergents and so on.

Before obtaining the final product, the material undergoes a processing unit.  Here it requires some pre-processing treatment to remove by products or contamination.

Objectives of separation of unwanted materials:

   The objectives of separation operation are listed below:

• Purification.
• Isolation.Recovery.
• Refining.
• Concentration
• Enrichment of material.

Factors affecting on separation:

        The separation process is mainly depending upon the following factors: 

•  Size
• Density
• Volatility
• Solubility
• Diffusivity
• Electric charge and so on.

Classification of Separation Process.

      Separation process is classified into basically two broad categories: 

•  Mechanical Separation.
• Diffusional Separation. (Our main concern is with this topic)

Mechanical Separation:

   Mechanical Separation are generally used for the separation of one phase from another phase or we can say that the separation of two heterogeneous phases is known as mechanical separation.

• Filtration (Separation of solid particles from a liquid)
• Screening (Separation of solid particles on the basis of size)
• Sedimentation [Separation of solid particles form a fluid (liquid, gas)]
• Magnetic Separation (Separation of particles having magnetic properties)
• Centrifuging (Separation of solids that form a porous cake from liquids)

Diffusional Separation:

   Diffusional separation can be defined as the separation of components of homogeneous phase.

How diffusional separation is done:

   Separation of heterogeneous phase is much easier, but in case of homogeneous phase the separation of component is a highly irreversible process. Mean this separation process does not occur spontaneously. For effective separation the material is to be transferred to a second phase. Certain components started to move from original solution to new phase, hence the separation is started.

Basically, the mass transfer operation is classified into 3 categories

• Gas-Liquid Separation.
• Liquid-Liquid Separation.
• Solid-Liquid/Gas Separation.

Gas-Liquid Separation

   As Clear from name this type of separation deals with gas and solid phase.

• Absorption
• Desorption
• Humidification
• Dehumidification
• Distillation

Absorption:

   This is the process in which gas mixture is contacted with a liquid for the purposes of preferentially dissolving one or more components of the gas into the liquid.

Example: Removal of NH₃ from by-product coke ovens using water.

Desorption:

   This is the reverse of absorption. In it there is removal of gas from gas solution using a gas.

Example: Removal of NH₃ from NH₃-water solution using air.

Humidification:

   The transfer of liquid to a gas phase containing one or more components by contacting dry gas with pure liquid is known as humidification

Example: Presence of water vapours in air.

Dehumidification:

   Transfer of vapour component from gas-vapour mixture to a liquid phase by contacting them is known as dehumidification.

Example: Transfer of water vapour from air-water vapour mixture to liquid water.

Distillation:

   It is the method in which the components of a liquid mixture are separated by distributing the substances between a gas and a liquid phase. OR the separation of components of a liquid on the basis of its boiling points.

Example:  Separation of petroleum fractions.

Liquid-Liquid Separation:

   In this type of separation, the both phases we are dealing with are liquids.

• Extraction

Extraction:

   Separation of a component from a liquid mixture by adding another insoluble or partially miscible solvent is known as extraction.

Example: Separation of dioxane from waterdioxane solution using benzene.

Solid-Liquid/Gas Separation:

In this type of separation, the phases which we can concern with are solid and liquid/gas.

• Leaching
• Adsorption
• Desorption
• Drying
• Crystallization

Leaching:

   Leaching is a process in which there is separation of soluble solute from a solid mixture by contacting it with a solvent is known as leaching.

Example: Separation of sugar from suger beets using hot water.

Adsorption:

   Adsorption involves contact of solid with either a liquid or a gaseous mixture in which a specific substance from the mixture concentrates on the solid surface.

Example: removal of moisture from air by silica gel.

Desorption:

   Desorption is simply the reverse process of adsorption operation.

Drying:

   Drying means the removal of moisture from a substance.

Example:  removal of water from clothes.

Crystallization:

   The process of forming solid particles within a homogeneous phase is called crystallization.

Example: the formation of crystals of sugar from a concentrated sugar solution.