Introduction to Mass Transfer II

 As previously defined (Click here) Mass transfer is the movement of components under a chemical potential gradient from an area of high concentration to that of a lower concentration. Once the gradient equals zero then equilibrium is established.

Mass transfer depends on the diffusion of molecules from one distinct phase to another. It is based on the differences in physical and/or chemical properties of the molecules in motion. These properties include solubility and vapor pressure. For interphase mass transfer, a concentration gradient exists between the bulk and interface. Under steady state conditions an interface equilibrium is assumed. An interface is the boundary between different phases.

Many industrial processes depend on mass transfer which aids in the movement of materials from one homogeneous phase to another. These include:

Adsorption and desorption – This process uses the ability of molecules to move from either gas or liquid phase to the surface of solid particles. Adsorption does not qualify as a true inter phase mass transfer operation as the fluid adheres to the solid surface instead of dissolving in the solid. Desorption is the opposite of adsorption as mass transfer occurs from the solid surface (adsorbent) to the liquid or gas phase (adsorbates).

This process is applied in:

• Removing toxic gases and smells from the air.
• Solvent recovery
• Removing ions from solutions

Adsorption is also discussed here.

Absorption and stripping – Absorption is the transfer of materials from a gas to a liquid phase. The gas is absorbed by a liquid in which the solute gas is more or less soluble from its mixture with an inert gas, as well as more or less insoluble gas. The liquid is immiscible in the gas phase. An example of absorption is the separation of ammonia from an air-ammonia mixture using water with the solute recovered from the solution using distillation. Absorption is also discussed here.

Stripping is the separation of a gas solute from a liquid phase.

Distillation – This is a process where a miscible, volatile liquid mixture is separated into its individual components using partial vaporization. The components vaporize when their boiling points are reached then are condensed into their liquid states. This is widely used in the purification of crude oil into gasoline, kerosene, fuel oil and lubricating oil.

In industrial applications, distillation commonly occurs in a distillation column. Some majore aspects of a distillation column are discussed here.

Extraction – This is a process where the separation of the constituents of a liquid solution is achieved by contact with another insoluble liquid. The liquid used to achieve this process is called a solvent while the solution to be extracted is called a feed. The product which is solvent rich is referred to as the extract while the residual liquid from which the solute is removed is called the raffinate.

Real world applications include the separation of aromatics from kerosene-based fuel oils, the production of fuels in the nuclear industry and the separation of penicillin from fermentation mixtures.

Leaching – This is the treatment of finely divided solids with a liquid.

Examples include oilseed extraction, extraction of sugar beets with hot water and extraction of medicinal compounds from plant roots, leaves and stems.

Humidification and dehumidification – Humidification refers to the increase of the vapor content of a gas stream by passing it over a liquid. Dehumidification on the other hand, involves the transfer of water vapor from the gas phase to the liquid phase. 

Membrane separation – This process involves the diffusion of a solute from one fluid stream through a semi-permeable membrane into another fluid stream. The components are selectively separated from the original solution from one side of the membrane to the other.

A membrane can be defined as a heterogeneous phase acting as a barrier to the flow of molecules and ionic species in liquid or vapor phases. If one component of the mixture travels faster in the membrane, a separation can be achieved. Based on their nature, heterogeneous barrier membranes can be classified into solid and liquid membranes.

Reverse osmosis and electrodialysis are examples of process that use membrane separation.

Crystallization – This is a process where a solid is formed from a liquid solution based on the difference in the solute concentration and its solubility at a certain temperature. In this process the solute transfer occurs from the liquid solution to a pure solid crystalline phase. When the concentration of the solute becomes higher than its solubility at a certain temperature, then the solute comes out of the solution is the form of a crystal.

Salt is extracted from sea water using crystallization.

Drying and Evaporation – Drying is the process of removing a small amount of water or other liquids from a solid material. The water is removed at a temperature below the boiling point of water by circulating air or another carrier gas over the material.  Evaporation is the removal of a large amount of water from solutions. The water is removed as a vapor at its boiling point.

In summary:

Separation Process	Separating Agent	Typical Applications
Adsorption	Adsorbent Solid	Separation of organics from gas.
Gas Absorption & Stripping	Solvent	Removal of CO2 from synthesis gas and CO2 and H2S from natural gas.
Distillation	Heat	Fractionation of crude oil.
Liquid-liquid Extraction	Solvent	Removal of aromatics from gasoline.
Solid-liquid Extraction	Solvent	Extraction of caffein from coffee.
Membrane separation	Membrane	Desalination of water.
Crystallization	Removal of heat	Production of salts and sugar.
Drying	Heat/Drying gas	Drying of fruits and polymer beads.