No one is perfect… that’s why pencils have erasers.
The same is true for reactions. They are not perfect in the sense that we do not always get a 100% yield. The reasons for this are:
- side reactions
- excess raw material
- loss of reactants through by-product formation/charring due to their sensitivity towards operating conditions
- some of the reactants go unreacted
To solve these problems, one may use solvents. But then the product obtained would still be in a diluted state. This is why we need separation.
Various separation processes exist in an industry and depending on the applicability, one process is chosen over the rest. Some of these processes are:
- Evaporation (e.g. recovering salts from solution)
- Absorption (e.g. separation of NH3 from a mixture)
- Crystallization (e.g. purification of solid compounds)
- Distillation (e.g. separation of crude oil into fractions)
- Chromatography (e.g. analysis in the lab)
- Filtration (e.g. desalination)
- Settling (e.g. waste-water treatment)
One can separate components of a mixture depending on the following properties:
- Density (e.g. gravity separation)
- Magnetic property/polarity (e.g. separation of minerals)
- Boiling point/Melting point/Vapor pressure (e.g. distillation)
- Viscosity/Solubility (e.g. separation of N2 and O2 can be done by absorption of N2 in a liquid as O2 leaves)
- States of material (e.g. again – separation of N2 and O2 can be done by absorption of N2 in a liquid as O2 leaves)
Applications in water purification
With increase in the number of water-stressed regions such as India, the need for water purification is more than ever. Equally important are small scale and the large scale water purification systems. Small scale systems include the portable water purification systems such as SODIS that uses solar energy to disinfect disease causing biological agents or a homemade waterfilter. Disinfection is one of the many steps involved in the purification process. Large scale systems includes an array of processes such as pre-treatment, sedimentation, filtration, disinfection, desalination and many more that require bigger assemblies.
Recently, DOW Technology helped the Largest Desalination Plant in Spain operating with pressurized ultrafiltration to deliver freshwater for municipal use. Ultrafiltration is one of the advanced separation processes, which is a type of membrane filtration. Apart from water purification, such systems are used in industry was various reasons. Ultrafiltration is used for concentrating target molecules, clarification needed in wastewater treatment processes, desalination such as that used in the plant in Spain or for fractionation of peptides in dilute samples.
In a chemical industry or a chemical laboratory, separation processes are required before and after each stages. For example, before entering the system, the raw materials are purified. After reaction, the process may give out materials in different phases -gas/liquid/solid. These have to be separated so as to reach the recycle stream and the effluent stream. The effluent stream again has to undergo recovery of certain substances before it reaches the environment.
Treatment of industrial waste is growing evidently stringent as time passes by. This requires such separation processes which aid pollution prevention. In fact, the entire waste water treatment mechanism is based on physical separation of effluent entities.