What are unit operations?
When in the chemical industry or related areas you want to prepare or synthesize a product from raw material through a long and complicated process, it is convenient to divide this process into simple individual stages that are easy to analyze separately. These individual stages that involve a single change or transformation are called unit operations.
A unit operation can involve both physical and chemical changes. These changes can be very varied, ranging from something as simple as the drying of a solid product to a controlled catalyzed polymerization reaction in a chemical reactor.
Chemical engineering is based on unit operations. In fact, the way in which these are related to each other and the equipment used in them, provide the basis for the design, construction, and control of chemical plants.
Types of unit operations
Unit operations are very varied and can be classified according to four different criteria. These are:
- The type of transport that govern them.
- According to its nature.
- Your work regime.
- The end they pursue.
These criteria are described below:
1. Classification according to the type of transport that govern them
This is the main classification criterion used by chemical engineers. Unit operations seek to modify the input materials to obtain what is wanted in the output.
This implies either modifying its mass or composition, which implies a mass transfer; modifying its level of quality of energy, which implies energy transfer; or modifying its speed or direction of movement, which implies momentum transfer.
According to which or which of these types of transport control the unit operation, these can be classified into:
Matter transfer operations
These are those in which there is a mass transfer to or from the substances involved in the process. These unit operations include:
- Both solid-liquid and liquid-liquid extraction processes.
- The rectification processes.
- Absorption and desorption.
- Adsorption and desorption.
Power transfer operations
These operations include all processes in which heat is transferred to or from the system, but without loss or gain of matter. In other words, there is only one input and one output current, and a physical change may or may not occur. These unit operations include:
- Heat exchange without phase or state change.
- Evaporation and condensation processes.
- The sublimation and deposition processes.
- Fusion and solidification processes.
Mass and energy transfer operations
In these unit operations, the transfer of energy, generally in the form of heat, is used to carry out the separation or union of several phases, which is why a mass transfer also occurs. Some examples of energy and mass transfer operations include:
- Humidification and dehumidification.
- Freeze drying.
Momentum Transfer Operations
These unit operations are those that involve moving a body from one place to another without carrying out any other transformation. These types of operations include:
- The circulation of fluids.
- The transport of solids.
These are additional operations to those already mentioned and they do not fit well in any of the other classes. Some examples are:
- Circulation of fluids through porous beds.
- Agitation of liquids.
2. Classification according to its nature
According to this criterion, unit operations can be classified into:
Physical unit operations
These unit operations involve any physical change in which the nature of the substances involved remains constant. That is, no chemical reactions of any kind occur in them.
Chemical unit operations
As the name implies, these unit operations involve the occurrence of chemical reactions that transform the chemical substances of the raw material into other different substances. These unit operations are the essence and the raison d’être of the chemical industry.
Biological unit operations
They are similar to chemical unit operations, but the only difference is that chemical reactions are carried out by living biological organisms such as bacteria, fungi, and yeast.
3. Classification according to your work regime
In addition to the above classification, both physical, chemical, and biological unit operations can also be separated into two types depending on whether or not they are carried out continuously:
Batch unit operations
They are those operations in which production or transformation occurs in discrete sequential steps. They are generally used in small plants or when looking for fine or high purity products.
Both physical, chemical, and biological unit operations can be carried out in batches. In the case of chemical operations, generally involve the use of a closed reactor into which the raw material is added, closed to carry out the chemical reaction for a preset period of time, and then opened to remove the products.
Continuous unit operations
They are those that are carried out uninterruptedly. In these cases, both the feeding of raw material and the removal or elimination of the product are done continuously and hence its name.
4. Classification according to the purpose they pursue
Finally, different types of unit operations can be distinguished according to their purpose. In this case, 3 main categories can be distinguished:
These include the operations of mixing two or more substances or phases to obtain a single final phase that is as homogeneous as possible.
They correspond to operations in which chemical transformations are carried out.
They are generally used to purify reagents prior to a reaction or to separate the products from unreacted reagents, solvents (or moisture), or any impurities prior to the packaging process.
Examples of unit operations
It is a unit operation that involves the transfer of heat and matter, which consists of the separation of liquid mixtures based on differences in their volatility (for which they also have different boiling points ). The liquid mixture is heated to its boiling point at a preset pressure.
The vapor phase will be richer in the more volatile components, so by re-condensing, it is possible to separate these from the less volatile substances that remain at the bottom of the still.
There are different types of distillation with different degrees of complexity to separate different types of mixtures:
- Simple distillation: consists of a single vaporization stage and a single condensation. It is used when the volatility differences are large or when one of the components is much more volatile than the others.
- Fractional distillation: it is basically a technique in which many small and staggered distillations are carried out in which the components are progressively separated into different fractions. It is used when the vapor pressures of the components are similar.
- Vacuum distillation: Vacuum lowers the boiling point of liquids, allowing them to be distilled at a lower temperature. This is desirable when there is a risk of decomposition at high temperatures. It also serves to separate azeotropic mixtures such as some ethanol-water mixtures.
- Steam distillation: It consists of bubbling steam in a mixture of liquids in which one of which is slightly volatile and insoluble in water. Water vapor literally drags the vapor of said liquid and when condensing they separate into two phases. It is also used to distill thermally unstable organic liquids below their normal boiling point.
It is a unit mass transfer operation in which the solvent (often water) that soaks up a solid is transferred by evaporation to the gas phase to be eliminated, leaving the solid drier than before.
Drying can be carried out by passing a stream of the wet solid through a drying tunnel in which hot dry air is passed counter currently. In this case, it is a continuous drying process.
It can also be carried out on trays that are placed in closed cabinets through which hot air is passed. In this case, we are in the presence of batch drying.
Finally, there are also rotary dryers and pressure dryers, as well as spraying in a stream of hot air. The latter is used in some cases for the preparation of instant coffee.
It is a surface phenomenon in which a dissolved substance or in the gaseous phase adheres to the surface of a solid. Solids are porous adsorbent materials with very high surfaces that allow the adsorption of large amounts of solute or gas. Activated carbon is a common example of an adsorbent used both in the laboratory and in industry.
An example of the application of adsorption on activated carbon is in the decaffeination process of coffee and tea.
Absorption consists of dissolving a component of a gaseous mixture in a liquid. It is a unit mass transport operation that is widely used to remove pollutants such as carbon dioxide or hydrogen sulfide from the air before it is discharged into the environment.
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