UNIT OPERATIONS: TYPES AND EXAMPLES - CHEMISTRY - 2024

The unit operations are those that involve physical treatments to the feedstock in order to obtain the desired product from this. All of these operations obey the laws of conservation of mass and energy, as well as momentum.

These operations facilitate the transport of the raw material (this in liquid, solid or gaseous state) towards the reactors, as well as its heating or cooling. They also promote the effective separation of a specific component from a product mix.

Water dams are examples of unit operations

Unlike unitary processes that transform the chemical nature of matter, operations seek to modify its condition through the gradient of one of its physicochemical properties. This is achieved by generating a gradient in mass, energy, or quantity movement.

Not only in the chemical industry there are countless examples of these operations, but also in the kitchen. For example, whipping a portion of liquid milk produces cream and skim milk.

On the other hand, if an acid solution (citric acid, vinegar, etc.) is added to this same milk, it causes the denaturation of its proteins, this being a process (acidulation) and not a unit operation.

Types of unit operations

Material transfer operations

Unit operations of this type transfer mass through a diffusion mechanism. In other words: the raw material is subjected to a system that generates a variation in the concentration of the component to be extracted or separated.

A practical example is considering the extraction of a natural oil from some seeds.

Because the oils are essentially apolar in nature, they can be extracted with an apolar solvent (such as n-hexane), which bathes the seeds but does not react (theoretically) with any of the components of their matrix (shells and nuts.).

Heat transfer operations

Here, heat is transferred from the body that is warmer to the body that is colder. If the raw material is the cold body and it is essential to raise its temperature to, for example, lower its viscosity and facilitate a process, then it is put into contact with a hot flow or surface.

However, these operations go beyond a "simple" transfer of heat, since energy can also be transformed into any of its manifestations (light, wind, mechanical, electrical, etc.).

An example of the above can be seen in hydroelectric plants, where water currents are used to generate electricity.

Mass and energy transfer operations simultaneously

In this type of operation, the two previous phenomena occur at the same time, transferring mass (concentration gradient) to a temperature gradient.

For example, if sugar is dissolved in a pot of water and then the water is heated, the crystallization of the sugar occurs when it is slowly cooled.

Here a transfer of the dissolved sugar to its crystals occurs. This operation, known as crystallization, allows obtaining solid products with a high degree of purity.

Another example is the drying of a body. If a hydrated salt is subjected to heat, it will release the water of hydration in the form of steam. This again produces a change in the mass concentration of the water in the salt as the salt increases in temperature.

Examples

Distillation

Distillation consists of the separation of the components of a liquid mixture based on their volatilities or boiling points. If A and B are miscible and form a homogeneous solution, but A boils at 50 ° C and B at 130 ° C, then A can be distilled from the mixture through simple distillation.

The image above represents a typical setup of a simple distillation. At industrial scales, distillation columns are much larger and have other characteristics, which allow the separation of compounds with very close boiling points (fractional distillation).

A and B are in the still bottle (2), which is heated in an oil bath (14) by the heating plate (13). The oil bath guarantees a more homogeneous heating throughout the body of the ball.

As the mixture increases its temperature around 50 ° C, the vapors of A escape and generate a reading on the thermometer (3).

Then, the hot vapors from A enter the condenser (5) where they are cooled and condensed by the action of the water that circulates around the glass (enters at 6 and leaves at 7).

Finally, the collecting balloon (8) receives condensed A. It is surrounded by a cold bath to prevent possible leakage of A to the environment (unless A is not very volatile).

Absorption

Absorption allows the separation of harmful components from a gaseous stream that is subsequently released into the environment.

This is accomplished by passing the gases into a column filled with liquid solvent. Thus, the liquid selectively solubilizes harmful components (such as SO ₂, CO, NO _x and H ₂ S), leaving the gas that emerges from it “clean”.

Centrifugation

In this unitary operation, the centrifuge (instrument in the upper image) exerts a centripetal force that exceeds thousands of times the acceleration of gravity.

As a result, the suspended particles settle to the bottom of the tube, facilitating the subsequent decantation or sampling of the supernatant.

If centripetal force did not operate, gravity would separate the solid at a very slow speed. Also, not all particles have the same weight, size, or surface area, so they do not settle into a single solid mass at the bottom of the tube.

Sifting

Like absorption, adsorption is useful in the purification of liquid and solid streams. However, the difference is that the impurities do not penetrate into the adsorbent material, which is a solid (like the bluish silica gel in the image above); instead it sticks to its surface.

Likewise, the chemical nature of the solid is different from that of the particles it adsorbs (even if there is a great affinity between the two). For this reason, adsorption and crystallization - the crystal adsorbs particles to grow - are two different unit operations.

References

1. Fernández G. (November 24, 2014). Unit operations. Retrieved on May 24, 2018, from: industriaquimica.net
2. Carlos A. Bizama Fica. Unit Operations: Unit 4: Types of Unit Operations.. Retrieved on May 24, 2018, from: academia.edu
3. Course: Chemical Technology (Organic). Lecture 3: Basic Principles of UnitProcesses and Unit Operations in Organic Chemical Industries.. Retrieved on May 24, 2018, from: nptel.ac.in
4. Shymaa Ali Hameed. (2014). Unit Operation.. Retrieved on May 24, 2018, from: ceng.tu.edu.iq
5. RL Earle. (1983). Unit Operations in Food Processing. Retrieved on May 24, 2018, from: nzifst.org.nz
6. Mikulova. (March 1, 2008). Slovnaft - New polypropylene plant.. Retrieved on May 24, 2018, from: commons.wikimedia.org
7. Rockpocket. (March 13, 2012). Thermo centrifuge.. Retrieved on May 24, 2018, from: commons.wikimedia.org
8. Mauro Cateb. (2016, October 22). Blue silica gel.. Retrieved on May 24, 2018, from: flickr.com