These operations facilitate the transport of the raw material (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.
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 done by generating a gradient of mass, energy or momentum.
There are countless examples of these operations, not only in the chemical industry, but also in the kitchen. For example, whipping a portion of liquid milk gives cream and skim.
On the other hand, if an acidic solution (citric acid, vinegar, etc.) is added to the same milk, it causes the denaturation of its proteins, which is a process (acidification) and not a unitary operation.
Types of unit operations
Matter transfer operations
Such unit operations transfer mass through a diffusion mechanism. In other words: the raw material is submitted to a system that generates a variation in the concentration of the component to be extracted or separated.
A practical example is to consider extracting a natural oil from some seeds.
As oils are essentially non-polar in nature, they can be extracted with a non-polar solvent (such as n-hexane), which bathes the seeds but does not (theoretically) react with any of their matrix components (shells and nuts). )
heat transfer operations
Here, heat is transferred from the warmer body to the cooler body. If the raw material is the cold body and it is essential to raise its temperature, for example, to reduce its viscosity and facilitate a process, it is subject to contact with a hot flux or surface.
However, these operations go beyond a “simple” heat transfer, as 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) before a temperature gradient.
For example, if sugar is dissolved in a pan of water and the water is heated, when allowed to cool slowly, crystallization of the sugar occurs.
Here a transfer of the dissolved sugar to its crystals takes place. This operation, known as crystallization, allows to obtain solid products with a high degree of purity.
Another example is drying a body. If a hydrated salt is subjected to heat, it will release the hydrating water as steam. This again produces a change in the mass concentration of water in the salt as its temperature increases.
Distillation consists of separating 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 by simple distillation.
The image above represents a typical setup for a simple distillation. On industrial scales, distillation columns are much larger and have other characteristics, which allow the separation of compounds with boiling points very close to each other (fractional distillation).
A and B are in the still flask (2), which is heated in an oil bath (14) by the heating plate (13). The oil bath guarantees a more homogeneous heating throughout the entire body of the ball.
As the mixture increases its temperature around 50 °C, the vapors A escape and generate a reading on the thermometer (3).
Then, the hot vapors A enter the condenser (5), where they are cooled and condensed by the action of water circulating around the glass (entering through 6 and exiting through 7).
Finally, the collector ball (8) receives A. condensed. It is surrounded by a cold bath to prevent possible leakage of A into the environment (unless A is not very volatile).
Absorption allows the separation of harmful components from a gas stream that is later released into the environment.
This is done by passing the gases into a column filled with liquid solvent. Thus, the liquid selectively solubilizes the harmful components (such as SO 2 , CO, NO x and H 2 S), leaving the gas that emerges from it “clean”.
In this unitary operation, the centrifuge (instrument in the image above) exerts a centripetal force that exceeds the acceleration of gravity thousands of times.
As a result, suspended particles settle to the bottom of the tube, facilitating subsequent decantation or sampling of the supernatant.
If centripetal force didn’t work, gravity would separate the solid at a very slow speed. Also, not all particles have the same weight, size or surface area; therefore, they do not settle into a single solid mass at the bottom of the tube.
Sieving consists of separating a solid and heterogeneous mixture, depending on the size of its particles. Thus, small particles will pass through the sieve (or sieve) openings, while large particles will not.
Like absorption, adsorption is useful in purifying liquid and solid streams. However, the difference is that the impurities do not penetrate 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 absorbs (even if there is a strong affinity between them). For this reason, adsorption and crystallization – the crystal adsorbs particles to grow – are two different unit operations.