Unit Operations

A process that does not include any chemical reaction is known as a unit operation. Unit operations deal with only physical changes in the substances involved in the process. They are equipment that produces physical changes in materials.

Physical alterations are made for a variety of reasons. In general, unit operations steps are performed before putting materials into chemical reactions to ensure that chemical reactions go smoothly.

Physical changes can include evaporation, condensation, crystallization, and other phase changes. Because condensation and evaporation take place inside the column, distillation is a unit operation process. Crystallizers and evaporators are also part of the unit operations.

Mechanical processes such as physical separations, size reduction, grinding, and mixing are also performed using unit operation equipment. The mass transfer and heat transfer processes may happen together without any chemical process.

The unit operations are classified in the following manner [1]:

  • Fluid flow operations: Fluidization, compression, pumping, etc.
  • Mechanical operations: Size reduction, mixing agitation, size enlargement, blending, classification-separation, filtration, etc.
  • Mass transfer: Distillation, crystallization, evaporation, leaching, adsorption, absorption, extraction, etc.
  • Heat transfer: When materials are handled, heat transfer can occur by any fundamental mechanism; convection, conduction, or radiation. Usually, two fundamental mechanisms occur simultaneously.

1 Distillation

In process engineering, the controlled boiling and condensation of a constituent in a liquid mixture is termed distillation. It is a separation technique that may either increase the concentration of a certain component in a mixture or extract (almost) pure constituents from it. When two liquids have different boiling points, distillation takes advantage of this by forcing one of the liquids into the gaseous state

On a lab scale, liquid mixtures are commonly distilled in batches, but industrial distillation operations are usually continuous, requiring a consistent mixture composition to be maintained.

Dalton and Raoult’s Laws in Distillation

For the boiling of a liquid, it needs to raise the temperature of the liquid to the point where vapor pressure becomes equal to the ambient pressure. At the boiling temperature, vapor bubbles begin to develop at the bottom of the vessel, which causes the liquid to evaporate.

The distillation process for a combination of liquids is governed by Raoult and Dalton laws. Raoult’s law states that an ideal combination of liquid components has a single component with the same molecular weight as the other components and the same vapor pressure [2]. Using the law of partial pressure, which was developed by William Dalton, we can calculate the overall pressure exerted by a gas combination [3].

The vapor pressure of the different components increases when a mixture of liquids is heated, resulting in an increase in the overall vapor pressure. As a result, the combination cannot have numerous boiling points with a given composition and pressure.

Impossibility of Complete Purification of a Component from Mixture by Distillation

When a combination of liquids reaches its boiling point, all volatile constituents boil. The amount of a component in the resulting vapor, on the other hand, is measured by its contribution to the overall vapor pressure of the mixture. As a result, chemicals with greater partial pressures can have a higher fraction in the vapor phase, whereas those with lower partial pressures can have a high fraction in the liquid phase.

It is challenging and almost impossible to extract a perfectly pure component from a combination by distillation because a component in the liquid mixture cannot comprise zero partial pressure. When one of the components in the combination has a partial pressure near zero, high purity components can be obtained.

Types of Distillation

The following are some of the most important distillation types:

1.1 Fractional Distillation

Fractional distillation is commonly utilized for liquids mixture with comparable boiling points. Several vaporization-condensation stages are involved (which take place in a fractional column). This is also referred to as the correction.

When the liquid mixture is put under heat, it evaporates and rises into the fractionating column. The vapors are then condensed by cooling on the walls of the condenser. The heated vapors from the distilling unit can heat the condensed vapor again, resulting in the formation of fresh vapors, and some fractions of these vapors are sent back to the distillation column. There are many of these vaporization-condensation cycles, and the quality of the distillate increases with each one.

1.2 Steam Distillation

When a liquid mixture contains components that are sensitive to heat, steam distillation is often employed to separate them. This is operationalized by passing steam through the slightly heated mixture to evaporate part of it. This approach produces a high heat-transfer rate without necessitating high temperatures. Condensation of the resulting vapor gives the requisite distillate. Steam distillation is a process for isolating essential oils and herb distillates from a range of aromatic plants and flowers.

1.3 Vacuum Distillation

In order to separate desired components from a liquid mixture with high boiling points, vacuum distillation is ideal. Boiling these substances at high temperatures is an inadequate method of separating them. As a need, the external pressure is reduced. The component can boil at lower temperatures with reduced external pressure. When the vapor pressure of a component equals the ambient pressure, it transforms into a vapor. Condensing the vapors and collecting them produces distillate. The vacuum distillation technique can also be utilized to acquire high-purity compounds that deteriorate at high temperatures when heated for a longer duration [4].

1.4 Air-Sensitive Vacuum Distillation

The vacuum distillation technique is used for substances that are sensitive to air and easily react with it, but the vacuum must be substituted with an inert gas after the procedure is over [5]. This approach is known as air-sensitive vacuum distillation.

1.5 Short Path Distillation

Short path distillation is being used to purify a little amount of a volatile molecule at high temperatures. This type of distillation is performed at lower pressures, and the distillate usually travels a short distance before becoming collected (thus the name “short path”). Because the distillate travels a shorter distance, the equipment walls experience a lesser rate of loss.

1.6 Zone Distillation

Zone distillation partly compels a substance and condenses the vapors to obtain a pure distillate. With the use of a zone heater, this is accomplished in a long vessel.

1.7 Important Applications

Many water purification procedures rely heavily on distillation. Several desalination facilities use this approach to get drinking water from saltwater. Distilled water is utilized in a variety of applications, including lead-acid batteries and low-volume humidifiers. This method is used to cleanse a wide range of fermented products, including alcoholic beverages.

Distillation is used to make several fragrances and food flavorings from plants and herbs. Oil stabilization is a sort of distillation that lowers the vapor pressure of crude oil, allowing it to be safely stored and transported.

The cryogenic distillation method may be used to separate air into nitrogen, oxygen, and argon. On a larger scale, distillation is used to purify liquid products derived through chemical synthesis.

2 Extraction

Extraction is a basic method for isolating a single chemical from a mixture. Liquid/liquid, liquid/solid, and acid/base (also known as chemically active extraction) are the three most prevalent extraction methods [6].

2.1 Liquid-Liquid Extraction

Two immiscible liquids are used in a liquid/liquid extraction. Immiscible liquids do not dissolve in one another; instead, they form layers when put in the same vessel. Immiscibility occurs when two liquids with opposite polarity mix. Diethyl ether or ether and water are the most commonly utilized extraction solvents. Polarity is a subjective concept; ether is nonpolar, whereas water is polar. Two phases appear when one is added to the other due to their distinct polarity. The density of an extraction solvent determines its position (top or bottom layer). Because ether has a density of 0.71 g/cm3 and H2O has a density of 1.0 g/cm3, ether is always the upper phase when the extraction solvent combination is ether and water.

2.2 Solid-Liquid Extraction

The soluble compounds of a solid matter, consisting of an inert matrix and an active agent, are recovered by a solvent in the solid-liquid extraction principle. The extract can be incorporated in either solid or liquid form in the extraction materials. It can be found in cells, such as oil in seeds, or as a thin dispersion over solid substances, such as caffeine in coffee.

2.3 Acid-Base Extraction

Acid-base extraction is also referred to as liquid-liquid extraction. It generally involves different degrees of water solubility and an organic solvent. As an organic solvent, any carbon-based liquid that does not dissolve well in water can be employed; prominent examples include ethyl acetate, ether, and dichloromethane [7].

The acid-base extraction technique is often dependent on the acid-base characteristics of the constituents. The approach is based on the idea that most organic substances are more soluble in organic solvents than in water. When an organic compound becomes ionic, it becomes more water-soluble than it was in the organic solvent. Adding or removing a proton (to form an H+ ion) will easily transform these substances into ions (a negative ion).

3 Gas Absorption and Desorption

The action of contacting a gas mixture with a liquid to selectively dissolve one or more gas mixture components and form a solution for them in the liquid is known as gas absorption (also known as scrubbing).

As a result, the mass transfer of a gas component from the gas phase to the liquid phase may be observed. The solute is said to be absorbed by the liquid.

Gas desorption (or stripping) involves mass transfer from the liquid to the gas phase rather than the reverse. Both systems have the same basic concepts.

Therefore, the diffusion of solute from the gas phase through a stationary or non-diffusing liquid is a part of the gas absorption process.

Chemical vs. Physical Absorption

Chemical and physical adsorption are the two types of absorption processes, depending on whether there is a chemical or physical interaction between the solute and the solvent (absorbent).

Physical absorption happens when water or hydrocarbon oils are utilized as absorbents since there are no major chemical interactions between the absorbent and the solute.

A quick and irreversible neutralization process occurs in the liquid phase when a strong base is used as an absorbent to dissolve an acid gas. Processes such as chemical or reactive absorption are other names for the same phenomenon.

Processes for absorbing H2S and CO2 using an aqueous solution of monoethanolamine (MEA), diethylene glycol (DEG), diethanolamine (DEA), or trimethylene glycol (TEG), in which a reversible chemical reaction occurs in the liquid phase, are considered complicated types of chemical absorption. Chemical reactions can improve the rate of absorption, the solvent’s absorption capacity, and selectivity, allowing just particular components of the gas to be dissolved and transforming a harmful chemical into a harmless product.

4 Crystallization

When a solution crystallizes, a solid phase separates from the liquid phase. The dispersion phase, which comprises many solid particles, acts as the final product, ensuring that the requirements are satisfied. The process of crystallization, then, maybe thought of as one of generating solids. In order to meet customers’ ever-increasing demands on particulate properties such as distribution of particle size, crystal form, caking behavior, degree of coagulation, and purity, the crystallization process is meticulously regulated and monitored. Additional filterability and washability criteria might be implemented because the particles must be easily removed from the mother liquid.

Solid particle creation is a lengthy process because the solid phase’s most rigid structure and large vessels are usually required to obtain an acceptable production rate. This stiff structure prevents extraneous components or solvent molecules from entering the system, resulting in a pure solid product only after one separation stage.

Cooling or evaporative crystallization, precipitation, and melt crystallization are all referred to as crystallization. However, the three forms of crystallization differ significantly in terms of processing procedures and associated equipment. Solute and antisolvent are dissolved in a solvent, precipitating the solid phase by mixing the two input streams. As a result, the hydrodynamics of the process has a considerable impact on precipitation in terms of product quality.

Melt crystallization takes advantage of the capability of crystallization to generate a pure product, and the solid phase is remolten to achieve the final product. The uses are mostly in the ultra-purification of organic molecules or as a concentration technique for producing pure water.

5 Membrane Separation Processes

Membrane separations are separate kinds of unit operations. A semi-permeable membrane that separates components of a mixture or solution by selectively directing their migration from one side of the membrane to the other is used in the membrane process to partly separate a feed containing a mixture of two or more components. Particles or molecules bigger than the membrane pores are rejected on the upstream side in certain circumstances, while in others, they flow through the membranes at varying speeds based on their molecular weights or volumes, allowing separation

5.1 Classification of Membrane Processes

Membrane processes are divided into three categories of pressure-driven membrane filtration [8]:

  • Ultrafiltration (UF) is used to separate organics having molecular weights of more than 5000.
  • Nanofiltration (NF) separates divalent salts and organics while allowing monovalent salts like NaCl to permeate.
  • Microfiltration (MF) is used to separate colloidal and micron-sized particles and microorganisms.
  • Reverse Osmosis (RO) is used for highly effective salt separation, including NaCl and soluble organics.

All of these processes are triggered by external pressure. A particular transmembrane pressure is required to enable the water to flow through the membrane. Membranes having a characteristic porous structure are utilized in microfiltration and ultrafiltration operations. Surface filtering is the most common separation process in MF/UF (straining, size exclusion). Particulate materials, such as inorganic particles, germs, and viruses, may be removed via these membrane mechanisms.

Dense membranes with smaller holes are utilized in reverse osmosis and its low-pressure counterpart nanofiltration. Dissolution and diffusion are used to move molecules across the membrane (or at least, that is how it works). Particles and dissolved inorganic or organic contaminants may be removed from water using reverse osmosis and nanofiltration membranes. It should be emphasized that distinguishing between porous and nonporous (dense) membranes is difficult and mostly a matter of terminology. Nanofiltration membranes, in particular, occupy a middle ground. Nanofiltration membranes have a size-exclusion effect, while reverse osmosis membranes may remove practically all dissolved species. Small molecules and ions flow through the membranes, whereas larger ions and molecules are trapped. Furthermore, surface charges on nanofiltration membranes are common, especially negative ones induced by ionized functional groups like carbonic acid groups. Electrostatic effects also have an impact on separation efficiency in this scenario. Negatively charged ions are rejected owing to repulsion forces, while positive ions are denied on both sides of the membrane to preserve electroneutrality. As a result, the size and charge of the solutes impact the rejection. As a result, nanofiltration membranes selectively reject bigger bivalent ions (Ca+2, Mg+2, SO2-4). Desalination is commonly accomplished using reverse osmosis, whereas sulfate ions, hardness, and bigger organic molecules with a molecular mass of more than 150 g/mol are removed through nanofiltration.

6 References

[1]        McCabe, W., Smith, J., and Harriott, P. (2004). Unit Operations of Chemical Engineering (7th edition)(McGraw Hill Chemical Engineering Series). McGraw Hill.

[2]        Schmitz, K. S. (2017). Raoult’s Law ScienceDirect. https://doi.org/10.1016/b978-0-12-386537-3.00011-3

[3]        Legg, R. (2017). Dalton’s Law of Partial Pressure – an overview. ScienceDirect https://doi.org/10.1016/b978-0-7506-1713-0.50009-3

[4]        Hickman, K. (1945). Adventures in vacuum chemistry. American Scientist, 33(4), xxx-231.

[5]        Komiya, S., Tatsumi, K., Mashima, T., Ito, T., Hurano, M., Fukuoka, A., Ozawa, F., Maruoka, K., Miyaura, N., and Takeda, T. (1997). Manipulation of Air-sensitive Compounds. Synthesis of Organometallic Compounds, 35-55.

[6]        Woerfel, J. B. (1995). Extraction. In Practical handbook of soybean processing and utilization (pp. 65-92). Elsevier.

[7]        Schaller, C. (2021). Acid-Base Extraction. https://chem.libretexts.org/Ancillary_Materials/Demos_Techniques_and_Experiments/General_Lab_Techniques/Acid-Base_Extraction

[8]        Gwak, G., and Hong, S. (2018). Pressure-Driven Membrane Process ScienceDirect https://doi.org/10.1533/9780857093790.5.680