Table of Contents
The deposition of molecular species onto the surface is referred to as adsorption. Surface phenomena include physisorption and chemisorption. They are both exothermic in nature. Pressure swing adsorption is a method that separates individual gases from a gas mixture. Desorption is the process of removing adsorbents from the surface of the adsorbate. Adsorption of gases such as hydrogen, nitrogen, and so on the surface of adsorbents such as charcoal is an example of physisorption.
Adsorption:
The deposition of molecular species onto the surface is referred to as adsorption. It is not the same as absorption, which is a physical or chemical process in which atoms, molecules, or ions enter a bulk phase.
The adsorbent is the molecular species that become adsorbed on the surface, while the adsorbate is the surface on which adsorption happens. Clay, silica gel, colloids, metals, and other adsorbents are some examples. Adsorption is thus a surface phenomenon. Desorption is the process of removing adsorbents from the surface of the adsorbate.
Adsorption Mechanism:
It is an exothermic process, which implies that energy is released during it. Enthalpy is the quantity of heat that is released when one mole of adsorbate is adsorbed on an adsorbent. The enthalpy change is indicated as negative. The reason for this is that when adsorbate molecules are adsorbed on the surface, their freedom of movement is constrained, resulting in a decrease in entropy. Adsorption happens spontaneously at constant temperature and pressure.
Types of Adsorption:
Adsorption is typically a surface phenomenon, with the greatest illustration being the drying of air in the presence of silica gel. Water molecules are adsorbed on the gel’s surface during this process.
Adsorption is categorized into two categories based on the material being deposited and adsorbed:
- Physisorption
- Chemisorption
- Physisorption
Physical adsorption, commonly known as physisorption, is an exothermic process. It has a modest adsorption enthalpy, ranging from 20 to 40 kJ/mol. Physisorption normally involves the accumulation of gas on a solid surface owing to weak forces known as Van der Waals forces. Because the adsorbent (the surface or substance on which the adsorption process occurs) on the provided surface does not display any particular gas, physisorption lacks specificity. It is reversible in the sense that the physisorption of a gas by a solid may be reversed by the physisorption of a gas by a solid.
Adsorption of gases such as hydrogen, nitrogen, and others on the surface of adsorbents such as charcoal is a common example of physisorption.
The surface area of the adsorbent influences physisorption. The amount of adsorption rises with increasing surface area. Finely split metals and porous substances, for example, have a huge surface area. As a result, they are regarded as excellent adsorbents. It also relies on the adsorbate’s composition (the accumulation of molecular species or substances at the surface).
- Chemisorption
Chemical adsorption is another name for chemisorption. Adsorption occurs in adsorbed substances bound together by chemical bonds in chemisorption. Chemisorption has a high specificity, which means that it occurs only when there is a chemical connection between the adsorbent and the adsorbate. Chemisorption is irreversible in nature, and it usually prefers high pressure. Chemisorption has a high enthalpy of adsorption due to chemical bonding, ranging from 80 to 240 kJ/mol. At a higher temperature, the physisorption of a gas adsorbed at a lower temperature can be changed to chemisorption.
Chemisorption is proportional to the surface area. Chemisorption increases as the surface area of the adsorbent grow. Adsorption of hydrogen, nitrogen and other gases on the surface of adsorbents such as ferrous catalysts at high temperatures is an example of chemisorption.
Difference Between Physisorption And Chemisorption
| Physisorption | Chemisorption |
| It is due to the formation of van der Waals forces. | It is the result of chemical bond formation. |
| In nature, it is reversible. | In nature, it is irreversible. |
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Nature does not have a preference for physisorption.
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It is very specific in nature. |
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It has a modest adsorption enthalpy of around 20 to 40 kJ/mol.
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Chemisorption has a very high adsorption enthalpy, ranging from 80 to 240 kJ/mol. |
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It prefers low temperatures.
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It prefers high temperatures.
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| Physisorption decreases as temperature rises. | Chemisorption increases as temperature rises. |
| It produces a multimolecular layer. | It produces a unimolecular layer. |
| In physisorption, the activation energy is lower. | Chemisorption has high activation energy. |
Pressure Swing Adsorption (PSA)
PSA, or pressure swing adsorption, is a method that separates individual gases from a gas mixture. PSA is a non-cryogenic air separation method that is widely utilized in industry.
- It is a cost-effective and dependable technology for separating various gases and obtaining extremely high purity levels for them.
- PSA is mostly used in chemical and petrochemical operations.
- It is employed in the recovery of hydrogen from coking or conversion gases, as well as the separation of oxygen and nitrogen from air.
- Under pressure, gases are separated depending on their molecular properties and affinity for an adsorbent substance.
- It operates at near-ambient temperatures, as opposed to cryogenic distillation techniques of gas separation, which operate at extremely low temperatures.
- Under high pressure, certain adsorbent materials (e.g., zeolites, activated carbon, etc.) preferentially absorb the target gas species as a trap. The process subsequently turns to low pressure to desorb the adsorbed material.
- Adsorption occurs when gas molecules attach to an absorbent substance.
- The adsorbent bed is chosen specifically for the gas to be absorbed.
- Only the gas to be separated should be adsorbed, while all other gases in the mixture pass through the adsorbent bed.
The PSA method is employed in a variety of applications across many sectors. It is employed in the recovery and purification of hydrogen, the manufacture of oxygen, the removal and purification of carbon dioxide, the recovery of helium, the generation of nitrogen, the recovery of methane, and other processes. It is utilized in the chemical and petrochemical sectors, as well as the iron and steel industries and refining.
Working Principle of Pressure Swing Adsorption
First, air from the surrounding environment is compressed into high-pressure air. This gas is then transported to a tank or column containing the adsorbent material (activated carbon, zeolite, etc.). The adsorbent used is determined by the gas to be extracted. This system is then cyclically pressurized and depressurized, with the low sorbing gas leaving the column first, followed by the other gases.
The pressure swing adsorption process comprises four major phases:
- Adsorption:
Adsorption begins with the adsorber being pressured with pure gas. The polluted gas is introduced into the column containing the adsorber. Adsorption occurs, and the pure gas is discharged from the column’s top. This continues until the adsorber’s adsorption capacity is achieved.
- Depressurization:
The adsorber is depressurized in modest increments to recover any remaining pure gas in the adsorber. The desorbed impurities are deposited into the PSA off-gas line once all pure gas has been collected.
- Regeneration:
To further renew the adsorbent bed, the adsorbent is purged with high-purity gas at constant off-gas pressure.
- Repressurization:
The adsorber is repressurized with pure gas and is now ready to receive more feed gas to restart the operation.
FAQs
What are the similarities between physisorption and chemisorption?
Surface phenomena include physisorption and chemisorption. They are both exothermic in nature. They both rise as the surface area increases. And both are related to a decrease in entropy.
Is there a difference between adsorption and absorption?
Absorption and adsorption are fundamentally distinct processes that should not be confounded. During the adsorption process, a liquid or gas collects on a liquid or solid surface and creates an atomic or molecular layer. However, during the absorption process, a substance diffuses into a solid or liquid to produce a solution (i.e., molecules undergoing resorption are taken up by the volume, not by the surface). During the absorption process, a substance gathers and transforms energy, which can cause metals to corrode.
What is Surface Chemistry?
Surface chemistry refers to the chemical reactions or chemical changes that occur on the plane's two surfaces. In surface chemistry, the interface of two phases can be solid-gas, solid-liquid, liquid-gas, and so on. It is entirely dependent on the compounds or components that are involved in the reaction or changes. It has a wide range of uses, including analytical work, the medical profession, the paint business, and so on. It includes a wide range of processes like absorption, catalysis, colloids, emulsions, and many more.
