adsorption and desorption | concepts • DEC.AAS™

adsorption, desorption
concepts

Adsorption is the process of attracting and retaining molecules or atoms on a surface. The molecules or atoms that are attracted to the surface are called adsorbates, and the surface that attracts them is called the adsorbent.

Activated carbon is a type of adsorbent that is made from carbonaceous materials, after being processed to create a high surface area and porosity, which makes it very effective at adsorbing molecules and atoms.

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adsorption processes

Adsorption is the process of attracting and retaining molecules or atoms on a surface. There are two main types of adsorption:

physical adsorption: a weak interaction between the adsorbate and the surface of the solid adsorbent. It is caused by van der Waals forces, which are weak intermolecular forces that occur between all molecules. Physical adsorption is reversible, meaning that the adsorbate can be desorbed from the surface of the adsorbent without breaking any chemical bonds.

chemical adsorption: it is a strong interaction between the adsorbate and the surface of the solid adsorbent, caused by the formation of chemical bonds between the adsorbate and the surface of the solid. Chemical adsorption (also known as chemisorption) is irreversible, meaning that the adsorbate cannot be desorbed from the surface of the adsorbent without breaking chemical bonds.

Van der Waals forces

Van der Waals forces are weak forces of attraction between molecules: they are caused by the temporary uneven distribution of electrons in a molecule, which creates a temporary dipole. This temporary dipole can then interact with the temporary dipoles in another molecule, creating a net attractive force. Van der Waals forces are divided into three types:

London dispersion forces: the weakest type of van der Waals force. They occur between all molecules, regardless of their polarity.

Dipole-dipole forces: stronger than London dispersion forces. They occur between polar molecules.

Ion-dipole forces: the strongest type of van der Waals force. They occur between an ion and a polar molecule.

Van der Waals forces are weak, but they can still be significant being responsible for the attraction between gas molecules and the surface of an adsorbent. This attraction is what allows gas molecules to be adsorbed onto the surface of a solid or liquid.

desorption processes

There are a number of different desorption processes that are used to regenerate the adsorbent (activated carbon) to remove adsorbates and restore its adsorption capacity. The choice of desorption process depends on a number of factors, including the type of adsorbate (VOCs), the desired regeneration rate, and the cost of the desorption process. Most used industrial processes are:

Temperature-swing adsorption (TSA): a process in which the temperature of the adsorbent is varied to control the adsorption of the adsorbate. The adsorbent, once saturated, is first heated to increase temperature (100-220 °C, typically using steam or hot inert gas - nitrogen), which causes the adsorbate to desorb. The desorption stream is then cooled to a lower temperature, which causes the adsorbate to condense and recovered for reuse.

Pressure-swing adsorption (PSA): process in which the pressure of the adsorbate is varied to control the adsorption of the adsorbate. The adsorbent, once saturated, is first placed under a low pressure, which causes the adsorbate to be desorbed. The desorption stream is then cooled to a lower temperature, which causes the adsorbate to condense and recovered for reuse.

Temperature-Vacuum Swing Adsorption (TVSA): a process that combines TSA (delta T) and VSA (delta P). The adsorbent, once saturated, is firstly heated to a high temperature (150-220 °C) using an inert gas (typically nitrogen), then pressure is decreased, which causes the adsorbate to desorb. The desorption stream is then cooled to a lower temperature, which causes the adsorbate to condense and recovered for reuse. Being vacuum is applied, size of the adsorbers and quantity of absorbent shall be limited, restricting the SRU™ design flexibility. Maintenance is also critical: tightness of the regeneration loop shall be kept to avoid entrance of air and reduce safety issues (oxygen and flammable VOCs at high concentration and temperatures, will lead to explosive conditions, see LEL).

adsorption isotherm

An adsorption isotherm is a curve that shows the relationship between the amount of adsorbate adsorbed and the equilibrium concentration of the adsorbate in the gas phase. There are many different types of adsorption isotherms, but the most common are:

Langmuir isotherm: based on the assumption that there is a finite number of adsorption sites on the surface of the adsorbent. The equation for the Langmuir isotherm is:

qe = qm * (K * Ce) / (1 + K * Ce)

where:

qe is the amount of adsorbate adsorbed at equilibrium

qm is the maximum amount of adsorbate that can be adsorbed

K is the Langmuir constant

Ce is the equilibrium concentration of the adsorbate in the surrounding solution

Freundlich isotherm: based on the assumption that the adsorption energy is not constant, but varies with the amount of adsorbate adsorbed. The equation for the Freundlich isotherm is:

qe = K * Ce^(1/n)

where:

qe is the amount of adsorbate adsorbed at equilibrium

K and n are constants

Ce is the equilibrium concentration of the adsorbate in the surrounding solution

Both isotherms are empirical equations (not based on a fundamental understanding of the adsorption process). They are not perfect models of adsorption but they are useful for predicting the amount of adsorbate that will be adsorbed on a surface under certain conditions, but not reliable at high pressures or when the adsorption is very strong. These isotherms do not take into account all of the factors that can affect adsorption, such as the temperature, the surface area of the adsorbent, and the interaction between the adsorbate and the adsorbent.

Despite these limitations, these isotherms are a useful tool for describing adsorption behavior. They are simple to use and can be applied to a wide variety of systems.

conclusions

Adsorption is a complex process that is influenced by a number of factors. The principles of adsorption and desorption, the Van der Waals forces, and the adsorption isotherm are all important concepts for understanding adsorption and for designing adsorption systems. Activated carbon is a versatile material that is used in a wide variety of applications. The understanding of adsorption for activated carbon is essential for the design and operation of adsorption systems, with solvent recovery (SRU™).