adsorption & desorption
concepts, processes & industrial applications

Adsorption is the accumulation of molecules or atoms at the surface of an adsorbent, while desorption is their controlled release from that surface. In industrial VOC control and solvent recovery, activated carbon is commonly used as the adsorbent, with regeneration by heat, pressure, vacuum or other process conditions selected according to the application.

Adsorption

VOCs or other adsorbates are retained on the surface and pore structure of an adsorbent such as activated carbon.

Desorption

The adsorbate is released from the adsorbent during regeneration, restoring adsorption capacity for the next cycle.

Activated carbon

A high-surface-area porous adsorbent widely used for industrial VOC adsorption and solvent recovery.

Industrial application

Adsorption and desorption form the core cycle of several VOC emission-control and solvent-recovery systems.

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.

    From adsorption to solvent recovery: in an industrial Solvent Recovery Unit (SRU), VOC-laden process exhaust is passed through activated-carbon adsorbers. Once loaded, the adsorbent is regenerated and the desorbed solvent is recovered, typically through condensation and downstream handling. See the Solvent Recovery Engineering Knowledgebase and DEC.SRU™ for the industrial application.

    Desorption processes and adsorbent regeneration

    There are a number of different desorption (also known as "regeneration") 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 desorption 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 N2), 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 critical: the regeneration loop’s tightness must be upheld to prevent air ingress, reducing safety risks from explosive conditions (oxygen mixed with flammable VOCs at high concentrations and temperatures can exceed the Lower Explosive Limit, LEL). Additionally, the vacuum pump’s fragility requires attention - poor loop integrity or pressure fluctuations can strain or damage it, causing failures that disrupt regeneration, elevate operational hazards, and increase maintenance costs.

    • Adsorption isotherms

    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.

    Why adsorption and desorption matter in industrial VOC control

    Adsorption and desorption are fundamental to the design and operation of industrial adsorption systems. The choice of adsorbent, adsorption conditions and regeneration method depends on the adsorbate, concentration, temperature, pressure, humidity, required capacity and process objective. Activated carbon is widely used for VOC adsorption, including solvent recovery applications. In a DEC.SRU™ Solvent Recovery Unit, adsorption and controlled desorption form the core cycle for recovering solvent from gas-phase process exhaust while controlling VOC emissions.

    Related DEC engineering topics
    Activated carbon Solvent recovery VOC concentration Thermal oxidation Distillation

    Further reading and contact DEC

    For more information on the DEC.GLX™ • glossary of terms & definitions, and how we can support your VOC emission control project, please contact DEC.

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