Activated carbon filters for the abatement of odours, VOCs and VICs

In activated carbon filters, the polluting molecules are removed through the principle of adsorption: a chemical-physical phenomenon that consists of the accumulation of one or more fluid substances on the surface of a condensate. With this technology, the airborne substances in the effluent are in fact trapped inside the solid adsorbent surface (the activated carbon), thus significantly reducing the polluting molecules which are in the flow to be treated.
The physical forces that make this phenomenon possible are called Van-der-Waals forces. However, in some cases, they are not sufficient to ensure the bond between the pollutants and the activated carbons as some molecules are too volatile to remain trapped in the cavities. For this reason, chemical bonding is necessary.
In the next section, we will explore this concept in detail by looking at the activated carbon activation mechanisms.

Activated carbons are amorphous materials deriving from carbon-rich raw materials such as wood, peat and coconuts. These materials are characterised by a porous structure and a high specific surface area, which allow several polluting molecules to be trapped within the pores. Two main methods are used to make carbons suitable for the adsorption of pollutants:

• physical activation: it takes place at temperatures between 400 and 600 °C in an oxygen-free environment. This process removes any impurities and allows the creation of “active sites” in which pollutants are trapped;
• chemical activation: it is based on the dehydrating action of compounds such as phosphoric acid and potassium hydroxide. The surface treatment allows the creation of “chemically activated sites” in which the molecules that are too volatile to remain bound to the active sites are also trapped. Carbons of this type are defined as “impregnated”.

These activation mechanisms make activated carbons extremely versatile and effective in removing a wide range of substances. Because of their chemical-physical characteristics, organic pollutants with particular activated groups such as -S- and -N- and their corresponding inorganic pollutants require impregnated activated carbons. In general, the following applies:

When designing an activated carbon adsorber, it is therefore essential to know the characteristics of the molecules to be removed, so as to be able to choose the most suitable carbons and the minimum equipment to install in order to minimise the risks which are intrinsic to the technology.
A typical phenomenon is in fact the formation of “hot spots”: hot spots in the bed where, due to particular conditions mainly linked to the classes of pollutants present (ketones, but also aldehydes and alcohols), embers are formed that can cause the entire filtering medium to catch fire.
Knowing these characteristics and how to deal with them is a key factor in choosing the environmental partner.

The accumulation of pollutants on the surface of the activated carbons leads to their saturation, thus reducing their adsorptive power. Depending on the nature and type of the treated molecules, activated carbons can in fact adsorb certain quantities of pollutants before they become saturated. But how can we understand when it is necessary to replace or regenerate carbons?
For this purpose, direct or indirect systems are used to measure the efficiency of the carbons. Direct systems measure the VOCs concentration during the emission, and they precisely indicate when to replace the carbons. However, the economic investment for these systems is significant, thus limiting their application to adsorption plants with internal regeneration. Indirect systems, on the other hand, measure the initial weight of the activated carbons and the variations over time using load cells located at the base of the adsorbers. As the pollutants are adsorbed, the activated carbons increase their weight until they reach a value that indicates saturation. This figure is set by our expert staff at the time of installation of the system, by taking into account the characteristics of the pollutants which are treated. In the presence of a mix of organic compounds, however, it is important to refine this parameter through a series of repeated analyses, so as to optimise maintenance.
After verifying the saturation of the activated carbons, it is necessary to replace them with newly produced virgin carbons or with regenerated carbons. The main difference between them, apart from the economic one, regards the residual adsorption capacity: it is total in the former, while in the latter it is reduced due to the regeneration process.
In internally regenerated adsorption plants, regeneration can take place by means of steam or other inert gases and it may result in the loss of some of the treated carbon, which will still have to be restored.

After exploring the operation of the adsorber and the activation and regeneration mechanisms of the activated carbons, it is now time to identify the advantages and disadvantages of this technology.

The effectiveness of the activated carbon filters is due to several factors, such as:

• adsorption surface area: the greater the surface area, the greater the possibility of adsorbing pollutants. However, it is important not to exaggerate, especially if the chemical composition and the type of the pollutants to be treated have not been assessed;
• variety of pore dimensions: the cavities in activated carbons have different dimensions. This characteristic allows them to adsorb molecules of different sizes, from the smallest to the largest ones;
• adsorption capacity: activated carbon adsorbers can adsorb different types of pollutants (odours, VOCs and some VICs);
• versatility: thanks to their high adsorption capacity, they can be used in different industrial contexts;
• saturation control: as we have seen, activated carbon filters can be equipped with special systems in order to check the saturation level of the carbons.

Activated carbon filters are a valuable ally in the elimination of many air pollutants, but they have some limitations:

• temperature: the operating principle behind this technology is physical adsorption through the Van der Waals forces. With a temperature increase, molecular agitation and thermal expansion lower the effectiveness of these forces, thus reducing the adsorption capacity. In case of high temperatures, impregnated activated carbons or those with special structures must therefore be considered;
• relative humidity: water vapour is one of the most common problems for filter performance. This is because in the presence of water vapour the pores are saturated, making the adsorption of other chemical species impossible;
• type of pollutant: the chemical-physical characteristics of the pollutants strongly affect the effectiveness and the operating efficiency of the system. For this reason, it is essential to know and take into consideration the dimensions, polarity and specific weight of the molecules to be removed;
• concentration of the pollutants: this parameter is fundamental to assess the sustainability of the investment, especially for non-impregnated filters;
• material: the choice of the adsorbent material must be made taking into account the characteristics of the pollutant and of the production system. In general, a filtering medium with mesopores is a good compromise that ensures a good internal surface area and reduces the risk of saturation by water vapour that usually occurs with very small pores;
• impregnant: as regards filters with impregnated carbons, the adsorbing capacity is related to the total surface area of the impregnant in contact with the fluid. The type of impregnant, on the other hand, determines its selectivity.

In conclusion of this article we can therefore state that activated carbon filtration is a valid solution for the treatment of air pollutants. However, in order to ensure the maximum efficiency, it is essential to know the characteristics of the molecules to be treated. For this reason, the abatement systems must be designed and manufactured by professionals who are able to choose the most suitable activated carbons according to the specific pollutants.
Contact Tecnosida® for your next system: we will manufacture a tailor made air pollutant abatement system for you!