Activated Carbon - Applications and Properties

What is activated carbon?

Activated carbon is a highly porous material with an extremely large surface area, ranging from approximately 300 to 2,000 m² per gram. Figure 1 illustrates the porous structure and internal channels within the carbon. Activated carbon is produced from various natural raw materials, including coconut shells, peat, lignite, wood, and others.

In many industries worldwide, activated carbon is widely used due to its very large surface area and strong affinity for organic compounds, which gives it a high adsorption capacity relative to its weight and volume. For this reason, it is commonly applied in industrial air filtration and purification systems for the removal of pollutants.

How activated carbon works

Activated carbon contains thousands of microscopic pores and channels through which air can flow. The air carries organic molecules (pollutants) into these pores, where the carbon effectively neutralizes them. This process is known as physical adsorption. For adsorption to be efficient, the carbon must have a very large surface area, with most of it located within the internal pores. In this respect, wood-based activated carbon is considered the most effective, followed by carbon produced from animal bones.

A contaminated gas stream containing volatile organic compounds (VOCs) - such as odors, aliphatic hydrocarbons, aromatic hydrocarbons, ketones, ethers, halogenated hydrocarbons, glycols, alcohols, esters, and aldehydes - passes through a deep bed of activated carbon. The large contact area, microscopic porosity, and electrostatic forces formed between the carbon surface and the organic molecules result in the adsorption of very small molecules in very large quantities, typically 500 - 1,200 mg of contaminant per gram of carbon. This process is referred to as adsorption.

The filtration efficiency of such systems typically reaches 98 - 99.9%. Adsorption units are commonly used when the treated gas stream contains relatively low pollutant concentrations, because although activated carbon has a high adsorption capacity, each adsorbed molecule reduces the remaining capacity of the carbon. Once the carbon becomes saturated, the system is no longer effective. In addition, there is a risk of hot spot formation within the carbon bed, which may lead to ignition of the saturated carbon.

To reduce fire risks, it is recommended to install CO and CO₂ sensors at the system outlet, along with integrated fire suppression systems inside the unit.

In modern industry, the use of volatile organic compounds (VOCs) - especially as solvents  is steadily increasing. The main applications of these substances include:

• As a primary or secondary component in chemical syntheses.
•  In pharmaceutical and cosmetics manufacturing, as part of synthesis processes or for coatings.
• In the paint and coatings industry, as a component of paints or for cleaning purposes.
• For degreasing and cleaning operations in the metal industry.
• In extraction processes within the process industry.
• For dry cleaning in the textile industry.
• In the printing industry, as a key component of printing inks.
• As a major component in the fuels industry and as an intermediate material in the rubber industry.
• As a primary component in the adhesives industry.

There are two main options for treating activated carbon. The first is replacement after use, according to regulations, the spent activated carbon must be disposed of at approved landfill sites or recycled, depending on the substances adsorbed onto it.

The second option is to stop the operation of the system and pass a flow of steam or hot air through it (a regenerative process). This process causes the adsorbed molecules to detach from the carbon. Such systems are extremely expensive, both to purchase and to maintain, and therefore are not commonly used in many parts of the world.