Activated Carbon: A traditional and promising adsorbent for gas and water purification

Activated carbon (AC) are highly porous adsorbents with a large surface area which can go up to 1500 m2/g, making them an effective material for a wide range of applications such as odor control, air purification, VOCs removal, methane and hydrogen storage and removal of contaminants present in process streams both in gas and liquid state. They are produced using various sources that are rich in carbon content with low ash content such as coconut shells, wood, peat, and coal.

At an industrial scale, ACs are manufactured in the form of granular, extruded, or powdered depending on their end use. Granular activated carbon (GAC) and powdered activated carbon (PAC) are the most widely utilized forms of AC. On one hand, PACs have better adsorption kinetics, higher surface areas, and smaller diffusion distances, but they are complex to integrate into industrial adsorption beds. As a result, they are widely utilized for batch systems followed by filtration and for liquid phase adsorption. However, GAC presents slower adsorption kinetics than PAC due to their size but they are much easier to introduce at large-scale adsorption columns for gas phase separation applications like CO2 capture. An additional variety of AC is the extruded pellet where the PAC is compressed to obtain a cylindrical shape pellet for gas-phase applications. These pellets have the potential to be manufactured in multiple shapes and sizes making them tailorable for specific applications. The pellet configuration of AC is considered more mechanically stable and provides lower dust content making them highly desirable.

To synthesize AC there are 2 main steps which are pyrolysis and activation. During the primary step of pyrolysis, the biomass is heated to extremely high temperatures to start the carbonization process. Pyrolysis results in an irreversible thermal degradation of any lignocellulosic matter through a series of chemical and physical changes at high temperatures (300–700 ◦C) in an oxygen-free environment in the presence of inert gases such as argon or nitrogen. As the temperature goes beyond 300 ◦C all other elements are eliminated causing an increase in carbon concentration and resulting in a carbon-rich material. After pyrolysis is complete, the carbon skeleton obtained needs to undergo an activation process. The activation can be done by one of the following processes: physical or chemical activation.

Physical activation: During this process, the carbon obtained previously (biochar) is developed into “activated carbon” by utilizing hot gases. Through the direct approach, the biomass undergoes heat treatment under N2 flow until the activation temperature is reached. Then the gaseous stream is switched to CO2, and the activation process occurs after the pyrolysis process is finished. On the other hand, during the two-step approach, pyrolysis occurs separately until the biochar is obtained. Afterward, the leftover material is cooled and collected. Next, in step 2 the biochar is heated again under the flow of CO2 or steam at a particular temperature and time till AC is obtained.

Chemical activation: In this method, the carbon material is impregnated with certain chemicals. The chemical utilized could be an acid, a strong base, or a salt (phosphoric acid, KOH, NaOH, potassium carbonate, CaCl2, and ZnCl2). Later the carbon is heated at extreme temperatures of 250–600 °C as the high temperature activates the carbon by forcing the material to open up and have more microscopic pores.

Now that we have understood the synthesis and applications of AC, we at MERYT hope to provide you with all types of AC such as: MERYT PAC (for water treatment), MERYT GAC (air and water treatment, with high surface area and low-pressure drop) and MERYT PEAC (pellets for industrial gas purification and solvent recovery).

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