Activated Carbon Adsorption: What It Is and How Filters Are Composed

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November 28, 2024 - Updated at December 5, 2024
Tiziano B.
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Adsorption is a fundamental process for ensuring effective air filtration through the hood. The key element that distinguishes filtering hoods is activated carbon, a highly porous material with an extremely large internal surface area.

Thanks to its porous structure, activated carbon can capture and neutralize pollutant molecules present in the air, improving the quality of the home environment. This entire process is known as adsorption.

But what exactly is this process? In this article, we will delve into the principle of adsorption and the role of activated carbon, providing you with all the useful information to understand what happens inside your kitchen hood.

What Is Adsorption?

Adsorption is a physico-chemical process where the molecules of a substance bind to the surface of a porous solid, such as activated carbon, without penetrating the material's structure. This phenomenon occurs because the molecules in a fluid (liquid or gas) adhere to the solid's surface due to molecular attraction forces, thereby trapping pollutants.

The difference between adsorption and absorption lies in how substances interact with the material. In absorption, the fluid substance is completely absorbed into the solid or liquid, distributing throughout the entire volume of the material. In adsorption, the substance is retained only on the surface of the solid.

In practical terms, for a kitchen hood with activated carbon filters, adsorption traps particles and gases, thus purifying the air. This makes adsorption an essential function for your hood.

What Is the Origin of Activated Carbon? Where Is It Sourced From?

Activated carbon can be derived from various organic materials with a high carbon content. The most common sources include:

Wood: About 130,000 tons produced annually
Fossil coal: Around 100,000 tons per year
Coconut shells: 35,000 tons annually
Peat: 35,000 tons annually

These raw materials, due to their composition, can be processed to produce activated carbon with high adsorption properties. However, to be suitable for production, they must meet certain fundamental characteristics:

A low amount of uncarbonized organic material
Long shelf life
The ability to maintain their properties

The selected raw material significantly affects the pore structure and surface area of the activated carbon, which determines the quality of the final product.

Production involves two main stages: carbonization and activation (as detailed below), both of which are critical to make the material porous and suitable for filtration.

Why Is It Called Activated Carbon?

The term "activated carbon" derives from the activation process the material undergoes. Initially, it is composed of carbon of vegetable or mineral origin, and it becomes "active" when treated in specialized furnaces with high-temperature steam.

This treatment creates tiny passages within the carbon granules, called micropores. The presence of these micropores gives carbon its adsorption properties, allowing it to effectively trap various molecules such as fumes, odors, and grease in the case of a kitchen hood.

The main characteristics that distinguish one type of activated carbon from another include the origin of the carbon, pore size, granule size, bulk density, and specific surface area, which can reach up to 1,000 m² per gram of material. Thanks to these properties, activated carbon works through chemical, physico-chemical, and physical mechanisms.

How Is Activated Carbon Produced?

The production of activated carbon involves several stages, from selecting the raw material to the final cooling and grinding phase. Let’s look at each step in detail:

Raw Material Selection: The choice of raw materials greatly influences the final characteristics of the carbon, such as porosity and adsorption capacity. Materials with high purity and low volatile substance content are preferred to achieve a high-quality product.
Carbonization: The selected material is heated in the absence of oxygen to allow volatile components (VOCs) to evaporate. This stage forms the porous base of the carbon, creating the initial micropores necessary for adsorption. Carbonization typically occurs in a closed container at low temperatures (around 400–800°C) for several hours.
Activation: After carbonization, the material undergoes activation, which can be achieved using two methods:
- Physical Activation: The carbon is treated with steam or gas at high temperatures (800–1100°C), further opening the pores and increasing the adsorption surface area.
- Chemical Activation: A chemical agent (such as phosphoric acid) is used to treat the carbon at lower temperatures (450–900°C). This method creates larger, more uniform pores. The carbon is then rinsed to remove any chemical residues.
Cooling and Grinding: Once activation is complete, the activated carbon is rapidly cooled to preserve its porous structure. It is then ground to achieve the desired particle size, ranging from fine powders to larger granules used in filters.

For a deeper understanding of its structure and the various sizes and forms in which it is available, continue reading the following sections.

What Is the Structure of Activated Carbon?

The structure of activated carbon resembles that of graphite, though it differs due to impurities and the activation process. Pure graphite consists of crystals formed by layers of hexagonal carbon atom arrangements, bonded by weak van der Waals forces, with strong carbon-carbon bonds within each layer.

In activated carbon, this arrangement is less organized than in graphite. Impurities and preparation techniques create a disorganized structure, resulting in a highly porous material. This porosity, a hallmark of activated carbon, depends on both the type of raw material used and the activation process applied.

Porosity of Activated Carbon

The porosity of activated carbon is one of its key features, determining its ability to adsorb harmful molecules and purify air (as in the case of kitchen hoods). Activated carbon’s porous structure is composed of a complex network of pores of various sizes, classified into three main categories:

Micropores (diameter less than 2 nm):
- Effective for adsorbing small molecules such as phenolic compounds, chlorine, and chloramines.
- These tiny pores trap volatile and toxic molecules present in water or air.
Mesopores (diameter between 2 nm and 50 nm):
- Ideal for adsorbing medium-sized molecules like proteins, peptides, and dyes.
- Useful for removing complex organic substances that negatively impact air and water quality.
Macropores (diameter greater than 50 nm):
- Capture large organic molecules and bulkier particles.
- While not primarily involved in chemical adsorption, these pores facilitate access to smaller pores and contribute to mechanical filtration.

This combination of pore sizes makes activated carbon a highly versatile material for removing a wide range of contaminants.

Size and Shape of Activated Carbon

Activated carbon is available in various shapes and sizes, tailored to the specific application it is used for. It can be produced as fine powder, granules, or compact blocks. The size of the granules directly affects the surface area available for adsorption.

Powdered Activated Carbon (PAC): Features very small particles with a large surface area, making it ideal for rapid adsorption processes.
Granular or Block Activated Carbon: Used in filters requiring greater mechanical strength and extended treatment times.

Granules typically range in size from 0.5 to 5 millimeters, depending on the filter design and specific adsorption needs.

In kitchen hood filters, the amount of activated carbon can vary but generally falls between 100 and 500 grams, sufficient for effectively adsorbing odors and airborne chemicals. Thus, the size and shape of the activated carbon also influence its efficiency in capturing pollutant molecules.

What Is the Structure of Activated Carbon Filters in Hoods?

Activated carbon filters typically feature a grid-like structure that allows air to pass through multiple layers of material. They are designed to trap and eliminate odors, fumes, and vapors generated during cooking. The main components are:

Granular or Powdered Activated Carbon: The primary substance responsible for adsorption.
Filter Support:
- The activated carbon is embedded in a support material, which may be made of nonwoven fabric, synthetic material, or cellulose.
- This support keeps the carbon in place and allows air to pass through the filter.
Container Structure:
- The filter is enclosed within a frame, usually made of plastic or metal, which ensures the carbon remains stable and facilitates easy installation in the hood.

This composition makes activated carbon filters effective in improving kitchen air quality by reducing odors and contaminants.

The Role of Activated Carbon in Kitchen Hood Filters

When polluted air passes through the filter, it comes into contact with the activated carbon granules. During this process, molecules from cooking fumes and grease adhere to the carbon's surface via adsorption.

This is a molecular-level, non-chemical process, meaning the substances are not destroyed but simply retained within the carbon's pores.

The filtration capacity of activated carbon ensures that the air is purified before being reintroduced into the environment, enhancing the health and safety of the kitchen space.

The Importance of Maintaining Activated Carbon Filters

What happens if you don’t properly maintain your activated carbon filters? Over time, these filters become clogged and stop effectively purifying the air.

For this reason, regular maintenance is essential, including replacement or regeneration, as in the case of long-life filters. Activated carbon filters are divided into two categories:

Standard Activated Carbon Filter:
- This type of filter needs regular replacement and cannot be washed.
Long-Life Activated Carbon Filter:
- This filter has a longer lifespan and can be regenerated. However, it still needs to be replaced after a certain period of time.

Need New Filters?

At Filtro Cappa, you’ll find filters and spare parts compatible with many different hood models. Visit our online store to find the right solution for you, or contact our support team via WhatsApp or email for any special requests.

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