Activated Carbon Basics
New to Activated Carbon Industry? You will find this is extremely helpful for you to get to know the basics of Activated Carbon
- What is Activated Carbon
- Key Properties of Activated Carbon
- Types of Activated Carbon
- Manufacturing Activated Carbon
- Selecting Activated Carbon
What is Activated Carbon?
Activated carbon is a carbonaceous, highly porous adsorptive medium that has a complex structure composed primarily of carbon atoms. The networks of pores in activated carbons are channels created within a rigid skeleton of disordered layers of carbon atoms, linked together by chemical bonds, stacked unevenly, creating a highly porous structure of nooks, crannies, cracks and crevices between the carbon layers.
Activated carbons are manufactured from coconut shell, peat, hard and soft wood, lignite coal, bituminous coal, olive pits and various carbonaceous specialty materials. Chemical activation or High Temperature Steam Activation mechanisms are used in the production of activated carbons from these raw materials.
The intrinsic pore network in the lattice structure of activated carbons allows the removal of impurities from gaseous and liquid media through a mechanism referred to as adsorption. This is the key to the performance of activated carbon.
Adsorption is the attachment or adhesion of atoms, ions and molecules (adsorbates) from a gaseous, liquid or solution medium onto the surface of an adsorbent – activated carbon. The porosity of activated carbons offers a vast surface on which this adsorption can take place. Adsorption occurs in pores slightly larger than the molecules that are being adsorbed, which is why it is very important to match the molecule you are trying to adsorb with the pore size of the activated carbon. These molecules are then trapped within the carbon's internal pore structure by Van Der Waals Forces or other bonds of attraction and accumulate onto a solid surface.
Typically 1 m3 of Activated Carbon with 0.3m3 of internal pores can adsorb 30m3 or more of a gas, even if present in low concentrations in a carrier.
Two Types of Adsorption
Physical Adsorption - During this process, the adsorbates are held on the surface of the pore walls by weak forces of attraction known as Van Der Waals Forces or London dispersion forces.
Chemisorption - This involves relatively strong forces of attraction, actual chemical bonds between adsorbates and chemical complexes on the pore wall of the activated carbon.
Key Properties of Activated Carbon
Surface Area - Generally, higher the internal surface area, higher the effectiveness of the carbon. The surface area of activated carbon is impressive, 500 to 1500 m2/g or even more; a spoonful of activated carbon easily equates the surface area of a soccer field.
It is in the activation process that this vast surface area is created. The most common process is steam activation; at around 1000°C steam molecules selectively burn holes into the carbonized raw material, thus creating a multitude of pores inside the carbonaceous matrix. In chemical activation, phosphoric acid is used to build up such a porous system at a lower temperature.
Total Pore Volume - Refers to all pore spaces inside a particle of activated carbon. It is expressed in milliliters per gram (ml/g), volume in relation to weight. In general, the higher the pore volume, the higher the effectiveness. However, if the sizes of the molecules to be adsorbed are not a good match to the pore size, some of the pore volume will not be utilized. Total pore volume (T.P.V.) differs by raw material source and type of activation method.
Pore Radius - The mean (average) pore radius, often measured in angstroms, differs by activated carbon type.
Pore Volume Distribution - Each type of carbon has its own unique distribution of pore sizes. They're referred to as micropores (small), mesopores (medium) and macropores (large). Carbons for adsorbing many types of gas molecules are microporous. The best carbons for decolorization have a higher distribution of mesopores.
- Micropores r < 1nm
- Mesopores r 1-25nm
- Macropores r > 25nm
nm = nanometer
Different Types of Activated Carbon
Activated carbon is mainly available in three forms or shapes: powder, granular and extruded. And each form is available in many sizes. Based upon the application and requirements, a specific form and size are recommended.
Granular Activated Carbon (GAC)
GAC are irregular shaped particles formed by milling and sieving. These products range from the sizes 0.2mm to 5 mm. They have the advantages of being harder and longer lasting than powdered activated carbons, clean to handle, purify large volumes of gas or liquids of a consistent quality, and can be reactivated and reused many times. GAC are used in both liquid and gas phase applications and in both fixed and moving systems.
In liquid phase uses, granular activated carbon is packed in columns and towers through which liquids flows. GAC are used where there is a single product to be refined or produced continuously in large quantities. In gas phase applications GACs have the advantage of having sufficient flow with an acceptable pressure drop through the carbon bed.
In addition, granular activated carbons are nearly always regenerated and reused, The period between reactivation varies significantly but is on average 18 months. Loss of material during reactivation ranges from 5% to 15%.
Powdered Activated Carbon (PAC)
PACs generally have a particle size distribution ranging from 5 to 150 Å, although coarser and finer grades are available. Advantages of powdered activated carbons are their lower processing costs and their flexibility in operation. The dosage of powdered activated carbon can be easily increased or decreased as process conditions vary. Powdered activated carbons are mainly used for liquid-phase adsorption. They are added to the liquid to be treated, mixed with the liquid and, after adsorption, are removed by sedimentation and filtration. Powdered activated carbons are generally used in batch process as the amount added can be easily altered and powder can be easily removed. The wet powder cake is not regenerated because of the problems associated with recycling the carbon, but incinerated or placed in landfills.
These are cylindrical pellets with diameter ranging from 1mm to 5mm. The extrusion process, together with the raw material used, ensures that the end product is hard and suitable for heavy duty applications. The extruded pellet form gives a low system pressure drop, which is an important consideration in the gas-phase uses. Markets lie in solvent recovery, gas purification and automotive emission control, where the high volume activity, low pressure drop and high stock resistance of extruded carbon enable them to last the entire life of the vehicle.
Manufacturing Activated Carbon
Carbonaceous materials are activated using 2 methods
- Steam Activation
- Chemical Activation
Steam activation is the most widely used process because it is generally used to activate both coconut shell and coal based carbons. Steam activated carbons are produced in a two-stage process. Firstly the raw material, in the form of lumps, pre sized material, briquettes or extrudates, is carbonized by heating in an inert atmosphere such as flue gas, so that dehydration and devolatilization of the carbon occur. For this stage temperatures usually do not exceed 700 C. Carbonization reduces the volatile content of the source material to under 20%. A coke is produced which has pores that are either small or too restricted to be used as an adsorbent.
The second stage is the activation stage which enlarges the pore structure, increases the internal surface area and makes it more accessible. The carbonized product is activated with steam at a temperature between 900C and 1100C. The chemical reaction between the carbon and steam takes place at the internal surface of the carbon, removing carbon from the pore walls and thereby enlarging the pores. The steam activation process allows the pore size to be readily altered and carbons can be produced to suit specific end-sues. For an example, the pore structure has to be opened up more for the adsorption of small molecules from a solution, as in water purification, than for the adsorption of large colour molecules in sugar decolorization.
Steam Activation produce activated carbon in the from of 1mm to 3mm pieces, which are crushed and screened to remove fines and dust to meet the specifications for granular activated carbons. To produce powdered activated carbons, the carbon pieces are further ground using a gentle pulverizing action.
Chemical Activation is generally used for the production of activate carbon from sawdust, wood or peat. Chemical activation involves mixing the raw material with an activating agent, usually phosphoric acid, to swell the wood and open up the cellulose structure. The paste of raw material and phosphoric acid is dried and then carbonized, usually in a rotary kiln, at a relatively low temperature of 400C to 500C. On carbonization, the chemical acts as a support and does not allow the char produced to shrink. It dehydrates the raw material resulting in the charring and amortization of the carbon, creating a porous structure and an extended surface area.
Activity is controlled by altering the proportions of raw material to reagent used. For phosphoric acid the ratio is usually between 1:0.5 and 1:4; activity increases with higher reagent concentration and is also affected by the temperature and residence time in the kiln.
Activated carbons produced by this method have a suitable pore distribution to be used as an adsorbent without further treatment. This is because the process used involves an “acid wash” which is used a purifying step in steam activated carbons, post activation. Chemically activated carbons, however, have a lower purity than specifically acid-washed steam activated carbons as they contain small amount of residual phosphate.
This chemical activation process mostly yields a powdered activated carbon. If granular material is required, granular raw materials are impregnated with the activating agent and the same method is used. The granular activated carbons produced have a low mechanical strength, however, and are not suitable for many gas phase uses. In some cases, chemically activated carbon is given a second activation with steam to impart additional physical properties.
Selecting Activated Carbon for your Application
Carbon Shape and System Design
For general adsorption system designs including Carbon in Leach (CIL) and Carbon in Pulp (CIP) systems granular carbons, column system for finer sizes and pellet activated carbons (1mm to 4mm) are used in fixed or moving bed filters. For activated carbon powders most suitable applications include flotation systems and batch process where used carbon is removed by filtration after an appropriate contact time.
Particle size and adsorption kinetics
Finer sizes accelerate diffusion rate of the adsorbates into the pores, hence improves kinetics, but wettability/filtration on powders, pressure drop or screening problems with granules, are limiting factors. Therefore particle size range and distribution selection is a compromise.
However, with cylindrical pellets the carbon has the advantage of better bed packing which eliminates some of the shortfall of granular carbon such as pressure drops and screening problems, but lesser hardness and greater cost compared to irregular carbons may be limiting factor.
Pore Sizes distribution and other parameters
In relation to the molecular size range of target adsorbates, the pore size distribution requirement changes. In gold extraction a carbon with a high micropore distribution would facilitate effective and efficient adsorption of gold-cyanide complexes. If you are targeting removal of large colored molecules and contaminant species, a carbon with greater distribution of meso and macro pores would be suitable.
The required adsorption capacity and purity of the activated carbon would depend on the end application. The hardness of the carbon needs also to be brought in early into the comparative studies. Such an approach has resulted in benefits to the consumer, increased service life, greater filter efficiencies, fewer regeneration cycles, economy on hardware costs through reduced filter sizes and variability of reactivation facilities.
The selection steps must necessarily be followed by actual plant trials or pilot tests stimulating all of the parameters of the industrial process, before purchase decisions are made.
Other Influences on Carbon Properties
Apart from the physical properties of pores, surface oxygen groups and the nonpolar nature of the surface frequently influence the adsorptive kinetics and capacity of a carbon. In addition to boost the adsorption efficiency for selected applications certain surface modifications, surface engineering and impregnations are carried out.