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.
Carbonaceous materials are activated using 2 methods
1. Steam Activation
2. 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
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.
How does activated carbon work?
Activated carbon works by a process of adsorption. This is an attraction of a molecule to the carbon's vast internal surface by weak forces, known as London forces. The molecule is held in place and cannot be removed, unless the process conditions change, for example heating or pressure. This can be useful as an activated carbon can be used to concentrate material on its surface, that can be later stripped and recovered. The use of activated carbon for gold recovery is one common example of this.
In some cases, the activated carbon is chemically treated to remove pollutants and in this case the resulting reacted compound is generally not recovered.
Activated carbon surface is also not completely inert, and a variety of catalytic processes can be achieved using and taking advantage of the extended internal surface area available.
What forms can activated carbon be supplied in?
Activated carbon can be manufactured commercially in granular, pelletised and powdered forms. Different sizes are defined for different applications. For example, in air or gas treatment, the restriction to flow is import, and so coarse particles are used to minimise pressure loss. In liquid treatment, where the removal process is slower, then finer particles are used to improve the rate, or kinetics, of the purification process.
What are the main uses of activated carbon?
There are hundreds of different applications for activated carbon ranging from odour control of cat litter to the preparation of the most modern pharmaceuticals.
Around the home, activated carbon may be present in domestic appliances; will most likely have treated the municipal water supply, purified the soft drinks in the refrigerator, and been used in the production of chemicals, in turn used to manufacture electronics, furniture and construction materials.
And more; our waste is burned to generate electricity, the gases from which are purified by activated carbon. Odour control again at sewage processing facilities, uses activated carbon, and the reclamation of precious metals from mining spoils is big business.
How can activated carbon be used?
How an activated carbon is used depends very much on the application duty, and its form. For example, powdered activated carbon (PAC) is used to treat drinking water, by simply adding the required amount directly to the water and then separating the resulting coagulation matter (as well as other solids) before sending the treated water to the network. The contact with the organics present results in adsorption of them and the purification of the water.
Granular carbons (or extruded pellets) are used in fixed filter beds, with the air, gas or liquid passing through it with a determined residence (or contact) time. During this contact the unwanted organics are removed and the treated effluent is purified.
How is activated carbon made?
Activated carbon is commercially manufactured from coal, wood, fruit stones (mainly coconut but also walnut, peach) and derivatives of other processes (gas raffinates). Of these coal, wood and coconut are the most widely available.
The product is manufactured by a thermal process, but in the case of raw materials such as wood, a promoter (such as an acid) is also used to develop the required porosity.
Downstream processes crush, screen, wash and/or grind the multitude of products to the client's requirements.
What are the benefits of using activated carbon?
If it is not already apparent, activated carbon purifies our world. It makes water drinkable; the air breathable and food stuffs edible. It ensures we have the best healthcare, and that we can enjoy the freedoms, and technological advances of the modern world.
With Xingyuan, we can all create a Pure World, just as we have been doing since 1990.
How to reactivation the spend activated carbon?
Spent (waste) granular or pelletized or Powder activated carbon can be recycled by thermal reactivation, in which the spent carbon is processed through a high temperature reactivation furnace at or above 850°C. The adsorbed organics on the carbon are thermally decomposed. The resulting gases are fed through an afterburner and a treatment system to allow emission to atmosphere in accordance with most stringent environmental regulations. Recycling by thermal reactivation is an environmentally responsible disposal method that helps to reduce CO2 emissions over the activated carbon and contributes to a sustainable use of the world's resources.
What equipment should equipped in a activated carbon factory?
A steam activation factory should be equipped its factory according to the procedure:
1. Carbonizing furnace, change the biomass into charcoal first.
2. Charcoal treatment, meshing and crushing.
3. Steam activation kiln.
4. Raw AC. Treatment system, crushing and milling, also including acid washing or other procedure.
5. A lab for testing.
Glossary of Terms for Activated Carbon
The activated carbon industry uses many terms and we know that it can sometimes be confusing. In this section we have collected the most common terms for your reference.
Abrasion resistance – the property of a particle to resist attrition or wearing away by friction.
Absorption – a process in which fluid molecules are taken up by a liquid or solid and distributed throughout the body of that liquid or solid.
Accelerated adsorption tests – adsorption tests in which the end point is hastened by testing at conditions more severe than those anticipated in service.
Accelerated service life – the elapsed time until the end point is reached in an accelerated adsorption test.
Acid extractable material – substances dissolved by an acid under specified conditions.
Activated carbon – a family of carbonaceous substances manufactured by processes that develop adsorptive properties.
Activation – any process whereby a substance is treated to develop adsorptive properties.
Activity – a generic term used to describe the capacity to adsorb in general; also, the adsorptive capacity of an adsorbent as measured by a standard test.
Adsorbate – any substance that is or can be adsorbed.
Adsorbent – any solid having the ability to concentrate significant quantities of other substances on its surface.
Adsorption – a process in which fluid molecules are concentrated on a surface by chemical or physical forces, or both.
Adsorption wave – see mass transfer zone.
Adsorption zone – see mass transfer zone.
Ash – residue after the combustion of a substance under specified conditions.
As is basis – as received.
Breakpoint – the appearance in the effluent of a specified concentration of an adsorbate.
Breakthrough – the first appearance in the effluent of an adsorbate of interest under specified conditions.
Channelling – the greater flow of fluid through passages of lower resistance which can occur in fixed beds or columns of particles due to non uniform packing, irregular sizes and shapes of the particles, gas pockets, wall effects, and other causes.
Chemical adsorption – see chemisorption.
Chemisorption (chemical adsorption) – the binding of an adsorbate to the surface of a solid by forces whose energy levels approximate those of a chemical bond.
Co adsorption – the adsorption of two or more components on an adsorbent, each affecting the adsorbability of the other (also termed as ‘competitive adsorption’)
Contact batch operation – an adsorption process in which an adsorbent is dispersed in a fluid to be treated and then separated when practical equilibrium is attained.
Continuous moving bed – an adsorption process characterized by flow of a fluid through a continuously moving bed of granular adsorbent with continuous withdrawal of spent adsorbent and continuous addition of reprocessed or virgin adsorbent.
Counter current adsorption – an adsorption process in which the flow of fluid is in a direction opposite to the movement of the adsorbent.
Critical bed depth – the minimum depth of an adsorbent bed required to contain the mass transfer zone.
Crushing strength – the property of a particle to resist physical breakdown when contained and subjected to a slowly increasing continuously applied force.
Degassing – removal of gases. density, absolute or true – the mass under specified conditions of a unit volume of a solid sorbent excluding its pore volume and inter – particle voids
Density, apparent (density, bulk) – the mass under specified conditions of a unit volume of a solid sorbent including its pore volume and inter – particle voids.
Density, block – see density, particle.
Density, bulk – see density, apparent.
Density, particle (density, block) – the mass under specified conditions of a unit volume of a solid sorbent including its pore volume but excluding inter – particle voids.
Desorption – the separation of an adsorbate as such from a sorbent.
Differential heat of adsorption – the heat evolved during the adsorption of an incremental quantity of adsorbate at a given level of adsorption.
Dosage – the quantity of substance applied per unit weight or volume of the fluid being treated.
Dry basis – exclusive of any moisture which may be present.
Dust – an imprecise term referring to particulates capable of temporary suspension in air or other gases; also, particles smaller than an arbitrarily selected size.
Dynamic adsorptive capacity – the quantity of a given component adsorbed per unit of adsorbent from a fluid, or fluid mixture moving through a fixed bed at the breakpoint for that component.
Effective size – the particle size, in millimetres, which corresponds to 10 percent finer on the cumulative particle size distribution curve.
Electrical conductivity of a particulate substance – the current flowing through a unit cross section for an imposed unit potential gradient under specified conditions of packing.
Electrophoresis – migration of dispersed solid, liquid or gaseous material to one of two electrodes under the influence of an impressed direct current voltage.
End point – the occurrence in the effluent of the maximum permissible concentration of an adsorbate of interest.
Equilibrium adsorptive capacity – the quantity of a given component adsorbed per unit of adsorbent from a fluid or fluid mixture at equilibrium temperature and concentration, or pressure.
Expanded bed – a bed of granular particles through which a fluid flows upward at a rate sufficient to slightly elevate and separate the particles without changing their relative positions.
Filterability – the rate at which particles can be separated from a slurry by means of a permeable medium under specified conditions.
Fines – particles smaller than the smallest nominal specification particle size.
Fixed bed – a bed of granular particles through which a fluid flows without causing substantial movement of the bed.
Fluidised bed – a bed of granular particles in which the fluid flows upward at a rate sufficient to suspend the particles completely and randomly in the fluid phase.
Freundlich adsorption isotherm – a logarithmic plot of quantity of component adsorbed per unit of adsorbent versus concentration of that component at equilibrium and at constant temperature, which approximates the straight line postulated by the Freundlich adsorption equation
X/M = kCn
where:
X = quantity adsorbed,
M = quantity of adsorbent, C = concentration, k and n = constants.
Granular activated carbon – activated carbon in particle sizes predominantly greater than 80 mesh.
Hardness – a generic term referring to the resistance of a particle to breakdown as measured by specific tests.
Heat of adsorption – the heat evolved during adsorption.
Hydrolytic adsorption – the adsorption of a weakly ionised acid or base formed by the hydrolysis of some types of salts in aqueous solution.
Hysteresis loop – the divergence between the paths of the adsorption and desorption isotherms.
Ignition temperature (kindling point) – the lowest temperature at which combustion will occur spontaneously under specified conditions.
Impact strength – the property of a particle to resist physical breakdown when subjected to a rapidly increasing applied force.
Integral heat of adsorption – the sum of the differential heats of adsorption from zero to a given level of adsorption.
Intermittent moving bed (pulse, slug) – an adsorption process characterized by upward flow of a fluid through a fixed bed of granular adsorbent with periodic withdrawal of spent adsorbent from the bottom of the bed and additions of reprocessed or virgin adsorbent to the top of the bed.
Irreversible adsorption – adsorption in which the desorption isotherm is displaced toward higher equilibrium adsorption capacities from the adsorption isotherm.
Isobar – a plot of quantity adsorbed per unit of adsorbent against equilibrium temperature when concentration or pressure is held constant.
Isotere – a plot of equilibrium concentration or pressure against temperature when the quantity adsorbed per unit of adsorbent is held constant.
Isotherm – a plot of quantity adsorbed per unit of adsorbent against equilibrium concentration, or pressure, when temperature is held constant.
Langmuir adsorption theory – the surface of an adsorbent has only uniform energy sites and adsorption is limited to a monomolecular layer.
Langmuir isotherm – a plot of isothermal adsorption data which to a reasonable degree fit the Langmuir adsorption equation.
Macropore – pores with widths exceeding 50 nanometres (500 angstrom units).
Mass transfer zone (adsorption wave) (adsorption zone) – the region in which the concentration of the adsorbate of interest in the fluid decreases from influent concentration to the lowest detectable concentration.
Mean particle diameter – the weighted average particle size, in millimetres, of a granular adsorbent computed by multi – plying the percent retained in a size fraction by the respective mean sieve openings, summing these values and dividing by 100.
Mesopore – pores of widths between 2 and 50 nanometres (20 and 500 angstrom units).
Micropore – pores with widths not exceeding 2 nanometres (20 angstrom units).
Moisture content – the water content of a substance as measured under specified conditions.
Monomolecular layer – an adsorbed film, one molecule thick.
Multimolecular layer – an adsorbed film more than one molecule thick.
Oven drying loss – the reduction in weight resulting when a substance is heated in an oven under specified conditions.
Pelleted activated carbon – a form of granular activated carbon that is cylindrical in shape.
Physical adsorption (van der Waals adsorption) – the binding of an adsorbate to the surface of a solid by forces whose energy levels approximate those of condensation.
Pore diameter – the diameter of a pore in a model in which the pores in a sorbent are assumed to be cylindrical in shape and which is calculated from data obtained by a specified procedure.
Pores – the complex network of channels in the interior of a particle of a sorbent.
Pore volume – volume of the pores in a unit weight of a sorbent.
Pore volume distribution – the distribution of pore volume among pores of different size or diameter.
Powdered activated carbon – activated carbon in particle sizes predominantly smaller than 200 mesh (75 microns)
Preferential adsorption – adsorption in which a certain component or certain components are adsorbed to a much greater extent than others.
Reactivation (revivification) – oxidation processes for restoring the adsorptive properties of a spent sorbent.
Regeneration – distillation or elution – type processes for restoring the adsorptive properties of a spent sorbent.
Relative efficiency – the rating of the adsorptive capacity of an adsorbent based on a comparison of its performance with that of a reference adsorbent in a defined test.
Retentivity – the ability of an adsorbent to resist desorption of an adsorbate.
Reversible adsorption – adsorption in which the desorption isotherm approximates the adsorption isotherm.
Revivification – see reactivation.
Service life (service time) – the elapsed time until the end point is reached in an adsorption process.
Service time – see service life.
Sorption – a process in which fluid molecules are taken up by absorption and adsorption.
Split feed – a liquid phase adsorption process in which a powdered adsorbent is added to the solution to be treated in two or more steps, with or without intermediate separation of the adsorbent.
Surface area (B.E.T.) – the total surface area of a solid calculated by the B.E. T. (Brunauer, Emmett, Teller) equation, from nitrogen adsorption or desorption data obtained under specified conditions.
Surface area distribution – the distribution of surface area according to some parameter such as pores of different size or diameter.
Surface oxides – oxygen containing compounds and complexes formed at the surface of an adsorbent.
Synthetic test solution – a solution of two or more components prepared under specified conditions for use in evaluation of adsorbents.
Threshold concentration – the minimum concentration at which a substance can be detected by the taste or odor test method employed.
Threshold odour test – a method of evaluating the odour level in a fluid by dilution under specified conditions with an odour free fluid.
Uniformity coefficient – the ratio of the particle diameter corresponding to 60 % finer on the cumulative particle size distribution curve to the particle diameter corresponding to 10 % finer on the same distribution curve.
Van der Waals adsorption – see physical adsorption.
Water – extractable material – substances dissolved from other substances by water under specified conditions.
Wettability – the rate at which particles can be made wet under specified conditions
