Activated Carbon Manufacture: Steam Activation

The use of steam for activation can be applied to virtually all raw materials. Steam activation is the most widely used process to activate carbonaceous materials. Steam activated carbons are produced in a two-stage process.

First, the raw material in the form of lumps, pre-sized material, briquettes or extrudates is carbonized by heating in an low oxygen atmosphere so that dehydration and devolatilization of the raw material occurs. Carbonization reduces the volatile content of the source material to under 20%. A coke or charcoal (depending on the raw material) is produced which has pores that are either small or too restricted to be used as an adsorbent.

A variety of methods have been developed but all of these share the same basic principle of initial carbonization at 500-600 degrees C，followed by activation with steam at 800-1,100 degrees C.

Since the overall reaction (converting carbon to carbon dioxide) is exothermic it is possible to utilize this energy and have a self-sustaining process：

C + H2O (steam) —> CO + H2 (-31 Kcal)
CO + ½ O2 —> CO2 (+67 Kcal)
H2 + ½ O2 —> H2O (steam) (+58 Kcal)
C + O2 —> CO2 (+94 Kcal)

A number of different types of kilns and furnaces can be used for carbonization/activation and include rotary (fired directly or indirectly), vertical multi-hearth furnaces, fluidized bed reactors and vertical single throat retorts. Each manufacturer has their own preference.

As an example, production of activated carbon using a vertical retort is described below.

Raw material is introduced through a hopper on top of the retort and falls under gravity through a central duct towards the activation zone. As the raw material moves slowly down the retort the temperature increases to 800-1000 degrees C and full carbonization takes place.

The second stage, which can take place later in the same kiln, is activation which enlarges the pore structure, increases the internal surface area and makes it more accessible. The carbonized product is activated with steam at very high temperatures. 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 activation zone, at the bottom of the retort, covers only a small part of the total area available and it is here that steam activation takes place. Air is bled into the furnace to convert the product gases, CO and H2 into CO2 and steam which, because of the exothermic nature of this reaction, reheats the firebricks on the downside of the retort, enabling the process to be self-supporting.

Every 15 minutes or so, the steam injection point is alternated to utilize the “in situ” heating provided by the product gas combustion. The degree of activation (or quality) of the product is determined by the residence time in the activation zone.

Activated Carbon Manufacturing: Chemical Activation

Chemical activation is generally used for the production of activated carbon from sawdust, wood or peat. The process involves mixing an organic chemical compound with the carbonaceous raw material, usually wood, and carbonizing the resultant mixture. The raw material and reagent are mixed into a paste, dried and carbonized in a rotary furnace at 600 degrees C. When phosphoric acid is the activating agent the carbonized product is further heated at 800- 1000 degrees C during which stage the carbon is oxidized by the acid. The acid is largely recovered after the activation stage and converted back to the correct strength for reuse.

Chemical activation

The raw material is mixed 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 400 to 500 degree Celsius. On carbonization, the chemical acts as a support and does not allow the charcoal produced to shrink. It dehydrates the raw material, resulting in the charring and amortization of the carbon, thereby creating a porous structure and an extended surface area.

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.

Activated carbons produced by this method have a suitable pore distribution to be used as an adsorbent without further treatment. The process used means that the activated carbons are acid washed carbons although they have a lower purity than specifically acid washed steam activated carbons. This chemical activation process normally 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. Granular activated carbons (GACs) produced have a low mechanical strength, and are not suitable for many gas phase uses. In some cases, chemically activated carbons are given a second activation with steam to impart additional physical properties.

Activity can be controlled by altering the proportion of raw material to activating agent, between the limits of 1:05 to 1:4. By increasing the concentration of the activating agent, the activity increases although control of furnace temperature and residence time can achieve the same objective.

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.

Raw Materials of Activated Carbon

A carbonaceous substance can be used as the raw material for activated carbon.
Kuraray Chemical selects materials considering the difficulty in obtaining the material, amount of material required, price, reactivity with gas or chemicals, and appropriateness of quality for the products.

Materials for activated carbon in use worldwide are as follows:

Powdered activated carbon	Sawdust Hard wood chips wood charcoal (carbon from sawdust) Grass ash (peat) etc…
Granulated activated carbon	Charcoal Coconut shell charcoal Coal (lignite, brown coal, bituminous coal, anthracite coal, etc.) Oil carbon Phenolic resin etc…
Fibrous activated carbon	Rayon Acrylonitril Coal tar pitch Petroleum pitch Phenolic resin, etc. etc…

Powdered activated carbon

Sawdust
Hard wood chips
wood charcoal (carbon from sawdust)
Grass ash (peat)

etc…

Granulated activated carbon

Charcoal
Coconut shell charcoal
Coal (lignite, brown coal, bituminous coal, anthracite coal, etc.)
Oil carbon
Phenolic resin

etc…

Fibrous activated carbon

Rayon
Acrylonitril
Coal tar pitch
Petroleum pitch
Phenolic resin, etc.

etc…

Coconut shell

In addition to the more common raw materials discussed earlier, others can include waste tires, phenol formaldehyde resin, rice husks, pulp mill residues, corn cobs, coffee beans and bones.

Most of the developed nations have facilities to activate coconut shell, wood and coal. Third world countries have recently entered the industry and concentrate on readily available local raw materials such as wood and coconut shell. Coconut shell contains about 75% volatile matter that is removed, largely at source by partial carbonization, to minimize shipping costs. When producing coconut shell activated carbon from coconuts, only the shell (see fig.) is used and 50000 coconuts are needed to produce 1 ton of activated carbon.

The cellulosic structure of the shell determines the end product characteristics, which (at 30-40% yield on the carbonized basis) is a material of very high internal surface area consisting of pores and capillaries of fine molecular dimensions.

The ash content is normally low and composed mainly of alkalis and silica. Coal is also a readily available and reasonably cheap raw material. The activate obtained depends on the type of coal used and its initial processing prior to carbonization and activation.

It is normal procedure to grind the coal and reconstitute it into a form suitable for processing, by use of a binder such as pitch, before activation. (This is typical for extruded or pelletized carbon). An alternative method is to grind the coal and utilize its volatile content to fuse the powder together in the form of a briquette.

This method allows for blending of selected materials to control the swelling power of the coals and prevents coking. If the coal is allowed to “coke” it leads to the production of an activate with an unacceptably high proportion of large pores.

Blending of coals also allows a greater degree of control over the structure and properties of the final product. Wood may be activated by one of two methods, i.e. steam or chemical activation, depending on the desired product. A common chemical activator is phosphoric acid, which produces a char with a large surface area suitable for decolorization applications.

The carbon is usually supplied as a finely divided powder which since produced from waste materials such as sawdust, is relatively cheap and can be used on a “throw-away” basis. Since activated carbon is manufactured from naturally occurring raw materials, its properties will obviously be variable. In order to minimize variability it is necessary to be very selective in raw material source and quality and practice a high level of manufacturing quality control.

Activated Carbon Adsorption Mechanism

How the adsorbate is absorbed ?

Activated carbon can be considered as a material of phenomenal surface area made up of millions of pores – rather like a “molecular sponge”. Activated carbon is a microporous inert carbon matrix with a very large internal surface (700 to 1 500 m²/g). The internal surface is ideal for adsorption. Activated carbon is made from materials containing amorphous carbon, such as wood, coal, peat, coconut shells… It is formed via a thermal process, where volatile components are removed from the carbon-laden material (raw material) in the presence of oxygen.

The process by which such a surface concentrates fluid molecules by chemical and/or physical forces is known as ADSORPTION (whereas, ABSORPTION is a process whereby fluid molecules are taken up by a liquid or solid and distributed throughout that liquid or solid).

In the physical adsorption process, molecules are held by the carbon’s surface by weak forces known as Van Der Waals Forces resulting from intermolecular attraction. The carbon and the adsorbate are thus unchanged chemically. However, in the process known as CHEMISORPTION molecules chemically react with the carbon’s surface (or an impregnant on the carbon’s surface) and are held by much stronger forces – chemical bonds.

In general terms, it is necessary to present the molecule to be adsorbed to a pore of comparable size. In this way the attractive forces coupled with opposite wall effect will be at a maximum and should be greater than the energy of the molecule.

For example, a fine pored coconut shell carbon has poor decolorizing properties because color molecules tend to be larger molecular species and are thus denied access to a fine pore structure. In contrast, coconut shell carbons are particularly efficient in adsorbing small molecular species. Krypton and Xenon, for instance, are readily adsorbed by coconut shell carbon but readily desorb from large pored carbons such as wood.

Maximum adsorption capacity is determined by the degree of liquid packing that can occur in the pores. In very high vapor pressures, multi-layer adsorption can lead to capillary condensation even in mesopores (25A).

Activated Carbon Adsorption Capacity

The effectiveness at which activated carbon can remove contaminants from a stream is not based on the quantity of carbon, but, the activated carbon adsorption capacity. The greater the capacity, the more contaminants the activated carbon will be able to adsorb in volume. However, due to natural carbon’s limitations, it is not able to adsorb certain contaminants, as there molecular weight are to low to be treated through this process alone.

Active carbon is most effective against compounds that hold a high molecular weight and low solubility due to activated carbon having a high molecular weight as well. If there is ever an uncertainty if a specific contaminant will be removed in the adsorption process, referral is to be made to the solubility and molecular weight of said containment.

If adsorption capacity is plotted against pressure (for gases) or concentration (for liquids) at constant temperature, the curve so produced is known as an ISOTHERM. Adsorption increases with increased pressure and also with increasing molecular weight, within a series of a chemical family. Thus, methane (CH4) is less easily adsorbed than propane (C3H8).

Efficiency is determined by the type of pollutant, the type of activated carbon which is used and the temperature and humidity of the waste gases. An effective installation can be expected to realise a yield between 95 – 98 % for input concentrations of 500 – 2 000 ppm.

If effective, concentrations can typically be brought from 400 – 2 000 ppm to under 50 ppm.

In foundries, an end concentration of 20 mg/Nm³ VOC has been established

Mercury can be brought down to less than 0.05 mg/Nm³. Dioxins to less than 0.1 ng TEQ/Nm³ and, for odour and H₂S, yields of 80 – 95 % have been established

This is a useful fact to remember when a particular system has a number of components.

Activated carbon adsorption mechanism

After equilibrium, it is generally found that, all else being equal, the higher molecular weight species of a multi-component system are preferentially adsorbed. Such a phenomenon is known as competitive or preferential adsorption – the initially adsorbed low molecular weight species desorbing from the surface and being replaced by higher molecular weight species. Physical adsorption in the vapor phase is affected by certain external parameters such as temperature and pressure.

The adsorption process is more efficient at lower temperatures and higher pressures since molecular species are less mobile under such conditions. Such an effect is also noticed in a system where moisture and an organic species are present. The moisture is readily accepted by the carbon surface but in time desorbs as the preferred organic molecules are selected by the surface.

This usually occurs due to differences in molecular size but can be also attributable to the difference in molecular charge. Generally speaking, carbon surfaces dislike any form of charge – since water is highly charged (ionic) relative to the majority of organic molecules the carbon would prefer the organic to be adsorbed.

Primary amines possess less charge on the nitrogen atom than secondary amines that in turn have less than tertiary amines. Thus, it is found that primary amines are more readily adsorbed than tertiary amines.

High levels of adsorption can be expected if the adsorbate is a reasonably large bulky molecule with no charge, whereas a small molecule with high charge would not be expected to be easily adsorbed.

Molecular shape also influences adsorption but this is usually of minor consideration. In certain situations, regardless of how the operating conditions can be varied, some species will only be physically adsorbed to a low level. (Examples are ammonia, sulfur dioxide, hydrogen sulfide, mercury vapor and methyl iodide). In such instances, the method frequently employed to enhance a carbon’s capability is to impregnate it with a particular compound that is chemically reactive towards the species required to be adsorbed.

Since carbon possesses such a large surface (a carbon granule the size of a “quarter” has a surface area in the order of ½ square mile!) coating of this essentially spreads out the impregnant over a vast area. This, therefore, greatly increases the chance of reaction since the adsorbate has a tremendous choice of reaction sites. When the adsorbate is removed in this way the effect is known as CHEMISORPTION.

Unlike physical adsorption the components of the system are changed chemically and the changed adsorbate chemically held by the carbon’s surface and desorption in the original form is nonexistent. This principle is applied in many industries, particularly in the catalysis field, where the ability of a catalyst can be greatly increased by spreading it over a carbon surface.

The effect of activated carbon on the adsorbate in water comes from two aspects: on the one hand, physical adsorption, the internal force of the activated carbon is in a balanced state under the force from all directions of the water body, and the external molecules are not balanced, so that the molecules adsorb to the activated carbon On the surface; on the other hand, it is chemical adsorption, because there is a chemical interaction between activated carbon and the adsorbed substance.

The adsorption of activated carbon on pollutants in water is the result of the combined action of the above two kinds of adsorption. There are four steps in the adsorption process of activated carbon on the adsorbate in water: first, due to the convection effect of the water body, the adsorbate diffuses onto the surface of the activated carbon; second, the adsorbate molecules diffuse into the large pores of the activated carbon through the liquid film; Third, the adsorbate molecules reach the micropores due to surface diffusion; fourth, the adsorbent molecules in water are adsorbed on the surface of the activated carbon pores.

Activated carbon adsorption equilibrium is a state of dynamic equilibrium. When the adsorption rate and the desorption rate of activated carbon in the solution are equal, that is, when the amount of activated carbon adsorption per unit time is equal to the amount of desorption, the concentration of the adsorbed substance in the solution and the concentration on the surface of the activated carbon will no longer change. For adsorption equilibrium.

Adsorption capacity and adsorption speed are two important indicators to measure the adsorption process of activated carbon. The adsorption capacity is reflected by the adsorption amount qe, which is mainly affected by the pore size and structure of activated carbon. In addition, temperature and pH value also affect the adsorption capacity of activated carbon.

Adsorption speed refers to the amount of material adsorbed per unit weight of adsorbent per unit time, which is mainly determined by the contact time of water and adsorbent. Because the adsorption reaction is an exothermic reaction, low temperature is usually beneficial to accelerate the adsorption rate.

Activated Carbon Structure

In order to explain the capabilities of activated carbon an appreciation of its structure is most useful. Much of the literature quotes a modified graphite-like structure; the modification resulting from the presence of microcrystallites, formed during the carbonization process, which during activation have their regular bonding disrupted causing free valencies which are very reactive.

In addition, the presence of impurities and process conditions influence the formation of interior vacancies, in the microcrystalline structures. Such theory generally explains pores as the result of faults in crystalline structures.

However, more recent research studies provide a more feasible explanation of the carbon structure. The generally accepted graphite-like structure theory falls down since the hardness of activated carbon is not in keeping with the layered structure of graphite.

Furthermore, the manufacturing conditions are different; in particular the temperature range utilized for activated carbon production is lower than that required for graphitization. Supporters of the graphite-like structure generally only explain the modified microcrystalline structure and ignore photographic and other methods of examining the residual macro structure. High magnification electron scanning microscopy, at 20,000x magnification, has revealed the presence of residual cellular structures.

These were previously unseen and unsuspected, except in the case of wood based activates which have sufficiently open structures visible to the naked eye. Cellular units are built from sugars, the most important being glucose.

Sugars ultimately will build to cellulose (the most important single unit in cellular construction) and cellulose polymers cross-link to form the wall of individual plant cells. Glucose units are wound into very tight helical spirals and under polarized light these exhibit anisotropy – demonstrating the presence of crystalline structures.

Although not as yet proven, it has been postulated that in the areas of maximum strain in cellulose chains it is conceivable that smaller crystalline units could be produced. In addition to cellulose, other materials also exist in cell wall structure.

Hemi-cellulose, which undergoes degradation more easily than cellulose and Lignin (the structure of which is still unproven) also exists and this is the most resistant to oxidation. Most theories attribute the structure of activated carbon to be aromatic in origin, thus, allowing the carbon structure itself to be described as aromatic in order to explain active centers, etc. Structures of the size of cell dimensions obviously do not influence physical adsorption but illustrate that the only material available for oxidation lies within the cell walls themselves.

Final activates consist almost entirely of elemental carbon together with residual ash which, in the case of wood and coconut, originate from minerals within the vessels of living tissues; silica being the only constituent actually incorporated within the cell wall tissue matrix.

The ash content of coal is of different composition and due to intrusion of inorganic materials during coalification. Thus, the overall structure consists of a modified cellular-like configuration with varying ash components depending on the particular raw material. The cellular-like structure theory offers a logical explanation for the differences in apparent density between activates of wood, coal and coconut.

Wood activates have a very open structure with thin wall cells whereas coconut activates show very thick walls with many pits. Furthermore, measurements taken from photomicrographs of coconut show good agreement with mercury penetration data. It is known that the carbonization and activation processes destroy, to varying degrees, intercellular walls and sieve plates between cells.

The end result on wood is a very open, sponge-like macrostructure seriously reducing the probability of adsorbate contact with cell walls. Activation of coconut produces a composition of rod-like cells in very close contact and large surface cavities are formed by destruction of dividing walls but these are shallow and do not extend through the activate’s granule.

The coconut activates thus differ significantly from wood activates in mechanical strength and density. Coconut activates exhibit extensive micropore volume, whereas wood activates have a definite trend to mesopores/macropores and a corresponding change in their basic properties. In the case of coal based carbons, pre-treatment of the raw coal is necessary in order for it to be processed, since raw coal swells during heating to produce coke-like structures. Control of this is achieved by first grinding the raw coal and mixing it with various additives, such as pitch, before it is introduced to the activation furnace.

However, the grinding process destroys the mechanical strength of coal – therefore, ground coal is reconstituted into briquettes prior to processing. Despite such pre-treatment, mercury penetration data for coal activates support the presence of structures similar to those identified in activates of wood and coconut, but to date no detection of residual plant structures has been found in coal activates.

Isotherm determinations reveal extensive micropore structures, although coal activates’ pore spectra are different to those of coconut activates with a tendency toward mesopores at lower activation.

The most reliable carbon structure model suggested to date is similar to that of polyamantane (C66 H59) which allows for a large degree of non-aromaticity, electron transfer and resonance. Progressive activation would tend to increase the number of active sites, and in turn the surface activity, similar to observed reactions with higher activates.

AWWA Standard for Granular Activated Carbon

This standard covers the use of granular and extruded activated carbons as a filter medium and adsorbent in water treatment. It involves the selection, placement, and use of granular activated carbon (GAC) in filter-adsorbers where the GAC must function as both a filter medium and adsorbent, as well as those systems where the primary function is adsorption. Section 1 discusses scope, purpose and application. Sections 2 and 3 list references and definitions.

Section 4 lists requirements, including: physical requirements; performance criteria; and, impurities. Section 5 discusses verification including: sampling; test procedures; and, rejection. Section 6 discusses delivery including: marking; packaging and shipping; and, affidavit of compliance. Section 7 discusses placing of GAC filter material including: preparation; placement of support media; placement of GAC; top surface elevation; and, contamination. Section 8 discusses preparation of filter for service including: backwashing; scraping; disinfection; cleaning; and, safety. Appendices provide a bibliography, and methods for adsorptive capacity testing including tannin and phenol.

The major revisions in this edition of ANSI/AWWA B604-96 include the following: the format has been changed to AWWA standard style; and, parameters required to specify GAC as a filter/adsorber, as well as an adsorbent, have been included.

Activated charcoal Medication

Activated charcoal is used in the emergency treatment of certain kinds of poisoning. It helps prevent the poison from being absorbed from the stomach into the body. Sometimes, several doses of activated charcoal are needed to treat severe poisoning. Ordinarily, this medicine is not effective and should not be used in poisoning if corrosive agents such as alkalis (lye) and strong acids, iron, boric acid, lithium, petroleum products (e.g., cleaning fluid, coal oil, fuel oil, gasoline, kerosene, paint thinner), or alcohols have been swallowed, since it will not prevent these poisons from being absorbed into the body.

Some activated charcoal products contain sorbitol. Sorbitol is a sweetener. It also works as a laxative, for the elimination of the poison from the body.Products that contain sorbitol should be given only under the direct supervision of a doctor because severe diarrhea and vomiting may result.

Activated charcoal has not been shown to be effective in relieving diarrhea and intestinal gas.

Activated charcoal may be available without a doctor’s prescription; however, before using this medicine, call a poison control center, your doctor, or an emergency room for advice.

Brand names includeActidose-Aqua, Aqueous Charcodote Adult, Aqueous Charcodote Pediatric,CharcoCaps, Charcoal, Charcodote, Charcodote Pediatric, Charcodote Tfs,Charcodote Tfs Pediatric, Di-Gon II, Diarrest, Donnagel, Donnagel-Mb, EZ-Char,Kao-Con, Kaodene NN, Kaolinpec, Kaopectate, Kaopek, Kerr Insta-Char,Parepectolin, Reese’s Charcoal, Requa Activated Charcoal.

Possible side effects

Call your doctor right away if you notice any of these side effects:

Allergic reaction: Itching or hives, swelling in your face or hands, swelling or tingling in your mouth or throat, chest tightness, trouble breathing

New or worsening abdominal pain, or severe constipation

Severe diarrhea

If you notice these less serious side effects, talk with your doctor:

Dark urine, or urinating less often than usual

Dark or black stools

Nausea or vomiting

If you notice other side effects that you think are caused by this medicine, tell your doctor.

Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

How Does Activated Carbon Work?

Activated carbon, also called activated charcoal, activated coal, or carbo activatus, is a form of carbon processed to have small, low-volume pores that increase thesurface area available for adsorption or chemical reactions. Activated is sometimes substituted with active.

Due to its high degree of microporosity, just one gram of activated carbon has a surface area in excess of 500 m², as determined by gas adsorption. An activation level sufficient for useful application may be attained solely from high surface area; however, further chemical treatment often enhances adsorption properties.

Physical adsorption is the primary means by which activated carbon works to remove contaminants from water. Carbon’s highly porous nature provides a large surface area for contaminants (adsorbates) to collect. In simple terms, physical adsorption occurs because all molecules exert attractive forces, especially molecules at the surface of a solid (pore walls of carbon), and these surface molecules seek other molecules to adhere to.

Activated carbon attracts organic chemicals from vapor and liquid streams cleaning them of unwanted chemicals. It does not have a great capacity for these chemicals, but is very cost effective for treating large volumes of air or water to remove dilute concentrations of contamination. For a better perspective, when individuals ingest chemicals or are experiencing food poisoning, they are instructed to drink a small amount of activated carbon to soak up and remove the poisons.

The large internal surface area of carbon has many attractive forces that work to attract other molecules. Thus, contaminants in water are adsorbed (or held) to the surface of carbon by surface attractive forces similar to gravitational forces. Adsorption from solution occurs as a result of differences in adsorbate concentration in the solution and in the carbon pores.

The adsorbate migrates from the solution through the pore channels to reach the area where the strongest attractive forces are. With this understanding of how the adsorption process works, we must then understand why it works, or why water contaminants become adsorbates. Water contaminants adsorb because the attraction of the carbon surface for them is stronger than the attractive forces that keep them dissolved in solution.

Those compounds that are more adsorbable onto activated carbon generally have a lower water solubility, are organic (made up of carbon atoms), have a higher molecular weight and a neutral or non-polar chemical nature. It should be pointed out that for water adsorbates to become physically adsorbed onto activated carbon, they must be both dissolved in water and smaller than the size of the carbon pore openings so that they can pass into the carbon pores and accumulate.

What Is Activated Carbon?

Activated carbon is a highly porous substance that attracts and holds organic chemicals inside it. The media is created by first burning a carbonaceous substance without oxygen which makes a carbon “char”. Next, the “char” is treated chemically or physically to develop an interconnected series of “holes” or pores inside the carbon. The great surface area of this internal pore network results in an extremely large surface area that can attract and hold organic chemicals.

The primary raw material used for activated carbon is any organic material with a high carbon content (coal, wood, peat, coconut shells). Granular activated carbon media is most commonly produced by grinding the raw material, adding a suitable binder to give it hardness, re-compacting and crushing to the correct size.

The carbon-based material is converted to activated carbon by thermal decomposition in a furnace using a controlled atmosphere and heat. The resultant product has an incredibly large surface area per unit volume, and a network of submicroscopic pores where adsorption takes place.

The walls of the pores provide the surface layer molecules essential for adsorption. Amazingly, one pound of carbon (a quart container) provides a surface area equivalent to six football fields.

Almost all materials containing a high fixed carbon content can potentially be activated. The most commonly used raw materials are coal (anthracite, bituminous and lignite), coconut shells, wood (both soft and hard), peat and petroleum based residues.

Many other raw materials have been evaluated such as walnut shells, peach pits, babassu nutshell and palm kernels but invariably their commercial limitation lies in raw material supply. This is illustrated by considering that 1,000 tons of untreated shell type raw material will only yield about 100 tons of good quality activated carbon.

Most carbonaceous materials do have a certain degree of porosity and an internal surface area in the range of 10-15 m2/g. During activation, the internal surface becomes more highly developed and extended by controlled oxidation of carbon atoms – usually achieved by the use of steam at high temperature.

After activation, the carbon will have acquired an internal surface area between 700 and 1,200 m2/g, depending on the plant operating conditions.

The internal surface area must be accessible to the passage of a fluid or vapor if a potential for adsorption is to exist. Thus, it is necessary that an activated carbon has not only a highly developed internal surface but accessibility to that surface via a network of pores of differing diameters.

As a generalization, pore diameters are usually categorized as follows:

micropores <40 Angstroms
mesopores 40 – 5,000 Angstroms
macropores >5,000 Angstroms (typically 5000-20000 A)

During the manufacturing process, macropores are first formed by the oxidation of weak points (edge groups) on the external surface area of the raw material. Mesopores are then formed and are, essentially, secondary channels formed in the walls of the macropore structure. Finally, the micropores are formed by attack of the planes within the structure of the raw material.

All activated carbons contain micropores, mesopores, and macropores within their structures but the relative proportions vary considerably according to the raw material.

A coconut shell based carbon will have a predominance of pores in the micropore range and these account for 95% of the available internal surface area. Such a structure has been found ideal for the adsorption of small molecular weight species and applications involving low contaminant concentrations.

In contrast wood and peat based carbons are predominantly meso/macropore structures and are, therefore, usually suitable for the adsorption of large molecular species. Such properties are used to advantage in decolorization processes.

Coal based carbons, depending on the type of coal used, contain pore structures somewhere between coconut shell and wood.

In general, it can be said that macropores are of little value in their surface area, except for the adsorption of unusually large molecules and are, therefore, usually considered as an access point to micropores.

Mesopores do not generally play a large role in adsorption, except in particular carbons where the surface area attributable to such pores is appreciable (usually 400 m2/g or more).

Thus, it is the micropore structure of an activated carbon that is the effective means of adsorption. It is, therefore, important that activated carbon not be classified as a single product but rather a range of products suitable for a variety of specific applications

Adsorption/Adsorbents/Activated Carbon

Since adsorption is a comparatively specialized technology, a capsule definition of terms may be helpful. Adsorption is a surface phenomenon, in which molecules of adsorbate are attracted and held to the surface of an adsorbent until an equilibrium is reached between adsorbed molecules and those still freely distributed in the carrying gas or liquid. While the atoms within the structure of the adsorbent are attracted in all directions relatively equally, the atoms at the surface exhibit an imbalanced attractive force which the adsorbate molecules help to satisfy. Adsorption can then be understood to occur at any surface, such as window glass or a table top. The characteristic which typifies an adsorbent is the presence of a great amount of surface area; normally via the wall area or slots, capillaries or pores permeating its structure, in a very small volume and unit weight.

The type of adsorption which is dependent primarily on surface attraction, in which factors such as system temperature, pressure, or impurity concentration may shift the adsorption equilibrium, is given the further classification of physical adsorption. The electronic forces (Van der Waal’s forces) responsible for adsorption are related to those which cause like molecules to bind together, producing the phenomena of condensation and surface tension. Conceptually, some prefer the analogy of physical adsorption being like iron particles attracted to, and held by, a magnet. Physical adsorption is the most commonly applied type, but an important sub-classification is chemisorption. Chemisorption refers to a chemical reaction between the adsorbate and the adsorbent , or often reaction with a reagent which may be impregnated on the extensive adsorbent surface (see Impregnated Carbons, below). Thus physical adsorption/desorption retains the chemical nature of the adsorbate, while chemisorption alters it.

The surface phenomenon of adsorption may now be contrasted with apsorption, in which one material intermingles with the physical structure of the other; for example, phenol dissolving into fibers of cellulose acetate (absorption) versus being adhered by surface attraction to the outer layer of the fibers (adsorption).

Activated charcoal) is an adsorbent derived from carbonaceous raw material, in which thermal or chemical means have been used to remove most of the volatile non-carbon constituents and a portion of the original carbon content, yielding a structure with high surface area. The resulting carbon structure may be a relatively regular network of carbon atoms derived from the cellular arrangement of the raw material, or it may be an irregular mass of crystallite platelets, but in either event the structure will be laced with openings to appear, under electron micrographic magnification, as a sponge like structure. The carbon surface is characteristically non-polar, that is, it is essentially electrically neutral. This non-polarity gives the activated carbon surface high affinity for comparatively non-polar adsorbates, including most organics. As an adsorbent, activated carbon is this respect contrasts with polar desiccating adsorbents such as silica gel and activated alumina. Granular Activated carbon will show limited affinity for water via capillary condensation, but not the surface attraction for water of a desiccant.

Activity Level

Activity level is often expressed as total surface area per unit weight, usually in square meters per gram. This total exposed surface will typically be in the range of 600-1200 m2/g. Toward the higher end of this range, one might better visualize one pound, about a quart in volume, of granular activated carbon with a total surface area of 125 acres.

To be useful in adsorption, surface area must be present in openings large enough to admit the adsorbate molecule(s). To provide some guidance on this topic, and for quality control purposes, the carbon industry has developed additional standardized vapor and liquid adsorption tests, using adsorbates of varying molecular size and chemical nature such as iodine, phenol, methylene blue, carbon tetrachloride, benzene and the color in standard black strap molasses. However activity level is measured, it is most meaningful when considered with additional characteristics described in the following sections.

Pore Structure

While openings into the carbon structure may be of various shapes, the term “pore,” implying a cylindrical opening, is widely used. A description of the minute distances between walls of these pores, normally expressed as a function of the total surface area or total pore volume presented by pores of various “diameters,” is the pore structure curve. The following sketches show some sample pore structure curves and what approximate pore shapes are described by the curves. Please note that the average pore shape depicted is derived from a summation of pores of various sizes and shapes. Thus no pore within the activated carbon is likely to have precisely the average shape, but the granular activated carbon overall will often perform as if all its surface area were in pores of that shape.

The smallest diameter pores make up the micropore structure, and are the highest adsorption energy sites. Microporosity is helpful in adsorbing lower molecular weight, lower boiling point organic vapors, as well as in removing trace organics in water to non-detectable levels. Larger pore openings make up the macroporosity, which is useful in adsorbing very large molecules and aggregates of molecules, such as “color bodies” in raw sugar solutions. Another important function of the macropore structure is in assisting diffusion of fluids to adsorption sites in the interior of the carbon particle.

Given the above, pore structure. (1) would be effective in adsorbing high volatility solvents, for certain types of odor control, and in removing trace organics from water; the latter with the liability of marginal diffusion characteristics. Pore structures along the lines of. (2) offer a good balance of selectivity for molecules of various sizes, ability to reduce vaporous and liquid contamination to ultra low levels, and good diffusion characteristics. Structure (3) would allow excellent diffusion and can accommodate very large molecular sizes, but has little micro- pore structure and would have very poor retentivity for most organics.

Activated carbon properties

Activated carbon is a non-hazardous carbon-bearing product with a porous structure and a very large internal surface area. The chemical structure of activated carbon can be defined as a crude form of graphite, with a random amorphous structure that is highly porous over a range of pore sizes, from visible cavities and gaps to those of molecular dimensions.

Treatment with activated carbon is based primarily on the phenomenon known as adsorption, in which molecules of a liquid or gas adhere to an external or internal surface of a solid substance. Activated carbon has a very large internal surface area (up to 1,500 m²/g) which makes it highly suitable for adsorption. Activated carbon can be impregnated with certain chemicals in order thus to enhance its properties for certain applications.

Applications:

Water and liquid applications for:

municipal drinking water treatment (taste, odour and micro pollutant removal e.g. pesticides, …),
domestic water treatment (in-line and cartridge filters),
process water (de-chlorination and de-ozonation),
ground water remediation,
waste water treatment – tertiary treatment (trace organics and COD removal, deodourisation and decolourisation, powdered as bio-flock improvement in an aerobic or anaerobic biological waste water treatment plant, as an additive for physical- chemical treatments),
raw material purification (purification of oils and fats, alcoholic and softdrinks, dyestuffs, …, decolourisation of sugar and glucose, food, chemicals, pharmaceuticals) and
catalytic processes.

Air and gas applications for:

air purification and environmental protection (removal of solvents and hydrocarbons, deodourisation, air conditioning, cooker hoods, flue gases, in powdered form the removal of dioxins, mercury and other trace elements from flue gases),
cleaning process gases (removal of contaminants from hydrogen, natural gas, carbon dioxide, landfill gas, solvent recovery, …),
respiration protection (gas masks, removal of harmful of toxic compounds),
tank venting,
ground water remediation,
molecular sieves.

Is There A Difference Between Activated Carbon And Activated Charcoal?

Most people have a misunderstanding that there is a difference between activated carbon and activated charcoal. Both of these terms can and are used interchangeably. As well, active carbon is another similar word used for activated carbon and activated charcoal. All of these phrases are synonymous and commonly found in our field.

What Does Activated Carbon Do?

Activated carbon attracts and holds organic chemicals from vapor and liquid streams cleaning them of unwanted chemicals. It does not have a great capacity for these chemicals, but is very cost effective for treating large volumes of air or water to remove dilute concentrations of contamination. For a better perspective, when individuals ingest chemicals or are experiencing food poisoning, they are instructed to drink a small amount of activated carbon to soak up and remove the poisons.

What Will Activated Carbon Remove?

Organic chemicals are attracted to carbon the best. Very few inorganic chemicals will be removed by carbon. The molecular weight, polarity, solubility in water, temperature of the fluid stream and concentration in the stream are all factors that affect the capacity of the carbon for the material to be removed. VOCs such as Benzene, Toluene, Xylene, oils and some chlorinated compounds are common target chemicals removed through use of carbon. Other large uses for activated carbon are the removal of odors and color contamination.

What Is Activated Carbon Made From?

Granular activated carbon can be produced from various carbonaceous raw materials, each of which will impart typical qualities to the finished pro-duct. Commercial grades are normally prepared from coconut and other nut shells, bituminous and lignite coals, petroleum coke, and sawdust, bark and Other wood products.

In general, nut shells and petroleum cokes will produce very hard carbons with a pore structure characterized by.(1) above, coals a (2) type structure in comparatively hard carbons, and wood (3) structure in carbons lacking great crush and abrasion resistance. It should be emphasized that specific production techniques may yield carbons that depart from the norm of a given raw material.Here at General Carbon, we carry activated carbon made from bituminous coal, lignite coal, coconut shell and wood.