Bulk chemicals and pharmaceuticals have a spectra of economically important industrial clusters in India, which include oils, organic chemicals, emulsions, dyes, aromatics, chlor-alkali and catalysts. A variety of products are used as daily chemicals for laundry, cleaning, cooking and fuel. The economic importance of biochemical industries is undermined for generation of variety of raw materials for other businesses and interrelated articles of pharmaceutical, medical and agriculture use.
Activated carbon or activated charcoal or activated coal is essentially simple and cheap raw material, which can be converted into more valuable product by combination of physical and chemical process. Thousands of companies we foresee today as processing chemicals can become a major part of this business.
Carbon is naturally occurring nonmetal element that exhibits two major allotropic forms of graphite and diamond. It exists freely as a natural inorganic resource in form of coal and petroleum. Carbon is chief chemical constituent of biological compounds. Compounds obtained from living organisms i.e. animals and plants are collectively called as organic compounds. These are formed due to property of self-bonding (catenation) and covalent bonding with metals and nonmetals of carbon.
Carbon atoms can form linear chains, branched chains and cyclic structures that provide basic structural framework of organic compounds. Organic functional groups like alcohols, acid, ester, amine, aldehyde, ketones are covalently linked to carbon atoms in aqueous solution that impart specific chemical property to the molecule. Macromolecules of hydrocarbons (carbon & hydrogen) and high molecular weight polymers can be assembled from relatively simple precursors (organic entities) in limitless varieties. Carbohydrates are the most abundant biomolecules on earth.
Each year, photosynthesis converts more than 100 billion metric tons of CO2 and H2O into cellulose and other plant products. Insoluble carbohydrates polymers like peptidoglycans, chitins, cellulose serve as structural and protective elements in the cell walls of bacteria and plants and in the connective tissues of animals. Some organic compounds of commercial importance to pharmaceutical industry are sugars, starch, cellulose, dyes, alkaloids, acetic acid and hydrocarbons.
Preparation and properties of activated carbon
Activated charcoal is carbon produced from carbonaceous materials such as nutshells, coconut husk, peat, wood, coir, lignite, and coal through physical reactivation (pyrolysis) or chemical reactivation (combustion – treatment with strong acid, base or salt at high temperatures). Chief constituent of paper (waste paper) being carbon can be reproduced through pyrolysis.
Charcoal can be wood charcoal, animal charcoal and sugar charcoal. Wood charcoal is obtained by heating wood in limited supply of air. Animal charcoal is obtained by destructive distillation of bones and animal wastes, which is a combination of carbon (approx. 10%) and calcium phosphate. Sugar charcoal is obtained by action of sulphuric acid on sugar. Lamp black or carbon soot is amorphous and microcrystalline allotropic form of carbon obtained from rich carbon sources such as coal, charcoal, kerosene oil, or petroleum by oxidation in limited supply of air. Coke is a grayish black solid obtained from destructive distillation of coal (heating out of contact of air), which is used as main fuel and reducing agent in metallurgical operations.
Volatile organic compounds are combustible that convert into corresponding oxides (CO2, NO2, SO2) leaving behind inorganic ash. Hydrocarbons burn with a sooty flame and leave behind soot of carbon black. Dry cellulose of waste paper or dried pulp can be converted into inorganic elemental carbon by complete combustion (pyrolysis) or partial combustion.Activated carbon obtained from complete combustion of charcoal or cellulose is processed carbon with low-volume pores and high surface area that makes it a useful catalyst for surface adsorption process in gas and petrochemical industry.
Powdered activated carbon (PAC - R1) is a complex product, which is difficult to classify on the basis of behavior, surface characteristics and other fundamental criteria. PAC is a fine powder of granules less than 1 mm size and average diameter of 0.2 nm. It is considered ideal for diffusion and adsorption process due to large surface area (SA): volume (V) ratio. These can be easily retained on a 50 mesh sieve screen (0.297 mm). Finer particles can even pass through 80 mesh sieve (0.177 mm). PAC is generally added directly to gravity filters and water basins for water purification.
Granular activated carbon (GAC) is relatively larger that PAC and presents lesser outer surface area (SA). GAC cannot pass through standard mesh size of 20 (0.84 mm) and finer ones through 40 mesh sieve (0.42 mm). These are particularly suitable for adsorption of volatile gases and vapors in oil industries. GAC can be used in sewage water treatment for decolorization of dissolved components and separation of organic compounds. GAC is designated by sizes such as 8×20, 20×40, or 8×30 for liquid phase applications and 4×6, 4×8 or 4×10 for vapor phase applications.
Activated carbon can be extruded to form large blocks, which minimize dust and increase mechanical strength for gas phase operations. Extruded activated carbon (EAC) combines powder of activated carbon with a binder, which are fused together at high temperatures and molded into desired shapes to form large blocks of typical size 0.8 mm – 130 mm. EAC can be combined with chlorine and aromatic compounds to improve odor and taste of final liquid or gas product.
Bead activated carbon (BAC) obtained from petroleum pitch is similar to EAC in size but possess smaller grain size and texture, which makes its suitable for fluidized bed operations such as water filtration. Activated carbon from animal and wood charcoal also contains impregnated inorganic cations such as silver (Ag), aluminum (Al), manganese (Mn), Silicon (Si), Iron (Fe). These are widely known for its antimicrobial and antiseptic activity. Carbon blocks can be coated with compatible biopolymer to give smooth and permeable coat for number of pharmaceutical adsorption processes like removal of toxic alkaloids from potent drug substance by adsorption.
Properties of activated carbon
Activated carbon is characterized by Iodine number (mg of iodine adsorbed per gram of activated carbon). They are very suitable for dechlorination and iodine extraction. In addition, they can also adsorb molasses (decolorizing molasses), organic tannins (in ppm range), carbon tetrachloride and methylene blue. They possess apparent density of 0.4 – 0.5 g/cm3 due to presence of air spaces and solid or skeletal density of approximately 2 g/cm3. Apparent surface area from 500 m2 – 1500 m2 is achievable due to presence of large interstitial air spaces. Microscopic examination reveals intensely convoluted spherical particles with variation in porosity. This indicates that individual particles bind each other through van der wall forces or London dispersion forces of attraction. Particles can be combined with ionic compounds for ion-exchange chromatography process. Activated carbon is an excellent abrasive and resistant to erosion. It is widely useful for processes where friction is desired for rubbing, abrasion and cleaning of metals. A major drawback to activity of activated carbon is the ash content. Ash reduces the overall activity or efficiency. Leaching of metal oxides such as ferric oxide from activated carbon is indicated by discoloration and presence of ash.
Application and uses
Activated carbon is capable of producing large number of organic products chiefly used in biotechnology industries. As a medicine it is used to treat metal poisoning and drug overdose followed by oral ingestion. Activated carbon by oral ingestion in form of tablet or capsule can help resolve gastrointestinal problems of flatulence, diarrhea and constipation. It is a good compound for decaffination process in medicine or industrial coffee production. It is used to treat a number of oral poisonings including effect of strong acids, metal poisoning and organic compounds like ethanol, alkaloids, pesticides.
Activated carbon is antidote prepared in combination with starch and other binders.However, activated carbon is considered carcinogenic if inhaled through respiratory tract as it interferes in pulmonary circulation of air in lungs and oxygen in blood. Activated carbon is widely used in making air filters and also used as a component of medicine in pharmaceutical industries.
Carbon filtering is a technique that uses bed of activated carbon to remove air contaminants and impurities by chemical adsorption. It is very effective in removing hazardous vapors and many toxic gases of atmosphere such as oxides of nitrogen (NOx), oxides of sulfur (SOx), oxides of carbon (COx), chlorinated compounds and inert gases. Typical particle size that can be removed by carbon filtration ranges from 0.5 - 50 micrometers. The particle size is used as part of the filter description to indicate its filtration capacity.
Each particle or granule embedded in layer provides a large surface area of internal pores for removal of contaminants with maximum possible exposure to the active sites within the filter media.
One pound (450 g) of activated carbon contains surface area of approximately 100 acres (40 hectares) for filtration activity.
Activated carbon is commonly used in laboratory-scale purification of organic molecules, which contain unwanted color of organic impurities. It is used for decolorization of sugar and other organic matter. Activated carbon is used in filtration of beverages (alcohol and non-alcoholic). It filters vodka and whiskey from organic impurities which can affect color, taste, and odor. Activated carbon (charcoal) is an allowed substance used by organic farmers in both livestock production and wine making.
In livestock production it is used as a pesticide, animal feed additive, processing aid, nonagricultural ingredient and disinfectant. In organic winemaking, activated carbon is allowed for use as a processing agent to absorb brown color pigments from white grape concentrates. Powdered Activated Carbon Treatment (PACT) is a wastewater technology in which powdered activated carbon is added to an anaerobic or aerobic treatment system.
The carbon in the biological treatment process acts as a "buffer" against the effects of toxic organics in the wastewater. The addition of the powdered activated carbon stabilizes biological systems against upsets and shock loading, controls color and odor, and may reduce disposal costs while removing soluble organics.
(The author is M.D. Of VMG Biotech Consultants, New Delhi)