Activated carbons are versatile adsorbents. Their adsorptive properties are due to their high surface area, a microporous structure, and a high degree of surface reactivity. They are, used, therefore, to purify, decolorize, deodorize, dechlorinate, separate, and concentrate in order to permit recovery and to filter, remove, or modify the harmful constituents from gases and liquid solutions. Consequently, activated carbon adsorption is of interest to many economic sectors and concern areas as diverse as food, pharmaceutical, chemical, petroleum, nuclear, automobile, and vacuum industries as well as for the treatment of drinking water, industrial and urban waste water, and industrial flue gases.
Activated carbon in its broadest sense includes a wide range of processed amorphous
carbon-based materials. It is not truly an amorphous material but has a microcrystalline structure. Activated carbons have a highly developed porosity and an extended interparticulate surface area. Their preparation involves two main steps: the carbonization of the carbonaceous raw material at temperatures below 800°C in an inert atmosphere and the activation of the carbonized product. Thus, all carbonaceous materials can be converted into
activated carbon, although the properties of the final product will be different, depending on the nature of the raw material used, the nature of the activating agent, and the conditions of the carbonization and activation processes.
During the carbonization process, most of the noncarbon elements such as oxygen, hydrogen, and nitrogen are eliminated as volatile gaseous species by the pyrolytic decomposition of the starting material. The residual elementary carbon atoms group themselves into stacks of flat, aromatic sheets cross-linked in a random manner. These aromatic sheets are irregularly arranged, which leaves free interstices. These interstices give rise to pores, which make activated carbons excellent adsorbents.
During carbonization these pores are filled with the tarry matter or the products of decomposition or at least blocked partially by disorganized carbon. This pore structure in carbonized char is further developed and enhanced during the activation process, which converts the carbonized raw material into a form that contains the greatest possible number of randomly distributed pores of various sizes and shapes, giving rise to an extended and extremely high surface area of the product. The activation of the char is usually carried out in an atmosphere of air, CO2, or steam in the temperature range of 800°C to 900°C. This results in the oxidation of some of the regions within the char in preference to others, so that as combustion proceeds,
a preferential etching takes place. This results in the development of a large internal surface, which in some cases may be as high as 2500 m2/g.
Activated carbons have a microcrystalline structure. But this microcrystalline structure differs from that of graphite with respect to interlayer spacing, which is 0.335 nm in the case of graphite and ranges between 0.34 and 0.35 nm in activated carbons. The elemental composition of a typical activated carbon has been found to be 88% C, 0.5% H, 0.5% N, 1.0% S, and 6 to 7% O, with the balance representing inorganic ash constituents. The oxygen content of an activated carbon can vary, however, depending on the type of the source raw material and the conditions of the activation process.
The most important application of activated carbon adsorption where large amounts of activated carbons are being consumed and where the consumption is ever increasing is the purification of air and water. There are two types of adsorption systems for the purification of air. One is the purification of air for immediate use in inhabited spaces, where free and clean air is a requirement. The other system prevents air pollution of the atmosphere from industrial exhaust streams. The former operates at pollutant concentrations below 10 ppm, generally about 2 to 3 ppm. As the concentration of the pollutant is low, the adsorption filters can work for a long
time and the spent carbon can be discarded, because regeneration may be expensive. Air pollution control requires a different adsorption setup to deal with larger concentrations of the pollutants. The saturated carbon needs to be regenerated by steam, air, or nontoxic gaseous treatments. These two applications require activated carbons with different porous structures. The carbons required for the purification of air in inhabited spaces should be highly microporous to affect greater adsorption at lower concentrations. In the case of activated carbons for air pollution control, the pores should have higher adsorption capacity in the concentration range 10 to 500 ppm.
For personal protection when working in a hostile environment, the
activated carbons used in respirators are also different. When working in the chemical industry, the respirators can use ordinary activated carbons because the pollutants are generally of low toxicity. However, for protection against warfare gases such as chloropicrin, cynogen chloride, hydrocynic acid, and nerve gases, special types of impregnated activated carbons are used in respirators and body garments. These activated carbons can protect by physical adsorption, chemisorption, and catalytic decomposition of the hazardous gases.
More than 800 specific organic and inorganic chemical compounds have been identified in drinking water. These compounds are derived from industrial and municipal discharge, urban and rural runoff, natural decomposition of vegetable and animal matter, and from water and waste water chlorination practices. Liquid effluents from industry also discharge varying amounts of a variety of chemicals into surface and ground water. Many of these chemicals are carcinogenic and cause many other ailments of varying intensity and character. Several methods such as coagulation, oxidation, aeration, ion exchange, and activated carbon adsorption have been used for the removal of these chemical compounds.
Active carbons in the form of carbonized wood charcoal have been used for many centuries. The Egyptians used this charcoal about 1500 BC as an adsorbent for medicinal purposes and also as a purifying agent. The ancient Hindus in India purified their drinking water by filtration through charcoal. The first industrial production of active carbon started about 1900 for use in sugar refining industries. This active carbon was prepared by the carbonization of a mixture of materials of vegetable origin in the presence of metal chlorides or by activation of the charred material by CO2 or steam. Better quality gas-adsorbent carbons received attention during World War I, when they were used in gas masks for protection against hazardous gases and vapors.
Nearly 80% (~300,000 tons/yr) of the total active carbon is consumed for liquid-phase applications, and the gas-phase applications consume about 20% of the total production. Because the active carbon application for the treatment of waste water is picking up, the production of active carbons is always increasing. The consumption of activecarbon is the highest in the U.S. and Japan, which together consume two to four times more active carbons than European and other Asian countries. The per capita consumption of active carbons per year is 0.5 kg in Japan, 0.4 kg in the U.S., 0.2 kg in Europe, and 0.03 kg in the rest of the world. This is due to the fact that Asian countries by and large have not started using active carbons for water and air pollution control purposes in large quantities.
Coconut shell and palm shell are the best raw materials for manufacturing of activated carbon because of its hardness. Indonesia is the largest coconut plantation with about 4 million hectares and the largest CPO producer in the world with palm oil plantations more than 7 million ha, which is the ideal location of the source of raw material for production of activated carbon. Indonesian coconut plantation area occupies 31.4% of the world with vast coconut plantations of coconut production of approximately 12.915 billion items (24.4% of world production). Coconut shell weight reached 12% of the weight of coconuts. With weight of coconut average 1.5 kg, the potential of Indonesia, namely coconut shell 2.3 million tons / year. The number of palm oil mills in Indonesia and Malaysia more than 800 units. When an palm oil mill with a capacity of 30 tons of fresh fruit bunches per hour, it will produce 1.95 tonnes of palm shells / hour or about 46.8 tons / day. Certainly the number of very abundant for the production of activated carbon.
Our continuous pyrolysis technology with capacity 60 up to 200 ton/day INPUT would very reliable in the process of carbonization. Integration our continuous pyrolysis technology in activated carbon plant would be very beneficial because the most efficient processes (self sustaining process with syngas) and all products can be drawn.
Quantitatively differences between the pyrolysis, gasification and combustion: based on the needs of the air needed during the process, i.e. as follows
Pyrolysis have a number of advantages are as follows:
