This column is based on “Activated Carbon Production Process – Cost Analysis,” a report published by Intratec. It can be found at: www.intratec.us/analysis/activated-carbon-production-cost.
Activated carbon, also known as activated charcoal or activated coal, is an inert solid composed mostly of carbon atoms, processed to have porous structure and a large internal surface area. These unique characteristics impart adsorptive properties that make activated carbon attractive for use in a range of liquid- and gas-phase applications.
The following paragraphs describe activated carbon production from coconut shells, which involves two main steps: carbonization of coconut shells into shell charcoal, followed by steam activation at high temperatures. Figure 1 presents a simplified flow diagram of the process.
Carbonization. Initially, coconut shells are fed to a crusher, where the size of the shells are reduced. The crushed material is fed to a flash dryer to remove moisture, and then fed to a rotary kiln to be carbonized. The shell fragments are heated under nitrogen in such a way that non-carbonaceous material — elements such as hydrogen and oxygen — is volatilized and eliminated from the precursor. Carbonization produces charcoal, with bio-oil vapors, steam and incondensable gases as byproducts. At this point, a carbon skeleton possessing a latent pore structure is formed. Oils and tars separated as byproduct are used for fuel.
Steam activation. In a fluidized-bed reactor, the charcoal is activated by reaction with steam at a temperature of 900–1,100°C under controlled atmosphere. The activation promotes the elimination of volatile components and the simultaneous oxidation of the outer surface of the charcoal, forming active sites. The gas escapes from the charcoal, leaving behind pores in the carbon solid. Air is also fed to the reactor, for the combustion of the carbon monoxide and hydrogen formed during activation. Carbon monoxide and H 2 are converted to steam and carbon dioxide.
Then the activated carbon undergoes a sequence of steps in which it is cooled and ground to the desired particle size. The activated carbon is cooled to ambient temperature by indirect cooling, and then ground into powder of specified mesh size. At this point, the finished powdered activated carbon is directed to a packing system.
Activated carbon can be produced from several carbon-rich raw materials, such as coal, lignite, wood, pitches and agricultural and forestry wastes. When it comes to manufacturing processes, there are basically two main activation methods: chemical activation, based on the dehydrating action of chemicals; and physical activation, based on the use of gases, such as steam or carbon dioxide. Several different pathways for activated carbon are presented in Figure 2.
The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce activated carbon was about $2,400 per ton of activated carbon in the first quarter of 2014. The analysis was based on a plant constructed in the U.S. with capacity to produce 10,000 metric ton per year of activated carbon.
Edited by Scott Jenkins
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