Activated Carbon and Activation Process

Activated carbons are defined as a group of highly porous materials with high carbon content and surface area. They are widely used, such as industrial gas separation, odor control, potable water and waste water treatment. The high efficiency in removing pollutants make activated carbon more suitable for adsorption purposes wherever the latter are needed. (Prakash et al. 1994; Mochida et al. 2000; Wu et al. 2005). 

Аctivated carbons are available in two forms: granular activated carbon (GAC) and powdered activated carbon (PAC), which depends on activation process techniques. There are two basic methods of producing activated carbon from carbonaceous materials: physical activation, which is also called thermal activation, and chemical activation. In physical activation, normally steam and carbon dioxide are used as activating agents, the activation temperature being maintained at 700 °C to 1100 °C. In case of chemical activation, some selected salts, bases and acids are used as activation agents; these agents are often called impregnating chemicals. In both cases, the procedure involves two stages of production: carbonization and activation. Chemical activation has been reported to result in producing activated carbon of higher quality compared to physical activation techniques while the precursors used were the same (Paredes et al. 2006; Ajayi and Olawale 2009).

Activated Carbon Preparation

Activated carbon preparation comprises two consecutive stages: carbonization and activation. Carbonization involves the endothermic decomposition of non-carbon precursors by means of volatilization. To be more precise, in the carbonization stage, precursors are heated in the absence of oxygen to produce charcoal-like material as a result of removing non-carbonaceous substances. The carbonization stage takes place in the inert gas atmosphere by purging gaseous nitrogen and in a stationary rotary or fluidized bed oven (Bansal et al. 1988). At this stage, carbon char formation is largely due to the nature of the carbonaceous material, the ultimate temperature of heating, the rate and the time of heating, as well as the current atmospheric conditions. Among these, temperature is a key factor. A low rate of heating leads to lessening volatilization and shrinkage of carbon precursors, which has a positive effect on the formation of micropores, whereas the high rate of heating damages micropores, enlarges mesopores and successive macropores (Bansal et al. 1988; Ji et al. 2007). Carbonization at a higher ultimate temperature may also cause the formation of graphite which is difficult to activate rather than carbon char. The temperature of carbonization normally ranges from 400 °C to 600 °C (Jankowska et al. 1991; Guo and Lua 2000; Bouchelta et al. 2008). Some author report carbonization temperatures as high as 1000 °C (Wang et al. 2009; Yang et al. 2010). This paper presents the results of the research on petroleum coke carbonization techniques within three hours at the temperature of 500 to 700 degrees.

The activation stage involved the production of porous carbon (activated carbon) with desirable physical and chemical properties in the course of the reaction between carbon char and the activating agents at high temperature. At this stage, the decomposed substances produced during carbonization and trapped inside the developed pores were removed. As a result, micropores increase in number. Physical activation and chemical activation are the main processes which occur throughout the stage and which are involved in the degradation, decomposition and removal of non-carbon products as well as unstructured carbons. The overall reaction rate is limited by the mass transport rate and the rate of chemical reaction between the activation agent and the carbonaceous material (Bansal et al. 1988).

Physical Activation

Physical activation occurs due to the reaction between the carbon char produced during carbonization and the activating agent at a high temperature varying from 800 °C to 1000 °C. As activating agents, steam, CO2, N2, air, the combination of steam and CO2, or steam and N2 combination, are used.

Oxygen is the most active agent, but it is not commonly applied because the reaction is very difficult to control. A process control when oxygen was used as an activating agent is reported by in 2008. It was an exothermic process (Williams and Parkes 2008). At higher temperatures, carbon atoms at the precursor’s surface as well as in the incipient micropores react with the activating agents and are removed mainly as carbon monoxide or carbon dioxide. This loss of carbon enlarges the existing micropores and develops new micropores, which results in producing a highly porous material.

Among various activating agents used for physical activation, steam is the most widely used when it comes to activated carbon production. The reactions which take place in the course of steam activation are as follows (Jankowska et al. 1991):

C + H2O = CO + H2

C + 2H2O = CO2 + 2H2

CО + H2O = CO2 + H2

C + 2H2 = CH4

C + CO2 = 2CO

Listed below are the reactions for CO2 and oxygen which are normally considered throughout the activation process (Bansal et al. 1988):

C + CO2 = 2CO [-39 Kcal]

C + O2 = 2CO [+53 Kcal]

C + O2 = CO2 [+92,4 Kcal]