Decolorization with Granulated Activated Carbons

Granulated activated carbons are also frequently used for the decolorization of sugar solutions. The decolorization process can be carried out by several methods such as the fixed-bed method, the moving-bed method, and the countercurrent continuous methods. 

In the moving-bed method, the adsorbent carbon is periodically removed in small proportions from the bottom of the filter during the decolorization operation, and a fresh portion of the adsorbent is simultaneonsly added to the top of the bed. The adsorbent bed is thus gradually displaced in the countercurrent direction with respect to the continuous flow of the sugar liquor. 

In the countercurrent continuous method, which is very commonly used in many sugar processing plants, the sugar solution to be decolorized flows upward through the granular carbon bed that continuously moves downward in the adsorption column. The flow rate of the solution is so chosen that it expands the bed of the adsorbent 

The countercurrent continuous method is the most economical, because it utilizes fully the adsorption capacity of the carbon and requires much smaller space and building, and the amount of sweet water is reduced to a minimum. It also reduces sugar losses and the consumption of fuel. 

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Although bone char was historically used for sugar decolorization, the use of activated carbon has grown very rapidly. Color improvement is key. but the activated carbon also removes nitrogenous substances and other colloids. This improves both fiiterability and crystallization rate. In the dextrose industry, granulated carbon is used, and the spent carbon is reactivated with steam and air in rotary multiple-hearth furnaces. 

It is manufactured either by carbonization of carbonaceous materials with simultaneous chemical activation or by gas treatment of already carbonized material (e. g., charcoal). Chemical treatment is carried out at 400-1000 °C with phosphoric acid or zinc chloride and gas activation at 800-1000 °C in the presence of water or carbon dioxide. Several types of kilns and furnaces are in use. A. is available as powder, granules and special shaped products. The absorbent property of a. is used in gas and water purification and in solvent recovery. Well known is its use for decoloration and purification in food, chemical and pharmaceutical industries. 

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