Decolorization with Powdered Activated Carbons

Decolorization of sugar solutions using powdered active carbons can be carried out by two methods contact batch method and the continuous layer filtration method. There is a third method, which is a combination of both the methods. 

The contact batch method is a batch process in that a given amount (5 to 10 kg of syrup) of the activated carbon is added to the sugar syrup placed in a container. 

FIGURE 5.1 Schematic diagram of a two-step sugar syrup purification. (From Jankowska, H., Swiatkowski, A., and Choma, J., in Active Carbon, Ellis Howard, Ltd. England, 1991. With permission.) 

The resulting suspension is kept at a temperature of 80 to 90°C for about 20 min, which is sufficient time to attain adsorption equilibrium. The suspension is then pumped into filter presses to remove the active carbon. During filtration the thickness of the filter cake (carbon bed) increases to 25 to 35 mm, and this results in an increase in the hydrodynamic resistance from 2 x 10 to 4 x 10 Pa. Therefore, the pressure during filtration has to be increased from 2 atm to 4 atm. About 600 kg of carbon is collected in Im of filter volume. When the filter press is full, the active carbon is sweetened off by washing with 6 to 9 times its weight of water. The active carbon is then removed and the filter press assembled again. 

Continuous layer filtration involves filtering the sugar syrup through a layer of activated carbon. Several types of filters are used, such as pressure leaf filters with metal frames on which a filter cloth that may be cotton, polyamide, or wire mesh is fixed rotary leaf filters or bed filters in which the filtering medium is a ceramic or sintered plate, wire mesh, or finely perforated metal plate. The latter filters are usually coated with a layer of filter aid that may be a diatomaceous earth. A suspension of active carbon in water or liquor is passed through the filter until a uniform layer of active carbon bed 10 to 15 mm thick builds up. The filter is then ready for filtration of the liquor that must flow to the filter at a uniform rate to avoid breaking the layer. 

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Alternative solutions to the problem of pH control in dextrose refining are the use of powdered activated carbon, which decolorizes the liquor but removes little ash or the use of ion exchangers, which effect a substantially quantitative removal of ash, plus treatment with powdered activated carbon to remove color. 

Complex mycelium pellets of Trametes versicolor with activated carbon powder were investigated for decolorization of Acid Violet 7 [49]. The complex pellets showed the best dye removal. The dye was almost completely removed in 6 h. For complex pellets, maximum decolorization rate (Vmax mg/L h) and half velocity concentration (Ks mg/L) was calculated 130.5 and 345.0 in batch system, respectively. 

New preparation. Carpino1 reports that an active form of manganese dioxide can be prepared by treating aqueous permanganate solutions with decolorizing carbon.4 A brown-black powder is precipitated on the excess carbon. If this... 

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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