Granulated active carbon beds

Recovery/Removal via Granulated Active Carbon Bed Granulated active carbon (GAC) beds are used to remove undesired fluorinated low molecular weight substances from manufacturing plant effluents. The first application of this method to perfluorinated emulsiflers was described by Russian authors [36]. Although... 

Granulated activated carbon is thus preferred, if a high volume stream continuous purification operation makes it necessary. Thus gas purification is almost exclusively carried out in fixed-bed adsorbers with granulated carbon. Potable water in Western Europe and Japan is preferably freed of organic impurities by percolation over granulated activated carbon. 

Small particle size activated carbon will produce a high pressure drop through the activated-carbon bed. Figure 22.1.12 compares the pressure drop of cylindrically-shaped activated carbon pellets with activated-carbon granulates. The activated carbon particle diameter must not be excessively large, because the long diffusion distances would delay adsorption and desorption. Commercially, cylindrical pellets with a particle diameter of 3 to 4 mm have been most efficient. 

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. 

The performance of an activated carbon, in industrial use, depends not only on the capacity of an activated carbon but also on its retentivity, that is for how long can the carbon retain the adsorbed species, under operating conditions, without it being subsequently desorbed. Activated carbons are used in beds with the gas, vapor or liquid passing through the bed. From a practical point of view it is desirable to operate by minimization of the frequency of replacement of the activated carbon bed. The characterizations, which have been elaborated upon (Section 4.2) clearly indicate the desirability to maximize the porosity of maximum adsorption potential accessible to the adsorptive at those low relative concentrations of adsorptive which are found in industrial situations. Other factors include rates of adsorption which are dependent on intra- and inter-diffusion of the adsorbate and granules and bed construction. At the same time, these adsorbed materials must remain within the activated carbon for as long as possible. 

Loosely filled granulated activated carbon is better suited to large deep-bed filters, which often require media depths of up to 1 m. Low depth, loosely filled filters can prove less effective due to their tendency to settle and allow the air to bypass the activated carbon medium or for the activated carbon itself to escape from the filter in the air stream and form into carbon dust. Loosely filled systems will almost always be specified for highly contaminated applications, such as some industrial processes, where deep-bed filters are required. 

The adsorbers are usually built of steel, and may be lagged or left unlagged the horizontal type is shown in Figure 28. The vapor-laden air is fed by the blower into one adsorber which contains a bed of 6- to 8-mesh activated carbon granules 12 to 30 inches thick. The air velocity through the bed is 40 to 90 feet per minute. The carbon particles retain the vapor only the denuded air reaches the exit, and then the exhaust line. The adsorption is allowed to continue until the carbon is saturated, when the vapor-laden air is diverted to the second adsorber, while the first adsorber receives low-pressure steam fed in below the carbon bed. The vapor is reformed and carried out by the steam. The two are condensed and if the solvent is not miscible with water, it may be decanted continuously while the water is run off similarly. After a period which may be approximately 30 or 60 minutes, all the vapor has been removed, the adsorbing power of the charcoal has been restored, and the adsorber is ready to function again, while adsorber No. 2 is steamed in turn. 

The sorbitol solution produced from hydrogenation is purified in two steps [4]. The first involves passing the solution through an ion-exchange resin bed to remove gluconate and other ions. In the second step, the solution is treated with activated carbon to remove trace organic impurities. The commercial 70% sorbitol solution is obtained by evaporation of the water under vacuum. The solid is prepared by dehydration until a water-free melt is obtained which is cooled and seeded. The crystals are removed continuously from the surface (melt crystallization). The solid is sold as flakes, granules, pellet, and powder forms in a variety of particle size distributions. 

Absorption of harmful organic compounds by activated carbons from gas and liquid media is of interest and importance for human and environmental protection purposes.1"21 The influence of the texture of carbon granules (size and volume of pores, specific surface area, granule size d, and carbon bed depth V), gas stream humidity and velocity, and amounts of pre-adsorbed water are investigated on adsorption of organics in different media.1 21... 

Powdered activated carbon (PAC) is used for the same purposes as GAC. The main difference between PAC and GAC is the smaller particle size of the powders (typically around 44 pm versus 0.6-4.0 mm for granules), which allows faster rates of adsorption [67]. However, because of the greater difficulty in handling, fine powders cannot be used in fixed-bed operations without incurring a high pressure drop, and there are associated problems in regenerating them for reuse. Hence, they are invariably used as disposable additives. 

Powdered activated carbons offer the advantage of low cost compared to granules in terms of both purchase price and capital expenditure (investment in adsorber units, pumps, etc). The cost of PAC is about 1.00/kg versus 2/kg for GAC [19]. For a system treating 4 million/day the cost is about 0.03/1,000 L [67], A wider range of impurity removal levels can be attained with powdered carbon, where the dose of carbon per batch can be adjusted, depending on the type and concentration of the contaminants [68]. GAC is normally used in continuous flow deep beds and is advantageous when variations in adsorption condi-... 

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