Granular-bed filters

Self and co-workers, "Electric Augmentation of Granular Bed Filters," EPA-600/9-80-039c, p. 309, 1980. 

Granular-bed filters may be divided into three classes ... 

Cleanable Granular-Bed Filters The principal objective in the development of cleanable granular-bed filters is to produce a device that can operate at temperatures above the range that can be tolerated with fabric filters. In some of the devices, the granules are circulated continuously through the unit, then are cleaned of the collected dust and returned to the filter bed. In others, the granular bed remains in place but is periodically taken out of service and cleaned by some means, such as backflushing with air. 

Calcium oxide (lime) Rotary kilns, vertical and shaft kilns, fluidized bed furnaces Particulate matter Cyclones plus secondary collectors (baghouse, ESP, wet scrubbers, granular bed filters, wet cyclones)... 

Granular bed filters are used in ten coil coating plants to remove residual solids from the clarifier effluent, and are considered to be tertiary or advanced wastewater treatment. Chemicals may be added upstream to enhance the solids removal. Pressure filtration is also used in this industry to reduce the solids concentration in clarifier effluent and to remove excess water from the clarifier sludge. Figure 7.4 shows a granular bed filter and Table 7.13 presents the heavy metal removal data of a lime clarification and filtration system. 

Granular bed filters are used in porcelain enameling wastewater treatment to remove residual solids from clarifier effluent (sedimentation effluent or flotation effluent). Filtration polishes the effluent and reduces suspended solids and insoluble precipitated metals to very low levels. Fine sand and coal are media commonly utilized in granular bed filtration. The filter is backwashed after becoming loaded with solids and the backwash is returned to the treatment plant influent for removal of solids in the clarification step.10-12... 

Fixed Granular-Bed Filters Fixed-bed filters composed of granules have received considerable theoretical and experimental study [Thomas and Yoder, AMA Arch. Ind. Health, 13, 545 (1956) 13, 550 (1956) Knettig and Beeckmans, Can. J. Chem. Eng., 52, 703 (1974) Schmidt et al.,/. AirPollut. Control Assoc., 28,143 (1978) Tar-dos et al., J. Air Pollut. Control Assoc., 28, 354 (1978) and Gutfinger and Tardos, Atmos. Environ., 13,853 (1979)]. The theoretical approach is the same as that used in the treatment of deep-bed fibrous filters. 

FIG. 17-61 Electrically augmented granular-bed filter. Combustion Power... 

For the purification of water supplies and for waste water treatment where the solid content is about 10 g/m3 or less, as noted by Cleasby(23) granular bed filters have largely replaced the former very slow sand filters. The beds are formed from granular material of grain size 0.6-1.2 mm in beds 0.6-1.8 m deep. The very fine particles of solids are removed by mechanical action although the particles finally adhere as a result of surface electric forces or adsorption, as Ives 24 points out. This operation has been analysed by Iwasaki 25) who proposes the following equation ... 

Let us apply the interpolation procedure to a case involving an electric field. It is well known that the efficiency of the granular bed filters can be significantly increased by applying an external electrostatic field across the filter. In this case, fine (<0.5-/rm) particles deposit on the surface of the bed because of Brownian motion as well as because of the electrostatically generated dust particle drift [51], The rate of deposition can be calculated easily for a laminar flow over a sphere in the absence of the electrostatic field [5]. The other limiting case, in which the motion of the particles is exclusively due to the electric field, could also be treated [52], When, however, the two effects act simultaneously, only numerical solutions to the problem could be obtained [51],... 

PRESSURE DROP AND SPECIFIC DEPOSIT FOR A GRANULAR-BED FILTER 14.19 REFERENCES 14.22... 

For filtering dust streams, a fixed-granular-bed filter with a length, L, of 1 m is filled with glass spheres of 5-mm (0.005-m) diameter, dm. The dust to be captured is limestone with a 0.1 -mm mean diameter, dd. The limestone density, pd, is 2700 kg/m3, and the gas velocity, u, is 0.5 m/s. The initial porosity, Cq, of the filter is 0.4, and the filtration process stops when the porosity has dropped to a level, e, of... 

As early as 2000 b.c., the Chinese used fixed granular beds for the filtration of wine. During the 20" century, granular bed filters have been widely used in the industry for cold and hot dry gas filtration. 

Table 3 Con arison between different granular bed filter types ... 

Figure 10 reports the filtration efficiency of the particles naturally contained in air at ambient temperature under the NMGBF prototype operating conditions mentioned in Table 4. Figure 10 shows that the gas-particle separation efficiency for submicronic particle is comprised between 54 to 96 %. Submicronic particles removal efficiency is the major limitation of the NMGBF as for granular bed filters. With particle diameters ranging from I to 10 pm, the efficiency reaches nearly 96%. 

Lipert, T. E. et al. (1981) Testing and verification of granular bed filters for removal of particulates and alkalies. High Temperature, High Pressure Particulate and Alkali Control in Coal Combustion Process Streams. In Proceedings U.S. DOE Contractors Meeting, CONF-810249, pp. 471-489. 

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