Filter/diyer

Slurries of free-filtering sohds that are difficult to suspend are sometimes filtered on a top-feed drum filter or filter-diyer. An example application is in the production of table salt. An alternative for slurries of extremely coarse, dense solids is the internal drum filter. In the chemical-process industiy both top-feed and internal drums (which are described briefly by Emmett in Schweitzer, op. cit., p. 4-41) have largely been displaced by the horizontal vacuum filter (q.v). 

On all rotating diyers, the vapor-outlet tube is stationary it enters the shell through a rotating gland and is fitted with an elbow and an upward extension so that the vapor inlet, usually protected by a felt dust filter, will be at all times near the top of the shell. 

Two-fluid nozzles do not operate efficiently at high capacities and consequently are not used widely on plant-size spray diyers. Their chief advantage is that they operate at relatively low pressure, the hq-uid being 0 to 400 kPa/m" pressure, while the atomizing fluid is usually no more than 700 kPa/m" pressure. The atomizing fluid may be steam or air. Two-fluid nozzles nave been employed for the dispersion of thick pastes and filter cakes not previously capable of being handled in ordinaiy atomizers [Baran, Ind. Eng. Chem., 56(10), 34-36 (1964) andTurba, Brit. Chem. Eng., 9(7), 457-460 (1964)]. 

In filter operation, it is essential that the gas be kept above its dew point to avoid water-vapor condensation on the bags and resiilting plugging of the bag pores. However, fabric filters have been used successfully in steam atmospheres, such as those encountered in vacuum diyers. In such cases, the housing is generally steam-traced. 

Of the three categories, the packed column is by far the most commonly used for the absorption of gaseous pollutants. Miscellaneous gas-absorption equipment could include acid gas scrubbers that are commonly classified as either wet or diy. In wet scrubber systems, the absorption tower uses a hme-based sorbent liquor that reacts with the acid gases to form a wet/solid by-product. Diy scrubbers can be grouped into three catagories (1) spray diyers (2) circulating spray diyers and (3) dry injection. Each of these systems yields a diy product that can be captured with a fabric filter baghouse downstream and... 

Commercially available centrifugal filters and spin-diyers are of very diverse geometries. We do not describe them here, but we wished to point out how the dimensioning of these apparatus is akin to that of a filtration device, and how the centrifugal force intervenes as the driving force of filtration. 

Spray diyer/hibric filter systems remove other air pollutants along with the S02- Heavy metals in particulate form are removed. Mercury, which can be present in several forms, may also be removed depending on the form and other factors. Very high mercury removal can be achieved with the injection of lime slurry containing activated carbon. Hydrochloric and hydrofluoric acids are removed with spray dryers (as they are in wet FGD systems) (Brown and Felsvang, 1991). SO3/H2SO4 are also removed with very high efficiency (but are not removed efficiently in wet FGD systems). 

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