Plate Tower Diameter

The empirical constant, /, for different types of trays are shown in Table 7.5. 

Gas absorber is used to remove SOj from a gas stream with pure water in a tray tower. The gas stream inlet molar flow rate is 206 kmol/h (3% SOj, 97% air) and the water inlet molar flow rate is 12,240 kmol/h. The temperature is 293 K and the pressure is 101.32 kPa. The column tray type is bubble cap. The concentration of SO2 in the exit air should be less than 500 ppm. Determine the number of bubble cap trays and tower diameter. 

Source Data from Calvert, S. et al. 1972. Wet Scrubber System Study, Vol. 1, Scrubber Handbook, EPA-R2-72-118a, U.S. Environmental Protection Agency, Washington, DC. 

The minimum acceptable diameter of the plate tower is determined using the following equation  

Selection of the suitable thermodynamic fluid package is very important to get the correct answer. NRTF was selected as the property estimation system using 

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If the tower diameter is <3 ft use a packed tower instead of plates... 

PM Impingement-plate tower collection efficiencies range from 50 to 99 percent, depending upon the application. This type of scrubber relies almost exclusively on inertial impaction for PM collection. Therefore, collection efficiency decreases as particle size decreases. Short residence times will also lower scrubber efficiency for small particles. Collection efficiencies for small particles (< 1 fim in aerodynamic diameter) are low for these scrubbers hence, they are not recommended for fine PM control. 

Reduetion in top reflux permits a reduction in the tower diameter in the section above the pump-around, thereby reducing investment. However, the reduction in reflux requires the addition of more fractionating plates to maintain the same separation efficiency. 

Random-packed towers are seldom designed with diameters larger than 4 ft, and diameters of commercial plate towers are seldom less than 2 ft. 

When column diameters are less than 0.6 m (2.0 ft) packed towers can be considerably cheaper. However, if alloy metals are necessary, plate towers may result in less cost. Using ceramic or other similar resistant materials for packing and materials of construction, packed towers can serve to handle corrosive materials and acids. Because the gas flow in packed towers may offer less degree of agitation, packed tower operation may be better for liquids that tend to foam. When liquids are thermally sensitive, packed columns may offer less holdup and thus prevent changes taking place in the liquids due to thermal reaction. 

W. L. Bolles Rapid Graphical Method of Estimating Tower Diameter and Tray Spacing of Bubble-Plate Fractionators, Pet. Refiner, 25(12) 103 (1946). 

Equilibrium-stage methods are usually adequate for nearly ideal distillation systems when coupled with calculations of plate efficiency to estimate actual trays or, in the case of packed towers, when HETS (height equivalent of a theoretical stage) or HETP (height equivalent to a theoretical plate) values are known from experience or from experiment to enable the estimation of packed height. For absorbers, strippers, and nonideal distillation systems, mass-transfer models are preferred, but their use requires a value for the tower diameter and a tray layout or type and size of packing. Even when mass-transfer models are preferred, initial calculations are usually made with equilibrium-stage models. Also, note that data for reliable mass-transfer coefficients is often difficult to obtain. 

Perforated-plate Towers. In the perforated-plate, or sieve-plate, column, the dispersed phase is repeatedly coalesced and redispersed by causing it to flow through a series of trays in which a large number of small holes have been punched or drilled. In the simplest type, the plates are similar to the side-to-side baffles described above, except that they are perforate. Hunter and Nash (42) describe a successful installation of this type for dephenolating gas liquor consisting of a 46-ft,-high shell, 5 ft. in diameter, in which the baffles each contain two hundred holes. 

Operating and optimum reflux ratio. For the case of total reflux, the number of plates is a minimum, but the tower diameter is infinite. This corresponds to an infinite cost of tower and steam and cooling water. This is one limit in the tower operation. Also, for minimum reflux, the number of trays is infinite, which again gives an infinite cost. These are the two limits in operation of the tower. 

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