Sieve plates mass transfer

Pulsed Columns. The efficiency of sieve-plate or packed columns is increased by the appHcation of sinusoidal pulsation to the contents of the column. The weU-distributed turbulence promotes dispersion and mass transfer while tending to reduce axial dispersion in comparison with the unpulsed column. This leads to a substantial reduction in HETS or HTU values. 

The pulsed-plate column is typically fitted with hori2ontal perforated plates or sieve plates which occupy the entire cross section of the column. The total free area of the plate is about 20—25%. The columns ate generally operated at frequencies of 1.5 to 4 H2 with ampHtudes 0.63 to 2.5 cm. The energy dissipated by the pulsations increases both the turbulence and the interfacial areas and greatly improves the mass-transfer efficiency compared to that of an unpulsed column. Pulsed-plate columns in diameters of up to 1.0 m or mote ate widely used in the nuclear industry (139,140). 

Three principal vapor—Hquid contacting devices are used in current crossflow plate design the sieve plate, the valve plate, and the bubble cap plate. These devices provide the needed intimate contacting of vapor and Hquid, requisite to maximizing transfer of mass across the interfacial boundary. 

Earlier studies in mass transfer between the gas-liquid phase reported the volumetric mass-transfer coefficient kLa. Since kLa is the combination of two experimental parameters, mass-transfer coefficient and mterfacial area, it is difficult to identify which parameter is responsible for the change of kLa when we change the operating condition of a fermenter. Calderbank and Moo-Young (1961) separated kta by measuring interfacial area and correlated mass-transfer coefficients in gas-liquid dispersions in mixing vessels, and sieve and sintered plate column, as follows ... 

Spray, packed, and sieve-plate columns give poor mass-transfer rates for consequently require greater height. The mass transfer in such columns can be significantly improved by providing mechanical agitation. Remen (1951) and Oldshue and Rushton (1952) introduced the rotating-disk contactor (see Fig. 26b) and the mixed column (see Fig. 26c). 

Batch reactors can also be used for studies of gas-liquid reactions. A common procedure, sometimes called "semibatch," is to conduct the reaction as batch with respect to the liquid and bubble the gas through at constant composition and pressure. Effective gas-liquid contacting is essential in order to avoid mass-transfer limitation with respect to the reactant supplied by the gas phase. Good ways of introducing the gas are through a fine-pore sieve plate or through the hollow shaft and arms of a stirrer. 

Plate columns are used for operations requiring a large number of transfer units, high pressure, high gas flow rates and low liquid flow rates, when it is necessary to supply or to remove heat, when solids are present in the liquid (or gas), and when the diameter is greater than 70 cm. They have the ability to handle large variations in gas and liquid flow rates. Mass-transfer data will be presented here for the most common designs— bubble-cap plates and sieve plates. 

The use of ozone os on oxidant for industrial wastes containing cyanides and other reducible toxic substances appears worthy of careful investigation. The oxidation of simple cyanides by ozone is rapid and complete. Mass transfer controls the absorption. The use of packed towers or sieve plate towers is indicated, and the maintenance of a pH of at least 9.0 is recommended. The destruction of cyanates and cyanide complexes is slower than the cyanide oxidation. These substances are destroyed if sufficient contact time and proper pH control are maintained so that these slower reactions can take place. The use of redox potential to control the degree of oxidation appears promising. Proper interpretation of the redox potential of the treated waste will give an excellent indication of the effectiveness of the treatment and the degree of removal of cyanide and cyanate. 

FIG. 15-38 Mass-transfer data for sieve plate and modified bubble plate columns. System benzoic acid + water + toluene, except where noted. To convert feet to meters, multiply by 0.3048 to convert inches to centimeters, multiply by 2.54. [Data taken from AUerton, Strmn, and Treybal,Tram. AIChE,39jp. 361 (1943) Row, Koffolt, and Withrow, Trans. AIChE, 37, p. 559 (1941) and Treyhal and Dumoulin, Ind. Eng. Chem.,34,p. 709 (1942).]... 

In pulsed sieve-plate towers, the entire column cross-section is occupied with trays, and thus the lighter phase passes through the holes in the upward stroke and the heavy phase in the downward stroke. This will continuously create new interfaces, which improves the mass transfer. By low pulsation intensities the dispersed phase is discon-tinuously moving through the holes (mixer-settler mode). The appropriate relation... 

Plate efficiency is a function of the rate of mass transfer between liquid and vapor. The prediction of mass-transfer coefficients in sieve trays and their relationship to plate efficiency are discussed in Chap. 21. Some published values of the plate efficiency of a 1.2-m column are shown in Fig. 18.34. This column had sieve trays with 12.7-mm holes and 8.32 percent open area, a 51-mm weir height, and... 

DISTILLATION PLATE EFFICIENCY. The two-film theory can be applied to mass transfer on a sieve tray to help correlate and extend data for tray effieiency. The bubbles formed at the holes are assumed to rise through a pool of liquid that is vertically mixed and has the local composition x. The bubbles change in composition as they rise, and there is assumed to be no mixing of the gas phase in the vertical direction. For a unit plate area with a superficial velocity the moles transferred in a thin slice dz are... 

Sieve-tray towers are very effective, both with respect to liquid-handling capacity and extraction efficiency, particularly for systems of low interfacial tension which do not require mechanical agitation for good dispersion. The general assembly of plates and downspouts is much the same as for gas-liquid contact except that a weir is not required. Towers packed with the same random packing used for gas-liquid contact have also been used for liquid extractors however, mass-transfer rates are poor. It is recommended instead that sieve-tray towers be used for systems of low interfacial tension and mechanically agitated extractors for those of high interfacial tension (Treybal, 1980). 

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