Spray-columns backmixing

Liquid Dispersion Spray columns are used with slurries or when the reaction product is a solid. The absorption of SO9 by a hme slurry is an example. In the treatment of phosphate rock with sulfuric acid, offgases contain HF and SiF4. In a spray column with water, solid particles of fluorosilic acid are formed but do not harm the spray operation. The coefficient /cl in spray columns is about the same as in packed columns, but the spray interfacial area is much lower. Considerable backmixing of the gas also takes place, which helps to make the spray volumetri-caUy inefficient. Deentrainment at the outlet usually is needed. 

Baffle Tray Columns Baffle tray columns are similar to spray columns except that baffles are added to reduce backmixing. The... 

Spray Column. The simplest differential extractor is the spray column (Fig. 18a.), which depends upon the initial dispersion of the dispersed phase to create favorably sized droplets. There is no means provided to redisperse this phase if any coalescence occurs. Although the equipment is simple and inexpensive, it is difificultto obtain more than one stage extraction. The passage of the dispersed phase ind uces considerable backmixing of the continuous phase, particularly in larger diameter columns. 

Spray Columns. As in gas absorption, axial dispersion (backmixing) in the... 

Continuous, Cocurrent, Countereurrent Cross-flow Completely backmixed ( ) Plug flow Spray column... 

Spray contactors ate paiticularly important for the absorption of impurities from large volumes of flue gas where low pressure drop is of key importance. They are used where materials in die liquid phase (e.g., particles of limestone) or in the gas phase (e.g., droplets of tar) may cause plugging of packing or trays. Other important applications of spray contactors (which are outside the scope of this discussion) include particulate removal and hot gas quenching. When used for absoiption, spn devices are not applicable to difficult separations and generally are limited to about four transfer units even with countercurrent spray column designs. The low efficiency of spray columns is believed to be due to entrainment of droplets in the gas and backmixing of the gas induced by the sprays. 

At low operating pressures, due to the large gas volumes, plate columns exhibit pronounced droplet entrainment, which lowers the separation performance due to backmixing of the liquid. The throughput can be increased by introducing into the spray layer a 5-10 high packed layer that acts as a mist collector. 

---