Extraction baffle towers

One of the most widely used spray-drying techniques for coffee extract involves spraying percolated coffee extract at 40°F into the top of a baffled tower of hot air at 480°F initially. The cooler wet air is removed toward the bottom of the tower and finally a powder of the coffee extract solids leaves the tower at about 90°F. These solids are in the form of hollow beads and contain 3% moisture. 

FIG. 15-41 Baffle towers, (a) Side-to-side flow at each tray, b) Center-to-center flow fdisk-and-doughnut style), fc) Center-to-side flow. [Reprinted frcm Treybai, Liquid Extraction (McGraw-HtU, 1963), with permisswn. Cepyright 1963 McGraw-Hill, Inc.]... 

BAFFLE TOWERS. These extraction towers contain sets of horizontal baffle plates. Heavy liquid flows over the top of each baffle and cascades to the one beneath light liquid flows under each baffle and sprays upward from the edge through the heavy phase. The most common arrangements are disk-and-doughnut baffles and segmental, or side-to-side, baffles. In both types the spacing between baffles is 1(K) to 150 mm (4 to 6 in.). 

Marked axial dispersion in both the liquid and solid phases has been observed in continuous countercurrent leaching systems. The solid-phase dispersion is probably caused by nonuniform conveying and by backmixing caused by the baffles which are used to prevent solid beds from turning en masse. VnliD n values of 16.1 m and 20 m, respectively, have b n repotted for sugar beet extraction in tower and slope extractors. Local fiow nonuniformity and larger-scale flow maldistribution are the primary factors that cause axial dispersion in the extract. 

Refer to Fig. 15-39. The tower is formed into compartments by horizontal doughnut-shaped or annular baffles, and within each compartment agitation is provided by a rotating, centrally located, horizontal disk. Somewhat similar devices have been known for some time. The features here are that the rotating disk is smooth and flat and of a diameter less than that of the opening in the stationaiy baffles, which facihtates fabrication and apparently improves extraction rates. The typical proportions of the internals of the RDC are as follows ... 

Baffles. Baffle mixers have frequently been used in conjunction with each of the flow-mixer types described above to provide longer contact time in the dispersed condition with relatively little expenditure of power. They are essentially orifice mixers with the orifices replaced by segmental baffles, the opening representing to 3 of the cross-sectional area of the pipe in which they are installed, and they are placed downstream from the mixer. The more moderate turbulence they create will frequently maintain a dispersion sufflciently well to permit rapid extraction. Alternatively, the downstream pipe may be filled with tower packing such as Raschig rings to accomplish the same purpose (10, 30, 50). 

Fig. 10.56, is the same in principle but is used in the larger diameter towers to reduce the length of travel of the liquid film on the baffle. Baffle spacing in these towers is ordinarily 4 to 6 in. The towers are versatile, capable of handling a wide variety of liquids and flow rates, and have had application in the extraction of acetic acid from pyroligneous liquors and from solutions used in cellulose acetate rayon manufacture, in the extraction of caffeine in the food industries, and for general chemical recovery in the synthetic organic chemicals industries. 

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