Spray towers liquid extraction

Extraction (sometimes called leaching) encompasses liquid-liquid as well as liquid-solid systems. Liquid-liquid extraction involves the transfer of solutes from one liquid phase into another liquid solvent it is normally conducted in mixer settlers, plate and agitated-tower contacting equipment, or packed or spray towers. Liquid-solid extraction, in which a liquid solvent is passed over a solid phase to remove some solute, is carried out in fixed-bed, moving-bed, or agitated-solid columns. 

Figure 2.26. Spray tower for extractions, (a) Light liquid dispersed, (h) Heavy liquid dispersed.

Spray Towers These are simple gravity extractors, consisting of empty towers with provisions for introducing and removing liquids at the ends (see Fig. 15-32). The interface can be run above the top distributor, below the bottom distributor, or in the middle, depending on where the best performance is achieved. Because of severe axial back mixing, it is difficult to achieve the equivalent of more than one or two theoretical stages or transfer units on one side of the interface. For this reason they have only rarely been applied in extraction applications. 

Ruby, C. L. and Elgin, J. C. Mass transfer—Transport properties. Chem. Eng. Prog. Symp. Series No. 16, 51 (1955) 17. Mass transfer between liquid drops and a continuous liquid phase in a countercurrent fluidized system. Liquid-liquid extraction in a spray tower. 

Equipment suitable for reactions between liquids is represented in Fig. 23 37. Almost invariably, one of the phases is aqueous with reactants distributed between phases for instance, NaOH in water at the start and an ester in the organic phase. Such reactions can be carried out in any kind of equipment that is suitable for physical extraction, including mixer-settlers and towers of various kinds empty or packed, still or agitated, either phase dispersed, provided that adequate heat transfer can be incorporated. Mechanically agitated tanks are favored because the interfacial area can be made large, as much as 100 times that of spray towers, for instance. Power requirements for L/L mixing are normally about 5 hp/1,000 gal and tip speeds of turbine-type impellers are 4.6 to 6.1 rn/s (15 to 20 ft/s). 

Spray or packed extraction towers, as shown in Figure 5.2, are similar to those used in gas absorption with the exception that two liquids, rather than a gas and a liquid, are flowing countercurrent to each other. The less dense, or lighter, liquid enters the tower bottom through a distributor, which creates small drops flowing upwards. These rise through the... 

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.). 

Not many operating data of large-scale liquid/liquid reactions are published. One study was made of the hydrolysis of fats with water at 230 to 260°C (446 to 500°F) and 41 to 48 atm (600 to 705 psi) in a continuous commercial spray tower. A small amount of water dissolved in the fat and reacted to form an acid and glycerine. Then most of the glycerine migrated to the water phase. The tower was operated at about 18 percent of flooding, at which condition the HETS was found to be about 9 m (30 ft) compared with an expected 6 m (20 ft) for purely physical extraction (Jeffreys, Jenson, and Miles, Trans. Inst. Chem. Eng., 39, 389-396 [1961]). A similar mathematical treatment of a batch hydrolysis is made by Jenson and Jeffreys Inst. Chem. Engrs. Symp. Ser, No. 23 [1967]). 

Heat Transfer, The use of an extraction tower to transfer heat between the dispersed and continuous liquids is an interesting variation of the operations under consideration. Rosenthal (69) and the author have found this a convenient means of comparing the tower performances for different conditions of operation. Typical spray-tower data are shown in Fig. 10.34,... 

Type C In order to overcome the disadvantage of loss of extract involved in processes of type B, the adsorption stages may be replaced by absorption into a liquid. In processes of this type the extract which has been dissolved in the solvent is absorbed by a wash fluid flowing countercurrent to the ex-tract/solvent stream. The absorption takes place in high pressure vessels which may either be operated as spray towers or as packed columns. Quick-acting closures are not necessary for these vessels. 

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