Continuous Multistage Countercurrent Extraction

Figure 12.7-3. Method to perform overall material balance for Example 12.7-1. Sec. 12.7 Continuous Multistage Countercurrent Extraction...

Multistage countercurrent contacting is the most effective mode for carrying out separation processes. It reduces the amount of solvent and makes possible the continuous production of extract. Real countercurrent contact is not easily established for solids, because special effort is necessary for moving the solid, with increased difficulties at elevated pressure. Therefore, it is easier not to move the solid material and to achieve countercurrent contact by other measures. For SFE from solids, a well known and often used configuration is to have several fixed beds in countercurrent contact with the solvent. 

Three steps required for multistage solvent extraction, i.e., phase mixing, phase settling, and transfer of the mobile phase, are defined clearly in the discontinuous countercurrent distribution process using the Craig apparatus. These basic requirements are essentially fulfilled by the use of a coiled tube in a continuous fashion. Solvent extraction using a coiled column is most efficiently performed with a horizontally laid coil that rotates about its own axis. In this horizontal coil orientation, the rotation induces the well known Archimedean screw force, which can be utilized for performing countercurrent solvent extraction. 

Continuous Countercurrent Multistage Fractional Extraction. The industrial process utilizing the double-solvent principles is continuous and is carried out according to the flowsheet of Fig. 7.18 (2, 15). The feed, consisting principally of a mixture of B and C to be separated, is introduced into the central portion of a cascade of stages, 1 to 1. To facilitate... 

These operations are all similar in that the mixture to be separated is brou t into contact with another insoluble phase, the adsorbent solid, and the unequal distribution of the ori al constituents between the adsorbed phase on the solid surface and the bulk of the fluid then permits a s aration to be made. All the techniques previously found valuable in the contact of insoluble fluids are useful in adsorption. Thus we have batchwise single-stage and continuous multistage separations and separations analogous to countercurrent absorption and stripping in the field of gas-liquid contact and to rectification and extraction with the use of reflux. In addition, the rigidity and immobility of a bed of solid... 

Figure 8.1.37. (a) Continuous countercurrent multistage solvent extraction cascade of N stages (b) graphical determination of stage numbers in such a cascade of equilibrium extraction stages. 

Now consider a multiple-stage process with countercurrent flow of the raffinate and extract phases. Fig. 2B. Countercurrent is the most efficient multistage configuration. One can write a mass balance around the nth stage, as indicated by envelope 1, using the following units for a continuous flowing process ... 

In some cases, especially with multiple solutes and complex phase equilibria, it may be useful to perform laboratory batch experiments to simulate a continuous, countercurrent, multistage process. These experiments can be used to test/verify calculation results and determine the correct distribution of components. For additional information, see Treybal, Chap. 9 in Liquid Extraction, 2d ed. (McGraw-Hill, 1963), pp. 359-393, and Baird and Lo, Chap. 17.1 in Handbook of Solvent E raction (Wiley, 1983 Krieger, 1991). 

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