Experiment 4

Experiment 4 - 4. LIQUID-LIQUID EXTRACTION: Partially...

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33 4. LIQUID-LIQUID EXTRACTION: Partially miscible liquids Introduction You have learned the principles of liquid-liquid extraction, especially, the partially-miscible liquid-liquid extraction, in ChE 315, Mass Transfer I. Simply speaking, liquid-liquid extraction is a mixing-separation process where two phases of liquids are mixed and then one or more desired components are transferred or separated from one liquid phase to the other to some extent due to the difference of their solubility in the two phases. This separation process is widely used in industry. The following photo shows a liquid-liquid extraction unit used in Uranium mill. (Photo taken by Hui Wang at Cameco’s Rabbit Lake Mill, 2007) Extraction Devices Due to the wide diversity of application, the industrial devices of liquid-liquid extraction vary greatly. They includes mixer-settlers, spay columns, packed columns, plate columns and columns with mechanically assisted agitation. Regardless of the type of equipment, the calculation of the equipment design maintains the same if extraction is considered as an equilibrium-staged process. The number of stages of an extraction system is determined by equilibrium calculation, and the size of devices is obtained using the HETS,or height equivalent to a theoretical stage.
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34 Principles of Liquid-Liquid Extraction Let’s recall the fundamental principles of the liquid-liquid extraction in brief. The task of separation is to transfer one or more components from one liquid phase to the other to a desired extent. The components to be removed or separated are called solute. The former liquid is called diluent and the latter is solvent. The two liquid phases are immiscible or partially miscible most of the time. At a condition where the solubility of a solute, A, in both phases, y A and x A , will be in equilibrium, the ratio of y A to x A , represented by K d , is called distribution ratio. The extraction can be regarded as one or more contacting stages. To simplify the extraction calculation, these stages are assumed as equilibrium ones, as shown in Fig. 1. Figure 1. Equilibrium staged liquid-liquid extraction. Extraction usually take places as a counter-current flow regime as shown in Figure 1. The feed diluent becomes raffinate and the solvent becomes extract. The term ―diluent‖ is also used to represent the component of the feed in which the solutes are dissolved. The equilibrium data of an extraction system is generally transferred into a triangular graph as shown in Fig. 2. The two known corresponding equilibrium points in the two phases are connected by lines called tie line. From a set of known compositions of one phase, you can determine the compositions of the other phase if you know how to draw the tie line from a known composition point. This is an important skill to learn when you want to determine the number of stages of an extraction contactor or device. 1
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This note was uploaded on 02/13/2012 for the course CHEM 4113 taught by Professor Dalai during the Fall '11 term at Minnesota.

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Experiment 4 - 4. LIQUID-LIQUID EXTRACTION: Partially...

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