Crosscurrent extraction

A theoretical or equihbrium stage is a device or combination of devices that accomplishes the effect of intimately mixing two immiscible liquids until equilibrium concentrations are reached, then physically separating the two phases into clear layers. Crosscurrent extraction (Fig. 15-4) is a cascade, or series of stages, in which the raffinate R from one extraction stage is contacted with additional fresh solvent S in a subsequent stage. 

After a single-stage liquid-liquid contact the phase remaining from the feed sohition (raffinate) can be contacted with another quantity of fresh extraction solvent. This crosscurrent extraction scVieme (Fig. 15-4) is an excellent laboratoiy procedure because the extrac t and raffinate phases can be analyzed after each stage to generate equihbrium data. Also, the feasibility of solute removal to lowTevels can be demonstrated. 

Multistage Crosscurrent Extraction In multistage crosscurrent extraction, the raffinate is successively contacted with fresh solvent which can be done continuously or in batch (Figure 10.4). With the given flow rates of the feed and the solvent streams, the... 

FIGURE 5 Multistage extraction processes (a) crosscurrent extraction, (b) countercurrent extraction, and (c) countercurrent fractional extraction. 

Crosscurrent extraction (Fig. 5a) consists of repeated contacts of the feed solution with fresh solvent, resulting in a series of extract streams of gradually diminishing concentration. It is a simple arrangement and can be readily applied batchwise (as a laboratory operation) or with continuous flow of feed and solvent. It is more effective than a single-stage fed with the same total flows of solvent and feed. 

Countercurrent extraction (Fig. 5b) is more effective than crosscurrent extraction because all the fresh solvent is contacted with the weakest raffinate. This arrangement is almost universally used in industrial extraction equipment. 

Crosscurrent extraction was used initially at Rocky Flats. This mode of extraction has the following disadvantages (1) the solvent capacity of the salt is not used effectively, (2) plutonium loss to the salt is high because two salts must be... 

Countercurrent extraction is favored over crosscurrent extraction at a fixed salt composition for the following reasons (1) less salt is required to obtain the same separation, (2) plutonium loss to the salt is lower, (3) less magnesium metal by-product is generated, and (4) salt feed to the salt recovery line is cut in half. 

Calculation of Stage Requirements 463 Single Stage Extraction 463 Crosscurrent Extraction 464 Immiscible Solvents 464... 

Calculate flows and compositions for multistage crosscurrent extraction. 

This is an extension of single-stage extraction wherein the raffinate is successively contacted with fresh solvent, and may be done continuously or in batches. Figure 7.9 is a schematic diagram for a three-stage crosscurrent extraction process. A single final raffinate results, and the extracts can be combined to provide a composited extract, as shown. As many stages as necessary can be used. 

The composited extract flows at the rate of 52.58 kg/h, and its composition is 5.4 wt% water, 45.1 wt% chloroform, and 49.5 wt% acetone. Notice that the acetone content of the final raffinate (18.5 wt%) is slightly lower than the acetone content of the raffinate obtained in Example 7.2 in a single-stage extraction with 50 kg/h of chloroform. Remarkably, this is true even though the total amount of solvent used in the multistage crosscurrent extraction process is only 3 x 8 = 24 kg/h. 

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