Multistage Crosscurrent Extraction

The feed of Example 7.2 is extracted three times with pure chloroform at 298 K, using 8 kg/h of solvent in each stage. Determine the flow rates and compositions of the various streams. 

Stage Raffinate Extract Acetone Chloroform Acetone Chloroform 

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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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... 

Calculate flows and compositions for multistage crosscurrent extraction. 

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

For a solute with a relatively poor partition coefficient, one of the two arrangements of multistage extraction, crosscurrent or countercurrent may be selected to improve efficiencies (Fig. 10.8). To decide which would be the advantageous choice, assume we have a 1,000 L/hr aqueous waste stream containing 0.02 M benzoic acid which is to be extracted with 300 L/hr of benzene for benzoic acid recovery. For the extractant benzene, Kp = 4.3 at... 

Example 1 In an API manufacture, a stirred tank of 5 m capacity is used as a reactor. It is charged with a solvent and cooled to 5 °C from the ambient temperature. The reactants are added and the reaction is carried about for 8 h under constant stirring. At the end of the reaction, the reaction mixture is washed with the same volume of water at 10 °C and then the wash water is decanted. The washing is repeated 6 times. Then, the vessel is cleaned for the next operation to meet regulatory requirements. The total batch time is about 40 h. Thus the vessel is occupied for 40 h. These washings amount to batch crosscurrent multistage extraction. There is a loss of the API as well as of the solvent (in which the API is held) in each of these washing. The yield is only about 60%. 

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