670 kg h–1 of a slurry containing 120 kg solute and 50 kg solvent is to be extra
ID: 532129 • Letter: 6
Question
670 kg h–1 of a slurry containing 120 kg solute and 50 kg solvent is to be extracted. The maximum permitted amount of solute in the final raffinate is 5 kg h–1. When a simple mixer-settling unit is used to separate extract and raffinate, the amount of solvent retained by the solid is 50 kg. Assuming perfect mixing and a constant ratio of solvent in extract and raffinate, determine the number of stages and the strength of the total extract for each of the following conditions: (i) Simple multiple contact is used for the extraction with a solvent addition of 100 kg h–1 per stage (ii) The same total amount of solvent as in (i) is provided in countercurrent operation (iii) Half the total amount of solvent in (i) is used in countercurrent operation. (b) Which of these methods would you recommend for the extraction? Give three reasons for your choice. (c) Suggest two alternative ways of carrying out the extraction and the circumstances under which they might be used.
Explanation / Answer
The separation achieved for a given number of ideal stages in a counter flow cascade can be determined by using a triangular diagram and special graphical techniques but a modified McCabe Thiele method, which is the approach, used here, is simple to use and has satisfactory accuracy for most cases. The method focuses on the concentration of solute in extract and raffinate phases, and the diagram does not show the concentration of the diluents in the extract or the concentration of solvent in the raffinate. However, these minor components of both phases are accounted for in determining the total flow of extract and raffinate, which affects the position of operating line.
If the number of ideal stages is specified, the fraction of solute extracted and the final compositions are determined by trial and error. The fraction extracted or the final extract composition is assumed, and the curved operating line is constructed. If too many stages are required, a smaller fraction extracted is assumed and the calculations are repeated. Computer programs for extraction calculations are available
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