A multi-stage extraction installation with countercurrent operation has to be designed to treat 500kg/h of a powdery product with a composition of 20% by weight of oil, 3% by weight of benzene and 77% of inert matter. Benzene with 2% by weight of oil must be used as solvent, and 90% of the oil contained in the feed must be separated from the extract. From experiments carried out in the laboratory under analogous conditions, it has been found that the amount of liquid retained by solids depends on the concentration in this way: If 400kg of solvent are used, calculate: a) The concentration of the extract. b) The amount of extract. c) The number of theoretical stages.

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Concentration kg of oil/kg of solution Retained solution kg/kg of solid 0,0 0,1 0,2 0,3 0,4 0,40 0,41 0,42 0,43 0,45 0,47 0,49 0,52 0,5 0,6 0,7