Fish liver oil must be extracted in a continuous countercurrent extraction apparatus, using ethyl ether with 3% oil as solvent. 500kg/h of livers containing 25% by weight of oil (on solvent free basis) enter the extraction system. If the amount of solvent used is 400kg/h, and the composition of the raffinate separated as underflow is 1% by weight of oil (on a free basis in solvent), calculate: a) The percentage of oil recovered in the extract. b) The necessary number of theoretical stages. The amount of solution retained by the livers, determined experimentally, is given by the following table:

kg of oil/kg of solution kg of solution/kg of oil-free liver 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,72 0,19 0,23 0,26 0,31 0,37 0,45 0,55 0,67 0,74