Question

A mixture containing 42% benzene and 58% toluene by mass is fed to a distillation column. A second mixture is also fed to the distillation column and has a composition of 65% toluene and 35% acetone by mass. The flowrates of the mixtures are 2500 kg h1 and 1500 kg h1 respectively. An overhead distillate exit stream of 800 kg h-1 containing 23% acetone and a bottom exit stream of 25% benzene are produced. Determine the bottom exit flowrate and the compositions of both exit streams. (a) (14 marks) (b) An evaporator concentrates 1000 kg h1 of an aqueous feed from a solids concentration of 3 wt% to a concentration of 12 wt%. The evaporation is carried out at atmosphere pressure. The feed is at 25°C. The liquid thermal properties may be assumed to be the same as water. Using the enthalpy values for a saturated steam-water system provided in Table Q1, calculate the vapour and liquid product flow rates and the heat load. (11 marks) Table Q1 Temp (C) Pressure (bar) hr (kJ kg) he (kJ kg he kJ kg") 25 100 0.03166 1.01325 1.5 104.8 419.1 467 2441.8 2256.7 2226 2546.6 2675.8 2693

Answer 1

(a) Lets assume x, y, z are fraction of benzene, tolerance, acetone by mas implies feed (F_1) = 2500 kg/h, x F_1 = 0.42 Y_F_1 = 0.58 Z F_1 = 0 implies feed (F_2) = 1500 kg/h, x F_2 = 0 Y F_2 = 0.65 Z F_2 = 0.35 implies Distillate (D) = 800 kg/h, Z_D = 0.23 (b) Bottom (w) x_w = 0.25 Now apply overall balance on column: - F_1 + F_2 = D + w implies 2500 + 1500 = 800 + w implies w = 3200 kg/h Now apply acetone balance: - implies (2500) (0) + 1500 (0.35) = 800 (0.23) + 3200 (Z_w) implies Z_w = 0.106 Now in bottom we know: - x_w + y_w + z_w = 1 implies x_w = 0.25 than 0.25 + y_w + z_w = 1 implies y_w = 1 - 0.25 - 0.106 = 0.644 Now similar apply benzene balance: - implies (2500) (0.42) + 1500 (0) = 800 (x_D) + 3200 (0.25)