Consider a separation train consisting of two distillation columns which are des
ID: 596462 • Letter: C
Question
Consider a separation train consisting of two distillation columns which are designed to separate a threecomponentmixture of benzene, toluene, and xylene into three streams, each rich in one of the species asshown below. For the first unit the feed rate is 1,000 mol/h containing 20 mol % benzene, 30 mol %toluene mixture, the bottoms product is of 2.5 mol % benzene, 35 mol% and all of the xylene fed to the
unit. The overhead product of the second unit has 8 mol% benzene, 72 mol % toluene, and benzene in
this stream is 100 % of benzene fed to this unit. Determine how much material will be processed by each
unit and how this material will be divided among the outlet streams. ( All compositions labeled below are
mol fractions.)
Explanation / Answer
Balances On UNIT 1: Xylene: 500 = 0.625*n3 Solve this first for n3 - Use this because Xylene is a tie component unit: 1000 = n2 + n3 Next solve this equation for n2 Benzene: 200 = n2*xB(2) + n2*0.025 Finally solve this equation for xB(2) Answers: n3 = 800 mol/h n2 = 200 mol/h xB(2) = 0.90 Now the DOF for UNIT 2 is 0. Balances on UNIT 2: Benzene: 0.025*800 mol/h = 0.08*n4 Solve this first for n4 - Use this because benzene is a tie component here unit: 800 mol/h = n4 + n5 Next solve this equation for n5 Toluene: 0.35*800 mol/h = 0.72*n4 + xT(5)*n5 Finally solve this equation for xT(5) Answers: n4 = 250 mol/h n5 = 550 mol/h xT(5) = 0.182 CHECK: