Flash Vaptorization of Benzene-Toluene

A liquid mixture containing 50 mole % each of benzene and toluene at 40oC is to be continuously flash vaporized to vaporize 60 mole % of the feed. The residual liquid product contains 35 mole % benzene. If the enthalpies per mole of feed, the liquid product and the vapor product are respectively 2,5 and 30 kJ/mole,

  1. calculate the heat added in kJ per mole of vapor product
  2. represent the process on a H-x-y diagram

Data:

Basis: 100 mole of feed

Feed composition, mole fraction of more volatile component (zF) = 0.5

Feed (F) = 100 mole

Since 60% of the feed is vaporized,

Vapor (D) = 60 mole

Liquid (W) = 40 mole

Mole fraction of more volatile component in the liquid (xW) = 0.35

Formulae:

F = D + W à 1

FzF = DxD + WxW à 2

FHF + Q = DHD + WHW à 3

Calculations:

From equations, 1 and 2

100 x 0.5 = 60xD + 40 x 0.35

Therefore, xD = 0.6

From equation 3,

100 x 2 + Q = 60 x 30 + 40 x 5

Therefore, Q = 1800 kJ

Heat to be added per mole of vapor product = Q/D = 1800/60 = 300 kJ

The H-x-y diagram for the above process is given below: