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316 Chapter 11 Distillation

11.5 Design illustration e fractionator
Design Problem
3
Benzene is proposed to be recovered by fractionating a 20 m /hr stream containing 35%, 25% and
40% mol benzene, toluene and ortho-xylene (O-x). At least 75% of the benzene in feed needs to be
recovered with 95% purity. Component property data are provided in Table 11.7.
Make a quick design of the fractionator.

Table 11.7 Component property data.
Benzene (B) Toluene (T) Ortho-xylene (O-x)
Molecular weight (MW) 78 92 106
N.B.P. (K) 353.05 383.6 417.4
Density, r (gm/cc) 0.8787 0.8636 0.8800
sat
Constants in Antoine’s equation log 10 (p ) ¼ a b/(T þ c);p sat (saturated vapor pressure) in kPa, T in C.

a 6.01905 6.08436 6.12699
b 1204.637 1347.620 1476.753
c 220.089 219.787 213.911
Cp l (kJ/kmol K), 134.8 155.96 187
average
Cp v (kJ/kmol K), 82.44 103.7 132.5
average
Heat of vaporisation at 30.77 38.06 36.24
NBP, l (kJ/kmol)

The l value at temperatures T 1 and T 2 are related by the equation
(l 1 /l 2 ) ¼ {(T c T 1) /(T c T 2 )} 0.38 , where T c is the critical temperature of the component.
Solution
Feed

MW F ¼ð:35 78 þ :25 92 þ :4 106Þ¼ 92:7
r ¼ð35 78 þ 92 25 þ 106 40Þ=
F
3
ð35 78 = :8787 þ 92 25 = :8636 þ 106 40 = :88Þ¼ 0:8755 kg=L ¼ 875:5kg=m
Hence, F ¼ 875:5 20=92:7 ¼ 1888:9 kg mol/hr; (B ¼ 661.1, T ¼ 472.2, O-x ¼ 755.6 kg mol/hr)

z F ¼ 0:35=0:25=0:4; mole fraction
Deciding light and heavy key components: As per product specification, Benzene and Toluene are
set as light and heavy key components. This means that distillate D to have minimum 90 mol%
Benzene and the rest is Toluene.
Distillate Composition: Based on 75% benzene recovery and 95% mole purity of benzene,