Page 325 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das
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11.7 Design illustration e flash distillation 327
Table 11.11 Properties of the B-T-(O-x) system.
Benzene (B) Toluene (T) Ortho-xylene (O-x)
Molecular Weight 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 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)
% w/w in feed 1.1 1.4 97.5
Mole% in feed (z F ) 0.0149 0.0160 0.9691
l 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.
Flash drum operating temperature and pressure
Since the temperature at which the feed may be available is not mentioned, it is assumed that the
feed is from a tank at ambient temperature, say w32 C. The N.B.P. of the limiting component benzene
is 78 C. This is not too high and easily attainable with heating using low pressure steam. Therefore, we
adopt drum temperature in the range 70e110 C so that it is achievable after preheating with steam or
any other hot stream. For a particular drum temperature (T), the bubble (P Bub:F ) and the dew point
(P Dew:F ) pressure of the feed are calculated from pure component vapour pressures and feed mole
fractions as
X sat X sat
z Fi p .& P Dew:F ¼ 1= z Fi = p
P Bub:F ¼ i i
i¼1;3 i¼1;3
Flashing of feed at a pressure P between the bubble point pressure P Bub:F and dew point pressure
P Dew:F would give rise to vapour and liquid phases coexisting in the drum at temperature T. The V/F
value and the corresponding liquid mole fraction (x i ) are to be calculated from Eqs. 11.33 and 11.34.
Solution of Eq. 11.33 for V/F requires trial or use of a convergence routine, e.g., NewtoneRaphson.
The acceptable P with which the drum can be operated shall be the one with benzene concentration
0.5% w/w or lower in the liquid. It may be noted that selecting an even lower pressure would result in
benzene concentration below 0.5%w/w, which is acceptable but this will be at the cost of product