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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
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