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31 0                                        Multiple Reactions   Chap. 6





             TO var~ TCSTR
              one can vary
             either vo for a
           fixed Vor varyV
             for a fixed  vo                   C
                                              .-
                                               0
                                              c
                                                 0.5
                                                   0.0       1 .o      2.0
                                                            ‘CSTR
                                       Figure E6-7.1  Concentrations as a function of  space time.


                            We resolve Equations (E6-7.17) through (E6-7.19) for different values of  z to arrive
                            at Figure E6-7.1.
                                The moles of hydrogen consumed in reaction 1 are equal to the moles of mes-
                            itylene consumed. Therefore, the conversion of hydrogen in reaction 1 is
                                             CMO-~M 0.0105-0.0029  = *,,  = o.36
                                                      -
                                        xlH  =  c,,   -     .02 1
                                               ’
                            The conversion of  hydrogen in reaction 2 is
                                            X,,   X, - X,,   0.58 - 0.36 = 0.22

                                Next, we determine the selectivity and yield. First, consider the rate selectiv-
                            ity parameter, SxT , at the optimum space-time. At zopt (see Figure E6-7.1), the con-
                            centration of xylene is a maximum. Therefore,




                           Thus, the rate (i.e., instantaneous) selectivity parameter of  xylene relative to toluene is





                           Similarly, the xylene yield based on reaction rates is also zero. Consequently, we see
                           that  under  these  conditions  ( zOpt ) the  instantaneous  selectivity and  instantaneous
                           yield, which are based on reactio?  rates, are not very meaningful parameters and we
                           must use the overall selectivity SXT and the overall yield  YXT , which are based on
                           molar flow  rates. The yield of  xylene from  mesitylene based on molar flow rates
                           exiting the CSTR for  ‘c  = 0.5 is
                                        -       Fx    =   cx    =    0.003 13
                                        YMX  = -
          Overall selectivity,                F,,  - FM  CMo - CM  0.0105 - 0.0029
           s, and yield,  ? .           -    0.41 mol xylene produced
                                        yMx  =  mole mesitylene reacted
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