Page 339 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das
P. 339

11.8 Batch distillation  341




               instead of total liquid amount (L) where the component amount (l i ) is defined as l i ¼ Lx i , and the
               liquid concentrations replaced by x i ¼ðl i =LÞ
                                                     dL  1 dl i  1 dl k
                                           This gives  ¼      ¼                            (11.58)
                                                     L   K i l i  K k l k
                  For ideal systems with constant relative volatility (a LK ) with reference to the heaviest component k,

                                                  dl i     dl k
                                                      ¼ a lk
                                                   l i      l k
                  On integration
                                                        !
                                                          1=a jk

                                               1=a ik
                                             l i        l j       l k
                                                                                           (11.59)
                                                   ¼          ¼
                                            l fi       l fj       l fk
               where the amount of liquid in the still is L ¼ Sðl i Þ, and the concentration of each component in the
               still is x i ¼ðl i =LÞ.
                  The same approach can be applied to multistage distillation of ternary/multicomponent mixtures
               but distillate composition (x Di ) replaces equilibrium composition (y i ) (similar to multistage distil-

               lation of binary system) to give
                                            dB      dx Bi     dx Bk
                                                                                           (11.60)
                                                          ¼
                                               ¼
                                            B     x Di   x Bi  x Dk   x Bk
                  In general, determination of distillate composition (x Di ) requires rigorous column simulation and
               the results are reliable only if the liquid holdup in the column is much lower than the liquid holdup in
               the still. Batch distillation of ternary (or multicomponent) mixtures is preferentially performed with
               constant reflux ratio although operation with periodically constant distillate concentration is also
               possible.


               11.8.2 Design deliverables
               a) Configuration
               e   Single stage or multistage e number of stages
               e   Refluxing arrangement and reflux ratio e constant reflux/constant reflux ratio or constant
                   composition of top product
               e   Operating pressure
               e   Initial and final temperature of column top and still
               e   Initial charge and final liquid holdup volume in still
               e   Collection of top product in receivers in case of multiple grades of top product
               e   Heat load for still heating e peak, average, variation with time

                b) Equipment dimensions
                  Charge tank/still size
                  Volume: The still is chosen to be vertical or horizontal based on considerations same as in flash
               distillation. This is already elaborated in Section 11.6.2.
   334   335   336   337   338   339   340   341   342   343   344