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Separator Design                                                323

            the height of a packed column from Equation 6.21.7P.
                 The  calculation  procedure  in  Table  6.22 could  also  be  used  for  a  multi-
            component  mixture. After  calculating the number  of  stages  for separating the  key
            component from  the mixture, then the  composition of all other components in the
            exit  stream  can  be  calculated  using  the  Kremser  equation,  Equation  6.21.5A  for
            absorbers or 6.21.5S for strippers.
                The tray spacing, Z T, calculated from  Equation 6.21.9T, depends on the pres-
            sure  [75]. To  obtain HETS from  Equation  6.21.9P requires the  column  diameter.
            To  obtain the column diameter, calculate the maximum allowable  gas velocity to
            prevent  entrainment  of  liquid.  First,  find  the  maximum value  of the parameter k,
            from  Equation  6.23.4  in  Table  6.23, which  occurs  when  the  column  is  about  to
            flood.  The column is then designed  to operate below the  flood  point.  The  maxi-
            mum  value of  k is  found  in Figure 6.18 for trays. Fair  [52]  recommends that the
            flooding  parameter, obtained from  Figure 6.18 for tray columns, be multiplied by
            0.9 for nonfoaming liquids. Treybal [29] recommends 0.75 for foaming liquids.  In
            Figure 6.18, the flooding parameter requires correcting for surface  tension and tray
            geometry using corrections given by Fair [52]. Figure 6.18 was developed by Fair
            [52]  for  fractionators.  Henley  and  Seader  [65]  used  an  earlier  version  of  Figure
            6.18  for absorbers and strippers. Fair [52] lists  restrictions on Equation 6.23.5 for
            tray columns. He  also  corrects  k  for tray  geometry, but  for preliminary  estimates
            we want to avoid designing trays.
                If  the  column  diameter  >  2.5  ft  (0.762  m),  use  a  tray  column.  Because  of
            maintenance, a tray column has to be internally accessible [75, 31].  The minimum
            diameter column that is accessible is 2.5 ft. For packed columns, Figure 6.19 only
            gives factors  for a limited number of packings. For other packings use the flooding
            ratios  in  Table  6.26. To  obtain  k  for a packing  listed  in Table  6.26 multiply  the
            flooding  ratio  by  the  flooding  factor  for  50  mm  (2  in)  Pall  rings  obtained  from
            Figure  6.19.  For  packed  columns  use  0.7  for  nonfoaming  liquids,  a  commonly
            accepted  value,  and  0.4  for  foaming  liquids  [6]. After  obtaining k v,  the  column
            diameter is then calculated from  equations listed in Table 6.23.


            Table  6.23  Summary  of  Equations  for  Sizing  Absorbers,  Strippers,  or
            Fractionators - Column Diameter______________________

            Subscripts: L = liquid —  V = vapor

            Column Diameter

            A = VV'/V V                                           (6.23.1)
                  2
            A = 7iD /4                                            (6.23.2)
            v v = k v [ (PL' -  Pv') / Pv' l" 2  "  v s = 0.9 v v  (6.23.3)




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