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272                       Applied Process Design for Chemical and Petrochemical Plants

                                                                  dency for rings to channel liquid to the tower wall. Saddle-
                                                                  type packing does not require the use of the cone.
                                                                    In packing a tower dry, high hydrostatic heads are thus
                                                                  avoided on joint connections; however, extra care must be
                                                                  taken to avoid ceramic breakage. The packing will proba-
                                                                  bly settle after installation, but it should not be pressed or
                                                                  tamped in place. This will cause extra breakage. The pack-
                                                                  ing should not be allowed to fall more than two feet to the
                                                                  bed  surface. It  should be  dumped  at random  to  avoid
           Figure 9-16A.  Stacked packing: square pattern (S). Used by permis-
           sion of U.S. Stoneware Co. (now, Norton Chemical Process Products   developing any  pattern.  The  dry  packed  tower  will  be
           Corp.).                                                more  dense  than  the  wet  packed  and  should  not  be
                                                                  pressed or tamped in place.  The pressure drop for dry
                                                                  packed beds can be as much as 50-60%  greater than for
                                                                  wet packed. Dry packing is not the preferred method due
                                                                  to significant settling that will occur.
                                                                    Metal and plastic packing can be dumped dry into the
                                                                  tower; however, reasonable care must be exercised, and the
                                                                  drop should not be more than 10-15  ft, using a chute with
                                                                  a cloth sock (open) on the end to cushion the impact.
            Figure 9-166.  Stacked packing: diamond pattern (D).  Used by per-   Repacking a tower will usually show a variation in pres-
            mission of U.S. Stoneware Co. (now, Norton Chemical Process Prod-   sure drop. For small 8-in. dia. units the variation may be as
            ucts Corp.).                                          much as 100%. In larger columns 24in. dia. and up, this
                                                                  variation is noticed, but only to about 50% or less.

                                                                           Packing Selection and Performance

                                                                  Guide Lines: Trays versus Packings

                                                                    Kister et al. [136] prepared one of the few comprehen-
                                                                  sive  distillation studies for  the  application  selection of
                                                                  valve  and sieve trays compared to random or structured
                                                                  packing. This reference is based on a more comprehen-
                                                                  sive evaluation of accumulated data by  the same authors
                                                                  [137]. Many  separate studies have  been  conducted for
                                                                  trays  [138] including bubble caps as well as various pack-
                                                                  ings, but few, if any attempt to establish similar conditions
                                                                  to make a viable comparison as is attempted in References
                                                                  136,  137. There  are  four  main  differences  related  to
                                                                  capacity and separation [ 1361 when considering:

                                                                    1. An  optimal tray design, one that balances tray and
            Figure 9-16C. Stacked packing: packing assembly. Used by permis-
            sion of U.S. Stoneware Co. (now, Norton Chemical Process Products   downcomer area so that neither prematurely restricts
            Corp.).                                                   capacity, and set weir  height, weir  geometry, clear-
                                                                      ance under the downcomer, and fractional hole area
            water for ceramic packings after installation of the bottom   so as to maximize efficiency and capacity.
            support  arrangement,  including any  stacked rings, and   2. Optimal packing design, which  configures distribu-
            the loose packing floated down to rest on top of the sup-   tors, supports, and bed heights to avoid loss of  effi-
            port. The fall should be as gentle as possible since broken   ciency to maldistribution and no premature capacity
            packing tightens the bed and increases pressure drop.     restriction occurs [ 1361.
              One manufacturer  [22]  suggests loading ceramic ring
            type packing using a sheet metal cone in the tower there-   The four main differences are [ 1361 :
            by  causing the rings to slide off the cone and fill to  the
            edges first. The cone is lifted as the rings are floated or   1. Differences between the capacity and efficiency of an
            dropped in. This technique is believed to reduce the ten-   optimal tray and an optimal packed tower design.
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