Page 18 - Chemical Process Equipment - Selection and Design
P. 18

RULES  OF  THUMB:  SUMMARY  XV

             20.  Packings  of  random  and  structured  character  are  suited   An  85%  free  cross  section  is  taken  for  design  purposes.  In
               especially to towers under 3 ft dia and where low pressure drop   countercurrent flow, the exit gas is  10-20°C  above the solid; in
               is  desirable.  With  proper  initial  distribution  and  periodic   parallel flow, the temperature of the exit solid is 100°C. Rotation
                redistribulion, volumetric efficiencies can be made greater than   speeds  of  about  4rpm  are  used,  but  the  product  of  rpm  and
               those of  tiray towers. Packed internals are used as replacements   diameter in feet is typically between 15 and 25.
               for achieving greater throughput or separation in existing tower   4.  Drum dryers for pastes and slurries operate with contact times of
                shells.                                           3-12  sec, produce flakes 1-3 mm  thick with evaporation rates of
            21  For  gas  rates  of  500 cfm,  use  1 in.  packing;  for  gas  rates  of   15-30  kg/m2 hr.  Diameters  are  1.5-5.0 ft;  the  rotation  rate  is
              *
               2000 cfm or more, use 2 in.                        2-10rpm.  The  greatest  evaporative capacity is  of  the  order  of
            22.  The ratio of  diameters of  tower and packing should be at least   3000 lb/hr in commercial units.
                15.                                             5.  Pneumatic conveying dryers normally take particles 1-3 mm  dia
            23. Because of  deformability, plastic packing is limited to a 10-15 ft   but up to 10 mm when the moisture is mostly on the surface. Air
               depth unsupported,  metal to 20-25  ft.            velocities  are  10-30 m/sec.  Single  pass  residence  times  are
               Liquid  redistributors  are  needed  every  5-10  tower  diameters   0.5-3.0 sec but with normal recycling the average residence time
               with  pall rings but  at  least  every 20ft.  The number  of  liquid   is brought up to 60 sec. Units in use range from 0.2 m dia by 1 m
               streams should be 3-5/sqft  in towers larger than 3 ft dia (some   high  to  0.3m  dia  by  38m  long.  Air  requirement  is  several
               experts say 9-12/sqft),  and more numerous in smaller towers.   SCFM/lb of  dry product/hr.
            25.  Height  equivalent  to  a  theoretical  plate  (HETP)  for   Fluidized bed dryers work best on particles of  a few tenths of  a
               vapor-liquid  conlacting  is  1.3-1.8ft  for  1 in.  pall  rings,   mm dia, but up to 4 mm dia have been processed. Gas velocities
               2.5-3.0  f: for 2 in. pall rings.                  of  twice  the  minimum  fluidization  velocity  are  a  safe
            26.  Packed  towers  should operate  near  70%  of  the  flooding rate   prescription. In continuous operation,  drying times  of  1-2  min
               given by the correlation of  Sherwood, Lobo, et al.   are  enough, but  batch  drying of  some pharmaceutical products
            27-  Reflux drums usually are horizontal, with  a liquid holdup of  5   employs drying times of  2-3  hr.
               min half  full.  A takeoff pot for a second liquid phase, such as   Spray dryers:  Surface moisture is removed in  about  5 sec,  and
               water  in hydrocarbon systems, is  slzed for  a linear velocity of   most drying is completed in less than 60 sec. Parallel flow of  air
               that phase of  0.5 ft/sec.  minimum diameter of  16 in.   and  stock is  most  common.  Atomizing nozzles have  openings
               For  towers  about  3ft  dia,  add  4ft  at  the  top  for  vapor   0.012-0.15  in.  and  operate  at  pressures  of  300-4000 psi.
               disengagement  and  6ft  at  the  bottom  for  liquid  level  and   Atomizing  spray  wheels  rotate  at  speeds  to  20,000rpm  with
               reboiler return.                                   peripheral speeds of  250-600  ft/sec.  With nozzles, the length to
            29.  Limit the tower height to about 175 ft max because of wind load   diameter ratio of  the dryer is 4-5;  with spray wheels, the ratio is
               and  foundation  considerations. An  additional criterion is  that   0.5-1.0.  For the final design, the experts say, pilot tests in a unit
               L/D be less than 30.                               of  2 m dia should be made.
              RIVERS AND  POWER RECOVERY EQUIPMENT
                                                                EVAPORATORS
            1. Efficiency  IS  greater  for  larger  machines. Motors  are  85-95%;
               steam  turbines  an:  42-78%;  gas  engines  and  turbines  are   1.  Long  tube  vertical  evaporators  with  either  natural  or  forced
               28-38%.                                            circulation  are  most  popular.  Tubes  are  19-63mm  dia  and
            2.  For  under  100HP, electric motors  are used  almost exclusively.   12-30 ft long.
              They are made for up to 20,000 HP.                2.  In  forced  circulation,  linear  velocities  in  the  tubes  are
            3.  Induction  motors  are  most  popular.  Synchronous motors  are   15-20  ft/sec.
               made  for  speeds  as  low  as  150rpm and  are  thus  suited  for   3.  Elevation  of  boiling  point  by  dissolved  solids  results  in
               example for  low  speed  reciprocating compressors, but  are  not   differences of  3-10°F  between solution and saturated vapor.
               made  smaller than 50 MP.  A  variety of  enclosures is  available,   4.  When the boiling point rise is appreciable, the economic number
               from weather-proof to explosion-proof.             of effects in series with forward feed is 4-5.
            4.  Steam turbines  are  competitive above 100HP. They are  speed   5. When the boiling point rise is small, minimum cost is  obtained
               controllable. Frequently they are employed as spares in case of   with 8-10  effects in series.
               power failure.                                   6. In backward feed the more concentrated solution is heated with
            5.  combustion  engines and  turbines  are  restricted  to  mobile  and   the  highest  temperature  steam  so  that  heating  surface  is
               remote locations.                                  lessened, but the solution must be pumped between stages.
            5.  Gas expanders for power recovery may be justified at capacities   7.  The  steam  economy  of  an  N-stage  battery  is  approximately
               of several lhundred HP; otherwise any needed pressure reduction   0.8N lb evaporation/lb of  outside steam.
               in process is effected with throttling valves.   8.  Interstage  steam  pressures  can  be  boosted  with  steam  jet
                                                                  compressors of  20-30%  efficiency or with  mechanical compres-
             RYING OF  SOLIDS                                     sors of  70-75%  efficiency.
            1. Drying times range from a few seconds in spray dryers to 1 hr or   EXTRACTION,  LIQUID-LIQUID
              less in rotary dryers, and up to several hours or even several days
              in tunnel shelf or belt dryers.                   1. The  dispersed  phase  should  be  the  one  that  has  the  higher
            2. Continuous tray and belt dryers for granular material of  natural   volumetric  rate  except  in  equipment  subject  to  backmixing
              size or pellleted to 3-15  mm  have drying times in  the  range of   where it  should be  the one with  the smaller volumetric rate. It
              10-200 mnn.                                         should be  the  phase that  wets the material of  construction less
            3.  Rotary cylindrical dryers operate with superficial air velocities of   well.  Since the  holdup  of  continuous phase  usually is  greater,
              S-10  ft/sec,  sometimes  up  to  35 ft/sec  when  the  material  is   that  phase  should  be  made  up  of  the  less  expensive or  less
              coarse. Residence times are 5-90  min. Holdup of  solid is 7-8%.   hazardous material.
   13   14   15   16   17   18   19   20   21   22   23