Page 19 - Chemical Process Equipment - Selection and Design
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XVi  RULES  OF  THUMB:  SUMMARY
              2.  There  are  no  known  commercial  applications  of  reflux  to   attrition, sizes in the range 50-500pm  dia,  a spectrum of  sizes
                extraction processes, although the theory is favorable (Treybal).   with ratio of  largest to smallest in the range of  10-25.
              3.  Mixer-settler  arrangements  are  limited  to  at  most  five  stages.   2.  Cracking catalysts are members of  a broad class characterized by
                Mixing  is  accomplished  with  rotating  impellers  or  circulating   diameters of  30-150  pm, density of  1.5 g/mL or so, appreciable
                pumps.  Settlers  are  designed  on  the  assumption that  droplet   expansion  of  the  bed  before  fluidization  sets  in,  minimum
                sizes are about  150pm dia.  In open vessels, residence times of   bubbling velocity greater than  minimum fluidizing velocity, and
                30-60  min or superficial velocities of 0.5-1.5 ft/min are provided   rapid disengagement of  bubbles.
                in settlers.  Extraction  stage efficiencies commonly are taken as   3.  The other extreme of  smoothly fluidizing particles is typified by
                80%.                                                coarse sand and glass beads both of  which have been the subject
              4.  Spray  towers  even  20-40  ft  high  cannot  be  depended  on  to   of  much  laboratory  investigation. Their  sizes are  in  the  range
                function as more than a single stage.               150-500 pm, densities 1.5-4.0 g/mL, small bed expansion, about
              5.  Packed towers are employed when 5-10  stages suffice. Pall rings   the  same  magnitudes  of  minimum  bubbling  and  minimum
                of  1-1.5in.  size are best.  Dispersed phase loadings should not   fluidizing velocities, and also have rapidly disengaging bubbles.
                exceed 25 gal/(min)  (sqft). HETS of  5-10  ft may be  realizable.   4.  Cohesive particles and large particles of  1 mm  or more  do not
                The dispersed phase must be redistributed every 5-7  ft. Packed   fluidize well and usually are processed in other ways.
                towers are not satisfactory when the surface tension is more than   5.  Rough  correlations  have  been  made  of  minimum  fluidization
                 10 dyn/cm.                                         velocity, minimum bubbling velocity, bed  expansion, bed  level
              6. Sieve  tray  towers  have  holes  of  only  3-8mm  dia.  Velocities   fluctuation,  and  disengaging  height.  Experts  recommend,
                through the holes are kept below 0.8 ft/sec to avoid formation of   however, that any real design be based on pilot plant work.
                small drops.  Redispersion of  either  phase  at  each  tray  can be   6.  Practical operations  are conducted at two or more multiples of
                designed for. Tray spacings are 6-24in.  Tray efficiencies are in   the  minimum  fluidizing  velocity.  In  reactors,  the  entrained
                the range of  20-30%.                               material is recovered with cyclones and returned  to process. In
              7.  Pulsed packed and sieve tray towers may operate at frequencies   dryers,  the  fine  particles  dry  most  quickly  so  the  entrained
                 of  90 cycles/min and amplitudes of  6-25  mm. In large diameter   material need not be recycled.
                 towers, HETS of  about 1 m has been observed. Surface tensions
                 as high as 30-40  dyn/cm have no adverse effect.   HEAT EXCHANGERS
              8.  Reciprocating tray towers can have holes 9/16in. dia,  50-60%   1. Take true countercurrent flow in a shell-and-tube exchanger as
                 open  area,  stroke  length  0.75 in.,  100-150  strokes/min,  plate
                                                                     a basis.
                spacing normally 2in. but  in  the  range  1-6in.  In a  30in.  dia   2.  Standard  tubes  are  3/4 in.  OD,  1 in.  triangular  spacing, 16 ft
                 tower, HETS is 20-25  in. and throughput is 2000 gal/(hr)(sqft).
                Power requirements are much less than of  pulsed towers.   long; a shell 1 ft dia accommodates 100 sqft; 2ft dia, 400 sqft,
                                                                     3 ft dia, 1100 sqft.
              9.  Rotating  disk contactors or other rotary  agitated towers realize   3.  Tube side is  for corrosive, fouling, scaling, and high  pressure
                 HETS  in  the  range  0.1-0.5m.  The  especially efficient  Kuhni   fluids.
                with perforated disks of  40% free cross section has HETS 0.2 m   4.  Shell side is for viscous and condensing fluids.
                 and a capacity of  50 m3/m2 hr.
                                                                   5.  Pressure  drops  are  1.5psi  for  boiling  and  3-9psi  for  other
                                                                     services.
              FILTRATION
                                                                   6. Minimum temperature  approach is 20°F with normal coolants,
              1. Processes  are  classified  by  their  rate  of  cake  buildup  in  a   10°F or less with refrigerants.
                 laboratory  vacuum leaf  filter: rapid,  0.1-10.0  cm/sec;  medium,   7.  Water inlet temperature is 90°F, maximum outlet 120°F.
                 0.1-10.0  cm/min; slow, 0.1-10.0  cm/hr.          8.  Heat   transfer   coefficients  for   estimating  purposes,
              2.  Continuous  filtration  should  not  be  attempted  if  1/8in.  cake   Btu/(hr)(sqft)("F):  water to liquid, 150; condensers, 150; liquid
                thickness cannot be formed in less than 5 min.       to liquid, 50; liquid to gas, 5; gas to gas, 5; reboiler, 200. Max
              3.  Rapid  filtering is  accomplished with  belts,  top  feed  drums,  or   flux in reboilers, 10,000 Btu/(hr)(sqft).
                 pusher-type centrifuges.                          9.  Double-pipe  exchanger  is  competitive  at  duties  requiring
              4.  Medium  rate  filtering  is  accomplished with  vacuum  drums  or   100-200  sqft.
                 disks or peeler-type centrifuges.                10.  Compact  (plate  and  fin)  exchangers have  350sqft/cuft,  and
              5.  Slow  filtering  slurries  are  handled  in  pressure  filters  or   about 4 times the heat transfer per cuft of  shell-and-tube units.
                 sedimenting centrifuges.                         11.  Plate  and  frame  exchangers  are  suited  to  high  sanitation
              6.  Clarification with  negligible cake buildup is  accomplished with   services, and are 2540% cheaper in stainless construction than
                 cartridges, precoat drums, or sand filters.         shell-and-tube units.
              7.  Laboratory  tests  are  advisable  when  the  filtering  surface  is   12. Air  coolers: Tubes  are  0.75-1.00in.  OD, total  finned surface
                 expected  to  be  more  than  a  few  square  meters,  when  cake   15-20  sqft/sqft  bare  surface,  U = 80-100  Btu/(hr)(sqft  bare
                 washing is critical, when cake drying may be a problem, or when   surface)("F),  fan  power  input  2-5  HP/(MBtu/hr),  approach
                 precoating may be needed.                           50°F or more.
              8.  For finely ground ores and minerals, rotary drum filtration rates   W.  Fired  heaters:  radiant  rate,  12,000 Btu/(hr)(sqft);  convection
                 may  be  1500  lb/(day)(sqft),  at  20rev/hr  and  18-25in.  Hg   rate, 4000; cold oil tube velocity, 6 ft/sec; approx equal transfers
                 vacuum.                                             of  heat in the two sections; thermal efficiency 70-75%;  flue gas
              9.  Coarse  solids  and  crystals  may  be  filtered  at  rates  of  6000   temperature 250-350°F  above feed inlet; stack gas temperature
                 lb/(day)(sqft)  at 20 rev/hr, 2-6  in. Hg vacuum.   650-950°F.
                                                                  INSULATION
              FLUIDIZATION OF  PARTICLES WITH  GASES
                                                                  1.  Up to 650"F, 85% magnesia is most used.
              1.  Properties of  particles that  are conducive to smooth fluidization   2.  Up  to  1600-1900"F,  a  mixture  of  asbestos  and  diatomaceous
                include: rounded  or smooth shape,  enough toughness to  resist   earth is used.
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