Page 21 - Chemical Process Equipment - Selection and Design
P. 21

XViii  RULES  OF  THUMB:  SUMMARY

               4.  Power input to a homogeneous reaction stirred tank is 0.5-1.5   strokes/min at 28 mesh. Solids content is not critical, and that of
                 HP/lOOOgal,  but  three  times this  amount when heat  is  to be   the overflow may be 2-20%  or more.
                 transferred.                                     7.  Hydrocyclones  handle  up  to  600cuft/min  and  can  remove
               5.  Ideal  CSTR  (continuous  stirred  tank  reactor)  behavior  is   particles in the range  of  300-5  pm from dilute suspensions. In
                 approached  when  the  mean  residence time is 5-10  times the   one  case,  a  20in.  dia unit  had  a  capacity of  1OOOgpm with  a
                 length  of  time  needed  to  achieve  homogeneity,  which  is   pressure drop of  5 psi and a cutoff between 50 and 150 pm.
                 accomplished with 500-2000  revolutions of  a properly designed
                 stirrer.                                         UTILITIES: COMMON  SPECIFICATIONS
               6.  Batch reactions are  conducted in stirred  tanks for  small daily
                 production rates  or when the reaction times are long or when   1. Steam:  15-30 psig,  250-275°F;  150 psig, 366°F; 400 psig, 448°F;
                 some  condition  such  as  feed  rate  or  temperature  must  be   600 psig, 488°F or with  100-150°F  superheat.
                 programmed in some way.                          2.  Cooling water: Supply at 80-90°F  from cooling tower, return at
               7.  Relatively slow reactions of  liquids and slurries are conducted   115-125°F;  return  seawater at llO"F, return tempered water or
                 in continuous stirred tanks. A battery of  four or five in series is   steam condensate above 125°F.
                 most economical.                                 3.  Cooling air supply at 85-95°F;  temperature approach to process,
               8. Tubular  flow  reactors  are  suited  to  high  production  rates  at   40°F.
                 short  residence times  (sec or  min)  and when substantial heat   4.  Compressed air at 45, 150, 300, or 450 psig levels.
                 transfer  is  needed.  Embedded  tubes  or  shell-and-tube   5.  Instrument air at 45 psig, 0°F dewpoint.
                 construction then are used.                      6.  Fuels:  gas  of  1000 Btu/SCF  at  5-10  psig,  or  up  to  25 psig  for
               9.  In  granular  catalyst  packed  reactors,  the  residence  time   some types of burners: liquid at 6 million Btu/barrel.
                 distribution often is  no better  than  that  of  a five-stage CSTR   7.  Heat  transfer  fluids:  petroleum  oils  below  600"F,  Dowtherms
                 battery.                                           below 750"F,  fused salts below  1100"F, direct fire or electricity
              10. For  conversions  under  about  95%  of  equilibrium,  the   above 450°F.
                 performance  of  a  five-stage  CSTR  battery  approaches  plug   8.  Electricity: 1-100 Hp, 220-550  V; 200-2500 Hp, 2300-4000 V.
                 flow.
                                                                  VESSELS  (DRUMS)
               REFRIGERATION
                                                                   1. Drums are relatively small vessels to provide surge capacity or
               1. A ton of  refrigeration is the removal of  12,000 Btu/hr of  heat.   separation of  entrained phases.
               2.  At various temperature levels: 0-50"F,  chilled brine and glycol   2.  Liquid drums usually are horizontal.
                 solutions;  -50-40"F,  ammonia,  freons,  butane;  -150--50"F,   3.  Gas/liquid separators are vertical.
                 ethane or propane.                                4.  Optimum  length/diameter=3,  but  a  range  of  2.5-5.0  is
               3.  Compression refrigeration with  100°F condenser requires  these   common.
                 HP/ton at various temperature levels: 1.24 at 20°F; 1.75 at 0°F;   5. Holdup time is 5 min half  full for reflux drums, 5-10  min for a
                 3.1 at -40°F;  5.2 at -80°F.                        product feeding another tower.
               4.  Below -80"F,  cascades of  two or three refrigerants are used.   6. In drums feeding a furnace, 30 min half full is allowed.
               5.  In single stage compression, the compression ratio is limited to   7.  Knockout drums ahead of compressors should hold no less than
                 about 4.                                            10 times the liquid volume passing through per minute.
               6. In multistage compression, economy is improved with interstage   8.  Liquid/liquid  separators  are  designed for  settling velocity  of
                 flashing and recycling, so-called economizer operation.   2-3  in./min.
               7.  Absorption refrigeration (ammonia to -30"F,  lithium bromide to   9.  Gas velocity in gas/liquid separators,  V = k  v  m  ft/sec,
                 +45"F)  is economical when waste steam is available at 12 psig or   with  k = 0.35  with  mesh  deentrainer,  k = 0.1  without  mesh
                 so.                                                 deentrainer.
                                                                  10.  Entrainment  removal  of  99%  is  attained  with  mesh  pads  of
               SIZE  SEPARATION OF  PARTICLES                        4-12  in. thicknesses; 6 in. thickness is popular.
                                                                  11.  For  vertical  pads,  the  value  of  the  coefficient  in  Step  9  is
               1.  Grizzlies  that  are  constructed  of  parallel  bars  at  appropriate   reduced by a factor of  2/3.
                 spacings are used to remove products larger than 5 cm dia.   12. Good performance can be expected at velocities of  30-100%  of
               2.  Revolving cylindrical screens rotate at 15-20 rpm and below the   those calculated with the given k; 75% is popular.
                 critical velocity; they are suitable for wet or dry screening in the   13.  Disengaging  spaces  of  6-Bin.  ahead  of  the  pad  and  12in.
                 range of  10-60 rnm.                                above the pad are suitable.
              3.  Flat  screens are vibrated  or  shaken or impacted with bouncing   14.  Cyclone separators can be designed for 95% collection of  5 pm
                 balls. Inclined screens vibrate  at 600-7000  strokes/min  and are   particles, but usually only droplets greater than 50 pm need be
                 used  for  down  to  38pm  although  capacity drops  off  sharply   removed.
                 below  200 ym.  Reciprocating  screens  operate  in  the  range
                 30-1000  strokes/rnin  and handle  sizes  down to 0.25mm  at the   VESSELS  (PRESSURE)
                 higher speeds.
               4.  Rotary sifters operate at 500-600 rpm and are suited to a range   1. Design  temperature  between  -20°F  and  650°F  is  50°F  above
                 of  12 mm to 50 yrn.                                operating  temperature;  higher  safety margins  are used  outside
               5.  Air classification is preferred for fine sizes because screens of  150   the given temperature range.
                 mesh and finer are fragile and slow.              2.  The design pressure is 10% or 10-25 psi over the maximum oper-
               6.  Wet classifiers mostly are used to make two product size ranges,   ating pressure,  whichever  is  greater.  The  maximum operating
                 oversize and  undersize,  with  a  break  commonly in  the  range   pressure, in turn, is taken as 25 psi above the normal operation.
                 between 28 and 200 mesh. A rake classifier operates at about 9   3.  Design  pressures  of  vessels  operating  at  0-1Opsig  and  600-
                 strokes/min  when  making  separation  at  200  mesh,  and  32   1000°F are 40 psig.
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