Page 17 - Chemical Process Equipment - Selection and Design
P. 17

xiv  RULES  OF THUMB:  SUMMARY
                 are  0.1-0.3%.  Blowdown  of  2.5-3.0%  of  the  circulation  is   DISTILLATION AND GAS ABSORPTION
                 necessary to prevent excessive salt buildup.
                                                                   1.  Distillation usually is the most economical method of separating
                                                                      liquids,  superior  to  extraction,  adsorption,  crystallization, or
                                                                      others.
               CRYSTALLIZATION FROM SOLUTION                       2.  For  ideal  mixtures,  relative  volatility  is  the  ratio  of  vapor
               1.  Complete  recovery  of  dissolved  solids  is  obtainable  by   pressures n12 = P2/P,.
                 evaporation,  but  only  to  the  eutectic  composition by  chilling.   3.  Tower  operating  pressure  is  determined  most  often  by  the
                 Recovery by  melt  crystallization also is limited by  the  eutectic   temperature  of  the  available condensing medium, 100-120°F  if
                 composition.                                         cooling  water;  or  by  the  maximum  allowable  reboiler
               2.  Growth  rates  and  ultimate  sizes  of  crystals  are  controlled  by   temperature, 150 psig steam, 366°F.
                 limiting the extent of  supersaturation at any time.   4.  Sequencing  of  columns  for  separating  multicomponent  mix-
               3.  The  ratio  S = C/C,,  of  prevailing  concentration  to  saturation   tures:  (a) perform the  easiest separation first, that  is, the one
                 concentration is kept near the range of  1.02-1.05.   least demanding of  trays and reflux, and leave the most difficult
               4.  In  crystallization by  chilling, the  temperature  of  the  solution is   to  the  last;  (b)  when  neither  relative  volatility  nor  feed
                 kept  at  most  1-2°F  below  the  saturation  temperature  at  the   concentration vary widely, remove the components one by one
                 prevailing concentration.                            as  overhead  products;  (c)  when  the  adjacent  ordered
               5.  Growth rates of  crystals under satisfactory conditions are in the   components  in  the  feed  vary  widely  in  relative  volatility,
                 range of  0.1-0.8  mm/hr. The growth rates are approximately the   sequence  the  splits  in  the  order  of  decreasing volatility;  (d)
                 same in all directions.                              when the concentrations in the feed vary widely but the relative
               6.  Growth rates are influenced greatly by the presence of impurities   volatilities  do  not,  remove  the  components  in  the  order  of
                 and of  certain specific additives that vary from case to case.   decreasing concentration in the feed.
                                                                   5.  Economically  optimum  reflux  ratio  is  about  1.2  times  the
                                                                      minimum reflux ratio R,.
                                                                    6.  The economically optimum number  of  trays is  near  twice the
               DISINTEGRATION                                         minimum value N,,,.
               1.  Percentages of  material greater  than  50% of  the  maximum size   7.  The  minimum  number  of  trays  is  found  with  the  Fenske-
                 are  about  50%  from  rolls,  15%  from  tumbling mills,  and  5%   Underwood equation
                 from closed circuit ball mills.
               2.  Closed circuit grinding  employs external  size  classification and
                 return  of  oversize  for  regrinding.  The  rules  of  pneumatic
                 conveying are applied to design of  air classifiers. Closed circuit is   8.  Minimum reflux for binary or pseudobinary mixtures is given by
                 most common with ball and roller mills.              the  following when  separation  is  esentially complete (xD = 1)
               3.  Jaw crushers take lumps of several feet in diameter down to 4 in.   and D/F is the ratio of  overhead product and feed rates:
                 Stroke rates  are 100-300/min.  The average feed is subjected to
                 8-10  strokes  before  it  becomes  small  enough  to  escape.   R,D/F  = l/(n - l),   when feed is at the bubblepoint,
                 Gyratory crushers are suited to slabby feeds and make  a more   (R, + 1)D/F = n/(a - l),  when feed is at the dewpoint.
                 rounded product.
               4.  Roll  crushers are made  either  smooth  or  with  teeth.  A  24in.   9.  A safety factor of  10% of the number of  trays calculated by the
                 toothed  roll  can  accept  lumps  14in. dia.  Smooth  rolls  effect   best means is advisable.
                 reduction ratios up to about 4. Speeds are 50-900  rpm. Capacity   10.  Reflux pumps are made at least 25% oversize.
                 is  about  25%  of  the  maximum corresponding to  a  continuous   11. For reasons of  accessibility, tray spacings are made 20-24  in.
                 ribbon of  material passing through the rolls.    12. Peak  efficiency  of  trays  is  at  values  of  the  vapor  factor
               5.  Hammer mills beat the material until it is small enough to pass   F, = u  6  in the range 1.0-1.2  (ft/sec) m. This range of
                 through the screen at the bottom of  the casing. Reduction ratios   F,  establishes  the  diameter  of  the  tower.  Roughly,  linear
                 of  40  are feasible. Large units operate at 900 rpm, smaller ones   velocities  are  2ft/sec  at  moderate  pressures  and  6ft/sec  in
                 up  to  16,000rpm. For  fibrous materials the  screen is  provided   vacuum.
                 with cutting edges.                               13.  The  optimum value  of  the  Kremser-Brown  absorption factor
               6.  Rod  mills  are  capable  of  taking  feed  as  large  as  50mm  and   A = K(V/L) is in the range 1.25-2.0.
                 reducing it to 300 mesh, but  normally the product range is 8-65   14.  Pressure drop per tray is of the order of  3 in. of  water or 0.1 psi.
                 mesh.  Rods  are  25-150mm  dia.  Ratio  of  rod  length  to  mill   15.  Tray  efficiencies  for  distillation  of  light  hydrocarbons  and
                 diameter is about 1.5. About 45% of the mill volume is occupied   aqueous  solutions  are  60-90%;  for  gas  absorption  and
                 by rods. Rotation is at 50-6570  of  critical.       stripping, 10-20%.
               7.  Ball mills  are better  suited than  rod mills to fine grinding. The   16.  Sieve trays have holes 0.25-0.50 in. dia, hole area being 10% of
                 charge is of  equal weights of  1.5, 2, and 3 in. balls for the finest   the active cross section.
                 grinding.  Volume  occupied  by  the  balls  is  50%  of  the  mill   17. Valve trays have holes 1.5in. dia each provided with a liftable
                 volume. Rotation  speed is 7040% of  critical. Ball mills have a   cap, 12-14  caps/sqft of  active cross section. Valve trays usually
                 length to diameter  ratio  in the  range  1-1.5.  Tube mills have a   are cheaper than sieve trays.
                 ratio of  4-5  and are capable of  very fine grinding. Pebble mills   18.  Bubblecap  trays  are  used  only  when  a  liquid  level must  be
                 have ceramic grinding elements, used when contamination with   maintained  at  low  turndown  ratio;  they  can  be  designed  for
                 metal is to be avoided.                              lower pressure drop than either sieve or valve trays.
               8.  Roller mills employ cylindrical or tapered surfaces that roll along   19.  Weir heights are 2in., weir lengths about 75% of tray diameter,
                 flatter  surfaces and  crush nipped  particles. Products  of  20-200   liquid rate  a  maximum of  about  8gpm/in.  of  weir; multipass
                 mesh are made.                                       arrangements are used at high liquid rates.
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