Page 20 - Chemical Process Equipment - Selection and Design
P. 20

RULES  OF  THUMB:  SUMMARY  Xvii
               Ceramic refractories at higher temperatures.       through a die followed by cutting. An  8 in. screw has a capacity
               Cyrogenic equipment (-200OF)  employs insulants with fine pores   of 2000 Ib/hr  of  molten plastic and is  able to extrude tubing at
               in which air is trapped.                           150-300ft/min  and  to  cut  it  into  sizes  as  small  as  washers  at
               Optimum  thickness varies  with  temperature:  0.5 in.  at  200"F,   8000/min. Ring  pellet  extrusion  mills  have  hole  diameters  of
               1.Oin. at 400"F, 1.25in. at 600°F.                 1.6-32mm.  Production  rates  cover  a  range  of  30-200
               Under  windy  conditions  (7.5  miles/hr),  10-20%  greater   lb/(hr)(HP).
               thickness of insulation is justified.            7.  Prilling towers convert molten materials into droplets and allow
                                                                  them to solidify in contact with an air stream. Towers as high as
            MIXING AND  AGlTATlOM                                 60 m  are  used.  Economically the  process  becomes  competitive
                                                                  with  other  granulation  processes  when  a  capacity  of  200-
            1.  Mild  agitation  is  obtained  by  circulating  the  liquid  with  an   400 tons/day is reached.  Ammonium nitrate prills, for example,
              impeller  a:  superficial velocities  of  0.1-0.2 ft/sec,  and  intense   are 1.6-3.5 mm dia in the 595% range.
              agitation ai 0.7-1.0  ft/sec.                     8.  Fluidized bed granulation is conducted in shallow beds 12-24 in.
            2.  Intensities  of  agitation  with  impellers  in  baffled  tanks  are   deep at air velocities of  0.1-2.5 m/s or 3-10  times the minimum
              measured by power input,  HP/1000 gal, and impeller tip speeds:   fluidizing  velocity,  with  evaporation  rates  of   0.005-
                    Operation      HP/lO(DO gal   Tip speed (ft/min)   1.0 kg/m2 sec. One product has a size range 0.7-2.4 mm dia.
               Blending              0.2-0.5
               Homogeneous reaction   0.5-1 ..5   7.5-10        PIPING
               Reaction with heat transfer   1.5-5,.0   10-15
               Liquid-liquid  mixtures   5        15-20         1. Line  velocities  and  pressure  drops,  with  line  diameter  D  in
               Liquid-gas  mixtures   5-10        15-20           inches:  liquid  pump  discharge,  (5 + D/3)  ft/sec,  2.0 psi/lOO ft;
               Slurries                10                         liquid pump suction,  (1.3 + D/6) ft/sec,  0.4 psi/100 ft; steam or
                                                                  gas, 200 ft/sec, 0.5 psi/100 ft.
            3.  Proportions  of  a stiirred tank  relative to the  diameter D: liquid   2.  Control valves require at least 10 psi drop for good control.
              level = D; turbine impeller diameter  D/3; impeller level above   3.  Globe valves  are used  for gases, for control and wherever tight
              bottom = W/3; impeller blade width == D/lS; four vertical baffles   shutoff is required. Gate valves are for most other services.
              with width = D/10.                                4.  Screwed  fittings  are  used  only  on  sizes  1.5in.  and  smaller,
            4.  Propellers are  made  a maximum of  18 in.,  turbine  impellers to   flanges or welding otherwise.
              9 ft.                                             5.  Flanges and  fittings are  rated  for  150, 300,  600, 900,  1500, or
            5.  Gas  bubbles  sparged  at  the  bottom  of  the  vessel will  result in   2500 psig.
              miid  agitalion  at  a  superficial gas  velocity  of  lft/min,  severe   6. Pipe schedule number = lOOOP/S,  approximately, where P is the
              agitation at 4 ft/min.                              internal  pressure  psig  and  S  is  the  allowable working  stress
            6. Suspension  of  solids  with  a  settling  velocity  of  0.03ft/sec  is   (about 10,000 psi for A120 carbon steel at 500°F). Schedule 40 is
              accomplished  with  either  turbine  or  propeller  impellers,  but   most common.
              when the  settling velocity is  above 0.15 ft/sec intense  agitation
              with a propeller is needed.                       PUMPS
            7. Power to drive a mixture of  a gas and  a liquid can be 2550%
              less than the power to drive the liquid alone.    1.  Power  for  pumping  liquids: HP = (gpm)(psi difference)/(l714)
            8.  In-line blenders are adequate when a second or two contact time   (fractional efficiency).
              is sufficient, with power inputs of  0.1-0.2HP/gal.   2.  Normal pump suction head (NPSH) of  a pump must be in excess
                                                                  of  a certain number,  depending on  the  kind of  pumps  and  the
            PARTICLE SIIZE  ENLARGEMENT                           conditions, if  damage is to be avoided. NPSH = (pressure at the
                                                                  eye of  the impeller - vapor pressure)/(density).  Common range
            1. The chief methods of  particle size enlargement are: compression   is 4-20  ft.
              into  a  mold,  extrusion  through  a  die  followed  by  cutting  or   3.  Specific speed N, = (r~m)(gpm)'.~/(head in ft)0.75. Pump may be
              breaking  to  size,  globulation  of  molten  material  followed  by   damaged if  certain limits  of  N,  are  exceeded,  and  efficiency is
              solidification,  agglomeration  undeir  tumbling  or  otherwise   best in some ranges.
              agitated conditions with or without binding agents.   4.  Centrifugal pumps:  Single  stage  for  15-5000 gpm,  SOOft  max
            2.  Rotating drum granulators have length to diameter ratios of 2-3,   head; multistage for 20-11,000  gpm, 5500 ft max head. Eficiency
              speeds of  10-20  rpm, pitch as much  as 10". Size is controlled by   45% at 100 gpm, 70% at 500 gpm, 80% at 10,000 gpm.
              speed,  residence  tiime,  and  amount  of  binder;  2-5mm  dia  is   5.  Axial pumps for 20-100,000  gpm, 40 ft head, 65-85%  efficiency.
              common.                                           6. Rotary  pumps  for  1-5000 gpm,  50,000 ft  head,  50-80%
            3.  Rotary disk granulators produce a more nearly uniform product   efficiency.
              than  drum  granulators.  Fertilizer is made  1.5-3.5 mm; iron ore   7. Reciprocating pumps for 10-10,000 gpm, 1,000,000 ft head max.
              10-25 mm dia,                                       Efficiency 70% at 10 HP, 85% at 50 HP, 90% at 500 HP.
            4.  Roll compacting and briquetting is done with  rolls ranging from
              130mm  dia  by  50mm  wide  to  910mm  dia  by  550mm  wide.   REACTORS
              Extrudates are made 1-10  mm thick and are broken down to size
              for any needed processing such as feed to tabletting machines or   1. The rate of  reaction in every instance must be established in the
              to dryers.                                           laboratory,  and  the  residence  time  or  space  velocity  and
            5.  Tablets are  made  in  rotary  compression machines that  convert   product distribution eventually must be found in a pilot plant.
              powders  and  granules  into  uniform  sizes.  Usual  maximum   2.  Dimensions of  catalyst particles are 0.1 mm  in  fluidized beds,
              diameter  is  about  1.5in., but  special sizes up  to  4in.  dia  are   1 mm in slurry beds, and 2-5  mm in fixed beds.
              possible~ Machines  operate  at  lOOrpm  or  so  and  make  up  to   3.  The  optimum  proportions  of  stirred  tank  reactors  are  with
              10,000 tablets/min.                                  liquid level equal  to  the  tank  diameter,  but  at high  pressures
            6.  Extruders  make  pellets  by  forcing  powders,  pastes,  and  melts   slimmer proportions are economical.
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