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.