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.