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Heat Exchangers |
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A. For the heat exchanger
equation, Q = UAF (LMTD), use F = 0.9 when charts for the LMTD
correction
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factor are not available
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B. Most commonly used
tubes are 3/4 in. (1.9 cm) in outer diameter on a 1 in triangular
spacing at 16 ft (4.9 m) long.
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C. A 1 ft (30 cm) shell
will contains about 100 ft2 (9.3 m2)
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A 2 ft (60 cm) shell will
contain about 400 ft2 (37.2 m2)
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A 3 ft (90 cm) shell will
contain about 1100 ft2 (102 m2)
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D. Typical velocities in
the tubes should be 3-10 ft/s (1-3 m/s) for liquids and30-100 ft/s (9-30
m/s) for gases
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E. Flows that are
corrosive, fouling, scaling, or under high pressure are usually placed
in the tubes
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F. Viscous and condensing
fluids are typically placed on the shell side.
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G. Pressure drops are
about 1.5 psi (0.1 bar) for vaporization and 3-10 psi (0.2-0.68 bar) for
other services
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H. The minimum approach
temperature for shell and tube exchangers is about 20 �F (10 �C) for
fluids and
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10 �F (5 �C) for
refrigerants.
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I. Cooling tower water is
typically available at a maximum temperature of 90 �F (30 �C) and should
be
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returned to the tower no
higher than 115 �F (45 �C)
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J. Shell and Tube heat
transfer coefficient for estimation purposes can be found in many
reference books
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or an online list can be
found at one of the two following addresses:
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K. Double pipe heat
exchangers may be a good choice for areas from 100 to 200 ft2 (9.3-18.6
m2) |
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L. Spiral heat exchangers
are often used to slurry interchangers and other services containing
solids
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M. Plate heat exchanger
with gaskets can be used up to 320 �F (160 �C) and are often used for
interchanging
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duties due to their high
efficiencies and ability to "cross" temperatures.
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Tray Towers
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A. For ideal mixtures,
relative volatility can be taken as the ratio of pure component vapor
pressures
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B. Tower operating
pressure is most often determined by the cooling medium in condenser or
the
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maximum allowable
reboiler temperature to avoid degradation of the process fluid
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C. For sequencing
columns:
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1. Perform the easiest
separation first (least trays and lowest reflux)
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2. If relative volatility
nor feed composition vary widely, take products off one at time
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as the overhead
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3. If the relative
volatility of components do vary significantly, remove products in order
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of decreasing volatility
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4. If the concentrations
of the feed vary significantly but the relative volatility do not,
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remove products in order
of decreasing concentration.
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D. The most economic
reflux ratio usually is between 1.2Rmin and 1.5Rmin
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E. The most economic
number of trays is usually about twice the minimum number of trays.
The minimum number of trays is
determined with the Fenske-Underwood Equation.
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F. Typically, 10% more
trays than are calculated are specified for a tower.
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G. Tray spacings should
be from 18 to 24 inches, with accessibility in mind
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H. Peak tray efficiencies
usually occur at linear vapor velocities of 2 ft/s (0.6 m/s) at moderate
pressures,
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or 6 ft/s (1.8 m/s) under
vacuum conditions.
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I. A typical pressure
drop per tray is 0.1 psi (0.007 bar)
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J. Tray efficiencies for
aqueous solutions are usually in the range of 60-90% while gas
absorption and
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stripping typically have
efficiencies closer to 10-20%
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K. The three most common
types of trays are valve, sieve, and bubble cap. Bubble cap trays are
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typically used when
low-turn down is expected or a lower pressure drop than the valve or
sieve
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trays can provide is
necessary.
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L. Seive
tray holes are 0.25 to 0.50 in. diameter with the total hole area being
about 10% of the total active tray area. |
M. Valve
trays typically have 1.5 in. diameter holes each with a lifting cap.
12-14 caps/square foot of tray is a good benchmark. Valve trays usually cost less than seive trays. |
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N. The most common weir
heights are 2 and 3 in and the weir length is typically 75% of the tray
diameter
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O. Reflux pumps should be
at least 25%
overdesigned
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P. The optimum Kremser
absorption factor is usually in the range of 1.25 to 2.00
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Q. Reflux drums are
almost always horizontally mounted and designed for a 5 min holdup at
half of the
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drum's capacity.
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R. For towers that are at
least 3 ft (0.9 m) is diameter, 4 ft (1.2 m) should be added to the top
for vapor
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release and 6 ft (1.8 m)
should be added to the bottom to account for the liquid level and
reboiler return
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S. Limit tower heights to
175 ft (53 m) due to wind load and foundation considerations.
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T. The Length/Diameter
ratio of a tower should be no more than 30 and preferrably below 20
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U. A rough estimate of
reboiler duty as a function of tower diameter is given by:
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Q = 0.5 D2 for
pressure distillation
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Q = 0.3 D2 for
atmospheric distillation
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Q = 0.15 D2
for vacuum distillation
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where Q is in Million
Btu/hr and D is tower diameter in feet
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Packed Towers |
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A. Packed towers almost
always have lower pressure drop than comparable tray towers.
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B. Packing is often
retrofitted into existing tray towers to increase capacity or
separation.
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C. For gas flowrates of
500 ft3/min (14.2 m3/min) use 1 in (2.5 cm) packing, for gas flows
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of 2000 ft3/min (56.6
m3/min) or more, use 2 in (5 cm) packing
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D. Ratio of tower
diameter to packing diameter should usually be at least 15
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E. Due to the possibility
of deformation, plastic packing should be limited to an unsupported
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depth of 10-15 ft (3-4 m)
while metallatic packing can withstand 20-25 ft (6-7.6 m)
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F. Liquid distributor
should be placed every 5-10 tower diameters (along the length) for pall
rings
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and every 20 ft (6.5 m)
for other types of random packings
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G. For redistribution, there should be 8-12 streams per sq. foot of
tower area for tower larger than
three feet in diameter. They should be even more numerous in
smaller towers. |
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H. Packed columns should
operate near 70% flooding.
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I. Height Equivalent to
Theoretical Stage (HETS) for vapor-liquid contacting is 1.3-1.8 ft
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(0.4-0.56 m) for 1 in
pall rings and 2.5-3.0 ft (0.76-0.90 m) for 2 in pall rings
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J. Design pressure drops
should be as follows:
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Service
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Pressure drop (in
water/ft packing)
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Absorbers and
Regenerators
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Non-Foaming Systems
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0.25 - 0.40
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Moderate Foaming Systems
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0.15 - 0.25
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Fume Scrubbers
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Water Absorbent
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0.40 - 0.60
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Chemical Absorbent
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0.25 - 0.40
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Atmospheric or Pressure
Distillation
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0.40 - 0.80
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Vacuum Distillation
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0.15 - 0.40
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Maximum for Any System
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1.0
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Reactors
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A. The rate
of reaction must be established in the laboratory and the residence time
or space velocity
will eventually have to be determined in a pilot plant. |
B. Catalyst
particle sizes: 0.10 mm for fluidized beds, 1 mm in slurry beds, and 2-5
mm in fixed beds. |
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C. For homogeneous
stirred tank reactions, the agitor power input should be about
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0.5-1.5 hp/1000 gal
(0.1-0.3 kW/m3), however, if heat is to be transferred,
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the agitation should be
about three times these amounts.
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D. Ideal CSTR behavior is
usually reached when the mean residence time is 5-10 times
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the length needed to
achieve homogeneity. Homogeneity is typically reached with
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500-2000 revolutions of a
properly designed stirrer.
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E. Relatively slow
reactions between liquids or slurries are usually conducted most
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economically in a battery
of 3-5 CSTR's in series.
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F. Tubular flow reactors
are typically used for high productions rates and when the
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residence times are
short. Tubular reactors are also a good choice when significant
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heat transfer to or from
the reactor is necessary.
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G. For conversion under
95% of equilibrium, the reaction performance of a 5 stages
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CSTR approaches that of a
plug flow reactor.
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H. Typically the chemical
reaction rate will double for a 18 �F (10 �C) increase in
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temperature.
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I. The reaction rate in a
heterogeneous reaction is often controlled more by the rate of
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heat or mass transfer
than by chemical kinetics.
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J. Sometimes, catalysts
usefulness is in improving selectivity rather than increasing
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the rate of the reaction.
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Refrigeration and Utilities
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A. A ton of refrigeration
equals the removal of 12,000 Btu/h (12,700 kJ/h) of heat
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B. For various
refrigeration temperatures, the following are common refrigerants:
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Temp (�F)
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Temp (�C)
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Refrigerant
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0 to 50
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-18 to -10
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Chilled brine or glycol
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-50 to -40
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-45 to -10
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Ammonia, freon, butane
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-150 to -50
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-100 to -45
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Ethane, propane
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C. Cooling tower water is
received from the tower between 80-90 �F (27-32 �C)
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and should be returned
between 115-125 �F (45-52 �C) depending on the size
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of the tower. Seawater
should be return no higher than 110 �F (43 �C)
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D. Heat transfer fluids
used: petroleum oils below 600 �F (315 �C), Dowtherms
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or other synthetics below
750 �F (400 �C), molten salts below 1100 �F (600 �C)
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E. Common
compressed air pressures are: 45, 150, 300, and 450 psig |
F.
Instrument air is generally delivered around 45 psig with a dewpoint 30
�F below the coldest expected ambient temperature. |
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