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194 Applied Process Design for Chemical and Petrochemical Plants
need for circulating viscous materials, points toward • Installation of a valve in the liquid circulation line as
forced-circulation reboilers for vacuum fractionators. shown on the illustration can aid in overcoming insta-
• For steam distillation columns, it is desirable to sparge bility and variations in reboiler duty.
the steam uniformly to all reboiler tubes. Because this • In the physical arrangement, make certain that the
provides full tube length for two-phase flow, thermal cir- pressure balance level, plus an allowance for froth,
culation is permitted. establishes a height that is below the bottom tray of the
• In reboiler design, film boiling should be avoided. column to avoid flooding the column. In addition, the
However, such rules-of-thumb as 10—12,000 Btu/(hr) estimated froth height on top of the liquid should still
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(ft ) maximum heat flux are frequently quite be below the level of the vapor return from the reboiler.
conservative. •“Mist” type flow usually does not occur in an economic
design, even though the recirculation rates may be
45
For heating, Fair suggests the Dittus-Boelter equation: high.
• In vacuum service, the large fraction of the tube length
13,600D t G t 2 0.8 1c l l 2 0.4
k l used for sensible heating leaves little density difference
h L 0.023 (10-182)
1 l 2 1k l 2
D i for thermal circulation. This fact, plus the frequent
need for circulating viscous materials, points towards
See symbols previously listed. forced-circulation reboilers for vacuum service.
The two-phase flow heat transfer coefficient is deter- • For steam distillation columns, it is desirable to sparge
mined from: 45 the steam uniformly into all reboiler tubes. This then
provides full length for two-phase flow, and thermal cir-
h tp / h L 3.5 (1/X tt ) 0.5 (10-183) culation is permitted.
• Film boiling should be avoided; however, nucleate
For the short-cut calculations: boiling often can be found at heat flux values greater
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_ than the rule-of-thumb values of 10—12,000 Btu/hr-ft .
( E )/2, at average condition (10-184) These are often conservative values. See Figure 10-119
from Fair. 46
E is evaluated at exit conditions
is evaluated at 40% exit vaporization 46
Examples from Fair (by permission) include the following:
Boiling coefficients: Determine from the McNelly, Gilmour,
Example 10-19. C 3 Splitter Reboiler
Kern, or Yilmaz equations previously given, or Fair’s sugges-
tion of the Bliss or Levy equations, which were not given due
A thermosiphon reboiler is to be designed for a fraction-
to constants not being available.
ator that separates propane as the bottoms product. The
Process the side boiling heat transfer coefficient:
conditions below the bottom tray are 401 psia and 164°F. A
total of 17,600 lb/hr vapor is to be produced.
L BC h L L CD h v
h p , see Figure 10-110 for lengths (10-185) Physical data for propane:
L t
L 24.80 lb/ft 3
(10-186)
h v h b h tp g 4.40 lb/ft ft 3
L 0.157 lb/(ft) (hr) or 0.065 cp
Evaluate h b at average inside T and h tp at 40% of the exit g 0.036 lb/(ft.) (hr) or 0.015 cp
vaporization. ( t/ p) s 0.24°F/psi
c L 0.85 Btu/(lb) (°F)
The 40% value is based upon integration of the h tp /h L
equation previously given. k L 0.068 Btu/(hr) (ft)(°F)
96.9 Btu/lb
-4
Subscripts: 1.24 (10 ) lb/ft
tp two phase 0.49
v vaporization
b boiling Preliminary Design
p process side (boiling) coefficient
3
t tube, tube side Boiling is to be inside / 4 -in. 16 BWG steel tubes, 8 ft long.
average correction Condensing steam at 25 psig is available for heating. An over-
_
2
bar over symbol average value all coefficient U 300 Btu/(hr) (ft )(°F) is expected. For the
given duty and a total driving force of 20°F, the inside surface
Fair emphasizes some helpful points: required is 285 ft This is equivalent to 96 tubes.
2