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Heat Transfer 169

Reboiler Heat Balance

Because the reboiler is usually used in conjunction with
distillation columns, the terminology and symbols used here
will relate to that application. Assume a column with an
overhead total condenser and a bottoms reboiler (see Fig-
ures 10-96D and 10-96E). Assuming all liquid feed, the heat
balance is 70

(10-142)
Q R (R 1 ) W D E D(V) RW D E D(l) W B E B(l) W f E f(l)
where
R reflux ratio, mol condensate returned to column/
mol product withdrawn
V RB vapor formed in reboiler, lb/hr
W flow rate, lb/hr
E D enthalpy of overhead product removed from
column., Btu/lb
Figure 10-103. Kern correlation for natural circulation boiling and
Q c heat load of overhead condenser (removed in
sensible film coefficients—outside and inside tubes. (Used by per-
condenser), Btu/hr mission: Kern, D.Q. Process Heat Transfer, 1 Ed., ©1950. McGraw-
st
Q R reboiler duty or heat added, Btu/hr Hill Book Company. All rights reserved.)
L latent heat of vaporization, Btu/hr
E B enthalpy of bottoms, Btu/lb
E F enthalpy of feed, Btu/lb
1,237.5 0.945 W B
W B 13,095 lb/hr total bottoms
Subscripts:
W D distillate total 25,000 13,095 11,905 lb/hr
B bottoms product
c condensing
Assume using enthalpy and latent heat tables/charts.
v vapor
l liquid
E B(l) 160 Btu/lb
R reboiler
E F(l) 100 Btu/lb
D overhead distillate product
E D(v) 302 Btu/lb
F feed
L v 135 Btu/lb
70
For a heat balance around the entire column with the
Then, substituting in equation 10-142,
feed being liquid or vapor; that is, Heat In Heat Out:
Q R (3.0 1.0)(11,905)(302) (3.0)(11,905)(95)
(10-143)
W F E F1l or V2 Q R Q C W B E B1l2 W D E D1l2
(13,095)(160) 25,000(100)
14,381,240 3,392,925 2,095,200 2,500,000
Example 10-17. Reboiler Heat Duty after Kern 70 10,583,515 Btu/hr
See Figure 10-103. Total vapor to be generated by reboiler:
Assume 25,000 lb/hr of a 50-50 mixture of light hydro-
carbons to be separated to a 99.5% (wt) light HC overhead 10,583,315/ 135 78,394 lb/hr
and bottoms of 5% (wt) heavier HC. The reflux ratio deter-
mined separately for the column is 3.0 mol reflux/mol of Kettle Horizontal Reboilers
overhead distillate.
Kettle horizontal reboilers consist of either a U-bundle or
Overall material balance:
a shell and tube bundle inserted into an enlarged shell. The
enlarged shell provides disengaging space for the vapor out-
25,000 W D W B
side and above the liquid, which is usually held by level con-
HC 1 (more volatile): 25,000(0.50) 0.995W D .05 W B
trol at the top level of the tube bundle or possibly a few in.
Then simultaneously: below the top of the tubes. The heating medium is inside
the tubes.
(25,000)(0.05) 0.995(25,000 W B ) 0.05(W B ) Internal reboilers are similar in concept and designed
12,500 24,875 0.995W B 0.05(W B ) accordingly to the kettle units, but this style does not have a