Unformatted text preview: = 1,000 lbmol/h
Estimated reflux rate = 1,000 lbmol/h
Estimated temperature of stage 1 = 90oF
Estimated temperature of bottom stage = 90oF
Estimated temperature of stage 2 = 90oF
Convergence was achieved only with great difficulty after 29 iterations. The problem was also
solved with the SCDS Column model (simultaneous correction method) in only 8 iterations. The
percent absorption of each component is as follows:
Component
Methane
Ethane
Propane
nButane
nPentane % Absorbed
4.8
21.0
60.6
97.3
99.95 The other results are presented on the next two pages. Exercise 10.37 (continued)
Analysis: (continued) Exercise 10.37 (continued)
Analysis: (continued) Exercise 10.38
Subject: Effect of intercooler location on the absorption of a light hydrocarbon mixture.
Given: Feed gas and absorbent oil of Figure 10.17 of Example 10.4. Incooler with a duty of
500,000 Btu/h.
Assumptions: SRK for Kvalues and enthalpies. Absorbent oil is nC12.
Find: Effectiveness of the intercooler and its preferred location. Possible change in duty if
intercooler stage temperature is at least 100oF.
Analysis: The calculations of Example 10.4 were first repeated using the TOWER model of
Chemcad. The following results differ from those of Example 10.4 because of the different
methods used for computing Kvalues and enthalpies:
Component
C1
C2
C3
nC 4
nC 5
nC12
Total:
Temp., oF lbmol/h:
Feed gas
160.00
370.00
240.00
25.00
5.00
0.00
800.00
105 Absorbent
0.00
0.00
0.00
0.05
0.78
164.17
165.00
90 Gas out
146.25
268.83
96.41
1.11
0.21
0.15
512.96
144.9 Liquid out
13.75
101.17
143.59
23,94
5.57
164.02
452.04
144.4 Compared to results of Example 10.4, more is absorbed, but the temperature rise is less.
For the effect of the intercooler, the calculations were also made with the Tower model (Insideout method) of Chemcad. The specifications for an intercooler of 500,000 Btu/h located at
Stage 2 from the top were as follows, where it should be noted that the intercooler is treated as a
feed between the absorbent and feed gas.
No condenser
Top pressure = 400 psia
Column pressure drop = 0 psi.
Number of stages = 6
Absorbent to stage 1
Intercooler with 0.500 MMBtu/h enthalpy (as a feed) at stage 2
Feed gas to stage 6
At the top, no condenser
At the bottom, no reboiler
Estimated distillate rate = 500 lbmol/h
Estimated reflux rate = 200 lbmol/h Exercise 10.38 (continued)
Analysis: (continued)
Estimated temperature of stage 1 = 120oF
Estimated temperature of bottom stage = 130oF
Estimated temperature of stage 2 = 120oF
Convergence was achieved in 6 iterations. The calculations were repeated for the intercooler
located at stages 3, 4, and 5. The following results show that the outlet gas temperature was
reduced and the amount of absorption was considerably increased, but the effect of stage location
is not significant, although a location at stage 4 or 5 appeared to be very slightly best. Case
Base  no intercooler
(a)  intercooler stage 2
(b)  intercooler stage 3
(c)  intercooler stage 4
(d) ...
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 Spring '11
 Levicky
 The Land

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