Unformatted text preview: intercooler stage 5 Liquid out,
500.74 Temp., gas out, oF
138.6 Because the temperature was still significantly higher than the lower limit of 100oF, a higher
intercooler heat duty of -2,000,000 Btu/h was tried at stage 5. However, the temperature of that
stage dropped to 84oF. For a duty of -1,500,000 Btu/h, the temperature of stage 5 was an
acceptable value of 102oF. The liquid out increased to 599.68 lbmol/h and the temperature of the
gas out decreased to 127.5oF. The corresponding component absorption rates compared to those
of the base case (no intercooler) were as follows: lbmol/h:
Component No intercooler -1,500,000 Btu/h, stage 5
Thus, the effect of the intercooler is very significant. The corresponding stage temperatures
compared to those of the base case are also of interest:
-1,500,000 Btu/h - stage 5 F: 1
114.9 Exercise 10.39
Subject: Design of a an absorber with two absorbent oil feeds.
Given: Feed gas at 90oF and 400 psia with the composition given below. Column has 8
equilibrium stages. Lean oil, assumed to be nC12, at 80oF and 400 psia at 250 lbmol/h enters the
top stage. Secondary oil at 80oF and 400 psia, of the composition given below, enters stage 7
from the top. 150,000 Btu/h (-0.15 MMBtu/h) is removed by intercooling from stage 7. The
feed gas enters stage 8.
Assumptions: SRK for K-values and enthalpies.
Find: Product rates and compositions, stage temperatures, and interstage vapor and liquid flow
rates and compositions.
Analysis: The calculations were made with the Tower model (Inside-out method) of Chemcad,
with the following specifications:
Top pressure = 400 psia
Column pressure drop = 0 psi.
Number of stages = 8
Lean oil (1st feed) to stage 1
Secondary oil (2nd feed) to stage 4
Intercooler (3rd feed) at stage 7
Feed gas (4th feed) to stage 8
At the top, no condenser
At the bottom, no reboiler
Estimated distillate rate = 450 lbmol/h
Estimated reflux rate = 350 lbmol/h
Estimated temperature of stage 1 = 110oF
Estimated temperature of bottom stage = 100oF
Estimated temperature of stage 2 = 100oF
Note that the intercooler is treated as the 3rd feed with a specification as a stream of -0.15 MM
Btu/h. Convergence was achieved in 4 iterations. The results are shown on the following two
pages. Note that the degree of absorption was good with following results:
nC 5 % in Feed gas
100.00 Exercise 10.39 (continued)
Analysis: (continued) Exercise 10.39 (continued)
Analysis: (continued) Exercise 10.40
Subject: Reboiled absorber for the separation of a hydrocarbon mixture.
Given: Absorbent oil of nC9 at 60oF and 230 psia and 40 lbmol/h. Feed at 120oF and 230 psia
with the composition given below. Column has 15 equilibrium stages plus a parti...
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