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Unformatted text preview: as assumed that the same number of stages and feed stage location could be used as in
Exercise 10.25. Also, the same reflux rate and distillate rate as specified for Exercise 10.25 were
given for initial estimates. It was understood that the Chemcad TOWER model would adjust the
reflux rate and distillate rate in an attempt to achieve the desired LK and HK recoveries. Thus,
the initial specifications for the TOWER model of the Chemcad program were:
Partial condenser
Top pressure = 120 psia
No condenser or stage pressure drop. Therefore, bottom pressure also = 120 psia
Number of stages = 11 (includes partial condenser and partial reboiler)
Feed stage = 6 from the top
At the top, component recovery of nbutane (component 3) = 0.98
At the bottom, component recovery of isopentane (component 4) = 0.98
Estimated distillate rate = 48.9 lbmol/h (actual rate)
Estimated reflux rate = 126.1 lbmol/h (actual rate)
Estimated temperature of stage 1 = 120oF
Estimated temperature of bottom stage = 250oF
Estimated temperature of stage 2 = 140oF
The feed is given as a bubblepoint liquid at 120 psia. Using the GraysonStreed correlation, the
temperature is computed to be 180.8oF. After 50 iterations, the calculations were not converged,
even though the reflux rate was increased from 126.1 lbmol/h to 8000.4 lbmol/h. The 98%
recoveries of the two key components could not be achieved. This indicated that the number of
stages was probably below the minimum number. Exercise 10.28 (continued) Analysis: (continued) To obtain a better estimate of the stage requirements, the Fenske equation can be applied in the
following manner, where the separation achieved in Exercise 10.25 for 11 stages is ratioed to the
separation desired here. That separation in Exercise 10.25 is 93.66% recovery of nC4 and 81.4%
recovery of iC5. Thus, applying Eq. (912) in this manner:
log
N= log 98
2
93.7
6.3 98
2
81.5
8.5 (11) = 17. 3 equilibrium stages With this result, it was decided to try 19 stages (including the partial condenser and
partial reboiler) with the feed to stage 10. Otherwise, the specifications for the TOWER model
were unchanged from above. This time convergence was achieved rapidly in just 2 iterations.
The reflux rate was increased by TOWER from 126.1 to 161.0 lbmol/h and the distillate rate was
reduced from 48.9 lbmol/h to 44.93 lbmol/h. Other results were as follows:
Condenser duty = 1,195,400 Btu/h
Reboiler duty = 1,586,600 Btu/h
Top temperature = 147.9oF compared to assumed value of 120oF
Bottom temperature = 232.3oF compared to assumed value of 250oF
Stage 2 temperature = 152.9oF compared to assumed value of 140oF Exercise 10.28 (continued)
Analysis: (continued) Analysis: (continued) Exercise 10.28 (continued) Exercise 10.29
Subject: Distillation of a light hydrocarbon mixture with a specified split. Given: Saturated liquid feed at 125 psia of 200 lbmol/h of 5 mol% iC4, 20 mol% nC4, 35 mol%
iC5, and 40 mol% nC5. Column is equipped with a total condenser and a partial reboiler,
operating at 125 psia. Reco...
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 Spring '11
 Levicky
 The Land

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