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Column is to contain 200 trays. Therefore, bottoms pressure = 280 + 2 + 20 = 302 psia. Reflux
ratio is approximately 15.9. Feed-stage location is approximately tray 140 from the top tray.
Assumptions: SRK equation of state for K-values and enthalpies.
Find: Reflux ratio for the optimal feed tray location.
Analysis: The calculations were made with the SCDS Column model (Simultaneous-correction
method) because of the small number of components and the large number of stages. It was
understood that the Chemcad SCDS model would adjust the reflux rate and distillate rate to
achieve the specified distillate and bottoms purities. Thus, the initial specifications for the
SCDS Column model of the Chemcad program were:
Top pressure = 280 psia
Condenser pressure drop = 2 psia. Column pressure drop = 20 psia.
Number of stages = 202 (includes partial condenser and partial reboiler)
Feed stage =140 from the top as a first estimate of the optimal feed stage
At the top, distillate purity = 99 mol% propylene
At the bottom, bottoms purity = 95 mol% propane
Estimated distillate rate = 350 lbmol/h
Estimated reflux rate = 5582 lbmol/h
Estimated temperature of stage 1 = 120oF
Estimated temperature of bottom stage = 140oF
Estimated temperature of stage 2 = 122oF
Estimated temperature of next to bottom stage = 138oF
When flashed, the feed was computed to be 69.11 mol% vaporized.
Convergence of the calculations was achieved rapidly in 6 iterations, with a converged reflux
ratio of 14.0047 compared to the initial estimate of 5582/350 = 15.95.
The effect of feed-stage location on the reflux ratio was then studied with the results shown
below. Analysis: (continued) Exercise 10.33 (continued)
Feed stage from top
110 Reflux ratio
13.646 As shown above, the optimal feed stage is in the range of stages 117 to 119. For the feed stage at
117 (116 in the column), the product distribution is as follows, accompanied by:
Condenser duty = 25,727,400 Btu/h
Reboiler duty = 25,434,800 Btu/h
The vapor and liquid compositions in the region of the feed stage, shown below, prove that a
pinch condition does not exist, but the mole fractions are not changing rapidly. Stage from top
119 Mole fraction
C3 in liquid
0.42073 Exercise 10.34
Subject: Use of a stabilizer to remove hydrogen and methane from an aromatic mixture.
Given: A feed mixture of composition given below at 240oF and 275 psia. Column to contain
about 10 equilibrium stages, with a partial condenser and a partial reboiler. Condenser outlet
temperature = 100oF. Bottoms to contain no more than 0.05 mol% methane plus ethane. Tray
efficiency = 70%. Column top pressure = 128 psia. Column bottom pressure = 132 psia.
Suggested estimates of 2 for the reflux ratio and 49.4 lbmol/h
Assumptions: SRK for K-values and enthalpies
Find: A suitable design with number of...
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