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Unformatted text preview: ed) Exercise 7.39 (continued) Summing the above results,
Number of equilibrium stages in the rectifying section = 28 + 23.6 = 51.6 stages
Number of equilibrium stages in the stripping section = 34.3 + 13.5 = 47.8 stages
Call it 99 stages in the column plus a partial reboiler with the feed to stage 52 from the top.
This compares to 102 stages in the column plus a partial reboiler with the feed to stages
50 from top as determined by the Fenske-Underwood-Gilliland method. Analysis: (continued) Exercise 7.39 (continued) Exercise 7.39 (continued)
Analysis: (continued) Exercise 7.40
Use of a McCabe-Thiele diagram to determine stage requirements for the
distillation of air into nitrogen and oxygen using a Linde double column.
Given: As shown in Fig. 7.45, the distillation consists of an upper column (UC) operating at 1
atm, on top of a lower column (LC) operating at 4 to 5 atm. Compressed air, containing 79
mol% N2 is condensed to supply heat in the reboiler of LC, and then is fed as liquid air to an
intermediate tray of LC. Bottoms liquid from LC, containing about 55 mol% N2, is the feed to an
intermediate tray in UC. The reboiler of UC is the condenser for LC. Condensate from the top
of LC is nearly pure N2, which is sent as reflux to the top of UC. The reboiler at the bottom of
UC provides almost pure O2 boilup for UC. Nearly pure liquid O2 is withdrawn from the UC
reboiler sump at the bottom of UC. The UC has no condenser. Nearly pure gaseous N2 leaves
the top of UC. This is consistent with the fact that N2 with a normal boiling point of -195.8oC
(77.4 K) is more volatile than O2 with a normal boiling point of -183oC (90.2 K).
Assumptions: Constant molar overflow. Constant relative volatility at each pressure.
Find: Construction lines on a McCabe-Thiele diagram that enable the determination of stage
Analysis: In LC, the N2 composition ranges from 55 mol% at the bottom to about 99 mol% at
the top, with a feed of 79 mol%. Based on calculations using K-values from the SRK equation of
state at 4.5 atm, the average relative volatility in LC is 2.5. In UC, the N2 composition ranges
from about 1 mol% at the bottom to 99 mol% at the top, with a feed of 55 mol%. Based on
calculations using K-values from the SRK equation of state at 1 atm, the average relative
volatility in UC is 4.0. Using Eq. (7-3), equilibrium curves for these constant α cases are shown
in the McCabe-Thiele diagram on the next page. However, so as not to clutter the diagram, the
curve for UC at 1 atm is based on N2, using,
yN 2 = α N 2 − O2 xN 2
1+ xN 2 α N 2 − O 2 − 1 = 4 xN 2
1+ 3xN 2 (1) while the curve for LC at 4.5 atm is based on O2, using,
yO 2 = α O 2 − N 2 xO 2
1+ xO 2 α O 2 − N 2 − 1 = (1 / 2.5) xO 2
1 + xO 2 1 / 2.5 − 1 = 0.4 xO 2
1- 0.6 xO 2 (2) Note that the equilibrium curve for 1 atm is above the 45o line, while that for 4.5 atm is below
the 45o line. Typical operating lines and q-lines are shown for determining the stage
requirements. Analysis: (continued) Exercise 7.40 (continued) Exercise 7.41
Comparison of measured with predicted plate eff...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
- Spring '11
- The Land