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Unformatted text preview: eed stage. These two operating lines are shown in the McCabeThiele
diagram below, where both pass through the point y = x = 0.02. Stepping off stages from the
bottom, it is seen that 3 stages are needed below the feed stage. The first stage is the partial
reboiler. The interreboiler is located at the second equilibrium stage. A total of 8 equilibrium
stages is required, just slightly more than that when all of the heat input is to the partial reboiler
at the bottom of the column.
The vapor composition of the interreboiler stage is 0.345, which from the given Tyx
phase equilibrium data corresponds to a temperature of approximately 173oC. The interreboiler
could also be located at the third stage from the bottom. The would increase the number of
stages by about half of a stage and lower the temperature of the interreboiler stage to 162oC. Exercise 7.37 (continued)
Analysis: (continued)
Alternative unit with Interreboiler: Exercise 7.38
Subject:
Effect of the addition of an intercondenser and interreboiler to a distillation
column separating nbutane and npentane.
Given: Distillate and bottoms compositions of actual operation (before addition of
intercondenser and interreboiler) compared to design specification.
Assumptions: Constant molar overflow. Constant relative volatility. Column is large enough
in diameter to handle increased reflux and boilup. Raoult's law (ideal solutions and ideal gas
law).
Find: Whether the addition can improve the operation because of the increased reflux and
boilup produced by the intercondenser and interreboiler.
Analysis:
First, estimate the average relative volatility for nC4/nC5. Assume a distillate
temperature of 120oF so that cooling water can be used in the condenser. This corresponds to a
saturation pressure of about 70 psia. Using Fig. 2.8 with Eqs. (221) and (244), the relative
volatility of butane with respect to pentane is α = 1.1/0.38 = 2.9. Assuming a 5 psi pressure
drop, gives a bottoms pressure of 75 psia and a corresponding temperature of 200oF. Using Fig.
2.8 with Eqs. (221) and (244), α = 2.1/0.9 = 2.3. Take the average relative volatility as 2.6 and
draw a yx equilibrium curve using Eq. (73),
y= 2.6 x
αx
=
1 + x (α − 1) 1 + 16 x
. (1) The equilibrium curve, based on Eq. (1) is shown below in a McCabeThiele diagram. Included
on the diagram are arbitrary operating lines and a qline for an equimolar feed that is 50 mol%
vaporized. Using these lines, 15 equilibrium stages are stepped off between the compositions of
the actual operation, xD = 1  0.1349 = 0.8651 and xB = 0.0428. The slope, L/V, of the rectifying
section operating line is 0.52. Exercise 7.38 (continued) Analysis: Actual operation before addition (continued) Exercise 7.38 (continued)
Analysis: Addition (continued)
When an interreboiler is added between the reboiler and the feed stage and an
intercondenser is added between the feed stage and the overhead condenser, the column is made
up of 4 sections instead of 2. Each section has its own operating line as shown in the McCabeThiele diagram below. In order to maintain the same reflux ratio and boilup ratio, the
intercondenser is designed to condense a molar flow rate equal to that produced b...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
 Spring '11
 Levicky
 The Land

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