Unformatted text preview: d total condenser. Vaporliquid
equilibrium data.
Assumptions: Constant molar overflow.
Find: For a distillate of 95 mol% CS2 and a bottoms of 5 mol% CS2, determine:
(a) Minimum reflux ratio, minimum boilup ratio, and minimum number of stages.
(b) Number of trays.
Analysis: (a) In the McCabeThiele plot on the next page, rectifying section and stripping
section operating lines are shown for determining minimum reflux and boilup ratios. Note that
the qline has a slope given in Eq. (726) as q/(q  1) = 0.5/(0.5  1) = 1 and intersects the point
{0.4, 0.4}. The rectifying section operating line intersects the point {0.95, 0.95} and the point
where the equilibrium curve and the qline intersect. The slope of that line is measured to be L/V
= 0.642. From Eq. (79), L/V = R/(R + 1). Rearranging,
Rmin = (L/V)/[1  (L/V)] = 0.642/(1  0.642) = 1.79
The stripping section operating line intersects the point {0.05, 0.05} and the point where the
equilibrium curve and the qline intersect. The slope of that line is measured to be L / V =
2.043. From Eq. (714), L / V = (VB + 1)/VB. Rearranging,
V
1
1
=
=
= 0.959
(VB )min =
B min ( L / V ) − 1 2.043 − 1
The McCabeThiele plot for minimum stages at total reflux is also shown on the next page. The
operating lines are coincident with the 45o line. It is seen that 6 equilibrium stages are needed.
(b) For a reflux ratio, R = L/D, of 4, the slope of the rectifying section operating line
from Eq. (79) is L/V = R/(R + 1) = 4/5 = 0.8. This line and the stripping section operating line
are shown on the third McCabeThiele diagram below. Except for the reboiler stage, the stages
are stepped off from an efficiency line, which for a Murphree vapor efficiency of 0.8 is
positioned 80% of the vertical distance from the operating line to the equilibrium curve, as
governed by Eq. (741), EMV = 0.8 = (yn  yn+1)/(yn*  yn+1), where yn+1 is the location on the
operating line, yn is the location on the efficiency line, and yn* is the location on the equilibrium
line. However, the reboiler is assumed to have a 100% efficiency. As seen in the plot, just over
9 trays are required plus the partial reboiler. Call it 10 trays plus the reboiler. The feed plate is 7
from the top. Analysis: (a) (continued) Exercise 7.27 (continued) Analysis: (a) (continued) Exercise 7.27 (continued) Analysis: (b) (continued) Exercise 7.27 (continued) Exercise 7.28
Subject: Preliminary design calculations for the distillation of a benzenetoluene mixture. Given: Bubblepoint feed of 50 mol% benzene and 50 mol% toluene. Equipment to include a
partial reboiler, total condenser, and a bubblecap tray column with an overall plate efficiency of
65%. Column to operate at 1 atm to produce a distillate of 95 mol% benzene and a bottoms of
95 mol% toluene. Vaporliquid equilibrium data from Exercise 7.13. Enthalpy data for reboiler.
Assumptions: Constant molar overflow and saturated liquid reflux.
Find: (a)
(b)
(c)
(d)
(e)
(f) Minimum reflux ratio (infinite s...
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 Spring '11
 Levicky
 The Land

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