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at the bottoms, both mole fractions in the liquid phase. Accordingly, it was believed that the
FUG method, the ShortCut Column of Chemcad, could give good estimates. Using the above
material balance table, the specifications were 5.85/5.91 = 0.9898 for the recovery of water to the
distillate, and 42.91/72.91 = 0.5885 for the recovery to the distillate of ethanol. The feed to
Column 2 is the distillate from Column 1 after being pumped to a pressure of 350 kPa for entry
into Column 2 and heating to the bubble point at that pressure.
The results are:
Minimum number of equilibrium stages = 47.2
Minimum reflux ratio = 5.85
Operating reflux ratio = 1.3(5.85) = 7.6
Number of equilibrium stages from Gilliland correlation = 86.9
Feed stage = 9
Using some small modifications to these values, the input data to the Tower model of Chemcad
was as follows:
Top pressure = 300 kPa
Condenser ∆P = 5 kPa
Column ∆P = 70 kPa
No. of stages = 88 (includes total condenser and partial reboiler)
Feed stage = 10 from the top
Reflux ratio = 7.6
Bottoms molar flow rate = 30.06
Estimated distillate rate = 48.76 mol/s (specified value)
Estimated reflux rate = 15D = 7.6(48.76) = 370.6 mol/s
Estimated temperatures: Stage 1, 10835oC Stage 12, 116oC Stage 2, 110oC
With these input data, a converged solution was obtained, but an ethanol bottoms product of only
98.8 mol% purity was obtained compared to the desired 99.8 mol%. Also a pinch region of
about 10 stages was evident just below the feed. Therefore, a second run was made with the
Feed stage = 20 from the top (moved down 10 stages)
Distillate flow rate = 78.82 mol/s, in place of reflux ratio
Mole fraction of ethanol in bottoms = 0.998, in place of bottoms flow rate
Convergence was achieved in 7 iterations, with a reflux ratio = 7.34. The material balance was
in agreement with the above material balance table for the system. Although the feed location
could probably be improved, no pinch region was now evident, as shown in the liquid-phase
composition profile on the next page.
Column 2 was sized with Chemcad for valve trays, with a 24-inch tray spacing.
The resulting column inside diameter was determined to be 7.5 feet for a nominal 85% of
flooding. The condenser duty was computed to be 14.88 MW, with a reboiler duty of 14.86
MW. For the 86 theoretical plates, the total pressure drop was computed to be 74.6 kPa. For a
final design, the computations should be repeated taking into account tray pressure drop and tray
efficiency Exercise 11.15 (continued)
Analysis: Column 2 (continued) Exercise 11.16
Subject: Feasibility of the separation of benzene (B) from cyclohexane (C) using homogeneous
azeotropic distillation with methanol (M) as the entrainer.
Given: Fresh feed of 100 kmol/h of 25 mol% B and 75 mol% C. Reference to an article by
Ratliff and Strobel.
Assumptions: Feed is at the bubble point at 1 atm. Homogeneous azeotropic distillation at 1
atm using M.
Find: Whether M can make possible the separation of B from C.
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
- Spring '11
- The Land