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Unformatted text preview: very of nC4 to the distillate = 95%. Recovery of iC5 to the bottoms =
95%. Number of equilibrium stages = 2 times that of the minimum stages by the Fenske
equation. SRK for K-values and enthalpies.
Find: A suitable design. Assume this consists of products, profiles, and stage compositions.
Analysis: The Shortcut model of Chemcad computes a minimum number of stages = 10.7 and a
minimum reflux ratio of 3.36. Therefore, select 2(10.7) = 21.4 or 22 stages (including the partial
reboiler). Estimate the reflux ratio at 1.3(3.36) = 4.4.
For the rigorous TOWER model of chemcad, use the following specifications:
Top pressure = 125 psia
No condenser or stage pressure drop. Therefore, bottom pressure also = 125 psia
Number of stages = 23 (includes total condenser and partial reboiler)
Feed stage = 12 from the top
At the top, component recovery of n-butane (component 2) = 0.95
At the bottom, component recovery of isopentane (component 3) = 0.95
Estimated distillate rate = 50 lbmol/h (actual rate)
Estimated reflux rate = 50(3.36) = 168 lbmol/h (actual rate)
Estimated temperature of stage 1 = 120oF
Estimated temperature of bottom stage = 250oF
Estimated temperature of stage 2 = 140oF
The feed is given as a bubble-point liquid at 125 psia. Using the SRK correlation, the
temperature is computed to be 210oF. After 4 iterations, the calculations are converged, while
increasing the reflux rate to 267.9 lbmol/h, and the distillate rate to 52.11 lbmol/h, at which the
95% recoveries of the two key components are achieved. The computed results are as follows on
this page and the next pages.
Condenser duty = 2,517,800 Btu/h
Reboiler duty = 2,582,500 Btu/h
Top temperature = 160.8oF compared to assumed value of 120oF
Bottom temperature = 233.2oF compared to assumed value of 250oF
Stage 2 temperature = 165.8oF compared to assumed value of 140oF Exercise 10.29 (continued)
Analysis: (continued) Analysis: (continued) Exercise 10.29 (continued) Exercise 10.30
Subject: Design of a depropanizer distillation column. Given: Feed at column average pressure of 315 psia and 66 mol% vaporized. Composition of
feed in lbmol/h is as follows: 26 C1, 9 C2, 25 C3, 17 nC4, 11 nC5, 12 nC6. Separation is between
C3 (0.4 lbmol/h in bottoms) and nC4 (0.3 lbmol/h in distillate). Cooling water available at 65oF
and steam available at 315 psia. Total column pressure drop = 2 psi.
Assumptions: Partial reboiler. Grayson-Streed correlation for K-values and Redlich-Kwong
equation for enthalpies.
Find: (a) Type condenser.
(b) Feed temperature. K-values and relative volatilities at feed temperature and pressure.
(c) Reflux ratio for 1.3 times minimum reflux ratio. Number of equilibrium stages in
rectifying and stripping sections.
(d) Separation. Separation for reflux ratio = 1.5 times minimum, 15 theoretical stagess,
feed stage at stage 9.
(e) Temperature and compositions at each stage for Part (c). Effect on these results if
reflux and stages of Part (d) are used.
Analysis: Use the Flash, Shortcut, and TOWER models of Chemcad to make the...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
- Spring '11
- The Land