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Unformatted text preview: trays, feed location, and reflux ratio.
Analysis: A valve is used to drop the feed pressure to column pressure. The calculations were
made with the Tower model (Inside-out method) of Chemcad. It was understood that the
Chemcad TOWER model would adjust the reflux rate and distillate rate to achieve the specified
distillate temperature and bottoms impurities. Thus, the initial specifications for the TOWER
model of the Chemcad program were as follows, where it was assumed that the main impurity in
the bottoms product would be ethane.
Top pressure = 128 psia
Condenser pressure drop = 0 psi. Column pressure drop = 4 psi.
Number of stages = 12 (includes partial condenser and partial reboiler)
Feed stage =3 from the top as a first estimate.
At the top, distillate temperature = 100oF.
At the bottom, bottoms purity = 0.0005 mole fraction of ethane
Estimated distillate rate = 50 lbmol/h
Estimated reflux rate = 100 lbmol/h
Estimated temperature of stage 1 = 100oF (actual specification)
Estimated temperature of bottom stage = 400oF
Estimated temperature of stage 2 = 300oF
A flash of the feed at feed conditions upstream of the feed valve showed it to be liquid.
Convergence of the calculations was achieved rapidly in 6 iterations, with a converged reflux
ratio, however of only 0.186, with a distillate rate of 48.715 lbmol/h. Thus, the reflux rate to the
top stage was only 9.06 lbmol/h compared to 1748.4 lbmol/h of feed. This is considered to be
unsatisfactory. Therefore, a second run was made with the feed entering the top stage (Stage 2)
and reducing the number of stages to 7 (including the partial condenser and partial reboiler). A
converged result shown on the next two pages was obtained in 5 iterations. This design is
considered more satisfactory and achieves the specifications. The impurity in the bottoms is
mainly ethane, as assumed. Other results are: reflux ratio = 0.0.211, condenser duty = 2,515,000
Btu/h, and reboiler duty = 12,976,000 Btu/h. The column will contain 5/0.7 = 8 trays. Analysis: (continued) Exercise 10.34 (continued) Exercise 10.34 (continued)
Analysis: (continued) Exercise 10.35
Subject: Simulation of a four-stage, nearly isothermal distillation process by flash calculations.
Given: Problem specifications in Figure 10.36, consisting of the feed conditions and
temperature and pressure for each of four isothermal flashes.
Assumptions: Grayson-Streed correlation for K-values and Redlich-Kwong equation for
Find: Component flow rates for all equilibrium vapor and liquid streams.
Analysis: The Chemcad simulator was used with its FLASH model in the isothermal flash
mode. Referring to Figure 10.36, the recycle process was simulated with L1, V3, and V4 as the
cut (tear) streams, with the sequence of calculations being Stages 2, 1, 3, and 4. No initial
estimates were given for the cut streams. Convergence was achieved in 19 iterations using direct
substitution. The resulting stream compositions are given below: Analysis (continued) Exercise 10.35 (continued) Exercise 10.36
Subject: Effect of...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
- Spring '11
- The Land