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Unformatted text preview: he desired value of 0.0025. This is
a huge difference and illustrates the importance of using reliable methods for estimating K-values
in the supercritical region. In this case, the PR EOS with the Wang-Sandler mixing rules does
not appear to be applicable. Exercise 11.28
Subject: Supercritical extraction of 100 mol/min of 10 mol% acetone in water by carbon
dioxide in a staged contactor operating in the supercritical region to recover 98% of the acetone
Given: Design of a supercritical extractor using the Group-Contribution EOS of SkjoldJorgensen, which is in good agreement with the experimental VLE data for acetone-CO2 at 313
K, as shown in the cited article by Cygnarowicz and Seider.
Find: Design of a similar extractor using the Peng-Robinson EOS with the Wang-Sandler
Analysis: In the Cygnarowicz and Seider (C-S) article, the experimental phase equilibria data
for the acetone-CO2 system at 313 K is shown in their Fig. 1. The Aspen Plus program was used
to prepare a similar plot based on the PR EOS with the Wang-Sandler mixing rules (PRWS).
The result is shown on the next page. Comparison of that plot with Fig. 1 in the C-S article
shows that the liquid composition is not predicted well. For example, when the pressure is 70
atm, Fig. 1 in the C-S article gives a carbon dioxide mole fraction of 0.84 in the liquid phase,
while the PRWS prediction in the figure on the next page gives a mole fraction of 0.94.
Therefore, the design given in Fig. 3 of the C-S article may not give the desired results for
PRWS. Consequently, a new design was sought based on the following specifications in the C-S
Extractor pressure > 70 atm
Extractor temperature > 310 K
CO2 feed rate > 200 mol/min
Acetone recovery of at least 98%
Water recovery of at least 6%.
The Aspen Plus program was applied with the RADFRAC model, using the non-ideal algorithm
with severe damping because of the difficulty in converging the calculations. However, initially,
a single-stage isothermal flash was used to establish preliminary conditions of extractor pressure
and temperature, and CO2 feed rate that would insure the presence of both vapor and liquid
phases. One set of conditions that accomplished this was 350 K, 70 atm, and 300 mol/min of
CO2. However, when these feed conditions were used with a 10-stage column, an acetone
recovery of only 68.8% was achieved. Consequently, other combinations of feed conditions were
tried, with some of the results as follows, where a solvent feed rate of 300 mol/min of CO2 was
used in all cases:
Feed % Extraction % Extraction
atm temperatures, K
The last result is the best, because it satisfies the specifications, except that not quite 98% of the
acetone is extracted. The material balance for this case is given on the next page. Exercise 11.28 (continued)
Feed Solvent Raffinate
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