Separation Process Principles- 2n - Seader & Henley - Solutions Manual

Correspondingly the occluded component vapor flow

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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 mixing rules. 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 article: 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: Pressure, Feed % Extraction % Extraction atm temperatures, K of acetone of water 70 350 68.8 4.53 70 360 79.1 5.64 70 370 89.7 6.97 80 370 97.9 7.37 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) Analysis: (continued) Mol/min: Component Feed Solvent Raffinate Carbon dioxide 0.0 300.0 8.62 Acetone 10.0 0.0 0....
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