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

4 1 iic5 85 1 15 84 i ic5 85 15 85 1 i ic5 85 fi 2

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Unformatted text preview: 0 69.2 0.801 0.350 69.6 0.796 0.300 70.2 0.789 0.250 71.2 0.777 0.200 73.1 0.754 0.150 76.8 0.705 0.100 84.9 0.578 0.050 95.1 0.361 0.020 107.1 0.000 0.000 The McCabe-Thiele plot for these data are shown on the next page. The equilibrium stages for total reflux are stepped off between the equilibrium curve and the 45oline, which represents the operating line, from the acetone xD = 0.95 down to the acetone xB = 0.02. From the plot, there are 5 minimum stages, which is 35% higher than the 3.7 minimum stages computed from the Fenske equation. The Fenske equation is not reliable for highly nonideal binary systems. Exercise 9.6 (continued) Analysis: McCabe-Thiele method (continued) Exercise 9.7 Subject: Minimum equilibrium stages and distribution of nonkey components using the Fenske equation for distillation of a paraffin hydrocarbon mixture. Given: Column pressure of 700 kPa. Feed composition and split for two key components in Fig. 9.23. K-values from Figs. 2.8 and 2.9. Find: Minimum number of equilibrium stages. Distribution of nonkey components at total reflux. Analysis: To apply the Fenske equation, the geometric mean relative volatility between the distillate and bottoms is needed. From the following crude estimate of the split of all nonkey components, compute the bubble-point temperatures of the distillate and bottoms and use the Kvalues at those temperatures to get the relative volatilities. Use the bubble-point equation, Eq. (4-12). The following results are obtained for a distillate bubble point of 68oF (20oC) and a bottoms bubble point of 257oF (125oC): Component Propane Isobutane n-Butane Isopentane n-Pentane n-Hexane n-Heptane n-Octane Total Feed, kmol/h 2500 400 600 100 200 40 50 40 3930 Distillate, kmol/h 2500 399 594 15 5 0 0 0 3513 K-value, distillate 1.250 0.490 0.340 0.138 0.108 0.038 0.013 0.005 K xD 0.8895 0.0557 0.0575 0.0006 0.0002 1.0035 Bottoms, kmol/h 0 1 6 85 195 40 50 40 417 K-value, bottoms 5.70 3.25 2.65 1.45 1.23 0.65 0.33 0.19 K xB 0.0078 0.0382 0.2955 0.5751 0.0623 0.0222 0.0177 1.0188 For the two key components, nC4 and iC5, the relative volatilities at the top and bottom are, respectively (0.34/0.138) = 2.46 and (2.65/1.45) = 1.83. The geometric mean αnC4,iC5 = [(2.46)(1.83)]1/2 = 2.12. From the Fenske equation, (9-12), log N min = d nC4 biC5 d iC5 bnC4 log α nC4 ,iC5 log = 594 85 6 15 log 2.12 = 8.4 To compute the distribution of nonkey components at total reflux, use for the lighter than light key components, LLK, Eq. (9-15), and use Eq. (9-16) for the heavier than heavy key, HHK, with iC5 as the reference component, r, and the above value of Nmin. Analysis: (continued) Exercise 9.7 (continued) fi Thus, for the LLK, i, bi = 1+ fi For the HHK, i, di = 1+ d iC5 biC5 d iC 5 biC5 d iC5 biC5 = α N min i,iC5 α iN min ,iC 5 α iN min ,iC5 fi 15 8.4 1+ α i,iC5 85 (1) 15 8.4 α i ,iC5 85 = 15 8.5 1+ α i ,iC5 85 fi (2) Using the above K-values to compute geometric mean values of αi, iC5 , followed by use of Eqs. (1) and (2) with the material balance, fi = di + bi , the following re...
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