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

Analysis because the pressure is close to 1 atm

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Unformatted text preview: int, the pressure = 165 psia = 23.9 psia. Assume a saturated liquid feed. The bubble-point temperature is estimated to be 215o F. Take an average temperature = 205oF, at which the vapor pressures for benzene and toluene, respectively, are 23 psia and 9 psia respectively. From Eq. (2-44), the corresponding K-values at the average section pressure of 21.5 psia are 1.07 and 0.42. The relative volatility, from Eq. (221) = 1.07/0.42 = 2.55. From the Fenske equation (9-12), applied between the distillate and feed compositions: dB fT 257 80 log log dT fB 0.1 260 N min = = = 7.1 stages log α B,T log 2.55 (b) For the section between the feed and the vapor sidestream, again take the LK as benzene and the HK as toluene. The pressure at the vapor sidestream = 180 kPa = 26.1 psia. A dew-point on the vapor sidestream gives a temperature of 265oF. Assume an average temperature for the section of 240oF, at which the vapor pressures for benzene and toluene, respectively, are 37 psia and 16 psia respectively. From Eq. (2-44), the corresponding K-values at the average section pressure of 25 psia are 1.48 and 0.64. The relative volatility, from Eq. (2-21) = 1.48/0.64 = 2.31. Exercise 9.5 (continued) Analysis: (b) (continued) From the Fenske equation (9-12), applied between the feed and vapor sidestream compositions: log N min = fB fT vsT vsB log α B,T log = 260 79.4 80 3 log 2.31 = 5.3 stages (c) In the section between the vapor sidestream and bottoms, take toluene as the LK and biphenyl as the HK. At the bottom of the column, the pressure = 200 kPa = 29 psia. Biphenyl boils at 255oC = 491oF at 14.7 psia, which is much higher than toluene. From Perry's Handbook, the following vapor pressure data are obtained for biphenyl: T, oF Vapor pressure, psia 243 0.19 274 0.39 307 0.77 165.2 1.16 329 1.93 400 3.87 445 7.74 A bubble point on the bottoms pressure gives 465oF. At this temperature, the relative volatility, αT,BP = vapor pressure of T/vapor pressure of BP = 220/10.6 = 20.8. At the temperature of the vapor sidestream, 265oF, αT,BP = vapor pressure of T/vapor pressure of BP = 27.9/0.37 = 75.4. Use a geometric mean for this wide range. Thus, αT,BP = [20.8(75.4)]1/2 = 40. From the Fenske equation (9-12), applied between the vapor sidestream and bottoms compositions: vsT bBP 79.4 4.8 log log vsBP bT 0.2 0.5 N min = = = 2.3 stages log α B,T log 40 Exercise 9.6 Subject: Comparison of minimum number of stages by Fenske equation and McCabeThiele method for a non-ideal system of acetone (A) and water (W). Given: A feed of 25 mol% A and 75 mol% W. Distillation at 130 kPa (975 torr) to obtain a distillate of 95 mol% A and a bottoms of 2 mol% A. Infinite-dilution liquid-phase activity coefficients of 8.12 for acetone and 4.13 for water. Find: Minimum stages by the Fenske equation and by the McCabe-Thiele method. Analysis: Because the pressure is close to 1 atm, estimate vapor-liquid equilibria from the modified Raoult's law, Eq. (4) in Table 2.3. Obtain vapor pressures fr...
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