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

006 and kf 0977 with xw 0497 and xf 0512 c to

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Unformatted text preview: oint, xW = zW = 0.5 and xF = zF = 0.5. Then, the only unknown in Eq. (9) is P. Equation (9) is linear in P and, thus, can be solved directly to give a bubble-point pressure of 719 torr. The K-values at the bubble point are 1.015 for W and 0.984. The composition of the vapor bubble is obtained from Eq. (2), which at the bubble point reduces to yi = xiKi = ziKi . This gives yW = 0.508 and yF = 0.492. (b) At the dew point, Ψ = V/F =1, and Eq. (1), combined with (4), becomes: f{xW, xF, P} = zW P P − 1 + zF −1 = 0 s o γ W {xW , xF } PW {107 C} γ F {xW , xF } PFs {107 o C} where because yW = zW = 0.5 and yF = zF = 0.5, xW , and xF =(1 - xW) are left as unknowns. The liquid phase mole fractions are from Eq. (3), xi = zi /Ki . Solving these equations by trial and error with a spread sheet, starting from P = 760 torr, xW = 0.5 and xF = 0.5, quickly leads to a dew-point pressure of 725.2 torr. The K-values at the dew point are computed to be KW = 1.006 and KF = 0.977, with xW = 0.497 and xF = 0.512. (c) To determine the existence of an azeotrope, where yi = xi , a series of bubble-point calculations can be made, using the procedure in part (a), starting from say, xW = 0.05 in increments of 0.05. If, in the range of xW from 0.05 to 0.95, the K-value of W switches from more than 1 to less than 1, then an azeotrope exists in this range. The calculations can then be refined. The result from a spreadsheet is a maximum-boiling azeotrope at 718.2 torr with a composition of 44.5 mol% W and 55.5 mol% F. Exercise 4.37 Subject: Bubble point, dew point, and equilibrium flash of a ternary mixture. Given: Mixture of 45 mol% n-hexane (1), 25 mol% n-heptane, and 30 mol% n-octane. Assumptions: Applicability of S-R-K method for estimating K-values. Find: (a) Bubble-point temperature at pressures of 5, 1, and 0.5 atm. Dew-point temperature at pressures of 5, 1, and 0.5 atm. (b) Temperature and phase compositions for flash of V/F = 0.5 at 5, 1, and 0.5 atm. (c) n-octane in the vapor if 90% of the n-hexane is vaporized at 1 atm. Analysis: The following results are obtained with the CHEMCAD simulator. (a) Bubble-point and dew-point temperatures: Pressure, atm Bubble-point T, oF Dew-point T, oF 0.5 147 178 1 187 216 5 307 330 (b) 50 mol% vaporization of feed: Pressure, atm Temperature, oF Vapor mole fractions: n-Hexane n-Heptane n-Octane Liquid mole fractions: n-Hexane n-Heptane n-Octane Moles V/mole F Moles L/mole F 0.5 162 1 201 5 318 0.615 0.231 0.154 0.599 0.233 0.168 0.554 0.239 0.207 0.285 0.269 0.446 0.5 0.5 0.301 0.267 0.432 0.5 0.5 0.346 0.261 0.393 0.5 0.5 (c) By iteration on the isothermal flash calculation, for 90 mol% of n-hexane to vapor, need a temperature of 210.2oF at 1atm. This gives 80.1 mol% vaporization with 65% vaporization of n-octane. Exercise 4.38 Subject: Vaporization of column bottoms in a partial reboiler. Given: 150 kmol/h of bubble-point liquid, L1, at 758 kPa, with a molar composition of 10% propane, 40% n-butane, and 50% n-pentane leaving bottom sta...
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