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

184440266 from fig 649 by a rough extrapolation moles

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Unformatted text preview: s: First, determine the molar flow rates of components in the exit gas. From the ideal gas law applied to the exiting gas at 1 atm and 68oF, ρ = P / RT = 1 / (0.7302)(68 + 460) = 0.00259 lbmol/ft3 Flow rate of exit gas = 1,000(0.00259) = 2.59 lbmol/h Gas contains 0.10(2.59) = 0.259 lbmol/h of NH3 and 0.90(2.59) = 2.331 lbmol/h of air. Next, determine the mole fractions in the inlet and exit liquid streams. Take as a basis, 100 lb of inlet liquid. Component H2O NH3 Total Inlet liquid: lb lbmol 80 4.44 20 1.18 100 5.62 mole fraction 0.79 0.21 1.00 Outlet liquid: lb lbmol 80.000 4.4400 0.808 0.0475 80.808 4.4875 mole fraction 0.9894 0.0106 1.0000 Determine the water flow rate, L', in and out by an ammonia balance: 0.21 0.0106 (1) L′ + 0 = L ′ + 0.259 0.79 0.9894 Solving Eq. (1), L' = 1.015 lbmol/h of H2O. NH3 in entering liquid = (0.21/0.79)1.015 = 0.270 lbmol/h NH3 in leaving liquid = 0.270 - 0.259 = 0.011 lbmol/h Because of the concentrated solution, take into account the bulk-flow effect. For a partial pressure driving force, can write the rate of mass transfer, V for volume, as, nNH 3 = 0.259 = Rearranging Eq. (1), V= KG aV * pNH 3 − pNH 3 ( yair ) avg 0.259( yair ) avg p * NH 3 − pNH 3 LM KG a LM (2) (3) Exercise 6.33 (continued) Analysis: (continued) First consider the bulk-flow effect. At the top of the column, in the bulk exiting air, yair = 0.90 To obtain the corresponding value of y in the vapor in equilibrium with the entering liquid at the top of the column, use Fig. 6.49. In entering bulk liquid, moles NH3/moleH2O =1.18/4.44=0.266. From Fig. 6.49, by a rough extrapolation, moles NH3/mole H2O in equilibrium gas = 0.266. Therefore, y*air = 1 - 0.266/1.266 = 1 - 0.210 = 0.79. The average yair = (0.90 + 0.79)/2 = 0.845. At the bottom of the column, in the bulk entering air, yair = 1.00 To obtain the corresponding value of y* in the vapor in equilibrium with the leaving liquid at the column bottom, use Fig. 6.49. In leaving bulk liquid, moles NH3/mol H2O = 0.0475/4.4 = 0.0108. From Fig. 6.49, moles NH3/mole H2O in equilibrium gas = 0.007 = mole fraction of NH3 Therefore, y*air = 1 - 0.007 = 0.993. The average yair = (1.000 + 0.993)/2 = 0.997 The (yair)avg = (0.845 + 0.997)/2 = 0.921 Now, determine the log mean partial pressure driving force for ammonia, using Dalton's law with the above mole fractions. At the top of the column, for ammonia, (p*- p)top = P (y*- y)top = (1.0)(0.210 - 0.10) = 0.11 atm At the column bottom, for ammonia, (p*- p)bottom = P (y*- y)bottom = (1.0)(0.007 - 0.0) = 0.007 atm Therefore, for ammonia, (p*- p)LM = 0.037 atm From Eq. (3), packed volume = V = 0.259(0.921) = 1.61 ft3 (0.037)(4) Exercise 6.34 Subject: Absorption of NH3 from air into water in a packed column. Given: Entering gas is 2,000 lb/h of air (dry basis) containing NH3 with a partial pressure of 12 torr and saturated with water vapor at 68oF and 1 atm. Equilibrium data in Fig. 6.49. Absorb 99.6% of the NH3. Assumptions: No absorption of air. No st...
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