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

5 in the nl correlation because it was not found

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Unformatted text preview: 0.1802) = 154.6 lbmol/h From Eq. (12-23): V V V J 3 = − J1 − J 2 = 316 − 154.6 = −123.0 lbmol / h . These three values are very close to those in Example 12.1: -32.7, 154.5, -121.8 (b) From Example 12.1, NT = Vn+1 - Vn = -54 lbmol/h From Eq. (12-19), but with diffusion and mass-transfer rates instead of fluxes, V N1V = J1V + z1 NT = −31.6 + 0.4654(-54) = -56.7 lbmol/h V V V N 2 = J 2 + z2 NT = 154.6 + 0.2100(−54) = 143.3 lbmol/h V V V N 3 = J 3 + z3 NT = −123.0 + 0.3246( −54) = −140.5 lbmol / h These three values are very close to those in Example 12.1: -57/8, 143.2, -139.4 (c) Approximate values of the Murphree vapor tray efficiency are obtained from (12-3), with K-values at phase interface conditions: E MVi = yi ,n − yi ,n +1 / KiI,n xi ,n − yi ,n +1 The three values are identical to those in Example 12.1 because they only involve the given mole fractions and K-values. In summary, the values calculated are the same as those computed in Example 12.1. The small differences are caused by round-off error. As would be expected, it makes no difference how the components are ordered. Exercise 12.7 Subject: Correlations for estimating binary-pair mass-transfer coefficients for trayed columns. Find: Advantages and disadvantages of the correlations Analysis: In Section 12.3, methods for trayed columns are mentioned as those of AIChE, Harris, Hughmark, Zuiderweg, Chan and Fair, and Chen and Chuang. To these may be added the method of Young and Stewart [AIChE J., 38, 592-602 (1992)] for sieve trays, and the method of Scheffe and Weiland [Ind. Eng. Chem. Res., 26, 228-236 (1987) for valve trays, but they are not considered here because they are not compared to other methods. The AIChE correlation, Ref. 20, was published in a book in 1958. The method is based on separate empirical equations for the individual number of transfer units, NV and NL, developed from experimental data on small bubble-cap columns of 2-ft diameter with a 2-ft tray spacing. The method gave favorable results when used to predict efficiencies of four commercial bubblecap columns of 4-ft, 5.5-ft, 8-ft, and 13-ft diameter. The method also gave favorable results for a commercial sieve-tray column of 13-ft diameter. The experimental data for the 2-ft diameter columns cover weir heights from 1 to 5 inches, F-factors from 0.2 to 2.6 (93% of flooding), liquid rates of 5-25 gpm/ft of average width, a vapor Schmidt number of 0.61, and a liquid diffusivity of 2.42 x 10-5 to 7.5 x 10-5 cm2/s. No bubble-cap design features were incorporated into the correlation. Round bubble-cap diameters ranged from 1.5 to 4 inches. Slot velocity in and cap-spacing for the bubble caps were varied widely. The transfer-unit correlation for the gas phase was developed from data on the absorption of ammonia from air into water at 1 atm and 20oC , which is controlled by mass-transfer in the gas phase because of the high solubility of ammonia in water. The correlation gives NV as a function only of the weir height, a superficial F-factor (see p. 313), the volumetric liquid ra...
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