Unformatted text preview: ˆ’ 0.1802) = 154.6 lbmol/h
From Eq. (1223):
V
V
V
J 3 = âˆ’ J1 âˆ’ J 2 = 316 âˆ’ 154.6 = âˆ’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. (1219), but with diffusion and masstransfer rates instead of fluxes,
V
N1V = J1V + z1 NT = âˆ’31.6 + 0.4654(54) = 56.7 lbmol/h
V
V
V
N 2 = J 2 + z2 NT = 154.6 + 0.2100(âˆ’54) = 143.3 lbmol/h
V
V
V
N 3 = J 3 + z3 NT = âˆ’123.0 + 0.3246( âˆ’54) = âˆ’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 (123), with
Kvalues at phase interface conditions:
E MVi = yi ,n âˆ’ yi ,n +1 / KiI,n xi ,n âˆ’ yi ,n +1
The three values are identical to those in Example 12.1 because they only involve
the given mole fractions and Kvalues. In summary, the values calculated are the same as those computed in Example 12.1. The small
differences are caused by roundoff error. As would be expected, it makes no difference how the
components are ordered. Exercise 12.7
Subject: Correlations for estimating binarypair masstransfer 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, 592602 (1992)] for sieve trays, and the method of
Scheffe and Weiland [Ind. Eng. Chem. Res., 26, 228236 (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 bubblecap columns of 2ft diameter with a 2ft tray spacing.
The method gave favorable results when used to predict efficiencies of four commercial bubblecap columns of 4ft, 5.5ft, 8ft, and 13ft diameter. The method also gave favorable results for
a commercial sievetray column of 13ft diameter. The experimental data for the 2ft diameter
columns cover weir heights from 1 to 5 inches, Ffactors from 0.2 to 2.6 (93% of flooding),
liquid rates of 525 gpm/ft of average width, a vapor Schmidt number of 0.61, and a liquid
diffusivity of 2.42 x 105 to 7.5 x 105 cm2/s. No bubblecap design features were incorporated
into the correlation. Round bubblecap diameters ranged from 1.5 to 4 inches. Slot velocity in
and capspacing for the bubble caps were varied widely.
The transferunit 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 masstransfer 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 Ffactor (see p. 313), the volumetric liquid ra...
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 Spring '11
 Levicky
 The Land

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