Page 249 - Separation process principles 2

214 Chapter 6 Absorption and Stripping of Dilute Mixtures

where magnitude, Figure 6.19 shows that values of EMv can be sig-
nificantly larger than Eov for large values of A.
Lewis [16] showed that when the equilibrium and operat-
ing lines are straight, but not necessarily parallel, the overall
stage efficiency, defined by (6-21), is related to the Murphree
The dimensionless Peclet number, Npe, which serves as a
vapor efficiency by
partial-mixing parameter, is defined by
log[l + EMV(A - 111
E, = (6-37)
log A
where ZL is the length of liquid flow path across the tray as
When the two lines are not only straight but parallel, such
shown in Figure 6.3, DE is the eddy diffusion coefficient in
that A = 1, (6-37) becomes E, = EM". Also, when EM" = 1,
the direction of liquid flow, OL is the average liquid residence
time on the tray, and u = ZL/OL is the mean liquid velocity then E, = 1 regardless of the value of A.
across the tray. Equation (6-34) is plotted in Figure 6.19 for
Scale-up from Laboratory Data
wide ranges of NPe and AEov. When Npe = 0, (6-3 1) holds;
when Npe = 00, (6-32) holds. When vapor-liquid equilibrium data for a system are unavail-
From (6-36), the Peclet number can be viewed as the ratio able or not well known, and particularly if the system forms
of the mean liquid bulk velocity to the eddy-diffusion veloc- a highly nonideal liquid solution with possible formation
ity. When Npe is small, eddy diffusion is important and of azeotropes, tray requirements are best estimated, and
the liquid approaches a well-mixed condition. When NPe is the feasibility of achieving the desired degree of separation
large, bulk flow predominates and the liquid approaches verified, by conducting laboratory tests. A particularly useful
plug flow. Experimental measurements of DE in bubble-cap apparatus is a small glass or metal sieve-plate column with
and sieve-plate columns [18-211 cover a range of 0.02 to center-to-side downcomers developed by Oldershaw [22]
0.20 ft2/s. Values of u/DE typically range from 3 to 15 ft-l.
Based on the second form of (6-36), Np, increases directly
with increasing ZL and, therefore, column diameter. A typi-
cal value of Npe for a 2-ft-diameter column is 10; for a 6-ft-
diameter column, Np, might be 30. For Np, values of this

0 1 .O 2.0 3.0 1 2 3 4 5 6 7 8 9 1 0
LEO" 'ZEOV
Figure 6.19 Effect of longitudinal mixing on Murphree vapor tray efficiency.