Packed Bed flooding.pdf - Youngstown State University's Personal ...

the pinching effect. The effects of maldistribution on efficiency are
therefore most severe in large-diameter columns and small-diameter
packings.
A good design practice is to seek a packing size that gives a D T/D p
between 10 and 40. This is often impractical, and higher ratios are
common. When D T/D p exceeds 100, avoiding efficiency loss due to
maldistribution is difficult. Either ratios exceeding 100 should be
avoided, or a special allowance should be made for loss of efficiency
due to maldistribution.
3. Wall flow effects become large when D T/D p falls below about 10.
Packing diameter should be selected such that D T/D p exceeds 10.
4. Columns containing less than five theoretical stages per bed are
relatively insensitive to liquid maldistribution. With 10 or more stages
per bed, efficiency can be extremely sensitive to maldistribution
(Strigle, Packed Tower Design and Applications, 2d ed., Gulf Publishing,
Houston, Tex., 1994) (Fig. 14-66). Beds consisting of small packings
or structured packings, which develop more theoretical stages
per bed, are therefore more sensitive to maldistribution than equaldepth
beds of larger packings. This is clearly demonstrated by FRI’s
experiments [Shariat and Kunesh, Ind. Eng. Chem. Res. 34(4), 1273
(1995)]. Lockett and Billingham (Trans. IChemE, vol. 81, Part A, p.
131, January 2003) concur with these comments when their procedure
(above) indicates high sensitivity to maldistribution, but allow a higher
number of stages per bed when the sensitivity is low.
5. Maldistribution tends to be a greater problem at low liquid flow
rates than at high liquid flow rates [Zuiderweg, Hoek, and Lahm, I.
ChemE Symp. Ser. 104, A217 (1987)]. The tendency to pinch and to
spread unevenly is generally higher at the lower liquid flow rates.
6. A packed column has reasonable tolerance for a uniform or
smooth variation in liquid distribution and for a variation that is totally
random (small-scale maldistribution). The impact of discontinuities or
zonal flow (large-scale maldistribution) is much more severe [Zuiderweg
et al., loc. cit.; Kunesh, Chem. Eng., p. 101, Dec. 7, 1987; Kunesh,
Lahm, and Yanagi, Ind. Eng. Chem. Res. 26(9), 1845 (1987)]. This is
so because the local pinching of small-scale maldistribution is evened
out by the lateral mixing, and therefore causes few ill effects. In contrast,
the lateral mixing either is powerless to rectify a large-scale
maldistribution or takes considerable bed length to do so (meanwhile,
efficiency is lost).
Figure 14-67 shows HETPs measured in tests that simulate various
types of maldistribution in FRI’s 1.2-m column containing a 3.6-m bed
of 1-in Pall ® rings. The y axis is the ratio of measured HETP in the
maldistribution tests to the HETP obtained with an excellent distributor.
Analogous measurements with structured packing were reported
by Fitz, King, and Kunesh [Trans. IChemE 77, Part A, p. 482 (1999)].
Generally, the response of the structured packings resembled that of
the Pall ® rings, except as noted below.
Figure 14-67a shows virtually no loss of efficiency when a distributor
uniformly tilts, such that the ratio of highest to lowest flow is 1.25
(i.e., a “1.25 tilt”). In contrast, an 11 percent chordal blank of a level
distributor causes packing HETP to rise by 50 percent.
Figure 14-67b compares continuous tilts with ratios of highest to
lowest flow of 1.25 and 1.5 to a situation where one-half of the distributor
passes 25 percent more liquid than the other half. The latter
(“zonal”) situation causes a much greater rise in HETP than a “uniform”
maldistribution with twice as much variation from maximum to
minimum.
Figure 14-67c shows results of tests in which flows from individual
distributor drip points were varied in a gaussian pattern (maximum/mean
= 2). When the pattern was randomly assigned, there was
no efficiency loss. When the variations above the mean were assigned
to a “high zone,” and those below the mean to a “low zone,” HETP
rose by about 20 percent. With structured packing, both random and
zonal maldistribution caused about the same loss of efficiency at the
same degree of maldistribution.
7. A packed bed appears to have a “natural distribution,” which is
an inherent and stable property of the packings. An initial distribution
which is better than natural will rapidly degrade to it, and one that is
worse will finally achieve it, but sometimes at a slow rate. If the rate is
extremely slow, recovery from a maldistributed pattern may not be
observed in practice (Zuiderweg et al., loc. cit.). Even though the
EQUIPMENT FOR DISTILLATION AND GAS ABSORPTION: PACKED COLUMNS 14-71
FIG. 14-67 Comparing the effects of “small-scale” and “large-scale” maldistribution
on packing HETP. (a) Comparing the effect of a simulated continuous
tilt (max/min flow ratio = 1.25) with the simulated effect of blanking a chordal
area equal to 11 percent of the tower area. (b) Comparing the effects of simulated
continuous tilts (max/min flow ratios of 1.25 and 1.5) with the effects of a
situation where one-half of the distributor passes 25 percent more liquid to the
other half. (c) Comparing the effects of random maldistribution with those of
zonal maldistribution. (Reprinted with permission from J. G. Kunesh, L. Lahm,
and T. Yahagi, Ind. Eng. Chem. Res., 26, p. 1845; copyright © 1987, American
Chemical Society.)
(a)
(b)
(c)