0 t 2 3 4 5 6 7 8 91011121314151617
Number of stages/ bed (at 100% distribution quality) («
Figura 9.6 Effect of irrigation quality on packing efficiency, (a) Case histories demonstrating efficiency enhancement with higher distribution quality rating, (o) Correlation of the effect of irrigation quality on packing efficiency. (From F. Moore and F. Rukooena, Chemical Plants & Processing, Europe edition, August 1987. Reprinted courtesy of Chemical Plants and Processing and of the Norton Co.)
a nonuniform profile will persist at least for some height, causing pinching similar to that described in Sec. 9.2.2.
4. The packed height through which a nonuniform profile persists is a function of column diameter (156). In pilot-scale columns, vapor maldistribution was found to persist for a bed height of the order of 1 ft (155,157). In large-diameter columns, this maldistribution persists to a much greater height (15,152,154). In a number of 15-ft-diameter absorbers (154), vapor maldistribution persisted through a 50-ft bed; the efficiency was about half that encountered during good vapor distribution.
5. Redistribution of vapor depends on a balance between the vertical and horizontal pressure gradients (157). The horizontal pressure gradient depends on column diameter, and diminishes rapidly as column diameter increases. This explains the strong effect of diameter in item 4 above. Another important factor cited by Porter and Ali (157) is vortex formation. This can cause downward flows of vapor in the bed. Downward flows were actually measured by Kabakov and Rozen (154) and Porter and Ali (157).
6. The maldistribution tendency correlates well with the ratio of packed height to packing size (157). Therefore, smaller sizes of packing require smaller packed depths for distribution to become uniform. However, the more open structure of the larger packings promotes horizontal movement giving further reduction in maldistribution tendency. Stikkelman and Wesselingh (155) demonstrated that the tendency to even out maldistribution increased with the openness of the packing structure.
7. Vapor nonuniformity may be troublesome with those structured packings that permit substantial radial spread only parallel to their sheets (155). The orientation of structured packing sheets usually alters every 8 to 12 in of bed depth, and the changes of orientation mitigate this nonuniformity. However, the disturbance created to the composition profile may linger for a greater depth.
8. Vapor maldistribution may be induced by liquid maldistribution (66) when vapor flows are high. Regions of high liquid holdup impede vapor rise and channel the vapor into the lighter-loaded regions (66). Since liquid tends to accumulate near the wall, vapor will tend to channel through the center.
9.2.8 Implications of maldistribution to packing design practice
This section presents the implications of the maldistribution studies to packing design practice. Implications of maldistribution studies to distributor design practices are outside the scope of this book and are discussed at length in a companion book (40). Conclusions that pertain to packing design practices are:
1. Three factors appear to set the effect of maldistribution on efficiency (Sees. 9.2.1 to 9.2.4).
■ The pinching effect. Local changes in LfV ratio, causing local composition pinches (Sec. 9.2.2).
■ Lateral mixing effect. Packing particles deflect both liquid and vapor laterally. This promotes mixing of vapor and liquid and counteracts the pinching effect (Sec. 9.2.3).
■ Liquid nonuniformity effect. Liquid flows unevenly through the packing and tends to concentrate at the wall (Sec. 9.2.4).
2. At small tower to packing diameter ratios (Dj/Dp < 10), the lateral mixing cancels out the pinching effect, and a greater degree of maldistribution can be tolerated without a serious efficiency loss (3,132,136). At high ratios (DjJDp < 40), the lateral mixing becomes too small to counteract the pinching effect (132). The effects of maldistribution on efficiency are therefore most severe in large-diameter columns and small-diameter packings.
A good design practice is to look for a packing size that will give a Dj1DP between 10 and 40. This is often impractical, and higher ratios are common. When D^Dp exceeds 40, there is an incentive to minimize it. When Dt/Dp exceeds 100, avoiding efficiency loss due to maldistribution is extremely difficult (116,137). Ratios exceeding 100 should either be avoided, or a special allowance should be made for loss of efficiency due to maldistribution.
3. Wall flow effects become large when D^Dp falls below about 10 (Sec. 9.2.4). Packing diameter should be selected such that D-ptDp 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 is extremely sensitive to maldistribution (15,152,158; see Fig. 9.6). Beds consisting of small packings or structured packings, which develop more theoretical stages per bed, are therefore more sensitive to maldistribution than equal-depth beds of larger packings.
5. Maldistribution tends to be a greater problem at low liquid flow rates than at high liquid flow rates (1,66,136). 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). However, the impact of discontinuities of zonal flow (large-scale maldistribution) is much more severe (51,131,136,137,139,158,159).
This is a particularly useful finding. In terms of the maldistribution models, small-scale maldistribution will be evened out by the lateral mixing, and therefore will cause few ill effects. On the other hand, the lateral mixing will either be powerless to rectify a large-scale maldistribution problem, or will take considerable bed length to do so (meanwhile, efficiency will be lost).
Figure 9.7 shows HETPs measured in tests that simulate various types of maldistribution in a 4-flt column containing a 12-ft bed of 1-in Pall® rings (131,160). The y-axis is the ratio of measured HETP in the maldistribution tests to the HETP obtained with an excellent distributor.
Figure 9.7a 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"). On the other hand, an 11 percent chordal blank of a level distributor causes packing HETP to rise by 50 percent.
Figure 9.76 compares continuous tilts with a ratio of maximum to minimum flow of 1.25 and 1.5 to a situation where one-half of the distributor is passing 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 9.7c 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.
7. A packed bed appears to have a "natural distribution," which is an inherent and stable property of the packings (15,131,136, 140,145,158,159). 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 (136,159).
8. In the presence of large-scale maldistribution, packing efficiency decreases as packing height increases (Fig. 9.6; also 15,131, 132,136). This is due to composition nonuniformity generated by pinching and to the development of wall flow. With small packings, the above may occur even in the absence of initial maldistribution (136).
POT of useable capacity (a)
POT of useable capacity (a)
1 INCH PALLRMOS 0,-Ct, 1 ATMOS
PCT of useable capacity
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