Maldistribution large scale

Finally, certain geometrical analogies should be kept within the following limits to avoid large-scale maldistribution of the flow (Treybal, 1980 Gunn, 1968, Carbeny 1976 Chen et al., 1968) ... 

General Generally, from a macroscopic point of view, maldistribution can be divided into two different phenomena (Stanek, 1994). The first one is small-scale maldistibution, which is connected mainly to the so-called preferred paths. It is the case where the liquid follows specific paths through bed and travels with velocities considerably higher than the mean. The same phenomenon is characterized as chaneling. The second case is large-scale maldistribution, which is connected to the nonhomogeneous (nonunifonn) initial distribution of the liquid and is referred to as wall effects. The concepts of distributor quality and liquid maldistribution in fixed beds are frequently found in the related technical literature, and these concepts are connected to each other—the better die distributor quality, the better the liquid distribution and flow into bed (Klemas and Bonilla, 1995). 

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 0.7 Comparing the effects of "small-scale and large-scale" maldistribution on packing HETP. (a) Comparing the effect of a simulated continuous tUt (max/ min flow ratio = 125 ) to the simulated effect of blanking a chordal area equal ts 11 percent of the tower area, (b) Comparing the effects of simulated continuous tilts (max/min flow ratios 125 and 1.5) to the effects of a situation where one-half of the distributor passes 25 percent more liquid to the other hen. (c) Comparing the effects of random maldistribution to these of zonal maldistribution. (Ae-...

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 or zonal flow ("large-scale maldistribution ) is much more severe (219, 221, 222, 442, 443). 

Any liquid distributor gives some maldistribution, because for practical reasons, liquid can only be divided into a limited number of streams. From these point sources the liquid spreads. The main considerations in selecting a distributor for a given service are compatibility with the service and avoiding large-scale maldistribution. 

In order to reduce the vapor-liquid interaction, standard weir riser pans are usually smaller than the column diameter and are supported on lugs, leaving an annular space for vapor rise between the distributor and the tower wall. This, however, creates an unirrigated region near the column wall, which may cause large-scale maldistribution. Other performance characteristics of this distributor are similar to those of the notched-trough distributor (below). 

With notched-trough distributors, poor irrigation is most likely to occur beneath wide troughs, wide vapor passages, supports, or near the column wall. Inadequate irrigation in such areas can cause large-scale maldistribution which may be detrimental to column efficiency. 

Since the main function of a redistributor is to counteract the effects of large-scale maldistribution, less frequent redistribution is required when large-scale maldistribution is absent or minimized. For instance, less frequent redistribution is needed when the ratio of column to packing diameter is small (443), because large-scale maldistribution is well-counteracted by lateral mixing in the packing (Sec. 3.1). 

When this arrangement is used, caution is required to ensure that the drawoff does not generate large-scale maldistribution of liquid to the bed below (see Sec. 3.1) and that unirrigated areas do not form on top of the packing under the drawoff pipe or sump. This is most likely to be a problem when a submerged drawoff nozzle or sump is used. 

In the presence of large-scale maldistribution, packing efficiency decreases as packing height increases (Fig. 9.6 also 16,131, 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). 

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