Maldistribution small scale

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... 

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). 

The real maldistribution for each of the phases, gas or liquid, over these specific cross-sections further is odled first type of maldistribution. The term small-scale maldistribution is also used [128]. This maldistribution depends... 

Hoek et al. [128] noted also that with random packings a semi stable flow distribution developed rapidly in the main body of the packing bed, which they called natural flow or small-scale maldistribution . It is characterized by severe channelling, or rivulet formations, of the liquid. Channelling is... 

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). 

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-...

Flow maldistribution of the phases can render the evaluation of RTD data very difficult. In some cases, maldistribution may exist in small units but it may not exist in large-scale units (e g., trickle-bed reactors). While in some other cases, such as three-phase fluidized-bed reactors, nonuniform gas distribution in large-scale units may cause undesirable recirculation and dead zones. Uniform gas distribution can usually be achieved in the small-scale fluidized-bed reactor. 

Hoek, P.J. Wesselingh, J.A. Zuiderweg, F.J. Small scale and large-scale liquid maldistribution in packed columns. Chem. Eng. Res. Des. 1986, 64, 431-449. 

Hoak, P. J., Large and Small Scale Liquid Maldistribution in Packed Column, Ph.D. Thesis, Delft University, Netherland, 1983. 

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). 

Zuiderweg, F. J., and P. J. Hoek, "The Effect of Small Scale Liquid Maldistribution on the Separating Efficiency of Random Packings, I. Chem. E. Symp. Ser. 104, 1987, p. B247. 

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). 

In summary, Fig. 5.19 shows that electrodes with exposed front and rear faces will exhibit significant maldistribution of current. With the small-scale electrolyzers commonly encountered in laboratory studies the effect may be significant and have considerable influence on scale-up. If we look at a typical current distribution on one electrode of an identical pair (Fig. 5.20), this distribution, in what is essentially a poor reactor design, exhibits a virtually constant current over about two-thirds of the front face, but at the edges currents rise to over three times that value. At the rear of the electrode, apart from the edges, there is only a small amount of current flowing. A limiting case of this analysis is when the electrodes are embedded into the rear wall, i.e., S = d. 

---