Zonal maldistribution

Figure 14-67c shows results of tests in which flows from individual distributor drip points were varied in a gaussian pattern (maxi-mum/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. 

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-...

The drip points should be evenly spread. The author has experienced cases where a distributor provided more liquid per unit area to some central regions than to some peripheral regions, resulting in maldistribution. Such zonal maldistribution is detrimental to column efficiency (221). 

The possible advantages of this system over the equilibrium-limited reactor system are smaller catalyst beds, lower gas recycle requirements, and lower capital requirements. The possible disadvantages of this system are (a) practically no turn-down since any turn-down would be equivalent to decreased space velocities, closer approach to equilibrium, and higher temperature rises (b) maldistribution of gases across the bed would give rise to excessive temperature rises in zones of low flow and (c) considerably shortened catalyst life because of possible high local or zonal temperature and, concurrently, greater chances for carbon laydown. 

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. 

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). 

---