Random 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 regular structure of the arranged catalysts prevents the formation of the random maldistributions characteristic of beds of randomly packed particles. This reduces the probability of the occurrence of hot spots resulting from flow maldistributions. Scale-up of monolithic and membrane reactors can be expected to be straightforward, since the conditions within the individual channels are scale invariant. 

The authors report that they have observed extremely high temperatures in a commercial reactor. In particular, this observation was not coupled with a known change in reactor conditions. However, the temperature levels observed could be explained by reduction of hydrogen flow to about 1/4 of the normal vdiich the authors surmise could have occurred due to a transitory partial localized plugging of the bed by dirt in the feed or by a flow obstruction caused by broken catalyst particles within the bed. Berkelew [2] also reported hot spotting due to localized random maldistributions in hydrotreaters. 

It may be that the random or structured packing efficiency is degraded by the need to limit the bed-height space required for liquid collection redistribution, and by maldistribution and channelling in the packing [33]. 

Pressure Generally, pressure has little effect on HETP of both random and structured packing, at least above 100 mbar abs. At deep vacuum (<100 mbar), there are data to suggest that efficiency decreases as pressure is lowered for random packings [Zelvinski, Titov, and Shalygin, Khim Tekhnol. Topi. Masel. 12(10) (1966)], but most of these data can also be explained by poor wetting or 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. 

Low liquid rates. With the aid of serrated weirs, splash baffles, reverse-flow trays, and bubble-cap trays, low liquid rates can be handled better in trays. Random packings suffer from liquid dewetting and maldistribution sensitivity at low liquid rates. 

On each of these, random and structured reactors behave quite differently. In terms of costs and catalyst loading, random packed-bed reactors usually are most favorable. So why would one use structured reactors As will become clear, in many of the concerns listed, structured reactors are to be preferred. Precision in catalytic processes is the basis for process improvement. It does not make sense to develop the best possible catalyst and to use it in an unsatisfactory reactor. Both the catalyst and the reactor should be close to perfect. Random packed beds do not fulfill this requirement. They are not homogeneous, because maldistributions always occur at the reactor wall these are unavoidable, originating form the looser packing there. These maldistributions lead to nonuniform flow and concentration profiles, and even hot spots can arise (1). A similar analysis holds for slurry reactors. For instance, in a mechanically stirred tank reactor the mixing intensity is highly non-uniform and conditions exist where only a relatively small annulus around the tip of the stirrer is an effective reaction space. 

Generally, the minimum wetting rate is at 0-5 to 2 gpm/ft2 for random packings, and 0.1 to 0.2 gpm/ft2 for structured packings (Sec. 8.2.15). It follows that point A is usually a distributor turndown limit. Regardless of which limit point A represents, it is extremely sensitive to maldistribution (Fig. 8.16b). When liquid distribution is poor, it will take more liquid to wet the entire bed, and point A will shift to the right. If distribution is very poor, point A may never be observed, and the curve will have no flat region at all. A V-shaped curve is not uncommon, and is indicative of poor distribution. 

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... 

The essence of monolithic catalysts is the very thin layers, in which internal diffusion resistance is small. As such, monolithic catalysts create a possibility to control the selectivity of many complex reactions. Pressure drop in straight, narrow channels through which reactants move in the laminar regime is smaller by two or three orders of magnitude than in conventional fixed-bed reactors. Provided that feed distribution is optimal, flow conditions are practically the same across a monolith due to the very high reproducibility of size and surface characteristics of individual monolith passages. This reduces the probability of occurrence of hot spots resulting from maldistributions characteristic of randomly packed catalyst beds. 

No maldistribution of gas or liquid in three-phase processes. Regarding application of the BSR concept to gas/liquid/solid processes, an important advantage of the BSR is that adjacent strings do not (necessarily) touch. Because of the liquid surface tension, liquid will not spill over from one BSR string to another. Consequently, the initial liquid distribution is maintained throughout the BSR module. This feature is especially advantageous when incomplete catalyst wetting (which results from liquid maldistribution in traditional, randomly packed trickle-flow reactors) would lead to hot spots and decreased selectivity. 

Depth of plastic random packings may be limited by the deformability of the packing elements to 10-15 ft. For metal random packings, this height can be 20-25 ft. For both random and structured packings, the height between redistributors is limited to 20-25 ft. because of the tendency of the phases to maldistribute. 

The drawbacks of randomly packed beds in microchannels are the high pressure drop and effects related to the nonuniform packing of the small catalyst particles, namely, channeling and maldistribution of the fluids. A large RTD results, which diminishes the reactor performance and, in the case of sequential reaction networks, the product selectivity. The reactor or the catalyst may be modified such that a structured bed is obtained. 

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