Maldistribution, packings, liquid

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. 

Very important for all types of packings is a uniform liquid distribution at the top of the bed and a limitation of bed height to 6 or 8 m. Beneath each bed, the liquid has to be collected, mixed, and redistributed. These measures intend to suppress the so-called maldistribution of liquid because it strongly affects mass transfer rates. The design of liquid distributors, liquid collectors, support grids, etc., should provide a large open area not to hinder the countercurrent flow of gas and liquid. 

Berner and Zuiderweg showed that a significant maldistribution of liquid occurred in a bed of Raschig ring packing [7]. Local flow rates differed from a normal pattern when measured at various liquid flows and with different size packings. They concluded that the bulk of the liquid... 

The above results are based on data obtained for optimized designs and under ideal test conditions. To translate our findings to the real world, one must factor in liquid and vapor maldistribution, which is far more detrimental to the efficiency of packings than trays. In addition. one also must account for poor optimization or restrictive internals, which are far more detrimental to the capacity of trays than packings. We also have cited several other factors that need to be considered when translating the findings of our analysis to real-world towers. ... 

The flow pattern of fluids in gas-liquid-solid (catalyst) reactors is often far from ideal. Special care must be taken to avoid by-passing of the catalyst particles near the reactor walls, where the packing density of the catalyst pellets is lower than in the centre of the bed. By-passing becomes negligible if the ratio of reactor to particles diameter is larger than 10 a ratio of 20 is recommended. Flow maldistributions might be serious in the case of shallow beds. Special devices must be used to equalize the velocity over the cross-section of the reactor before reactants are introduced onto the catalyst bed. 

Redistributors are used to collect liquid that has migrated to the column walls and redistribute it evenly over the packing. They will also even out any maldistribution that has occurred within the packing. 

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]. 

The packing cannot be higher than a maximum value because the liquid has the tendency to pass to the wall of the column. Therefore at given sections the liquid has to be collected and re-distributed to the next layer of packings. A maldistribution of the liquid results in low separation efficiency. 

Liquid and vapor are well distributed. Both liquid and vapor maldistribution have a major detrimental effect on packing efficiency. 

Empirical Prediction Moore and Rukovena [Chemical Plants and Processing (European edition), p. 11, August 1987] proposed the empirical correlation in Fig. 14-64 for efficiency loss due to liquid maldistribution in packed towers containing Pall rings or Metal Intalox packing. This correlation was shown to work well for several case studies (Fig. 14-64), is simple to use, and is valuable, at least as a preliminary guide. 

Maximum Liquid Maldistribution Fraction To characterize the sensitivity of packed beds to maldistribution, Lockett and... 

Liquid maldistribution lowers packing turndown. The 2-in Pall rings curve in Fig. 14-59 shows HETP rise upon reaching the distributor turndown limit. 

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. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

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