Liquid flow maldistribution

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. 

These theoretical dependences are in very good agreement with many experimental observations (see e.g. Porter et al., I968 Berner et al., 1978). It is, for example, well known that the liquid flow maldistribution grows when decreasing the liquid flowrate. 

In large size industrial trickle-bed reactors, it is difficult and practically impossible to obtain a perfect distribution of fluid flows. In these reactors, one may consider two types of gas-liquid flow maldistribution ... 

Such homogeneous or pseudo-homogeneous models are phenomenologically incorrect. Gas and liquid flow maldistributions have indeed been observed in trickle-bed reactors (17, 18)... 

When straight or serrated segmental weirs are used in a column of circiilar cross secdion, a correction may be needed for the distorted pattern of flow at the ends of the weirs, depending on liquid flow rate. The correction factor F from Fig. 14-33 is used direcdly in Eq. (14-112) or Eq. (14-119). Even when circular downcomers are utilized, they are often fed by the overflow from a segmental weir. When the weir crest over a straight segmental weir is less than 6 mm V in), it is desirable to use a serrated (notched) weir to provide good liquid distribution. Inasmuch as fabrication standards permit the tray to be 3 mm Vh in) out of level, weir crests less than 6 mm V in) can result in maldistribution of hquid flow. 

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. 

In addition to the reduction in performance, flow maldistribution may result in increased corrosion, erosion, wear, fouling, fatigue, and material failure, particularly for liquid flows. This problem is even more pronounced for multiphase or phase change flows as compared to single-phase flows. Flow distribution problems exist for almost all types of exchangers and can have a significant impact on energy, environment, material, and cost in most industries. 

Maldistribution tends to be a greater problem at low liquid flow rates than at high liquid flow rates [Zuiderweg, Hoek, and Lahm, 1. ChemE Symp. Ser. 104, A217 (1987)]. The tendency to pinch and to spread unevenly is generally higher at the lower liquid flow rates. 

It stands to reason that plumes are only formed under certain hydrodynamic conditions, e.g. ratios of flow rates of the liquids. Otherwise, simple bi-lamination with comparatively low specific interface may occur. In addition, a flow maldistribution within the array may occur for certain conditions, i.e. most flow passes the first row of nozzles at the expense of the residual holes. So far, there is, to the best of our knowledge, no detailed report on modeling these aspects or an experimental proof where indeed plume fluid structures are visualized only gross characterization of the mixing was given (see below). 

The simplest approach of splitting one stream into six streams is the use of a fluidic element that directly connects one inlet with six symmetrically arranged outlets [140, 141, 148], Commercial fluid distributors, however, suffer from flow maldistribution, as the smallest deviations in their manufacture have a large impact. Hence specially devices are required for flow splitting to micro devices. A liquid-flow distribution module was one first such device to be realized. 

A more detailed mass transfer study on the carbon dioxide absorption in sodium hydroxide solution was performed using a falling-film microreactor [319]. Experimental investigations were made at a liquid flow of 50ml/h, with three NaOH concentrations (0.1,1 and 2 M), at a fixed inlet molar ratio C02 NaOH of 0.4, and for a range of C02 concentration of 0.8-100%. A two-dimensional reactor model was developed, and the results are similar to the experimental data at low NaOH concentrations (0.1 and 1 M). The agreement is less pronounced for higher concentrations such as 2 M NaOH, which could be explained by either maldistribution of... 

Liquid flow patterns and maldistribution on large trays... 

Case studies were reported (170,174) of large-diameter (> 15-ft) towers with sieve trays not reaching the expected efficiency. Maldistribution was cited as the culprit or at least one of the causes. Improving liquid flow patterns, often among other modifications, was the fix. The only other evidence that channeling adversely affects tray efficiency comes from the above-mentioned theoretical models. 

In ideal trickle flow reactors, all particles in the catalyst bed take part in the overall reactor performance, since each is surrounded (wetted) by the liquid phase that flows around it. Situations in which the liquid flows preferentially through a certain part of the bed, while the gas phase flows predominantly through another part, should be avoided [23]. In this case, part of the bed is not contacted by the liquid reactant at all and docs not contribute to the overall conversion. To avoid this maldistribution, Gierman [20] proposed the following criterion for the wetting number Wtx for co-current downflow operation ... 

At low liquid flow rates, flow maldistributions such as channeling, bypassing, and incomplete... 

When both gas and liquid flow downward over a string of spheres, it can simulate a trickle-bed reactor if the liquid flows downward in the form of a thin film. The hydrodynamics for this type of reactor are reasonably well known. Both the hydrodynamics of the liquid flow over a single sphere and the phenomena taking place at the junctions of two spheres have been extensively studied. Flow maldistribution encountered in the pilot-scale trickle-bed reactor is eliminated in this type of reactor. Furthermore, good estimations of the various mass-transfer resistances can be ascertained. The reactor is successfully used by Satterfield et al.st) for the catalytic hydrogenation of a-methyl styrene. Their experimental setup is shown in Fig. 5-9. 

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