Residence time distribution flow maldistribution

The dispersion and stirred tank models of reactor behavior are in essence single parameter models. The literature contains an abundance of more complex multiparameter models. For an introduction to such models, consult the review article by Levenspiel and Bischoff (3) and the texts by these individuals (2, 4). The texts also contain discussions of the means by which residence time distribution curves may be used to diagnose the presence of flow maldistribution and stagnant region effects in operating equipment. 

All the preceding sections were concerned with one-dimensional voidage distribution in the vertical direction. However, maldistribution of solids in the radial direction, generally dilute in the center and dense next to the wall, often causes unfavorable residence time distributions for both the solids and the fluidizing gas, thus resulting in undesirable product distribution. Although it has long been known that in vertical flow of G/S systems solids are preferentially scattered toward the wall, accurate measurement has not been easy. 

If recirculation rates are 10 to 15 times the feed rate, the reactor would tend to operate nearly isothermally. High velocities past the bed of particles could eliminate almost completely any external mass-transfer influence on the reactor performance. By varying the circulation rates, the reaction condition for which the mass transfer effect is negligible can be established. Except for the rapidly-decaying catalyst system, steady state can be achieved effectively. Sampling and product analysis can be obtained as effectively as in the fixed-bed reactor. Residence-time distributions for the fluid phases can be measured easily. High fluid velocities would cause less flow-maldistribution problems. 

An issue that is not adequately addressed by most models (EQ and NEQ) is that of vapor and liquid flow patterns on distillation trays or maldistribution in packed columns. Since reaction rates and chemical equilibrium constants are dependent on the local concentrations and temperature, they may vary along the flow path of liquid on a tray, or from side to side of a packed column. For such systems the residence time distribution could be very important, as well as a proper description of mass transfer. On distillation trays, vapor will rise more or less in plug flow through a layer of froth. The liquid will flow along the tray more or less in plug flow, with some axial dispersion caused by the vapor jets and bubbles. In packed sections, maldistribution of internal vapor and liquid flows over the cross-sectional area of the column can lead to loss of interfacial area. 

To avoid flow maldistribution in micro packed beds the particle diameter should be less than a 10th of the tube diameter dp < df/lO). The residence time distribution in packed beds can be estimated with the following empirical relation valid... 

The drawback of randomly packed microreactors is the high pressure drop. In multitubular micro fixed beds, each channel must be packed identically or supplementary flow resistances must be introduced to avoid flow maldistribution between the channels, which leads to a broad residence time distribution in the reactor system. Initial developments led to structured catalytic micro-beds based on fibrous materials [8-10]. This concept is based on a structured catalytic bed arranged with parallel filaments giving identical flow characteristics to multichannel microreactors. The channels formed by filaments have an equivalent hydraulic diameter in the range of a few microns ensuring laminar flow and short diffusion times in the radial direction [10]. 

The maintenance of uniform flow distribution in fixed bed reactors is frequently a problem. Maldistribution leads to an excessive spread in the distribution of residence times with adverse effects on the reactor performance, particularly when consecutive reactions are involved. It may aggravate problems of hot-spot formation and lead to regions of the reactor where undesired reactions predominate. Disintegration or attrition of the catalyst may lead to or may aggravate flow distribution problems. 

We see from the preceding that even though the distribution has been broadened considerably, there is no evidence of an effect on the overall conversion. This makes sense because there are just as many fluid elements below the average as there are above and so we get average behavior— just as we intuitively know we should. What, however, happens when the flow is so maldistributed that the residence time is actually bimodal ... 

The distribution of gas to the catalyst bed, influenced by the change in direction of gas flow from axial to radial, can have an important effect on reactor performance. In the event of gas maldistribution there will be a spread of residence times in the bed, which can adversely affect both conversion and selectivity [1][2], and may cause local hot-spots. It is possible to force-the gas flow to be uniform, for example by using nozzles to feed the gas to the bed, however, it is of interest to see whether the unit can be designed and operated so as to minimize or eliminate maldistribution. 

The distribution of fluids to the multiple channels turns out to be complex, as every single molecule needs to experience the same conditions (i.e., temperature and residence time) to obtain optimal reactor performance, efficiency, and safety. Maldistribution in multichannel reactors results in loss of product selectivity and yield, as demonstrated for consecutive reactions [86]. The flow nonuniformities generally occur due to two reasons a poor reactor design and manufacturing... 

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