Backmixing vapor

In the case of unmixed vapors between the plates, the equations, being implicit in Ey, have also been solved numerically (112). The results depend on the arrangement of the downcomers and are not too different numerically from equation 93. In reaHty, however, the Hquid is neither completely backmixed nor can the tray be considered as a plug-flow device. 

Tray efficiency 0 j is supposed to represent a measure of the deviation from equilibrium-stage mass transfer assuming backmixed trays. However, the estimate of tray efficiency requires accurate knowledge of the equihbrium vaporization constant. Any deviations between the actual equihbrium relation and that predicted by the database will be embodied in the tray efficiency estimate. It is a tender trap to accept tray efficiency as a true measure of the mass transfer hmitations when, in fact, it embodies the uncertainties in the database as well. 

Zuiderweg, F. J. and D. E. Nutter, On the Evidence of Vapor Backmixing in Packed Columns in the Case of High Pressure Distillation, copyright by Institution of Chemical Engineers. Note undated and publication not given. 

High vapor velocities, combined with high foam levels, will cause the spray height to hit the underside of the tray above. This causes mixing of the liquid from a lower tray, with the liquid on the upper tray. This backmixing of liquid reduces the separation, or tray efficiency, of a distillation tower. 

Liquid Flow Patterns on Large Trays The most popular theoretical models (below) postulate that liquid crosses the tray in plug flow with superimposed backmixing, and that the vapor is perfectly mixed. Increasing tray diameter promotes liquid plug flow and suppresses backmixing. 

Zuiderweg and Nutter [IChemE Symp. Ser. 128, A481 (1992)] explain the loss of capacity/efficiency by a large degree of backmixing and vapor recycle at high flow parameters, promoted by the solid walls... 

Most popular theoretical models (such as the AlChE and the Chan and Fair models, Sec. 7.2.1) postulate that liquid crosses the tray in plug flow (Fig. 7.7a) with superimposed backmixing, and that vapor is perfectly mixed. Increasing tray diameter promotes liquid plug flow and suppresses backmixing. This should enhance efficiency in large-diameter columns, but such enhancement has not been observed (147,148). Liquid maldistribution is the common explanation to the observation. 

In contrast with vapor-liquid columns, tray efficiencies are very low (5 to 30%) in liquid-liquid systems. The trays do limit continuous phase backmixing as well as provide drop redispersion, but at the expense of reduced capacity. 

Entrainment. In a properly functioning column, much of the mass transfer takes place in a turbulent, high-interfacial-area froth layer which develops above the liquid on the plate. Inadequate disengagement of the liquid and vapor in the froth results in the backmixing of froth with the liquid from the tray above, and a lowered efficiency. Entrainment is frequently due to inadequate downcomer size or tray spacing. 

When trays are operating in the spray regime, different considerations apply. The liquid does not enter the downcomer by flowing over the outlet weir. Instead, it enters when liquid droplets suspended in the vapor space descend into the downcomer (Fig. 6.16). The closer the weir is to the holes, the easier it is for these suspended droplets to reach the downcomer. Further, the liquid pool on the tray floor near the outlet weir induces liquid backmixing onto the tray, which lowers tray efficiency (333). For these reasons, the outlet weir should be placed as close to the holes as possible for spray regime operation (90,... 

It is assumed that the gas phase is completely backmixed and losses of n-butyraldehyde due to vaporization are negligible. (Although there will be a significant change in the viscosity of the liquid when it is oxidized, for the sake of illustration, average property values are assumed.)... 

The mass balances (Eqs. (10.3) and (10.4)) assume plug-flow behavior for both the vapor and the liquid phase. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the column performance, for example via backmixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for catalytic distillation processes, we performed experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK [77]. 

Another result, which is interesting for model simplification, concerns dispersion. In catalytic vapor phase reaction the effect of backmixing on reactor efficiency generally proves to be negligible except for cases of high conversion and short beds. For concurrent trickle-flow operation Hears [47] has developed on the basis of a perturbation solution of the one-dimensional plugflow dispersion (PD-)model, a criterion for negligible (< 5 %) influence of axial dispersion in case of first order reactions ... 

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