Distillation columns liquid redistribution

We emphasize that the reactive section of a reactive distillation column can be viewed as a cascade-type two-phase reactor with the reactor temperature determined by the bubblepoint temperature of the tray liquid-phase composition. It is clear that the composition and temperature profiles will certainly affect the performance of the reactive zone, and the feed tray locations appear to be one of the most effective variables for these profiles redistribution. In this section, we are interested in how the composition profile will be affected by changing the feed tray location. Each individual feed tray location is changed while holding the other constant. 

The assembly shown in Fig. 2.104 illustrates a set-up for simple fractionation using a Vigreux column which has moderate fractionating efficiency and is probably one of the most widely used columns. The column consists of a glass tube with a series of indentations such that alternate sets of indentations point downwards at an angle of 45° in order to promote the redistribution of liquid from the walls to the centre of the column. The mixture to be fractionated is placed in a flask of convenient size (it should be one-third to one-half full), a few fragments of porous porcelain added and a water condenser attached to the side-arm. The distillate is collected in small flasks or in test tubes. The bulb of the thermometer... 

In spite of a frequent redistribution of the liquid it may occur that the packing is not unlforndy wetted the film itself may also have different forms. Stuke [161] has investigated this subject in detail, and has examined the influence of the nature of the packing, the properties of the liquid, the column load and the distillation pressure (with its associated temperature) on irregularities of film formation. His conclusion.s can be summarized as follows (see also chap. 4.2). 

Later experiments in small diameter columns have shown that a bed of random dumped packing develops a definite number of preferred paths of internal liquid flow [8]. If liquid is distributed onto the packed bed with a greater number of streams per square foot than number of preferred liquid paths, the liquid streams within the bed coalesce until the number of preferred liquid paths is reached. If liquid is distributed onto the packed bed with a smaller number of streams per square foot than the number of preferred liquid paths, the packing redistributes the liquid until the number of preferred liquid paths is established. In this latter case, the upper portion of the packed bed has a lower-than-nor-mal separating efficiency. Figure 10-8 shows the performance of such a packed bed in a binary distillation. Curve I represents the separation obtained with a high-performance liquid distribution system. Curve II shows the effect of inadequate liquid distribution to the top surface of the 10-ft deep packed bed. 

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