Extraction minimum reflux

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

The thermal quality of the solvent feed has no effect on the value of (S/F)mjn, but does affect the minimum reflux to some extent, especially as the (S/F) ratio increases. R nax occurs at higher values of the reflux ratio as the upper-feed quality decreases a subcooled upper feed provides additional refluxing capacity and less external reflux is required for the same separation. It is also sometimes advantageous to introduce the primary feed to the extractive distillation column as a vapor to help maintain a higher solvent concentration on the feed tray and the trays immediately below... 

The advantages that may justify the additional costs of reflux at the bottom of a solvent extraction tower are illustrated in Figure 6. Minimum reflux is represented by the tie line from / to fe. Maximum reflux would be represented by the line / to the extract layer which would exist for infinite solvent to feed ratio. Practical operation of the equipment will fall between the limits of minimum and maximum reflux, as represented by fy. The operating point for the enriching section of the extraction column is located by an intersection between the lines erE and fy, and the reflux ratio is the ratio of the distances k e /k e (22). 

Figure 14.7. Locations of operating points P and Q for feasible, total, and minimum extract reflux on triangular diagrams, and stage requirements determined on rectangular distribution diagrams, (a) Stages required with feasible extract reflux, (b) Operation at total reflux and minimum number of stages, (c) Operation at minimum reflux and infinite stages.

Separation constraints The separation in a column can be expressed as the impurity levels of the key components in the two products xg.LK in the bottoms and xD Hx in the distillate. Separation is limited by the minimum reflux ratio and the minimum number of trays. We must always have more trays than the minimum and a higher reflux ratio than the minimum. If the number of trays in the column is not large enough for the desired separation, no amount of reflux will be able to attain it and no control system will work. In extractive distillation columns, there is also a maximum reflux ratio limitation, above which the overhead stream becomes less pure as the reflux increases. 

It should also be noted that many extractive distillation systems exhibit a maximum reflux ratio as well as the conventional minimum reflux ratio. For a given solvent-to-feed ratio, if too much reflux is returned to the column., the solvent is diluted and the separation becomes poorer since not enough solvent is available to soak up component B. 

E.xample problems are included to highlight the need to estimate the entire set of products that can be reached for a given feed when using a particular type of separation unit. We show that readily computed distillation curves and pinch point cur es allow us to identify the entire reachable region for simple and e.xtractive distillation for ternary mixtures. This analysis proves that finite reflux often permits increased separation we can compute exactly how far we can cross so-called distillation boundaries. For extractive distillation, we illustrate how to find minimum. solvent rates, minimum reflux ratios, and, interestingly, ma.xinnim reflux ratios. 

Minimum reflux corresponds to the overlap of an operating line and a tie-line (infinite stages at a pinch point). This concept is similar to minimum solvent flowrate for an extraction process without reflux. Total reflux corresponds to the minimum number of stages. Remember that total reflux means that no streams are going into or out of the column, so that F, B, and D are zero, and A = A. ... 

The minimum reflux ratio of extract is 3.49. Using a right-triangle diagram, calculate ... 

A modified McCabe-Thiele method employed in extractive distillation has been described by Nagel and Sinn [78]. Kortiim and Faltusz [79] have dealt with a variety of problems involved in selective separating processes ranging from the design of an automatic apparatus of special steel for continuous operation to the calculation of the minimum reflux ratio and the required amount of additive. 

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