Extraction limiting reflux ratios

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). 

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. 

But by analogy with extractive distillation, it can be expected that a second feed point would drastically widen the product region at a finite reflux ratio and thus also increase the conversion. Between the two feed points, the column profile is perpendicular to the distillation lines (Fig. 2.5). Since this effect is based on the finite nature of the reflux ratio employed, we can expect product purity and conversion to first increase with an increase in the reflux ratio and then slowly decrease again. The limiting value that is established for an infinite reflux ratio is determined by the azeotrope concentration in the methyl acetate/methanol system. 

The most important limiting values of BED are the minimum feed flow rate (Fmn) at given reflux ratios (R), particularly at infinite reflux ratio (R=° ), minimum reflux ratio (Rmin). minimum number of theoretical stages in the extracting section and... 

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