Minimum solvent ratio

What does it mean to have two pinch point curves for A To operate as we wish, we need to keep the composition of the liquid on the trays between these two curves for the section of the column between the feeds. We shall now show that one of the curves dictates a minimum reflux ratio and the other a maximum reflux ratio that must be used to operate the column for a fixed solvent ratio. We shall then show that there is a minimum solvent ratio we can use—one where the pinch point trajectories just touch. We will also show that larger ratios move the pinch curves apart, making it easier to effect the separation. 

The slope of the operating line is defined by the solvent ratio. The minimum solvent ratio depicts the operating line in the loading diagram with a common point on the equilibrium curve. Indefinite theoretical stages on the operating line would be neces-... 

Consider the case of minimum reflux ratio (infinite stages). As the amount of solvent is reduced, point M (equal to S + F) in Fig. 11.9 moves towards F, and P (equal to D + So) moves towards D. Point P" (equal to B - S) moves away from the equilibrium curve. The maximum distance that points M, P, and P" can be moved is determined by the slope of the tie lines. The minimum solvent ratio, which corresponds to the minimum reflux ratio, is reached when a tie line and an operating line coincide. A pinch point can occur either in the enriching or in the stripping section of the column, so it is necessary to seek the highest value of the minimum reflux ratio by trial and error. In this example, it occurs at the feed stage. The minimum reflux ratio is 0.58 and the corresponding minimum solvent ratio is 0.74. 

BLM Balance line for the case of a minimum solvent ratio and a single solvent pass through the... 

The solvent ratio v is an important operating variable for absorption, similar to the reflux ratio for rectification. It controls the slope of the balance line for an absorption problem and has to be larger than a minimum solvent ratio is obtained by... 

If the feed, solvent, and extract compositions are specified, and the ratio of solvent to feed is gradually reduced, the number of ideal stages required increases. In economic terms, the effect of reducing the solvent-to-feed ratio is to reduce the operating cost, but the capital cost is increased because of the increased number of stages required. At the minimum solvent-to-feed ratio, the number of ideal stages approaches infinity and the specified separation is impossible at any lower solvent-to-feed ratio. In practice the economically optimum solvent-to-feed ratio is usually 1.5 to 2 times the minimum value. 

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). 

Because there is no azeotrope, these mixtures could be separated without adding a solvent. This, however, would be a difficult and expensive separation. Thus there is no minimum feed ratio (minimum solvent flow) and the only way to determine the optimal solvent-to-process feed ratio is by determining the sequence cost over a range of feed ratios. The best reflux ratios are again 1.2—1.5 times the minimum. 

The variable that has the most significant impact on the economics of an extractive distillation is the solvent-to-feed (S/F) ratio. For closeboiling or pinched nonazeotropic mixtures, no minimum-solvent flow rate is required to effect the separation, as the separation is always theoretically possible (if not economical) in the absence of the solvent. However, the extent of enhancement of the relative volatihty is largely determined by the solvent concentration and hence the S/F ratio. The relative volatility tends to increase as the S/F ratio increases. Thus, a given separation can be accomplished in fewer equihbrium stages. As an illustration, the total number of theoretical stages required as a function of S/F ratio is plotted in Fig. 13-75 7 for the separation of the nonazeotropic mixture of vinyl acetate and ethyl acetate using phenol as the solvent. 

Both maximum and minimum limits exist of the solvent/feed ratio. The maximum is the value that locates the mix point M on the binodal curve near the solvent vertex, such as point Mmla on Figure 14.7(b). When an operating line coincides with a tieline, the number of stages will be infinite and will correspond to the minimum solvent/feed ratio. The pinch point is determined by the intersection of some tieline with line RnS- Depending on whether the slopes of the tielines are negative or positive, the intersection that is closest or farthest from the solvent vertex locates the operating point for minimum solvent. Figure 14.9 shows the two... 

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. 

For dilute solutions and a high degree of solute removal, the minimum solvent to feed ratio (Smin/F) may be estimated from the inverse of the distribution coefficient. 

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. 

Countercurrent Operation 466 Minimum Solvent/Feed Ratio 468 Extract Reflux 468... 

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