Double feed pinch

Try this for yourself using DODS-SiCo and the TT design option, find a combination of product distribution for a set a that will result in the double-feed pinch. 

The double-feed pinch case occurs only at a specific selection of product purities. Although it may seem like a trivial ccmdition, the double-feed pinch scenario... 

FIGURE 5.29 Illustration of minimum reflux increase for a given feed and for product specifications laying either side of the double feed pinch product ecifications. 

Notice that in finding / a3min and /Jasmin that there was only one possibility in which the TTs could touch one another since the Xa s of the respective CSs were fixed. In the case of CS2 and CS4 however, the difference points are variable and hence it is necessary to take account of aU three of these pinching scenarios. Since a double-feed pinch case is merely the special condition where cases (a) and (b) in Figure 7.31 meet, there are essentially only two cases of minimum reflux across the feed. In order to find the minimum reflux for the CSs across the feed, it is thus necessary to consider both these cases. Holland et al. [27] treated this problem by evaluating the eigenvectors at the feed for liquid feeds only, but we will treat the general case here for any quality feed. 

In the first class, called preferred separation, the mid-component is distributed approximately evenly between the two products. There is a double pinch zone, above and below the feed (Fig. 9.36). Material balance line and concentration profile mtersect at the feed point. The reflux ratio has the lowest value of all possible separations. Hence, the preferred separation is the most favourable in ternary systems. 

For the special case of sharp splits, scenario (c) is known as the double-feedpinch as this mutual pinch point will occur at the feed composition (illustrated in Section 5.4.1). These three cases are dependent on the product and feed compositions one chooses, and are illustrated in Figure 5.14a c. 

In Fig. 5.2-29 the internal concentration profiles of preferred separation, low-boiler separation, and high-boiler separation are qualitatively shown. Limiting for the energy demand is the pinch (i.e., point of intersection of operating and equihbrium lines). At preferred separations a double pinch exists whose concentration is equal to the feed concentration (like in binary distillation). 

There are a number of features in this example drawn from industry that fall outside the conventional boxmdaries of textbook problems. The first item of note is the location of the pinch, which occurs at a point between feed and effluent conditions. The resulting minimum CO2 consumption is at the modest level of 11.9 kg C02/kg beans. Even doubling that amount for actual operating conditions still yields a consumption that is less than that calculated for a single-stage contact (Illustration 6.7). This is a clear-cut vindication of tire coxmtercurrent mode of operation. 

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