Sloppy split

Unlike simple columns, these complex configurations produce more than two products and feature more than a single light and heavy key component. A task-based representation of these schemes is accomplished with the ideas of hybrids and sloppy splits. Additional tasks are made up of different simple tasks. They are subsequently termed hybrid tasks and are defined as an ordered combination of simple distillation tasks, as illustrated in Figure 9. 

For example, the separations d,h, and d2h2 are sloppy splits with different amounts of A in the distillate, and accordingly with different recoveries. The separation of pure A at the top is represented by the split d h, which corresponds to a direct sequence . Accordingly, the separation of B/C in a second column is represented by the edge BC, on which h is the feed. Similarly, the first split in an indirect sequence , in which C is separated in bottoms and A/B at the top, is shown by the segment d"b". The locus of all splits between the above limit cases allows the regions of attainable products to be defined. 

Figure 3.16 displays a second alternative, with a sloppy split in the first column. The points I), and d, are on the same residue curve inside the same distillation region. Because d, can be pushed on the side AB, the next binary split will give high purity A and AB azeotrope, which is recycled. If the distillation border is high concave the point I), could approach closely the side BC. As a result, the third split can deliver good purity B and entrainer to be recycled. This alternative seems interesting but uncertain with respect to achievable purity. Fortunately, the simulation indicates that the distillation border can be deeply crossed. 

Finally, it is interesting to compare the alternatives as capital and operation costs (Figure 3.19). The most expensive columns are those crossing the boundary, as well as for separating the binary chloroform/toluene. The alternative with two columns is the cheapest It also has the lowest operation costs. The first alternative is the most expensive in operation. On the basis of total annual cost the best alternative is by far the one with only two columns. The alternative with a sloppy split is ranked in second place, because of its cheaper operation. 

In alternative (a) pure products are obtained in each column. Since the relative volatility diminishes with the pressure, higher reflux is needed in the H P column. The balance of duties can be obtained by adjusting the split of the feed. Roughly speaking, by double-effect distillation the energy consumption is divided by two. In alternative (b) there is a large temperature difference between top and bottom that may be exploited by a sloppy split in the HP column with the heavy component, while in alternative (c) this is done with the light component. Alternative (c) is the best for the present case study since it allows a lower temperature of the hot utility. 

With these elements, the alternatives can be compared on the basis of Total Annual Cost (Fig. 9.23). By far, the best alternative has only two columns. The alternative with a sloppy split is ranked in the second place, because of cheaper operation. 

The problem addressed in this paper can be stated as follows Given a number of components, that do not form azeotropes, to be separated into a predefined set of products. The objective is to find an appropriate and cost effective separation scheme. This scheme includes conventional columns, partially linked distillation systems -with any number of heat exchangers between 2 and 2(n-l)- that can produce prefractionators, sloppy splits, side columns etc. Without loss of generality, the products are listed in a decreasing order of volatilities. 

Sloppy first bottom split, second oolumn pressurized, in which the bottoms from the first column is further separated in the high-pressure column... 

Sloppy First Bottom Split Sloppy First Bottom Split Sloppy First Bottom Split... 

Let s examine another alternative. For example, the first split might be sloppy , meaning that both distillate and bottoms contain a large amount of the intermediate component. This would imply two more splits, in total three simple columns, obviously uneconomical. In consequence, the second column should deliver all three components, including the intermediate as side-stream. This scheme seems unusual, but it may be in fact more economically when the two columns are heat-integrated. 

Three columns sequence for separation in two distillation fields. First split sloppy. 

Figure 9.21 Separation of acetone and chloroform with toluene (Alternative 2). Three columns, first split sloppy. The simple distillation border is crossed in the first column.

---