Petlyuk columns

Consider now thermal coupling of the prefractionator arrangement from Fig. 5.116. Figure 5.16a shows a prefi-actionator arrangement with partial condenser and reboiler on the prefractionator. Figure 5.166 shows the equivalent thermally coupled prefractionator arrangement sometimes known as a Petlyuk column. To make the two arrangements in Fig. 5.16 equivalent, the thermally coupled prefractionator requires extra plates to substitute for the prefractionator condenser and reboiler. 

Fig. 3. Rearrangement of a conventional distillation sequence into the Petlyuk column...

As mentioned earlier, the Petlyuk column (Figure Ic) poses potential operational problems because of the two directions of the interconnecting vapor... 

In addition to the visual observations of the dynamic responses, a quantitative measure is needed to provide a better comparison. With such an objective, lAE values were evaluated for each closed-loop response. The PUL option shows the lowest lAE value of 5.607 x 10 , while the value for the Petlyuk column turns out to be 2.35 x 10. Therefore, the results of the test indicate that, for the SISO control of the heaviest component of the ternary mixture, the PUL option provides the best dynamic behavior and improves the performance of the Petlyuk column. Such result is consistent with the prediction provided by the SVD analysis. 

The results of the dynamic test for a positive change in the set point of the light component (A) are displayed in Figure 6. For a change in the set point from 0.987 to 0.991, the three systems are shown to be controllable and reach the new value of product composition, although the PUL scheme shows a quicker adjustment. lAE values were also calculated for each response the two best lAE values correspond to the new arrangements 4.20 x 10 for the PUL system, and 6.10 x 10 for the PUV system. The lAE value for the Petlyuk column was 2.35 x 10, which again shows a case in which the dynamic... 

Figure 7 shows the dynamic responses obtained when the set point for the intermediate component was changed from 0.98 to 0.984. One may notice the better response provided by the Petlyuk column in this case, which is faster than the other two systems and without oscillations. When the lAE values were calculated, a remarkable difference in favor of the Petlyuk system was observed 2.87 x 10 for the Petlyuk column, compared to 0.0011 for the PUL system and 0.0017 for the PUV system. The results from this test may seem unexpected, since the new arrangements have been proposed to improve the operation capabilities of the Petlyuk column. The SISO control of the intermediate component, interestingly, seems to conflict with that of the other two components in terms of the preferred choice from dynamic considerations. 

Thermally coupled systems can also be devised for multicomponent mixtures. Sargent and Gaminibandara (Optimization in Action, L. W. C. Dixon, ed.. Academic Press, London, 1976, p. 267) presented a natural extension of the Petlyuk column sequence to multi-component systems. Agrawal [Ind. Eng. Chem. Res., 35,1059 (1996) Trans. Inst. Chem. Eng., 78,454 (2000)] presented a method for generating an even more complete superstructure from which all the... 

In the previous examples, a one-to-one correspondence exists between the units and the tasks (e.g., in Fig. 2 each node performs a particular separation task). It is possible, however, to develop more general superstructure representations in which a one-to-many relationship exists between the units and the tasks. An example of a one-to-many relationship is the superstructure for separation shown in Fig. 6 proposed by Sargent and Gaminibandara (1976), this superstructure accommodates sharp splits and has the Petlyuk column embedded as an alternative design. Note, for instance, that column 1 does not have a prespecified separation task. From this example it is clear that superstructures that have one-to-many relationships between units and tasks tend to be richer in terms of embedded alternatives. On the other hand, the more restricted one-to-one superstructures tend to require simpler MINLP models that are quicker to solve. 

Petlyuk column has been extensively studied. The energy saving may be estimated roughly at 30% compared with simple columns, being more efficient than side-rectifier and side-stripper arrangements. However, the industry is reluctant. A reason is the difficulty to manage mechanical problems, as the split of the flows and the pressure drop between columns. More fundamental reasons seem to be the limitation in... 

Holland, S.T., et al. Complex column design by application of column profile map techniques Sharp split Petlyuk column design. Industrial Engineering Chemistry Research, 2010, 49(1) 327 349. 

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