Distillation Petlyuk column

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

Notice that Equation 8.16 is also a linear transform of the residue curve eqjuation, as the difference vector (S ) is not a function of the column s internal liquid composition (x). Hence, the mathematics describing the zero-order reactive distillation system is in fact identical to the infinite reflux Petlyuk column, even though the phenomena and equipment involved in the process are very different. 

A simulation of a DWC based on the equivalent Petlyuk column has two recycle loops, which requires two recycle blocks (R1 and R2), as shown in Figure 9.6b. The prefractionator column (Pref) has neither reboiler nor condenser, which can be simulated using an absorber column. This column has two recycle streams, which includes a recycled vapor stream to a prefractionator (Vapor pre in) at the bottom, and the recycled liquid stream to the prefractionator (Liquid pre in) at the top. These two streams need the initial estimates to solve the Pref column. A distillation unit for the main column, the material streams. Feed, Vapor pre out. Liquid pre out. Liquid Main out. Vapor Main out. Distillate, Side, and Bottom for Main column are added to the flow sheet. Two recycle blocks need to be added to the flow sheet one recycle block (Rl) for the Liquid Main out and another (R2) for Vapor Main out to initialize the recycle stream. Once the Pref is solved, the main column is simulated with an estimated number of stages and stage locations from the shortcut calculation. 

Industrial applications of the divided-wall (Petlyuk) column have expanded, so a new chapter has been added that covers both the design and the control of these more complex coupled columns. The use of dynamic simulations to quantitatively explore the safety issues of rapid transient responses to major process upsets and failures is discussed in a new chapter. A more stmctured approach for selecting an appropriate control structure is outlined to help sort through the overwhelmingly large number of alternative stmctures. A simple distillation column has five factorial (120) alternative structures that need to be trimmed down to a workable number, so that their steady-state and dynamic performances can be compared. 

We understand by distillation complex a countercurrent cascade with branching of flows, with recycles or bypasses of flows. Columns with side stripping or side rectifier and columns with completely connected thermal flows (the so-called Petlyuk columns ) are examples of distillation complexes with branching of flows. A column of extractive distillation, together with a column of entrainer regeneration, make an example of a complex with recycle of flows. Columns of this complex work independently of each other therefore, we do not examine it in this chapter, and the questions of its usage in separation of azeotropic mixtures and questions of determination of entrainer optimal flow rate are discussed in the following chapters. 

Three kinds of distillation complexes with thermal coupling flows (with branching of liquid and/or vapor flows) - columns with side stripping, columns with side rectifiers and complexes with full thermal coupling flows, called Petlyuk column -are used in industry at present. 

Figure 6.12. Some complex columns for ternary mixtures (a) with side rectifying (b) with side stripping (c) Petlyuk column (d) with prefractionator (e) more operable Petlyuk column (f) with divided wall and (g) with divided wall for extractive distillation.

Later, these columns were independently rediscovered (Petlyuk, Platonov, Slavinskii, 1965 Platonov, Petlyuk, Zhvanetskiy, 1970) on the basis of theoretical analysis of thermodynamically reversible distillation because this distillation complex by its configuration coincides with the sequence of thermodynamically reversible distillation of three-component mixture (see Chapter 4), but in contrast to this sequence it contains regular adiabatic columns. The peculiarities of Petlyuk columns for multicomponent mixtures are (1) total number of sections is n(n - 1) instead of 2(n - 1) in regular separation sequences (2) it is sufficient to have one reboiler and one condenser (3) the lightest and the heaviest components are the key components in each two-section constituent of the complex and (4) n components of a set purity are products. 

At design of Petlyuk columns, as of distillation columns and complexes of other types, the purity of separation products is a set (specified) parameter, while the tray numbers for all sections nn, nsi, itr2, aod (Fig. 7.15) and reflux... 

The comparison of various distillation complexes and of ordinary flowsheets is given in Tedder and Rudd (1978). Columns with side strippings and side rectifiers, Petl50ik columns, flowsheet with prefracionator, and also some other feasible configurations of two columns were examined. It was shown, in particular, that Petlyuk columns are preferable at big content of average volatile component. 

However, while estimating expenditures by thermodynamic efficiency rj (Agrawal Fidkowski, 1999), as has to be expected, the region of preferability of Petlyuk columns occupies only a small part of the area of the concentration triangle, compared with sequences of simple columns and other distillation complexes. 

Besides sequences of simple columns, some types of distillation complexes, each of which can replace two adjacent simple columns, were examined in work (Ghnos Malone, 1988). The following complex columns and distillation complexes were examined column with side output above the feed cross-section, column with side rectifier, column with side stripping flowsheet with prefractionator, Petlyuk column top and side flows from the first column into the second one (Fig. 8.3a),... 

The theory of trajectory tear-off was extended to the section located between the cross-section of entrainer input and that of the main-feeding input in columns of sharp extractive distillation (Petlyuk Danilov, 1999), which included it into the general method of presynthesis. 

Table 2 shows the lAE values obtained for each composition control loop of the distillation sequences under analysis. It is observed that the Petlyuk column offers the best dynamic behavior, which is reflected in the lowest values of lAE, for the control of the three product streams. The dynamic response of each control loop when the Petlyuk column was considered is displayed in Figure 2, where a comparison can be made to the response obtained with the widely-used direct sequence. One may notice in particular how the direct sequence is unable to control the composition of the intermediate component, while the Petlyuk column provides a smooth response, with a relatively short settling time. It is interesting to notice that for this mixture with an ESI = 1, and a low content of the intermediate component in the feed, the Petlyuk column offers the highest energy savings and also shows the best dynamic performance from the five distillation sequences under consideration. 

When the content of the intermediate component in the feed was raised from 20 to 70 percent, significant changes in the dynamic responses of the distillation systems were observed. The first remark is that the Petlyuk column does not provide the best choice from an operational point of view. A second observation is that the best choice depends on the control loop of primary interest. When the control of the light (A) or the heavy (C) component of the ternary mixture is of primary concern, then the TCDS-SS scheme provides the best option since it offers the lowest lAE values for these control loops. 

---