DIVIDED-WALL PETLYUK COLUMNS

The divided-wall column splits the middle section of a single vessel into two areas by inserting a vertical wall in the vessel at an appropriate position, not necessarily at the diameter. Feed is introduced into the prefractionator side of the wall. A sidestream is removed from the other side. The sidestream is mostly the intermediate boiling component of the ternary mixture. The lightest component goes overhead in the distillate product, and the heaviest component goes out in the bottoms product. 

At the bottom of the divided-wall section, the vapor is split between the two sides in proportion to the cross-sectional area of each side, which is fixed by the physical location of 

Distillation Design and Control Using Asperi Simulation, Second Edition. William L. Luyben. 2013 John Wiley Sons, Inc. Published 2013 by John Wiley Sons, Inc. 

The divided-wall column has many degrees of freedom at the steady-state design stage. The number of stages in the four different sections of the column, the locations of the feed and sidestream withdrawal points, and the location of the wall are seven of the parameters that must be specified and are all fixed by the physical equipment at the time of construction. They cannot be changed during operation. The location of the wall fixes how the vapor splits between the two sides of the wall, so the vapor split is not adjustable during operation for control purposes. 

In addition, there are four degrees of freedom that are adjustable during design and are also adjustable during operation of the column reflux flow rate (/ ), vapor boilup (V), sidestream flow rate (5), and the liquid split ratio (jSl = i-p/i-R)- The variable Lp is the liquid flow rate fed to the prefractionator side of the wall, and Lp is the total liquid leaving the bottom tray in the rectifying section. Of course, the rest of the liquid coming from the bottom of the rectification section is fed to the sidestream side of the column. Distillate and bottoms flow rates are used to maintain liquid levels in the reflux drum and column base, respectively. 

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DIVIDED-WALL (PETLYUK) COLUMNS TABLE 12.2 Controller Tuning Parameters... 

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. 

The example illustrates the optimization of a divided wall column. In order to avoid increasing the complexities of the model, we rnrnrtnize the heat flow in the reboiler. A divided wall column can be simulated using the thermodynamically equivalent configuration Petlyuk arrangement[6] (see Figure 1)... 

Energy reductions of up to 30% have been reported in some systems for Petlyuk and divided-wall column configurations compared with the direct-separation sequence. Figure 12.1 gives the flowsheet of a divided-wall column for the numerical benzene/toluene/xylene separation example considered later in this chapter. The material presented in this chapter is based on the paper that studied the control of divided-wall columns. ... 

The divided-wall column is an example of a complex column configuration whose industrial apphcations for separating ternary mixtures have expanded in recent years. Since there is only one column, one reboiler, and one condenser, capital costs are reduced compared with a conventional two-column configuration. An increasing number of industrial apphcations of the divided-wall column have been reported in recent years with about 100 columns reported to be in service. The divided-wall column is a practical way to implement the topology of the Petlyuk column. 

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.

Figure 6.13. Some Petlyuk columns for four-component mixtures (a) sequential column arrangement (b) more operable arrangement (c) with two divided walls and (d) in two columns (with intermediate split in first column).

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