Prefractionation arrangements

Consider a three-product separation as in Fig. 5.11a in which the lightest and heaviest components are chosen to be the key separation in the first column. Two further columns are required to produce pure products (see Fig. 5.11a). However, note from Fig. 5.11a that the bottoms and overheads of the second and third columns are both pure B. Hence the second and third columns could simply be connected and product B taken as a sidestream (see Fig. 5.116). The arrangement in Fig. 5.116 is known as a prefractionator arrangement. Note that the first column in Fig. 5.116, the prefractionator, has a partial condenser to reduce the overall energy consumption. Comparing the prefractionator arrangement in Fig. 5.116 with the conventional... 

Figure 5.11 Choosing nonadjacent keys leads to the prefractionator arrangement.

Figure 5.13 Compasition profiles for the middle product in the prefractionator arrangement show that there are no remixing effects. (From Triantafyllou and Smith, Trans. IChemE, part A, 70 118, 1992 repr uced by permission of the Institution of Chemical Engineers.)...

In addition, one other feature of the prefractionator arrangement is important in reducing mixing effects. Losses occur in distillation operations due to mismatches between the composition of the column feed and the composition on the feed tray. Because the prefractionator distributes component B top and bottom, this allows greater freedom to match the feed composition with one of the trays in the column to reduce mixing losses at the feed tray. 

The elimination of mixing losses in a prefractionator arrangement means that it is inherently more efficient than an arrangement using simple 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. 

Introduce complex distillation configurations. Introduce prefractionation arrangements (with or without thermal coupling), side-rectifiers, and side-strippers to the extent that operability can be... 

Prefractionator arrangements (both with and without thermal coupling) can be used to replace either direct or indirect pairings. 

This remixing that occurs in both sequences of simple distillation columns is a source of inefficiency in the separation. By contrast, consider the prefractionator arrangement shown in Figure 11.9. In the prefractionator, a crude split is performed so that Component B is distributed between the top and bottom of the column. The upper section of the prefractionator separates AB from C, whilst the lower section separates BC from A. Thus, both sections remove only one component from the product of that column section and this is also true for all four sections of the main column. In this way, the remixing effects that are a feature of both simple column sequences are avoided4. 

The elimination of mixing losses in the prefractionator arrangement means that it is inherently more efficient than an arrangement using simple columns. The same basic arguments apply to both distributed distillation and prefractionator arrangements, with the additional degree of... 

Figure 4.13. Prefractionation arrangements (a) removing light keys with absorber (b) removing heavy keys with stripper (c) heat pumping (d) vapor recompression (e) reboiler flashing. B bottom product, D distillate, V valve.

A prefractionator arrangement with no heat integration saves about 30% energy compared to the best of the direct or indirect sequence. A prefractionator with further heat integration where the columns are run at different pressures, can have savings of around 50% compared to the best of the direct or indirect sequence (Ding Luyben, 1990). 

For both the arrangements the energy saving is dependent on the recovery of the middle component from the prefractionator. Figure 2 shows a comparison of the minimum vapour flowrate required for the integrated and non-integrated prefractionator arrangement. 

Figure 2. Comparing Vmin for prefractionator arrangement with and without integration, sharp split, propane-butane-pentane zf = [0.15 0.7 0.15].

The objective of the study is to implement a simple optimal control scheme for the integrated prefractionator arrangement by finding and controlling the variables in the system that will directly ensure optimal economic operation. Then, when there are disturbances in the system, there is no need to re-optimize. 

The first important step in this systematic procedure is to analyse the number of degrees of freedom (DOF) for the system. For the integrated prefractionator arrangement there are eleven DOF, when assuming a fixed feedrate. These are the boilup in the HP column, the condensation rate in the HP column, reflux, distillate and bottom flowrate from both columns, sidestream flowrate in the LP column, boilup in the IP column and condensation rate in the LP column (see Figure 1). 

The method of self-optimizing control is applied to a heat integrated prefractionator arrangement. This system has a total of eleven degrees of freedom with six DOF available for optimization when variables with no-steady state effects have been excluded and the duties of the two columns are matched. From the optimization it is found that there is one degree of freedom left for which there is not an obvious choice of control variable. The method of self- optimizing control will be used to find a suitable control variable that will keep the system close to optimum when there are disturbances. 

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