Thermally Coupled Columns Side Rectifiers and Strippers

4 THERMALLY COUPLED COLUMNS SIDE RECTIFIERS AND STRIPPERS 

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THERMALLY COUPLED COLUMNS SIDE RECTIFIERS AND STRIPPERS 199 TABLE 6 Summary of the Pseudo Simple Column at the Thermally Coupled Units... 

Due to the tremendous costs associated to distillative separations, many alternate schemes to the simple column shown above have been proposed over the past several years both to improve on some of its inherent costs. Traditionally, when purifying a multicomponent mixture, an entire series of distillation columns are used in series, and the way in which these columns are sequenced may make a tremendous difference in the eventual process costs. However, due to the large energy requirements of even the most optimal sequence, more complex column arrangements have been proposed and subsequently utilized. These arrangements include thermally coupled columns such as side rectifiers and strippers, the fully thermally coupled columns (often referred to as the Petlyuk and Kaibel columns). 

Examples of such complex distillation structures are thus columns that have more than one feed point and/or more than two product streams, like distributed material addition/removal columns, and thermally coupled columns. Obviously, as the complexity of the distillation structure increases, so does the design itself thereof. This chapter will, as an introduction to complex column design, treat the design of elementary complex columns such as distributed feed and sidestream withdrawal columns, and side rectifiers, and strippers, before discussing more intricate complex columns like fully thermally coupled columns (sometimes referred to as the Petlyuk and Kaibel columns) in the subsequent chapter. Despite... 

The partitioned thermally coupled prefractionator in Figure 11.14c can be simulated using the arrangement in Figure 11.14b as the basis of the simulation. However, like side-rectifiers and side-strippers, fully thermally coupled columns have some important degrees of freedom for optimization. In the fully thermally coupled column, there are six column sections (above and below the partition, above and below the feed in the prefractionator and above and below the sidestream from the main column side of the partition). The degrees of freedom to be optimized in partitioned columns are ... 

Consider now ways in which the best arrangement of a distillation sequence can be determined more systematically. Given the possibilities for changing the sequence of simple columns or the introduction of prefractionators, side-strippers, side-rectifiers and fully thermally coupled arrangements, the problem is complex with many structural options. The problem can be addressed using the optimization of a superstructure. As discussed in Chapter 1, this approach starts by setting up a grand flowsheet in which all structural features for an optimal solution are embedded. 

The use of complex columns (side-strippers, side-rectifiers and thermally coupled prefractionators) reduces the overall heat duties for the separation at the expense of more extreme temperatures for reboiling and condensing. Heat integration benefits from smaller duties, but more extreme temperatures make the heat integration more difficult). 

Consider the use of a side-rectifier or side-stripper for the separation of a three-product mixture. Assume that thermally coupled columns operate at the same pressure. Also, assume the feed to be saturated liquid. Data for the operation of the two arrangements are given in Tables 21.9 and 21.10. 

By eliminating the reboiler and condenser in the prefractionator column in Fig. 13-67a (the column containing sections 1 and 2) we obtain a thermally coupled system, also known as a Petlyuk system, shown in Fig. 13-67b [Petlyuk, Platonov, and Slavinskii, Int. Chem. Eng., 5, 555 (1965)]. Side stripper, side rectifier, and Petlyuk systems can also be built as divided wml columns, as explained in detail in the subsection below on thermally coupled systems. 

By thermal coupling the heat is transferred by direct contact between vapour and liquid flows that connect sections of different columns. This is a major difference with heat integrated columns , where the heat exchange takes place by condenser/reboilers. Hence, thermal coupled columns have a more complex behaviour. Figure 11.21 illustrates two basic arrangements with side-columns. The first is the side-rectifier, derived irom a direct sequence. The second one is the side-stripper that corresponds to... 

The development of thermally coupled systems started with attempts to find energy-saving schemes for the separation of ternary mixtures into three products. One of the first industrial applications was the side rectifier configuration for air separation. The side stripper configuration followed naturally. By combining the two we obtain the fully thermally coupled system of Petlyuk, Platonov, and Slavinskii [Int. Chem. Eng., 5, 555 (1965)] see Fig. 13-67h. It consists of the prefractionator which accepts the ternary feed stream followed by the main column that produces the products (product column). 

The transition split divides direct-type sphts from indirect-type splits as discussed by Doherty and Malone (Conceptual Desisn of Distillation Systems, 2001, chaps. 4 andS) also see Fidkowski, Doherty, and Malone [AlChE J., 39,1301(1993)]. The upper line in Fig. 13-70 is the minimum vapor flow leaving the reboiler of the main column, which also corresponds to the minimum vapor flow for the entire system since all the vapor for the total wstem is generated by this reboiler. For P = 0 the minimum vapor flow for the entire thermally coupled system (i.e., main column) becomes equal to the minimum vapor flow for the side rectifier system (i.e., main column of the side-rectifier system see Fig. 13-65b or c) (Vsr) for P = 1 it is equal to the minimum vapor flow of the entire side stripper system (Vss) (which is the sum of the vapor flows from both the reboilers in this system see Fig. 13-66h or c). Coincidentally, the values of these two minimum vapor flows are always the same (Vsr), = (Vss)mm- For P = Pr the main column is pinched at both feed locations i.e., the minimum vapor flows for separations A/B and B/C are equal. 

FIG. 13-74 Feed composition regions of column configurations with highest thermodynamic efficiency DS—direct split, IS—indirect split, SR—side rectifier, SS—side stripper, FC—fully thermally coupled. Example for ttA = 4.0, OCb = 2.0, and ar = 1.0. 

Both configurations can be divided into four CSs for the case of a ternary system where we wish to obtain relatively pure distillate, bottoms, and sidestream products. For the sake of consistency, the side unithas been numbered as CS4 while the internal CS is labeled as CS2 in both configurations. In the side-stripper unit, the liquid coming from CSi is divided into two streams one that is directed to the main column body and another one that is directed toward the side-stripping unit. The vapor flow in CSi is also the sum of the vapor flows from the side stripping unit and the main column. Similarly, the vapor flow from CS3 in the side rectifier is directed toward the main column and the rectifying unit, while the liquid flowrate in CS3 is the sum of the liquid flowrates in the main column and the rectifier. The location of the sidestream withdrawal and addition at the thermally coupled junction are assumed to take place at the same location. This assumption can be relaxed however, an additional CS would be created and this case will not be further discussed in this text... 

The two internal degrees of fi eedom of each column have been specified through the appropriate split ratio and a single, reference reflux. Both Rm in the side stripper and Rm in the side rectifier have been chosen as the respective reference refluxes since they lie the closest to the thermally coupled sections. This choice is of course arbitrary, and one may equivalently choose any CS to serve as the reference CS. 

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