Distillation sequencing simple columns

Much work has been carried out to find methods for the synthesis of distillation sequences of simple columns that do not involve heat integration. However, heat integration may have a significant... 

TABLE 5.1 The Number of Possible Distillation Sequences Using Simple Columns... 

Distillation. There is a large inventory of boiling liquid, sometimes under pressure, in a distillation column, both in the base and held up in the column. If a sequence of columns is involved, then, as discussed in Chap. 5, the sequence can be chosen to minimize the inventory of hazardous material. If all materials are equally hazardous, then choosing the sequence that tends to minimize the flow rate of nonkey components also will tend to minimize the inventory. Use of the dividing-wall column shown in Fig. 5.17c will reduce considerably the inventory relative to two simple columns. Dividing-wall columns are inherently safer than conventional arrangements because they lower not only the inventory but also the number of items of equipment and hence lower the potential for leaks. 

Distillation sequencing. Unless there are constraints severely restricting heat integration, sequencing of simple distillation columns can be carried out in two steps (1) identify the best few nonintegr-ated sequences, and (2) study the heat integration. In most cases, there is no need to solve the problems simultaneously. ... 

Table 11.1 Number of possible distillation sequences using simple columns.

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. 

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. 

Table 11.10 presents some heuristics for using complex distillation columns to separate a ternary mixture into its pure component products. On the basis of these heuristics and those for simple columns, suggest two sequences containing complex columns that can be used to separate the mixture described in Table 11.9 into relatively pure products. 

Distillation sequencing. The separation of homogeneous nonazeotropic mixtures using distillation usually offers the degree of freedom to choose the distillation sequence. The choice between different sequences can be made on the basis of total vapor load, energy consumption, refrigeration shaft power for low-temperature systems, or total cost. However, there is often little to choose between the best few sequences in terms of such measures of system performance if simple distillation columns are used. 

Floquet et al. (1985) proposed a tree searching algorithm in order to synthesize chemical processes involving reactor/separator/recycle systems interlinked with recycle streams. The reactor network of this approach is restricted to a single isothermal CSTR or PFR unit, and the separation units are considered to be simple distillation columns. The conversion of reactants into products, the temperature of the reactor, as well as the reflux ratio of the distillation columns were treated as parameters. Once the values of the parameters have been specified, the composition of the outlet stream of the reactor can be estimated and application of the tree searching algorithm on the alternative separation tasks provides the less costly distillation sequence. The problem is solved for several values of the parameters and conclusions are drawn for different regions of operation. 

For Distillation, Favor Sequences that Remove Components One-by-Oneas Overhead Product. This advice is based on the need to condense or remove heat from overhead streams and add heat to bottom streams. Adding nonkey components to the overhead stream increases the cooling load on the overhead condenser and the heating load on the reboiler for a simple column. Removing components one-by-one in Older of decreasing volatility also minimizes the vapor flow in the column supporting a direct sequence. 

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). 

In all the systems considered so far, we have only been concerned with obtaining a relatively pure distillate or bottoms stream. However, in tamary systems, for example, it is often desirable to obtain three relatively pure products. The traditional approach for achieving this has been with a sequence of simple columns, although this has obvious disadvantages to it, because to achieve this separation at least two separate columns have to be erected (refer to Section 2.6.4) meaning an increase in both capital and operating costs. The removal of a sidestream product from the main column may however be more advisable in certain instances. 

For azeotropic mixtures, the main difficulty of the solution of the task of synthesis consists not in the multiplicity of feasible sequences of columns and complexes but in the necessity for the determination of feasible splits in each potential column or in the complex. The questions of synthesis of separation flowsheets for azeotropic mixtures were investigated in a great number of works. But these works mainly concern three-component mixtures and splits at infinite reflux. In a small number of works, mixtures with a larger number of components are considered however, in these works, the discussion is limited to the identification of splits at infinite reflux and linear boundaries between distillation regions Reg° . Yet, it is important to identify all feasible splits, not only the spUts feasible in simple columns at infinite reflux and at linear boundaries between distillation regions. It is important, in particular, to identify the spUts feasible in simple columns at finite reflux and curvilinear boundaries between distillation regions and also the splits feasible only in three-section columns of extractive distillation. 

Such an approach was used in the works (Petlyuk Platonov, 1965 Petlyuk, Platonov, Slavinskii, 1965) and in more detail in Agrawal and Fidkowski (1998, 1999). In the latter works the comparison not only of sequences of simple columns, but also of distillation complexes, was made for three-component mixtures. Capital costs can differ considerably for such separation flowsheets, but in this case it is not important because energy expenditures greatly exceed capital ones. 

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),... 

Probably the most common method used for sequence selection for simple distillation columns is heuristic. Many heuristics have been proposed, but they can be summarized by the following four ... 

Heat Integration of Sequences of Simple Distillation Columns... 

This remixing which occurs in both sequences of simple distillation columns is a source of inefficiency in the separation. By contrast. 

---