Distillation Sequencing - Summary

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 nonintegrated sequences and (2) study 

The best few nonintegrated sequences can be identified most simply using the total vapor load as a criterion. If this is not satisfactory, then the alternative sequences can be sized and costed using shortcut techniques. 

Separation Processes, 2d ed., McGraw-Hill, New York, 1980. 

Westerberg, A. W., The Synthesis of Distillation-Based Separation Systems, Comp. Chem. Eng., 9 421, 1985. 

and LinnhoflF, E., The Design of Separators in the Context of Overall Processes, Trans. IChemE. ChERD, 66 195, 1988. 

Crude oil distillation is carried out in a complex column sequence in which live steam is injected into the separation to provide the heat required and to reduce the partial pressure of the components to be distilled. The 

Table 11.9 shows the composition of a four-component mixture to be separated by distillation. The K-values for each component at the bubble point temperature of this mixture are given. The liquid- and vapor-phase mixtures of these 

Component Mole fraction in feed K-value Normal boiling point (°C) 

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. 

---

In summary, the BVDM or residue curve plots can be used to conceptualize feasible distillation sequences. It is particularly useful for azeotropic systems involving three conponents, either when there are three components in the feed or when an entrainer is added to break a binary azeotrope. This method has only been introduced in this section, so care must be taken when applying this method without additional reading. Reference [171 is su ested for further reading. 

An ideal mixture of n components requires a sequence of n - 1 conventional distillation columns (two product streams) to separate the components completely. The columns can be arranged sequentially without recycle between them. This picture changes when mixtures forming azeotropes must be separated. Nonideal systems sometimes require complex distillation arrangements involving more than n - 1 columns with recycle of material between the columns. For the analysis of such systems, we recommend the use of residue curve maps. We base the following summary on the excellent book by Doherty and Malone (1998), who pioneered the use of these techniques. 

Distillation columns and complexes entering into this sequence differ by splits and kinds of complexes. The best sequence is characterized by the smallest summary expenditures on separation (energy expenditures and capital costs, taking into consideration their payback period). 

To choose the entrainer among a number of alternative entrainers, it is necessary to carry out comparative estimation of expenditures on separation. For preliminary estimation at extractive distillation, the value of minimum flow rate of entrainer can be used and, at the sequence in Fig. 8.22b, the length of possible product composition segment at the side of the concentration triangle can be used. For more precise estimation, it is necessary to calculate the summary vapor flow in the columns in the mode of minimum reflux at several values of excess factor at the flow rate of entrainer. 

---