Heat integration distillation

No attempt should be made to optimize pressure, reflux ratio, or feed condition of distillation in the early stages of design. The optimal values almost certainly will change later once heat integration with the overall process is considered. 

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... 

Heat Integration of Sequences of Simple Distillation Columns... 

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... 

Stephanopoulos, G., Linnhoff, B., and Sophos, A., Synthesis of Heat Integrated Distillation Sequences, IChemE Symp. Ser., 74 111, 1982. 

Morari, M., and Faith, D. C., The Synthesis of Distillation Trains with Heat Integration, AlChEJ, 26 916, 1980. 

Kakhu, A. L, and Flower, J. R., Synthesi ng Heat-Integrated Distillation Systems Using Mixed Integer Programming, Trans. IChemE ChERD, 66 241, 1988. 

Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

Evolving the Design of Simpie Distillation Columns to Improve Heat Integration... 

The following steps are necessary in establishing a heat integrated distillation sequence... 

Establish the heat integration potential of simple columns. Introduce heat recovery between reboilers, intermediate reboilers, condensers, intermediate condensers, and other process streams. Shift the distillation column pressures to allow integration, where possible, using the grand composite curve to assess the heat integration potential. 

As pointed out in Chap. 5, replacing simple columns by complex columns tends to reduce the vapor (and heat) load but requires more of the heat to be added or removed at extreme levels. This means that the introduction of complex columns in the design might prejudice heat integration opportunities. Thus the introduction of complex distillation arrangements needs to be considered simultaneously with the heat integration. This can be carried out manually with some trial and error or using an automated procedure such as that of Kakhu and Flower. ... 

The appropriate placement of distillation columns when heat integrated is not across the pinch. The grand composite curve can be used as a quantitative tool to assess integration opportunities. 

Once the distillation is integrated, then driving forces between the composite curves become smaller. This in turn means the capital/energy tradeofiF for the heat exchanger network should be adjusted accordingly. 

LinnhoflF, B., Dunford, H., and Smith, R., Heat Integration of Distillation Columns into Overall Processes, Chem. Eng. Sci., 38 1175, 1983. 

Andrecovich, M. J., and Westerberg, A. W., A Simple S3mthesis Method Based on Utility Bounding for Heat-Integrated Distillation Sequences, A/CAE 7, 31(3) 363, 1985. 

The concept of the appropriate placement of distillation columns was developed in the preceding chapter. The principle also clearly applies to evaporators. The heat integration characteristics of distillation columns and evaporators are very similar. Thus evaporator placement should be not across the pinch. ... 

The task of process synthesis is to evaluate these schemes and select the best. The creative possibilities of design do not stop here. For example, these separations can be carried out by heat integration, by multiple-draw columns, or by use of processes other than distillation many other variations are possible. 

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