Heat-integrated distillation column

The separation of acetonitrile from acetonitrile by extractive distillation with water can be done in a more efficient two-column heat integrated setup. The separation of acrylonitrile from water, which is hindered by the existence of an azeotrope, can actually take advantage of the large immiscibility gap. Valuable byproducts, such as HCN and acetonitrile can be efficiently separated. Chemical conversion can solve the separation of difficult impurities, such as acroleine. 

In Design 4, the feed was sent to the low-pressure column, which produced a pure low-boiler distillate but a mixed underflow (again, a sloppy separation decreasing the temperature difference across the low-pressure column). The mixed bottoms was then completely separated in the high-pressure column. The condenser of the high-pressure column was the reboiler of the low-pressure column (heat integration in the opposite direction as flow). 

Sketch a control system for the two-column heat-integrated distillation system shown. 

The pressure-swing distillation system is well suited for heat integration because the temperatures in the high-pressure column are much higher than those in the low-pressure column. Therefore, it is often possible to use the hot vapor from the top of the high-pressure column to provide heat in the reboiler of the low-pressure column. Heat-integrated pressureswing systems are discussed in Chapter 6. 

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... 

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... 

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. 

The scope for integrating conventional distillation columns into an overall process is often limited. Practical constraints often prevent integration of columns with the rest of the process. If the column cannot be integrated with the rest of the process, or if the potential for integration is limited by the heat flows in the background process, then attention must be turned back to the distillation operation itself and complex arrangements considered. 

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

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. ... 

It was noted earlier that dryers are quite difierent in character from both distillation and evaporation. However, heat is still taken in at a high temperature to be rejected in the dryer exhaust. The appropriate placement principle as applied to distillation columns and evaporators also applies to dryers. The plus/minus principle from Chap. 12 provides a general tool that can be used to understand the integration of dryers in the overall process context. If the designer has the freedom to manipulate drying temperature and gas flow rates, then these can be changed in accordance with the plus/minus principle in order to reduce overall utility costs. 

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