Appropriate Placement of Distillation

Rgure 14.1 The appropriate placement of distillation columns. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)... 

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 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 appropriate placement of distillation columns when heat integrated is not across the heat recovery 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... 

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. 

If an inappropriately placed distillation column is shifted above the heat recovery pinch by changing its pressure, the condensing stream, which is a hot stream, is shifted from below to above the pinch. The reboiling stream, which is a cold stream, stays above the pinch. If the inappropriately placed distillation column is shifted below the pinch, then the reboiling stream, which is a cold stream, is shifted from above to below the pinch. The condensing stream stays below the pinch. Thus appropriate placement is a particular case of shifting streams across the pinch, which in turn is a particular case of the plus-minus principle (see Chapter 19). 

To ensure energy saving, the position of unit operations with respect to Pinch must respect the rules of the Appropriate Placement. For example, a heat engine has to be placed either above or below the Pinch, but not across it. Distillation columns should not be placed across the Pinch. Contrary, a heat pump is optimally placed across the Pinch. This subject will be discussed in more detail in the Chapter 11. 

The improvement of design can be realised by Appropriate Placement and Plus/Minus principle. Appropriate Placement defines the optimal location of individual imits against the Pinch. It applies to heat engines, heat pumps, distillation columns, evaporators, furnaces, and to any other unit operation that can be represented in terms of heat sources and sinks. 

Figure 11.9 Representation of a simple distillation column Appropriate Placement...

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