Plus/minus principle

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. 

Dryers are different in characteristic from distillation columns and evaporators in that the heat is added and rejected over a large range of temperature. Changes to dryer design can be directed by the plus/minus principle. 

Figure 19.1 The plus-minus principle guides process changes to reduce utility consumption. (From Smith R and Linnhoff B, 1988, Tram IChemE ChERD, 66 195 reproduced by permission of the Institution of Chemical Engineers.)...

Soft constraints should exploit the plus-minus principle to improve the targets. 

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). 

There are two theoretical principles to consider in bringing conceptual changes with energy saving impact Appropriate Placement and Plus/Minus Principle (Linnhoff et al., 1982, 1994). The first ensures an optimal use of energy provided that the placement of the unit operations in with respect to Pinch respects some rules. Chapter 11 will... 

Plus/Minus principle states that a reduction in the utility requirements can be obtained if the following actions are taken (Smith, 1995) ... 

Figure 10.41 The use of Plus/Minus principle for energy saving...

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. 

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