Optimization capital/energy tradeoff

Figure 6.12 The optimal setting of the capital/energy tradeoff for threshold problems.

Distillation capital costs. The classic optimization in distillation is to tradeoff capital cost of the column against energy cost for the distillation, as shown in Fig. 3.7. This wpuld be carried out with distillation columns operating on utilities and not integrated with the rest of the process. Typically, the optimal ratio of actual to minimum reflux ratio lies in the range 1.05 to 1.1. Practical considerations often prevent a ratio of less than 1.1 being used, as discussed in Chap. 3. 

The U.S. chemical industry achieved an annual reduction of 4.2% in energy input per unit of output for the period 1975—1985 (2). This higher reduction resulted from cost optimization, the tradeoff of increased capital for reduced energy use, that was driven by energy prices (4). In contrast, from 1985 to 1990, the energy input per unit of output has been almost flat (2) as a consequence of falling prices. The average price the U.S. chemical industry paid for natural gas fell by one-third between 1985 and 1988 (1,5). 

Cost optimization basically depends on the tradeoffs between the cost of energy (fuel) and capital investment, as seen in Figure 5.1. Fines for pollution and incentives for environment-friendly technologies may reduce the cost associated with exergy loss. 

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