Capital Cost Considerations

Traditionally, in the bulk chemical industries the capital cost of a plant may be scaled using the exponential costing approach  

The design changes suggested so far for distillation columns have been motivated by the incentive to reduce energy costs by more effective integration between the distillation column and the rest of the process. There are, however, 

Consequently, the optimum reflux ratio for an appropriately integrated distillation column will be problem specific and is likely to be quite different from that of a stand-alone column operated from utilities. 

This approach to the overall problem breaks down the design procedure into two steps of first determining the best nonintegrated sequence and then heat integrating. This assumes that the two problems of distillation sequencing 

It is important to emphasize that, when considering distillation sequences, the focus should not be exclusively on the single sequence with the lowest overall cost. Rather, because there is often little difference in cost between the best few sequences, and because of the uncertainties in the calculations and the fact that other factors need to be considered in a more detailed evaluation, the best few should be evaluated in more detail rather than just one. 

It thus appears that the flowrate of the nonkeys may account for the differences between sequences. Essentially, nonkey components have two effects on a separation. They cause8  

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. 

however, the column is appropriately integrated, then the reflux ratio often can be increased without changing the overall energy 

If complex distillation columns are being considered, then these also can bring about significant reductions in capital cost. The dividing-wall column shown in Fig. 5.17 not only requires typically 20 to 30 percent less energy than a conventional arrangement but also can be typically 30 percent lower in capital cost than a conventional two-column arrangement.  

The composite curves for this flowsheet are shown in Fig. 14.86. The composite curves are dominated by the reboilers and condensers of the two distillation columns and the feed vaporizer for the acetone feed. It is immediately apparent that the two distillation columns are both inappropriately placed across the pinch. Linnhoflf and Parker  

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Thus capital cost considerations reinforce the argument that the nonintegrated sequence with the lowest heat load is that with the lowest total cost. 

If nonlinear capital costs considerations are left out of the formulation, then the problem can be formulated as an MILP problem (see Chapter 3). Once nonlinear costs are included, the problem becomes an MINLP, with all of the problems associated with nonlinear optimizations. As with many such problems, if the required formulation turns out to be an MINLP, then an MILP can be used to provide a good initialization for the MINLP, simplifying its solution. 

The maximum turn-down ratio for an operating system is three-to-one. Operation in an on-off mode can also be employed to effectively increase this ratio. Another approach is the use of modular units. However, this latter option will increase capital costs considerably. 

Today the internal combustion engine reigns supreme Virtually all road transport, sea transport and aviation are dependent upon it. Only in the field of railways is there a viable alternative (electric traction) and even there electrification is by no means complete because of capital cost considerations. The overwhelming predominance of the internal combustion engine (including gas turbine in aviation) is associated with a total dependence on petroleum-derived fuels for our transport requirements. 

Required reliability Environmental constraints Capital cost considerations... 

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