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Classically, process optimization evaluates the trade-off of capital expenditure against reduced operating expenses. The standard approach involves an incremental analysis using a discounted cash-flow technique, such as net present value (NPV) or discounted cash-flow rate of return (DCFROR). However, this method is not applicable in all situations.
For example, when determining an optimum cleaning schedule for heat exchangers, one must factor in that a one-time expenditure can lead to both a reduction in expenses and a change in the life of the project. This is also a good example of an optimization where expenses vary with time (unsteady state). In this case, incremental analysis is still required, but the discounted cash...