In process systems engineering, mathematical models with different economic objective functions, e.g., total cost, profit, net present value, and the internal rate of return, are frequently used for process flow sheet optimization. In financial theory, however, the net present value is the only correct criterion when selecting from among mutually exclusive alternatives, while the others are not totally correct. This paper shows that only those process models with sufficient levels of accuracy are suitable for generating proper optimal designs using the correct economic criterion. An analysis method is proposed for testing the accuracy and adequacy of process flow sheet optimization models, in terms of appropriate trade-offs between invested funds and generated cash flows. This method analyzes the cash flow function vs investment, its derivative, and the differences between optimum solutions obtained by different economic criteria. Suitable process models generate concave monotonically increasing cash flow functions and flat derivative curves, while optimal designs obtained by different economic criteria are substantially different.