Process Design Tutoring

Senior design feels different from every earlier course. The problem is open-ended, the work is done in a team, and nothing in the back of the book tells you if the flowsheet is right. That freedom is what makes it hard to start. STEM Tutor 360 helps you break the project into steps you can check: define the basis, choose a process path, draw the flow diagram with material and energy balances, size the equipment, estimate cost, and judge the economics and safety.

Sessions cover design heuristics, equipment sizing for pumps, vessels, heat exchangers, and columns, and the thinking behind a process simulation in Aspen Plus, Aspen HYSYS, or a similar program: choosing a property method, getting a flowsheet to converge, and checking results by hand. We also cover cost estimation, NPV and payback, HAZOP, and writing the report. Your team does the design. We help you reason through it and check the numbers.

A typical process economics problem, worked

A plant addition costs $2.0 million to build in year 0. It returns a net cash flow after tax of $600,000 per year for 5 years. The company requires a 10% return. Is it worth building?

Discount the cash flows back to year 0. The annuity factor for 5 years at 10% is $\dfrac{1 - 1.10^{-5}}{0.10} = 3.7908$, so the five payments are worth $600{,}000 \times 3.7908 = \$2{,}274{,}472$ today. $$\text{NPV} = -\$2{,}000{,}000 + \$2{,}274{,}472 = \$274{,}472$$

A positive NPV means the project earns more than the required 10%. Simple payback is $2.0 / 0.6 = 3.3\ \text{years}$, but it ignores the time value of money. Discounting the cash flows, payback takes about 4.3 years. The IRR, the discount rate that makes NPV zero, is 15.2%.

One number is never enough. If capital cost runs 20% over, to $2.4 million, the NPV becomes -$125,528 and the answer flips. That is why reports include a sensitivity analysis.

Where students usually get stuck

• Trusting simulator output without a hand check. A wrong property method is a common cause of results that look reasonable but are not.
• Using old equipment prices without updating them with a cost index such as CEPCI, or scaling with the wrong exponent.
• Mixing before-tax and after-tax cash flows, or leaving out working capital, depreciation, and taxes.
• Leaving safety and the HAZOP for the last week instead of building them into the design.
• Reports that list results without the reasoning, such as the alternatives considered and why one was chosen.