23-Chem-A5 Chemical Plant Design and Economics · May 2013
Question 4 of 7: Process Selection
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exams — May 2013 — 04-Chem-A5 Chemical Plant Design and Economics. Three-hour, open-book exam; any non-communicating calculator permitted. The paper poses seven equally weighted essay questions and the candidate answers any five; only five are marked. All seven are answered below for completeness. These are conceptual design-and-economics questions — the solutions are written as organised prose (clarity and organisation are explicitly marked). The one numerical illustration (a Canadian Capital Cost Allowance schedule in Q2) is worked from stated assumptions.
Reference texts: M.S. Peters, K.D. Timmerhaus & R.E. West, Plant Design and Economics for Chemical Engineers (5th ed., McGraw-Hill) — the exam's named primary text (cost estimation, profitability, depreciation, optimisation); R. Turton et al., Analysis, Synthesis, and Design of Chemical Processes (4th ed., Prentice Hall) — process synthesis, safety, and economics; W.D. Seider et al., Product and Process Design Principles (3rd ed., Wiley) — separation-train synthesis and heuristics; supporting Canadian tax practice from the Canada Revenue Agency Capital Cost Allowance classes and the half-year rule.
At the synthesis stage the engineer is choosing among fundamentally different routes and flowsheets to the same product, and fixed-capital investment is only one axis of comparison. A sound comparison weighs the following additional factors:
Operating cost and raw-material economy. Over a plant's life the operating cost — dominated by feedstock consumption, reaction yield and selectivity, catalyst and solvent make-up, and utilities — usually outweighs capital. A route with higher capital but markedly better atom economy or milder utilities can be far cheaper overall. The economic potential (product value minus raw-material cost) is the first screen in Douglas's hierarchical synthesis.
Safety and hazard. The inherent hazard of each route — toxicity and flammability of the chemicals handled, and the severity of operating pressure and temperature. Inherently safer routes (lower inventory of hazardous intermediates, milder conditions) are preferred even at a cost premium.
Environmental impact and sustainability. Quantity and toxicity of emissions, effluent and solid waste; the cost and feasibility of treating them to meet regulatory limits; energy intensity and carbon footprint. Waste-minimising routes reduce both liability and treatment cost.
Reliability, operability and flexibility. How steadily the process runs, how tolerant it is of feed and rate variation, and its turndown range — a cheaper plant that runs erratically is a poor investment.
Product quality and purity achievable, and the ease of meeting the required specification.
Technical maturity / risk. Proven, licensable technology versus a novel route that promises lower cost but carries scale-up and performance risk.
Feedstock availability and flexibility — security and price stability of the raw-material supply, and the ability to switch feeds.
Controllability and process complexity — number of unit operations and recycles, and how readily the flowsheet can be held at its optimum.
By-product value and marketability, plot-area footprint, and the scope for heat integration.
In practice these factors are combined with capital cost either qualitatively (a weighted decision matrix screening alternatives) or, once the field is narrowed, quantitatively through a full after-tax discounted-cash-flow comparison that folds operating cost, capital, and the cost of environmental compliance into a single measure such as NPV. The best route is rarely the one with the lowest capital cost alone.
