23-Chem-A5 Chemical Plant Design and Economics · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Every separation exploits some difference in a physical or chemical property between the species to be separated. Three of the most common processes are discussed below.
1. Distillation. The refining workhorse, distillation separates on the basis of relative volatility — the difference in boiling point (vapour pressure) between components. Repeated partial vaporisation and condensation on trays or packing enriches the more volatile species in the vapour and the less volatile in the liquid; a crude tower fractionates petroleum into naphtha, kerosene, gas-oil and residue in exactly this way. It uses an energy-separating agent (heat) and needs no added chemical, which is why it is the default choice — but it becomes uneconomic when the relative volatility approaches unity and impossible at an azeotrope.
2. Absorption (and stripping). A gas-liquid contacting operation that separates on the basis of differential solubility: a liquid solvent selectively dissolves one component out of a gas stream (absorption), and the reverse operation recovers it (stripping). An amine unit removing H2S and CO2 from refinery gas is a classic example. Absorption uses a mass-separating agent (the solvent), so its economics hinge on solvent selectivity and the cost of regenerating and recycling that solvent.
3. Liquid–liquid extraction. Here separation rests on the difference in distribution (partition) between two immiscible liquid phases: a solvent preferentially extracts the target solute from the feed liquid. Extraction is chosen when distillation is unattractive — close-boiling mixtures, azeotropes, or heat-sensitive materials — such as recovering aromatics from a reformate with a selective solvent. Like absorption it needs a mass-separating agent and a downstream step (often distillation) to recover the solvent.
(Adsorption onto a solid — molecular sieves drying a gas, or a sieve separating linear from branched paraffins — is a fourth common process, exploiting differential affinity for a solid surface, used especially for trace removal and for separations distillation cannot make.)
Main criteria for selecting one over another. The selection turns on:
The governing heuristic is: use distillation (energy-separating agent) unless the relative volatility is too low, an azeotrope intervenes, or the material is heat-sensitive — in which case a mass-separating-agent process (absorption, extraction, adsorption) is selected and the added cost of regenerating that agent is accepted.
