23-Chem-A5 Chemical Plant Design and Economics · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2018 — 16-Chem-A5 Chemical Plant Design and Economics. Three-hour, closed-book exam; one two-sided aid sheet and an approved Sharp/Casio calculator permitted. Six equally weighted (20-mark) questions are posed and the candidate answers any five; only the first five are marked. All six are answered below for completeness. Question 1 is a process-synthesis flowsheet (catalytic propane dehydrogenation), Question 2 is a numerical discounted-cash-flow (DCFROR) minimum-selling-price calculation, and Questions 3–6 are qualitative design/economics essays (pilot-plant investigation, technical design factors, economic design factors, and a solvent-emission abatement scheme).
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 (pilot plants Ch. 3, general design considerations and plant-location factors Ch. 2–3, interest/depreciation/profitability Ch. 7–10); R.K. Sinnott & G. Towler, Chemical Engineering Design (Coulson & Richardson vol. 6) — economic analysis, cash-flow/DCFROR and flowsheeting; R. Smith, Chemical Process Design and Integration (2nd ed., Wiley) — reaction–separation–recycle structure; supporting Canadian practice from CCOHS, provincial OH&S regulation and Environment and Climate Change Canada air-emission guidance.
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.
Given. A reaction–separation–recycle process: fresh propane (1000 kg/h) plus a propane recycle are preheated to 670 °C, dehydrogenated over a packed catalyst bed, the 800 °C effluent (C3H8, C3H6, CH4, H2) is cooled to 110 °C, the C3 species are recovered by oil absorption/stripping, and a distillation column splits 98% propylene product from a 97% propane recycle.
Find. (a) A labelled block flowsheet of the whole plant, and (b) a statement of the process objective plus the function of each unit.
This is a classic reaction–separation–recycle synthesis problem: identify the reactor, the light-gas/heavy-gas separations, and the recycle of unconverted reactant, then describe why each unit exists. The flowsheet follows the process description unit by unit.
The figure above is the block flowsheet. Fresh propane (1000 kg/h) mixes with the 97%-propane recycle and enters the preheater, which raises the combined feed to 670 °C. The hot feed passes to the packed-bed reactor, whose 800 °C effluent (propane, propylene, methane and hydrogen) is cooled to 110 °C in the cooler. The cooled gas enters the oil absorber, where propane and propylene dissolve in a lean absorber oil while the light non-condensables (CH4 and H2) leave overhead as off-gas. The rich oil flows to the stripper, which is heated to drive the dissolved C3 gases back out; the regenerated lean oil is recycled to the absorber. The stripped C3 gas is raised in pressure by the compressor and fed to the distillation column (C3 splitter), which delivers a 98%-propylene product overhead and a 97%-propane bottoms that is recycled to the preheater.
Overall objective. To manufacture high-purity (98%) propylene from propane by catalytic dehydrogenation, recovering and recycling the unconverted propane so that the propane is converted essentially "to extinction" and raw-material use is minimised. Because a single pass through the equilibrium-limited reactor converts only part of the propane, the entire separation train exists to recover product, reject light by-products, and return unreacted propane to the reactor.
| Unit | Function |
|---|---|
| Preheater | Raises the combined (fresh + recycle) propane feed to the 670 °C reactor-inlet temperature; supplies the sensible heat needed before the strongly endothermic dehydrogenation. |
| Packed-bed reactor | Carries out the catalytic dehydrogenation C3H8 ⇌ C3H6 + H2 (endothermic, equilibrium-limited); partial per-pass conversion yields an effluent of propylene, unconverted propane, plus methane and hydrogen from cracking side-reactions. |
| Cooler | Cools the 800 °C effluent to 110 °C, quenching the reaction and lowering the temperature so the C3 species can be absorbed into the oil in the next unit. |
| Oil absorber | Contacts the gas with lean oil so propane and propylene dissolve, while the light gases methane and hydrogen (which are far less soluble) leave overhead — the easy bulk separation of C3s from light non-condensables. |
| Stripper | Heats the rich oil to release (desorb) the dissolved C3 gases, regenerating lean oil that returns to the absorber; separates the valuable C3 stream from the recirculating solvent. |
| Compressor | Raises the pressure of the stripped C3 gas to the level required for economical distillation (higher pressure raises the condensing temperature so cooling water can be used). |
| Distillation column (C3 splitter) | Performs the difficult close-boiling propane/propylene split: 98%-propylene product overhead and 97%-propane bottoms, which is recycled to the preheater. |
