23-Chem-A5 Chemical Plant Design and Economics · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams / EGBC — May 2019 — 16-Chem-A5 Chemical Plant Design and Economics. Three-hour closed-book examination; one aid sheet (both sides) and an approved calculator are permitted. Six questions are printed and any five constitute a complete paper (each worth 20 marks); all six are solved below for completeness. Three questions carry numbers (Q1 route economics, Q3 production cost, Q4 depreciation); the other three (Q2 supercritical extraction, Q5 the design hierarchy intrinsic to a chemical process, Q6 VOC-abatement P&ID) are answered as structured description with a supporting diagram where the paper asks for one.
Reference texts: M. S. Peters, K. D. Timmerhaus & R. E. West, Plant Design and Economics for Chemical Engineers (5th ed., McGraw-Hill) — total-product-cost anatomy, straight-line depreciation, after-tax cash flow, profitability; R. Turton, R. C. Bailie, W. B. Whiting & J. A. Shaeiwitz, Analysis, Synthesis, and Design of Chemical Processes (4th ed., Prentice Hall) — the economic-potential screen, reaction-path selection and the process flow diagram; J. M. Douglas, Conceptual Design of Chemical Processes (McGraw-Hill) — the level-2 economic-potential hierarchy and the balanced vinyl-chloride process; G. Towler & R. Sinnott, Chemical Engineering Design (Coulson & Richardson Vol. 6, 2nd ed.) — utilities, VOC control and product recovery; R. H. Perry & D. W. Green, Perry’s Chemical Engineers’ Handbook (9th ed.) — supercritical-fluid extraction. Depreciation is worked in the U.S. MACRS/straight-line framework the question specifies; the Canadian CCA declining-balance analogue is noted where relevant.
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.
Design basis. The stream is nitrogen saturated with both water vapour and acetone at 75–90 °C and ~1 atm; acetone is flammable and fully water-miscible. The target is <200 ppmv acetone in the vent, with the acetone recovered for reuse and the inert nitrogen recycled to the dryer (a closed-loop inert dryer avoids venting flammable vapour to air). Two consequences of the printed basis shape the scheme. First, the inlet is a band, 75–90 °C: saturated loading rises steeply with temperature, so the condenser and the absorber are sized on 90 °C and the analyser, not a fixed duty, is what guarantees the vent specification across the band. Second, the co-present water vapour is an asset rather than a nuisance, because acetone and water are completely miscible — the chilled condensate and the rich scrubbing water are the same kind of stream and go to one acetone/water stripping column, whose bottoms water returns to the absorber. Full water-miscibility therefore makes a water absorber the natural bulk-removal step, with a carbon guard bed as the final polish to hit the ppm limit.
Process description. The three-stage recovery train works down the concentration in series. (1) Chilled condenser — cooling the saturated nitrogen below the acetone dew point condenses the bulk of the solvent as liquid, together with most of the water vapour it carries; this is the cheapest removal because it needs only refrigeration. (2) Water absorber (scrubber) — the partially stripped gas is contacted counter-currently with water, which readily dissolves the residual acetone because the two are completely miscible, taking the vapour below the 200 ppm limit. (3) Activated-carbon guard bed — a final adsorption polish guarantees the vent specification and absorbs any upset; two beds allow regeneration in swing. The clean nitrogen is then recycled to the dryer. The condensate and the rich absorber water are combined and sent to an acetone stripper (distillation), which recovers solvent-grade acetone overhead for reuse and returns clean water to the absorber.
Instrumentation (P&ID control loops). A temperature indicating controller (TIC) on the condenser manipulates the refrigerant flow to hold the outlet below the dew point; a flow indicating controller (FIC) sets the absorber water rate in ratio to the gas flow; a level indicating controller (LIC) on the absorber sump and on the stripper reboiler/receiver manages inventory; the stripper reboiler carries a TIC on steam; and, critically, an acetone analyser (AIT) on the vent continuously measures ppm and alarms / diverts flow if the 200 ppm limit is approached. Because acetone is flammable, the nitrogen loop is kept oxygen-lean and the vent is interlocked to the analyser — an inherently safer, closed-loop design.
