23-Chem-A5 Chemical Plant Design and Economics · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2013 — 04-Chem-A5 Chemical Plant Design and Economics. Three-hour, open-book exam; any non-communicating calculator permitted. Six equally weighted questions are posed and the candidate answers any five; only the first five are marked. All six are answered below for completeness. Questions 1, 3 and 6 are conceptual design / management questions answered as organised prose; questions 2, 4 and 5 contain the numerical work (production capacity and pricing, simple- and compound-interest loan accounting, and sinking-fund depreciation) and every boxed figure.
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, interest and investment, depreciation, profitability, process synthesis, and plant safety); R. Turton et al., Analysis, Synthesis, and Design of Chemical Processes (4th ed., Prentice Hall) — flowsheet synthesis, separation selection, and safety; W.D. Seider et al., Product and Process Design Principles (3rd ed., Wiley) — separation-train synthesis; 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.
The condensed stream is a binary mixture of two mutually miscible liquid products that differ in volatility (boiling point). The natural and lowest-cost separation is therefore fractional distillation. The condensed mixture is fed to a distillation column operating between the two products' boiling points: the more volatile product is enriched up the column and taken overhead as the distillate (condensed, with part returned as reflux to set purity), while the less volatile product is concentrated down the column and withdrawn as the bottoms product by way of a reboiler. A single column with one feed and two draws separates a binary mixture into two reasonably pure products; the reflux ratio and number of stages are set by the required purities and the relative volatility.
Before the column, the condenser on the reactor effluent also acts as a phase separator: because the products leave the reactor as a vapour, condensing them drops the liquid products out while any unreacted gas (and inerts) remains in the vapour and is taken off the top of a knock-out drum for recycle or venting. The solid metal-oxide catalyst stays inside the well-mixed reactor (or is retained on a filter/settling leg), so it does not enter the separation train. Two engineering caveats are worth stating: if the two products are close-boiling, form an azeotrope, or are heat-sensitive, ordinary distillation becomes uneconomic or infeasible and an alternative is used — extractive or azeotropic distillation, or liquid–liquid extraction with a selective solvent followed by solvent recovery.
Every unit named in part (i) appears on the sheet together with the auxiliaries a working plant needs — the gas compressor and feed pump that raise the feeds to reactor pressure, the feed preheater, the unreacted-gas recycle with its purge (which stops inerts accumulating in the loop), the column's overhead condenser, reflux drum and reboiler, and the two product coolers that bring the products down to storage temperature. Each is itemised with its design data in part (iii).
Each unit is listed with the process information an engineer must fix before it can be sized and specified.
| Equipment | Information needed to design it |
|---|---|
| Feed compressor / pump | Gas and liquid flow rates, suction and discharge pressures, inlet temperature, fluid physical properties (density, viscosity), required head and driver power. |
| Catalytic reactor (well-mixed) | Reaction stoichiometry and kinetics (rate, activation energy), required conversion, operating T and P, heat of reaction and duty, residence time/volume, catalyst loading and geometry, materials of construction for high T/P. |
| Condenser (effluent) | Vapour flow and composition, dew point and condensing curve, coolant type and temperature, heat duty, allowable pressure drop, fouling and materials. |
| Knock-out / reflux drum | Vapour–liquid flow rates, vapour density and allowable velocity (for disengagement), holdup/residence time, operating P and T. |
| Distillation column | Feed rate, composition and thermal state; product purity specifications; relative volatility / VLE data; reflux ratio; number of theoretical stages and efficiency; column diameter (vapour load) and height; operating P and T. |
| Reboiler | Bottoms flow and composition, boil-up rate, heat duty, heating-medium temperature (steam pressure), heat-transfer area, materials. |
| Overhead condenser & product coolers | Overhead/product flow and composition, condensing/cooling curve, coolant temperature, heat duty, area, pressure drop. |
