Chemical Engineering Practice (Plant Design, Economics, Safety, Instrumentation)
Core concepts
This domain blends design methodology, cost estimation and process safety. Know the design hierarchy (batch vs continuous, flowsheet synthesis, heat integration, pinch analysis), and cost estimation levels (order-of-magnitude, study, preliminary, definitive). Master capital cost components (fixed and working capital), the six-tenths rule for scaling, and depreciation methods (straight-line, declining balance). Profitability measures include payback, ROI, NPV and IRR; understand time value of money and discounting. Safety: HAZOP, fault tree and event tree analysis, layers of protection, LOPA, and inherent safety principles (minimise, substitute, moderate, simplify). Instrumentation: sensors, transmitters, controllers (P, PI, PID), control valves, and process control loops; understand dead time, gain and tuning. Also cover relief systems, flammability limits, and material safety data.
Traps
Do not confuse fixed capital with total capital investment (which includes working capital). The six-tenths rule uses a cost ratio raised to ~0.6, not linear scaling. In HAZOP, guide words generate deviations, not solutions. Distinguish alarm from interlock and relief device. PID integral action removes offset; derivative reduces overshoot but amplifies noise.
Worked rule of thumb
Scaling cost: C2 = C1·(Q2/Q1)^0.6. Doubling capacity raises cost by 2^0.6 ≈ 1.52, i.e., about 52%, illustrating economy of scale. For profitability, NPV > 0 at the chosen discount rate means the project is acceptable.
Exam strategy
Show the cash-flow timeline, identify the correct cost basis, and state safety-analysis method names precisely.