Core chemical engineering questions
"What is a mass balance and how do you apply it?" Conservation of mass: mass in = mass out + accumulation. Steady-state: accumulation = 0, so mass in = mass out. Batch: mass in + generated = mass out + consumed + accumulated. Used to size equipment, determine recycle ratios, check process consistency, identify losses. Energy balances apply conservation of energy — used for heat exchanger design, reactor heating/cooling, column reboiler and condenser duties. "Explain the difference between batch, semi-batch, and continuous processes." Batch: all reactants charged, product removed at end — flexible, used for specialty chemicals and pharma. Continuous: steady-state feed and product removal — efficient at scale for commodity chemicals. Semi-batch: one reactant charged, another fed over time — controls selectivity or manages exothermic reaction safety.
Process design and equipment questions
"How does a distillation column work and what determines the number of theoretical stages?" Components separated by relative volatility: vapour rises, liquid falls in countercurrent contact on trays or packing, reaching equilibrium at each stage. Reboiler provides vapour at bottom; condenser provides liquid reflux at top. Number of stages determined by McCabe-Thiele or Fenske-Underwood-Gilliland methods. Key variables: feed composition, product specifications, relative volatility, reflux ratio. Fewer stages at higher reflux (more energy); more stages at lower reflux (more capital). "What is HAZOP?" Hazard and Operability Study: a structured risk assessment using guide words (No, More, Less, As Well As, Reverse, Other Than) applied to process parameters to identify deviations, causes, consequences, safeguards, and recommendations. Required for new plant design and significant modifications.
Pharmaceutical chemical engineering questions
Pharma chemical engineers work under GMP (Good Manufacturing Practice): processes must be validated, equipment qualified (IQ/OQ/PQ), changes managed through formal change control. "What is process validation?" Demonstrates a manufacturing process consistently produces product meeting its specifications. ICH Q8/Q9/Q10 framework: design space (proven acceptable ranges), control strategy (how process is monitored), continued process verification (ongoing monitoring post-validation). Without validated processes, regulators will not approve a product for commercial manufacture.
Behavioral questions
"Describe a time you identified a safety issue on a process plant or in the lab." Show: you spotted the hazard, took appropriate immediate action, reported through the correct channel (near miss system), and contributed to investigation and corrective action. "Tell me about a complex process problem you solved." Systematic approach: define the problem (symptom vs root cause), gather data (trends, lab analysis, P&ID review), generate hypotheses, test, implement, verify, document. Use technical detail — vague answers signal the candidate was not directly involved.