Tricky Chemical Engineering interview questions

75 questions with answers (CHE-426 to CHE-500). Try each one aloud before reading the answer.

Material & Energy Balances

CHE-426 · Since mass is conserved in a reactor, are moles also conserved?

Trap: Assuming moles balance like mass. Total moles change whenever stoichiometric coefficients differ, as in N₂ + 3H₂ → 2NH₃ (4 mol → 2 mol). Atom balances and total mass are conserved, but total moles are not.

CHE-427 · Does adding recycle increase a reactor's single-pass conversion?

Trap: Confusing single-pass and overall conversion. Recycle raises overall conversion based on fresh feed. Single-pass conversion often falls, because recycle increases reactor throughput and dilutes reactant concentration.

CHE-428 · At steady state nothing changes, so can any reaction be occurring?

Trap: Equating steady state with no activity. Steady state means accumulation is zero. Reaction, generation and consumption continue at constant rates, balanced by inflows and outflows.

CHE-429 · If combustion is incomplete, is percent excess air based on the oxygen actually used?

Trap: Basing excess on actual consumption. Percent excess air is always based on the theoretical O₂ for complete combustion of all fuel to CO₂ and H₂O, regardless of how much actually burns.

CHE-430 · Does "adiabatic" mean the temperature stays constant?

Trap: Confusing adiabatic with isothermal. Adiabatic means no heat transfer across the boundary. Temperature can change sharply, as in adiabatic compression or an adiabatic reactor with exothermic reaction.

CHE-431 · Is a boiler efficiency of 95% always better than one of 90%?

Trap: Ignoring the heating value basis. Efficiency on an LHV basis is higher than on an HHV basis for the same boiler, by about 10% for natural gas. Comparisons are meaningful only on the same basis.

CHE-432 · Is a 50:50 mixture by mass of ethanol and water also 50:50 by moles?

Trap: Treating mass and mole fractions as equal. With molar masses of 46 and 18 g/mol, 50 g each gives 1.09 mol ethanol and 2.78 mol water, so ethanol is only about 28 mol%.

Chemical Engineering Thermodynamics

CHE-433 · Why does a pressure cooker cook food faster if heat input is the same?

Trap: Thinking pressure itself cooks the food. Higher pressure (about 1 bar gauge) raises water's boiling point to about 120 °C, and the higher temperature speeds cooking reactions.

CHE-434 · Solids are denser than liquids, so why does ice float?

Trap: Assuming every solid is denser than its liquid. Hydrogen bonding locks water molecules into an open hexagonal lattice, so ice (about 917 kg/m³) is less dense than liquid water (about 1000 kg/m³).

CHE-435 · Can entropy ever decrease?

Trap: Applying the second law to every system. Entropy of a system can decrease, as when a refrigerator cools its contents or water freezes. Only the total entropy of an isolated system (system plus surroundings) cannot decrease.

CHE-436 · Are heat and temperature the same thing?

Trap: Using the terms interchangeably. Temperature is a state property measuring thermal intensity. Heat is energy in transit caused by a temperature difference, not something a body "contains."

CHE-437 · Does water boil at a higher temperature at high altitude because the sun is closer?

Trap: Thinking altitude raises boiling temperature. Lower atmospheric pressure lowers the boiling point, to about 90 °C near 3000 m, so food cooks more slowly.

CHE-438 · Do molecules in an ideal solution have no intermolecular forces?

Trap: Confusing ideal solution with ideal gas. In an ideal solution, like and unlike interactions are equal, so ΔH_mix = 0 and ΔV_mix = 0. Forces exist, and entropy of mixing is still positive.

CHE-439 · Can a frictionless Carnot engine reach 100% efficiency?

Trap: Thinking only friction limits efficiency. Even a reversible engine has η = 1 − T_c/T_h, which would be 100% only with a cold reservoir at absolute zero, which is unattainable.

Fluid Mechanics & Mechanical Operations

CHE-440 · Does doubling pipe diameter halve the pressure drop at the same flow?

Trap: Assuming a linear relationship. At constant flow, ΔP ∝ 1/D⁵ in turbulent flow (about 32 times lower) and ∝ 1/D⁴ in laminar flow (16 times lower).

CHE-441 · Can a pump located above a well lift water 20 m by suction?

Trap: Thinking a pump "pulls" liquid. Atmospheric pressure pushes water up, limiting theoretical suction lift to about 10.3 m at sea level and much less in practice. Deep wells need submersible pumps.

CHE-442 · Does Bernoulli's equation say faster-moving fluid has higher pressure?

Trap: Reversing the relationship. Along a streamline, higher velocity means lower static pressure, which is the basis of venturi meters and aerofoil lift.

CHE-443 · Is pipe flow at Re = 3000 laminar or turbulent?

Trap: Expecting a single sharp threshold. Re between about 2100 and 4000 is transitional, so flow can switch between laminar and turbulent depending on disturbances, inlet conditions and roughness.

CHE-444 · Can you reduce a centrifugal pump's flow by throttling its suction valve?

Trap: Treating suction and discharge throttling as equivalent. Throttling suction lowers NPSHa and causes cavitation. Flow should be controlled with a discharge valve, a speed drive or a recycle.

CHE-445 · Can a positive displacement pump be run against a closed discharge valve like a centrifugal pump?

Trap: Assuming all pumps tolerate shut-off. A positive displacement pump keeps displacing fluid, so pressure rises until something ruptures or the motor stalls. It needs a pressure relief valve on the discharge.

CHE-446 · Is viscosity a fixed property of a fluid at a given temperature?

Trap: Assuming all fluids are Newtonian. For non-Newtonian fluids, apparent viscosity changes with shear rate (shear-thinning paint or ketchup, shear-thickening cornstarch slurry) and sometimes with time (thixotropy).

Heat Transfer

CHE-447 · Does adding insulation to a pipe always reduce heat loss?

Trap: Ignoring critical radius. For small pipes or wires with an outer radius below r_c = k/h, added insulation increases surface area faster than resistance and heat loss rises until the critical radius is exceeded.

CHE-448 · Can the cold fluid leave an exchanger hotter than the hot fluid's outlet?

Trap: Thinking it is always impossible. In counter-current flow, the cold outlet can exceed the hot outlet temperature (temperature cross). It is impossible only in co-current flow.

CHE-449 · Is a black body necessarily black in color?

Trap: Judging emissivity by visible color. A black body is an ideal absorber and emitter at all wavelengths. Snow and white paint look white but have high infrared emissivity, about 0.9 or higher.

CHE-450 · To raise U, should you increase the larger film coefficient?

Trap: Improving the wrong side. U is dominated by the largest resistance (smallest h). Raising an already high coefficient changes U very little; improve the controlling, low-h side instead, for example with fins on the gas side.

CHE-451 · Is a metal bar colder than a wooden block when both feel different at room temperature?

Trap: Trusting touch as a thermometer. Both are at the same temperature. Metal feels colder because its high conductivity draws heat away from the skin faster.

CHE-452 · Does boiling heat flux always increase with wall superheat?

Trap: Extrapolating nucleate boiling. Beyond the critical heat flux, a vapor film forms and flux falls in transition boiling (the Leidenfrost effect) before rising again in film boiling at very high wall temperatures, risking burnout.

Mass Transfer

CHE-453 · Can wet-bulb temperature ever exceed dry-bulb temperature?

Trap: Thinking evaporation can heat air. Evaporative cooling means wet-bulb ≤ dry-bulb. They are equal only at 100% relative humidity.

CHE-454 · Is diffusion faster in liquids because molecules are packed more closely?

Trap: Equating closeness with speed. Gas diffusivities are about 10⁻⁵ m²/s while liquid diffusivities are about 10⁻⁹ m²/s, roughly 10,000 times slower, because constant collisions in liquids hinder movement.

CHE-455 · Will doubling drying air temperature double the drying rate throughout drying?

Trap: Applying constant-rate behavior everywhere. Hotter air mainly increases the constant-rate period. In the falling-rate period, internal moisture diffusion controls, so air conditions have much less effect.

CHE-456 · Does heating air remove moisture from it?

Trap: Confusing relative with absolute humidity. Heating lowers relative humidity, increasing drying capacity, but absolute humidity is unchanged. Moisture is removed by cooling below the dew point or by desiccants.

CHE-457 · Is adsorption endothermic because adsorbents are regenerated by heating?

Trap: Inferring heat of adsorption from regeneration. Physical adsorption is exothermic, so capacity falls with temperature. That is why heating releases adsorbed molecules in TSA.

CHE-458 · Does raising temperature improve gas absorption into a liquid?

Trap: Assuming heat speeds everything up. Gas solubility generally falls with temperature (Henry's constant rises), so absorbers are cooled. Higher temperature helps stripping, not absorption, unless chemical reaction rates dominate.

Chemical Reaction Engineering

CHE-459 · Does a catalyst shift equilibrium toward products?

Trap: Thinking a catalyst changes equilibrium. It speeds up the forward and reverse reactions equally, so equilibrium is reached faster but the equilibrium composition and K do not change.

CHE-460 · Does higher temperature always give more product?

Trap: Ignoring equilibrium. Higher temperature increases rate, but for exothermic reactions it shifts equilibrium backward (Le Chatelier), lowering maximum conversion, as in ammonia and SO₃ synthesis.

CHE-461 · Is reaction order always equal to the stoichiometric coefficient?

Trap: Applying elementary-reaction logic to all reactions. Only elementary reactions follow stoichiometry. H₂ + Br₂ → 2HBr, for example, has a complex rate law with fractional order.

CHE-462 · Is a PFR always better than a CSTR?

Trap: Generalizing from positive-order kinetics. For zero order, volumes are equal. For autocatalytic reactions at moderate conversion, or when low reactant concentration improves selectivity or temperature control, a CSTR can be better.

CHE-463 · Is space time always equal to the mean residence time?

Trap: Ignoring density change. τ = V/v₀ uses inlet volumetric flow. For gas-phase reactions with volume change or temperature change, actual residence time differs from τ. They are equal only for constant-density systems.

CHE-464 · Do exothermic reactions start on their own because they release heat?

Trap: Confusing thermodynamics with kinetics. Exothermic reactions still need activation energy. A methane–air mixture is stable until ignited, even though combustion is strongly exothermic.

Separation Processes

CHE-465 · Can an azeotrope ever be broken?

Trap: Thinking azeotropes are absolute barriers. They can be broken by pressure-swing distillation (when azeotropic composition shifts with pressure), an entrainer in azeotropic or extractive distillation, adsorption (molecular sieves) or membranes such as pervaporation.

CHE-466 · Total reflux gives the best purity, so why not operate there?

Trap: Forgetting production. At total reflux, no distillate or bottoms are withdrawn, so no product is made. It is used only for start-up and testing.

CHE-467 · Does adding more trays always improve product purity?

Trap: Ignoring pinches and feed location. Near a pinch, extra trays give almost no benefit, and with a wrong feed location they can even worsen separation. Reflux, feed condition and stage placement matter as much as tray count.

CHE-468 · If design shows 20 theoretical stages, are 20 trays installed?

Trap: Confusing theoretical stages with real trays. Real trays do not reach equilibrium. With 70% overall efficiency, about 20/0.7 ≈ 29 trays are needed, less any stage provided by the reboiler.

CHE-469 · Is a relative volatility of 1 an easy separation?

Trap: Misreading the scale. α = 1 means vapor and liquid have the same composition, so distillation is impossible. Separation gets easier as α moves further from 1.

CHE-470 · Does raising column pressure improve distillation?

Trap: Assuming pressure helps separation. Higher pressure usually lowers relative volatility and raises reboiler temperature. Pressure is chosen to allow cooling water or air in the condenser, or to avoid refrigeration, not to aid separation.

Process Control

CHE-471 · Is a higher controller gain always better because it reduces offset?

Trap: Ignoring stability. Higher gain reduces offset and speeds response, but too much increases oscillation, amplifies noise and eventually causes instability, especially with dead time.

CHE-472 · Does adding integral action make a loop more stable?

Trap: Thinking integral just "fixes" offset for free. Integral action adds phase lag, making the loop more oscillatory and less stable. Gain often has to be reduced when integral is added.

CHE-473 · Can feedforward control eliminate all disturbances on its own?

Trap: Trusting a perfect model. Feedforward handles only measured disturbances and depends on model accuracy. Unmeasured disturbances and model errors need feedback trim, so the two are combined.

CHE-474 · Is a fail-safe valve always one that fails closed?

Trap: Equating safe with closed. The safe position depends on service: fuel valves fail closed, but cooling water to an exothermic reactor fails open and some pressure relief paths fail open.

CHE-475 · Should derivative action be used on flow loops?

Trap: Assuming PID is always better than PI. Flow measurements are fast and noisy, so derivative action amplifies noise and wears valves. Flow loops normally use PI control with low gain and fast reset.

CHE-476 · Can dead time be eliminated by tuning the controller harder?

Trap: Thinking tuning removes delay. Dead time is physical and limits achievable performance; aggressive tuning causes oscillation. Reduce it by relocating sensors or shortening lines, or compensate with a Smith predictor or MPC.

Process Equipment Design

CHE-477 · A vacuum vessel only sees 1 bar pressure difference, so can its walls be thin?

Trap: Treating external pressure like internal pressure. Vacuum vessels fail by buckling at stresses well below yield, so they often need thicker walls or stiffening rings than a vessel at the same internal pressure.

CHE-478 · Does a hemispherical head need the same thickness as the cylindrical shell?

Trap: Assuming equal stress. A sphere carries pressure with stress PD/(4t), half the cylinder's hoop stress, so a hemispherical head needs roughly half the shell thickness.

CHE-479 · Should a relief valve be set at normal operating pressure for maximum protection?

Trap: Setting relief too low. It would lift constantly. Relief valves are set at or below MAWP (design pressure), with a margin above operating pressure, typically at least about 10%, to avoid simmering and leakage.

CHE-480 · Is a heavily oversized heat exchanger always safer?

Trap: Thinking extra area is free. Oversizing lowers velocity, increasing fouling, causes poor control at low duty and can over-cool or freeze fluids, as well as wasting capital.

CHE-481 · Is stainless steel immune to corrosion?

Trap: Thinking "stainless" means corrosion-proof. Austenitic stainless steels suffer chloride stress corrosion cracking, pitting and crevice corrosion, and are attacked by some acids. Material selection must match the specific process chemistry.

CHE-482 · Do baffles in an agitated tank waste energy?

Trap: Seeing baffles as obstructions. Without baffles, low-viscosity liquids swirl and form a vortex with poor top-to-bottom mixing. Baffles increase power draw but turn it into effective mixing.

Process Safety & HAZOP

CHE-483 · If a vapor is below its LFL it is safe, so is a mixture above UFL also safe?

Trap: Treating "too rich" as safe. A mixture above UFL becomes flammable when diluted with air during a leak or vessel opening, passing through the flammable range. Rich mixtures inside equipment are still hazardous.

CHE-484 · Nitrogen is inert, so is a nitrogen-filled vessel safe to enter?

Trap: Confusing non-flammable with non-hazardous. Nitrogen displaces oxygen and causes rapid asphyxiation without warning. Confined-space entry requires isolation, ventilation, gas testing and permits.

CHE-485 · Is the flash point the temperature at which a liquid ignites by itself?

Trap: Confusing flash point with autoignition temperature. At the flash point, vapors ignite only with an external source. Self-ignition occurs at the much higher autoignition temperature.

CHE-486 · A site has had no lost-time injuries for years, so is its process safety good?

Trap: Equating personal safety with process safety. Low injury rates say little about major accident hazards. The 2005 Texas City refinery explosion occurred at a site with good personal safety statistics.

CHE-487 · Does a HAZOP identify every possible hazard?

Trap: Treating HAZOP as complete. It depends on the team's experience, design information and the nodes studied, and is weaker for non-process hazards and human factors. It is combined with other studies such as SIL review and QRA.

CHE-488 · Does adding more safeguards always reduce risk?

Trap: Assuming protection layers simply multiply. Layers that share sensors, power or valves have common-cause failures and are not independent. Extra complexity can also cause spurious trips and bypassing. Effective protection layers must be independent, auditable and maintained.

Industrial Chemical Processes & Petrochemicals

CHE-489 · In the Contact process, is SO₃ absorbed in water to make sulfuric acid?

Trap: Assuming water absorption. SO₃ with water forms a persistent acid mist. It is absorbed in about 98% H₂SO₄, with water added separately to maintain concentration.

CHE-490 · Does fluid catalytic cracking use hydrogen?

Trap: Confusing FCC with hydrocracking. FCC operates without added hydrogen and rejects carbon as coke. Hydrocracking uses high-pressure hydrogen.

CHE-491 · Does high-octane gasoline contain more energy?

Trap: Equating octane with energy content. Octane number measures knock resistance, not heating value. Premium fuel gives benefit only in engines designed for higher compression or boost.

CHE-492 · Is ammonia conversion per pass in the Haber–Bosch loop nearly complete?

Trap: Assuming high conversion per pass. Equilibrium and kinetics limit single-pass conversion to typically well below 30%. Ammonia is condensed out and unreacted gas is recycled, with a purge for inerts.

CHE-493 · Can crude oil be separated in a single atmospheric distillation tower?

Trap: Ignoring thermal cracking. Heavy fractions would need temperatures above about 370 °C at atmospheric pressure, where cracking and coking begin. Atmospheric residue is therefore sent to a vacuum column.

CHE-494 · Is natural gas pure methane?

Trap: Assuming pipeline-grade composition at the wellhead. Raw gas contains ethane, propane, butanes, CO₂, H₂S, N₂, water and sometimes helium. Processing removes acid gases and water and recovers natural gas liquids.

Process Economics & Plant Design

CHE-495 · Is the project with the shortest payback period always the best?

Trap: Relying on payback alone. Payback ignores the time value of money and all cash flows after payback, so a project with a slower payback but much larger long-term returns can have a higher NPV.

CHE-496 · Under the six-tenths rule, does doubling capacity raise cost by 60%?

Trap: Misreading the exponent. Cost ratio = 2^0.6 ≈ 1.52, so doubling capacity raises cost by about 52%, not 60%.

CHE-497 · Is depreciation a cash outflow?

Trap: Treating an accounting entry as cash. Depreciation is a non-cash expense. It reduces taxable income, so it increases after-tax cash flow through the tax shield.

CHE-498 · Is the project with the highest IRR always the best choice?

Trap: Ranking by IRR alone. IRR ignores project scale, can give multiple values when cash flows change sign more than once, and assumes reinvestment at the IRR. For mutually exclusive projects, NPV is the correct criterion.

CHE-499 · Does buying the cheapest equipment minimize plant cost?

Trap: Ignoring life-cycle cost. Cheaper equipment may have lower efficiency, higher energy and maintenance costs, shorter life and more downtime. Selection should minimize total annualized or life-cycle cost.

CHE-500 · Is working capital depreciated like fixed capital?

Trap: Treating working capital as a wasting asset. Working capital (inventories, receivables and cash) is not depreciated. It is recovered at the end of the project's life.

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