Intermediate Chemical Engineering interview questions

150 questions with answers (CHE-151 to CHE-300). Try each one aloud before reading the answer.

Material & Energy Balances

CHE-151 · How do single-pass conversion and overall conversion differ in a recycle process?

Single-pass conversion is based on reactor inlet and outlet. Overall conversion is based on fresh feed and process outlet. With separation and recycle of unreacted feed, overall conversion can approach 100% even when single-pass conversion is low.

CHE-152 · How much air is needed to burn 1 mol of methane with 20% excess air?

CH₄ + 2O₂ → CO₂ + 2H₂O. Theoretical O₂ = 2 mol; with 20% excess, O₂ = 2.4 mol. Air = 2.4/0.21 ≈ 11.4 mol.

CHE-153 · What is degrees-of-freedom analysis in material balances?

DOF = number of unknowns − number of independent equations (balances, specifications, relations). A problem is solvable when DOF = 0; positive DOF means more data is needed and negative means over-specification.

CHE-154 · What is Hess's law?

The enthalpy change of a reaction depends only on initial and final states, not the path. Reaction heats can therefore be found by adding or subtracting known reactions, such as formation or combustion reactions.

CHE-155 · What is adiabatic flame temperature?

The temperature combustion products would reach if fuel burned completely with no heat loss and no work. It is highest with stoichiometric pure oxygen and falls with excess air, dilution or preheat removal.

CHE-156 · An evaporator concentrates 1000 kg/h of 10% NaCl solution to 25%. How much water is evaporated?

Salt balance: 100 kg/h salt. Product = 100/0.25 = 400 kg/h. Water evaporated = 1000 − 400 = 600 kg/h.

CHE-157 · How is the heat of reaction calculated from heats of formation?

ΔH°ᵣ = Σν·ΔH°f(products) − Σν·ΔH°f(reactants), using stoichiometric coefficients ν. A negative ΔH°ᵣ means the reaction is exothermic.

CHE-158 · How do you choose the purge fraction in a recycle loop?

A larger purge keeps inert concentration low, raising reactor performance, but loses more valuable reactant. A smaller purge saves reactant but lets inerts build up, increasing recycle flow and compression cost. The optimum balances these losses.

CHE-159 · What is an Orsat analysis?

A flue-gas analysis giving the volume percentages of CO₂, CO, O₂ and N₂ on a dry basis, because water vapor condenses before measurement. It is used to compute excess air and combustion efficiency.

CHE-160 · What is the difference between higher and lower heating value?

HHV (gross) assumes water in the products is condensed, so it includes latent heat. LHV (net) assumes water leaves as vapor. For natural gas, LHV is roughly 10% below HHV.

CHE-161 · What is the composition when 100 kg of 20% salt solution is mixed with 200 kg of 50% salt solution?

Salt = 20 + 100 = 120 kg in 300 kg total. Mass fraction = 120/300 = 40%.

CHE-162 · How is a steady-flow energy balance applied to a heat exchanger?

With no shaft work and negligible kinetic and potential energy changes, Q = ΔH. For an adiabatic exchanger, heat lost by the hot stream equals heat gained by the cold stream: ṁ_h·cₚ,h·ΔT_h = ṁ_c·cₚ,c·ΔT_c.

CHE-163 · How much heat is needed to raise 500 kg of water from 20 °C to 80 °C?

Q = m·cₚ·ΔT = 500 × 4.18 × 60 ≈ 125,400 kJ ≈ 125 MJ.

Chemical Engineering Thermodynamics

CHE-164 · What is the Carnot efficiency of an engine running between 500 K and 300 K?

η = 1 − T_c/T_h = 1 − 300/500 = 0.40, or 40%. This is the maximum possible efficiency for any heat engine between those reservoirs.

CHE-165 · What is the Joule–Thomson effect?

The temperature change of a real gas during adiabatic throttling at constant enthalpy. Below its inversion temperature, a gas cools on expansion (used in liquefaction). Hydrogen and helium warm at room temperature because their inversion temperatures are low.

CHE-166 · What do the constants a and b represent in the van der Waals equation?

In (P + a/V²)(V − b) = RT, a corrects for intermolecular attraction, which lowers pressure, and b corrects for the finite volume occupied by molecules.

CHE-167 · What is the Clausius–Clapeyron equation?

ln(P₂/P₁) = −(ΔH_vap/R)(1/T₂ − 1/T₁). It relates vapor pressure to temperature, assuming ideal vapor, negligible liquid volume and constant latent heat.

CHE-168 · Why are Maxwell relations useful?

They link partial derivatives of thermodynamic properties, such as (∂S/∂P)_T = −(∂V/∂T)_P, so unmeasurable quantities like entropy can be calculated from measurable P–V–T data.

CHE-169 · What is the Gibbs–Duhem equation?

At constant T and P, Σxᵢ·d(ln γᵢ) = 0. The activity coefficients in a mixture are interdependent, which is used to test the thermodynamic consistency of VLE data.

CHE-170 · How is the equilibrium constant related to Gibbs free energy?

ΔG° = −RT ln K. A negative ΔG° gives K > 1, favoring products; a positive ΔG° gives K < 1.

CHE-171 · What is the van't Hoff equation?

d(ln K)/dT = ΔH°/(RT²). For exothermic reactions (ΔH° < 0), K decreases with temperature; for endothermic reactions, K increases.

CHE-172 · What do positive and negative deviations from Raoult's law mean?

Positive deviation (γ > 1) means unlike molecules attract less than like ones, raising vapor pressure, and can form minimum-boiling azeotropes (ethanol–water). Negative deviation (γ < 1) means stronger unlike attraction and can form maximum-boiling azeotropes (HNO₃–water).

CHE-173 · What is a residual (departure) property?

The difference between a real-fluid property and its ideal-gas value at the same T and P, for example Hᴿ = H − Hⁱᵍ. It is calculated from an equation of state to correct ideal-gas enthalpy and entropy.

CHE-174 · What is isentropic efficiency of a compressor?

η_s = (isentropic work)/(actual work) = (h₂ₛ − h₁)/(h₂ − h₁). Real compressors need more work than the ideal reversible adiabatic process, so they have higher outlet temperatures.

CHE-175 · What is chemical potential and what is its role in phase equilibrium?

μᵢ = (∂G/∂nᵢ) at constant T, P and other nⱼ. At phase equilibrium, each component's chemical potential (or fugacity) is equal in all phases.

CHE-176 · How much work does 1 mol of ideal gas do when expanding isothermally and reversibly at 300 K to double its volume?

W = nRT ln(V₂/V₁) = 1 × 8.314 × 300 × ln 2 ≈ 1729 J ≈ 1.73 kJ.

Fluid Mechanics & Mechanical Operations

CHE-177 · What is the Hagen–Poiseuille equation?

For laminar flow in a pipe, ΔP = 32μLV/D². It gives the Darcy friction factor f = 64/Re and a parabolic velocity profile with maximum velocity twice the average.

CHE-178 · What is the Darcy–Weisbach equation?

h_f = f·(L/D)·V²/(2g), or ΔP = f·(L/D)·ρV²/2, where f is the Darcy friction factor (four times the Fanning factor). It gives frictional head loss in pipes.

CHE-179 · What does the Moody chart show?

The Darcy friction factor versus Reynolds number for various relative roughness values ε/D. It covers laminar (f = 64/Re), transition, and turbulent zones; at high Re, f depends only on ε/D.

CHE-180 · What are the pump affinity laws?

For a centrifugal pump with a fixed impeller diameter: Q ∝ N, H ∝ N² and Power ∝ N³. Similar relations apply for impeller diameter changes at fixed speed.

CHE-181 · How is a pump's operating point determined?

By the intersection of the pump head–flow curve and the system curve. The system curve is static head plus friction losses, which rise roughly with Q².

CHE-182 · How are minor losses in fittings calculated?

Using a loss coefficient h = K·V²/(2g) or an equivalent length Le/D added to the pipe length. Elbows, valves, entrances and exits each have characteristic K values.

CHE-183 · What is the Ergun equation?

It gives pressure drop through packed beds: ΔP/L = 150μV(1−ε)²/(ε³dₚ²) + 1.75ρV²(1−ε)/(ε³dₚ). The first term dominates in laminar flow, the second in turbulent flow.

CHE-184 · What is the constant-pressure filtration equation?

t/V = (K/2)V + B, where K depends on specific cake resistance and B on filter medium resistance. Plotting t/V against V gives a straight line from which both resistances are found.

CHE-185 · How do Rittinger's, Kick's and Bond's laws differ?

They relate size reduction energy to particle size. Rittinger's law (energy ∝ new surface area) suits fine grinding, Kick's law (energy ∝ log of size ratio) suits coarse crushing, and Bond's law (energy ∝ 1/√d) is intermediate and most widely used.

CHE-186 · How does a cyclone separator work?

Gas enters tangentially and spins, so centrifugal force throws particles to the wall, where they spiral down to the dust outlet. Efficiency rises with inlet velocity and particle size; the cut size is the diameter collected with 50% efficiency.

CHE-187 · What is the Reynolds number for water flowing at 2 m/s in a 50 mm pipe?

Re = ρVD/μ = 1000 × 2 × 0.05/0.001 = 100,000. The flow is fully turbulent.

CHE-188 · Why are multistage compressors used with intercooling?

Compression raises gas temperature. Cooling between stages brings the gas back near inlet temperature, reducing total work toward the isothermal minimum, limiting discharge temperature and allowing higher overall pressure ratios.

CHE-189 · How does particle shape affect settling and bed behavior?

Non-spherical particles have higher drag and settle more slowly than spheres of equal volume. Sphericity (surface area of equal-volume sphere ÷ actual surface area) is used in drag, Ergun and fluidization correlations.

Heat Transfer

CHE-190 · What is fouling in heat exchangers?

The build-up of deposits such as scale, biological growth, corrosion products or coke on heat transfer surfaces. It adds thermal resistance, reduces duty and raises pressure drop. A fouling factor is included in design.

CHE-191 · What is the LMTD for counter-current flow with the hot fluid going from 150 to 100 °C and the cold fluid from 30 to 70 °C?

ΔT₁ = 150 − 70 = 80 °C, ΔT₂ = 100 − 30 = 70 °C. LMTD = (80 − 70)/ln(80/70) = 10/0.1335 ≈ 74.9 °C.

CHE-192 · What is the LMTD correction factor F?

For multipass or crossflow exchangers, the true mean temperature difference = F × LMTD (counter-current), with F ≤ 1. Designs usually require F ≥ 0.75–0.8; lower values mean more shells in series are needed.

CHE-193 · When is the effectiveness–NTU method preferred over LMTD?

When outlet temperatures are unknown, as in rating an existing exchanger. Effectiveness ε = Q/Q_max is a function of NTU = UA/C_min and the heat capacity ratio C_min/C_max.

CHE-194 · What is the Dittus–Boelter equation?

Nu = 0.023·Re^0.8·Prⁿ for turbulent flow in smooth tubes, with n = 0.4 for heating and 0.3 for cooling the fluid. It is valid roughly for Re > 10,000 and 0.7 < Pr < 160.

CHE-195 · What is critical radius of insulation?

For a cylinder, r_c = k/h. Adding insulation to a pipe smaller than r_c increases heat loss, because extra surface area outweighs the added conductive resistance. Above r_c, insulation reduces heat loss.

CHE-196 · What is the Biot number and when is lumped analysis valid?

Bi = hL_c/k_solid, the ratio of internal conduction resistance to surface convection resistance. If Bi < 0.1, temperature inside the solid is nearly uniform, so lumped-capacitance analysis applies.

CHE-197 · What is the Grashof number?

Gr = gβΔTL³/ν², the ratio of buoyancy to viscous forces. It governs natural convection; the Rayleigh number Ra = Gr·Pr is used in its correlations.

CHE-198 · What does the pool boiling curve show?

Heat flux against wall superheat, passing through natural convection, nucleate boiling (high flux), the critical heat flux peak, unstable transition boiling, and film boiling where a vapor blanket insulates the surface.

CHE-199 · What is fin efficiency?

The ratio of actual heat transferred by a fin to the heat it would transfer if its whole surface were at the base temperature. It falls with fin length and with lower fin conductivity.

CHE-200 · What is steam economy in multiple-effect evaporation?

Steam economy = kg water evaporated per kg of live steam. Vapor from one effect heats the next effect at lower pressure, so economy rises roughly with the number of effects (about 0.8 per effect).

CHE-201 · What is the heat flow through a 0.2 m brick wall (k = 0.7 W/m·K) with 100 °C temperature difference per m²?

q = kAΔT/L = 0.7 × 1 × 100/0.2 = 350 W per m².

CHE-202 · How is the overall heat transfer coefficient calculated for a clean plane wall with fouling?

1/U = 1/hᵢ + Rf,ᵢ + x/k + Rf,ₒ + 1/hₒ. For tubes, each term is referred to a common area, usually the outside area, by multiplying inside resistances by Aₒ/Aᵢ.

Mass Transfer

CHE-203 · What is Fick's first law of diffusion?

J = −D·dC/dx. Molar diffusion flux is proportional to the concentration gradient and occurs from high to low concentration. Gas diffusivities are around 10⁻⁵ m²/s; liquid ones are around 10⁻⁹ m²/s.

CHE-204 · What is the two-film theory?

It assumes stagnant gas and liquid films on each side of the interface, where all resistance lies, with equilibrium at the interface. Flux N = k_G(p − pᵢ) = k_L(Cᵢ − C), and the mass transfer coefficient is proportional to D.

CHE-205 · How do penetration and surface-renewal theories differ from film theory?

They model unsteady diffusion into fluid elements briefly exposed at the interface, giving k ∝ D^0.5 rather than k ∝ D. They agree better with data for gas absorption into agitated liquids.

CHE-206 · What is a controlling resistance in gas absorption?

When the solute is highly soluble (small Henry's constant, such as NH₃ in water), gas-film resistance controls. When it is poorly soluble (O₂ or CO₂ in water), liquid-film resistance controls. 1/K_y = 1/k_y + m/k_x.

CHE-207 · What are HTU and NTU in packed column design?

Packed height Z = HTU × NTU. NTU (number of transfer units) measures separation difficulty from the driving forces. HTU (height of a transfer unit) measures packing and contacting efficiency, equal to G/(K_y·a).

CHE-208 · What is the equilibrium moisture content in drying?

The moisture a solid retains in equilibrium with air at given temperature and humidity; it cannot be removed by that air. Free moisture is the total moisture minus this equilibrium value.

CHE-209 · What is critical moisture content?

The moisture content at which drying changes from the constant-rate to the falling-rate period, when the surface can no longer stay fully wet. It depends on material, thickness and drying rate.

CHE-210 · What information does a psychrometric chart give?

For moist air at a given pressure, it relates dry-bulb, wet-bulb and dew-point temperatures, absolute and relative humidity, humid volume and enthalpy. It is used to design dryers, cooling towers and HVAC systems.

CHE-211 · Why is the wet-bulb temperature nearly equal to the adiabatic saturation temperature for air–water?

Because the Lewis number (Le = α/D ≈ h/(k_y·c_s)) is close to 1 for air–water. For other systems, such as organic vapors in air, the two temperatures differ.

CHE-212 · What are range and approach in a cooling tower?

Range is the difference between hot water inlet and cold water outlet temperatures. Approach is the difference between cold water outlet and the inlet air wet-bulb temperature, the theoretical limit. Smaller approach needs a much larger tower.

CHE-213 · How do the Langmuir and Freundlich isotherms differ?

Langmuir assumes monolayer adsorption on uniform sites: q = q_m·K·C/(1 + K·C), saturating at q_m. Freundlich is empirical, q = K_F·C^(1/n), describing heterogeneous surfaces without a saturation limit.

CHE-214 · What is the Chilton–Colburn analogy?

It links momentum, heat and mass transfer: j_H = St·Pr^(2/3) = j_D = St_m·Sc^(2/3) = f/2 (Fanning). It allows mass transfer coefficients to be estimated from heat transfer or friction data.

CHE-215 · What is the difference between nucleation and crystal growth?

Nucleation is the formation of new crystals, primary (from clear solution) or secondary (induced by existing crystals). Growth is the increase in size of existing crystals. High supersaturation favors nucleation and fine crystals; low supersaturation favors growth.

Chemical Reaction Engineering

CHE-216 · How do a CSTR and a PFR compare?

A CSTR is well mixed and operates at the uniform exit concentration. A PFR has no axial mixing, so concentration falls along its length. For positive-order reactions, a PFR needs a smaller volume for the same conversion.

CHE-217 · What are the design equations for a CSTR and a PFR?

CSTR: V = F_A0·X/(−r_A) evaluated at exit conditions. PFR: V = F_A0·∫₀ˣ dX/(−r_A). For a batch reactor: t = N_A0·∫dX/(−r_A·V).

CHE-218 · How long does a first-order batch reaction with k = 0.1 min⁻¹ take to reach 90% conversion?

t = ln[1/(1 − X)]/k = ln(10)/0.1 ≈ 23.0 min.

CHE-219 · What conversion does a CSTR achieve for a first-order reaction with k = 0.5 min⁻¹ and τ = 4 min?

X = kτ/(1 + kτ) = 2/(1 + 2) = 0.667, or about 66.7%.

CHE-220 · What conversion does a PFR achieve for the same first-order reaction (k = 0.5 min⁻¹, τ = 4 min)?

X = 1 − e^(−kτ) = 1 − e^(−2) ≈ 0.865, or 86.5%, higher than the 66.7% from a CSTR of the same volume.

CHE-221 · Why do CSTRs in series approach PFR performance?

Each tank operates at a progressively lower concentration, so the overall rate is higher than a single tank at exit concentration. As the number of tanks approaches infinity, the system behaves like a PFR.

CHE-222 · What are the Thiele modulus and effectiveness factor?

The Thiele modulus φ = L√(k/D_eff) compares reaction rate to pore diffusion rate. The effectiveness factor η is the actual rate ÷ the rate without diffusion limits. For large φ, η ≈ 1/φ, meaning strong internal diffusion limitation.

CHE-223 · What is Langmuir–Hinshelwood kinetics?

A heterogeneous rate model in which reactants adsorb on catalyst sites, react on the surface and desorb. Rates take forms like r = kK_AK_Bp_Ap_B/(1 + K_Ap_A + K_Bp_B)², showing saturation and inhibition.

CHE-224 · How do reactor choice and conditions affect selectivity in parallel and series reactions?

For parallel reactions, high concentration favors the higher-order path (use PFR or batch) and low concentration favors the lower-order path (use CSTR). For series reactions A → R → S, a PFR with limited conversion maximizes the intermediate R.

CHE-225 · What are the main causes of catalyst deactivation?

Poisoning (strong chemisorption of impurities like sulfur), fouling or coking (carbon deposits), sintering (loss of active surface at high temperature), and leaching or attrition of active material.

CHE-226 · What is the difference between the integral and differential methods of kinetic analysis?

The integral method assumes a rate law, integrates it and tests whether the data fit a straight line. The differential method finds rates directly from the slopes of concentration–time data and fits order and k without assuming a form.

CHE-227 · What is the expansion factor in gas-phase reactions?

ε_A = y_A0·δ, the fractional change in volume between no conversion and complete conversion, so V = V₀(1 + ε_A·X) at constant T and P. For A → 2B with pure A, ε_A = 1.

CHE-228 · What is the adiabatic temperature rise?

ΔT_ad = (−ΔHᵣ)·C_A0·X/(ρcₚ), the temperature increase if all reaction heat stays in the fluid. For an adiabatic reactor, temperature rises linearly with conversion along the adiabatic operating line.

Separation Processes

CHE-229 · What is an azeotrope?

A mixture whose vapor has the same composition as its liquid at a given pressure, so it cannot be separated beyond that point by simple distillation. Ethanol–water forms one at about 95.6 wt% ethanol at 1 atm.

CHE-230 · Why is reflux used in distillation?

Reflux returns liquid to contact rising vapor on each stage, enriching the vapor in the light component and improving purity. Higher reflux needs fewer stages but more condenser and reboiler energy.

CHE-231 · What are minimum reflux and total reflux?

At total reflux, no product is withdrawn and the minimum number of stages is needed. At minimum reflux, an operating line touches the equilibrium curve (a pinch), requiring infinite stages. Real operation lies between.

CHE-232 · What is the Fenske equation?

N_min = ln[(x_D/(1 − x_D))·((1 − x_B)/x_B)]/ln α, giving the minimum number of theoretical stages at total reflux for a binary with constant relative volatility.

CHE-233 · What is the q-line in McCabe–Thiele analysis?

The feed line y = [q/(q − 1)]x − z_F/(q − 1), where q is the fraction of feed that is liquid. q = 1 (saturated liquid) gives a vertical line; q = 0 (saturated vapor) gives a horizontal line. It meets the intersection of the two operating lines.

CHE-234 · What are flooding and weeping in tray columns?

Flooding occurs at high vapor or liquid rates when liquid backs up in the downcomer or is entrained upward, causing high ΔP and poor separation. Weeping occurs at low vapor rates when liquid leaks through tray holes.

CHE-235 · What is the equilibrium vapor composition for α = 2.5 and x = 0.4?

y = αx/[1 + (α − 1)x] = (2.5 × 0.4)/(1 + 1.5 × 0.4) = 1.0/1.6 = 0.625.

CHE-236 · What is the minimum liquid-to-gas ratio in absorption?

The L/G at which the operating line touches the equilibrium line, giving a pinch and requiring infinite height. Practical designs use about 1.2–1.5 times the minimum.

CHE-237 · What is a plait point on a ternary extraction diagram?

The point on the binodal (solubility) curve where the extract and raffinate phases have identical composition and the tie lines shrink to zero length. Extraction is impossible at that composition.

CHE-238 · What properties make a good extraction solvent?

High selectivity and distribution coefficient for the solute, low mutual solubility with the feed, sufficient density difference, moderate interfacial tension, easy recovery, low toxicity, flammability and cost, and chemical stability.

CHE-239 · What is the optimum reflux ratio?

The value minimizing total annual cost. Capital cost falls as reflux rises from minimum (fewer stages), but energy cost rises. The optimum is typically about 1.2–1.5 times minimum reflux.

CHE-240 · What is the Rayleigh equation for batch distillation?

ln(F/W) = ∫ from x_W to x_F of dx/(y − x), where F is initial charge and W is residue. It applies to simple differential distillation with vapor removed as formed.

Process Control

CHE-241 · Why does integral action eliminate offset, and what is reset windup?

Integral action keeps changing the output as long as any error exists, so steady state is reached only at zero error. Windup occurs when a large sustained error saturates the valve while the integral term keeps growing, causing overshoot when control returns.

CHE-242 · What is derivative kick and how is it avoided?

A spike in controller output caused by differentiating a step setpoint change. It is avoided by applying derivative action to the measured variable instead of the error. Derivative filtering also limits noise amplification.

CHE-243 · What is Ziegler–Nichols tuning?

A classical method in which, under P-only control, gain is increased until sustained oscillation to find the ultimate gain K_u and period P_u. Then for PID: K_c = 0.6K_u, τ_I = P_u/2, τ_D = P_u/8. It gives aggressive, quarter-decay response.

CHE-244 · How is closed-loop stability assessed?

A system is stable if all roots of the characteristic equation 1 + G_OL(s) = 0 have negative real parts. The Routh array, root locus and Bode or Nyquist criteria are used to test this.

CHE-245 · What are gain margin and phase margin?

Gain margin is how much loop gain can increase before instability, at the phase crossover frequency (−180°). Phase margin is the extra phase lag tolerable at the gain crossover frequency. Typical targets: GM 1.7–2, PM 30–45°.

CHE-246 · How does damping ratio affect a second-order system?

For ζ < 1 (underdamped) the response oscillates with overshoot; ζ = 1 is critically damped, the fastest without overshoot; ζ > 1 (overdamped) is sluggish and non-oscillatory.

CHE-247 · What are the main inherent characteristics of control valves?

Linear (flow ∝ travel), equal percentage (each equal travel increment changes flow by an equal percentage, common for varying ΔP) and quick opening (large flow change near closed, used for on/off service).

CHE-248 · What is split-range control?

One controller output drives two or more valves over different signal ranges, such as 0–50% opening a cooling valve and 50–100% opening a heating valve, to cover a wide operating range.

CHE-249 · What is override (selective) control?

Two or more controllers share one final element, and a high or low selector passes the safest signal. For example, a compressor flow controller is overridden by a discharge pressure controller near limits.

CHE-250 · What is averaging level control?

Loose, low-gain tuning of a surge tank level controller so level varies within limits while outflow changes smoothly, damping flow disturbances to downstream units instead of holding level tightly.

CHE-251 · What is the offset for a P-only controller with a unit step setpoint change?

Offset = 1/(1 + K_cK_p), where K_c is controller gain and K_p is process gain. It decreases as gain increases but never reaches zero.

CHE-252 · How is a typical distillation column controlled?

Column pressure is controlled by condenser duty or vapor flow; levels by distillate, bottoms or reflux flows; and composition is inferred from a sensitive tray temperature that adjusts reflux or reboiler duty.

Process Equipment Design

CHE-253 · What wall thickness does a cylindrical shell need under 1 MPa internal pressure, with inside radius 1000 mm, allowable stress 138 MPa and joint efficiency 1?

ASME formula t = PR/(SE − 0.6P) = 1 × 1000/(138 − 0.6) ≈ 7.3 mm, plus corrosion allowance, then rounded up to the next standard plate thickness.

CHE-254 · Why is longitudinal stress half of hoop stress?

For a closed cylinder, axial force P·πD²/4 acts on the wall cross-section πDt, giving σ_L = PD/(4t). Hoop force P·D·L acts on 2tL, giving σ_h = PD/(2t). Hoop stress therefore governs.

CHE-255 · What is weld joint efficiency?

A factor (E ≤ 1) reducing allowable stress to account for weld quality and degree of inspection. Fully radiographed butt welds can use E = 1.0, while spot or no radiography uses lower values.

CHE-256 · What is the purpose of hydrostatic testing?

To prove vessel strength and leak tightness before service by filling with water and pressurizing above design pressure. Under ASME VIII Div. 1, the test pressure is typically 1.3 × MAWP, adjusted for the temperature stress ratio. Water is used because it stores little energy.

CHE-257 · Which fluid should go on the tube side of a shell-and-tube exchanger?

The tube side is generally used for fluids that are more corrosive, more fouling, at higher pressure, hotter, or cooling water, because tubes are cheaper to make in alloy, easier to clean and better at containing pressure.

CHE-258 · What is the difference between a kettle reboiler and a thermosyphon reboiler?

A kettle reboiler boils liquid in a large shell, acting as a theoretical stage, and is tolerant but costly. A thermosyphon uses density difference for natural circulation, giving higher coefficients, compactness and short residence time, but needs adequate head.

CHE-259 · How do sieve, valve and bubble-cap trays compare?

Sieve trays are cheapest with good capacity but limited turndown. Valve trays have movable caps for better turndown at modest cost. Bubble-cap trays have excellent turndown and prevent weeping but are expensive and have high pressure drop.

CHE-260 · How is column diameter estimated?

From the flooding velocity, using the Souders–Brown relation u_f = C·√((ρ_L − ρ_V)/ρ_V). The design velocity is set at about 70–85% of flooding, and the area is vapor volumetric flow ÷ velocity.

CHE-261 · How is agitator power calculated?

P = N_p·ρ·N³·D⁵, where N_p is the power number, N the speed (rev/s) and D the impeller diameter. In turbulent flow with baffles, N_p is constant, about 5 for a Rushton turbine.

CHE-262 · How is material of construction chosen?

Based on corrosion resistance to the process fluid and its impurities, mechanical strength at design temperature, toughness at low temperature, fabricability, product contamination limits and life-cycle cost. Carbon steel, stainless steels, nickel alloys, titanium, FRP and linings are typical options.

CHE-263 · What are typical velocities for line sizing?

Rough guidelines: pump discharge liquids 1–3 m/s, pump suction lower (about 0.5–1.5 m/s) to protect NPSH, gases and vapors about 10–30 m/s, and steam about 20–40 m/s, balanced against pressure drop, erosion and noise.

CHE-264 · What loads must tall columns be designed for?

Internal or external pressure, dead weight of shell, internals, liquid and insulation, wind load, seismic load, thermal stresses and vortex-induced vibration. Combined stresses are checked at the skirt and base.

Process Safety & HAZOP

CHE-265 · What is LOPA?

Layer of Protection Analysis is a semi-quantitative method that estimates scenario frequency by multiplying the initiating event frequency by the probability of failure on demand of each independent protection layer, then compares it with a tolerable risk target.

CHE-266 · What is the difference between a fault tree and an event tree?

A fault tree works backwards from a top event using AND/OR gates to find combinations of causes. An event tree works forward from an initiating event through success or failure of safeguards to possible outcomes.

CHE-267 · What scenarios are considered for relief valve sizing?

Blocked outlet, external fire, cooling or reflux failure, control valve failure, exchanger tube rupture, runaway reaction, thermal expansion of trapped liquid and utility failure. The largest credible relief load sets the size.

CHE-268 · What is the dust explosion pentagon?

Fuel (combustible dust), oxygen, ignition source, dispersion of dust in air and confinement. All five are needed for a dust explosion; secondary explosions often follow when the first blast disperses settled dust.

CHE-269 · What is a runaway reaction?

An exothermic reaction in which heat generation, rising exponentially with temperature, exceeds heat removal, rising only linearly. Temperature and pressure accelerate uncontrollably, possibly rupturing the vessel.

CHE-270 · What is Management of Change?

A formal process requiring hazard review, approval, documentation and training before any change to equipment, chemicals, procedures, or operating limits, other than replacement in kind.

CHE-271 · What is Safety Integrity Level?

A measure of the risk reduction an SIS function provides, per IEC 61511. In low-demand mode, the average probability of failure on demand is: SIL 1: 10⁻² to 10⁻¹, SIL 2: 10⁻³ to 10⁻², SIL 3: 10⁻⁴ to 10⁻³, SIL 4: 10⁻⁵ to 10⁻⁴.

CHE-272 · Why are bonding and grounding important?

Flow, mixing and filling of low-conductivity liquids and powders generate static charge. Bonding equalizes potential between conductive objects, and grounding drains charge to earth, preventing sparks that can ignite flammable vapors.

CHE-273 · What are hazardous area zones for gases?

Zone 0: flammable atmosphere present continuously or for long periods. Zone 1: likely in normal operation. Zone 2: not likely, and only briefly if it occurs. Electrical equipment is selected with suitable protection for each zone.

CHE-274 · What lessons came from the Bhopal disaster?

In 1984, water entered a methyl isocyanate tank, causing a runaway reaction and toxic release that killed thousands. Lessons: minimize hazardous inventories, maintain safety systems (scrubber, flare and refrigeration were out of service), control siting near communities, and plan emergencies.

CHE-275 · What is a flame arrester?

A device with narrow passages, such as crimped metal ribbon, that removes heat from a flame front and quenches it, preventing flame propagation into tanks or pipelines through vents.

CHE-276 · What is a bow-tie analysis?

A diagram with the hazardous top event in the center, threats and preventive barriers on the left, and consequences and mitigative barriers on the right. It communicates how barriers manage a major accident hazard.

Industrial Chemical Processes & Petrochemicals

CHE-277 · Why is SO₃ absorbed in 98% sulfuric acid instead of water?

SO₃ reacts with water vapor to form a fine, stable acid mist that is difficult to capture. Absorption in 98–99% H₂SO₄ is efficient, and water is added to maintain strength. Double-contact double-absorption raises SO₂ conversion to about 99.7%.

CHE-278 · How is urea produced?

NH₃ and CO₂ react at high pressure to form ammonium carbamate (fast, exothermic), which dehydrates to urea and water (slow, endothermic). Unconverted carbamate is decomposed and recycled; the urea solution is concentrated and prilled or granulated.

CHE-279 · How is ethylene oxide made?

By direct oxidation of ethylene with oxygen over a silver catalyst at about 200–300 °C. Competing total combustion to CO₂ is strongly exothermic, so multitubular reactors with good cooling and inhibitors are used to control selectivity.

CHE-280 · How is methanol synthesized?

From syngas (CO, CO₂, H₂) over a Cu/ZnO/Al₂O₃ catalyst at about 50–100 bar and 200–300 °C: CO + 2H₂ → CH₃OH. It is exothermic and equilibrium-limited, so a recycle loop is used.

CHE-281 · How does hydrocracking differ from FCC?

Hydrocracking cracks heavy oils under high hydrogen pressure over bifunctional catalysts, giving saturated, low-sulfur diesel and jet fuel. FCC operates without hydrogen at low pressure, producing more gasoline and olefins and rejecting carbon as coke.

CHE-282 · What is alkylation in a refinery?

Reaction of isobutane with light olefins (propylene, butylenes) over strong acid catalysts (HF or H₂SO₄) to make alkylate, a high-octane, branched paraffinic gasoline blendstock with no aromatics or sulfur.

CHE-283 · What is isomerization in a refinery?

Conversion of straight-chain C₅/C₆ paraffins (light naphtha) into branched isomers over a platinum or acid catalyst, raising octane number without forming aromatics.

CHE-284 · What are delayed coking and visbreaking?

Both are thermal cracking of heavy residues. Visbreaking is mild, lowering viscosity of fuel oil. Delayed coking is severe: residue is heated and cracked in coke drums, producing lighter liquids, gas and solid petroleum coke.

CHE-285 · What is the difference between addition and condensation polymerization?

Addition (chain-growth) polymerization links unsaturated monomers without by-products, as in polyethylene or PVC. Condensation (step-growth) polymerization joins monomers with loss of small molecules like water, as in nylon and PET.

CHE-286 · How do LDPE and HDPE production differ?

LDPE is made by free-radical polymerization at very high pressure (about 1000–3000 bar), giving branched chains and low density. HDPE uses Ziegler–Natta or chromium catalysts at low pressure, giving linear chains, higher crystallinity and density.

CHE-287 · What is the Claus process?

Sulfur recovery from H₂S: about one-third of the H₂S is burned to SO₂, then 2H₂S + SO₂ → 3S + 2H₂O in a thermal stage and catalytic alumina converters. Tail-gas units raise overall recovery further.

CHE-288 · What is the water–gas shift reaction?

CO + H₂O ⇌ CO₂ + H₂, which is mildly exothermic. It increases hydrogen yield from syngas. High-temperature shift uses Fe–Cr catalyst and low-temperature shift uses Cu–Zn catalyst, reaching lower CO levels at equilibrium.

Process Economics & Plant Design

CHE-289 · If a 100 t/day plant costs $10 million, what is the estimated cost of a 200 t/day plant using the six-tenths rule?

C₂ = 10 × (200/100)^0.6 = 10 × 1.516 ≈ $15.2 million.

CHE-290 · A pump cost $20,000 when the cost index was 500. What does it cost when the index is 800?

Cost = 20,000 × (800/500) = $32,000.

CHE-291 · How do straight-line and declining-balance depreciation differ?

Straight-line depreciation charges equal amounts each year: (Cost − Salvage)/Life. Declining-balance depreciation charges a fixed percentage of remaining book value, giving larger deductions early and improving early cash flow.

CHE-292 · What is the NPV of a project costing $100 that returns $60 at the end of each of two years at 10% discount rate?

NPV = 60/1.1 + 60/1.21 − 100 = 54.55 + 49.59 − 100 ≈ $4.1. The project is marginally acceptable.

CHE-293 · What is the compound interest formula?

F = P(1 + i)ⁿ, where P is the present amount, i the interest rate per period and n the number of periods. The present value of a future sum is P = F/(1 + i)ⁿ.

CHE-294 · What is Return on Investment?

ROI = annual net profit ÷ total capital investment × 100. It is a simple profitability measure that does not account for the timing of cash flows.

CHE-295 · What are the typical classes of capital cost estimates?

Estimates range from order-of-magnitude or ratio estimates (roughly ±30–50% accuracy) through study and preliminary (budget) estimates to definitive and detailed estimates (about ±5–10%), with accuracy improving as design detail grows.

CHE-296 · Why is there an optimum pipe diameter?

A larger pipe costs more to buy and install but reduces pressure drop and pumping cost. The economic optimum minimizes the sum of annualized capital and operating cost.

CHE-297 · What is the payback period of a $10 million investment with $2.5 million annual net cash flow?

Payback = 10/2.5 = 4 years.

CHE-298 · What is capitalized cost?

The initial cost plus the present value of an infinite series of replacements, used to compare equipment with different lives: K = C_V + (C_V − V_s)/[(1 + i)ⁿ − 1], where V_s is salvage value.

CHE-299 · What is sensitivity analysis in project evaluation?

Varying key inputs (product price, raw material cost, capital, plant capacity) one at a time by a set percentage to see their effect on NPV or IRR, identifying which assumptions most influence profitability.

CHE-300 · What is the difference between fixed and variable operating costs?

Variable costs scale with production, such as raw materials, catalysts, utilities and packaging. Fixed costs do not, such as labor, maintenance, insurance, overheads and depreciation.

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