Reaction Engineering Interview Questions

Reaction rate is the speed at which reactants are consumed or products are formed per unit time. It can be expressed as: change in concentration per unit time (-dCA/dt for reactant A), moles reacted per unit volume per unit time, or moles reacted per unit mass of catalyst per unit time (for catalytic reactions). Rate is typically a function of temperature and concentrations of reactants, expressed through rate laws.

The Arrhenius equation describes how rate constant varies with temperature: k = A*exp(-Ea/RT), where A is the pre-exponential factor (frequency factor), Ea is activation energy, R is gas constant, and T is absolute temperature. It shows that rate constant increases exponentially with temperature. Activation energy represents the minimum energy barrier that reactant molecules must overcome to form products. Higher Ea means stronger temperature dependence.

Reaction order is the exponent to which reactant concentration is raised in the rate law: rate = k*CA^n*CB^m. Order with respect to A is n, with respect to B is m, and overall order is (n+m). It is determined experimentally by: method of initial rates (varying one concentration), integral method (fitting concentration-time data), or half-life method. Order can be zero, fractional, or negative for complex mechanisms, and does not necessarily equal stoichiometric coefficients.

A batch reactor is a closed vessel where reactants are charged, reaction proceeds over time, and products are removed after completion. Characteristics: no flow during reaction, composition changes with time (unsteady state), uniform composition throughout (well-mixed), suitable for small-scale production and slow reactions. Advantages: simple operation, flexible for multiple products, easy cleaning between batches. Applications: pharmaceuticals, specialty chemicals, and process development.

CSTR (Continuous Stirred Tank Reactor) has perfect mixing - composition everywhere equals outlet composition, concentration is uniform but low (at exit value). PFR (Plug Flow Reactor) has no axial mixing - fluid moves as plugs with concentration varying along length from inlet to outlet values. For positive-order reactions, PFR is smaller than CSTR for same conversion because average concentration is higher. CSTR is better for highly exothermic reactions (diluted conditions for temperature control).

Conversion (X) is the fraction of limiting reactant that has reacted: X = (moles reacted)/(moles fed). Selectivity (S) is the ratio of desired product to undesired product formed, or desired product to total products. Yield (Y) is the amount of desired product formed relative to amount that could form if all reactant converted to desired product: Y = conversion x selectivity. High conversion is not always desired if selectivity suffers - optimization balances both.

A catalyst is a substance that increases reaction rate without being consumed in the overall reaction. It works by providing an alternative reaction pathway with lower activation energy. The catalyst participates in intermediate steps but is regenerated. Catalysts do not change equilibrium position - they only help reach equilibrium faster. Types include homogeneous (same phase as reactants) and heterogeneous (different phase, typically solid catalyst with fluid reactants).

The seven steps are: (1) External diffusion - reactants diffuse from bulk fluid to catalyst surface, (2) Internal diffusion - reactants diffuse through pores to active sites, (3) Adsorption - reactants adsorb on catalyst surface, (4) Surface reaction - reaction occurs on catalyst surface, (5) Desorption - products desorb from surface, (6) Internal diffusion of products out through pores, (7) External diffusion of products into bulk fluid. The slowest step is rate-limiting and determines overall rate.

Space velocity (SV) is the ratio of volumetric flow rate to reactor volume: SV = v0/V, with units of inverse time (h-1). It indicates how many reactor volumes of feed are processed per unit time. Space time (tau) is the inverse: tau = V/v0, representing the time to process one reactor volume of feed. For continuous reactors, space time relates to residence time. Common terms: GHSV (gas hourly space velocity), LHSV (liquid hourly space velocity), WHSV (weight hourly space velocity based on catalyst weight).

Exothermic reactions release heat, raising temperature unless heat is removed. Higher temperature increases rate but may reduce equilibrium conversion - requires cooling to control temperature and optimize conversion. Endothermic reactions absorb heat, lowering temperature without heat addition. Higher temperature favors both rate and equilibrium - requires heating to maintain rate. Exothermic reactions pose runaway risks; endothermic reactions are self-limiting but may need significant heat input.

A fixed bed reactor contains stationary solid catalyst particles through which reactant fluid flows. The catalyst remains in place (not suspended or fluidized). Characteristics: high catalyst loading, good for high-throughput gas-phase reactions, temperature gradients possible (radial and axial), and pressure drop increases with smaller particles. Applications: ammonia synthesis, methanol synthesis, catalytic reforming, and hydroprocessing. May use multiple beds with intermediate heating/cooling for temperature control.

Advantages include: excellent heat transfer (can approach isothermal operation), good mass transfer between gas and solids, uniform temperature distribution (reduces hot spots), ability to continuously add and remove catalyst (for regeneration), handles fouling catalysts better (continuous circulation), and suitable for highly exothermic reactions. Applications: fluid catalytic cracking (FCC), polyethylene production, and coal combustion. Challenges include catalyst attrition, complex hydrodynamics, and potential for bypassing.

Main causes include: Poisoning - strong chemisorption of impurities on active sites (sulfur on metal catalysts), Fouling/Coking - physical blocking by deposits (carbon on cracking catalysts), Sintering - loss of surface area at high temperatures (metal particle growth), Attrition - physical breakdown of catalyst particles (fluidized beds), and Leaching - loss of active component in liquid phase. Deactivation can be reversible (regeneration possible) or permanent, and affects catalyst life and reactor performance.

Residence Time Distribution (RTD) describes how long different fluid elements spend in a reactor. It is determined by tracer experiments - injecting a tracer pulse or step and measuring outlet concentration over time. E(t) is the exit age distribution. RTD diagnoses non-ideal flow: bypassing (short-circuiting), dead zones (stagnant regions), and channeling. Ideal PFR has narrow RTD (all elements same time); ideal CSTR has exponential RTD. RTD is used to predict conversion in non-ideal reactors.

Key examples include: Ammonia synthesis reactor (high pressure, promoted iron catalyst, fixed bed with cold shot cooling), Steam reformer (high temperature, nickel catalyst, tubular reactor for hydrogen production), FCC regenerator-reactor (fluidized bed, zeolite catalyst, petroleum cracking), Ethylene cracker (thermal cracking, tubular furnace, no catalyst), Polymerization reactors (slurry, gas phase, or solution type for plastics), and Fermenters (stirred tanks, biological reactions for pharmaceuticals and biochemicals).

From steady-state mole balance on reactant A: (molar flow in) - (molar flow out) + (generation by reaction) = 0. FA0 - FA + rA*V = 0, where FA0 = CA0*v0, FA = CA*v0 = CA0*(1-X)*v0, and rA is negative for reactant consumption. Rearranging: V/FA0 = -X/rA or tau = CA0*X/(-rA). Since CSTR is uniform, rA is evaluated at exit conditions. This shows larger volumes needed for low concentrations (high conversion) where rate is low.

For first-order reaction: -rA = k*CA = k*CA0*(1-X). CSTR: V_CSTR = FA0*X/(k*CA0*(1-X)) = v0*X/(k*(1-X)). PFR: V_PFR = FA0*integral(dX/(-rA)) = v0/k * ln(1/(1-X)). Ratio: V_CSTR/V_PFR = X*(1-X)/[(1-X)*ln(1/(1-X))] = X/[(1-X)*ln(1/(1-X))]. At 90% conversion: V_CSTR/V_PFR = 0.9/[(0.1)*2.3] = 3.9. CSTR requires 3.9 times larger volume. The difference increases with conversion.

CSTRs in series approach PFR behavior as the number of tanks increases - concentration decreases stepwise rather than being immediately diluted to exit value. For n equal-volume CSTRs in series with first-order kinetics: CA/CA0 = 1/(1+k*tau_each)^n. For same total volume, multiple CSTRs achieve higher conversion than single CSTR. Practical limit is 3-5 tanks due to diminishing returns. Series arrangement also allows interstage heating/cooling, feeding at different stages, and staged catalyst ages.

A Levenspiel plot shows 1/(-rA) or FA0/(-rA) versus conversion X. It is used to graphically determine reactor volume: CSTR volume is a rectangle (height = 1/(-rA) at exit X, width = X), PFR volume is area under the curve from 0 to X. The plot illustrates: CSTR always larger than PFR for positive-order reactions (rectangle vs. area), optimal reactor sequencing for autocatalytic reactions (CSTR first may be beneficial), and minimum volume configurations for complex kinetics.

Adiabatic operation means no heat exchange with surroundings - all heat of reaction changes reactor temperature. Energy balance: rho*Cp*dT/dt = (-delta_H_rxn)*(-rA). Temperature rise relates to conversion via: T = T0 + (-delta_H_rxn)*CA0*X / (rho*Cp). For exothermic reactions, temperature increases with conversion (adiabatic temperature rise). The adiabatic operating line on X-T diagram shows this relationship. Maximum adiabatic temperature rise is when X=1. Multi-bed reactors with intercooling are used to control temperature.

Thiele modulus (phi) is dimensionless group comparing reaction rate to diffusion rate: phi = L*sqrt(k/De), where L is characteristic length, k is rate constant, and De is effective diffusivity. High phi means diffusion-limited; low phi means kinetically-limited. Effectiveness factor (eta) is actual rate divided by rate without diffusion limitation. For first-order reaction in sphere: eta = (3/phi)*(coth(phi) - 1/phi). When phi < 0.4, eta approaches 1 (kinetic control); when phi > 4, eta approaches 3/phi (diffusion control).

Langmuir-Hinshelwood model assumes: surface reaction is rate-limiting, adsorption follows Langmuir isotherm (limited sites, monolayer), and reaction occurs between adsorbed species. Rate expression: r = k*KA*PA*KB*PB / (1 + KA*PA + KB*PB + KP*PP)^2, for reaction A + B -> Products. Denominator accounts for site competition. At low pressures, rate increases with pressure; at high pressures, rate becomes independent (saturation). Model explains why rate passes through maximum with some partial pressures.

Recycle reactors return portion of exit stream to inlet, combining PFR advantages with better temperature control. Recycle ratio R = recycle flow / fresh feed flow. At R=0, pure PFR; as R increases, approaches CSTR behavior. Benefits: dilutes inlet concentration for exothermic reactions (temperature control), converts unused reactants (improves overall yield), and allows steady-state operation with deactivating catalysts (maintain conversion). Used in ammonia synthesis (high recycle ratio due to limited per-pass conversion).

Parallel reactions (A -> B desired, A -> C undesired): selectivity depends on relative rates. If n1 > n2 (B formation higher order), keep CA high - use PFR or batch. If n1 < n2, keep CA low - use CSTR or dilute feed. Series reactions (A -> B -> C, B desired): selectivity maximized at intermediate conversion. Use PFR or batch with optimal residence time. Monitor and stop at peak B concentration. For CSTR, multiple tanks allow staged optimization. Temperature also affects selectivity based on activation energies.

Key methods include: BET surface area (N2 adsorption measures total surface area), pore size distribution (BJH method from adsorption isotherm), chemisorption (metal dispersion and active sites), XRD (crystal structure and phase identification), TEM/SEM (particle size and morphology), TPR/TPO (reducibility and oxidation behavior), XPS (surface composition and oxidation states), and FTIR (adsorbed species identification). Combination of methods provides complete catalyst understanding for development and troubleshooting.

Runaway occurs when heat generation exceeds heat removal capability, causing accelerating temperature rise. Conditions: highly exothermic reaction, insufficient cooling capacity, loss of stirring/mixing, accumulation of reactants, and operating near critical temperature. Prevention: proper reactor sizing with adequate cooling, emergency cooling systems, controlled reactant addition rates, temperature monitoring and interlocks, relief systems sized for runaway scenario, and inherently safer design (limit inventory, dilute conditions). HAZOP and reaction calorimetry identify risks.

Ergun equation combines viscous and inertial losses: dP/dL = 150*mu*v*(1-epsilon)^2/(dp^2*epsilon^3) + 1.75*rho*v^2*(1-epsilon)/(dp*epsilon^3), where epsilon is bed void fraction and dp is particle diameter. First term dominates at low Reynolds number (laminar), second at high Re (turbulent). Smaller particles increase pressure drop significantly. For reacting systems, must account for molar expansion/contraction. High pressure drop requires balance: smaller particles improve effectiveness factor but increase dP.

Heat transfer options: external heat exchange (shell-side heating/cooling fluid, jackets), direct heating (fired heaters for endothermic like steam reforming), autothermal operation (exothermic reaction provides heat for endothermic), and adiabatic beds with interstage heat exchange. Design parameters: tube diameter (smaller improves radial heat transfer but increases pressure drop), tube length, number of tubes, and coolant flow pattern. Heat transfer coefficient depends on Reynolds number, packing, and fluid properties. Radial temperature gradients must be considered.

Methods depend on deactivation mechanism: Coke removal - controlled oxidation (burn coke with dilute O2 at elevated temperature), steam stripping, or hydrogen treating. Poison removal - washing or chemical treatment (remove sulfur by oxidation and washing). Redispersion - controlled oxidation-reduction cycles restore metal dispersion. Chlorination may restore acidity in zeolites. In-situ vs ex-situ regeneration depends on process constraints. FCC uses continuous regeneration in separate vessel; fixed beds use periodic regeneration cycles or swing reactors.

Types include: Stirred tank (good mixing, flexible, batch/continuous), Bubble column (simple, good heat transfer, limited mass transfer), Packed column (continuous, good mass transfer, low liquid holdup), Spray tower (simple, low pressure drop, limited contact time), and Trickle bed (catalyst pellets with liquid trickling down, gas-liquid-solid contact). Selection based on: reaction kinetics (fast reactions need high mass transfer), heat transfer needs, phase ratio requirements, pressure, and whether catalyst is needed. Gas-liquid mass transfer is often rate-limiting.

Microkinetic modeling describes reaction kinetics by elementary steps rather than overall reactions. It includes: adsorption/desorption steps, surface reactions between adsorbed species, and diffusion steps. Each step has rate parameters from theory (DFT calculations) or experiments. Benefits: mechanistic insight, extrapolation to new conditions, rational catalyst design, and identifies rate-limiting step. Challenges: many parameters needed, computational complexity, and validation difficulty. Used in catalyst development and process optimization, particularly for complex reactions like Fischer-Tropsch.

Membrane reactors combine reaction and separation in one unit. Types: catalytic membrane (membrane is catalyst), packed bed with membrane wall (selective product removal), and membrane with external catalyst. Advantages: shift equilibrium-limited reactions by removing product (e.g., H2 removal in dehydrogenation), improve selectivity by controlled reactant addition, reduce downstream separation, and intensify process. Challenges: membrane stability at reaction conditions, cost, sealing, and scale-up. Applications: hydrogen production, dehydrogenation, and partial oxidation.

Method: measure rate constants at multiple temperatures, then use Arrhenius plot. Plot ln(k) vs 1/T - slope equals -Ea/R. Linear regression gives activation energy from slope. Requirements: kinetically-controlled conditions (no mass transfer limitations), accurate temperature measurement, sufficient temperature range (typically 20-50K span), and verified reaction order. Differential reactor or initial rate method preferred. Compare apparent activation energy: ~40-80 kJ/mol for chemical kinetics, <20 kJ/mol indicates diffusion control, >100 kJ/mol for strong temperature dependence.

For equilibrium-limited reactions, conversion is limited by thermodynamics regardless of kinetics or reactor size. Strategies: operate at favorable conditions (lower temperature favors exothermic equilibrium, higher pressure favors mole-reducing reactions), remove products to shift equilibrium (membrane reactors, reactive distillation), or use excess of one reactant. Temperature optimization: balance high equilibrium conversion (low T for exothermic) with acceptable rate (higher T). May use staged reactors with intercooling and near-equilibrium operation in each stage.

Isothermal: constant temperature throughout reactor, simplifies kinetic analysis, requires heat exchange to maintain temperature, achievable in well-stirred or thin tubular reactors with high heat transfer. Non-isothermal: temperature varies with position/time, represents most industrial fixed bed reactors, coupling of mass and energy balances, adiabatic is limiting case. Non-isothermal design more complex but realistic. Hot spots in fixed beds can cause sintering or runaway. Temperature profile optimization important for selectivity and catalyst life.

Diagnostic approach: Check operating conditions (T, P, feed composition vs design), verify instrumentation accuracy, sample catalyst for analysis (coke content, metal dispersion, poison accumulation, physical integrity). Pressure drop trend indicates fouling or bed movement. Temperature profile changes reveal deactivation patterns - uniform decline suggests sintering, inlet-concentrated suggests feed contamination. Compare with fresh vs used catalyst activity tests. Mass balance closure verifies analytical accuracy. Historical correlation with feed source changes may identify poisons.

Key considerations: tube diameter (smaller improves radial heat transfer but increases tube count and cost), tube length (conversion vs pressure drop), coolant selection and flow pattern (co-current vs counter-current vs isothermal bath), coolant temperature optimization (higher improves kinetics but may create hot spots), number of tubes for required capacity, hot spot temperature prediction (parametric sensitivity), runaway avoidance (operating diagram), and catalyst activity profile (consider graded activity). Use 2D pseudo-homogeneous or heterogeneous models for detailed design.

Methodology: design experiments covering relevant range (T, P, compositions, conversions), use differential reactor or integral reactor with appropriate analysis, propose mechanisms and derive rate expressions, perform regression to estimate parameters with statistical validation (confidence intervals, correlation matrix), check model discrimination (compare rival models using F-test or information criteria), validate against independent data. Use weighted least squares if error variance varies. Global optimization algorithms help avoid local minima. Sensitivity analysis identifies critical parameters.

Scale-up challenges: mass transfer regime may change (lab kinetically-controlled, plant diffusion-limited), heat transfer limitations develop in larger particles or tubes, flow distribution becomes non-ideal (bypassing, maldistribution), pressure drop increases with bed depth, and external gradients develop. Strategies: use representative particle size in lab, include transport calculations (Weisz-Prater criterion, Mears criterion), pilot at intermediate scale, develop models validated across scales, and account for flow maldistribution. Full-scale catalyst loading and startup procedures also differ.

CFD applications: complex geometries where plug flow/well-mixed assumptions fail, multiphase reactors (bubble columns, fluidized beds), mixing studies in stirred tanks, spray systems, and local phenomena (hot spots, dead zones). Approach: geometry creation, mesh generation, select turbulence model and multiphase approach (Euler-Euler, Euler-Lagrange), couple with reaction kinetics (species transport with source terms), solve energy equation for non-isothermal, and validate against experimental data. Challenges: computational cost, model parameter selection, and validation difficulty.

Considerations: catalyst particle size (smaller improves mass transfer but complicates separation), solid loading (higher gives more activity but increases viscosity, affects mass transfer), gas-liquid mass transfer (often rate-limiting, depends on agitation, sparging), liquid-solid mass transfer, heat removal (internal coils, external circulation), catalyst attrition and makeup rate, and product/catalyst separation (settling, filtration, magnetic separation). Hydrodynamics: bubble size distribution, solid suspension, and flow regime. Used for hydrogenation, Fischer-Tropsch, and polymerization.

Reaction calorimetry measures heat generation rate under controlled conditions to determine: heat of reaction (total energy release), adiabatic temperature rise (maximum temperature if cooling fails), time to maximum rate (TMR) for thermal stability, and critical temperatures (onset of decomposition). Equipment: RC1 for normal operation conditions, ARC/DSC for runaway scenarios. Data used for: reactor sizing with adequate cooling, emergency relief sizing (using DIERS methodology), defining safe operating windows, and identifying incompatibilities. Essential for new chemistry development.

Deactivation models: activity decay expressed as a = f(time, T, coke content). Common forms: first-order (da/dt = -kd*a), concentration-dependent, temperature-accelerated. Incorporate into reactor model by multiplying intrinsic rate by activity. Design implications: oversizing for end-of-run operation, determining cycle length, optimizing operating temperature (trade-off between rate and deactivation), and designing for swing operation. Moving bed or fluidized bed allows continuous regeneration. Model validation requires extensive operating data correlation.

Optimization parameters: hydrogen partial pressure (higher improves hydrogenation but increases compression cost), temperature (higher improves kinetics but accelerates deactivation, affects selectivity), space velocity (balance conversion vs throughput), H2/oil ratio (ensure adequate hydrogen availability, minimize recycle), and catalyst grading (use different activities along bed to manage exotherms and optimize selectivity). Challenges: liquid maldistribution, hot spots, pressure drop with deactivation. Use pilot plant data for scale-up with proper accounting for wall effects.

Intensification approaches: Microreactors (high surface area/volume, precise temperature control, enhanced mixing, safer for hazardous reactions), Spinning disk reactors (thin films, excellent mass transfer), Oscillatory baffled reactors (plug flow at low velocities), Reactive distillation (combine reaction and separation), Membrane reactors (selective product removal), and Flow chemistry (continuous vs batch for pharma). Benefits: smaller equipment, faster mixing/heat transfer, improved selectivity, safer operation, and reduced capital/inventory. Challenges: scale-up, reliability, and fouling.

Autothermal reforming (ATR) combines partial oxidation (exothermic) with steam reforming (endothermic) to achieve thermal balance. Oxygen reacts with hydrocarbon at top producing CO2, H2O, and heat; this heat drives subsequent steam reforming reactions. Design considerations: oxygen-to-carbon ratio (thermal balance), steam-to-carbon ratio (carbon formation prevention, hydrogen yield), operating temperature (typically 900-1100C), catalyst selection (partial oxidation zone: no catalyst; reforming zone: nickel-based), and refractory lining for high temperature. Used in syngas/hydrogen production and GTL.

Key considerations: electrode selection (material, surface area, catalyst loading), electrolyte composition and conductivity, cell potential optimization (overpotential minimizes energy loss), current density (balance conversion rate vs efficiency), mass transfer to/from electrodes (agitation, flow-through electrodes), bubble management (gas evolution reactions), temperature control (affects kinetics and conductivity), and cell configuration (monopolar vs bipolar, flow-by vs flow-through). Scale-up maintains electrode spacing and current distribution. Applications: chlor-alkali, electrowinning, and emerging electrolysis.

Challenges: highly exothermic reactions with low thermal conductivity polymer, viscosity increases dramatically with conversion (affects mixing, heat transfer), molecular weight distribution depends on temperature history (requires tight control), multiple product grades requiring transitions, fouling on heat transfer surfaces, and runaway potential. Control strategies: monomer feed rate manipulation, jacket temperature control, reflux cooling, controlled initiator addition, and grade transition scheduling. Continuous processes use multiple zones; batch uses programmed temperature trajectories. Online monitoring (viscosity, Raman) enables quality control.

Key considerations: light source selection (UV lamps, LEDs - wavelength matching absorption spectrum), reactor geometry for uniform light distribution, photon efficiency (quantum yield), light attenuation in absorbing medium (Beer-Lambert law), residence time in illuminated zone, heat management (from lamps and reaction), scale-up maintaining uniform illumination (thin films, annular designs), and catalyst photostability for photocatalytic reactions. Challenges: photon delivery efficiency decreases with scale. Emerging: micro-photoreactors, solar concentrators, and flow photochemistry for pharmaceuticals.

Optimization approach: define objective (maximize selectivity, minimize cost, maximize yield), map concentration-temperature attainable region using reaction invariants, apply optimal temperature and mixing policies based on kinetics (PFR for positive order, CSTR for negative order), use attainable region theory for geometric insights, formulate as MINLP (mixed integer nonlinear programming) for discrete decisions (reactor types, sequence), or use superstructure optimization. Consider practical constraints: heat integration, controllability, and operability. Software tools include GAMS, Aspen for optimization.