Basic Chemical Engineering interview questions

150 questions with answers (CHE-001 to CHE-150). Try each one aloud before reading the answer.

Material & Energy Balances

CHE-001 · What is a material balance?

An accounting of mass for a system: Input − Output + Generation − Consumption = Accumulation. For total mass without nuclear reactions, generation and consumption are zero.

CHE-002 · What is the difference between steady-state and unsteady-state processes?

At steady state, no variable changes with time, so accumulation is zero. In an unsteady (transient) process, quantities such as holdup, temperature or composition change with time.

CHE-003 · What is a basis of calculation?

A chosen reference amount, such as 100 kg of feed, 1 mol of fuel or 1 hour of operation, on which all balance calculations are performed. Results are scaled afterwards.

CHE-004 · What is a tie component?

A material that passes through a process unchanged and enters and leaves in a single stream each, such as N₂ in combustion or dry solids in drying. It links stream flows easily.

CHE-005 · What is a recycle stream?

A stream that returns unreacted feed or material from downstream back to an earlier point in the process. It raises overall conversion, recovers valuable reactants and controls conditions.

CHE-006 · What is a purge stream?

A small stream withdrawn from a recycle loop to prevent the build-up of inerts or impurities, such as argon and methane in an ammonia synthesis loop.

CHE-007 · What is a bypass stream?

Part of a feed that skips a process unit and rejoins the outlet. It is used to control the final composition or temperature of the product.

CHE-008 · What is a limiting reactant?

The reactant present in less than its stoichiometric proportion relative to the others. It is consumed first and limits the maximum amount of product.

CHE-009 · What is percent excess of a reactant?

% excess = (moles fed − moles theoretically required)/(moles theoretically required) × 100, where the theoretical requirement is based on complete reaction of the limiting reactant.

CHE-010 · How do conversion, yield and selectivity differ?

Conversion is the fraction of a reactant consumed. Yield is product formed relative to reactant fed (or consumed). Selectivity is desired product formed relative to undesired product, or to reactant converted.

CHE-011 · What is the difference between mole fraction and mass fraction?

Mole fraction is moles of a component divided by total moles. Mass fraction is its mass divided by total mass. They differ unless all components have the same molar mass.

CHE-012 · What is enthalpy?

H = U + PV, a state function representing internal energy plus flow work. For open systems, the change in enthalpy measures heat transferred at constant pressure.

CHE-013 · What is the standard heat of formation?

The enthalpy change when 1 mol of a compound forms from its elements in their standard states at 25 °C and 1 bar. Elements in their standard states have ΔH°f = 0.

Chemical Engineering Thermodynamics

CHE-014 · What is the first law of thermodynamics for a steady-flow open system?

Q − Wₛ = ΔH + ΔKE + ΔPE per unit mass flow. Energy entering as heat and shaft work equals the change in enthalpy, kinetic and potential energy of the stream.

CHE-015 · What does the second law of thermodynamics state?

The total entropy of an isolated system never decreases; real processes generate entropy. Heat cannot flow spontaneously from cold to hot, and no engine converts heat fully to work.

CHE-016 · What is the difference between intensive and extensive properties?

Intensive properties (T, P, density) do not depend on system size. Extensive properties (mass, volume, total enthalpy) scale with the amount of material.

CHE-017 · What is Gibbs free energy?

G = H − TS. At constant T and P, a process is spontaneous if ΔG < 0 and at equilibrium when ΔG = 0, which makes G the key function for phase and reaction equilibrium.

CHE-018 · What is fugacity?

An "effective pressure" that replaces pressure in equations for real fluids so that μ = μ° + RT ln(f/f°). For an ideal gas, fugacity equals partial pressure; φ = f/P is the fugacity coefficient.

CHE-019 · What is the ideal gas law?

PV = nRT, with R = 8.314 J/(mol·K). It assumes point molecules with no intermolecular forces and works well at low pressure and high temperature.

CHE-020 · What is the compressibility factor?

Z = PV/(nRT). Z = 1 for an ideal gas; Z < 1 means attractive forces dominate and Z > 1 means repulsive forces dominate. It is often read from generalized charts using reduced T and P.

CHE-021 · What is Raoult's law?

For an ideal solution, the partial pressure of a component equals its liquid mole fraction times its pure-component vapor pressure: pᵢ = xᵢPᵢˢᵃᵗ. It suits chemically similar mixtures like benzene–toluene.

CHE-022 · What is Henry's law?

For a dilute solute, partial pressure is proportional to liquid mole fraction: pᵢ = Hᵢxᵢ. It describes sparingly soluble gases such as O₂ or CO₂ in water.

CHE-023 · What is the Gibbs phase rule?

F = C − P + 2 for non-reacting systems, where F is degrees of freedom, C components and P phases. Pure water at its triple point has F = 1 − 3 + 2 = 0.

CHE-024 · What is the critical point?

The temperature and pressure above which liquid and vapor phases become indistinguishable. For water it is about 374 °C and 22.1 MPa.

CHE-025 · What is an activity coefficient?

γᵢ measures deviation of a component from ideal-solution behavior: aᵢ = γᵢxᵢ. γ = 1 is ideal; γ > 1 indicates positive deviation and γ < 1 negative deviation.

CHE-026 · What is the difference between sensible heat and latent heat?

Sensible heat changes temperature without phase change (Q = m·cₚ·ΔT). Latent heat changes phase at constant temperature, such as about 2257 kJ/kg to vaporize water at 100 °C.

Fluid Mechanics & Mechanical Operations

CHE-027 · What is the Reynolds number?

Re = ρVD/μ, the ratio of inertial to viscous forces. Pipe flow is laminar below about 2100 and turbulent above about 4000, with a transition zone in between.

CHE-028 · What is the difference between Newtonian and non-Newtonian fluids?

Newtonian fluids (water, air) have shear stress proportional to shear rate, so viscosity is constant. Non-Newtonian fluids (slurries, polymer melts, ketchup) have viscosity that varies with shear rate or time.

CHE-029 · What is Bernoulli's equation?

P/ρg + V²/2g + z = constant along a streamline for steady, incompressible, frictionless flow. It expresses conservation of pressure, kinetic and potential energy heads.

CHE-030 · What is the difference between dynamic and kinematic viscosity?

Dynamic viscosity μ (Pa·s) relates shear stress to shear rate. Kinematic viscosity ν = μ/ρ (m²/s). Water at 20 °C has μ ≈ 1 mPa·s and ν ≈ 1 × 10⁻⁶ m²/s.

CHE-031 · What is NPSH?

Net Positive Suction Head is the pressure head at the pump suction above the liquid's vapor pressure. NPSH available must exceed NPSH required by the pump to avoid cavitation.

CHE-032 · How do centrifugal and positive displacement pumps differ?

Centrifugal pumps add kinetic energy with an impeller; flow varies strongly with head. Positive displacement pumps (gear, piston, diaphragm) trap fixed volumes, giving nearly constant flow regardless of pressure, suited to viscous fluids and metering.

CHE-033 · What is cavitation?

Formation and violent collapse of vapor bubbles when local pressure falls below vapor pressure, typically at the impeller eye. It causes noise, vibration, pitting and loss of head.

CHE-034 · What is the difference between a venturi meter and an orifice meter?

Both measure flow from a pressure drop across a restriction (Q ∝ √ΔP). A venturi has gradual sections, high discharge coefficient (~0.98) and low permanent loss; an orifice is cheap but has Cd ≈ 0.61 and high permanent loss.

CHE-035 · What is a rotameter?

A variable-area flow meter in which a float rises in a tapered vertical tube until drag and buoyancy balance its weight. Float height indicates flow rate at nearly constant pressure drop.

CHE-036 · What is the purpose of size reduction equipment?

To break solids into smaller particles to increase surface area, liberate components or meet product specs. Crushers (jaw, gyratory) handle coarse feed; mills (ball, hammer, roller) produce fine particles.

CHE-037 · What is filtration and what is a filter aid?

Filtration separates solids from fluid by passing the slurry through a porous medium that retains a cake. A filter aid such as diatomaceous earth forms a porous, incompressible layer that prevents blinding by fine or slimy solids.

CHE-038 · What is Stokes' law?

Terminal settling velocity of a small sphere in laminar flow: vₜ = g·d²·(ρₚ − ρ)/(18μ). It applies at low particle Reynolds numbers (roughly below 1).

CHE-039 · What is fluidization?

The condition when upward gas or liquid flow through a bed of particles creates drag equal to the bed's effective weight, so particles behave like a fluid. It gives excellent mixing and heat transfer.

Heat Transfer

CHE-040 · What are the three modes of heat transfer?

Conduction (molecular energy transfer within a medium), convection (transfer by bulk fluid motion) and radiation (electromagnetic emission requiring no medium).

CHE-041 · What is Fourier's law?

q = −kA·dT/dx. Conductive heat flow is proportional to thermal conductivity, area and temperature gradient. Copper has k ≈ 400 W/(m·K); still air about 0.026 W/(m·K).

CHE-042 · What is Newton's law of cooling?

q = hA(Tₛ − T∞), where h is the convective heat transfer coefficient in W/(m²·K). Typical h: free convection of gases 2–25, forced liquid convection 50–20,000, boiling or condensing much higher.

CHE-043 · What is the Stefan–Boltzmann law?

A black body emits q = σAT⁴, with σ = 5.67 × 10⁻⁸ W/(m²·K⁴) and T in kelvin. Real surfaces emit εσAT⁴, where ε is emissivity.

CHE-044 · What is the overall heat transfer coefficient?

U combines all series resistances (inside film, wall, outside film, fouling) so that Q = UA·ΔT. It is the reciprocal of total thermal resistance per unit area.

CHE-045 · What is LMTD?

Log Mean Temperature Difference = (ΔT₁ − ΔT₂)/ln(ΔT₁/ΔT₂), the effective driving force for a heat exchanger with terminal differences ΔT₁ and ΔT₂. Q = UA·LMTD.

CHE-046 · What is the difference between co-current and counter-current flow?

In co-current (parallel) flow, both fluids enter at the same end. In counter-current flow, they enter at opposite ends, giving a higher LMTD, smaller area and allowing closer temperature approach.

CHE-047 · What is a shell-and-tube heat exchanger?

A bundle of tubes inside a cylindrical shell: one fluid flows through the tubes, the other over them in the shell, guided by baffles. It is the most common industrial exchanger, handling high pressures and temperatures.

CHE-048 · What is a black body?

An ideal surface that absorbs all incident radiation and emits the maximum possible radiation at each temperature (ε = 1). Real "grey" bodies have constant emissivity below 1.

CHE-049 · What is the Nusselt number?

Nu = hL/k, the ratio of convective to conductive heat transfer across a fluid layer. It is the dimensionless heat transfer coefficient used in correlations.

CHE-050 · What is the Prandtl number?

Pr = cₚμ/k = ν/α, the ratio of momentum diffusivity to thermal diffusivity. Gases have Pr ≈ 0.7, water about 7 at 20 °C, and oils much higher.

CHE-051 · What is an evaporator?

Equipment that concentrates a solution by boiling off solvent, usually water. Types include short-tube (calandria), long-tube rising and falling film, forced circulation and agitated thin-film.

CHE-052 · What is the difference between filmwise and dropwise condensation?

In filmwise condensation, condensate wets the surface as a continuous film that resists heat transfer. Dropwise condensation forms beads on non-wetting surfaces and gives several times higher coefficients but is hard to sustain.

Mass Transfer

CHE-053 · What is the difference between absorption and adsorption?

Absorption takes a substance into the bulk of another phase, like CO₂ dissolving in amine solution. Adsorption holds molecules on a surface, like organics on activated carbon.

CHE-054 · What is the difference between molecular and eddy diffusion?

Molecular diffusion is driven by random molecular motion in still fluids or laminar flow. Eddy (turbulent) diffusion results from turbulent mixing and is usually orders of magnitude faster.

CHE-055 · What is a mass transfer coefficient?

k relates molar flux to a concentration driving force: N = k·ΔC. It lumps diffusion and flow effects near an interface, analogous to h in heat transfer.

CHE-056 · What is the Schmidt number?

Sc = μ/(ρD), the ratio of momentum diffusivity to mass diffusivity. It is the mass-transfer counterpart of the Prandtl number; gases have Sc ≈ 1, liquids around 1000.

CHE-057 · What is the Sherwood number?

Sh = kL/D, the ratio of convective to diffusive mass transfer. It is the mass-transfer analogue of the Nusselt number.

CHE-058 · What is the difference between equimolar counter-diffusion and diffusion through a stagnant film?

In equimolar counter-diffusion (as in binary distillation), A and B move in opposite directions at equal molar rates. In diffusion through stagnant B (as in evaporation or absorption), only A moves, adding bulk flow that raises its flux.

CHE-059 · What are absolute humidity and relative humidity?

Absolute humidity is mass of water vapor per mass of dry air (kg/kg). Relative humidity is the ratio of the water vapor partial pressure to saturation vapor pressure at the same temperature, expressed as a percentage.

CHE-060 · What are dry-bulb temperature, wet-bulb temperature and dew point?

Dry-bulb is the air temperature measured normally. Wet-bulb is the steady temperature of a wetted thermometer cooled by evaporation. Dew point is the temperature at which air becomes saturated on cooling at constant humidity.

CHE-061 · What are the constant-rate and falling-rate periods in drying?

In the constant-rate period, surface moisture evaporates like free water at the wet-bulb temperature. Below the critical moisture content, the falling-rate period begins and internal moisture movement controls the slower rate.

CHE-062 · What is a cooling tower?

A unit that cools water by direct contact with air, mainly by evaporating a small fraction of the water. Types include natural draft and mechanical draft (forced or induced), with counterflow or crossflow fill.

CHE-063 · What is crystallization?

Formation of solid crystals from a supersaturated solution, melt or vapor. Supersaturation is created by cooling, evaporation, adding an antisolvent or reaction, and drives nucleation and crystal growth.

CHE-064 · What is leaching?

Solid–liquid extraction, in which a solvent dissolves a soluble component from a solid matrix, such as sugar from sugar beets or copper from ore.

CHE-065 · What is membrane separation?

Separation by a semi-permeable barrier that passes some species faster than others. Examples include microfiltration, ultrafiltration, nanofiltration, reverse osmosis and gas separation membranes.

Chemical Reaction Engineering

CHE-066 · What is a catalyst?

A substance that speeds up a reaction by providing a lower activation energy pathway without being consumed. Examples are iron in ammonia synthesis and zeolites in catalytic cracking.

CHE-067 · How is the rate of reaction defined?

The moles of a species formed or consumed per unit time per unit volume (homogeneous) or per unit mass of catalyst (heterogeneous). For reactant A, −rₐ is positive when A is consumed.

CHE-068 · What is the difference between order and molecularity?

Molecularity is the number of molecules colliding in an elementary step and is always a whole number. Order is the empirical exponent on concentration in the rate law and can be zero, fractional or negative.

CHE-069 · What is an elementary reaction?

A reaction that occurs in a single step, so its rate law follows directly from its stoichiometry. Non-elementary reactions proceed through multiple steps and their rate laws must be found experimentally.

CHE-070 · What are the units of the rate constant for an nth-order reaction?

(concentration)^(1−n)·time⁻¹. For example, first order: s⁻¹; second order: m³/(mol·s) or L/(mol·s); zero order: mol/(m³·s).

CHE-071 · What is activation energy?

The minimum energy reacting molecules must have to form products, in kJ/mol. A higher Ea makes the rate more sensitive to temperature.

CHE-072 · What is a batch reactor?

A closed vessel in which reactants are charged, react for a set time and are discharged. Composition changes with time. It suits small-scale, multiproduct or pharmaceutical production.

CHE-073 · What is a CSTR?

A Continuous Stirred-Tank Reactor in which feed enters continuously and contents are perfectly mixed, so outlet composition equals tank composition and the reaction runs at the lowest (exit) concentration.

CHE-074 · What is a plug flow reactor?

A tubular reactor in which fluid moves as plugs with no axial mixing and uniform radial composition. Concentration falls gradually along the length, and every element has the same residence time.

CHE-075 · What is the difference between space time and space velocity?

Space time τ = V/v₀ is the time needed to process one reactor volume of feed at inlet conditions. Space velocity is its reciprocal, 1/τ, often in h⁻¹.

CHE-076 · What is the half-life of a first-order reaction?

t½ = ln 2/k ≈ 0.693/k, independent of initial concentration. For other orders, half-life depends on initial concentration.

CHE-077 · What is the difference between homogeneous and heterogeneous reactions?

Homogeneous reactions occur in a single phase, such as gas-phase cracking. Heterogeneous reactions involve two or more phases, such as gas reacting on a solid catalyst surface.

CHE-078 · What is residence time distribution?

RTD, E(t), describes how long fluid elements stay in a vessel. Measured by tracer tests, it reveals bypassing, dead zones and how close a reactor is to ideal plug flow or perfect mixing.

Separation Processes

CHE-079 · What is the principle of distillation?

Separation of a liquid mixture based on differences in volatility (boiling points). The vapor formed is richer in the more volatile component, and repeated vaporization–condensation stages build purity.

CHE-080 · What is relative volatility?

α = (yₐ/xₐ)/(y_b/x_b), the ratio of K-values of two components. For ideal systems, α = Pₐˢᵃᵗ/P_bˢᵃᵗ. The larger α is above 1, the easier the separation.

CHE-081 · What is a theoretical (ideal) stage?

A stage in which the leaving vapor and liquid streams are in equilibrium. Real trays achieve only part of this, so actual trays = theoretical stages ÷ overall efficiency.

CHE-082 · What is Murphree tray efficiency?

The actual change in vapor composition across a tray divided by the change that would occur if the leaving vapor were in equilibrium with the leaving liquid: E_MV = (yₙ − yₙ₊₁)/(yₙ* − yₙ₊₁).

CHE-083 · What is the difference between bubble point and dew point?

Bubble point is the temperature at which a liquid mixture forms its first vapor bubble at a given pressure. Dew point is the temperature at which a vapor mixture forms its first drop of liquid.

CHE-084 · What is flash distillation?

A single-stage separation in which a feed is partly vaporized, usually by pressure reduction or heating, and the vapor and liquid are separated in a drum at equilibrium.

CHE-085 · What is steam distillation?

Distillation with live steam so that a high-boiling, water-immiscible organic vaporizes below its normal boiling point, because total pressure is the sum of both vapor pressures. It is used for essential oils.

CHE-086 · What is the difference between packed and tray columns?

Tray columns use discrete plates and suit large diameters, high liquid loads and fouling service. Packed columns use random or structured packing, giving lower pressure drop, suiting vacuum, corrosive and small-diameter duties.

CHE-087 · What is liquid–liquid extraction?

Separation of a solute from a liquid feed by contacting it with an immiscible solvent in which the solute is more soluble. The solvent-rich phase is the extract; the depleted feed phase is the raffinate.

CHE-088 · What is the distribution coefficient in extraction?

K_D = concentration of solute in the extract phase ÷ concentration in the raffinate phase at equilibrium. A higher value means less solvent is needed.

CHE-089 · What is the difference between absorption and stripping?

Absorption transfers a component from a gas into a liquid solvent, such as H₂S into amine. Stripping (desorption) is the reverse, removing a dissolved gas from liquid using steam, air or heat.

CHE-090 · What is reflux ratio?

R = L/D, the ratio of liquid returned to the column from the condenser to the distillate withdrawn as product.

Process Control

CHE-091 · What is the difference between open-loop and closed-loop control?

Open-loop control acts without measuring the output, as with a timer. Closed-loop (feedback) control measures the controlled variable, compares it with the setpoint and adjusts the manipulated variable.

CHE-092 · What is the difference between feedback and feedforward control?

Feedback acts after the controlled variable deviates. Feedforward measures a disturbance (for example, feed flow) and corrects before the output is affected. They are often combined.

CHE-093 · What are setpoint, process variable and manipulated variable?

The setpoint is the desired value. The process (controlled) variable is the measured quantity, such as temperature. The manipulated variable is what the controller changes, such as steam flow via a valve.

CHE-094 · What do P, I and D actions do in a PID controller?

Proportional acts on present error. Integral acts on accumulated past error and removes offset. Derivative acts on rate of change of error, anticipating trends and adding damping.

CHE-095 · What is offset in process control?

The steady-state difference between setpoint and controlled variable that remains after a disturbance or setpoint change with proportional-only control. Integral action eliminates it.

CHE-096 · What do fail-open and fail-closed mean for control valves?

They describe the valve position on loss of air or signal. Fail-closed (air-to-open) suits fuel to a furnace; fail-open (air-to-close) suits cooling water to a reactor. The choice is based on safety.

CHE-097 · What is a transfer function?

The ratio of the Laplace transform of the output to that of the input for a linear system with zero initial conditions, G(s) = Y(s)/X(s). It describes dynamic behavior.

CHE-098 · What is the time constant of a first-order system?

For G(s) = K/(τs + 1), τ is the time to reach 63.2% of the final change after a step input. The response is about 98% complete after 4τ.

CHE-099 · What is dead time?

A pure delay between a change in input and the first response in output, often from transport through pipes or analyzers. It degrades controllability because the controller acts on old information.

CHE-100 · What is cascade control?

A control structure in which a primary (master) controller's output sets the setpoint of a fast secondary (slave) loop, such as reactor temperature resetting jacket temperature. It rejects inner-loop disturbances quickly.

CHE-101 · What is ratio control?

A strategy that keeps one flow in a fixed proportion to another measured (wild) flow, such as fuel-to-air in a burner or reflux-to-feed in a column.

CHE-102 · What is a DCS?

A Distributed Control System spreads control functions across networked controllers in the plant, linked to operator workstations. It is standard for continuous chemical plants with many interacting loops.

Process Equipment Design

CHE-103 · What is the difference between design pressure and operating pressure?

Operating pressure is the normal running value. Design pressure is a higher value used to calculate thickness, typically with a margin above the maximum expected operating pressure; relief devices are set at or below it.

CHE-104 · What is corrosion allowance?

Extra wall thickness added beyond the calculated pressure thickness to allow for metal loss over the equipment's design life. Values of 1.5–3 mm are common for carbon steel.

CHE-105 · What is hoop stress in a thin-walled cylinder?

σ_h = PD/(2t), the circumferential stress from internal pressure. It is twice the longitudinal stress (PD/4t), so cylinders fail along longitudinal seams.

CHE-106 · What types of heads are used on pressure vessels?

Hemispherical heads are strongest and thinnest but costly to form. 2:1 semi-ellipsoidal heads are common for moderate pressures. Torispherical heads are cheapest and suited to lower pressures. Flat heads need great thickness.

CHE-107 · What are the main types of shell-and-tube exchangers?

Fixed tubesheet (cheap, but limited thermal expansion and shell cleaning), U-tube (allows expansion, hard to clean tube bends), and floating head (allows expansion and bundle removal, most expensive).

CHE-108 · Why are baffles used on the shell side of a heat exchanger?

They direct shell fluid across the tubes, increasing velocity and the heat transfer coefficient, and support the tubes against sagging and vibration. Segmental baffles are most common.

CHE-109 · What is the difference between triangular and square tube pitch?

Triangular pitch fits more tubes per shell and gives higher coefficients but is hard to clean mechanically outside. Square pitch allows lane cleaning and is preferred for fouling shell-side fluids.

CHE-110 · What is the difference between a pressure relief valve and a rupture disc?

A relief valve is spring-loaded and recloses once pressure drops. A rupture disc is a thin membrane that bursts at a set pressure and must be replaced. Discs suit fast pressure rises, corrosive or fouling service.

CHE-111 · What types of agitators are used in mixing vessels?

Propellers (axial flow, low viscosity), turbines like Rushton (radial, gas dispersion) and pitched-blade (axial), paddles, and anchors or helical ribbons for high-viscosity fluids.

CHE-112 · Why are baffles fitted in agitated vessels?

They prevent swirling and vortex formation, convert rotational flow into top-to-bottom mixing and increase power input. Four baffles of about one-tenth to one-twelfth of tank diameter is standard.

CHE-113 · What is the difference between fixed-roof and floating-roof storage tanks?

Fixed-roof tanks have a permanent roof and vapor space, suiting low-volatility liquids. Floating-roof tanks have a roof resting on the liquid surface, minimizing vapor space and evaporation losses for volatile products like crude and gasoline.

CHE-114 · What is the difference between a PFD and a P&ID?

A Process Flow Diagram shows main equipment, streams and mass/energy balance data. A Piping and Instrumentation Diagram shows all piping, valves, instruments, control loops and safety devices in detail.

Process Safety & HAZOP

CHE-115 · What is the difference between hazard and risk?

A hazard is anything with the potential to cause harm, such as a flammable inventory. Risk combines the likelihood of harm with its severity: Risk = Frequency × Consequence.

CHE-116 · What are flash point, fire point and autoignition temperature?

Flash point is the lowest temperature at which vapor ignites briefly with an ignition source. Fire point is slightly higher, where burning is sustained. Autoignition temperature is where the mixture ignites with no external source.

CHE-117 · What are flammability limits?

The lower (LFL) and upper (UFL) flammable limits are the fuel concentrations in air between which a mixture can burn. Methane, for example, is about 5% to 15% by volume.

CHE-118 · What is the fire triangle?

Fire requires fuel, oxygen (oxidizer) and an ignition source. Removing any one prevents or extinguishes fire. The fire tetrahedron adds the chemical chain reaction.

CHE-119 · What is a Safety Data Sheet?

An SDS (formerly MSDS) is a standardized document giving a chemical's hazards, safe handling, PPE, first aid, firefighting, storage and disposal information. The GHS format has 16 sections.

CHE-120 · What is a BLEVE?

A Boiling Liquid Expanding Vapor Explosion. A vessel of pressurized liquid, often LPG, fails (frequently due to fire weakening the shell), the liquid flashes violently and a fireball follows if it is flammable.

CHE-121 · What is the purpose of a flare system?

To safely collect and burn relief, vent and emergency depressurization gases at a remote, elevated point, converting flammable and toxic hydrocarbons to CO₂ and water.

CHE-122 · What is a permit-to-work system?

A formal written procedure that authorizes specific hazardous work (hot work, confined-space entry, line breaking) after hazards are identified, isolations are made and controls are agreed and signed off.

CHE-123 · What is lockout/tagout?

LOTO isolates energy sources (electrical, pressure, chemical) and secures them with personal locks and tags so equipment cannot be energized or operated while maintenance is in progress.

CHE-124 · What is a Threshold Limit Value?

The TLV is the airborne concentration of a substance to which nearly all workers can be repeatedly exposed without adverse effect. Common forms are TWA (8-hour), STEL (15-minute) and Ceiling.

CHE-125 · What is inerting or nitrogen blanketing?

Replacing or covering the vapor space of a vessel with an inert gas such as N₂ so oxygen stays below the limiting oxygen concentration, preventing flammable atmospheres and oxidation.

CHE-126 · What is a Safety Instrumented System?

An SIS is an independent set of sensors, logic solver and final elements that automatically brings the process to a safe state when a dangerous condition is detected, separate from the basic process control system.

Industrial Chemical Processes & Petrochemicals

CHE-127 · What is the difference between unit operations and unit processes?

Unit operations are physical changes, such as distillation, filtration and drying. Unit processes are chemical changes, such as oxidation, nitration and polymerization.

CHE-128 · What is the Contact process?

Manufacture of sulfuric acid: sulfur is burned to SO₂, which is oxidized to SO₃ over a V₂O₅ catalyst at about 400–450 °C, then absorbed in concentrated H₂SO₄ to form oleum or acid.

CHE-129 · What is the Ostwald process?

Manufacture of nitric acid: ammonia is oxidized with air over a platinum–rhodium gauze to NO, which is further oxidized to NO₂ and absorbed in water to form HNO₃.

CHE-130 · What does the chlor-alkali process produce?

Electrolysis of brine produces chlorine at the anode and hydrogen plus sodium hydroxide at the cathode: 2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH.

CHE-131 · What is the Solvay process?

Production of sodium carbonate (soda ash) from brine, limestone and ammonia. CO₂ is passed through ammoniated brine to precipitate NaHCO₃, which is calcined to Na₂CO₃; ammonia is recovered and recycled.

CHE-132 · Why does crude oil refining use both atmospheric and vacuum distillation?

The atmospheric column separates gases, naphtha, kerosene, diesel and atmospheric residue. The residue is distilled under vacuum to recover heavy gas oils at lower temperatures, avoiding thermal cracking.

CHE-133 · What is fluid catalytic cracking?

FCC breaks heavy gas oils into gasoline, LPG and light olefins using a hot, fluidized zeolite catalyst in a riser reactor. Coke deposited on the catalyst is burned off in a regenerator.

CHE-134 · What is catalytic reforming?

A process that converts low-octane naphtha into high-octane reformate rich in aromatics using a platinum-based catalyst, producing hydrogen as a valuable by-product.

CHE-135 · What is steam cracking?

Thermal cracking of ethane, LPG or naphtha with steam at high temperature and short residence time to produce ethylene, propylene and other olefins, followed by rapid quenching.

CHE-136 · What is hydrotreating?

Catalytic treatment of petroleum fractions with hydrogen to remove sulfur (as H₂S), nitrogen (as NH₃), and to saturate olefins. Hydrodesulfurization is key to meeting low-sulfur fuel specifications.

CHE-137 · What is the difference between octane number and cetane number?

Octane number measures a gasoline's resistance to knock in spark-ignition engines (iso-octane = 100, n-heptane = 0). Cetane number measures a diesel's ignition quality, meaning short ignition delay, in compression-ignition engines.

CHE-138 · What is syngas and how is it made?

Syngas is a mixture of H₂ and CO. It is made mainly by steam methane reforming (CH₄ + H₂O → CO + 3H₂), partial oxidation or coal and biomass gasification, and is used for ammonia, methanol and Fischer–Tropsch fuels.

Process Economics & Plant Design

CHE-139 · What is the difference between CAPEX and OPEX?

CAPEX is capital expenditure on building the plant: equipment, installation and buildings. OPEX is recurring operating cost: raw materials, utilities, labor and maintenance.

CHE-140 · What is the difference between fixed capital and working capital?

Fixed capital covers the installed plant and facilities. Working capital is money tied up in raw material and product inventory, receivables and cash needed to run the plant, typically recovered at project end.

CHE-141 · What is depreciation?

The allocation of a fixed asset's cost over its useful life as it loses value. It is a tax-deductible, non-cash expense that reduces taxable income.

CHE-142 · What is payback period?

The time required for cumulative cash inflows to recover the initial investment. It is simple, but ignores the time value of money and cash flows after payback.

CHE-143 · What is Net Present Value?

NPV = Σ CFₜ/(1 + i)ᵗ − initial investment, the sum of all cash flows discounted to today at the required rate. A project with NPV > 0 creates value.

CHE-144 · What is Internal Rate of Return?

The discount rate at which a project's NPV equals zero. A project is attractive if its IRR exceeds the company's cost of capital or hurdle rate.

CHE-145 · What is the six-tenths rule?

A scaling rule for estimating capital cost: C₂ = C₁(Q₂/Q₁)^0.6. It reflects economies of scale; actual exponents vary roughly from 0.4 to 0.8 by equipment type.

CHE-146 · What is a cost index?

An index, such as the Chemical Engineering Plant Cost Index (CEPCI), used to update old cost data to the present: Cost_now = Cost_then × (Index_now/Index_then).

CHE-147 · What is the break-even point?

The production rate at which total revenue equals total cost (fixed + variable), so profit is zero. Break-even quantity = Fixed cost ÷ (price − variable cost per unit).

CHE-148 · What factors govern plant layout?

Process flow sequence, safety distances between hazardous units, access for operation, maintenance and firefighting, prevailing wind direction, future expansion, utility routing and piping cost.

CHE-149 · What factors affect site selection for a chemical plant?

Raw material and market proximity, transport links, water and power availability, land cost, labor, climate, environmental regulations, waste disposal, community acceptance and incentives.

CHE-150 · What is the Lang factor?

A multiplier applied to total purchased equipment cost to estimate total fixed capital. Classic values are about 3.1 for solids processing, 3.6 for solid–fluid and 4.7 for fluid processing plants.

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