Q: What is Chemical Engineering?
Chemical Engineering is the art and science of transforming materials and energy into valuable products safely and efficiently.
Chemical Engineering is the branch of engineering that deals with the conversion of raw materials into useful products through chemical, physical, or biological processes.
It combines principles of chemistry, physics, mathematics, and engineering to design, optimize, and operate industrial processes.
The focus is on process design, equipment development, heat & mass transfer, reaction engineering, and process control.
Chemical engineers work to ensure safe, economical, and sustainable production in industries such as oil & gas, petrochemicals, pharmaceuticals, food, fertilizers, and energy.
In short, it’s about turning laboratory chemistry into large-scale, efficient, and safe industrial operations.
Q: What are the major branches of Chemical Engineering?
Chemical Engineering branches connect chemistry, biology, and engineering to design safe, efficient, and sustainable industrial processes.
Process Engineering:
Focuses on design, optimization, and operation of chemical processes and plants.
Thermodynamics & Transport Phenomena:
Deals with heat, mass, and momentum transfer and energy balance in chemical systems.
Reaction Engineering:
Involves design and analysis of chemical reactors for efficient conversion and yield.
Biochemical Engineering:
Applies chemical engineering principles to biological systems, like fermentation and bioprocessing.
Environmental Engineering:
Focuses on pollution control, waste treatment, and sustainable process design.
Materials Engineering:
Deals with development and processing of new materials such as polymers, ceramics, and nanomaterials.
Process Control & Instrumentation:
Ensures automation, safety, and stability of chemical plants through control systems.
Q: What are the Key Responsibilities of a Chemical Engineer in Industry?
A Chemical Engineer ensures that industrial processes run safely, efficiently, and sustainably — from concept to production.
Process Design & Development:
Design and develop efficient chemical processes for production.
Optimization:
Improve process performance to enhance yield, safety, and cost-effectiveness.
Plant Operation & Monitoring:
Supervise and control plant operations to ensure smooth and continuous production.
Safety & Environmental Compliance:
Ensure all operations meet safety, health, and environmental standards.
Troubleshooting:
Identify and resolve process or equipment issues to minimize downtime.
Scale-up & Commissioning:
Translate lab-scale processes to pilot and industrial scale successfully.
Quality Control:
Maintain product quality through process control and analytical checks.
Innovation & Sustainability:
Develop eco-friendly, energy-efficient, and sustainable process solutions.
Q: What are the Major Differences Between Chemistry and Chemical Engineering?
Focus Area:
Chemistry focuses on studying substances, their properties, and reactions at a molecular level.
Chemical Engineering focuses on applying those reactions on an industrial scale to produce useful products.
Objective:
Chemists aim to discover and understand new compounds and reactions.
Chemical engineers aim to design and operate processes that make those compounds economically and safely.
Work Environment:
Chemists mostly work in laboratories conducting experiments.
Chemical engineers work in plants, refineries, and design offices managing large-scale operations.
Core Skills:
Chemistry emphasizes theoretical and experimental knowledge.
Chemical Engineering emphasizes process design, thermodynamics, heat & mass transfer, and control systems.
End Goal:
Chemists create new chemical knowledge or compounds.
Chemical engineers turn that knowledge into practical, large-scale production.
Q: What are Unit Operations and Unit Processes?
Unit Operations handle how materials are processed physically, while Unit Processes define what chemical transformation occurs.
Unit Operations:
Refer to physical steps involved in a chemical process where no chemical reaction occurs.
Involve separation, transfer, or change of physical state of materials.
Examples: Distillation, Filtration, Evaporation, Drying, Heat Exchange, Mixing.
Focus on mass transfer, heat transfer, and fluid flow principles.
Unit Processes:
Involve chemical reactions that bring about a chemical change in the substance.
Responsible for producing new chemical compounds from raw materials.
Examples: Oxidation, Hydrogenation, Nitration, Sulphonation, Polymerization.
Focus on reaction kinetics, catalysis, and reactor design.
Q: Give Examples of Common Unit Operations in Process Plants
Unit operations are the physical backbone of process industries — enabling separation, heat transfer, and material transformation efficiently.
Distillation: Separation of liquid mixtures based on boiling point differences.
Absorption & Stripping: Gas–liquid contact operations for removing or recovering specific components.
Filtration: Solid–liquid separation using porous media.
Evaporation: Concentration of solutions by removing solvent as vapor.
Drying: Removal of moisture from solids using heat or air flow.
Crystallization: Formation of solid crystals from a solution or melt.
Heat Exchange: Transfer of heat between process streams for energy efficiency.
Mixing & Agitation: Ensures uniform composition and reaction conditions.
Size Reduction: Reduction of particle size through crushing or grinding.
Extraction: Separation based on solubility differences between two immiscible phases.
Q: What is a Process Flow Diagram (PFD)?
A PFD is the blueprint of a chemical process — showing how materials and energy flow through key Equipment
A Process Flow Diagram (PFD) is a schematic representation of the major equipment and flow of materials in a chemical process.
It shows how raw materials are converted into final products through various process steps.
Includes major equipment, process lines, flow directions, operating conditions, and key process data (like temperature, pressure, and flow rates).
Helps engineers understand, design, and optimize the process layout before detailed engineering.
Acts as a communication tool between process engineers, designers, and operators.
Does not include minor details like valves, fittings, or instrumentation — those are shown in P&IDs (Piping and Instrumentation Diagrams).
Essential for process design, safety review, and plant operation.
Q: What is a Piping and Instrumentation Diagram (P&ID)?
A P&ID is the heart of plant engineering — showing how equipment, piping, and instruments work together to safely control the process
A Piping and Instrumentation Diagram (P&ID) is a detailed engineering drawing that shows the complete piping layout, equipment, valves, and instrumentation of a process plant.
It represents the relationship between process equipment, control systems, and pipelines.
Includes details like valves, pumps, sensors, control loops, pressure/temperature indicators, and instrumentation tags.
Used for design, operation, maintenance, and safety analysis of process systems.
Serves as a guide for plant commissioning, troubleshooting, and modification.
Unlike a PFD, a P&ID provides precise control and interlock information, not just process flow.
It is a key document for process safety and HAZOP studies.
Q: What is Meant by Mass Balance and Energy Balance?
“Mass balance tracks the flow of matter, while energy balance tracks the flow of energy — both ensure process efficiency, accuracy, and control.”
Mass Balance:
It is the accounting of all material entering and leaving a process system.
Based on the law of conservation of mass — “Mass can neither be created nor destroyed.”
Ensures that input = output + accumulation + losses in a process.
Used to determine flow rates, compositions, and efficiencies of unit operations.
Example: Calculating feed, product, and by-product quantities in a distillation column.
Energy Balance:
It is the calculation of all energy entering and leaving a system.
Based on the first law of thermodynamics — “Energy can neither be created nor destroyed, only transformed.”
Accounts for heat, work, kinetic and potential energy in a process.
Used to design heaters, coolers, reactors, and overall process efficiency.
Example: Estimating steam required for heating or energy recovered from condensate.
🔹 Mass & Energy Balance
Q: What is the Law of Conservation of Mass?
“The law of conservation of mass means — what goes into a system must come out, nothing is lost or created, only transformed.”
The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical or physical process.
The total mass of reactants equals the total mass of products in any closed system.
It forms the foundation of mass balance in chemical engineering.
Mathematically expressed as:
Input = Output + Accumulation + Losses (for any system).
This law ensures that all material entering a process must be accounted for, either as product, by-product, or waste.
Essential for process design, reactor calculations, and environmental assessments.
Q: How Do You Perform a Mass Balance Over a Process Unit?
“Mass balance is performed by defining the system, applying mass conservation, solving for unknowns, and verifying that all material entering equals material leaving.”
1. Define the System Boundary:
Identify the process unit (e.g., reactor, distillation column) and mark all inlet and outlet streams.
2. Collect Process Data:
Obtain flow rates, compositions, and operating conditions for all input and output streams.
3. Apply the Law of Conservation of Mass:
Use the fundamental equation:
Input = Output + Accumulation + Losses.
For steady-state systems, Accumulation = 0, so Input = Output.
4. Write Component Balances:
Perform balance for each individual component (e.g., A, B, C) as:
Input of A = Output of A + Accumulation of A.
5. Solve the Equations:
Use algebraic methods to find unknown flow rates or compositions.
6. Verify the Results:
Check for mass closure — ensure total input ≈ total output (within acceptable error limits).
7. Interpret and Optimize:
Use the balance to analyze efficiency, detect losses, or improve yield.
Q: What is a Limiting Reactant?
“The limiting reactant is the one that runs out first, controlling how much product a reaction can produce.”
The limiting reactant is the reactant that gets completely consumed first in a chemical reaction.
It limits the amount of product that can be formed — once it is used up, the reaction stops.
Other reactants may remain in excess after the reaction is complete.
Determined by comparing mole ratios of reactants with their stoichiometric ratios from the balanced equation.
Identifying the limiting reactant helps in calculating theoretical yield and optimizing raw material usage.
Crucial for cost control, process efficiency, and reactor design in chemical industries.
Example: In the reaction 2H₂ + O₂ → 2H₂O, if hydrogen is less than stoichiometric proportion, H₂ is the limiting reactant.
Q: What is Conversion, Yield, and Selectivity?
“Conversion tells how much reacted, Yield shows how much desired product formed, and Selectivity reveals how efficiently it was formed.”
Conversion:
It measures how much of a reactant is consumed in a chemical reaction.
Defined as the fraction or percentage of reactant reacted out of the total fed.
Formula:
Conversion (X) = (Moles reacted / Moles fed) × 100%
Indicates reaction completeness and process efficiency.
Yield:
It represents the amount of desired product formed compared to the theoretical maximum possible.
Formula:
Yield (Y) = (Actual moles of desired product / Theoretical moles possible) × 100%
Reflects overall process performance and raw material utilization.
Selectivity:
It indicates how effectively the reaction forms the desired product over undesired by-products.
Formula:
Selectivity (S) = (Moles of desired product / Moles of undesired product) × 100%
Important in reactions with multiple possible products.
Q: What is the Difference Between Open and Closed Systems?
“In an open system, both mass and energy cross the boundary; in a closed system, only energy can — mass stays constant.”
Open System:
Allows both mass and energy (heat or work) to enter or leave the system
Common in continuous processes where materials flow in and out.
Example: Boiler, distillation column, or cooling tower.
Used when continuous operation and material exchange are required.
Closed System:
Allows energy exchange (heat or work) but no mass transfer across the boundary.
The quantity of matter remains constant, though its energy may change.
Example: Sealed reactor, piston cylinder (without mass exchange).
Used for batch processes and thermodynamic analysis.
Q: How Do You Perform an Energy Balance?
“An energy balance is performed by accounting for all heat, work, and enthalpy changes — ensuring total energy entering equals total energy leaving the system.”
1. Define the System Boundary:
Identify the equipment or process unit (e.g., reactor, heat exchanger) and mark all energy inputs and outputs.
2. Apply the First Law of Thermodynamics:
The basis is energy conservation —
Energy In = Energy Out + Accumulation + Losses.
3. Identify All Energy Forms:
Include heat (Q), work (W), enthalpy of streams, kinetic, and potential energy.
4. Write the Energy Balance Equation:
For steady-state:
Σ (Energy In) = Σ (Energy Out)
For unsteady-state:
Σ (Energy In) – Σ (Energy Out) = Accumulation
5. Substitute Known Data:
Use flow rates, temperatures, pressures, and specific heat values to calculate enthalpy or energy changes.
6. Solve for Unknowns:
Determine required heat duty, work done, or outlet temperature as needed.
7. Verify the Balance:
Ensure energy input ≈ energy output (within acceptable error).
Q: What is Sensible and Latent Heat?
“Sensible heat changes temperature without phase change; latent heat changes phase without temperature change.”
Sensible Heat:
It is the heat absorbed or released by a substance that changes its temperature without changing its phase.
Can be measured directly using a thermometer.
Formula: Q = m × Cp × ΔT
Example: Heating water from 30°C to 90°C — temperature rises, phase remains liquid.
Important in heat exchangers, heaters, and coolers.
Latent Heat:
It is the heat absorbed or released during a phase change (solid ↔ liquid ↔ gas) without temperature change.
Cannot be measured by temperature difference.
Types: Latent heat of fusion, vaporization, and condensation.
Example: Boiling of water at 100°C — temperature constant while vapor forms.
Crucial in evaporation, condensation, and refrigeration processes.
Q: Define Enthalpy and Entropy
“Enthalpy measures the system’s total heat energy, while Entropy measures the system’s disorder and energy dispersion.”
Enthalpy (H):
It is the total heat content of a system, representing the sum of internal energy and the energy required to displace the surroundings.
Mathematically: H = U + PV
U = Internal energy, P = Pressure, V = Volume.
Indicates the energy change during heating, cooling, or chemical reactions at constant pressure.
Measured in kJ/mol or kJ/kg.
Example: Heat absorbed during vaporization or reaction heat in a process.
Important for energy balance, reaction design, and thermodynamic calculations.
Entropy (S):
It is a measure of the degree of disorder or randomness in a system.
Represents the unavailable energy that cannot be converted into useful work.
Mathematically related by: ΔS = ΔQ / T (for reversible processes).
Increases in irreversible or spontaneous processes.
Indicates process direction and efficiency in thermodynamics.
Measured in kJ/mol·K.
Q: What are Heat Losses and How Are They Minimized?
“Heat losses are unwanted energy escapes from a system — minimized through insulation, heat recovery, good design, and preventive maintenance.”
Heat Losses:
Refer to the unwanted transfer of heat energy from a system to its surroundings.
Occur due to conduction, convection, and radiation from equipment surfaces, pipelines, or storage vessels.
Result in energy inefficiency, higher operating costs, and reduced process performance.
Common sources: Uninsulated pipelines, heat exchangers, furnaces, and storage tanks.
Methods to Minimize Heat Losses:
Thermal Insulation:
Use of insulating materials (e.g., glass wool, mineral wool, ceramic fiber) to reduce heat transfer.
Proper Equipment Design:
Compact design and heat recovery systems (like economizers or recuperators).
Maintenance:
Regular checking for leaks, corrosion, and insulation damage.
Heat Integration:
Utilize waste heat recovery for preheating feed streams.
Surface Coatings:
Use reflective or heat-resistant coatings to minimize radiation losses.
Operational Control:
Maintain optimal operating conditions to avoid excess energy usage.
Q: What is the Difference Between Specific Heat and Heat Capacity?
“Specific heat is heat per unit mass per degree change, while heat capacity is total heat required for the entire body per degree change.”
Specific Heat (c or Cp):
It is the amount of heat required to raise the temperature of 1 unit mass of a substance by 1°C (or 1 K).
It is an intensive property — independent of the mass of the material.
Units: kJ/kg·K or J/g·K.
Example: Water has a specific heat of 4.18 kJ/kg·K, meaning it requires 4.18 kJ to heat 1 kg of water by 1°C.
Heat Capacity (C):
It is the total amount of heat required to raise the temperature of a given body or system by 1°C (or 1 K).
It is an extensive property — depends on the total mass of the system.
Units: kJ/K or J/K.
Formula relation: C = m × c, where m = mass, c = specific heat.
🔹 Fluid Mechanics
Q: What is Laminar and Turbulent Flow?
“In laminar flow, fluid moves smoothly in layers; in turbulent flow, it moves chaotically with intense mixing and energy loss.”
Laminar Flow:
Flow in which fluid particles move in smooth, orderly layers (streamlines) with minimal mixing between them.
Occurs at low velocities and low Reynolds numbers (Re < 2100).
Characterized by steady, predictable flow behavior.
Example: Flow of viscous fluids like oil or glycerin in small pipes.
Advantages: Low friction losses, stable flow, and easy to analyze mathematically.
Turbulent Flow:
Flow in which fluid particles move randomly and chaotically, with significant mixing and velocity fluctuations.
Occurs at high velocities and high Reynolds numbers (Re > 4000).
Characterized by eddy currents and irregular motion.
Example: Flow of water or air in large pipelines or industrial systems.
Advantages: Better mixing and heat transfer, but higher energy losses due to friction.
Q: What is Reynolds Number?
Definition:
Reynolds number (Re) is a dimensionless number that indicates whether the flow of a fluid is laminar or turbulent.
Formula:
Where,
μ = fluid density
V = fluid velocity
D = characteristic diameter
ρ = dynamic viscosity
Purpose:
It represents the ratio of inertial forces to viscous forces in a fluid flow.
Flow Regimes:
Laminar flow: Re < 2000
Transitional flow: 2000 ≤ Re ≤ 4000
Turbulent flow: Re > 4000
Significance:
Helps in predicting flow patterns, designing pipelines, heat exchangers, and reactors.
Example:
Used to decide whether flow will be smooth (laminar) or mixed and chaotic (turbulent).
Q: What are Major and Minor Losses in a Pipe?
Definition:
In fluid flow through pipes, energy (head) losses occur due to friction and disturbances — these are classified as major and minor losses.
1. Major Losses:
Caused by friction between the fluid and the pipe wall along the pipe length.
Formula:
f = friction factor
L = length of pipe
D = diameter
v = flow velocity
Depends on: pipe roughness, flow velocity, viscosity, and Reynolds number.
2. Minor Losses:
Occur due to fittings, bends, valves, expansions, contractions, and entries/exits.
Formula:
K = loss coefficient (depends on fitting type).
Though smaller in magnitude, they become significant in short or complex piping systems.
Significance:
Both losses reduce pressure and energy in the system.
Must be considered in pump selection, pipe design, and system efficiency.
Q: How do you calculate pressure drop in a pipeline?
Determine flow regime → find f→ apply Darcy–Weisbach → add minor losses.
Definition: Pressure drop is the loss of pressure as fluid flows through a pipe due to friction and other resistances.
Calculation Method:
The Darcy–Weisbach equation is commonly used:
ΔP = Pressure drop (Pa)
f = Friction factor (depends on flow regime & pipe roughness)
L = Length of pipe (m)
D = Diameter of pipe (m)
ρ = Fluid density (kg/m³)
v= Flow velocity (m/s)
Friction Factor (f):
For laminar flow (Re < 2000):
For turbulent flow (Re > 4000): use Moody chart or Colebrook equation
Additional Losses:
Include minor losses due to fittings, bends, valves, etc.:
Key Point:
Accurate pressure drop estimation ensures energy-efficient design, pump sizing, and safe pipeline operation.
Q: What is Bernoulli’s Equation?
Bernoulli’s equation is a fundamental principle in fluid mechanics that relates pressure, velocity, and elevation in a steady, incompressible, and frictionless flow.
Equation:
P = Pressure energy
½ρv² = Kinetic energy per unit volume
ρgh = Potential energy per unit volume
Meaning:
It states that the total mechanical energy of a fluid remains constant along a streamline if there are no energy losses.
Assumptions:
Steady flow
Incompressible fluid
No friction or viscous losses
Along a single streamline
Applications:
Used in flow measurement (Venturi meter, Orifice meter, Pitot tube)
Pump and turbine analysis
Pressure–velocity relationship in pipelines
Q: What is a Pump Head?
Pump head is the height of a fluid column that a pump can raise the fluid to, representing the energy imparted by the pump to the fluid.
It is expressed in meters (m) or feet (ft) of fluid.
Formula:
ΔP = Pressure increase across the pump (Pa)
ρ = Fluid density (kg/m³)
g = Gravitational acceleration (9.81 m/s²)
Types of Pump Head:
Static Head:
Difference in elevation between suction and discharge.
Friction Head:
Losses due to pipe friction and fittings.
Total Dynamic Head (TDH):
Significance:
Determines the pump’s ability to overcome system resistance.
Crucial for pump selection and performance evaluation.
Q: What are the types of pumps used in chemical plants?
Pumps are mechanical devices used to move liquids through pipelines by converting mechanical energy into hydraulic energy.
Main Types of Pumps:
1. Centrifugal Pumps (Dynamic Type):
Most common in chemical industries.
Fluid is accelerated by an impeller to convert velocity into pressure.
Used for: Low to medium viscosity fluids, large flow rates.
Examples: Single-stage, multistage, self-priming, and magnetic drive pumps.
2. Positive Displacement Pumps:
Deliver a fixed volume of fluid per cycle irrespective of discharge pressure.
Used for: High-viscosity fluids, precise metering, and high-pressure applications.
Subtypes:
Reciprocating Pumps: Piston or diaphragm type; used for dosing or high-pressure transfer.
Rotary Pumps: Gear, screw, or lobe type; smooth and steady flow.
3. Specialty Pumps:
Peristaltic Pumps: Ideal for corrosive or shear-sensitive fluids.
Diaphragm Pumps: Leak-free, suitable for hazardous or toxic chemicals.
Magnetically Driven Pumps: No shaft seal → prevents leakage and contamination.
Selection Criteria:
Depends on fluid properties (viscosity, corrosiveness), flow rate, pressure, and process safety requirements.
Q: What is a Centrifugal Pump Curve?
A centrifugal pump curve is a graphical representation of pump performance, showing how the head, flow rate, efficiency, and power vary with operating conditions.
Key Parameters on the Curve:
1. Head vs. Flow Rate (Q–H Curve):
Shows how discharge head decreases as flow rate increases.
Helps identify the Best Efficiency Point (BEP).
2. Efficiency Curve:
Indicates the pump’s efficiency at various flow rates.
BEP is where efficiency is maximum.
3. Power Curve:
Shows power consumption at different flow rates.
4. NPSH (Net Positive Suction Head) Curve:
Indicates the minimum suction head required to avoid cavitation.
Purpose / Importance:
Helps in selecting the right pump for the desired operating range.
Ensures efficient and reliable operation close to BEP.
Assists in troubleshooting issues like cavitation or overload.
Q: What is Cavitation and How is it Prevented?
Cavitation is a destructive phenomenon caused by low pressure at the pump suction, and it is best prevented by ensuring sufficient NPSH and smooth suction flow conditions.
Cavitation is the formation and collapse of vapor bubbles in a liquid when local pressure falls below the liquid’s vapor pressure, causing damage and loss of performance in pumps or turbines.
Causes:
Low suction pressure or high fluid velocity at the pump inlet.
High fluid temperature (increases vapor pressure).
Improper pump selection or excessive suction lift.
Effects:
Pitting and erosion of impeller surfaces.
Vibration and noise during operation.
Drop in flow rate, head, and efficiency.
Shortened pump life.
Prevention Methods:
1. Maintain adequate NPSH (Net Positive Suction Head):
Ensure NPSH available > NPSH required.
2. Reduce suction lift:
Keep pump close to liquid source.
3. Minimize friction losses:
Use shorter and larger-diameter suction pipes.
4. Avoid high temperature at suction side.
5. Use proper pump selection:
Choose pump suitable for fluid type and flow conditions.
6. Install air release valves to remove trapped air or vapors.
Q: What is NPSH (Net Positive Suction Head)?
NPSH determines a pump’s ability to avoid cavitation — maintaining sufficient NPSH is essential for pump reliability and longevity.
NPSH is the measure of the absolute pressure at the pump suction compared to the liquid’s vapor pressure, used to determine if the liquid will vaporize (cause cavitation) during pumping.
Types of NPSH:
1. NPSH Available (NPSHa):
The actual suction head provided by the system to the pump.
Depends on suction conditions, liquid level, and friction losses.
2. NPSH Required (NPSHr):
The minimum suction head the pump needs to avoid cavitation.
Provided by the pump manufacturer based on pump design.
Formula
Psuction = Suction pressure
Pvapour = Vapor pressure of the liquid
Pstatic = Static head
hloss= Friction losses in suction line
Key Condition:
NPSH Available (NPSHa) > NPSH Required (NPSHr)
Importance:
Ensures smooth, cavitation-free operation of the pump.
Helps in proper pump selection and installation.
🔹 Heat Transfer
Q: What are the Modes of Heat Transfer?
In real systems, heat transfer often occurs by a combination of all three modes, and understanding them is vital for thermal design and process efficiency.
Heat transfer is the process of energy movement due to temperature difference between two bodies or regions.
There are three main modes of heat transfer:
1. Conduction:
Heat transfer through a solid medium without movement of particles.
Occurs due to molecular vibration and energy exchange.
Example: Heat flow through a metal rod.
Governing Law: Fourier’s Law —
2. Convection:
Heat transfer by bulk movement of fluid (liquid or gas).
Can be natural (due to density difference) or forced (using fans/pumps).
Example: Heat transfer from hot water to surrounding air.
Governing Law: Newton’s Law of Cooling —
3. Radiation:
Heat transfer through electromagnetic waves without any medium.
Occurs even in vacuum.
Example: Heat from the Sun reaching the Earth.
Governing Law: Stefan–Boltzmann Law —
Q: What is Fourier’s Law?
Fourier’s Law describes the rate of heat conduction through a material — it states that heat flows from a region of higher temperature to a region of lower temperature, and the rate is proportional to the temperature gradient and area of heat transfer.
Mathematical Expression:
where:
Q = Rate of heat transfer (W)
k = Thermal conductivity of the material (W/m·K)
A = Cross-sectional area (m²)
dT/dx = Temperature gradient (K/m)
Negative sign (–) indicates heat flows in the direction of decreasing temperature.
Key Points:
Applicable to steady-state conduction.
The higher the thermal conductivity (k), the faster the heat transfer.
Fundamental in designing heat exchangers, insulation, and thermal systems.
Q: What is an Overall Heat Transfer Coefficient (U)?
The Overall Heat Transfer Coefficient (U) represents the total ability of a system to transfer heat through a combination of materials and layers (like fluids, walls, and fouling).
Higher U value → better heat transfer efficiency.
U depends on fluid properties, flow conditions, and material characteristics.
It combines the effects of conduction, convection, and fouling resistances into a single value.
Mathematical Expression:
Q = Rate of heat transfer (W)
U = Overall heat transfer coefficient (W/m²·K)
A = Heat transfer area (m²)
∆T = Temperature difference across the surface (K)
Combined Resistance Formula:
h1, h2= Convective heat transfer coefficients (inner & outer sides)
δ = Wall thickness
k = Thermal conductivity of wall material
Rf = Fouling resistance
Importance:
Indicates how effectively heat is transferred through a surface or heat exchanger.
Used in the design and performance evaluation of heat exchangers.
Q: What is the difference between Parallel Flow and Counter-Flow Heat Exchangers?
Counter-flow design is preferred in most industrial applications due to its higher thermal efficiency and better energy utilization.
Definition: The terms parallel flow and counter-flow describe the direction of fluid movement relative to each other in a heat exchanger.
1. Parallel Flow Heat Exchanger:
Both hot and cold fluids enter from the same end and move in the same direction.
Temperature difference between fluids is maximum at the inlet and decreases along the length.
Less efficient heat transfer due to smaller average temperature difference.
Outlet temperatures of both fluids approach each other but never cross.
Used for: Applications needing uniform wall temperature, like viscous fluid heating.
2. Counter-Flow Heat Exchanger:
Hot and cold fluids move in opposite directions.
Maintains a higher average temperature difference across the length.
More efficient — allows greater heat transfer and closer approach to inlet temperatures.
Outlet of cold fluid can be hotter than the outlet of the hot fluid.
Used for: High-efficiency applications like condensers, boilers, and process heat recovery.
Key Differences:
Q: What causes fouling in heat exchangers?
“Fouling is mainly caused by deposition, reaction, or biological growth on heat transfer surfaces — and controlling fluid quality, temperature, and velocity are the best ways to prevent it.”
Definition: Fouling is the unwanted deposition of materials on heat exchanger surfaces, reducing heat transfer efficiency and increasing pressure drop.
Main Causes:
1. Sedimentation: Deposition of suspended solids or particles from the fluid on the surface due to low velocity.
2. Scaling: Precipitation of dissolved salts (like CaCO₃) when temperature or concentration exceeds solubility limits.
3. Corrosion: Formation of corrosion products (rust, oxides) that adhere to surfaces.
4. Biological fouling: Growth of microorganisms, algae, or biofilms in cooling water systems.
5. Chemical reaction fouling: Polymerization or decomposition of process fluids forming sticky layers.
6. Freezing fouling: Solidification of fluid components on cold surfaces (e.g., wax or ice).
Key Note:
Prevention: Proper filtration, chemical treatment, maintaining flow velocity, surface coatings, and regular cleaning schedules help minimize fouling.
Q: What is LMTD (Log Mean Temperature Difference)?
“LMTD represents the true average temperature difference between fluids — a key parameter ensuring precise and efficient heat exchanger design.”
Definition: LMTD is the average effective temperature difference between the hot and cold fluids in a heat exchanger, driving the heat transfer process.
Key Points:
It accounts for the temperature variation along the heat exchanger length.
Used in the formula:
Formula:
Applicable to: Both parallel flow and counter flow heat exchangers (LMTD is higher in counter flow).
Significance:
Provides a realistic measure of the temperature driving force.
Essential for accurate heat exchanger design and performance evaluation.
Q: What is effectiveness in a heat exchanger?
“Effectiveness tells how close a heat exchanger comes to its ideal performance — a direct measure of its thermal efficiency.”
Definition: Effectiveness (ε) is a measure of how efficiently a heat exchanger transfers heat compared to the maximum possible heat transfer.
Key Points:
It is the ratio of actual heat transfer to the maximum possible heat transfer:
Depends on heat exchanger type, flow arrangement (parallel, counter, cross), and NTU (Number of Transfer Units).
Range: 0 < ε < 1
ε = 1 → Ideal (perfect heat exchange)
Lower ε → Less efficient performance
Significance:
Indicates how effectively a heat exchanger utilizes the available temperature difference.
Useful for performance evaluation and design optimization without detailed temperature data.
Q: What is the difference between conduction and convection?
“Conduction is heat transfer by contact, while convection is heat transfer by fluid movement — conduction dominates in solids, convection in fluids.”
1. Conduction:
Heat transfer through direct contact between molecules.
Occurs in solids, liquids, or stationary fluids.
Governed by Fourier’s Law:
Example: Heat flow through a metal rod.
2. Convection:
Heat transfer by bulk movement of fluid (liquid or gas).
Involves both conduction and fluid motion.
Governed by Newton’s Law of Cooling:
Example: Cooling of hot water by air circulation.
Key Differences:
Medium: Conduction needs direct contact; convection requires fluid motion.
Mechanism: Molecular energy transfer vs mass flow of fluid.
Rate of Transfer: Convection is generally faster due to fluid motion.
Q: How do you calculate the heat duty of a heat exchanger?
“Heat duty quantifies the total heat exchanged — calculated either from fluid properties (ṁCpΔT) or design parameters (UAΔTlm), ensuring energy balance and performance check of the exchanger.”
Definition:
Heat duty is the total amount of heat transferred per unit time in a heat exchanger.
Formula Methods:
1. Using mass flow rate and specific heat:
Q = heat duty (kW or kcal/hr)
ṁ= mass flow rate
Cp= specific heat capacity
Tₒᵤₜ – Tᵢₙ= temperature change of the fluid
2. Using overall heat transfer approach:
U = overall heat transfer coefficient
A = heat transfer area
LMTD = log mean temperature difference
Key Points:
The same Q applies for both hot and cold sides (neglecting losses).
Hot fluid loses heat = Cold fluid gains heat.
Can be used to determine exchanger size or check performance.
🔹 Mass Transfer
Q: What is Diffusion? What is Fick’s law?
Diffusion is the natural process of molecular mixing driven by concentration difference, vital in mass transfer and reaction engineering.
Diffusion is the spontaneous movement of molecules from a region of higher concentration to lower concentration until equilibrium is reached.
It occurs due to the random thermal motion of particles.
It is a mass transfer process important in gases, liquids, and solids.
The driving force for diffusion is the concentration gradient.
Described mathematically by Fick’s Laws of Diffusion:
Fick’s First Law: relates diffusion flux to concentration gradient.
Fick’s Second Law: predicts concentration change with time.
Example: Oxygen transfer from air to water, or solute movement in a solution.
Key factors affecting diffusion: temperature, molecular size, and medium viscosity.
Q: What is Absorption and Stripping?
Absorption adds gas into liquid; Stripping removes solute from liquid using gas — both are key gas–liquid mass transfer operations in chemical industries.
Absorption:
Absorption is a mass transfer operation where a gas component is transferred into a liquid solvent.
The solute gas dissolves into the liquid phase due to concentration or partial pressure difference.
Common example:
CO₂ removal from flue gas using amine solution.
Driving force: Difference between the gas-phase partial pressure and the liquid-phase equilibrium pressure.
Basic formula:
Nₐ = kₐ × (pₐ - pₐ*)
where,
Nₐ = rate of absorption,
kₐ = mass transfer coefficient,
pₐ = partial pressure of solute in gas,
pₐ* = equilibrium partial pressure at liquid interface.
Equipment used:
Absorption column, packed tower, tray column.
Stripping:
Stripping is the reverse of absorption.
Here, a dissolved component is removed from a liquid by contacting it with an inert gas or vapor.
The solute transfers from liquid phase to gas phase.
Common example:
Ammonia removal from wastewater using air.
Driving force:
Difference between solute concentration in liquid and its equilibrium concentration in gas.
Basic formula:
Nₐ = kₐ × (Cₗ - Cₗ*)
where,
Cₗ = actual solute concentration in liquid,
Cₗ* = equilibrium concentration,
kₐ = liquid-phase mass transfer coefficient.
Equipment used: Stripping column, packed tower, steam stripper.
Q: What is the difference between Distillation and Extraction?
Distillation separates components by boiling point difference, while Extraction separates by solubility difference — distillation uses heat, extraction uses a solvent.
Distillation:
Distillation is a separation process based on differences in volatility or boiling points of components in a liquid mixture.
Components are separated by vaporization and condensation.
It requires heat energy for vapor generation.
Common example:
Separation of ethanol and water.
Driving force:
Difference in vapor–liquid equilibrium composition.
Basic formula (Raoult’s Law):
Pᵢ = xᵢ × Pᵢ°
where,
Pᵢ = partial pressure of component i,
xᵢ = mole fraction of component i in liquid,
Pᵢ° = vapor pressure of pure component i.
Equipment used: Distillation column, reboiler, condenser.
Extraction:
Extraction is a mass transfer process based on solubility difference of components in two immiscible phases (usually liquid–liquid or solid–liquid).
No vaporization is needed — separation occurs by selective dissolution.
Common example: Separation of acetic acid from water using ether.
Driving force: Concentration gradient between two phases.
Basic formula (Distribution Law):
C₁ / C₂ = Kd
where,
C₁ = solute concentration in phase 1,
C₂ = solute concentration in phase 2,
Kd = distribution coefficient.
Equipment used: Mixer–settler, extraction column, centrifuge extractor.
Q: What is Relative Volatility?
Relative volatility quantifies how much more volatile one component is than another — a key factor determining the feasibility and efficiency of distillation.
Relative volatility is a measure of the ease of separation of two components in a distillation process.
It expresses the ratio of vapor–liquid equilibrium tendencies (volatilities) of two components.
A higher relative volatility indicates easier separation.
When relative volatility = 1, separation is impossible (both components have equal volatility).
Driving principle:
Components with higher vapor pressure vaporize more readily.
Basic formula:
α₍ᵢⱼ₎ = (yᵢ / xᵢ) ÷ (yⱼ / xⱼ)
where,
α₍ᵢⱼ₎ = relative volatility of component i to j,
yᵢ, yⱼ = mole fractions in vapor phase,
xᵢ, xⱼ = mole fractions in liquid phase.
Simplified form (using K-values):
α₍ᵢⱼ₎ = Kᵢ / Kⱼ
where K = y/x is the equilibrium ratio for each component.
Typical range: For effective separation, α > 1.5 is desirable.
Q: What is a Distillation Column?
A distillation column is the heart of separation processes, enabling efficient component separation by exploiting boiling point differences through vapor–liquid equilibrium stages.
A distillation column is a mass transfer equipment used to separate liquid mixtures into components based on differences in their volatilities (boiling points).
It operates on the principle of repeated vaporization and condensation inside the column.
The column provides contact between rising vapor and descending liquid, allowing equilibrium stages for separation.
Two main types:
Tray (plate) column – uses trays or bubble caps for vapor–liquid contact.
Packed column – uses packing materials (Raschig rings, saddles) for contact surface.
Key components: Reboiler (heats liquid), Condenser (cools vapor), Feed point, Trays/packings, and Reflux system.
Driving force: Difference in vapor–liquid composition between the stages.
Basic formula (Fenske Equation – for minimum stages):
Nmin = log[(xD / (1 − xD)) × ((1 − xB) / xB)] ÷ log(αavg)
where,
Nmin = minimum number of theoretical stages,
xD = mole fraction of more volatile component in distillate,
xB = mole fraction of more volatile component in bottoms,
αavg = average relative volatility.
Applications: Separation of ethanol-water, petroleum fractions, air separation, etc.
Q: What is Reflux Ratio?
Reflux ratio is the balance between purity and efficiency in distillation — it defines how much condensed liquid is recycled to enhance separation quality.
The reflux ratio is a key parameter in distillation column design and operation, representing the amount of liquid returned to the column compared to the liquid product withdrawn from the top.
It controls the purity of the distillate and the energy efficiency of the separation.
Definition:
It is the ratio of reflux liquid (L) returned to the column to the distillate (D) taken out.
Formula:
Reflux Ratio (R) = L / D
where,
L = liquid returned to the column as reflux,
D = distillate product withdrawn.
Effect of reflux ratio:
Higher reflux ratio: better separation, more stages required, higher energy use.
Lower reflux ratio: lower energy consumption, but reduced separation efficiency.
Optimum reflux ratio gives desired purity with minimum energy cost.
In practice, operating reflux ratio is typically 1.2 to 1.5 times the minimum reflux ratio.
Q: What is Flooding in a Distillation Column?
Flooding is a condition where excess vapor flow obstructs liquid downflow, leading to inefficient separation and column instability — controlled by maintaining operation below the flooding velocity.
Flooding is an undesirable condition in a distillation column where liquid flow is hindered due to excessive vapor velocity.
It occurs when the vapor flow rate becomes so high that it carries liquid upward, preventing proper vapor–liquid contact and reducing separation efficiency.
Result:
High pressure drop, poor mass transfer, foaming, and sometimes liquid carryover in the distillate.
Main causes:
Excessive vapor load or heat input at the reboiler.
Improper design (tray spacing or packing).
High reflux rate or liquid holdup.
Indications of flooding:
Sudden pressure drop increase across the column.
Reduced tray efficiency and unstable operation.
Liquid entrainment or poor product purity.
Preventive measures:
Operate below flooding velocity (usually 60–80% of flooding limit).
Optimize reflux and reboiler duty.
Ensure proper tray or packing design.
Empirical formula (Flooding velocity):
Vf = C × √((ρL − ρV) / ρV)
where,
Vf = flooding vapor velocity,
C = capacity coefficient (depends on tray/packing type),
ρL = liquid density,
ρV = vapor density.
Q: What is the difference between adsorption and absorption?
Adsorption occurs at the surface, while absorption occurs into the bulk — Ad → At surface, Ab → All through.
Adsorption is a surface phenomenon, where molecules of a substance (adsorbate) accumulate only on the surface of another material (adsorbent).
Absorption is a bulk phenomenon, where molecules penetrate and get distributed throughout the volume of the absorbing material.
In adsorption, the concentration of adsorbate is high at the surface and decreases with depth.
In absorption, the concentration of absorbed substance remains uniform throughout the bulk.
Adsorption can be physical (physisorption) or chemical (chemisorption) in nature.
Absorption generally involves physical dissolution or diffusion of one substance into another.
The rate of adsorption decreases with time as the surface becomes saturated.
The rate of absorption remains nearly constant until equilibrium is achieved.
Example of adsorption: Gas molecules adsorbed on activated charcoal.
Example of absorption: Ammonia gas absorbed in water.
Formulas:
For adsorption: x/m = f(p, T)
where, x = mass of adsorbate, m = mass of adsorbent, p = pressure, T = temperature.
For absorption: N = kₗ (C - C)*
where, N = rate of absorption, kₗ = liquid-phase mass transfer coefficient, C** = interface concentration, C = bulk concentration.
🔹 Reaction Engineering
Q: What are the types of chemical reactors?
✅ In short:
Batch → closed system;
CSTR → continuous with mixing;
PFR → continuous without mixing;
PBR → catalytic solid bed;
Fluidized Bed → solid particles in motion for better transfer.
Chemical reactors are vessels where chemical reactions take place under controlled conditions of temperature, pressure, and mixing.
The main types of reactors are classified based on mode of operation and phase of reactants.
✅ 1. Batch Reactor:
Reactants are charged, reaction occurs, and products are withdrawn after completion.
No inflow or outflow during the reaction.
Suitable for small-scale or batch production and time-dependent reactions.
Formula: Rate of reaction → r = (1/V) × (dN/dt)
where, r = rate of reaction, V = reactor volume, N = moles of reactant, t = time.
✅ 2. Continuous Stirred Tank Reactor (CSTR):
Reactants enter and products leave continuously with uniform mixing inside the reactor.
Operates at steady state, composition inside the reactor is same as outlet.
Suitable for liquid-phase reactions and exothermic processes.
Formula: Material balance → Fₐ₀ - Fₐ + V·rₐ = 0
where, Fₐ₀ = inlet molar flow rate, Fₐ = outlet molar flow rate, V = volume, rₐ = rate of reaction.
✅ 3. Plug Flow Reactor (PFR):
Reactants flow continuously through a tubular reactor with no back-mixing.
Composition changes along the reactor length.
Ideal for large-scale continuous operations and high conversion.
Formula: -rₐ = (Fₐ₀/V) × (dX/dτ)
where, rₐ = rate of reaction, Fₐ₀ = inlet molar flow rate, X = conversion, τ = residence time.
✅ 4. Packed Bed Reactor (PBR):
Contains solid catalyst particles packed in a column; reactants pass through the bed.
Common in gas-solid catalytic reactions like hydrogenation or oxidation.
Formula: dFₐ/dW = rₐ'
where, Fₐ = molar flow rate, W = weight of catalyst, rₐ' = rate per unit weight of catalyst.
✅ 5. Fluidized Bed Reactor:
Solid catalyst particles are suspended by upward flow of gas or liquid.
Provides excellent heat and mass transfer.
Used in catalytic cracking and combustion processes.
Q: What is the difference between Batch Reactor, CSTR, and PFR?
✅ In short:
Batch Reactor: Closed, unsteady, good for small-scale or kinetic studies.
CSTR: Continuous, well-mixed, steady-state operation.
PFR: Continuous, un-mixed flow, ideal for high conversion and large-scale production.
The three main types of chemical reactors are Batch Reactor, Continuous Stirred Tank Reactor (CSTR), and Plug Flow Reactor (PFR).
They differ in mode of operation, flow pattern, and mixing characteristics.
✅ 1. Batch Reactor:
Operates in closed mode — reactants are loaded, reaction proceeds, and products are removed after completion.
No material enters or leaves during the reaction.
Conditions (temperature, pressure) can be varied with time.
Suitable for small-scale, batch, or laboratory operations.
Formula: Rate of reaction → r = (1/V) × (dN/dt)
where, r = rate of reaction, V = reactor volume, N = moles of reactant, t = time.
✅ 2. Continuous Stirred Tank Reactor (CSTR):
Operates in continuous mode with steady-state conditions.
Reactants continuously enter and products continuously leave the reactor.
Assumes perfect mixing — concentration inside the reactor is the same as outlet.
Suitable for liquid-phase and exothermic reactions.
Formula: Material balance → Fₐ₀ - Fₐ + V·rₐ = 0
where, Fₐ₀ = inlet molar flow rate, Fₐ = outlet molar flow rate, V = volume, rₐ = rate of reaction.
✅ 3. Plug Flow Reactor (PFR):
Operates in continuous mode with no back-mixing.
Reactants move through the reactor in plugs, and concentration changes along the reactor length.
Ideal for large-scale continuous operations with high conversion efficiency.
Formula: -rₐ = (Fₐ₀/V) × (dX/dτ)
where, rₐ = rate of reaction, Fₐ₀ = inlet molar flow rate, X = conversion, τ = residence time.
Q: What is Reaction Rate and Rate Constant?
Reaction Rate tells how fast a reaction occurs.
Rate Constant (k) tells how inherently reactive a system is under given conditions.
✅ Reaction Rate:
Reaction rate is the speed at which reactants are converted into products in a chemical reaction.
It indicates how fast the concentration of a reactant decreases or product increases with time.
It depends on factors like concentration, temperature, catalyst, and pressure.
Expressed as the change in concentration per unit time.
Formula:
Rate = - (1/a) × (d[A]/dt) = (1/b) × (d[B]/dt)
where, [A] = concentration of reactant, [B] = concentration of product, a and b = stoichiometric coefficients.
Unit: mol·L⁻¹·s⁻¹ (for concentration-based rate).
✅ Rate Constant (k):
Rate constant is a proportionality constant in the rate law that relates reaction rate to reactant concentrations.
It is specific for a given reaction at a particular temperature.
Depends on temperature (increases with temperature) but independent of concentration.
For a general reaction A → B,
Rate = k × [A]ⁿ
where, n = order of reaction.
Arrhenius Equation:
k = A × e^(-Ea/RT)
where, A = frequency factor, Ea = activation energy, R = gas constant, T = temperature (K).
Q: What is the order of reaction?
Order of reaction defines how reactant concentrations influence the rate, and helps in understanding reaction kinetics and mechanism.
Order of reaction is the sum of the powers of concentration terms of reactants in the rate law expression.
It shows how the rate of reaction depends on the concentration of reactants.
Determined experimentally (cannot be deduced from stoichiometry directly).
It indicates the sensitivity of the reaction rate to changes in reactant concentrations.
✅ Formula:
For a reaction A + B → Products,
Rate = k × [A]ᵐ × [B]ⁿ
where,
m = order with respect to A,
n = order with respect to B,
Total order = (m + n).
✅ Key Points:
Zero-order reaction:
Rate is independent of reactant concentration. (Rate = k)
First-order reaction:
Rate is directly proportional to one reactant. (Rate = k[A])
Second-order reaction:
Rate depends on the square of one reactant or product of two reactant concentrations. (Rate = k[A]² or k[A][B])
Fractional or mixed orders can also exist in complex reactions.
Unit of rate constant (k) changes with the order of reaction.
Q: What is Activation Energy?
Activation energy is the threshold energy required for reactants to transform into products — it controls reaction feasibility and speed.
Activation energy (Ea) is the minimum amount of energy that reacting molecules must possess to successfully collide and form products.
It represents the energy barrier that must be overcome for a chemical reaction to occur.
Reactions with low activation energy proceed faster, while those with high activation energy are slower.
It depends on the nature of reactants and can be lowered by using a catalyst.
Catalyst provides an alternative reaction pathway with a lower activation energy.
Activation energy determines the temperature sensitivity of a reaction — higher Ea means greater effect of temperature on rate.
✅ Formula (Arrhenius Equation):
k = A × e^(-Ea / RT)
where,
k = rate constant,
A = frequency factor,
Ea = activation energy,
R = gas constant,
T = absolute temperature (K).
✅ Alternative form (for two temperatures):
ln(k₂ / k₁) = (Ea / R) × [(1/T₁) - (1/T₂)]
Q: What is Arrhenius Equation?
The Arrhenius equation explains how temperature and activation energy influence the speed of a chemical reaction, making it a key tool in reaction kinetics.
The Arrhenius equation explains how temperature and activation energy influence the speed of a chemical reaction, making it a key tool in reaction kinetics.
The Arrhenius Equation expresses the relationship between the rate constant (k) of a reaction and temperature (T).
It shows that reaction rate increases with temperature because more molecules gain sufficient energy to overcome the activation energy barrier.
The equation was proposed by Svante Arrhenius to explain the temperature dependence of reaction rates.
It helps in determining activation energy (Ea) and predicting reaction behavior at different temperatures.
Formula:
k = A × e^(-Ea / RT)
where,
k = rate constant,
A = frequency factor (collision frequency and orientation),
Ea = activation energy,
R = universal gas constant,
T = absolute temperature (K),
e = base of natural logarithm.
Logarithmic Form:
ln k = ln A - (Ea / R) × (1/T)
→ A straight-line form used to determine Ea experimentally from a plot of ln k vs. 1/T.
Alternate Temperature Form:
ln(k₂ / k₁) = (Ea / R) × [(1/T₁) - (1/T₂)]
Q: What is Residence Time?
Residence time is the average duration a reactant stays inside the reactor, directly influencing reaction completion and efficiency.
Residence time is the average time a fluid element or reactant molecule spends inside a reactor.
It indicates how long reactants remain in contact under reaction conditions before leaving the reactor.
It is a key parameter in reactor design as it affects conversion, yield, and reactor performance.
Also known as mean residence time (τ) or space time in continuous reactors.
Residence time depends on reactor volume and volumetric flow rate of the fluid.
Longer residence time usually leads to higher conversion, while too long may cause undesired side reactions.
Formula:
τ = V / Q
where,
τ = residence time,
V = reactor volume,
Q = volumetric flow rate of the fluid.
For batch reactor:
Residence time equals reaction time since the system is closed.
Q: What is a Catalyst and its Role?
A catalyst speeds up reactions by lowering activation energy and improving process efficiency without being consumed in the reaction.
A catalyst is a substance that increases the rate of a chemical reaction without being consumed or permanently altered in the process.
It works by providing an alternative reaction pathway with a lower activation energy (Ea).
By reducing the energy barrier, more molecules attain the required energy to react, thus accelerating the reaction rate.
A catalyst does not change the equilibrium position; it only helps the system reach equilibrium faster.
It is effective in small quantities and can be reused multiple times.
Catalysts are broadly classified as homogeneous (same phase as reactants) and heterogeneous (different phase).
In industrial processes, catalysts are essential for increasing efficiency, selectivity, and reducing energy consumption.
Formula (based on Arrhenius Equation):
k = A × e^(-Ea / RT)
→ A catalyst increases k by lowering Ea, while A and R remain constant.
Example:
Platinum in catalytic converters (automotive).
Iron in the Haber process (NH₃ synthesis).
Q: What causes Catalyst Deactivation?
Catalyst deactivation is caused by coking, poisoning, sintering, leaching, or structural damage, leading to a decline in activity and efficiency over time.
Catalyst deactivation is the loss of catalytic activity and selectivity over time during a chemical process.
It occurs due to physical, chemical, or mechanical changes in the catalyst or its surface.
Deactivation reduces the reaction rate, product yield, and process efficiency.
Major Causes of Catalyst Deactivation:
1. Fouling or Coking:
Deposition of carbon (coke) or heavy hydrocarbons on the catalyst surface blocks active sites.
Common in hydrocarbon processing and cracking reactions.
2. Poisoning:
Irreversible adsorption of impurities (e.g., sulfur, lead, chlorine) on active sites.
Even small amounts of poisons can deactivate catalysts significantly.
3. Sintering (Thermal Degradation):
High temperatures cause catalyst particles to agglomerate, reducing surface area and active sites.
Common in metal-based catalysts.
4. Leaching:
Loss of active material due to dissolution in liquid phase or reaction with the medium.
Seen in liquid-phase catalytic reactions.
5. Structural Changes:
Alteration in crystal structure or phase transformation reduces catalytic performance.
Can be due to temperature, pressure, or chemical attack.
General Deactivation Rate Expression:
- (da/dt) = k_d × aⁿ
where,
a = catalyst activity,
k_d = deactivation rate constant,
n = order of deactivation,
t = time.
Q: What is Selectivity in Reaction Engineering?
Selectivity defines how selectively a reaction produces the desired product, ensuring maximum yield, minimal waste, and better process control in chemical engineering.
Selectivity is a measure of how efficiently a reactant is converted into a desired product compared to undesired or side products.
It indicates the preference of a reaction pathway toward forming a specific product when multiple reactions occur simultaneously.
High selectivity means more desired product and fewer by-products — a key factor for process efficiency and economic viability.
Selectivity depends on factors like temperature, pressure, catalyst, and residence time.
It is especially important in parallel and consecutive reactions where competing reactions can reduce yield.
Formula:
Selectivity (S) = (Moles of desired product formed) / (Moles of undesired product formed) or, in terms of reactant conversion:
S = (Rate of desired reaction) / (Rate of undesired reaction)
For multiple products:
Sᵢ = (νᵢ × Fᵢ) / Σ(νⱼ × Fⱼ)
where,
νᵢ = stoichiometric coefficient of product i,
Fᵢ = molar flow rate of product i.
🔹 Thermodynamics
Q: What is the First Law of Thermodynamics?
The First Law of Thermodynamics ensures that the total energy of a system is conserved, linking heat, work, and internal energy in every process.
The First Law of Thermodynamics is the law of conservation of energy applied to thermodynamic systems.
It states that energy can neither be created nor destroyed, but can only be transformed from one form to another.
The total energy of an isolated system remains constant during any physical or chemical process.
Energy may appear as heat (Q) or work (W), but their sum equals the change in internal energy (ΔU) of the system.
This law forms the basis for energy balance in all engineering and thermodynamic calculations.
Formula:
ΔU = Q - W
where,
ΔU = change in internal energy of the system,
Q = heat added to the system,
W = work done by the system.
Sign Convention:
Q is positive when heat is added to the system.
W is positive when work is done by the system.
Q: What is the Second Law of Thermodynamics?
The Second Law of Thermodynamics states that entropy of the universe always increases, defining the direction, feasibility, and limits of thermodynamic processes.
The Second Law of Thermodynamics defines the direction of energy transfer and introduces the concept of entropy (S).
It states that heat cannot spontaneously flow from a colder body to a hotter body without external work.
It implies that every natural process has a preferred direction and tends toward increased disorder or entropy.
While the First Law deals with energy conservation, the Second Law deals with energy quality and irreversibility.
It explains why 100% conversion of heat to work is impossible and sets the limit for efficiency in engines and refrigerators.
Formula (for reversible process):
dS = δQ_rev / T
where,
dS = change in entropy,
δQ_rev = reversible heat transfer,
T = absolute temperature (K).
For an isolated system:
ΔS ≥ 0
Equality holds for reversible processes,
Inequality holds for irreversible processes.
Q: What is an Isothermal, Adiabatic, and Isobaric Process?
Isothermal: Temperature constant → Q = W
Adiabatic: No heat transfer → Q = 0
Isobaric: Pressure constant → Q = nCpΔT
Each describes how energy and work interact under specific thermodynamic constraints.
Isothermal Process:
An isothermal process occurs at constant temperature throughout.
Since temperature is constant, the internal energy (ΔU) of an ideal gas remains zero.
Any heat added (Q) to the system is used to do work (W).
Common in slow processes with good heat exchange with surroundings.
Formula:
Q = W = nRT ln(V₂/V₁)
where, n = moles of gas, R = gas constant, T = temperature, V₁, V₂ = initial and final volumes.
Adiabatic Process:
An adiabatic process occurs with no heat transfer between the system and surroundings (Q = 0).
All energy change appears as work done and change in internal energy.
Rapid processes or perfectly insulated systems approximate adiabatic behavior.
Formula:
P·V^γ = constant or T·V^(γ−1) = constant
where, γ = Cp/Cv (ratio of specific heats).
Work done:
W = [P₁V₁ - P₂V₂] / (γ - 1)
Isobaric Process:
An isobaric process takes place at constant pressure.
The heat added or removed changes both internal energy and volume.
Common in systems like expansion of gases in pistons.
Formula:
Q = ΔU + PΔV = nCpΔT
where, Cp = specific heat at constant pressure, ΔT = temperature change.
Q: What is Gibbs Free Energy?
Gibbs free energy represents the usable energy available to do work and serves as a key indicator of reaction spontaneity under constant temperature and pressure conditions.
Gibbs Free Energy (G) is a thermodynamic potential that measures the maximum reversible work obtainable from a system at constant temperature and pressure.
It determines the spontaneity and feasibility of a chemical or physical process.
A decrease in Gibbs free energy indicates that a reaction can occur spontaneously.
It combines both enthalpy (H) and entropy (S) changes to predict the overall energy balance of a system.
Gibbs free energy is widely used in chemical thermodynamics, reaction equilibrium, and phase transformations.
Formula:
G = H − T·S
where,
G = Gibbs free energy,
H = enthalpy,
T = absolute temperature (K),
S = entropy.
Change in Gibbs Free Energy:
ΔG = ΔH − T·ΔS
Spontaneity Criteria:
ΔG < 0: Reaction is spontaneous.
ΔG = 0: Reaction is at equilibrium.
ΔG > 0: Reaction is non-spontaneous.
Q: What is the difference between Reversible and Irreversible Processes?
A reversible process is ideal, infinitely slow, and perfectly efficient, while an irreversible process is real, spontaneous, and always accompanied by entropy generation.
Reversible and irreversible processes are two fundamental types of thermodynamic processes based on how energy transfer occurs between a system and its surroundings.
Reversible Process:
A reversible process is an idealized process that occurs infinitely slowly, allowing the system to remain in thermodynamic equilibrium at all times.
Both the system and surroundings can be restored to their initial states without any net energy loss.
It is a theoretical concept used for maximum efficiency (e.g., Carnot cycle).
No entropy generation occurs in a reversible process.
Formula:
dS_system + dS_surroundings = 0
Example: Isothermal reversible expansion of an ideal gas.
Irreversible Process:
An irreversible process occurs rapidly and involves finite driving forces such as friction, heat loss, or unrestrained expansion.
The system cannot be restored to its initial state without external intervention.
Always involves entropy generation and loss of useful energy.
It represents real, natural processes occurring in daily life.
Formula:
dS_system + dS_surroundings > 0
Example: Free expansion of gas, mixing of gases, combustion, etc.
Q: What is Vapor–Liquid Equilibrium (VLE)?
Vapor–Liquid Equilibrium defines the balance between vapor and liquid compositions at specific conditions and is crucial for designing efficient separation and purification processes.
Vapor–Liquid Equilibrium (VLE) is the condition where a liquid and its vapor phase coexist in equilibrium at a given temperature and pressure.
At VLE, the rate of evaporation equals the rate of condensation, and the chemical potential of each component is the same in both phases.
It is fundamental in distillation, absorption, and other separation processes in chemical engineering.
VLE helps determine the composition of vapor and liquid phases, which differ due to different volatilities of components.
It depends on temperature, pressure, and intermolecular interactions.
Condition for Equilibrium:
fᵢᴸ = fᵢⱽ
where,
fᵢᴸ = fugacity of component i in liquid phase,
fᵢⱽ = fugacity of component i in vapor phase.
Using Raoult’s Law (for ideal systems):
Pᵢ = xᵢ · Pᵢ⁰
and
yᵢ = (xᵢ · Pᵢ⁰) / P_total
where,
Pᵢ = partial pressure of component i,
xᵢ = mole fraction in liquid phase,
yᵢ = mole fraction in vapor phase,
Pᵢ⁰ = vapor pressure of pure component i.
For non-ideal systems (using activity coefficients):
Pᵢ = γᵢ · xᵢ · Pᵢ⁰
where γᵢ = activity coefficient.
Q: What is Raoult’s Law?
Raoult’s Law states that each component in a liquid mixture contributes to the total vapor pressure in proportion to its mole fraction, serving as a key principle for ideal solution behavior and phase equilibrium analysis.
Raoult’s Law describes the relationship between the vapor pressure of a component in a liquid mixture and its mole fraction in the liquid phase.
It applies to ideal solutions, where the intermolecular forces between unlike molecules are similar to those between like molecules.
It states that the partial vapor pressure of each component is directly proportional to its mole fraction in the liquid phase.
The total pressure of the system is the sum of the partial pressures of all components.
Raoult’s Law is fundamental in vapor–liquid equilibrium (VLE) calculations and distillation design.
Formula:
Pᵢ = xᵢ × Pᵢ⁰
where,
Pᵢ = partial vapor pressure of component i,
xᵢ = mole fraction of component i in the liquid phase,
Pᵢ⁰ = vapor pressure of pure component i at the same temperature.
Total Pressure:
P_total = Σ(xᵢ × Pᵢ⁰)
For Non-Ideal Solutions (Modified Raoult’s Law):
Pᵢ = γᵢ × xᵢ × Pᵢ⁰
where γᵢ = activity coefficient (accounts for non-ideality).
Q: What is a Phase Diagram?
A phase diagram illustrates how a substance changes phase with temperature and pressure, helping predict equilibrium states and transition points for chemical and physical processes.
A phase diagram is a graphical representation that shows the equilibrium relationship between different phases (solid, liquid, and vapor) of a substance under varying temperature and pressure conditions.
It helps identify the state of matter and the conditions for phase transitions such as melting, boiling, and sublimation.
Each line or curve on the diagram represents the boundary where two phases coexist in equilibrium.
It is an essential tool in thermodynamics, chemical engineering, and material science for understanding phase behavior and designing separation or cooling processes.
Main Regions and Points:
Solid Region: Stable at low temperature and high pressure.
Liquid Region: Exists at moderate temperature and pressure.
Vapor Region: Stable at high temperature and low pressure.
Triple Point: The unique condition where solid, liquid, and vapor coexist in equilibrium.
Critical Point: The condition beyond which liquid and vapor phases become indistinguishable.
General Relation (Clausius–Clapeyron Equation):
dP/dT = ΔH_vap / (T × ΔV)
where,
dP/dT = slope of phase boundary,
ΔH_vap = enthalpy of vaporization,
T = absolute temperature,
ΔV = change in volume during phase change.
Q: What is a Refrigeration Cycle?
A refrigeration cycle transfers heat from a cold region to a hot region using a refrigerant and external work, with performance measured by its COP — the higher the COP, the more efficient the system.
A refrigeration cycle is a thermodynamic process that removes heat from a low-temperature region and rejects it to a high-temperature region, maintaining the desired cooling effect.
It operates on the principle of heat absorption during evaporation and heat rejection during condensation of a refrigerant.
The cycle requires external work input, usually from a compressor, to drive the process.
It is widely used in refrigerators, air conditioners, and industrial cooling systems.
Main Components:
1. Evaporator: Absorbs heat from the space to be cooled; refrigerant evaporates.
2. Compressor: Compresses vapor to high pressure and temperature.
3. Condenser: Rejects heat to surroundings; vapor condenses into liquid.
4. Expansion Valve: Reduces pressure and temperature of refrigerant before entering evaporator.
Basic Steps of Vapor-Compression Refrigeration Cycle:
1. Evaporation: Low-pressure liquid absorbs heat → converts to vapor.
2. Compression: Vapor compressed to high pressure.
3. Condensation: High-pressure vapor releases heat → becomes liquid.
4. Expansion: Liquid expands, pressure drops → cycle repeats.
Performance Parameter (Coefficient of Performance, COP):
COP = Q_L / W
where,
COP = coefficient of performance,
Q_L = heat absorbed from refrigerated space,
W = work input to compressor.
Q: What is the difference between a Heat Engine and a Heat Pump?
Heat Engine: Converts heat → work, operates naturally from hot to cold.
Heat Pump: Uses work → move heat, operates from cold to hot.
Both obey thermodynamic laws but serve opposite purposes — one produces power, the other transfers heat.
Both heat engines and heat pumps operate on the principles of thermodynamics, but their objectives and directions of heat flow are opposite.
Heat Engine:
A heat engine converts heat energy into useful mechanical work.
It operates by receiving heat (Q₁) from a high-temperature source, converting part of it into work (W), and rejecting the remaining heat (Q₂) to a low-temperature sink.
The process is cyclic and follows the Second Law of Thermodynamics.
Used in power plants, automobiles, and turbines.
Formula (Efficiency):
η = W / Q₁ = 1 − (Q₂ / Q₁)
where,
η = thermal efficiency,
W = work output,
Q₁ = heat absorbed,
Q₂ = heat rejected.
Heat Pump:
A heat pump works in the reverse direction of a heat engine.
It uses work input (W) to transfer heat (Q₁) from a low-temperature reservoir to a high-temperature reservoir.
The main goal is heating or cooling, not work generation.
Commonly used in air conditioners and refrigeration systems.
Formula (Coefficient of Performance, COP):
COP_HP = Q₁ / W = 1 + (Q₁ − Q₂) / (Q₁ − Q₂)
Simplified as,
COP_HP = 1 + (T_L / (T_H − T_L))
🔹 Process Design & Equipment
Q: What is Process Simulation?
Process simulation is a virtual representation of real industrial operations used to predict performance, enhance design accuracy, and minimize cost and risk before plant implementation.
Process simulation is the use of computer-based models to analyze, design, and optimize chemical or industrial processes without physically performing them.
It involves mathematical modeling of mass, energy, and momentum balances to predict the performance of a process under various conditions.
The goal is to improve efficiency, safety, and economics before implementing the process on a large scale.
It helps engineers understand process behavior, equipment sizing, and control strategies.
Process simulation is a key tool in chemical, petrochemical, and environmental engineering.
Key Steps in Process Simulation:
1. Define process flow diagram (PFD) – identify units, streams, and connections.
2. Select thermodynamic models – choose equations of state or activity models.
3. Input process data – feed composition, pressure, temperature, and flow rates.
4. Run material and energy balances – solve for unknown variables.
5. Analyze results – check conversion, yield, energy consumption, and cost.
Common Simulation Software:
Aspen Plus – steady-state simulation.
HYSYS – gas processing and refinery systems.
CHEMCAD, PRO/II, DWSIM – general chemical process modeling tools.
Basic Energy Balance Formula Used:
Σ (ṁ × h)_in = Σ (ṁ × h)_out + Q − W
where,
ṁ = mass flow rate,
h = specific enthalpy,
Q = heat added,
W = work done.
Q: What are commonly used process simulators (Aspen Plus, HYSYS)?
Aspen Plus is best for chemical and steady-state design, while Aspen HYSYS excels in hydrocarbon and dynamic process simulation — both are essential tools for modern process engineers.
Process simulators are powerful computer-based tools used to model, design, and optimize chemical, petrochemical, and industrial processes.
They help engineers perform mass and energy balances, thermodynamic calculations, and equipment design virtually before actual implementation.
Among the most widely used simulators are Aspen Plus and Aspen HYSYS, developed by AspenTech.
Aspen Plus:
Used mainly for steady-state simulation of chemical and petrochemical processes.
Handles complex reactions, phase equilibria, and separation units like distillation, extraction, and absorption.
Based on rigorous thermodynamic models such as Peng–Robinson, NRTL, and UNIQUAC.
Ideal for process design, optimization, and sensitivity analysis.
Commonly applied in chemical, polymer, and specialty chemical industries.
Aspen HYSYS:
Designed for oil, gas, and refinery processes, focusing on hydrocarbon systems and energy industries.
Supports both steady-state and dynamic simulations, useful for process control and safety studies.
Excellent for modeling compressors, heat exchangers, separators, and pipelines.
Widely used in natural gas processing, LNG, and power generation.
Uses equations of state (EOS) like Peng–Robinson and Soave–Redlich–Kwong.
Key Benefits of Using Process Simulators:
Reduce design time and cost by virtual testing.
Optimize process performance and energy efficiency.
Support scale-up, troubleshooting, and operator training.
Enable “what-if” analysis for process improvement.
Q: What is Design Pressure and Design Temperature?
Design Pressure ensures the equipment can handle the maximum expected internal pressure.
Design Temperature ensures it can withstand the maximum expected operating temperature — both together guarantee safety, reliability, and compliance in plant design.
Design pressure and design temperature are critical parameters used in the mechanical design of process equipment, such as vessels, reactors, heat exchangers, and pipelines.
They ensure that equipment can safely withstand operating conditions, including possible fluctuations and upsets.
Design Pressure:
Design pressure is the maximum gauge pressure that equipment is designed to safely handle at the design temperature.
It is usually higher than the normal operating pressure to provide a safety margin.
Determined based on the worst-case pressure scenario, such as blocked outlet, fire exposure, or control valve failure.
It serves as the basis for mechanical thickness, material selection, and pressure relief device sizing.
Typical rule:
Design Pressure = Operating Pressure + Safety Margin (≈ 10–25%)
Design Temperature:
Design temperature is the maximum temperature at which the equipment is expected to operate safely under design pressure.
It is selected higher than the normal operating temperature to allow for process upsets and heat transfer variations.
Also used to determine material strength, thermal expansion, and allowable stress.
Typical rule:
Design Temperature = Operating Temperature + Safety Margin (≈ 10–20°C)
Formula (Basic Relation):
P_design ≥ P_operating
T_design ≥ T_operating