Chemical Reactor Related interview Questions contained in this blog which are asked in chemical industry
What are the main parts of a Chemical Reactor?
A chemical reactor consists of the vessel, agitator, baffles, heating/cooling system, inlets/outlets, instrumentation, safety devices, and manways.
1. Reactor Vessel (Body):
The main container where the chemical reaction takes place.
Ensures homogeneous mixing of reactants.
Reduces concentration and temperature gradients.
3. Baffles:
Vertical strips inside the reactor to improve mixing and prevent vortex formation.
4. Heating / Cooling Jacket or Coils:
Provides temperature control via steam, water, or other heat transfer fluids.
5. Inlets & Outlets (Nozzles):
For feeding reactants, sampling, adding catalysts, and product discharge.
6. Instrumentation & Sensors:
Can be connected to DCS/PLC for control.
7. Safety Devices:
8. Manway / Manhole:
For cleaning, inspection, and maintenance.
Q: What are the main types of chemical reactors?
Closed system, reactants charged and products removed after reaction.
Suitable for small-scale, flexible operations, pharmaceuticals, specialty chemicals.
Continuous Stirred Tank Reactor (CSTR)
Continuous input and output, well-mixed, uniform composition.
Used for liquid-phase reactions, moderate conversion per pass.
Reactants flow in one direction without back-mixing.
High conversion, suitable for large-scale continuous processes.
Contains catalyst pellets, gas or liquid flows through the bed.
Common in petrochemical and catalytic processes.
Catalyst particles suspended by upward flowing fluid.
Excellent mixing, heat transfer, used in catalytic cracking.
Combination of batch and continuous (feeds added during operation).
Useful for controlling reaction rate and heat release.
Q: When would you select a batch reactor versus a continuous reactor?
Batch Reactor is preferred when:
Production is small-scale or multiproduct.
Processes require flexibility (different recipes/batches).
Reaction time is long or uncertain.
High-value, low-volume products like pharmaceuticals, specialty chemicals.
Continuous Reactor (CSTR/PFR) is preferred when:
Production is large-scale and single product.
Need for consistent quality and steady operation.
Reaction is fast and well-defined.
Common in petrochemicals, bulk chemicals, and fuels.
Q: Why do we use stainless steel reactors in industries?
Stainless steel reactors are preferred for their strength, corrosion resistance, cleanliness, and versatility, ensuring safe, efficient, and high-quality industrial operations.
Corrosion Resistance:
Stainless steel offers excellent resistance against acids, chemicals, and moisture, increasing reactor life.
High Strength and Durability:
It can withstand high pressure and temperature, making it ideal for batch and continuous reactions.
Hygienic and Easy to Clean:
Its smooth surface prevents contamination and bacterial growth, which is crucial in pharmaceutical and food industries.
Chemical Compatibility:
Suitable for handling a wide range of reactants, solvents, and catalysts without affecting product quality.
Good Heat Transfer:
Stainless steel allows efficient heating and cooling through jackets or coils, ensuring uniform reaction temperature.
Long Service Life and Low Maintenance:
Provides high reliability and reduced maintenance costs compared to mild steel or lined vessels.
Why steam enters top side of jacket in reactor?
If we pass the steam from bottom side the condensate that is formed after
losing the heat won’t have a comfortable passage to get out of the system.
In turn the entering steam will start to heat the returning condensate rather than heating the reactor surface. That’s why we have to pass it from the top.
Why the hot liquid in heat exchanger, reactor jacket should flow from
bottom to top?
If we pass the liquid from top to bottom, it will flow fast by gravity
itself. So it will have less contact time with the heat exchanger/reactor
surface which will result in poor heat transfer.That’s why the hot fluid should be passed from bottom to top to maximize the contact time. The same is applicable for cold fluid also in reactors.
What is the
function of catalyst in chemical industry?
A catalyst improves speed of
chemical reaction by providing an alternate reaction pathway with lower
activation energy.
Since the activation energy is lower, more products will be
formed in the same amount of time.
Q: What is the use of a Glass-Lined Reactor?
A Glass-Lined Reactor is used to safely handle corrosive and high-purity chemical reactions, ensuring durability, cleanliness, and product quality in critical chemical processes.
1. Purpose:
Used for chemical reactions involving corrosive or highly reactive materials.
Combines the strength of steel with the corrosion resistance of glass.
2. Key Uses:
Acidic and alkaline reactions where metal reactors would corrode.
Pharmaceutical, chemical, and agrochemical industries for synthesis and formulation.
Batch reactions, crystallization, polymerization, and mixing of sensitive chemicals.
3. Advantages:
Excellent corrosion and chemical resistance.
Smooth glass surface prevents product contamination and sticking.
Easy to clean and maintain high purity in multiproduct plants.
Can withstand vacuum and pressure conditions safely.
Q: What is Thermal Shock in a Reactor?
Thermal Shock is a stress failure due to sudden temperature change, mainly in glass-lined reactors, and is prevented by gradual temperature control and uniform heating/cooling.
1. Definition:
Thermal Shock occurs when a reactor experiences a sudden temperature change, causing rapid expansion or contraction of the reactor material.
2. Cause:
Sudden heating or cooling of the reactor (e.g., charging cold material into a hot vessel or vice versa).
Uneven temperature distribution between glass lining and steel shell in glass-lined reactors.
3. Effects:
Cracking or damage to the glass lining.
Loss of corrosion resistance and risk of contamination.
Reduced reactor life and possible leakage or equipment failure.
4. Prevention:
Maintain controlled heating and cooling rates.
Avoid sudden addition of hot or cold materials.
Use temperature monitoring and gradual ramping during process changes.
Thermal Shock is a stress failure due to sudden temperature change, mainly in glass-lined reactors, and is prevented by gradual temperature control and uniform heating/cooling.
Difference
between Reversible and Irreversible chemical reactions.
In reversible reaction, reactants
react to form a new product & are get back the original product or
reactant.
While in irreversible reaction, it’s impossible to get back original
product or reactant.
In reversible
reaction, changes take place very slowly
through a series of intermediate step in the equilibrium state.
While in irreversible reaction, there is no equilibrium state.
Q: What is Glass Transition Temperature (Tg)?
1. Definition:
The Glass Transition Temperature (Tg) is the temperature at which an amorphous material (like polymer or glass) changes from a hard, brittle (glassy) state to a soft, rubbery (flexible) state.
2. Nature of Transition:
It is not a sharp phase change like melting but a gradual change in physical properties (hardness, flexibility, heat capacity).
3. Applicable Materials:
Mainly observed in amorphous polymers, glass, and resins.
Crystalline materials have melting points, not Tg.
4. Importance:
Determines operating temperature limits of materials.
Helps in material selection for coatings, plastics, and reactors.
Below Tg → material is rigid; above Tg → soft and flexible.
5. Example:
PTFE: Tg ≈ –100°C
Polystyrene: Tg ≈ 100°C
Q: How do you check the RPM of an agitator on a reactor?
The most reliable way is using a tachometer, but you can also estimate RPM from motor data, gearbox ratio, or VFD frequency if direct measurement isn’t possible.
1. Tachometer Method:
Use a digital or contact tachometer to measure the agitator shaft speed directly.
Place the tachometer tip on the shaft (for contact type) or point the laser (for non-contact type) while the agitator is running.
The device displays RPM instantly and accurately.
2. Motor Nameplate & Gear Ratio Method:
Check the motor RPM from the nameplate.
If a gearbox is present, calculate agitator RPM using:
3. Frequency to RPM Conversion (for VFD):
If the motor is controlled by a VFD, you can calculate approximate RPM from frequency:
4. Mechanical Counter (Manual):
Mark a point on the shaft and count rotations per minute manually using a stopwatch (used only for approximate checking).
How much power required for agitation?
It is a function of RPM.
It is also depends on
- Viscosity of fluid
- Density of fluid.
- Dimensions of vessels and Impeller
It is related by dimensionless form as:
P=power required for agitation
n = rotational speed of impeller
Da= diameter of impeller
९= density of fluid
For baffled vessels, Power number (Np) does not depend on the value of Froude number (Nfr). and it is only a function of Reynolds number (NRe).
Empirical correlations and charts are available with the dimensional groups to get Np. from which power required for agitation is calculated. This required power is supplied by means of electrical drive / gearbox assembly and transmitted to the vessels by the impeller attached with the shaft coupled to the drive.
The mechanical design of agiated vessel is calculated as per pressure vessel design codes such as ASME Section VIII Division 1 or IS 2825.
How do you decide the RPM of an agitator in reactors?
The RPM of an agitator is decided based on the purpose of mixing, fluid properties, and reactor design.
Key factors include viscosity, density, and phase (liquid–liquid, solid–liquid, or gas–liquid).
Low RPM is used for blending and maintaining suspension, while higher RPM is needed for dispersion or mass transfer.
Too low speed leads to poor mixing; too high causes vortex formation, high power consumption, and equipment damage.
Generally, scale-up correlations, Reynolds number, and power number are used to select the optimum speed that achieves efficient mixing with minimal energy consumption and mechanical stress.
What is the voltage used in spark test of glass lined reactor?
Spark test of glass-lined reactor is usually carried out at ~15 kV (range 10–20 kV) to check lining integrity.
Purpose: To check integrity of glass lining and detect pinholes, cracks, or defects.
Test Method: High voltage spark tester is used.
Typical Voltage Range:
10 kV to 20 kV (commonly applied).
Standard practice → around 15 kV for most glass-lined equipment.
Procedure: Voltage is applied between the conductive substrate (metal shell) and probe. Spark indicates defect.
Key Point: Voltage should be sufficient to detect flaws but not so high that it damages the glass lining.
Q: What factors influence the rate of a chemical reaction inside reactors?
Concentration of reactants – higher concentration increases collision frequency.
Temperature – higher temperature accelerates reaction by increasing kinetic energy.
Pressure – important for gas-phase reactions; higher pressure raises reaction rate.
Catalyst presence – lowers activation energy, speeds up reaction.
Mixing and mass transfer – ensures uniform reactant distribution and avoids limitations.
Heat transfer – controls temperature, prevents hot spots or quenching.
Reactor design (Batch, CSTR, PFR, etc.) – affects residence time and conversion.
Q: How does temperature affect reaction rate and reactor selection?
Reaction Rate:
Higher temperature increases molecular collisions → faster reaction.
Follows Arrhenius equation: rate rises exponentially with temperature.
Too high temperature may cause side reactions or catalyst deactivation.
Reactor Selection:
For exothermic reactions → prefer CSTR/Fluidized Bed (better temperature control).
For endothermic reactions → Tubular/PFR with external heating.
Temperature control is critical in scale-up to ensure safety and selectivity.
Q: Explain the concept of reactor yield and selectivity.
Yield
Ratio of desired product formed to the theoretical or possible maximum.
Indicates overall efficiency of conversion.
Example: If 100 mol reactant → 80 mol desired product, yield = 80%.
Selectivity
Ratio of desired product to undesired/by-products formed.
Reflects how well the reaction pathway is controlled.
High selectivity = less waste, more economical process.
Key Difference
Yield = how much desired product you got.
Selectivity = how cleanly you got it without side products.
Q: How do you approach safe operation and risk assessment in reactor processes?
Process Hazard Identification – use HAZOP, FMEA, or What-if analysis to identify risks.
Thermal & Pressure Control – ensure proper cooling, venting, and pressure relief systems.
Instrumentation & Automation – apply alarms, interlocks, and emergency shutdown systems.
Operating Procedures – follow SOPs, batch records, and safety checklists.
Material Compatibility – check corrosion, reactivity, and catalyst stability.
Emergency Preparedness – plan for leaks, runaway reactions, or power failure.
Training & Culture – ensure operators are trained and safety-first mindset is enforced.
Q: Why are glass-lined reactors not used for base reactions?
Glass-lined reactors are avoided for base reactions because strong alkalis corrode the glass lining, damage the equipment, reduce durability, and compromise safety.
Glass lining is sensitive to strong alkalis: Strong bases like NaOH or KOH can attack and corrode the glass layer, causing damage.
Risk of erosion and delamination:
Prolonged exposure to alkaline media can lead to peeling, cracking, or erosion of the glass lining.
Loss of reactor integrity:
Once the glass layer is damaged, the reactor loses its corrosion resistance, leading to contamination and potential failure.
Reduced service life:
Continuous use with basic reactions significantly shortens the equipment’s lifespan and increases maintenance costs.
Safety concerns:
Damaged glass lining may lead to leakage, pressure issues, or product contamination, posing safety hazards.
Q: How do you calculate the required steam for reactor load calculation?
→ Determine heat load
→ Apply energy balance
→ Convert to steam requirement
→ Add losses and verify with system capacity.
To calculate steam required for heating or maintaining temperature in a reactor:
1️⃣ Identify Data Required:
Reactor mass of material (m) in kg
Specific heat (Cp) of material in kCal/kg°C or kJ/kg°C
Initial and final temperature (T₁, T₂) in °C
Heating time (t) in hours or seconds
Latent heat of steam (λ) in kCal/kg (typically 540 kcal/kg for saturated steam)
2️⃣ Calculate Heat Required (Q):
Q = m × Cp × (T₂ - T₁)
3️⃣ Convert Heat Energy to Steam Requirement:
4️⃣ Include Additional Loads:
Heat losses through insulation (~5–10%)
Jacket/coil condensate load
Agitator and reaction exotherm/endotherm if applicable
5️⃣ Final Check:
Compare calculated steam load with steam trap and control valve capacity
Ensure sufficient steam pressure to meet temperature rise rate
Example:
For 2000 kg material, Cp = 0.6 kcal/kg°C, ΔT = 60°C, λ = 540 kcal/kg, efficiency = 0.85:
Q = 2000 × 0.6 × 60 = 72,000 kcal
Q: What is the importance of reaction kinetics in reactor design and optimization?
Reaction kinetics is the foundation of reactor design — it defines how fast reactions occur, enabling efficient, safe, and optimized reactor operation.
Definition:
Reaction kinetics is the study of the rate of chemical reactions and how it depends on concentration, temperature, and catalysts.
Importance in Reactor Design & Optimization:
1. Determines Reactor Type & Size:
Kinetic data helps decide whether to use a batch, plug flow (PFR), or continuous stirred tank reactor (CSTR) based on the required conversion and rate.
2. Establishes Rate Laws:
Defines the rate equation and order of reaction, essential for design equations and performance prediction.
3. Predicts Conversion & Yield:
Enables accurate calculation of reactant conversion, product yield, and selectivity under given conditions.
4. Optimizes Operating Conditions:
Helps in setting optimum temperature, pressure, and residence time for maximum productivity and minimum by-products.
5. Guides Catalyst Selection:
Kinetic studies indicate catalyst effectiveness and activation energy, aiding in catalyst design and regeneration strategy.
6. Ensures Process Safety & Control:
Understanding reaction rates helps prevent runaway reactions and maintain stable operation.
Q: How do you troubleshoot a chemical reactor process issue?
Troubleshooting a reactor involves a structured check of process data, equipment, materials, and controls to pinpoint and correct the root cause effectively.
Approach:
Troubleshooting a reactor involves a systematic analysis to identify the root cause of deviation in performance, safety, or product quality.
Step-by-Step Method:
1. Define the Problem:
Clearly identify the issue (e.g., low conversion, temperature rise, foaming, pressure fluctuation).
2. Review Operating Data:
Check process parameters — temperature, pressure, feed rate, agitation speed, pH, and flow rates.
Compare with design and normal operating values.
3. Verify Instrumentation & Controls:
Ensure sensors, transmitters, and control loops are functioning correctly and calibrated.
4. Check Feed Quality & Composition:
Analyze raw material purity, contamination, or ratio deviations that can affect reaction rate.
5. Evaluate Reaction Kinetics & Catalyst Condition:
Inspect for catalyst deactivation, fouling, or aging that may alter reaction efficiency.
6. Inspect Mechanical Integrity:
Check agitator, baffles, seals, jackets, and heat exchangers for blockages or leakage.
7. Assess Heat Transfer Performance:
Look for inefficient cooling/heating leading to temperature maldistribution or hot spots.
8. Perform Material Balance:
Validate input–output consistency to identify unaccounted losses or side reactions.
9. Take Corrective Actions:
Adjust process parameters, clean or replace catalyst, repair equipment, or recalibrate instruments.
10. Document & Prevent Recurrence:
Record findings, update SOPs, and train personnel to avoid repetition.
Q: How would you scale up a chemical reaction from lab to industrial scale?
Scaling up a reaction involves understanding kinetics, validating through pilot studies, managing heat and mass transfer, ensuring safety, and optimizing operating conditions for consistent industrial performance.
To transfer a chemical process from laboratory to pilot and industrial scale while maintaining product quality, yield, and safety.
Step-by-Step Approach:
1. Understand Reaction Kinetics:
Study reaction rate, order, heat of reaction, and mechanism to predict behavior on a large scale.
2. Material and Energy Balance:
Perform detailed mass and heat balance to estimate raw material requirements, heat load, and cooling needs.
3. Identify Scale-Up Parameters:
Maintain geometric similarity, mixing intensity (Reynolds number), and residence time.
Evaluate mass transfer and heat transfer limitations.
4. Pilot Plant Trials:
Conduct tests in a pilot reactor to validate lab data and identify operational challenges such as foaming, fouling, or heat buildup.
5. Equipment Selection:
Choose appropriate reactor type, agitator, and heat exchange system based on reaction phase (liquid–liquid, gas–liquid, solid–liquid).
6. Safety and Hazard Evaluation:
Perform HAZOP, thermal stability, and runaway reaction studies to ensure safe scale-up.
7. Optimization:
Fine-tune temperature, pressure, feed rate, and catalyst loading for best yield and energy efficiency.
8. Control Strategy Development:
Design automation and control loops to maintain stable operation at scale.
9. Documentation and Validation:
Record all parameters, develop SOPs, and validate process reproducibility at industrial scale.
Q: What role does heat transfer and mixing play in chemical reactor performance?
Efficient heat transfer controls temperature, while proper mixing ensures uniform reaction — both are vital for stable, safe, and high-performance reactor operation.
1. Heat Transfer:
Maintains Reaction Temperature:
Controls reaction rate and selectivity by maintaining desired temperature.
Prevents Hot Spots or Runaway:
Ensures uniform temperature distribution, avoiding thermal degradation or unsafe conditions.
Affects Conversion and Yield:
Poor heat removal can reduce conversion or cause side reactions.
Design Importance:
Efficient heat exchangers, jackets, or coils are critical for exothermic/endothermic reactions.
2. Mixing:
Ensures Uniform Composition:
Promotes homogeneous distribution of reactants, catalysts, and temperature.
Improves Mass Transfer:
Enhances reactant contact and reaction rate, especially in multiphase systems.
Prevents Concentration Gradients:
Avoids local zones of excess reactant that can cause undesired by-products.
Supports Catalyst Efficiency:
Ensures proper dispersion of catalyst particles in heterogeneous systems.
3. Combined Effect:
Interlinked Functions:
Proper mixing supports effective heat transfer, and good heat control stabilizes reaction kinetics.
Directly Impacts Reactor Performance:
Determines conversion efficiency, product quality, and safety.