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.
Can be glass-lined, stainless steel, or alloy depending on corrosiveness.
2. Agitator / Stirrer:
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:
Temperature, pressure, pH, level sensors for process monitoring.
Can be connected to DCS/PLC for control.
7. Safety Devices:
Pressure relief valves, rupture discs, vents to handle overpressure.
8. Manway / Manhole:
For cleaning, inspection, and maintenance.
Q: What are the main types of chemical reactors?
Batch Reactor
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.
Plug Flow Reactor (PFR) / Tubular Reactor
Reactants flow in one direction without back-mixing.
High conversion, suitable for large-scale continuous processes.
Packed Bed Reactor
Contains catalyst pellets, gas or liquid flows through the bed.
Common in petrochemical and catalytic processes.
Fluidized Bed Reactor
Catalyst particles suspended by upward flowing fluid.
Excellent mixing, heat transfer, used in catalytic cracking.
Semi-Batch Reactor
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.
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.
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.
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
μ=viscosity 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.