1. What is a scrubber and why is it used?
A scrubber is an air pollution control device that removes harmful gases, vapors and particulates from exhaust streams.
Used to treat acidic, alkaline, solvent and dust-laden emissions.
Essential in chemical and pharma plants to meet environmental norms and ensure safe workplace air quality.
2. What is the working principle of a scrubber?
Works on gas–liquid contact, where pollutants are absorbed or reacted in the scrubbing liquid.
Uses mechanisms like absorption, neutralization and condensation.
Efficiency depends on contact area, droplet size and gas–liquid interaction time.
3. What are the types of scrubbers used in industries?
Packed bed scrubbers for gas absorption and chemical neutralization.
Venturi scrubbers for fine particulate removal.
Spray tower scrubbers for simple gas cooling and bulk removal.
Dry scrubbers for powder-based neutralization where water cannot be used.
4. What is a packed bed scrubber?
Uses packing material to increase surface area for gas–liquid interaction.
Pollutant-laden gas passes upward while scrubbing liquid flows downward.
Highly efficient for acid gas removal such as HCl, SO₂, NH₃ and Cl₂.
5. What is the function of packing material in scrubbers?
Provides large surface area for mass transfer between gas and liquid.
Enhances absorption efficiency and reduces liquid consumption.
Common materials include PP saddles, rings and structured packings.
6. What is scrubbing efficiency?
Indicates how effectively a scrubber removes pollutants from gas streams.
Depends on gas flow rate, liquid flow rate, packing design and reaction chemistry.
7. Why pH control is important in scrubbers?
pH determines reaction rate and absorption capacity of scrubbing liquid.
Incorrect pH reduces neutralization efficiency and increases outlet emissions.
Automatic dosing systems maintain optimal pH for stable operation.
8. What is a Venturi scrubber and where is it used?
Uses high-velocity gas through a throat to atomize liquid into fine droplets.
Effective for capturing fine particulates, fumes and sticky dust.
Commonly used for dust-generating processes, reactors and dryers.
9. What is the role of recirculation in scrubber systems?
Reuses scrubbing liquid to reduce water and chemical consumption.
Maintains consistent performance by ensuring continuous wetting of packing.
Requires periodic blowdown to control contamination and dissolved solids.
10. What causes pressure drop in scrubbers?
Caused by packing blockage, high gas flow rate or fouling of nozzles.
Excessive pressure drop increases power consumption and reduces efficiency.
Regular cleaning and optimized design maintain stable operation.
11. What are common scrubbing liquids used in scrubbers?
Water is used for soluble gases and cooling applications.
Caustic solution (NaOH) removes acidic gases like HCl, SO₂ and Cl₂.
Acid solutions remove alkaline vapors such as NH₃ and amines.
Specialized chemicals are used for VOCs, solvents and odorous gases.
12. What is gas–liquid contact time and why is it important?
Contact time is the duration gas stays in the scrubber interacting with liquid.
Longer contact time improves absorption efficiency and reaction completion.
Defined by scrubber height, gas velocity and packing design.
13. What is L/G ratio in scrubbers?
L/G ratio is Liquid-to-Gas ratio indicating how much liquid is used per unit gas.
Higher L/G increases absorption efficiency but increases utility cost.
Optimal L/G ensures effective scrubbing with minimal chemical consumption.
14. What is the purpose of a demister pad?
Removes entrained liquid droplets from cleaned gas before release.
Prevents water carryover to chimney and downstream equipment.
Improves scrubbing efficiency and reduces liquid losses.
15. What causes foaming in scrubbers?
Caused by organic vapors, surfactants, high dissolved solids or microbial growth.
Leads to liquid carryover, reduced efficiency and operational instability.
Controlled by antifoam dosing, maintaining blowdown and pH balance.
16. What is the function of a circulation pump in scrubber systems?
Delivers scrubbing liquid to spray nozzles or packing area.
Maintains required flow rate and pressure for proper gas–liquid contact.
Pump reliability directly affects scrubbing performance.
17. What factors affect scrubber performance?
Packing type and height, liquid distribution, pH level and L/G ratio.
Gas velocity, contaminant concentration and nozzle condition.
Cleanliness of packing and proper recirculation flow also influence efficiency.
18. What is the role of makeup water in scrubbers?
Replaces water lost via evaporation, drift and blowdown.
Maintains liquid volume and concentration of chemicals in the system.
Prevents excessive buildup of dissolved solids.
19. What is blowdown in scrubber operation?
Controlled removal of contaminated scrubbing liquid to maintain quality.
Prevents scaling, foaming and increase of dissolved solids.
Essential for stable and efficient long-term scrubber operation.
20. Why is a scrubber stack (chimney) required?
Provides safe height for the release of treated gas into the atmosphere.
Ensures proper dispersion of residual pollutants.
Prevents backflow of exhaust gases into the scrubber or plant area.
21. What is the purpose of spray nozzles in scrubbers?
Spray nozzles atomize scrubbing liquid into fine droplets for efficient gas–liquid contact.
Ensure uniform distribution over packing material.
Nozzle clogging reduces scrubbing efficiency and increases pressure drop.
22. What causes scaling inside scrubbers?
Scaling occurs due to high hardness, dissolved salts or improper blowdown.
Deposits reduce packing porosity and block nozzles.
Controlled by water softening, chemical dosing and periodic cleaning.
23. What is a dual-stage scrubber?
A two-stage system using different scrubbing liquids or mechanisms.
First stage removes particulates or acidic gases; second stage neutralizes remaining contaminants.
Used for multi-component emissions requiring high removal efficiency.
24. What is mass transfer in scrubbers?
Movement of pollutants from gas phase to liquid phase.
Enhanced by high surface area, small droplets and proper turbulence.
Determines overall scrubbing efficiency and chemical consumption.
25. What is the role of ORP control in scrubbers?
ORP (Oxidation Reduction Potential) indicates oxidation capacity of scrubbing liquid.
Used in chlorine, H₂S and odor control scrubbers.
Correct ORP ensures complete neutralization or oxidation of pollutants.
26. What causes high pressure drop in packed bed scrubbers?
Fouled packing, high gas velocity or liquid maldistribution.
Mineral scaling or sticky organic deposits in packing.
Leads to increased energy cost and reduced scrubbing efficiency.
27. What is a caustic scrubber?
Uses NaOH solution to neutralize acidic gases like HCl, SO₂, CO₂ and Cl₂.
Produces salts such as NaCl or Na₂SO₃ depending on reaction.
Provides high removal efficiency for corrosive acidic emissions.
28. What is a fume scrubber?
Removes hazardous fumes generated during reactions, storage or transfer.
Prevents toxic exposure and corrosion of plant equipment.
Commonly used for acid fumes, solvent fumes and cyanide vapors.
29. What is liquid distribution system in scrubbers?
Includes nozzles, headers and pipelines that distribute liquid evenly over packing.
Poor distribution creates dry spots and reduces absorption area.
Proper design ensures uniform wetting and efficient scrubbing.
30. What is stack monitoring in scrubber systems?
Measurement of pollutant concentration in treated exhaust gas.
Ensures compliance with environmental standards.
Parameters monitored include pH, SO₂, HCl, NH₃, VOCs and particulate levels.
31. Why is pH monitoring essential in scrubber operation?
pH directly affects absorption and neutralization reactions.
Low pH reduces efficiency in acid gas scrubbers; high pH affects alkaline scrubbers.
Continuous pH monitoring ensures stable performance and correct chemical dosing.
32. What is a mist eliminator and how does it work?
A mist eliminator removes fine liquid droplets from scrubber outlet gas.
Works using mesh pads or vane-type separators to capture mist.
Prevents liquid carryover to chimney and protects downstream equipment.
33. What is a scrubber recirculation tank?
Stores scrubbing liquid for continuous recirculation.
Allows settling of solids and dilution before pumping.
Maintains consistent chemical concentration during operation.
34. What is gas velocity and why is it important?
Gas velocity is the speed at which gas moves through the scrubber.
High velocity reduces contact time and causes carryover.
Low velocity reduces turbulence and mass transfer efficiency.
35. What is a chemical dosing system in scrubbers?
Automatically adds chemicals like NaOH, H₂SO₄ or oxidizers into scrubbing liquid.
Maintains optimal pH, ORP and chemical balance.
Improves efficiency and reduces manual handling.
36. What causes liquid carryover in scrubbers?
High gas velocity, foaming, nozzle malfunction or poor demister performance.
Leads to water loss and corrosion in downstream ducts.
Controlled through correct design and maintenance.
37. What is the purpose of bleed-off (blowdown) in scrubbers?
Removes concentrated scrubbing liquid containing dissolved solids.
Prevents scaling, fouling and foaming in the system.
Keeps scrubbing liquid effective over long operation.
38. What is absorption efficiency?
Absorption efficiency measures how well pollutants transfer from gas to liquid.
Depends on L/G ratio, packing area and reaction kinetics.
High efficiency ensures compliance with emission norms.
39. What is the effect of temperature on scrubbing performance?
High temperature reduces gas solubility and absorption efficiency.
Increases vapor load and may cause stripping of absorbed gases.
Cooling gas before scrubbing improves overall performance.
40. Why is proper nozzle selection important?
Determines droplet size, spray pattern and coverage area.
Incorrect nozzle reduces liquid distribution and scrubbing efficiency.
Nozzle material must resist corrosion from scrubbing chemicals.
41. What is flooding in packed bed scrubbers?
Flooding occurs when excessive liquid flow blocks gas passage through the packing.
Causes sharp increase in pressure drop and reduced efficiency.
Prevented by maintaining correct L/G ratio and ensuring clean packing.
42. What is channeling in scrubbers?
Channeling happens when gas flows through preferred paths instead of uniformly across packing.
Results from poor liquid distribution or fouled packing.
Reduces gas–liquid contact area and lowers scrubbing performance.
43. What is the function of a Venturi throat?
The throat accelerates gas velocity to create intense turbulence.
Breaks scrubbing liquid into fine droplets for particulate removal.
Controls differential pressure and scrubber efficiency.
44. Why is packing height important?
Packing height determines gas–liquid contact time and absorption area.
Higher packing increases removal efficiency but raises pressure drop.
Optimized based on pollutant concentration and required performance.
45. What is drift loss in scrubbers?
Drift loss is the escape of liquid droplets with cleaned gas.
Caused by poor demister performance or high gas velocity.
Increases water consumption and causes downstream corrosion.
46. What is the purpose of level control in the recirculation tank?
Maintains sufficient liquid volume for pump suction.
Prevents pump dry running and cavitation.
Ensures stable L/G ratio for effective scrubbing.
47. What causes pump cavitation in scrubber systems?
Low liquid level, blocked suction line or high temperature.
Produces vibration, noise and damage to impeller.
Ensured by proper NPSH, cooling and suction design.
48. Why are corrosion-resistant materials required for scrubbers?
Scrubbers handle acidic or alkaline gases that attack metals.
Materials like FRP, PP, PVC and SS316 prevent corrosion.
Improves service life and reduces maintenance cost.
49. What is the role of instrumentation in scrubbers?
Includes pH meter, ORP meter, flow meters and pressure gauges.
Helps maintain optimal conditions for pollutant removal.
Ensures safety, efficiency and compliance with emission norms.
50. What are common problems in scrubber operation?
Scaling, foaming, nozzle clogging, poor distribution and high pressure drop.
Caused by improper blowdown, variable gas load or dirty packing.
Regular monitoring and maintenance prevent performance decline.
51. What is a quench tower in scrubbing systems?
A quench tower rapidly cools hot gas before it enters the scrubber.
Prevents damage to packing and reduces vapor load.
Essential for processes with hot acidic or solvent vapors.
52. Why is droplet size important in scrubbing?
Smaller droplets increase surface area for gas absorption.
Too fine droplets may carry over; too large reduce efficiency.
Nozzle type and pressure determine optimal droplet size.
53. What causes odor breakthrough in scrubbers?
Insufficient chemical dosing, low pH/ORP or poor liquid circulation.
Fouled packing or exhausted absorbent solution.
Correcting chemical concentration and improving distribution restores performance.
54. What is the purpose of interstage cooling in multi-stage scrubbers?
Reduces gas temperature between stages.
Improves solubility and reaction rates of pollutants.
Enhances overall removal efficiency for complex emissions.
55. What are the advantages of FRP scrubbers?
High corrosion resistance against acids, alkalis and solvents.
Lightweight, easy to install and low maintenance.
Suitable for aggressive environments and outdoor installations.
56. What is mist loading and why is it a concern?
Mist loading refers to the concentration of entrained liquid droplets leaving the scrubber.
High mist loading leads to environmental issues and stack corrosion.
Controlled by efficient demisters and correct gas velocity.
57. What is foam fractionation in scrubbers?
Unwanted separation where foam traps pollutants, reducing efficiency.
Occurs with surfactant-rich or organic vapors.
Prevented by antifoam agents and controlling blowdown.
58. What is chemical consumption rate in scrubbers?
The rate at which neutralizing chemicals are used during pollutant removal.
Depends on pollutant concentration, L/G ratio and reaction stoichiometry.
Accurate monitoring reduces cost and improves efficiency.
59. What is the function of sampling ports in scrubbers?
Allow gas or liquid sampling for performance testing.
Used to measure pollutant removal efficiency and chemical strength.
Necessary for environmental compliance and troubleshooting.
60. What is gas residence time in scrubbers?
The time gas spends inside the scrubber for absorption.
Longer residence time improves pollutant removal.
Controlled through scrubber height, gas velocity and internal design.
61. What causes unequal liquid distribution in scrubbers?
Blocked spray nozzles or uneven header pressure.
Improper alignment of nozzles or damaged distribution trays.
Leads to dry zones, channeling and low absorption efficiency.
62. What is a neutralization reaction in scrubbers?
Chemical reaction between acidic or alkaline gases and scrubbing liquid.
Converts harmful gases (HCl, SO₂, NH₃) into harmless salts.
Efficiency depends on pH, reaction kinetics and contact time.
63. Why is recirculation flow rate important?
Ensures continuous wetting of packing and stable L/G ratio.
Low flow causes dry spots and poor absorption.
High flow increases pressure drop and causes flooding.
64. What is a counter-current scrubber?
Gas flows upward while liquid flows downward.
Provides maximum gas–liquid contact and high removal efficiency.
Most common design for acid and solvent scrubbing.
65. What is a cross-flow scrubber?
Gas flows horizontally while liquid flows vertically.
Lower pressure drop compared to counter-current design.
Used where gas contains solids or where low energy use is needed.
66. What causes gas bypassing in scrubbers?
Poorly sealed ducts, gaps in packing support or improper tray installation.
Gas escapes without contacting scrubbing liquid.
Results in reduced pollutant removal and stack emissions.
67. Why is ventilation important around scrubber units?
Prevents accumulation of toxic fumes released from leaks.
Ensures safe working environment for operators.
Protects instruments and motors from corrosive vapors.
68. What is dissolved solid buildup in scrubbers?
Continuous chemical reactions produce salts in the scrubbing liquid.
High TDS increases scaling, fouling and pump wear.
Controlled by regular blowdown and fresh water addition.
69. Why is gas cooling beneficial before scrubbing?
Cooler gas has higher solubility for pollutants.
Reduces vapor load and prevents stripping of dissolved gases.
Improves mass transfer and overall scrubbing performance.
70. What is scrubber turndown ratio?
Ratio of maximum to minimum gas flow the scrubber can handle effectively.
High turndown allows stable operation under varying loads.
Important for batch processes with fluctuating emissions.
71. What is a caustic consumption calculation in scrubbers?
Determines the amount of NaOH needed to neutralize acidic gases.
Based on pollutant concentration, gas flow rate and reaction stoichiometry.
Helps optimize chemical usage and reduce operating cost.
72. What is a spray tower scrubber?
Uses spray nozzles to create liquid droplets without packing.
Suitable for coarse gas cleaning, cooling and bulk absorption.
Lower efficiency than packed beds but simpler and low maintenance.
73. What causes nozzle clogging in scrubbers?
High dissolved solids, scaling, rust particles or poor filtration.
Reduces liquid distribution and absorption efficiency.
Prevented by strainer cleaning and regular blowdown.
74. What is the purpose of an inspection window in scrubbers?
Allows visual check of spray pattern, foam level and packing condition.
Helps diagnose operational issues without shutdown.
Improves safety and maintenance planning.
75. What is entrainment in scrubbers?
Carryover of liquid droplets with the gas stream.
Caused by high velocity, excess foaming or poor demisting.
Leads to downstream corrosion and water loss.
76. What is gas scrubbing stoichiometry?
Defines chemical reaction ratios between pollutants and scrubbing chemicals.
Ensures correct dosing of NaOH, H₂SO₄ or oxidizers.
Prevents excessive chemical use and maintains optimum pH.
77. What causes high chemical consumption in scrubbers?
Overdosing due to poor control of pH/ORP.
High pollutant load, wrong stoichiometry or excessive blowdown.
Optimized dosing and monitoring reduce cost.
78. What is internal recirculation inside a scrubber?
Movement where part of gas or liquid loops back inside the scrubber.
Enhances mixing and absorption efficiency.
Common in high-energy scrubbers like Venturi designs.
79. What is the importance of inlet gas distribution?
Ensures gas enters the scrubber uniformly across cross-section.
Prevents channeling and uneven loading on packing.
Improves removal efficiency and reduces pressure drop.
80. What is spent scrubbing liquid and how is it handled?
Liquid containing dissolved pollutants, salts and reaction products.
Must be neutralized and treated before disposal.
Handled through effluent treatment plants or chemical neutralization systems.