Elutriation tank is an essential solid-liquid separation technique widely used in pharmaceutical engineering for particle classification. This concise guide explains the principle, construction, working, uses, merits and demerits of the elutriation tank, tailored for B. Pharm students. Learn how fluid velocity and particle settling interact in the elutriation tank to achieve size-based separation, and review design elements such as inlet geometry, overflow weir, and collection zones. The content emphasizes practical uses in granule classification, dust removal, and formulation development, plus merits and demerits relevant to lab and industrial scales. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the basic principle governing separation in an elutriation tank?
- Difference in particle electrical charge
- Difference in particle terminal settling velocity
- Magnetic susceptibility of particles
- Centrifugal force applied to the mixture
Correct Answer: Difference in particle terminal settling velocity
Q2. Which flow direction is typically used in an elutriation tank to separate particles?
- Downward flow with particles moving downwards
- Horizontal flow across the tank
- Upward counter-current flow of fluid
- Tangential swirling flow
Correct Answer: Upward counter-current flow of fluid
Q3. Which design component controls the fluid velocity in an elutriation tank?
- Overflow weir height
- Feed hopper material
- Tank wall thickness
- External agitator speed
Correct Answer: Overflow weir height
Q4. In elutriation, particles that have higher terminal velocities compared to the upward fluid velocity will:
- Be carried to the overflow
- Remain suspended indefinitely
- Settle downward and be collected as coarse fraction
- Dissolve into the liquid
Correct Answer: Settle downward and be collected as coarse fraction
Q5. Which law is most commonly used to estimate settling velocity of small spherical particles in an elutriation tank?
- Fick’s law
- Stokes’ law
- Boyle’s law
- Henry’s law
Correct Answer: Stokes’ law
Q6. What is the main advantage of using an elutriation tank for powder classification in pharmaceutical processes?
- High thermal efficiency
- Low shear and gentle classification of particles
- Complete sterilization of particles
- Ability to process only gaseous mixtures
Correct Answer: Low shear and gentle classification of particles
Q7. Which parameter is NOT directly influencing separation efficiency in an elutriation tank?
- Upward fluid velocity
- Particle density
- Particle colour
- Particle shape
Correct Answer: Particle colour
Q8. What is a common use of elutriation tanks in pharmaceutical formulation development?
- Dry heat sterilization
- Granule size classification
- Lyophilization of liquids
- Tablet coating
Correct Answer: Granule size classification
Q9. Which of the following is a typical demerit of the elutriation tank?
- Produces extremely high temperatures
- Limited throughput and scale-up challenges for narrow size ranges
- Generates strong magnetic fields
- Requires vacuum conditions
Correct Answer: Limited throughput and scale-up challenges for narrow size ranges
Q10. The settling velocity of a particle in elutriation depends on particle diameter, density difference and:
- Ambient light intensity
- Fluid viscosity
- Container colour
- Electrical conductivity
Correct Answer: Fluid viscosity
Q11. In construction, which section of an elutriation tank prevents re-entrainment of settled particles?
- Feed inlet nozzle
- Settling zone with baffling
- Top mixing impeller
- Heating jacket
Correct Answer: Settling zone with baffling
Q12. Elutriation is best suited for separating particles that differ primarily in:
- Colour intensity
- Size and density
- Magnetic properties
- Solubility in organic solvents
Correct Answer: Size and density
Q13. When scaling up an elutriation tank, which factor requires careful consideration?
- Upscaling of electrical systems only
- Maintaining geometric similarity and velocity profiles
- Replacing liquid with gas
- Changing particle chemistry
Correct Answer: Maintaining geometric similarity and velocity profiles
Q14. What is the role of the overflow weir in an elutriation tank?
- To introduce feed into the tank
- To control the height of the ascending fluid and discharge fines
- To heat the tank contents
- To magnetically separate metal particles
Correct Answer: To control the height of the ascending fluid and discharge fines
Q15. Which fluid is most commonly used in elutriation tanks for pharmaceutical powders?
- Compressed air
- Water or aqueous media
- Liquid nitrogen
- Hydraulic oil
Correct Answer: Water or aqueous media
Q16. In an elutriation tank, if the upward fluid velocity equals the particle terminal velocity, the particle will:
- Rise rapidly to the overflow
- Remain suspended at the equilibrium level
- Soon dissolve into the liquid
- Instantly settle to the bottom
Correct Answer: Remain suspended at the equilibrium level
Q17. Which measurement helps to predict whether a particle will be carried over with the fluid?
- Terminal settling velocity compared to upward fluid velocity
- Particle melting point
- Particle optical refractive index
- Ambient room humidity
Correct Answer: Terminal settling velocity compared to upward fluid velocity
Q18. Which advantage makes elutriation preferable for fragile pharmaceutical granules?
- High shear mixing
- Low mechanical stress during separation
- Use of extreme temperatures
- High centrifugal forces
Correct Answer: Low mechanical stress during separation
Q19. What is the effect of increasing fluid viscosity on particle separation in an elutriation tank?
- Increases settling velocity for all particles
- Decreases settling velocity, making separation of small particles easier
- No effect on settling
- Causes immediate coagulation
Correct Answer: Decreases settling velocity, making separation of small particles easier
Q20. Which operational variable can be adjusted to retain finer particles in the overflow stream?
- Increase upward fluid velocity
- Decrease feed concentration only
- Lower fluid temperature drastically
- Change the tank colour
Correct Answer: Increase upward fluid velocity
Q21. Which of the following best describes the ‘cut size’ (d50) in elutriation?
- The size at which 50% of particles are magnetic
- The particle size at which half the particles report to overflow and half to underflow
- The diameter of the tank inlet
- The maximum particle size that will dissolve
Correct Answer: The particle size at which half the particles report to overflow and half to underflow
Q22. A poorly designed inlet in an elutriation tank will most likely cause:
- Improved classification precision
- Turbulence and re-entrainment reducing separation efficiency
- Complete filtration of fines
- Instant particle coagulation
Correct Answer: Turbulence and re-entrainment reducing separation efficiency
Q23. Which factor makes elutriation less effective for particles of similar sizes?
- Large density differences
- Small difference in terminal velocities
- Use of water as fluid
- Presence of magnetic additives
Correct Answer: Small difference in terminal velocities
Q24. Compared to centrifugation, elutriation tanks generally provide:
- Higher mechanical stress on particles
- Lower energy consumption and gentler separation
- The ability to separate gases
- Faster separation for submicron particles
Correct Answer: Lower energy consumption and gentler separation
Q25. Which instrument/parameter would you use to calculate Reynolds number for particles in elutriation?
- pH meter reading
- Particle characteristic length, fluid velocity and fluid viscosity
- Thermocouple temperature only
- Mass spectrometer output
Correct Answer: Particle characteristic length, fluid velocity and fluid viscosity
Q26. In pharmaceutical applications, elutriation helps to remove which undesirable component from granules?
- Active pharmaceutical ingredient
- Fines and dust
- Tablets
- Solvent residues
Correct Answer: Fines and dust
Q27. Which maintenance practice is important for consistent elutriation performance?
- Regular calibration of fluid flow meters and cleaning of inlet and weir areas
- Painting the outer tank monthly
- Replacing the tank every week
- Adding surfactants daily regardless of feed
Correct Answer: Regular calibration of fluid flow meters and cleaning of inlet and weir areas
Q28. What happens when feed concentration (solids loading) in the elutriation tank is too high?
- Separation improves due to enhanced settling
- Increased collisions and hindered settling reduce efficiency
- The fluid turns into gas
- Particles dissolve completely
Correct Answer: Increased collisions and hindered settling reduce efficiency
Q29. Which phrase best describes the working zone where upward fluid carries fines out?
- Coarse collection zone
- Overflow or elutriation zone
- Heating chamber
- Compression zone
Correct Answer: Overflow or elutriation zone
Q30. How is particle density difference (ρp − ρf) important in elutriation?
- It determines electrical conductivity only
- It directly affects the terminal settling velocity and separation behavior
- It has no role in settling
- It only affects colour of particles
Correct Answer: It directly affects the terminal settling velocity and separation behavior
Q31. Which modification can improve separation of very fine particles in an elutriation tank?
- Reduce fluid viscosity drastically
- Use co-flowing gas instead of liquid
- Decrease upward velocity slightly and use multiple stages
- Increase feed temperature indiscriminately
Correct Answer: Decrease upward velocity slightly and use multiple stages
Q32. What is the likely effect of particle agglomeration on elutriation performance?
- Agglomeration increases apparent particle size and may shift particles to underflow
- Agglomeration reduces density to zero
- Agglomeration makes particles magnetic
- No effect at all
Correct Answer: Agglomeration increases apparent particle size and may shift particles to underflow
Q33. Which type of elutriator setup is often used for laboratory-scale particle classification?
- High-speed rotating drum only
- Vertical elutriation column or small elutriation tank
- Large industrial cyclone only
- Freeze-dryer
Correct Answer: Vertical elutriation column or small elutriation tank
Q34. Which metric quantifies the sharpness of separation in elutriation processes?
- Separation index or grade efficiency curve slope
- Boiling point elevation
- pH gradient
- Colorimetric index
Correct Answer: Separation index or grade efficiency curve slope
Q35. Why might air elutriation (gas elutriation) be used instead of liquid elutriation for certain powders?
- To increase solubility of particles
- To avoid wetting and dissolution of moisture-sensitive powders
- To subject powders to high temperatures
- To induce chemical reactions
Correct Answer: To avoid wetting and dissolution of moisture-sensitive powders
Q36. Which operational control is crucial to maintain steady separation during continuous elutriation?
- Stable upward fluid velocity and steady feed rate
- Frequent sudden changes in feed composition
- Intermittent vacuum pulses
- Random heater cycling
Correct Answer: Stable upward fluid velocity and steady feed rate
Q37. In elutriation, what does ‘underflow’ refer to?
- The overflow fines stream
- The settled coarse fraction collected at the bottom
- A measurement instrument
- The tank lid
Correct Answer: The settled coarse fraction collected at the bottom
Q38. A pharmaceutical elutriation process needs to minimize shear to protect API crystals. Which choice is appropriate?
- Use mechanical high-shear mixers in the tank
- Employ gentle upward flow elutriation with smooth inlets
- Apply ultrasonic cavitation
- Introduce rotating blades into the settling zone
Correct Answer: Employ gentle upward flow elutriation with smooth inlets
Q39. Which disadvantage applies when separating particles with similar densities but different shapes?
- Shape affects drag and complicates prediction of settling behavior
- Shape has no effect on settling
- Shape only influences colour
- Shape makes particles radioactive
Correct Answer: Shape affects drag and complicates prediction of settling behavior
Q40. How can you empirically determine the cut size of an elutriation tank?
- Measure magnetic susceptibility of the overflow
- Run a graded sample test and analyze particle size distribution of overflow and underflow
- By visual inspection only
- By tasting the overflow
Correct Answer: Run a graded sample test and analyze particle size distribution of overflow and underflow
Q41. In lab practice, which analytical method is commonly used to characterize particle size before and after elutriation?
- Titration
- Laser diffraction or sieve analysis
- Gas chromatography
- UV-visible spectroscopy
Correct Answer: Laser diffraction or sieve analysis
Q42. Which safety concern is relevant when elutriating toxic powder APIs with water?
- Formation of flammable vapours only
- Potential contamination of effluent and need for containment and effluent treatment
- Risk of explosion due to magnetic fields
- Instant neutralization of API
Correct Answer: Potential contamination of effluent and need for containment and effluent treatment
Q43. What is an effective way to improve separation selectivity in an elutriation system without changing tank size?
- Introduce surfactants to completely dissolve particles
- Operate cascade or multistage elutriation with adjusted velocities
- Remove the overflow weir
- Use higher temperatures to melt particles
Correct Answer: Operate cascade or multistage elutriation with adjusted velocities
Q44. Which of the following describes a merit of elutriation compared to screening?
- Insensitive to particle size differences
- Can classify irregular-shaped particles and separate by effective settling behavior
- Requires large vibrating screens
- Only useful for gaseous mixtures
Correct Answer: Can classify irregular-shaped particles and separate by effective settling behavior
Q45. Which condition indicates operation in the Stokes regime for particle settling?
- Very high particle Reynolds number (>2000)
- Low Reynolds number (Re << 1) and laminar drag conditions
- Particles are molten
- Flow is fully turbulent and chaotic
Correct Answer: Low Reynolds number (Re << 1) and laminar drag conditions
Q46. Which corrective action would you take if coarse particles are being carried over to the overflow?
- Increase upward fluid velocity
- Decrease upward fluid velocity or adjust inlet to reduce turbulence
- Replace feed with smaller particles only
- Change the fluid to a gas without adjusting velocity
Correct Answer: Decrease upward fluid velocity or adjust inlet to reduce turbulence
Q47. Which type of pharmaceutical material is least likely suitable for wet elutriation?
- Water-insoluble granules
- Hygroscopic or water-sensitive APIs that degrade on contact with water
- Inert dust
- Non-reactive salts
Correct Answer: Hygroscopic or water-sensitive APIs that degrade on contact with water
Q48. In elutriation literature, what does the term ‘hindered settling’ refer to?
- Enhanced settling due to external fields
- Reduced settling velocity due to high particle concentration interactions
- Settling of particles in vacuum
- Settling that occurs only at high temperatures
Correct Answer: Reduced settling velocity due to high particle concentration interactions
Q49. Which comparison is true: elutriation vs cyclone separation?
- Elutriation relies on terminal velocity and low-shear fluid flow; cyclone uses centrifugal forces and is better for dry separation of fines
- Both methods are identical in principle
- Cyclones use magnetic fields while elutriation uses sound waves
- Elutriation is only for gases and cyclone only for liquids
Correct Answer: Elutriation relies on terminal velocity and low-shear fluid flow; cyclone uses centrifugal forces and is better for dry separation of fines
Q50. Which experimental variable would you monitor to check process stability during continuous elutriation?
- Overflow particle size distribution and mass flow rate
- Colour of the tank paint
- Number of people in the lab
- Time of day only
Correct Answer: Overflow particle size distribution and mass flow rate
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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