Semi-continuous centrifuge – principle, construction, working, uses, merits, demerits MCQs With Answer
A semi-continuous centrifuge is a vital separation device in pharmaceutical processing, combining centrifugation principles with intermittent solids discharge for efficient solid-liquid separation. This introduction covers the principle (centrifugal sedimentation), main constructional parts (bowl, scroll/rotor, feed, discharge), working parameters (speed, feed rate, differential speed, g-force), and typical pharmaceutical uses such as isolation of APIs, fermentation broths, and washing of crystalline products. Also discussed are merits (high throughput, improved washing) and demerits (cake handling, maintenance). Keywords: semi-continuous centrifuge, principle, construction, working, uses, merits, demerits, B.Pharm, MCQs. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the fundamental principle behind a semi-continuous centrifuge?
- Magnetic attraction of particles
- Gravitational settling only
- Centrifugal sedimentation due to high rotational speed
- Evaporation of solvent
Correct Answer: Centrifugal sedimentation due to high rotational speed
Q2. Which component directly receives the feed in most semi-continuous centrifuges?
- Beach section
- Bowl inlet or feed pipe
- Peel mechanism
- Drive motor casing
Correct Answer: Bowl inlet or feed pipe
Q3. In a scroll-type semi-continuous centrifuge, what causes solids to move toward the discharge end?
- Thermal gradient along the bowl
- Axial thrust generated by differential speed between bowl and scroll
- Magnetic fields in the scroll
- Gravity acting inside the rotating frame
Correct Answer: Axial thrust generated by differential speed between bowl and scroll
Q4. Which parameter is used to express the effective centrifugal force in a centrifuge?
- Relative Centrifugal Force (RCF)
- Boiling point elevation
- Mass flow rate
- Turbulence index
Correct Answer: Relative Centrifugal Force (RCF)
Q5. What is the correct formula for Relative Centrifugal Force (RCF) in terms of radius (r, cm) and speed (N, rpm)?
- RCF = 1.118 × 10^-5 × r × N^2
- RCF = r × N / 60
- RCF = π r^2 N
- RCF = 9.81 × r × N
Correct Answer: RCF = 1.118 × 10^-5 × r × N^2
Q6. A key advantage of semi-continuous centrifuges over batch centrifuges is:
- They always operate at lower speeds
- They allow intermittent solids discharge without stopping the rotor
- They require no maintenance
- They eliminate the need for a drive motor
Correct Answer: They allow intermittent solids discharge without stopping the rotor
Q7. Which pharmaceutical application often utilizes a semi-continuous centrifuge?
- Tablet coating only
- Isolation of crystalline active pharmaceutical ingredients (APIs)
- Lyophilization
- Capsule filling
Correct Answer: Isolation of crystalline active pharmaceutical ingredients (APIs)
Q8. In the construction of a semi-continuous centrifuge, the “bowl” is usually made of:
- Wood
- Polyethylene foam
- High-strength stainless steel or corrosion-resistant alloy
- Glass
Correct Answer: High-strength stainless steel or corrosion-resistant alloy
Q9. Which operational variable affects cake dryness during semi-continuous centrifugation?
- Ambient humidity only
- Rotational speed (g-force) and residence time
- Color of the bowl
- Operator height
Correct Answer: Rotational speed (g-force) and residence time
Q10. What is a common demerit of semi-continuous centrifuges in pharmaceutical production?
- Impossible to scale up
- Frequent cake collapse and complex cake discharge leading to product loss
- Total elimination of solvent use
- They are sterile by design without cleaning
Correct Answer: Frequent cake collapse and complex cake discharge leading to product loss
Q11. The “beach” in a centrifuge refers to:
- The external housing of the motor
- A conical or cylindrical zone where the clarified liquid and cake separate
- A packaging tray
- The electrical control panel
Correct Answer: A conical or cylindrical zone where the clarified liquid and cake separate
Q12. Differential speed in a scroll centrifuge means:
- The bowl and scroll rotate at exactly the same speed
- The scroll rotates slightly slower or faster than the bowl to convey solids
- The difference between feed and discharge pressure
- The speed difference between input pump and motor
Correct Answer: The scroll rotates slightly slower or faster than the bowl to convey solids
Q13. A semi-continuous centrifuge intended for sensitive APIs should allow which design consideration?
- Rough internal surfaces to trap particles
- Easy dismantling and cleaning for GMP and cleaning validation
- Permanent welded joints preventing access
- Use of untreated cast iron for bowl
Correct Answer: Easy dismantling and cleaning for GMP and cleaning validation
Q14. Which wash technique is commonly used in semi-continuous centrifugation to purify crystalline solids?
- Counter-current washing
- Lyophilic dehydration
- Thermal cracking
- Magnetic rinsing
Correct Answer: Counter-current washing
Q15. What is the principal difference between a continuous and a semi-continuous centrifuge?
- Continuous centrifuges never separate solids
- Semi-continuous intermittently discharges solids; continuous discharges continuously
- Semi-continuous is always larger than continuous
- They are identical in operation
Correct Answer: Semi-continuous intermittently discharges solids; continuous discharges continuously
Q16. Which factor does NOT directly influence separation efficiency in a semi-continuous centrifuge?
- Particle size and density difference
- Viscosity of the liquid
- Color of the operator’s uniform
- Rotational speed
Correct Answer: Color of the operator’s uniform
Q17. Cake washing in a semi-continuous centrifuge primarily aims to:
- Increase the particle size
- Remove mother liquor and impurities from the solid cake
- Change the crystal polymorph
- Reduce rotational speed
Correct Answer: Remove mother liquor and impurities from the solid cake
Q18. Which maintenance activity is critical for safe, long-term operation of semi-continuous centrifuges?
- Periodic balancing and inspection of rotor and bowl
- Painting the exterior weekly
- Replacing bearings every hour
- Cleaning only when the machine fails
Correct Answer: Periodic balancing and inspection of rotor and bowl
Q19. What is the typical symptom of an unbalanced load in a centrifuge?
- Increased vibration and noise
- Improved separation efficiency
- Decreased feed rate only
- Automatic reduction of particle size
Correct Answer: Increased vibration and noise
Q20. In scaling up a semi-continuous centrifuge process, which criterion is most useful?
- Maintaining similar RCF and residence time
- Using identical part numbers as the lab unit
- Doubling motor power automatically
- Keeping the same physical footprint
Correct Answer: Maintaining similar RCF and residence time
Q21. Solid discharge in a semi-continuous machine is achieved by which mechanism in many designs?
- Thermal melting of cake
- Mechanical scraping or peeler that removes cake intermittently
- Permanent open slot where solids leak continuously
- Manual scooping through a sealed bowl
Correct Answer: Mechanical scraping or peeler that removes cake intermittently
Q22. Which safety feature is essential in centrifuge operation?
- Vibration sensors and automatic shutdown
- No guarding to allow visual checks
- Manual starter without interlocks
- Open bowl operation
Correct Answer: Vibration sensors and automatic shutdown
Q23. Which property of the feed most strongly affects sedimentation velocity in centrifugation?
- Particle density relative to fluid and particle size
- Operator skill
- Ambient light levels
- Instrument color coding
Correct Answer: Particle density relative to fluid and particle size
Q24. For sticky suspensions where cake adherence is problematic, a design modification would be:
- Use of smooth, polished bowl surfaces and adjustable differential speed
- Use of rough, pitted surfaces to trap the cake
- Removal of scroll to stop conveyance
- Increasing ambient humidity
Correct Answer: Use of smooth, polished bowl surfaces and adjustable differential speed
Q25. Which cleaning requirement is especially important for pharmaceutical-grade semi-continuous centrifuges?
- Compatibility with CIP/SIP and validation for cleaning
- Cleaning only with dry brushes
- No cleaning between batches
- Use of flammable solvents for rapid removal
Correct Answer: Compatibility with CIP/SIP and validation for cleaning
Q26. Which is a sign of wear in scroll or conveyor parts?
- Decreased differential speed capability and poor solids conveyance
- Increased product purity
- Faster cake formation with no vibration
- Permanent reduction in bowl diameter
Correct Answer: Decreased differential speed capability and poor solids conveyance
Q27. What is the role of feed distribution in semi-continuous centrifugation?
- Ensure even radial distribution to avoid imbalance and promote uniform cake formation
- To cool the motor only
- To lubricate the scroll bearings
- To provide electrical grounding
Correct Answer: Ensure even radial distribution to avoid imbalance and promote uniform cake formation
Q28. Which solid property makes centrifugation most effective?
- High density difference from the liquid and larger particle size
- Neutral color
- Low melting point
- Electrical conductivity
Correct Answer: High density difference from the liquid and larger particle size
Q29. To decrease residual solvent in the cake after centrifugation, you would:
- Increase wash volume and adjust centrifuge speed and residence time
- Reduce centrifuge speed to zero
- Stop washing entirely
- Cover the bowl with insulating material
Correct Answer: Increase wash volume and adjust centrifuge speed and residence time
Q30. Which maintenance check helps prevent catastrophic rotor failure?
- Non-destructive testing (NDT) and regular inspection for cracks
- Painting the rotor annually
- Changing the rotor color
- Removing all balancing weights permanently
Correct Answer: Non-destructive testing (NDT) and regular inspection for cracks
Q31. Which type of centrifuge is most similar in principle to semi-continuous scroll centrifuges?
- Decanter centrifuge
- Refrigerator
- Gel electrophoresis unit
- Magnetic stirrer
Correct Answer: Decanter centrifuge
Q32. What is the effect of increasing feed rate beyond design in a semi-continuous centrifuge?
- Improved clarity and lower solids load
- Possible overload, poorer clarification, and thinner cake formation
- Automatic reduction of bowl speed
- No change in performance
Correct Answer: Possible overload, poorer clarification, and thinner cake formation
Q33. Which documentation is essential for pharmaceutical centrifuge qualification?
- Design qualification (DQ), installation qualification (IQ), operational qualification (OQ), and performance qualification (PQ)
- Only a handwritten toolkit list
- Employee birthdays
- Only a power bill
Correct Answer: Design qualification (DQ), installation qualification (IQ), operational qualification (OQ), and performance qualification (PQ)
Q34. Which operating condition will reduce the cake porosity and help achieve drier cake?
- Higher g-force and longer residence time
- Lower g-force and shorter residence time
- Eliminating washing
- Operating at zero rpm
Correct Answer: Higher g-force and longer residence time
Q35. What is commonly used to prevent corrosion in centrifuge bowls handling acidic slurries?
- Stainless steel or special corrosion-resistant coatings
- Uncoated aluminum
- Wooden liners
- Leaving the bowl untreated
Correct Answer: Stainless steel or special corrosion-resistant coatings
Q36. Which process parameter is adjusted to control shear-sensitive products in centrifugation?
- Reduce rotational speed (g-force) and use gentler feed introduction
- Increase temperature drastically
- Use glass bowls only
- Increase differential speed to maximum
Correct Answer: Reduce rotational speed (g-force) and use gentler feed introduction
Q37. What is the typical reason for using semi-continuous centrifuges in fermentation broth processing?
- To isolate biomass efficiently while allowing washing and discharge without stopping
- To sterilize the broth by heating
- To dissolve solids into solvents
- To aerate the broth
Correct Answer: To isolate biomass efficiently while allowing washing and discharge without stopping
Q38. Which design feature improves wash efficiency in semi-continuous centrifuges?
- Proper baffling or distribution of wash liquid and counter-current washing stages
- Removing all inlet nozzles
- Using colored wash solutions only
- Operating without a scroll
Correct Answer: Proper baffling or distribution of wash liquid and counter-current washing stages
Q39. Excessive vibration during operation is MOST likely caused by:
- Imbalanced load, misalignment, or damaged rotor
- High product purity
- Properly balanced rotor
- Clean and polished bowl
Correct Answer: Imbalanced load, misalignment, or damaged rotor
Q40. What is one environmental drawback of centrifugation in pharma?
- Potential generation of solvent-laden cake and wastewater requiring proper disposal
- It eliminates all waste
- It converts solids into harmless gases
- No energy consumption
Correct Answer: Potential generation of solvent-laden cake and wastewater requiring proper disposal
Q41. Which monitoring parameter helps in controlling cake thickness online?
- Torque or differential speed feedback and feed rate monitoring
- Color of the outlet pipe
- Time of day
- Ambient noise level
Correct Answer: Torque or differential speed feedback and feed rate monitoring
Q42. Which operational strategy aids in handling fine colloidal particles that are hard to sediment?
- Use of flocculants or coagulants to increase effective particle size
- Decrease rpm to near zero
- Avoid any pre-treatment and rely on single pass
- Use only visual inspection
Correct Answer: Use of flocculants or coagulants to increase effective particle size
Q43. When selecting a semi-continuous centrifuge, which material compatibility concern is primary?
- Chemical resistance of wetted parts to solvents, acids or bases in the feed
- Color matching with factory walls
- Whether it can be carried by one person
- Shape of external bolts
Correct Answer: Chemical resistance of wetted parts to solvents, acids or bases in the feed
Q44. Which operational parameter controls the residence time of solids inside the bowl?
- Feed rate and scroll conveyance speed
- Ambient room color
- Motor insulation type
- Operator shoe size
Correct Answer: Feed rate and scroll conveyance speed
Q45. What is a common troubleshooting step when clarification quality suddenly declines?
- Check feed concentration, rpm, and possible blockage or wear in feed distribution
- Change the color of the feed
- Increase operator count
- Randomly adjust control knobs without data
Correct Answer: Check feed concentration, rpm, and possible blockage or wear in feed distribution
Q46. Which metric is useful to quantify centrifuge throughput in industrial practice?
- Mass or volume of solids recovered per hour (kg/h or L/h)
- Number of paint coats on the machine
- Operator commute time
- Frequency of cleaning only
Correct Answer: Mass or volume of solids recovered per hour (kg/h or L/h)
Q47. Which type of rotor imbalance is most dangerous?
- Static and dynamic imbalance at high speed without shutdown
- Color imbalance
- Imbalance corrected by software only
- Light dust on the rotor
Correct Answer: Static and dynamic imbalance at high speed without shutdown
Q48. What operational practice reduces contamination risk between batches?
- Validated cleaning (CIP/SIP) and proper changeover procedures
- Never cleaning the bowl
- Only wiping with a dry cloth
- Using the same solvent for all products without validation
Correct Answer: Validated cleaning (CIP/SIP) and proper changeover procedures
Q49. For shear-sensitive bioproducts, which centrifugation approach is preferred?
- Lower g-forces, gentle feed, and possibly alternative separation (e.g., filtration)
- Maximum rpm to reduce processing time
- High differential speed to shear particles
- Using an unbalanced rotor
Correct Answer: Lower g-forces, gentle feed, and possibly alternative separation (e.g., filtration)
Q50. Which advantage makes semi-continuous centrifuges attractive for pilot-scale pharmaceutical manufacture?
- They balance continuous operation with flexibility for batch-specific discharge and washing
- They are the cheapest option regardless of process
- They eliminate the need for process optimization
- They are maintenance-free
Correct Answer: They balance continuous operation with flexibility for batch-specific discharge and washing
