Semi-continuous centrifuge – principle, construction, working, uses, merits, demerits MCQs With Answer

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

G S Sachin

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.

Mail- Sachin@pharmacyfreak.com