Molecular distillation – principle and methodology MCQs With Answer is a concise, exam-focused guide for B. Pharm students exploring high-vacuum, short-path distillation used to separate thermally sensitive and high-boiling pharmaceuticals, lipids, and natural products. This technique relies on molecular flow under very low pressures, minimizing residence time and thermal decomposition while achieving high purity. Key concepts include mean free path, Knudsen regime, evaporator-condensor geometry, wiped-film and short-path configurations, vacuum systems, and process variables like temperature, feed rate and condenser efficiency. Ideal for practical labs and competitive exams, this resource emphasizes both fundamental theory and applied methodology. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the fundamental operating principle of molecular distillation?
- Separation based on azeotrope formation
- Separation under high pressure to raise volatility
- Separation by exploiting molecular free-path at very low pressures
- Separation using chemical reactions between components
Correct Answer: Separation by exploiting molecular free-path at very low pressures
Q2. Which flow regime predominates in molecular distillation?
- Turbulent flow
- Laminar viscous flow
- Free molecular flow (Knudsen regime)
- Transitional flow
Correct Answer: Free molecular flow (Knudsen regime)
Q3. In molecular distillation, the mean free path of vapor molecules should be:
- Much smaller than the distance between evaporator and condenser
- Comparable to or greater than the distance between evaporator and condenser
- Equal to the liquid film thickness always
- Negligible compared to molecular diameter
Correct Answer: Comparable to or greater than the distance between evaporator and condenser
Q4. Molecular distillation is particularly suitable for separating:
- Low-boiling volatile solvents only
- Thermally unstable, high-boiling compounds
- Simple inorganic salts
- Gaseous mixtures at atmospheric pressure
Correct Answer: Thermally unstable, high-boiling compounds
Q5. Which of the following configurations is commonly used in molecular distillation?
- Tray column
- Short-path evaporator
- Packed column
- Bubble column
Correct Answer: Short-path evaporator
Q6. A key advantage of molecular distillation over conventional distillation is:
- Longer residence time for thermal equilibration
- Ability to operate at atmospheric pressure
- Minimal thermal degradation of heat-sensitive materials
- Higher energy consumption always
Correct Answer: Minimal thermal degradation of heat-sensitive materials
Q7. Which parameter directly reduces boiling points in molecular distillation?
- Increased feed viscosity
- Reduced operating pressure (vacuum)
- Higher column packing
- Longer reflux ratio
Correct Answer: Reduced operating pressure (vacuum)
Q8. The typical vacuum level for effective molecular distillation is in the range of:
- Atmospheric pressure (760 Torr)
- 10–100 Torr
- 0.001–1 Torr (very low pressure)
- 1000 Torr
Correct Answer: 0.001–1 Torr (very low pressure)
Q9. Which component is essential to achieve required low pressure in molecular distillation?
- Heating mantle only
- Efficient vacuum pump (e.g., diffusion or molecular drag pump)
- Reflux drum
- Perforated trays
Correct Answer: Efficient vacuum pump (e.g., diffusion or molecular drag pump)
Q10. In a wiped-film molecular distillation unit, the wiper serves to:
- Create a thin, continuous film for rapid evaporation
- Increase bulk residence time
- Act as a condenser
- Introduce reflux to the system
Correct Answer: Create a thin, continuous film for rapid evaporation
Q11. Short path between evaporator and condenser in molecular distillation is important to:
- Allow extensive gas-phase diffusion and mixing
- Prevent recondensation on the evaporator and reduce residence time
- Increase vapor pressure near the evaporator
- Encourage nucleation of solids
Correct Answer: Prevent recondensation on the evaporator and reduce residence time
Q12. Which material property of the feed is most critical for efficient molecular distillation?
- Color only
- Vapor pressure and thermal stability
- Electrical conductivity
- Magnetic susceptibility
Correct Answer: Vapor pressure and thermal stability
Q13. The driving force for separation in molecular distillation is:
- Difference in chemical reactivity
- Difference in vapor pressure (volatility) of components
- Difference in solubility in packing material
- Difference in optical properties
Correct Answer: Difference in vapor pressure (volatility) of components
Q14. Which measurement often indicates successful molecular distillation operation?
- High condenser temperature
- Reduced distillate purity
- Low product degradation and high recovery of target compound
- Increased feed viscosity
Correct Answer: Low product degradation and high recovery of target compound
Q15. The condenser in a molecular distillation setup must be:
- Located far from the evaporator to allow mixing
- Efficient and placed very close to the evaporator
- Operated at very high temperature
- Made of porous packing
Correct Answer: Efficient and placed very close to the evaporator
Q16. One limitation of molecular distillation is:
- Inability to separate thermally sensitive compounds
- High throughput for very low-viscosity feeds
- Low throughput and high equipment cost for industrial scale
- Requirement for high-pressure operation
Correct Answer: Low throughput and high equipment cost for industrial scale
Q17. Which of the following is NOT an application of molecular distillation in pharmaceutical processing?
- Purification of essential oils and phytochemicals
- Separation of vitamin concentrates (e.g., vitamin E)
- Fractionation of triglycerides and lipids
- Sterilization of aqueous vaccine solutions
Correct Answer: Sterilization of aqueous vaccine solutions
Q18. In molecular distillation, reducing residence time minimizes:
- Mass transfer rates
- Thermal degradation and polymerization
- Vapor formation
- Vacuum level
Correct Answer: Thermal degradation and polymerization
Q19. Which variable can be adjusted to control the evaporation rate in molecular distillation?
- Feed color only
- Feed rate and surface temperature
- Ambient light intensity
- Evaporator material magnetic field
Correct Answer: Feed rate and surface temperature
Q20. The Knudsen number (Kn) is used to characterize flow in molecular distillation. A Kn >> 1 indicates:
- Continuum viscous flow
- Transitional flow
- Free molecular flow where molecular collisions with surfaces dominate
- That molecular diameter is negligible
Correct Answer: Free molecular flow where molecular collisions with surfaces dominate
Q21. Why are anti-oxidants sometimes added before molecular distillation of lipids?
- To increase boiling point
- To prevent oxidative degradation during heating
- To improve vacuum pump efficiency
- To form azeotropes
Correct Answer: To prevent oxidative degradation during heating
Q22. Which instrument is commonly used to monitor vacuum level in molecular distillation?
- Thermometer
- Pressure gauge (e.g., Pirani or Penning gauge)
- Refractometer
- pH meter
Correct Answer: Pressure gauge (e.g., Pirani or Penning gauge)
Q23. What is the effect of increasing evaporator surface temperature in molecular distillation?
- Decrease in vapor pressure of components
- Increase in evaporation rate and potential for thermal degradation
- Instantaneous solidification of distillate
- No change to process dynamics
Correct Answer: Increase in evaporation rate and potential for thermal degradation
Q24. In molecular distillation, the term “short-path” refers to:
- Short time to install the equipment
- Minimal distance between evaporation surface and condenser
- Short electrical wiring
- Short duration of feed preparation
Correct Answer: Minimal distance between evaporation surface and condenser
Q25. Which of the following best describes the mass transfer mechanism in molecular distillation?
- Bulk convective transfer through packed bed
- Diffusion-limited transfer under viscous regime
- Free molecular transfer of vapor molecules directly from surface to condenser
- Mass transfer via liquid-liquid extraction
Correct Answer: Free molecular transfer of vapor molecules directly from surface to condenser
Q26. For very viscous feeds, which molecular distillation variant is preferable?
- Fractionating column
- Wiped-film molecular distillation
- Simple batch boiling
- Vacuum crystallization
Correct Answer: Wiped-film molecular distillation
Q27. Which safety consideration is important during molecular distillation of organic compounds?
- Ensuring oxygen presence to aid evaporation
- Preventing leaks and air ingress to avoid oxidation and pump damage
- Operating at high pressures without relief valves
- Removing all cooling on the condenser
Correct Answer: Preventing leaks and air ingress to avoid oxidation and pump damage
Q28. What role does condenser temperature play in molecular distillation?
- No role; condenser temperature is irrelevant
- Controls condensation efficiency and product collection
- Only affects color of the distillate
- Makes the process operate at atmospheric pressure
Correct Answer: Controls condensation efficiency and product collection
Q29. In evaluating separation efficiency, which parameter is commonly used for molecular distillation?
- Number of theoretical plates (not directly applicable)
- Percentage recovery and purity of distillate
- Reflux ratio solely
- Color change
Correct Answer: Percentage recovery and purity of distillate
Q30. Which of the following is a common feed pretreatment before molecular distillation?
- Removal of non-volatile solids and pre-filtration
- Adding salts to increase boiling point
- Increasing ambient humidity
- None; feeds are always used crude
Correct Answer: Removal of non-volatile solids and pre-filtration
Q31. Molecular distillation differs from fractional distillation primarily because it:
- Operates by increasing pressure to separate components
- Relies on conventional column separation theory
- Uses extremely low pressures and very short path lengths to avoid vapor–liquid equilibrium dependence
- Is only suitable for gases
Correct Answer: Uses extremely low pressures and very short path lengths to avoid vapor–liquid equilibrium dependence
Q32. Which variable tends to decrease product residence time in the evaporator?
- Lower feed rate
- Higher film thickness
- Higher rotational speed of wiper (in wiped-film units)
- Poor thermal contact
Correct Answer: Higher rotational speed of wiper (in wiped-film units)
Q33. A vacuum leak during molecular distillation will most likely cause:
- Improved separation due to increased pressure
- Oxidative degradation and reduced distillate purity
- Instant freezing of the product
- Increased mean free path improving yields
Correct Answer: Oxidative degradation and reduced distillate purity
Q34. Which pump combination is often used to achieve high vacuum for molecular distillation?
- Peristaltic pump alone
- Rotary vane backing pump with diffusion or turbomolecular pump
- Gear pump at atmospheric pressure
- Diaphragm pump only
Correct Answer: Rotary vane backing pump with diffusion or turbomolecular pump
Q35. Which analytical method is commonly used to assess purity after molecular distillation?
- Gas chromatography (GC) or HPLC
- Thermogravimetric analysis only
- Microscopy
- pH titration only
Correct Answer: Gas chromatography (GC) or HPLC
Q36. The evaporator surface in molecular distillation is typically heated to:
- Temperatures below ambient always
- A temperature sufficient to vaporize desired component without excessive decomposition
- Extremely high temperatures to carbonize impurities
- Exactly the same as condenser temperature
Correct Answer: A temperature sufficient to vaporize desired component without excessive decomposition
Q37. Which statement about recondensation and fouling is true?
- Fouling is not a concern in molecular distillation
- Proper temperature control and short path minimize recondensation and fouling
- Recondensation is encouraged to improve yields
- Fouling only occurs at atmospheric pressure
Correct Answer: Proper temperature control and short path minimize recondensation and fouling
Q38. What effect does increasing condenser cooling efficiency have?
- Reduces condensation of target vapor
- Improves collection efficiency and prevents backflow
- Increases vapor pressure in the evaporator
- Destroys vacuum pumps
Correct Answer: Improves collection efficiency and prevents backflow
Q39. In scale-up of molecular distillation, which factor becomes critical?
- Maintaining short path and adequate vacuum at higher throughput
- Using the same lab heater without adjustment
- Eliminating the condenser altogether
- Operating at atmospheric pressure
Correct Answer: Maintaining short path and adequate vacuum at higher throughput
Q40. The evaporation flux in molecular distillation primarily depends on:
- Gravity only
- Surface temperature, vapor pressure and accommodation coefficient
- Color of the feed
- Electrical conductivity
Correct Answer: Surface temperature, vapor pressure and accommodation coefficient
Q41. What is the role of an oil trap or cold trap in the vacuum line?
- To heat the distillate
- To protect vacuum pump from condensable vapors and oils
- To increase system pressure
- To act as a reflux drum
Correct Answer: To protect vacuum pump from condensable vapors and oils
Q42. Which observation suggests inadequate vacuum during a run?
- High distillate purity
- Presence of heavy fraction condensation near the evaporator and poor separation
- Faster evaporation than normal
- Clear, odorless emissions
Correct Answer: Presence of heavy fraction condensation near the evaporator and poor separation
Q43. Why is molecular distillation favored for purifying cannabinoids and essential oils?
- It requires high oxygen levels
- It enables separation at low temperatures minimizing thermal degradation and preserving bioactivity
- It always removes all water content
- It uses chemical solvents for extraction
Correct Answer: It enables separation at low temperatures minimizing thermal degradation and preserving bioactivity
Q44. Which of the following best improves throughput without compromising product quality?
- Increasing evaporator path length
- Optimizing feed distribution and maintaining vacuum and temperature control
- Raising condenser temperature significantly
- Allowing intermittent air ingress
Correct Answer: Optimizing feed distribution and maintaining vacuum and temperature control
Q45. Which parameter would you monitor to detect polymerization during distillation of sensitive lipids?
- Increase in viscosity and formation of residues on the evaporator
- Decrease in feed color only
- Rise in system pressure to atmospheric instantly
- Drop in condenser cooling water temperature
Correct Answer: Increase in viscosity and formation of residues on the evaporator
Q46. What is the accommodation coefficient in the context of molecular distillation?
- A measure of how well vapor molecules stick to the condenser surface upon collision
- A measure of solution acidity
- The ratio of feed viscosity to density
- Temperature difference between feed and ambient
Correct Answer: A measure of how well vapor molecules stick to the condenser surface upon collision
Q47. Which operational change would most likely increase selectivity between two close-boiling components?
- Increase residence time dramatically
- Minimize temperature difference between feed and condenser
- Use staged molecular distillation or multiple passes with optimized temperature and vacuum
- Introduce strong oxidants into feed
Correct Answer: Use staged molecular distillation or multiple passes with optimized temperature and vacuum
Q48. What maintenance is essential for reliable molecular distillation performance?
- Ignoring seals and gaskets
- Regular checking of vacuum seals, pump oil, condenser cleaning and removing deposits
- Never changing coolant in condenser
- Leaving the system permanently vented to atmosphere
Correct Answer: Regular checking of vacuum seals, pump oil, condenser cleaning and removing deposits
Q49. When designing a molecular distillation process for a heat-sensitive drug intermediate, which approach is most appropriate?
- Operate at highest temperature possible to speed process
- Use low-pressure, short-path setup with minimal residence time and staged fractionation if necessary
- Employ long-packed columns at atmospheric pressure
- Add reactive agents to force separation
Correct Answer: Use low-pressure, short-path setup with minimal residence time and staged fractionation if necessary
Q50. Successful molecular distillation scale-up should prioritize which combination?
- Higher temperature and ignoring vacuum
- Maintaining geometric short-path, vacuum integrity and reproducible film formation
- Replacing condenser with open air cooling
- Reducing monitoring to save costs
Correct Answer: Maintaining geometric short-path, vacuum integrity and reproducible film formation
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