Mechanism of drying process MCQs With Answer

Mechanism of drying process MCQs With Answer offers B. Pharm students a focused, practical guide to the fundamentals of pharmaceutical drying. This introduction covers drying mechanism, heat and mass transfer, constant and falling rate periods, moisture diffusion, equilibrium moisture content, and common industrial dryers used in drug manufacturing. Emphasis on drying kinetics, critical moisture content, effective diffusivity, and energy considerations helps students link theory to formulation and process design. Ideal for exam prep and practical understanding, these MCQs reinforce key concepts used in tablet granule drying, lyophilization basics, and dryer selection. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which mechanism predominates during the constant rate period of drying?

• Internal diffusion of bound moisture
• Evaporation from a free water surface
• Solid-state diffusion of moisture
• Vapor-phase condensation inside pores

Correct Answer: Evaporation from a free water surface

Q2. What defines the transition from constant to falling rate period?

• Onset of external convection
• Reaching equilibrium moisture content
• Disappearance of free surface moisture (critical moisture content)
• When drying temperature exceeds boiling point

Correct Answer: Disappearance of free surface moisture (critical moisture content)

Q3. During the falling rate period, the drying rate is mainly controlled by:

• Surface evaporation limited by vapor pressure
• Internal moisture diffusion to the surface
• External air humidity only
• Conduction through solid walls

Correct Answer: Internal moisture diffusion to the surface

Q4. Effective moisture diffusivity in solids typically increases with:

• Decreasing temperature
• Increasing bound water content only
• Increasing temperature
• Higher external air humidity

Correct Answer: Increasing temperature

Q5. Which of the following best describes equilibrium moisture content (EMC)?

• Moisture content when drying rate is maximum
• Moisture content in equilibrium with surrounding air humidity and temperature
• Moisture content at zero temperature
• Moisture content immediately after wetting

Correct Answer: Moisture content in equilibrium with surrounding air humidity and temperature

Q6. Wet basis moisture content is calculated as:

• Mass of water divided by dry mass (g water / g dry solid)
• Mass of water divided by wet mass (g water / g wet solid)
• Dry mass divided by total mass
• Mass of solids divided by water mass

Correct Answer: Mass of water divided by wet mass (g water / g wet solid)

Q7. The primary driving force for drying in convective dryers is:

• Temperature gradient inside the solid
• Partial pressure (vapor pressure) difference between surface and bulk air
• Gravity-induced flow of moisture
• Chemical potential of solvents other than water

Correct Answer: Partial pressure (vapor pressure) difference between surface and bulk air

Q8. In a drying curve, the slope during the constant rate period represents:

• Change in moisture content per unit time (drying rate)
• Increase in temperature only
• Change in porosity
• Heat capacity of the sample

Correct Answer: Change in moisture content per unit time (drying rate)

Q9. Which dryer uses heat by conduction and direct contact between heated surface and product?

• Fluidized bed dryer
• Tray dryer
• Drum dryer
• Spray dryer

Correct Answer: Drum dryer

Q10. Spray drying is preferred for pharmaceuticals primarily because it:

• Is suitable for heat-sensitive proteins only
• Produces uniform fine powders from solutions or suspensions quickly
• Requires minimal energy per kg of water removed
• Always preserves crystalline structure

Correct Answer: Produces uniform fine powders from solutions or suspensions quickly

Q11. The internal resistance to moisture movement in porous solids is often modeled by:

• Fick’s laws of diffusion
• Newton’s law of cooling
• Raoult’s law only
• Bernoulli’s equation

Correct Answer: Fick’s laws of diffusion

Q12. In drying kinetics, an Arrhenius-type relationship is used to describe the temperature dependence of:

• Surface tension
• Effective moisture diffusivity or drying rate constant
• Air density
• Relative humidity only

Correct Answer: Effective moisture diffusivity or drying rate constant

Q13. During pharmaceutical drying, bound water refers to:

• Water trapped in macropores behaving like free water
• Water chemically or physically adsorbed to solid matrix, not easily removed
• Water that evaporates in constant rate period
• Bulk solvent used in granulation

Correct Answer: Water chemically or physically adsorbed to solid matrix, not easily removed

Q14. Which parameter best quantifies the energy required to remove a unit mass of water during drying?

• Drying rate constant
• Specific heat of the product
• Specific energy consumption (kJ/kg water removed)
• Relative humidity

Correct Answer: Specific energy consumption (kJ/kg water removed)

Q15. A dryer operating under reduced pressure (vacuum) mainly benefits by:

• Increasing boiling point of solvent
• Reducing the boiling point of water to enable lower temperature drying
• Eliminating the need for heat input
• Increasing air density for convection

Correct Answer: Reducing the boiling point of water to enable lower temperature drying

Q16. In a fluidized bed dryer, fluidization enhances drying by:

• Reducing surface area of particles
• Promoting good gas-solid contact and uniform temperature
• Limiting mass transfer to conduction only
• Compressing the particles to reduce porosity

Correct Answer: Promoting good gas-solid contact and uniform temperature

Q17. The critical moisture content is best described as:

• The moisture content at which sample ignites
• The moisture content at which surface moisture is depleted and falling rate begins
• The moisture content corresponding to EMC
• The initial moisture content of a wet material

Correct Answer: The moisture content at which surface moisture is depleted and falling rate begins

Q18. Which factor does NOT significantly affect drying rate in convective drying?

• Air velocity
• Air relative humidity
• Sample color
• Surface area of the sample

Correct Answer: Sample color

Q19. Moisture sorption isotherms relate moisture content to:

• Temperature only
• Water activity or relative humidity at constant temperature
• Drying time directly
• Particle size distribution

Correct Answer: Water activity or relative humidity at constant temperature

Q20. The driving force for internal diffusion of moisture is:

• Thermal conductivity gradient
• Chemical potential gradient of water within the solid
• Electrical potential across the material
• External air turbulence only

Correct Answer: Chemical potential gradient of water within the solid

Q21. In thin-layer drying studies, drying constants are primarily used to:

• Predict color changes
• Model drying kinetics and estimate drying time for scale-up
• Calculate viscosity of the solvent
• Determine mechanical strength

Correct Answer: Model drying kinetics and estimate drying time for scale-up

Q22. The falling rate period can be divided into multiple falling-rate stages because:

• Air temperature fluctuates randomly
• Different moisture binding states and transport mechanisms dominate at different contents
• Evaporation suddenly stops and restarts
• Particles fuse together forming a single pore

Correct Answer: Different moisture binding states and transport mechanisms dominate at different contents

Q23. Lyophilization (freeze-drying) removes water mainly by:

• Liquid evaporation at atmospheric pressure
• Sublimation of ice under reduced pressure
• Conduction through heated shelves only
• Osmotic dehydration

Correct Answer: Sublimation of ice under reduced pressure

Q24. Which of these is a common measure of drying performance for a dryer?

• Residence time distribution only
• Throughput and specific energy consumption
• Pore size distribution exclusively
• Number of fans used

Correct Answer: Throughput and specific energy consumption

Q25. In porous pharmaceutical granules, moisture moves via:

• Only vapor diffusion through external air
• Liquid capillary flow and vapor diffusion depending on moisture level
• Only surface diffusion
• Electrophoresis of water molecules

Correct Answer: Liquid capillary flow and vapor diffusion depending on moisture level

Q26. The moisture diffusivity in porous solids is often called:

• Thermal diffusivity
• Effective moisture diffusivity
• Mass velocity
• Moisture conductivity

Correct Answer: Effective moisture diffusivity

Q27. A high inlet air temperature in drying will usually:

• Increase drying rate but may degrade heat-sensitive drugs
• Decrease drying rate always
• Have no effect on drying kinetics
• Increase humidity of the drying air

Correct Answer: Increase drying rate but may degrade heat-sensitive drugs

Q28. In drying design, the Biot number for mass transfer (ratio of internal to external resistance) indicates:

• Whether heat conduction is negligible
• Whether internal diffusion or external mass transfer controls drying
• Electrical resistance of the sample
• The viscosity of the solvent

Correct Answer: Whether internal diffusion or external mass transfer controls drying

Q29. What is the role of humidity control during pharmaceutical drying?

• Controls crystallinity and prevents overdrying of hygroscopic materials
• Has no impact on product quality
• Only affects color of product
• Decreases drying time regardless of other conditions

Correct Answer: Controls crystallinity and prevents overdrying of hygroscopic materials

Q30. The constant rate period typically ends when:

• Product temperature drops below ambient
• Surface moisture is no longer continuous and capillary flow ceases
• Air velocity falls to zero
• Internal diffusion becomes infinitely fast

Correct Answer: Surface moisture is no longer continuous and capillary flow ceases

Q31. Which drying model explicitly accounts for internal diffusion resistance and external mass transfer?

• Newton model only
• Two-resistance model (internal + external mass transfer)
• Henderson-Pabis empirical model exclusively
• Raoult’s law

Correct Answer: Two-resistance model (internal + external mass transfer)

Q32. During pharmaceutical drying, shrinkage of particles affects drying mainly by:

• Changing internal diffusion path length and surface area
• Increasing vapor pressure of water
• Causing chemical reactions with air
• Eliminating bound water entirely

Correct Answer: Changing internal diffusion path length and surface area

Q33. In a packed bed dryer, channeling can occur due to:

• Uniform airflow distribution
• Poor air distribution causing preferential paths and uneven drying
• Excessively small particle sizes only
• Low humidity air only

Correct Answer: Poor air distribution causing preferential paths and uneven drying

Q34. The mass transfer coefficient for external drying increases with:

• Decreasing air velocity
• Increasing air velocity and turbulence
• Higher product moisture content only
• Lower temperature always

Correct Answer: Increasing air velocity and turbulence

Q35. Which technique estimates moisture content as a function of relative humidity for pharmaceuticals?

• Thermogravimetric analysis (TGA) only
• Moisture sorption isotherm measurement
• Particle size analysis
• Chromatography

Correct Answer: Moisture sorption isotherm measurement

Q36. In drying, latent heat of vaporization is important because it:

• Represents energy required to heat solids only
• Is the energy required to vaporize water and dominates energy consumption
• Is negligible compared to sensible heat always
• Only applies in freeze-drying

Correct Answer: Is the energy required to vaporize water and dominates energy consumption

Q37. Which dryer is most suitable for thermolabile pharmaceutical products to avoid high temperatures?

• Rotary drum dryer at high temperature
• Vacuum tray dryer or freeze dryer
• Direct-fired spray dryer with high inlet temp
• Open sun drying

Correct Answer: Vacuum tray dryer or freeze dryer

Q38. In adsorption isotherms for pharmaceuticals, Type II or III isotherms indicate:

• Non-adsorbing surfaces only
• Multilayer adsorption and capillary condensation in porous materials
• Only monolayer adsorption
• Absence of bound water

Correct Answer: Multilayer adsorption and capillary condensation in porous materials

Q39. The concept of psychrometry is important in drying because it relates:

• Air properties: dry-bulb temperature, wet-bulb temperature, humidity, and enthalpy
• Only the color of air
• Solid-state chemistry
• Particle porosity and hardness

Correct Answer: Air properties: dry-bulb temperature, wet-bulb temperature, humidity, and enthalpy

Q40. Which process parameter directly affects the surface vapor pressure of water on a particle?

• Particle color
• Particle surface temperature
• Ambient light intensity
• External static pressure only

Correct Answer: Particle surface temperature

Q41. For scale-up of a drying process, key factors to consider include:

• Only the dryer brand
• Heat and mass transfer coefficients, residence time, air flow patterns, and loading
• Color of product packaging
• Supplier of raw materials only

Correct Answer: Heat and mass transfer coefficients, residence time, air flow patterns, and loading

Q42. Which drying law is commonly used for the constant rate period in drying models?

• Newton’s law of cooling adapted for drying (linear relation between drying rate and driving force)
• Fick’s second law only
• Henderson-Hasselbalch equation
• Arrhenius law without temperature dependence

Correct Answer: Newton’s law of cooling adapted for drying (linear relation between drying rate and driving force)

Q43. In convective drying, increasing air humidity will generally:

• Increase drying rate
• Reduce the driving force for mass transfer and lower drying rate
• Have no effect on drying
• Always cause product crystallization

Correct Answer: Reduce the driving force for mass transfer and lower drying rate

Q44. Which is an indicator that internal diffusion is the rate-controlling step during drying?

• Drying rate is independent of sample thickness
• Drying rate strongly depends on sample thickness and particle size
• Drying occurs only at the surface instantly
• External air velocity controls the drying solely

Correct Answer: Drying rate strongly depends on sample thickness and particle size

Q45. The Henderson-Pabis and Page models are used to:

• Describe mechanical strength of tablets
• Empirically fit thin-layer drying kinetics data
• Measure porosity directly
• Calculate solvent polarity

Correct Answer: Empirically fit thin-layer drying kinetics data

Q46. In pharmaceutical drying, control of product temperature is critical because:

• It does not influence product stability
• High temperatures can degrade actives and change polymorphic forms
• Temperature only affects air density
• Lower temperatures always increase drying rate

Correct Answer: High temperatures can degrade actives and change polymorphic forms

Q47. Desorption isotherm hysteresis indicates:

• There is no bound water in the material
• Differences between adsorption and desorption paths due to pore structure and bonding
• Moisture content is independent of humidity
• Material is chemically inert to water

Correct Answer: Differences between adsorption and desorption paths due to pore structure and bonding

Q48. In the context of drying, specific surface area influences drying by:

• Decreasing vapor pressure of water
• Affecting available surface for evaporation and rate of external mass transfer
• Changing chemical composition of the product
• Only affecting electrical conductivity

Correct Answer: Affecting available surface for evaporation and rate of external mass transfer

Q49. The term “falling rate period” implies that:

• Drying rate increases with time
• Drying rate decreases with time as moisture becomes harder to remove
• Drying stops immediately
• Only surface moisture is present throughout

Correct Answer: Drying rate decreases with time as moisture becomes harder to remove

Q50. Which analytical method is commonly used to determine residual moisture content of dried pharmaceuticals?

• Karl Fischer titration
• UV-Vis spectroscopy of solids
• Gas chromatography for nonvolatile solids only
• Particle size analysis

Correct Answer: Karl Fischer titration

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