Unit Operation: Evaporation – evaporator types and factors MCQs With Answer

Introduction

This quiz set focuses on Unit Operation: Evaporation — covering evaporator types, operational factors, design considerations, and performance parameters relevant to M.Pharm students studying MPC 204T Pharmaceutical Process Chemistry. The questions concentrate on practical and theoretical aspects such as single- and multiple-effect systems, film evaporators, vacuum operation, heat transfer limitations, boiling point rise, fouling and scaling, steam economy, and methods to improve energy efficiency. Each MCQ is crafted to test conceptual understanding and problem-solving ability used in pharmaceutical concentrating processes, formulation drying, and plant design decisions. Use these to prepare for exams, viva voce, and real-world equipment selection and optimization.

Q1. Which evaporator type is most suitable for heat-sensitive pharmaceutical products requiring short residence time and gentle processing?

• Rising film evaporator
• Wiped film (agitated thin film) evaporator
• Calandria shell-and-tube evaporator
• Plate heat exchanger evaporator

Correct Answer: Wiped film (agitated thin film) evaporator

Q2. In a multiple-effect evaporator, which factor primarily determines the steam economy?

• The number of effects and inter-effect temperature drop
• The material of construction of the tubes
• The feed pH
• The color of the product

Correct Answer: The number of effects and inter-effect temperature drop

Q3. Boiling point rise (BPR) in evaporators is most directly caused by which of the following?

• Increase in steam pressure supplied to the evaporator
• Presence of non-volatile solutes lowering vapor pressure
• Use of a larger heat transfer area
• High feed flow rate only

Correct Answer: Presence of non-volatile solutes lowering vapor pressure

Q4. Which evaporator configuration minimizes liquid holdup and thermal degradation for highly viscous pharmaceutical solutions?

• Falling film evaporator
• Batch kettle evaporator
• Wiped film evaporator
• Multiple-effect calandria evaporator

Correct Answer: Wiped film evaporator

Q5. What is the principal advantage of vacuum evaporation in pharmaceutical concentration?

• Increased boiling point of the solution
• Lowering of solvent boiling point to protect heat-sensitive compounds
• Higher fouling rate on heat transfer surfaces
• Requirement for higher steam pressure

Correct Answer: Lowering of solvent boiling point to protect heat-sensitive compounds

Q6. Which factor most significantly reduces overall heat transfer coefficient (U) in an evaporator when scaling occurs?

• Decrease in steam pressure
• Formation of an insulating fouling layer on the heat transfer surface
• Use of larger diameter tubes
• Increasing feed temperature

Correct Answer: Formation of an insulating fouling layer on the heat transfer surface

Q7. For a single-effect evaporator, which operational change will increase the evaporation rate most effectively without changing feed properties?

• Reducing steam temperature while keeping pressure constant
• Increasing the temperature difference (delta T) between heating steam and boiling liquor
• Decreasing heat transfer area
• Lowering feed flow rate to zero

Correct Answer: Increasing the temperature difference (delta T) between heating steam and boiling liquor

Q8. Which evaporator type is typically used when vapour recompression (MVR or TVR) is to be applied for energy efficiency?

• Wiped film evaporator only
• Multiple-effect evaporator or single-effect evaporator with compressor integration
• Open kettle evaporator exclusively
• Batch crystallizer

Correct Answer: Multiple-effect evaporator or single-effect evaporator with compressor integration

Q9. What is the main drawback of using very high vacuum in an evaporator for aqueous pharmaceutical feeds?

• Decrease in boiling point of water
• Excessive air ingress and increased risk of product oxidation
• Increased latent heat of vaporization
• Improved steam economy

Correct Answer: Excessive air ingress and increased risk of product oxidation

Q10. In a falling film evaporator, what governs the film thickness and heat transfer performance most strongly?

• Surface tension and flow rate of the liquor forming the film
• Color of the feed
• Length of the vapour outlet only
• Material used in distillation column packing

Correct Answer: Surface tension and flow rate of the liquor forming the film

Q11. Which parameter is critical when designing an evaporator to avoid entrainment and carryover of product into the vapour?

• Residence time only
• Proper vapour–liquid separation design and appropriate vapour velocity
• Using the thinnest possible tubes irrespective of feed
• Maximizing feed concentration at inlet

Correct Answer: Proper vapour–liquid separation design and appropriate vapour velocity

Q12. Heat transfer coefficient in film evaporators is generally higher than in flooded shell-and-tube evaporators because:

• Film evaporators use colder steam
• The thin liquid film provides lower resistance to heat transfer and promotes turbulence
• They have a larger residence time
• They always operate at atmospheric pressure

Correct Answer: The thin liquid film provides lower resistance to heat transfer and promotes turbulence

Q13. Which feed property most increases the risk of scaling and fouling in evaporators?

• Low total dissolved solids (TDS)
• High concentration of sparingly soluble salts and organic solids
• Neutral pH only
• Low viscosity at operating temperature

Correct Answer: High concentration of sparingly soluble salts and organic solids

Q14. What is the effect of increasing feed viscosity on evaporator design and operation?

• Facilitates formation of thin films improving heat transfer
• Increases resistance to heat transfer, requires forced circulation or wipers
• Eliminates the need for vacuum operation
• Reduces boiling point rise to zero

Correct Answer: Increases resistance to heat transfer, requires forced circulation or wipers

Q15. In multiple-effect evaporation, to maximize efficiency under a fixed steam supply, what is a common design approach?

• Use a single very large-effect evaporator
• Match boiling point drops so that vapour from one effect becomes the heating medium for the next
• Keep all effects at identical pressure and temperature
• Operate each effect with different feeds simultaneously

Correct Answer: Match boiling point drops so that vapour from one effect becomes the heating medium for the next

Q16. Which method is commonly used to control foaming and entrainment in pharmaceutical evaporators?

• Increasing boiling point rise deliberately
• Installing demisters, foam breakers, or antifoam addition and controlling vapour velocity
• Operating at the highest possible temperature
• Removing all volatile components before evaporation

Correct Answer: Installing demisters, foam breakers, or antifoam addition and controlling vapour velocity

Q17. Total heat transfer area required for an evaporator is calculated primarily from which formula basis?

• U = Q × A × ΔT
• Q = U × A × ΔT, where Q is heat duty, U is overall heat transfer coefficient, ΔT is driving temperature difference
• Area is independent of heat duty
• A = ΔT / (Q × U)

Correct Answer: Q = U × A × ΔT, where Q is heat duty, U is overall heat transfer coefficient, ΔT is driving temperature difference

Q18. Thermal degradation of active pharmaceutical ingredients during evaporation can be minimized by:

• Prolonged high-temperature exposure to ensure complete water removal
• Using vacuum operation, lower temperature, reduced residence time, and gentle film evaporators
• Increasing feed solids concentration abruptly at inlet
• Reducing heat transfer area significantly

Correct Answer: Using vacuum operation, lower temperature, reduced residence time, and gentle film evaporators

Q19. What is the primary advantage of thermal vapour recompression (TVR) compared to mechanical vapour recompression (MVR) in some pharmaceutical plants?

• TVR requires electric compressors and is more expensive to operate in all cases
• TVR uses a steam ejector and can be simpler with lower maintenance where steam is readily available
• TVR always gives higher energy efficiency than MVR
• TVR eliminates the need for vacuum systems

Correct Answer: TVR uses a steam ejector and can be simpler with lower maintenance where steam is readily available

Q20. Which design consideration is most important when selecting materials of construction for an evaporator handling corrosive pharmaceutical feeds?

• Selecting materials only based on cost
• Compatibility with product and cleaning agents, corrosion resistance, and hygiene requirements
• Choosing the heaviest available metal
• Using glass-lined materials for all high-temperature alcoholic feeds irrespective of chemistry

Correct Answer: Compatibility with product and cleaning agents, corrosion resistance, and hygiene requirements

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