Flash distillation – principle and methodology MCQs With Answer

Flash distillation – principle and methodology MCQs With Answer is an essential topic for B. Pharm students studying separation processes and pharmaceutical engineering. This concise introduction explains the principle of flash distillation, key methodology steps, equilibrium concepts, K-values, mass and energy balances, and common applications in drug processing. You will learn about single-stage (flash) separation, isenthalpic/adiabatic flashes, vacuum flashing for heat-sensitive compounds, and computational methods like Rachford–Rice for vapor fraction calculation. Emphasis is placed on practical design considerations, assumptions, and limitations relevant to pharmaceutical operations. Clear conceptual understanding helps in process optimization and quality control. ‘Now let’s test your knowledge with 50 MCQs on this topic.’

Q1. What is the primary principle behind flash distillation?

• Gradual multi-stage vapor-liquid contact
• Instantaneous equilibrium between vapor and liquid after a sudden pressure or temperature change
• Condensation followed by fractional reflux
• Membrane-based selective permeation

Correct Answer: Instantaneous equilibrium between vapor and liquid after a sudden pressure or temperature change

Q2. In a single-stage flash, the overall mass balance is given by which equation?

• F = V – L
• F = V + L
• F = V * L
• F = V / L

Correct Answer: F = V + L

Q3. For an ideal solution under total pressure P, the equilibrium ratio Ki (K-value) for component i is expressed as:

• Ki = xi / yi
• Ki = yi / xi
• Ki = Pi_sat / P
• Ki = P / Pi_sat

Correct Answer: Ki = Pi_sat / P

Q4. Which equation is commonly used to solve the vapor fraction (V/F) in multicomponent flash calculations?

• Raoult’s law directly
• Rachford–Rice equation
• Antoine equation
• Henderson–Hasselbalch equation

Correct Answer: Rachford–Rice equation

Q5. What does an isenthalpic (adiabatic) flash imply about the process?

• Temperature remains constant
• Total enthalpy of the feed equals the sum of enthalpies of vapor and liquid products
• Pressure is kept constant by heating
• Mass is not conserved

Correct Answer: Total enthalpy of the feed equals the sum of enthalpies of vapor and liquid products

Q6. Which property is most useful for predicting vapor pressures of pure components in flash calculations?

• Melting point
• Viscosity
• Antoine equation constants (A, B, C)
• Dielectric constant

Correct Answer: Antoine equation constants (A, B, C)

Q7. In pharmaceutical applications, why is vacuum flash commonly used?

• To increase boiling points
• To separate non-volatile solids
• To lower boiling points and protect heat-sensitive drugs
• To increase solvent polarity

Correct Answer: To lower boiling points and protect heat-sensitive drugs

Q8. Which assumption is typically made for a simple flash calculation?

• Infinite number of theoretical plates
• Complete chemical reaction between components
• Liquid and vapor leaving the flash drum are in equilibrium
• No heat transfer to the surroundings but chemical non-equilibrium

Correct Answer: Liquid and vapor leaving the flash drum are in equilibrium

Q9. Bubble point is defined as:

• The temperature where the first vapor dew forms on cooling a vapor
• The temperature at which a liquid begins to boil and first bubble appears at given pressure
• The temperature of the saturated vapor only
• The temperature at which the liquid solidifies

Correct Answer: The temperature at which a liquid begins to boil and first bubble appears at given pressure

Q10. Dew point corresponds to:

• The temperature where the first liquid droplet forms from a cooling vapor at given pressure
• The point where liquid completely vaporizes
• The freezing point of the vapor
• The point where vapor becomes supercritical

Correct Answer: The temperature where the first liquid droplet forms from a cooling vapor at given pressure

Q11. In a binary ideal solution flash at constant pressure, yi is related to xi by:

• yi = xi / Ki
• yi = Ki * xi
• xi = Ki * yi
• yi = xi * P

Correct Answer: yi = Ki * xi

Q12. The Rachford–Rice equation helps determine which unknown in flash calculations?

• Feed composition zi only
• Temperature only
• Vapor fraction (V/F) for a specified pressure and temperature
• Density of liquid phase

Correct Answer: Vapor fraction (V/F) for a specified pressure and temperature

Q13. For non-ideal liquid mixtures, which factor must be included in equilibrium relations?

• Surface tension
• Activity coefficients (γi)
• Color index
• Viscosity correction

Correct Answer: Activity coefficients (γi)

Q14. Which thermodynamic model is commonly used to estimate activity coefficients for polar pharmaceutical solvents?

• Ideal gas law
• Wilson, NRTL or UNIQUAC models
• Raoult’s law without correction
• Arrhenius equation

Correct Answer: Wilson, NRTL or UNIQUAC models

Q15. What is relative volatility (αij)?

• Ratio of vapor pressures of pure components at same temperature
• Ratio of K-values (Ki/Kj) indicating ease of separation between components i and j
• Ratio of densities
• Percentage of component i in feed

Correct Answer: Ratio of K-values (Ki/Kj) indicating ease of separation between components i and j

Q16. A relative volatility near 1 indicates:

• Very easy separation by simple flash
• No difference between liquid and vapor compositions; separation is difficult
• Complete immiscibility
• Ideal binary azeotrope always present

Correct Answer: No difference between liquid and vapor compositions; separation is difficult

Q17. Which instrument in a flash distillation unit allows vapor-liquid disengagement?

• Heat exchanger
• Flash drum (vessel)
• Pump
• Membrane module

Correct Answer: Flash drum (vessel)

Q18. When performing an adiabatic flash, what must be satisfied besides mass balances?

• Momentum balance only
• Overall energy (enthalpy) balance
• No need for energy considerations
• Only component-wise mass balances, no energy balance

Correct Answer: Overall energy (enthalpy) balance

Q19. Which of the following is NOT a typical assumption in simple flash calculations?

• Equilibrium between vapor and liquid leaving the drum
• Perfect mixing within each phase
• Chemical reactions between components
• Negligible pressure drop inside the drum

Correct Answer: Chemical reactions between components

Q20. In pharmaceutical processing, flash distillation is often used to:

• Sterilize solutions by autoclaving
• Remove volatile solvents quickly from extracts or formulations
• Synthesize active pharmaceutical ingredients via reaction
• Filter suspended solids

Correct Answer: Remove volatile solvents quickly from extracts or formulations

Q21. For a binary feed with known zi and Ki values, what is the first practical step in a flash calculation?

• Assume a value for V/F and iterate using balance equations
• Directly calculate densities
• Measure surface tension
• Set vapor fraction to zero always

Correct Answer: Assume a value for V/F and iterate using balance equations

Q22. Which graph is useful for visualizing vapor-liquid equilibrium in flash calculations?

• Titration curve
• T-x-y or x-y diagram (composition vs temperature or vapor composition)
• Van’t Hoff plot
• Nyquist plot

Correct Answer: T-x-y or x-y diagram (composition vs temperature or vapor composition)

Q23. If Pi_sat for component i is greater than total pressure P, what is expected in flash operation?

• Component i will remain entirely in the liquid
• Component i will have a vapor mole fraction tendency greater than liquid mole fraction (yi > xi)
• Component i will precipitate
• Component i will react chemically

Correct Answer: Component i will have a vapor mole fraction tendency greater than liquid mole fraction (yi > xi)

Q24. Which computational approach is commonly used for solving flash calculations when analytical solution is not possible?

• Direct integration
• Iterative numerical methods (Newton–Raphson, bisection)
• Graphical convolution only
• No computation is needed

Correct Answer: Iterative numerical methods (Newton–Raphson, bisection)

Q25. In multicomponent flash, the component material balance is:

• F zi = V xi + L yi
• F zi = V yi + L xi
• F = V + L only, no component balance
• F zi = F yi

Correct Answer: F zi = V yi + L xi

Q26. Which of the following describes a binary azeotrope in the context of flash distillation?

• A mixture that can be fully separated by single-stage flash
• A composition at which vapor and liquid have identical composition and conventional separation is difficult
• An ideal solution that obeys Raoult’s law perfectly
• A solid-liquid eutectic point

Correct Answer: A composition at which vapor and liquid have identical composition and conventional separation is difficult

Q27. What role do heat exchangers play upstream of a flash drum in pharmaceutical processes?

• They cool the vapor product to solid
• They adjust feed temperature and enthalpy to achieve desired flash conditions
• They separate immiscible liquids
• They carry out chemical reactions

Correct Answer: They adjust feed temperature and enthalpy to achieve desired flash conditions

Q28. If a feed is saturated liquid entering a flash drum with no heat input, the process is typically:

• Isothermal and isobaric with no vapor produced
• Adiabatic (isenthalpic) and will produce vapor depending on pressure drop
• Exothermic reaction vessel
• Always producing 100% vapor

Correct Answer: Adiabatic (isenthalpic) and will produce vapor depending on pressure drop

Q29. In design, larger residence time in a flash drum primarily helps to:

• Reduce thermal conductivity
• Improve phase disengagement and reduce entrainment
• Increase chemical reaction rates
• Enhance solubility of solids

Correct Answer: Improve phase disengagement and reduce entrainment

Q30. Which of the following is a limitation of single-stage flash compared to fractional distillation?

• Cannot separate mixtures with large relative volatility
• Limited separation efficiency for close-boiling or many-component mixtures
• Requires more stages than fractionation
• Always needs reflux

Correct Answer: Limited separation efficiency for close-boiling or many-component mixtures

Q31. For an ideal binary mixture at pressure P, if Ki for component 1 is 2 and xi1 = 0.3, what is yi1?

• 0.15
• 0.6
• 1.5
• 0.3

Correct Answer: 0.6

Q32. Which is a common method to handle non-ideality in vapor phase during flash calculations?

• Assume ideal gas always
• Use fugacity coefficients from an equation of state (e.g., Peng–Robinson)
• Ignore vapor-phase non-ideality completely
• Apply Raoult’s law with no corrections

Correct Answer: Use fugacity coefficients from an equation of state (e.g., Peng–Robinson)

Q33. In a pharmaceutical context, which feed characteristic most strongly influences choice of flash conditions?

• Color of the feed
• Heat sensitivity of active pharmaceutical ingredients (APIs)
• Magnetic susceptibility
• Electrical conductivity

Correct Answer: Heat sensitivity of active pharmaceutical ingredients (APIs)

Q34. Which statement about adiabatic flash temperature behavior is correct?

• Temperature always increases in adiabatic flash
• Temperature depends on feed enthalpy and splits; it may rise or fall relative to feed
• Temperature is fixed by ambient conditions only
• Temperature must equal the feed’s initial temperature always

Correct Answer: Temperature depends on feed enthalpy and splits; it may rise or fall relative to feed

Q35. What is the purpose of using a demister pad in a flash drum?

• To increase liquid vaporization
• To remove entrained liquid droplets from the vapor stream
• To catalyze chemical reactions
• To measure pressure

Correct Answer: To remove entrained liquid droplets from the vapor stream

Q36. For a multicomponent feed, what does zi represent?

• Vapor-phase mole fraction of component i in product
• Liquid-phase mole fraction of component i in product
• Overall feed mole fraction of component i
• Activity coefficient of component i

Correct Answer: Overall feed mole fraction of component i

Q37. Which numerical property must be between 0 and 1 and is often solved for in flash calculations?

• Relative volatility
• Vapor fraction (V/F)
• Antoine constant A
• Activity coefficient greater than 10 always

Correct Answer: Vapor fraction (V/F)

Q38. When calculating K-values at a guessed temperature, which pure component data are essential?

• Antoine constants to find Pi_sat and total pressure P
• Solid solubility data only
• Viscosity as a function of shear
• pH dependence

Correct Answer: Antoine constants to find Pi_sat and total pressure P

Q39. Which process variable is most commonly manipulated to control the extent of flashing?

• Feed color
• Operating pressure in the flash drum
• Magnetic field strength
• Feed ionic strength

Correct Answer: Operating pressure in the flash drum

Q40. If a component forms an azeotrope with solvent, single-stage flash will:

• Effectively separate the azeotrope completely
• Not be able to separate the azeotrope beyond its azeotropic composition
• Convert the azeotrope into solids
• Make the mixture ideal

Correct Answer: Not be able to separate the azeotrope beyond its azeotropic composition

Q41. In Rachford–Rice formulation, if Ki = 1 for all components, the vapor fraction V/F is:

• Undefined
• Equal to 0
• Any value satisfies the equation because yi = xi = zi
• Equal to 1

Correct Answer: Any value satisfies the equation because yi = xi = zi

Q42. Which safety concern is particularly relevant for industrial flash distillation of solvents?

• Formation of explosive vapor-air mixtures and solvent flammability
• Excessive magnetic fields
• Radioactivity emission
• Corrosion by neutrinos

Correct Answer: Formation of explosive vapor-air mixtures and solvent flammability

Q43. A benefit of performing a flash at reduced pressure is:

• Higher boiling point of all components
• Lower required temperature for vaporization of volatile solvents
• Increased polymerization rates
• Increased solid precipitation always

Correct Answer: Lower required temperature for vaporization of volatile solvents

Q44. Which measurement is most critical to verify equilibrium in pilot flash experiments?

• Vessel color
• Compositions of both vapor and liquid leaving the drum
• Electrical conductivity only
• Magnetic susceptibility only

Correct Answer: Compositions of both vapor and liquid leaving the drum

Q45. In practice, which factor can cause deviation from equilibrium assumptions in a flash drum?

• Perfect phase contact
• Insufficient residence time and poor mixing leading to non-equilibrium
• Exact Raoult’s law behavior
• Infinite methane content

Correct Answer: Insufficient residence time and poor mixing leading to non-equilibrium

Q46. Which of these is a typical observable product of flash distillation in pharma processing?

• High-boiling solvent in vapor stream
• Vapor rich in low-boiling solvent and a liquid residue enriched in non-volatiles
• Only solids
• Pure elemental oxygen

Correct Answer: Vapor rich in low-boiling solvent and a liquid residue enriched in non-volatiles

Q47. When performing flash calculations for aqueous-organic mixtures, special attention is needed for:

• Color stability
• Liquid-liquid immiscibility and possible two-liquid phases
• Magnetic properties
• Solid-state polymorphism only

Correct Answer: Liquid-liquid immiscibility and possible two-liquid phases

Q48. Which step is essential when validating a flash distillation model against experimental data?

• Compare predicted and measured phase compositions and temperatures
• Only check pump rates
• Ignore energy balances
• Measure only visual clarity

Correct Answer: Compare predicted and measured phase compositions and temperatures

Q49. For accurate flash design involving volatile impurities, one should include:

• Only pure component boiling points at 1 atm
• Activity coefficients and vapor-phase non-ideality (fugacity) corrections
• Only the feed color and odor
• Only the molecular weight

Correct Answer: Activity coefficients and vapor-phase non-ideality (fugacity) corrections

Q50. Which advantage makes flash distillation attractive in pharmaceutical manufacturing?

• High separation efficiency for components with near-equal volatilities
• Simple equipment and rapid removal of volatile solvents, minimizing thermal degradation
• Ability to fractionate mixtures into many narrow cuts like a full column
• Complete elimination of nonvolatile impurities

Correct Answer: Simple equipment and rapid removal of volatile solvents, minimizing thermal degradation

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