Iocl selection series distillation

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Distillation — 50 MCQ Test
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IOCL Selection Series 2025

Distillation — 50 Multiple Choice Questions

Q1. Distillation separates components primarily on the basis of

Density differences Boiling point differences Melting point differences Solubility differences

✅ Correct: Boiling point differences — Distillation uses volatility differences to separate components.

Q2. Azeotropic mixture is characterized by

Constant boiling point and same vapor-liquid composition Components immiscible Ideal Raoult’s law behavior Same melting points

✅ Correct: Constant boiling point and same vapor-liquid composition — azeotrope boils without change in composition.

Q3. Relative volatility (α) between components A and B is ratio of

Vapor pressures of A and B Their mole fractions in liquid Their vapor pressures (or K-values) — K_A / K_B Boiling points

✅ Correct: K_A / K_B — relative volatility = ratio of K-values (y/x) slopes, indicates ease of separation.

Q4. In continuous distillation, increasing reflux ratio generally

Decreases number of theoretical stages required Increases number of stages required Has no effect Lowers product purity

✅ Correct: Decreases number of theoretical stages — more reflux gives better separation per stage but increases energy use.

Q5. McCabe-Thiele method is used to determine

Column diameter Number of theoretical stages Reboiler duty Tray efficiency

✅ Correct: Number of theoretical stages — graphical method for binary distillation design.

Q6. Total reflux condition means

No vaporization All condensed vapor returned as reflux, no product withdrawn Zero reboiler duty Azeotropic distillation

✅ Correct: All condensed vapor returned — used to find minimum number of stages (infinite reflux).

Q7. Minimum reflux ratio (R_min) corresponds to

Infinite number of stages Zero stages Maximum distillate flow Minimum reboiler duty

✅ Correct: Infinite number of stages — at R_min you need infinitely many theoretical stages for desired separation.

Q8. A packed column is preferred over tray column when

Handling large vapour rates Small capacity, low pressure drop and fouling-prone systems Need for many stages in small column height Binary mixture only

✅ Correct: Small capacity & low pressure drop — packing gives low ΔP and is good for corrosive or fouling feeds.

Q9. Murphree tray efficiency refers to

Actual separation per tray vs ideal stage Column diameter efficiency Packing efficiency Heat transfer efficiency

✅ Correct: Actual vs ideal stage — measure of real tray performance relative to theoretical plate.

Q10. The feeding point where feed is introduced at its bubble point is called

Subcooled feed Superheated feed Saturated liquid feed Azeotropic feed

✅ Correct: Saturated liquid feed — feed at bubble point is saturated liquid and has q = 1 in McCabe-Thiele.

Q11. The q-line in McCabe-Thiele diagram represents

Operating line of rectifying section Feed condition (fraction liquid)** Reflux ratio Distillate composition only

✅ Correct: Feed condition — q-line slope indicates feed thermal state (q = fraction liquid).

Q12. Steam stripping in column is used to

Add heat at top Remove light ends from bottom product Reduce reflux Increase column diameter

✅ Correct: Remove light ends from bottom — live steam strips volatile components from bottom product.

Q13. Total condenser in distillation does which of the following?

Partially condenses vapor Completely condenses overhead vapor to liquid Acts as reboiler None of these

✅ Correct: Completely condenses overhead vapor — gives liquid distillate for reflux or product draw.

Q14. Partial reboiler means

All liquid boiled Only part of the liquid is vaporized and returned to column No vaporization Column flooding

✅ Correct: Only part vaporized — common in plant columns where reboiler serves as a feed/boiler.

Q15. In binary distillation, if relative volatility α ≈ 1, then

Easy separation Difficult separation Complete immiscibility No vapour formation

✅ Correct: Difficult separation — α close to 1 means components have similar volatilities.

Q16. Boiling point elevation of a solution is an example of

Ideal solution behavior Non-ideal solution behavior Dalton's law failure Raoult's law being exact

✅ Correct: Non-ideal behavior — presence of solute changes vapour pressure and boiling point.

Q17. Entrainment in trays leads to

Better separation Liquid carryover to the next stage (bad) Lower pressure drop Increased tray spacing

✅ Correct: Liquid carryover — entrainment reduces efficiency and may contaminate products.

Q18. Flooding in a distillation column is caused by

Too low vapor velocity Excessive vapor velocity leading to liquid hold-up Low reflux Proper tray spacing

✅ Correct: Excessive vapor velocity — causes liquid to be carried upwards, preventing proper contacting.

Q19. HETP stands for

Height Equivalent to a Theoretical Plate Heat Energy to Pressure Height Effectiveness per Tray None of these

✅ Correct: Height Equivalent to a Theoretical Plate — used for packed columns to relate packing height to plates.

Q20. For a binary mixture, if desired distillate purity increases, typically required

Reflux ratio decreases Number of stages decreases Reflux ratio increases or number of stages increases Feed flow increases

✅ Correct: Reflux ratio increases or more stages — higher purity needs more separation effort.

Q21. Steam distillation is useful for separating

Non-volatile solids High-boiling, heat-sensitive organic compounds Azeotropes only Metals

✅ Correct: High-boiling, heat-sensitive organics — steam lowers effective boiling point so compound distills at lower temperature.

Q22. A common industrial tray type is

Bubble cap tray Sieve tray Valve tray All of the above

✅ Correct: All of the above — bubble cap, sieve and valve trays are common, selected by duty and fouling.

Q23. Reboiler duty depends primarily on

Latent heat of vaporization and boil-up rate Number of feed points Column diameter Tray spacing

✅ Correct: Latent heat and boil-up rate — heat required to vaporize liquid returned to column.

Q24. When a non-ideal mixture forms an azeotrope, simple distillation cannot

Change composition of azeotrope by ordinary distillation Separate any components Operate Be performed

✅ Correct: Change composition of azeotrope — azeotropes distill at constant composition; special methods required.

Q25. Steam sparging is used in reboiler to

Reduce viscosity only Strip volatile components and provide heating Cool the column Increase tray spacing

✅ Correct: Strip volatiles and heat — it serves both as heating medium and stripping agent.

Q26. If vapour-liquid equilibrium (VLE) data is known, designer can determine

Number of stages and reflux ratio Column paint color Material of construction only Compressor power

✅ Correct: Number of stages and reflux — VLE is fundamental for distillation design.

Q27. Operating line in rectifying section relates

Liquid mol fraction in column to reflux Vapor and liquid compositions (y vs x) in rectifying section Reboiler duty to condenser duty Tray efficiency only

✅ Correct: y vs x — operating line gives relation between vapor and liquid compositions in section.

Q28. Steam jet ejectors are used in vacuum distillation to

Compress gases to high pressure Create and maintain vacuum without mechanical pumps Act as condensers Heat the feed

✅ Correct: Create vacuum — steam ejectors use motive steam to draw vapor and create low pressure.

Q29. Vapor-liquid contacting area in packed column is provided by

Trays Packings (saddles, rings) Reboiler Condenser

✅ Correct: Packings — increase surface area for mass transfer between phases.

Q30. Enthalpy balance around a distillation column is used to find

Material balance only Reboiler and condenser duties Tray number Column color

✅ Correct: Reboiler and condenser duties — energy balances determine heat duties required.

Q31. Vapor-liquid equilibrium for ideal binary solutions follows

Dalton's law only Raoult's law Henry's law only None

✅ Correct: Raoult's law — for ideal solutions with similar molecular interactions.

Q32. In vacuum distillation, boiling points are

Higher than atmospheric Lower than atmospheric Unchanged Dependent on tray type only

✅ Correct: Lower than atmospheric — reduced pressure lowers boiling points, useful for heat-sensitive materials.

Q33. Column flooding decreases column performance because

Better contacting occurs Excess liquid holdup prevents vapor flow and contacting Reflux increases None of the above

✅ Correct: Excess liquid holdup — vapor paths blocked and mass transfer reduced.

Q34. Lean solvent in extractive distillation acts to

Form azeotrope with light key Change relative volatilities to ease separation Remove non-volatiles only Heat the mixture

✅ Correct: Change relative volatilities — solvent alters VLE to separate difficult pairs.

Q35. In a distillation column, the bottoms product is taken from

Top tray Bottom (near reboiler) Feed point Condenser

✅ Correct: Bottom (near reboiler) — heavy components collected at bottom.

Q36. Azeotropic distillation with entrainer is used to

Break azeotropes by adding third component Increase azeotrope strength Reduce column height only Remove non-volatiles

✅ Correct: Break azeotropes — entrainer forms new VLE behaviour allowing separation.

Q37. If distillate composition vs stage is known, total number of theoretical stages indicates

Column length/height via tray spacing or packing HETP Reboiler duty only Feed composition only Condenser area

✅ Correct: Column height — stages × spacing or stages × HETP gives approximate column height.

Q38. In distillation control, increasing reflux typically causes

Higher top product purity Lower top product purity No change Column flooding

✅ Correct: Higher top purity — more reflux returns condensed liquid to column improving separation.

Q39. Reflux drum in distillation is used to

Separate vapor and liquid from condenser output Add heat to column Act as reboiler None of these

✅ Correct: Separate vapor & liquid — drum stores condensate and provides reflux or product draw.

Q40. Operating pressure of a column affects

Boiling points and relative volatilities Only column height Packing type only None

✅ Correct: Boiling points & relative volatilities — pressure choice important for separation and energy.

Q41. In distillation, light key (LK) and heavy key (HK) refer to

Two major impurities Components whose separation defines the column specifications Solvents used Tray names

✅ Correct: Components defining separation — LK lighter target and HK heavier target in distillate/bottoms.

Q42. In multicomponent distillation, shortcut methods like Fenske-Underwood-Gilliland are used to estimate

Column material Minimum stages, minimum reflux and approximate actual stages Condenser design Pump power

✅ Correct: Min stages & reflux etc. — F-U-G gives quick design estimates for multicomponent mixtures.

Q43. Overhead vapour composition equals liquid composition when

Ideal solution at boiling with equal K-values Azeotrope present and vapour-liquid compositions equal Total reflux only Never

✅ Correct: Azeotrope presence — at azeotrope y and x are equal for that composition (boil without change).

Q44. If feed is a saturated vapor, q (feed condition) equals

1 0 Between 0 and 1 Negative

✅ Correct: 0 — saturated vapor feed corresponds to q = 0 (all vapor, no liquid).

Q45. Murphree tray efficiency less than 1 indicates

Real tray does better than ideal stage Real tray is less effective than ideal stage Column flooded Full condensation

✅ Correct: Less effective — real trays never outperform ideal theoretical stage (efficiency ≤ 1).

Q46. The feed stage location affects

Required number of stages and energy consumption Only tray type Column material only None

✅ Correct: Number of stages & energy — optimal feed location minimizes stages and energy.

Q47. If a distillation column has high vapor traffic but low liquid load, problem likely is

Low tray efficiency Weeping (liquid falling through perforations) Flooding Good operation

✅ Correct: Weeping — too little liquid hold-up causes weeping and loss of contact.

Q48. Reflux ratio is defined as

Reflux flow divided by distillate flow Distillate divided by reflux Reboiler duty divided by condenser duty Feed divided by bottoms

✅ Correct: Reflux/Distillate — amount of liquid returned to column per unit product withdrawn.

Q49. For binary mixture, Fenske equation gives

Minimum reflux ratio Minimum number of theoretical stages at total reflux HETP Packing type

✅ Correct: Minimum number of theoretical stages at total reflux — Fenske gives N_min for specified separation.

Q50. Typical troubleshooting step when column shows dropping top purity is to

Increase reflux ratio or check for feed composition change/leaks Decrease reboiler duty only Change column color Remove condenser

✅ Correct: Increase reflux or investigate feed/leaks — usual action to restore top purity and check disturbances.

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IOCL Selection Series 2025 — Distillation MCQ Test