Distillation interview questions

Distillation interview questions blog contains interview questions regarding to distillation process and concept

Question 1: What is Distillation? What are Types of Distillation?

Definition: Distillation is a procedure in which vaporization is followed by fluid mixture to separate out phases of fluid mixture with the help of energy.

Types of Distillation

• Flash Distillation
• Batch Distillation
• Continuous Distillation 
• Steam Distillation
• Extractive Distillation
• Azeotropic Distillation

Read in details Distillation types discussed here

Q: What is the difference between Distillation and Extraction?

1️⃣ Basic Principle:

Distillation: Separation based on difference in boiling points of components.

Extraction: Separation based on difference in solubility of components in two immiscible phases.

2️⃣ Type of Process:

Distillation: Thermal separation (uses heat).

Extraction: Mass transfer separation (uses solvent, no heat needed).

3️⃣ Phase Involved:

Distillation: Occurs between liquid and vapor phases.

Extraction: Occurs between two liquid phases (liquid–liquid) or solid–liquid.

4️⃣ Energy Requirement:

Distillation: High energy due to heating and condensation.

Extraction: Low energy, mainly for mixing and solvent recovery.

5️⃣ Key Equipment:

Distillation: Column with trays or packing, reboiler, condenser.

Extraction: Mixer–settler, extraction column, or centrifuge.

6️⃣ Typical Applications:

Distillation: Separation of ethanol-water, crude oil fractions.

Extraction: Separation of heat-sensitive compounds, antibiotics, or aromatic recovery.

Question 2: What are the various graphical methods for the calculation of number of plates in distillation column?

Answer: There are three methods

1. Mccabe thiele method
2. Ponchon sevrit method
3. Lewis sorel methodWha

Question 3: What are the different types of tray efficiencies?

Answer: There are three types of tray efficiencies

1. Local or point efficiencies
2. Murphee plate efficiencies
3. Overall efficiency

In most of cases overall plate efficiencies is used which is founded by

Overall plate efficiency = No of ideal trays required / No of actual trays required

Question 4: What is relative volatility of fluids? How it impact distillation?

Answer: The relative volatility is the ratio of the K values for two components. It is denoted by ‘α’

               

                                α = k₁ /k₂

In distillation if relative volatility is high then it is easy to separate fluid mixture and few numbers of trays required but if relative volatility is near 1 then it’s difficult to distillate fluid mixture.

Relative volatility is reduced if column pressure is increased.

Question 5: Distinguish between liquid-liquid extraction and leaching?

Answer: In liquid-liquid extraction solvent is used as solvent to separate liquids and in leaching (also known as solid-liquid extraction) solute is using to separate like separate oil from oil cake using hexane.

Question 6: State operational problems in distillation column

Answer: Mainly there are four problems occurring

1. Flooding
2. Weeping/Dumping
3. Entrainment
4. Foaming

In Packed bed column there are main three problems

1. Channeling
2. Loading
3. Flooding

Question 7: What is flooding?

Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column. 

The increased pressure from excessive vapour also backs up the liquid in the down comer, causing an increase in liquid holdup on the plate above. 

Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.

Question 8: What is weeping in distillation column?

• This phenomenon is caused by low vapour flow. 
• The pressure exerted by the vapour is insufficient to hold up the liquid on the tray. 
• Therefore, liquid starts to leak through perforations. Excessive weeping will lead to dumping. 
• That is the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started. 
• Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency.

9. What is normal tray spacing (Distance between two plates) in distillation column?

Answer: As per thumb rule. Normal tray spacing is 2 ft. or 0.6m. Height should not exceed 30m. Hence maximum numbers of try is about 50 trays due to fabrication limit diameter of column should be less than 7m and height less than 35meter.

Question 9: What is Entrainment in distillation column?

• Entrainment refers to the liquid carried by vapour up to the tray above and is again caused by high vapour flow rates. 
• It is detrimental because tray efficiency is reduced: lower volatile material is carried to a plate holding liquid of higher volatility. 
• It could also contaminate high purity distillate. Excessive entrainment can lead to flooding.

Question 10: What is Foaming in distillation column?

• Foaming refers to the expansion of liquid due to passage of vapour or gas. 
• Although it provides high inter facial liquid-vapour contact, excessive foaming often leads to liquid buildup on trays.
• In some cases, foaming may be so bad that the foam mixes with liquid on the tray above. Whether foaming will occur depends primarily on physical properties of the liquid mixtures, but is sometimes due to tray designs and condition. 
• Whatever the cause, separation efficiency is always reduced.

Question 11: What is channeling in distillation column?

In packed column, misdistribution is detrimental to packing efficiency and turndown. 

Misdistribution occurs at low liquid and /or vapour flow, or if the liquid feed is not distributed evenly over the packing Misdistribution delivers less liquid to some area than to others, resulting in reduced mass transfer. 

For example, the column wall directly under the distributor is poorly irrigated. Down in the bed, the liquid tends to flow toward the wall. This condition is also known as channeling. 

At very low flow rates, there may be insufficient liquid to wet the surface of the packing.

Question 12: What is loading in distillation column?

It means the actual flow quantities up and down through the equipment. 

The "loadings" are then compared with the maximum allowable quantities as determined by the physical size of the equipment as well as the operating P and T and properties of the flowing fluids. 

For example, through a section of fractional trays, that comparison would be expressed as "percent of flood". Typical design is 80 to 85 percent of flood for a fractionator.

Question 13: What would be impact if we increase and decrease the size of packing in packed column?

Increase in size of packing will give lower mass transfer rate and lower pressure drop.
Decrease in packing size will lead to higher pressure drop and high mass transfer rates.

Question 14: What should be the packing size in packed columns?

The size of packing should be approximately 1/8th of the internal diameter of packed column for optimum pressure drop

Question 15: When to use absorption factor method to calculate no of plates?

•  If the operating data line and equilibrium line in the Mccabe thiele method runs parallel the no of theoretical plates would be infinite.

• So it would be impossible to find the numbers of plates as the both won’t touch at any point. So we need absorption factor method wherein no of theoretical plates can be found by Fenske equation.

Question 16: What is the use of plate efficiency in distillation column?

Plate efficiency in plate column is used to convert the theoretical number of plates into actual number plates. As any plate can’t perform ideal we have to multiply its efficiency with theoretical no of plates to get actual plates required.

Question 17: What is effect of reflux ratio on the no. of plate required in distillation column?

At high reflux ratio no. of plate required less  (Small  column  height) and at low reflux ratio no. of plate required more (Large column height).

Question 18: Why is Reflux done in distillation column? and Define Reflux Ratio for Increasing product purity.

Reflux: -It is amount of distillate which is resend to distillation column is known as a reflux.

Reflux Ratio: -Reflux  ratio  is  the ratio  of the portion of the overhead  liquid  product  from  a distillation column that is returned to the upper part of column to the portion of liquid

Question 18: Define: Distillation, Simple distillation, Steam distillation, vacuum distillation, Azeotropic distillation, Extractive distillation, Fractional distillation. Volatile liquid

Distillation

Distillation is unit operation in which liquid  mixture  is separated  based on their boiling point difference and relative volatility by means of thermal

Simple Distillation

when the boiling point difference  of two liquid  in  mixture  is  high then we can use simple
Ex: – A mixture of acetone (B.P. – 57 ˚c) & water (B.P. – 100 ˚c) can be separated by simple distillation because boiling point difference is high.

Steam distillation

Steam distillation is used for

• Separating high boiling components from Nonvolatile impurities by using
• For separating high boiling fraction where  there are chances  of decomposition  of material  at high

Vacuum Distillation

It is the type of distillation in which the liquid mixture is to be distilled out in the vacuum which is less than the atmospheric
Vacuum is pressure less than atmospheric pressure, when it is applied at that time  liquid  boils before its boiling point.
Vacuum: – Pressure below the atmospheric pressure is called vacuum.

Azeotropic Distillation

When Boiling  point  difference   is  very low
In Azeotropic distillation a third component called entrainer is added to the  mixture  which forms a new low boiling azeotrope with one of the components which is distilled out first.
Ex – Acetic acid (B.P. – 115 ˚c) and  water  (B.P.  - 100 ˚c) mixture, the butyl acetate (B.P. – 90 ˚c) is added as entrainer and it forms azeotrope with water in the mixture. Water and butyl acetate is distilled out while acetic acid is remain as residue.

Extractive Distillation

Also used when boiling point difference is very close
In Extractive distillation solvent is added which alter the relative volatility of the original components, thus permitting separation.
Ex – In Toluene (B.P – 111 ˚c) and Iso-octane (B.P. – 100 ˚c) liquid mixture, phenol  (B.P.  – 182 ˚c) is added as solvent, Iso-octane is  removed  as distillate  and  toluene  and  phenol removed as residue.

Fractional Distillation

It is used for separating more than two components from the liquid mixture
Ex – Crude oil

Volatile Liquid

It is a tendency of a liquid to vaporize.

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Question 19: What is the Relative Volatility? What is important of Relative Volatility?

• It is a ratio of concentration of more volatile component in vapour phase to liquid phase is called Relative Volatility
• For separation  of liquid  mixture  using  distillation,  Relative  volatility  should  be more  than 1.

Question 20: What is difference between Distillation and Extraction?

Both are used for separating liquid mixtures  in  liquid  extraction  the separation  is  done into 2 liquid phase while in distillation the  separation  is  done  into  a liquid  phase  & vapour phase.

Distillation	Extraction
Relative volatility should be more than 1	Selectivity should be more than 1
Gives pure product	Does not give pure product
Thermal energy has to be supplied	It can be carried out at room temperature
It is suitable for low boiling point and boiling point different if high.	It is suitable for heat sensitive materials
Distillation is costlier	It also required distillation For separation of Extract Solution.
weeping in distillation column

21. When reflux ratio to the column is minimum and zero, what are the requirements in column?

When reflux ratio is minimum, column requires maximum number of trays and minimum reboiler load for a required separation. To avoid this problem of infinite trays we use optimum reflux ratio.

When reflux ratio is zero, column requires infinite number of trays and minimum reboiler load for a required separation. To avoid this problem of infinite trays we use optimum reflux ratio.

22. Why distillation? Why not adsorbption or leaching ?

In distillation the new phase generated is different from the original by phase, or heat content only. This heat can be removed or added by easy operations. 

But in case of adsorption or leaching the a foreign substance is introduced to separate the phases. The new phase generated using these processes is a new solution which in turn may be separated using other separation methods unless the new solution is directly useful. 

This makes the distillation process to more economical.

Distillation process depends on the relative volatilities of the components. If the difference is too low separation is difficult and it makes the process as more expensive.

23. What is Differential Distillation, Simple Distillation, Rayleigh distillation,Rayleigh equation, material Balance equation?

Differential Distillation:

Simple distillation, also known as Rayleigh distillation or differential distillation, is the most elementary example of batch distillation. In this distillation system, the vapor is removed from the
still during a particular time interval and is condensed in the condenser. 

The more volatile component is richer in the vapor than in the liquid remaining in the still. Over time, the liquid remaining in the still begins to experience a decline in the concentration of the more volatile component, while the distillate collected in the condenser becomes progressively more enriched in the more volatile component.

Schematic representation of differential distillation is as shown in in fig.

Fig: Differential distillation

No reflux is returned to the still, and no stages or packing materials are provided inside the column; therefore, the various operating approaches are not applicable to this distillation system.

The early analysis of this process for a binary system, proposed by Rayleigh is given below. Let F be the initial binary feed to the still (mol) and xF be the mole fraction of the more volatile component (A) in the feed. Let B be the amount of compound remaining in the still, xB be the mole fraction of component A in the still, and xD be the mole fraction of component A in the vapor phase. The differential material balance for component A can then be written as:

xD dB = d ( B xB ) = B dxB + xB dB

Upon integration:

Or

In this simple distillation process, it is assumed that the vapor formed within a short period is in thermodynamic equilibrium with the liquid; hence, the vapor composition (xD) is related to the liquid composition (xB) by an equilibrium relation of the form xD = F(xB). 

The exact relationship for a particular mixture may be obtained from a thermodynamic analysis depending on temperature and pressure. 

For a system following the ideal behavior given by Raoult’s law, the equilibrium relationship between the vapor composition y (or xD) and liquid composition X (or xB) of the more volatile component in a binary mixture can be approximated using the concept of constant relative volatility (α), which is given by:

Substitution of the above equation in Equation (2)

Although the analysis of simple distillation historically represents the theoretical start of batch distillation research, a complete separation using this process is impossible unless the relative volatility of the mixture is infinite. 

Therefore, the application of simple distillation is restricted to laboratory-scale distillation, where high purities are not required, or when the mixture is easily separable

31. What is distillation, and why is it widely used in chemical industries?

Distillation is a physical separation process that separates liquid mixtures based on differences in volatility, i.e., boiling points of components. 

The lighter or more volatile component vaporizes first, and on condensation, we obtain a purified distillate. 

It is one of the most common separation techniques in industries because it provides

high purity products and is versatile, being used in petroleum refining, alcoholic beverages, essential oil extraction, and chemical manufacturing. 

For example, crude oil is separated into

petrol, diesel, and kerosene using fractional distillation.

32. In distillation, what do the terms "x" and "y" represent, and how are they related?

In vapor-liquid equilibrium (VLE), x denotes the mole fraction of a component in the liquid phase, while y represents the mole fraction of the same component in the vapor phase. 

The relationship between x and y depends on the relative volatility of the mixture and is

derived from VLE data. This relationship is critical for designing distillation columns because it determines how efficiently components separate. 

For example, if xA = 0.4 for a component A, its vapor mole fraction yA might be 0.62 depending on α, showing that vapor is richer in the more volatile component.

33. What is relative volatility (α), and how does it influence separation?

Relative volatility (α) is a measure of how easily two components can be separated by distillation. 

Mathematically, it is the ratio of their equilibrium vapor-to-liquid ratios (K-values). 

If α > 1.5, separation is generally easy, 

while α ≈ 1 indicates that the components

have nearly identical volatilities, making separation very difficult (e.g., azeotropes). 

For instance, benzene and toluene have α around 2, allowing feasible separation, but ethanol–water has α close to 1 at

azeotropic composition, requiring special methods.

34. What is vapor-liquid equilibrium (VLE), and why is it important in distillation?

Vapor-liquid equilibrium (VLE) is the condition at which a liquid mixture and its vapor phase are in equilibrium at a given temperature and pressure. 

In this state, the rate of molecules entering

the vapor equals those returning to liquid, ensuring stability. 

VLE data helps engineers design the number of trays, reflux ratios, and column dimensions. 

For example, the ethanol-water system at 1 atm

shows a well-studied VLE curve, crucial for designing separation strategies.

35. What is an azeotrope, and can you give a practical example?

An azeotrope is a unique mixture of two or more liquids that behaves as if it were a single substance, boiling at a constant composition and temperature. 

This makes further separation by simple distillation impossible beyond this composition. 

A common industrial example is the ethanol-water azeotrope at 95.6% ethanol by volume, which limits the maximum purity of ethanol through simple distillation. 

To overcome this, azeotropic or pressure-swing

distillation is used.

36. Can you explain Raoult’s Law and its role in distillation design?

Raoult’s Law states that the partial vapor pressure of a component in a liquid mixture equals its mole fraction in the liquid multiplied

by its pure component vapor pressure. 

This principle is used to predict the vapor composition in ideal mixtures. It is fundamental to calculating phase equilibrium in distillation, though real mixtures often deviate due to non-idealities. 

For example, in an ideal benzene- toluene system, Raoult’s law accurately predicts vapor pressures, but ethanol-water shows deviations due to hydrogen bonding.

37. How do simple distillation and fractional distillation differ?

Simple distillation involves vaporizing a liquid mixture and condensing the vapor, suitable when the boiling point difference is large (>60 °C). It is simple but limited in use for industrial purposes.

Fractional distillation, on the other hand, uses a column with trays or packing, enabling multiple vapor-liquid contacts for improved separation of close-boiling components. 

For example, separating acetone (56 °C) from water (100 °C) can be done by simple distillation, but separating benzene (80 °C) and toluene (110 °C) requires fractional distillation.

38. What is flash distillation, and where is it commonly applied?

Flash distillation is a single-stage partial vaporization process where a liquid mixture is suddenly exposed to reduced pressure or

higher temperature, causing partial vaporization. 

The vapor and liquid phases that form are then separated based on composition. It is widely

used in petroleum refining to pre-separate crude oil into lighter and heavier fractions before detailed refining. 

Its simplicity makes it

suitable where approximate separation is sufficient.

39. What do we mean by continuous distillation, and why is it preferred in large-scale industries?

Continuous distillation is a process where feed is introduced continuously, and products are withdrawn simultaneously, maintaining steady-state operation. 

Unlike batch distillation, the compositions of distillate and bottoms remain constant over time. 

It is the standard method in large-scale industries such as oil refineries and petrochemical complexes, where thousands of tons of crude are processed daily. 

This method ensures efficiency, consistency, and

economic feasibility.

40. How does batch distillation operate, and in what situations is it suitable?

Batch distillation starts with a fixed charge of liquid feed, which is distilled over time, and the composition of distillate changes as distillation progresses. 

It is ideal for small-scale operations, specialty chemicals, or pharmaceutical products, where flexibility and purity are important. 

For example, essential oils and fine

chemicals are often produced by batch distillation due to small volumes and frequent product changes.

41. What is molecular distillation, and why is it important for heat-sensitive products?

Molecular distillation is a high-vacuum distillation technique in which the mean free path of vapor molecules is larger than the

distance to the condenser. This allows separation at very low temperatures, preventing decomposition of heat-sensitive compounds.

It is used for purifying vitamins, pharmaceuticals, essential oils,

and fatty acids. For instance, vitamin E is commonly purified using molecular distillation to retain its bioactivity.

42. What is vacuum distillation, and where is it typically used?

Vacuum distillation is performed under reduced pressure, which lowers the boiling point of liquids and prevents thermal cracking or

degradation. 

It is crucial in industries where high-boiling components must be separated without decomposition. 

For example, in oil refineries, vacuum distillation is used to separate heavy gas oils and

lubricating oils from crude residue after atmospheric distillation. 

43. What is extractive distillation, and how does it work?

Extractive distillation involves adding a high-boiling solvent (entrainer) that modifies the relative volatility of components, enabling otherwise difficult separations. 

The solvent is chosen such that it does not form an azeotrope and can be easily recovered. 

A classic example is separating aromatics (benzene, toluene) from aliphatics using sulfolane as the entrainer. 

This technique is widely applied in the petrochemical industry.

44. What is steam distillation, and why is it used for essential oils?

Steam distillation passes steam through a liquid mixture, lowering the boiling point of volatile compounds and allowing separation at

lower temperatures. 

This prevents decomposition of heat-sensitive

materials. It is commonly used in the perfume and food industries for extracting essential oils like lavender, eucalyptus, and rose. 

The

advantage is that oils with boiling points above 200 °C can be distilled safely below 100 °C with steam.

45. What is reflux in a distillation column, and why is it important?

Reflux refers to the portion of condensed overhead vapor returned to the top of the column. 

It enhances vapor-liquid contact and

sharpens separation, leading to purer products. Without reflux, only a single theoretical separation could occur, making effective distillation impossible. 

For example, in crude distillation towers, a portion of condensed naphtha is returned as reflux to improve the separation of

light fractions.

46. What is reflux ratio, and how does it affect separation efficiency? 

The reflux ratio is defined as the ratio of liquid returned as reflux to the distillate withdrawn. 

A higher reflux ratio generally increases

purity but also raises energy consumption and operating cost.

Conversely, too low a reflux ratio reduces separation efficiency.

Industrial designers optimize reflux ratio to balance product purity with economic operation.

47. What is meant by minimum reflux ratio, and how is it determined?

The minimum reflux ratio is the lowest reflux ratio at which the desired separation can just be achieved, requiring an infinite number of stages in theory. 

It is determined using Underwood’s equations or graphical methods such as McCabe-Thiele.

Operating at this ratio is impractical, so actual reflux ratios are chosen higher (typically 1.2–1.5 times the minimum).

48. What does total reflux mean, and why is it useful?

Total reflux is the condition where all condensed vapor is returned to the column as reflux, with no distillate withdrawn. 

This results in the maximum possible separation for a given number of stages. It is

used during startup operations to establish steady profiles and in design calculations to estimate the minimum number of stages

required.

49. What is the q-line in the McCabe-Thiele method, and what does it represent?

The q-line represents the thermal condition of the feed in the McCabe-Thiele graphical method. The slope of the line is q/(q–1), 

where q is the fraction of feed that is liquid. For example, q=1

corresponds to a saturated liquid feed (vertical line), while q=0

corresponds to a saturated vapor feed (horizontal line). 

The q-line helps determine the correct feed tray location in the column. 

50. How is relative volatility defined in terms of K-values, and why is it important?

Relative volatility (αAB) can also be expressed in terms of K-values,

where K = y/x = vapor mole fraction/liquid mole fraction at equilibrium. 

Thus, αAB = KA / KB. This definition is particularly useful in multicomponent distillation because it simplifies equilibrium

calculations. 

A higher α means easier separation, while α ≈ 1 indicates difficulty, often requiring special separation techniques like azeotropic

distillation.

Section 2: Equipment & Internals

51. What are the main parts of a distillation column, and what functions do they serve?

A distillation column is a tall vertical shell designed to provide intimate contact between rising vapor and descending liquid, allowing repeated vapor-liquid equilibrium stages. 

The key components include:

• Column shell – the cylindrical vessel holding trays or packing.

• Trays or packing – internal contact devices for phase interaction.

• Feed inlet – where the mixture is introduced.

• Condenser – condenses overhead vapor.

• Reflux drum – stores condensed distillate and splits it into reflux and product.

• Reboiler – provides the heat at the bottom to generate vapor.

Each part plays a role in maintaining efficient mass transfer and

stable operation. 

52. What is a reboiler, and what types are commonly used?

A reboiler is a heat exchanger located at the base of a distillation column that supplies the heat needed to generate vapor, driving

separation. 

The main types are:

• Kettle reboiler – liquid is boiled in a shell with an external heater, simple design.

• Thermosyphon reboiler – uses natural circulation; commonly used in refineries.

• Forced circulation reboiler – requires a pump, suitable for viscous fluids.

• Fired reboiler – directly heated with fuel combustion, used in large petroleum units.

For example, crude distillation units typically use thermosyphon reboilers for efficiency.

53. What is a condenser, and what are its main types?

A condenser is a heat exchanger at the top of a distillation column that cools and condenses rising vapor into liquid. 

Two types exist:

• Total condenser – condenses all vapor into liquid, producing only liquid product.

• Partial condenser – condenses only part of the vapor, giving both vapor and liquid outputs (used when vapor is required as a

product).

For instance, in a crude oil distillation tower, the overhead vapors of naphtha and light gases are partially condensed to separate LPG from liquid naphtha.

54. What is a reflux drum, and why is it necessary?

A reflux drum, also called an accumulator, is a vessel that collects condensed vapors from the condenser. 

It stores liquid distillate and allows splitting of the stream between reflux (returned to the column) and distillate product (withdrawn). The reflux drum also enables gas– liquid disengagement when non-condensables are present. 

For example, in refinery distillation units, reflux drums stabilize pressure by removing dissolved gases before pumping reflux back.

55. What is a sieve tray, and where is it suitable?

A sieve tray is a flat tray with perforated holes through which vapor passes, bubbling into the liquid layer on the tray. 

It is simple in design, inexpensive, and provides good efficiency for moderate vapor-liquid loads.

However, it is prone to weeping at low vapor

rates. 

They are widely used in petrochemical plants where load conditions remain fairly stable.

56. What is a valve tray, and what advantages does it offer?

A valve tray contains movable valves that lift as vapor flows through, opening wider at higher vapor rates and closing at low rates. 

This self- adjusting feature provides flexibility and good turndown ratio.

Valve trays are more expensive than sieve trays but offer better efficiency across variable operating conditions. 

They are often used in multi-product refineries where feed rates and compositions

fluctuate.

57. What is a bubble cap tray, and why is it less common today?

A bubble cap tray consists of chimneys (risers) covered with caps that force vapor to bubble through the liquid. 

This ensures a positive

liquid seal, preventing vapor bypass. While bubble cap trays handle low vapor loads and give good mixing, they are costly, bulky, and

have higher pressure drop compared to sieve/valve trays. 

They are still used in cases where liquid backflow prevention is critical, such as in solvent recovery units. 

58. What is tray spacing, and what are typical values used?

Tray spacing is the vertical distance between consecutive trays in a distillation column. 

It ensures adequate disengagement of vapor and

liquid as they pass between trays. Typical values range from 18–24 inches (0.45–0.6 m), depending on column diameter and service. 

For vacuum distillation, larger spacings may be used to reduce entrainment.

59. What are downcomers and weirs in a tray column?

• Downcomer – a vertical channel that allows liquid to flow from one tray to the tray below.

• Weir – a small vertical barrier on the tray that maintains a liquid level, ensuring proper vapor–liquid contact.

Together, they regulate liquid holdup and contact time.

Without them, liquid might drain too quickly, reducing efficiency.

60. What is random packing, and where is it applied?

Random packing consists of small, randomly dumped pieces such as Raschig rings, Berl saddles, or Pall rings. 

These create a large surface area for vapor-liquid interaction. Random packing is

inexpensive and effective for small-diameter columns and when fouling is minimal. 

For example, random packing is common in

absorption towers and solvent recovery columns.

61. What is structured packing, and why is it used in vacuum distillation?

Structured packing is made of corrugated metal or plastic sheets arranged in a regular pattern, giving uniform liquid flow and high surface area. 

It provides low pressure drop, which is crucial in

vacuum distillation to avoid vapor pressure losses. 

It also gives higher efficiency (lower HETP). For instance, structured packing is widely used in lube oil vacuum towers to improve efficiency at very low pressures.

62. How do tray columns differ from packed columns?

• Tray columns: vapor and liquid interact in discrete equilibrium stages, better for large diameters, high liquid loads, or fouling

feeds.

• Packed columns: provide continuous contact surface, lower pressure drop, better suited for vacuum and small-diameter columns.

For example, crude oil fractionators (large scale) use trays, while pharmaceutical distillation often uses packed columns due to purity and low pressure needs.

53. What is HETP, and how is it used in column design?

HETP stands for Height Equivalent to a Theoretical Plate. It is the packing height that achieves the same separation as one ideal

equilibrium stage. HETP depends on packing type, fluid properties, and flow rates. 

For example, structured packing may have an HETP of 0.3–0.5 m, whereas random packing may be around 0.6–0.9 m. 

This concept is used to estimate column height when using packed sections.

64. When should trays be used instead of packing, and vice versa?

• Trays are better for large-diameter columns, dirty feeds, or high vapor-liquid loads.

• Packing is better for small diameters, vacuum distillation, and when pressure drop must be minimized. 

For example, atmospheric crude towers use trays due to scale, while vacuum towers use packing to reduce pressure drop.

65. What is entrainment in distillation columns, and why is it problematic?

Entrainment occurs when liquid droplets are carried upward by rising vapor into higher trays. 

This contaminates overhead products

with heavier components and reduces efficiency.

 

Entrainment usually happens at high vapor rates (close to flooding conditions). To control

it, demisters and lower vapor velocities are used.

66. What is weeping, and how does it affect column efficiency?

Weeping occurs when liquid leaks through tray perforations due to insufficient vapor flow.

Instead of being held on the tray for vapor-

liquid contact, liquid drains downward, reducing separation efficiency. 

It is often indicated by low tray pressure drop. Operators correct weeping by increasing vapor rate or reducing reflux.

67. What is flooding in a distillation column, and what are its symptoms?

Flooding happens when excessive vapor flow prevents liquid from descending properly through downcomers, leading to accumulation

and loss of separation. 

Symptoms include:

• Sudden increase in column ΔP (pressure drop).

• Liquid carryover into overhead.

• Instability in reflux drum levels.

Flooding is prevented by designing columns to operate at 60– 80% of flooding velocity. 

68. What is foaming, and in which cases does it occur?

Foaming is the formation of stable froth on trays due to surface- active agents in the feed (e.g., soaps, surfactants, heavy aromatics). 

It reduces separation efficiency and can cause entrainment and flooding.

Anti-foam chemicals or feed pretreatment can control foaming. For instance, foaming is a frequent issue in FCC gasoline fractionators.

69. What is dumping, and when does it occur?

Dumping occurs when liquid bypasses the tray active area and flows straight through downcomers without proper vapor-liquid

contact. 

It usually happens at very high liquid loads or improper tray hydraulics. 

Dumping drastically reduces efficiency and is corrected by adjusting liquid handling capacity.

70. What are demisters, and what purpose do they serve?

Demisters are devices, usually wire mesh pads or vane separators, installed at the top of distillation columns to remove entrained liquid

droplets from vapor. 

They prevent contamination of overhead

products and protect downstream compressors or equipment. 

For example, in amine regeneration units, demisters prevent amine carryover into the gas phase.

Section 3: Column Design & Methods

71. What are the key design parameters for a distillation column?

When designing a distillation column, engineers must consider:

• Feed composition and flow rate – determines number of stages.

• Relative volatility – affects ease of separation. • Reflux ratio – impacts both energy and equipment size.

• Tray/packing efficiency – influences column height.

• Operating pressure – affects relative volatility and condenser/reboiler design.

For example, a column separating benzene–toluene will require fewer trays than one separating close-boiling xylene isomers.

72. What is the McCabe–Thiele method, and why is it useful?

The McCabe–Thiele method is a graphical design approach used to determine the number of theoretical stages required for a binary

distillation. 

It plots equilibrium curve, operating lines, and feed line on an x–y diagram. By “stepping off” stages, one can estimate how many trays are needed for a given reflux ratio. 

Though approximate, it is a simple, visual, and effective tool for early design before rigorous

simulations.

73. What is the Fenske equation, and when is it applied?

The Fenske equation gives the minimum number of stages required for a desired separation under total reflux (no product withdrawn). 

It is expressed as:

Where xD,xBx_D, x_B are distillate and bottoms compositions, and α\alpha is relative volatility. 

It provides a baseline for column design,

usually combined with Underwood and Gilliland correlations.

74. What is the Underwood method used for in distillation design?

The Underwood method estimates the minimum reflux ratio needed for a given separation. It is especially useful for multicomponent distillation, where simple McCabe–Thiele charts

don’t apply. 

By solving a set of algebraic equations involving feed composition, relative volatility, and product purity, designers can size reflux systems and condensers correctly.

75. What is Gilliland’s correlation, and how is it applied?

Gilliland’s correlation relates the actual number of stages to the minimum number of stages (from Fenske) and the reflux ratio

(from Underwood). 

It provides a quick method to estimate real

column performance and tray count. Although empirical, it is still widely used for preliminary sizing before simulation.

76. What is reflux ratio, and why is it a critical parameter?

Reflux ratio is defined as the ratio of liquid returned to the column (reflux) to the distillate withdrawn.

• High reflux ratio → more separation, smaller column size, but higher energy cost.

• Low reflux ratio → larger column, lower energy, but harder separation.

For example, crude oil fractionators often use higher reflux ratios in the naphtha section to obtain sharper boiling cuts.

77. What is the minimum reflux ratio, and how does it affect design?

The minimum reflux ratio (Rmin) is the lowest reflux ratio at which desired separation can be achieved with an infinite number of trays.

Operating close to Rmin gives low energy usage but requires impractically tall columns. 

Hence, industrial design typically uses 

78. What is the total reflux condition, and when is it used?

Total reflux means all condensed distillate is returned as reflux and no product is withdrawn. This provides the maximum separation per

stage and is used for:

• Determining minimum number of stages (Fenske).

• Starting up a new distillation column until stable separation is achieved.

However, it is impractical for continuous operation due to no product recovery.

79. What are the common operating pressures for distillation columns?

• Vacuum distillation: below atmospheric pressure (e.g., lube oil fractionation).

• Atmospheric distillation: crude towers operate at 1–2 atm.

• High-pressure distillation: propane/propylene splitters may operate at 20–40 atm.

Pressure is selected to balance volatility, condenser cooling medium, and energy requirements.

80. How does pressure affect distillation column design?

Pressure influences:

• Relative volatility: higher pressure reduces volatility differences, making separation harder.

• Temperature levels: higher pressure raises boiling points, which may require stronger materials.

• Condenser/reboiler duties: pressure affects utilities used (e.g., water vs. refrigeration).

For example, propane–propylene separation is conducted at high pressure to allow water cooling in the condenser. 

81. What is multicomponent distillation, and how is it more complex than binary distillation?

Multicomponent distillation involves separating mixtures with more than two components (e.g., crude oil). 

Unlike binary, multiple light, intermediate, and heavy products must be taken out simultaneously.

This requires advanced methods like Underwood equations, rigorous simulations, or shortcut models (Fenske–Underwood– Gilliland). 

The design is more complex since multiple relative volatilities must be considered.

82. What is azeotropic distillation, and give an example.

Azeotropic distillation is used when two components form an azeotrope—a constant-boiling mixture that cannot be separated by

normal distillation. 

A third component (entrainer) is added to break

the azeotrope.

Example: Ethanol–water azeotrope (95.6% ethanol) is broken by adding benzene or cyclohexane, which alters relative volatility and

allows separation of pure ethanol.

83. What is extractive distillation, and where is it used?

Extractive distillation involves adding a solvent (entrainer) that alters the relative volatility of components without forming an azeotrope.

Example: Separation of close-boiling aromatics and aliphatics using sulfolane solvent.

It is widely used in petrochemical industries for butadiene extraction and aromatic separation.

84. What is reactive distillation, and why is it advantageous?

Reactive distillation combines chemical reaction and distillation in one unit. 

This improves efficiency by simultaneously shifting reaction equilibrium and separating products.

Example: Production of methyl tert-butyl ether (MTBE) from methanol and isobutene.

Advantages include reduced equipment cost, higher conversion, and energy savings.

85. What is pressure-swing distillation, and where is it applied?

Pressure-swing distillation exploits the fact that some azeotropes disappear at different pressures. 

By operating two columns at different

pressures, separation is achieved without entrainers.

Example: Ethanol–water and THF–water systems can be separated using pressure swing.

86. What is steam distillation, and what are its advantages?

Steam distillation introduces steam directly into the column to reduce partial pressures of components, lowering their boiling points.

Advantages:

• Prevents thermal decomposition of heat-sensitive compounds (e.g., essential oils).

• Reduces operating temperature in vacuum distillation.

• Common in crude distillation for kerosene/diesel stripping.

87. What is vacuum distillation, and where is it used?

Vacuum distillation operates at pressures below atmospheric to reduce boiling temperatures. 

This prevents decomposition of high-

boiling materials.

Examples:

• Lube oil distillation in refineries.

• Deodorization of edible oils. 

• Recovery of solvents.

It requires large diameter columns and careful sealing to avoid air leakage.

88. What is molecular distillation, and how is it different from conventional distillation?

Molecular distillation is a short-path vacuum distillation technique performed under very high vacuum (10⁻⁶ to 10⁻⁸ torr). 

The distance between evaporator and condenser is very small, allowing molecules to travel without collisions. 

It is used for separating thermally unstable compounds like vitamins, fatty acids, and pharmaceuticals.

89. What is batch distillation, and when is it preferred?

Batch distillation is a discontinuous process where a charge of liquid mixture is distilled over time. 

Composition of distillate changes as

distillation proceeds. 

It is preferred for:

• Small-scale production.

• Specialty chemicals, pharmaceuticals, perfumes.

• Variable feedstocks.

Although less efficient than continuous distillation, it is flexible for multiproduct operations.

90. What is continuous distillation, and why is it dominant in industry?

Continuous distillation feeds mixture continuously and withdraws products steadily. 

It is dominant because it handles large volumes,

stable operation, and lower per-unit cost. 

Examples include crude oil fractionation and large petrochemical plants. However, it is less

flexible for frequent product changes. 

91. What are side strippers in distillation columns, and why are they used?

Side strippers are small auxiliary columns connected to the main fractionator.

They remove lighter or heavier components from side draw streams, improving cut purity.

Example: In crude distillation, kerosene and diesel side strippers use steam to strip out lighter hydrocarbons, giving sharper boiling fractions.

92. What are pump arounds, and what purpose do they serve in crude towers?

Pump arounds are circulating reflux streams taken from intermediate trays, cooled in exchangers, and returned to the column. 

They:

• Improve fractionation by providing intermediate cooling.

• Reduce condenser duty by rejecting heat at lower temperatures.

• Control tower temperature profile.

They are a key feature of atmospheric crude distillation units.

93. What is a dividing wall column, and what benefits does it offer?

A dividing wall column (DWC) has a vertical wall inside the shell that splits vapor–liquid traffic, allowing three-product separation in a single shell.

Benefits:

• Energy savings (10–30%).

• Lower capital cost.

• Smaller footprint.

They are increasingly used for separating ternary mixtures in petrochemicals. 

94. What is a thermally coupled distillation column (Petlyuk column)?

A Petlyuk column is an advanced configuration where two columns share energy streams instead of operating independently. 

This reduces energy consumption and equipment size compared to conventional trains. 

It is essentially an extension of dividing wall

concept but with more flexibility.

95. What is shortcut distillation design, and when is it used?

Shortcut methods use simplified assumptions and correlations to estimate column requirements. 

Examples include Fenske– Underwood–Gilliland method. 

They are used in preliminary design

stages to size columns before rigorous simulation and detailed design.

96. What are equilibrium stages in distillation, and why are they important?

An equilibrium stage is a theoretical step where vapor and liquid phases achieve equilibrium.

The total number of such stages represents the ideal separation efficiency. Real trays or packing

achieve only a fraction of this efficiency, hence requiring more physical stages than theoretical.

97. What is Murphree tray efficiency, and how is it defined?

Murphree efficiency measures how close an actual tray approaches equilibrium. 

It is defined as:

where yn+1* is the equilibrium vapor composition, yn is the vapor entering the tray, and yn+1 is the vapor leaving the tray. 

For the liquid phase (EML), the formula is EML = (xn - xn+1) / (xn - xn+1), where xn* is the equilibrium liquid composition, xn is the liquid

entering the tray, and xn+1 is the liquid leaving the tray Typical values range from 50–90%, depending on tray design and operating conditions.

98. What is overall column efficiency, and how does it differ from Murphree efficiency?

Overall efficiency relates the actual number of trays to the theoretical stages required. 

It accounts for inefficiencies across the

entire column, whereas Murphree efficiency applies to individual trays.

Overall efficiencies are typically lower (30–70%). Designers use this to size total tray count.

99. What is relative volatility, and why is it crucial for separation?

Relative volatility (α\alpha) expresses the ease of separating two components:

α=KAKB\alpha = \frac{K_A}{K_B} n

Where KK is vapor–liquid equilibrium ratio. Higher α\alpha means easier separation. 

If α\alpha is close to 1, separation becomes very

difficult (e.g., xylene isomers). It is the cornerstone parameter for predicting distillation feasibility.

100. What are pinch points in distillation, and how do they affect design?

Pinch points occur when operating lines approach the equilibrium curve very closely, requiring many trays to achieve separation. 

This typically happens near minimum reflux or when separating close- boiling mixtures. 

Pinch points make columns taller and costlier,

which is why designers avoid operating too close to minimum reflux.

Section 4: Operation, Troubleshooting & Case Studies 

101. What are the main operating variables in a distillation column?

The key operating variables include:

• Reflux ratio – controls separation sharpness.

• Reboiler heat input – controls vapor generation.

• Feed rate and composition – directly affect product quality.

• Column pressure – influences relative volatility.

• Condenser duty – regulates reflux and overhead removal.

Operators monitor and adjust these to keep the column within safe and efficient limits.

102. How is temperature profile monitored and interpreted in a column?

Temperature gradually decreases from bottom to top of the column, reflecting boiling ranges of fractions. 

A stable profile indicates proper operation. Abnormalities such as:

• Flat profile → flooding or foaming.

• Sudden jump → feed composition change.

• Abnormally low top temperature → condenser overload.

Thus, the profile acts as a quick diagnostic tool.

103. How is pressure profile monitored in distillation columns?

Pressure should gradually drop from bottom to top. Sudden pressure increases suggest flooding or entrainment, while very low drops

indicate weeping. 

Monitoring ΔP across trays or packing sections

helps identify hydraulic problems early. 

104. What is column startup procedure in distillation?

Startup typically involves:

1. Ensuring equipment is leak-free and ready.

2. Charging column with initial liquid (for wetting trays/packing).

3. Starting reboiler and gradually increasing heat.

4. Establishing total reflux until steady separation is achieved.

5. Slowly introducing feed and adjusting reflux/product withdrawal.

This staged approach prevents upsets and ensures stable operation.

105. What is column shutdown procedure?

Shutdown involves:

• Gradually reducing feed and heat input.

• Returning to total reflux or reduced load.

• Draining liquid inventories.

• Isolating utilities and depressurizing safely.

Controlled shutdown avoids equipment stress and prevents product contamination.

106. What are common operational problems in distillation columns?

Typical problems include:

• Flooding – excessive vapor load.

• Weeping/dumping – low vapor flow.

• Foaming – presence of surfactants.

• Entrainment – liquid carryover. 

• Corrosion or fouling – impurities in feed.

Each of these affects separation efficiency and must be diagnosed with operating data.

107. How do you identify flooding in a distillation column?

Symptoms include:

• Sharp increase in pressure drop across the column.

• Fluctuating liquid levels in reflux drum.

• Poor product quality.

• Audible noise/vibration.

Flooding typically occurs at vapor rates beyond 80–90% of design.

108. What are the signs of weeping in a column?

Weeping is characterized by:

• Low pressure drop across trays.

• Reduced separation efficiency.

• Poor tray temperatures (lower than expected).

• Observation of liquid leakage during inspection.

It is usually corrected by increasing vapor rate or reducing liquid load.

109. What causes foaming, and how is it controlled?

Foaming is caused by surfactants, heavy aromatics, or impurities that stabilize bubbles.

Consequences include entrainment and reduced

separation. 

Control measures include:

• Feed pretreatment (removing surfactants).

• Antifoam additives. 

• Operating adjustments (reducing vapor velocity).

110. What is entrainment, and how is it minimized?

Entrainment is liquid droplets carried upward by vapor, contaminating overhead products. It is minimized by:

• Reducing vapor velocity.

• Installing demisters or mesh pads.

• Using adequate tray spacing.

• Properly designing downcomers.

111. What is column fouling, and what strategies reduce it?

Fouling occurs when deposits of polymers, salts, coke, or solids form on trays/packing. 

It reduces efficiency and increases ΔP. Strategies

include:

• Feed pretreatment (e.g., desalting crude oil).

• Additives (inhibitors, dispersants).

• Regular cleaning/maintenance.

Designers may also prefer trays over packing for dirty services.

112. How does feed composition variation affect column operation?

A sudden increase in heavy or light ends can shift product qualities and upset temperature profile. 

Operators must adjust reflux ratio,

heat input, and draw rates to maintain specifications. 

Automated control systems are often used to handle such variations.

113. How is column efficiency evaluated during operation?

Efficiency is evaluated by comparing actual product compositions 

• Collecting samples at different draws.

• Measuring tray/packing efficiency.

• Performing heat and mass balance checks.

Any deviations indicate possible hydraulic or mechanical issues.

114. What are energy-saving strategies in distillation operations?

Strategies include:

• Operating at optimum reflux ratio.

• Using heat integration (e.g., pumparounds, heat exchangers).

• Employing dividing wall columns.

• Switching to structured packing for low ΔP.

• Exploring alternative separation methods (membranes, adsorption).

115. What is pressure relief system requirement for distillation columns?

All columns must have pressure safety valves (PSVs) sized for credible overpressure scenarios:

• Loss of cooling in condenser.

• Fire exposure.

• Blocked outlet.

PSVs protect column integrity and ensure compliance with codes (ASME/API).

116. How is instrumentation used for control of distillation columns?

Key instruments include:

• Temperature controllers – regulate reboiler duty. 

• Level controllers – maintain reflux drum and reboiler levels.

• Pressure controllers – stabilize column pressure.

• Flow controllers – regulate reflux and product withdrawal.

Advanced DCS/PLC systems integrate all signals for smooth operation.

117. What is advanced control in distillation, and why is it applied?

Advanced control techniques (e.g., model predictive control, inferential composition control) help maintain product specs in

multicomponent, interacting systems. 

They minimize energy usage

and maximize throughput under disturbances. For example, modern

crude towers use inferential analyzers to control naphtha and diesel cut points.

118. What are the safety hazards in distillation columns?

Major hazards include:

• Overpressure → explosion risk.

• Leakage of flammable/toxic vapors.

• Thermal decomposition of high-boiling feeds.

• Tray collapse or packing damage under abnormal load.

Mitigation includes relief systems, monitoring, and regular inspection.

119. What are the main inspection and maintenance activities for distillation units?

Activities include:

• Visual inspection of trays/packing for damage or fouling.

• Checking for corrosion, erosion, and material thinning. 

• Cleaning deposits from reboilers and column internals.

• Pressure testing and leak checks.

Turnarounds are scheduled periodically to ensure reliability.

120. What is tray hydraulics analysis, and why is it important?

Tray hydraulics analysis determines whether trays can handle vapor and liquid loads without weeping, flooding, or entrainment. 

It involves calculating downcomer residence times, vapor velocities, and weir heights. 

It ensures that column operates safely within

design margins.

121. What is column revamping, and when is it needed?

Revamping means modifying an existing distillation column to handle new capacity, feed, or product requirements. It may involve:

• Adding trays or replacing with packing.

• Installing structured packing for energy savings.

• Upgrading reboilers/condensers.

Revamps are cheaper than new units and extend asset life.

122. What are heat integration opportunities in distillation plants?

Examples include:

• Using hot product streams to preheat feed.

• Employing pumparounds to reduce condenser load.

• Coupling reboilers with waste heat recovery.

Such integration reduces fuel and utility costs significantly.

123. What is column simulation, and what software is commonly used?

Column simulation uses process software to model separation, predict performance, and optimize design. Popular tools include:

• Aspen Plus / Aspen HYSYS.

• ChemCAD.

• PRO/II.

Simulations allow testing different feeds, pressures, and configurations before implementation.

124. How do you troubleshoot poor product purity in a distillation column?

Steps include:

1. Check feed composition (unexpected light/heavy ends).

2. Review operating variables (reflux ratio, reboiler duty).

3. Verify condenser/reboiler performance.

4. Inspect column hydraulics (weeping, flooding).

5. Examine internals for damage or fouling.

Systematic troubleshooting helps restore desired purity.

125. What is tray damage, and what are its consequences?

Tray damage may include broken valves, corroded weirs, or collapsed

downcomers. 

Consequences are poor vapor–liquid contact, short- circuiting, and reduced efficiency. This leads to off-spec products and requires column shutdown for repair.

126. What is column diameter determined by?

Column diameter is sized based on vapor and liquid traffic to avoid flooding. 

Empirical correlations (Souders–Brown) are often used:

Vmax=CρL−ρVρVV_{max} = C \sqrt{\frac{\rho_L -

\rho_V}{\rho_V}} Where CC is a capacity constant. Larger vapor rates require bigger

diameters.

127. What is column height determined by?

Column height is primarily a function of:

• Number of theoretical stages required.

• Tray spacing (18–24 in typical).

• Packing HETP (if packed).

Additional height is allowed for disengagement space above/below internals.

128. What is thermal coupling between reboiler and condenser?

Some advanced designs allow heat exchange between column sections, where condenser heat removal is reused to partially supply

reboiler duty. 

This reduces utility usage and increases overall energy efficiency.

129. What case study examples highlight distillation troubleshooting?

• Crude tower flooding due to high naphtha vapor load → solved

by reducing top reflux.

• Propane–propylene splitter poor purity → traced to weeping trays; resolved by increasing vapor rate.

• Vacuum tower coke formation → mitigated by lowering reboiler temperature and adding wash oil.

These cases show how field observation and design knowledge must work together. 

130. What future trends are emerging in distillation technology?

Future trends focus on sustainability and efficiency:

• Dividing wall and reactive distillation for process intensification.

• Hybrid systems combining membranes with distillation.

• AI-based control systems for real-time optimization.

• Advanced materials for corrosion resistance and high efficiency.

These innovations aim to cut energy use and improve environmental performance.

Distillation Column – Packed Design

131. What is the function of the vapor outlet in a packed distillation column?

The vapor outlet is located at the top of the column and allows the separated vapor product (usually the more volatile component) to exit.

In many cases, this vapor is condensed in a condenser to obtain distillate. 

For example, in crude distillation, light fractions like LPG or naphtha leave through this vapor line. 

It is critical to size the vapor

outlet correctly to avoid pressure drop and entrainment.

132. Why is a liquid distributor essential in packed columns?

The liquid distributor ensures that the feed liquid is evenly spread across the cross-sectional area of the packing. 

Uneven distribution can lead to channeling, poor mass transfer, and reduced column efficiency.

For example, in large diameter towers, spray nozzles or trough distributors are used to guarantee proper wetting of the packing

surface.

133. What is structured packing, and what advantages does it offer over random packing?

Structured packing is a type of packing material arranged in a regular, ordered geometry, often made of corrugated sheets. 

It provides high surface area, low pressure drop, and high separation efficiency. Unlike random packing (like Raschig rings or saddles), structured packing promotes uniform vapor–liquid contact. 

It is commonly used in applications where energy efficiency and high purity are critical, such as cryogenic air separation.

134. Explain the role of the liquid collector in the packed distillation column.

A liquid collector gathers liquid after it has flowed through one packed section, redistributes it, and directs it toward the next section.

This ensures uniform liquid flow across successive layers of packing.

For example, collectors are used between different types of packing (random and structured) to maintain hydraulic balance and prevent maldistribution.

135. What is the purpose of the ringed channel inside the column?

The ringed channel serves as an internal support and a guiding structure for redistributing liquids. 

It helps in collecting liquid from upper packing and delivering it properly to lower sections. This

prevents bypassing and enhances column efficiency. 

For instance, in tall towers, multiple ringed channels are installed to avoid liquid

maldistribution.

136. Why is random packing like rings and saddles used in some sections of the column?

Random packing, such as Raschig rings, Pall rings, or saddle packings, provides economical vapor–liquid contact and is easy to

install. 

They are particularly suitable for moderate separation duties and where fouling may occur.

For example, saddle packing has better

liquid spreading capability and is widely used in absorption and stripping columns where cost efficiency is vital.

137. What are hold-down grids, and why are they important in packed columns?

Hold-down grids are mechanical supports placed above the packing to prevent the packing material from being displaced by vapor flow or

hydraulic surges. 

They also help distribute liquid evenly. For instance, in vacuum distillation units, hold-down grids prevent floating or fluidization of structured packing under reduced pressure conditions.

138. What role does the reboiler return line play in this system?

The reboiler return line brings vapor generated in the reboiler back into the bottom of the column. 

This vapor provides the driving force

for separation, as it rises through the packing and contacts descending liquid. 

For example, in a refinery atmospheric distillation unit, the furnace acts as the reboiler, and its vapors enter the column through

the reboiler return line.

139. Why are hatches provided in distillation columns?

Hatches are manways or access points used for inspection, maintenance, and replacement of internals such as packing, grids, and

distributors. 

They are critical for safe operation and turnaround activities. 

For example, during shutdowns, operators use hatches to check for fouling or damage to packing materials.

140. What is the significance of the bottom line in a packed column?

The bottom line is the outlet for the heavier, less volatile components (bottoms product). 

It connects to downstream equipment such as

reboilers, pumps, or product storage tanks. For example, in crude distillation, heavy gas oil or residue exits through the bottom line for

further processing. 

Ensuring proper sealing and flow design at this

point is vital to avoid leakage and maintain system integrity.

Distillation Column – Advanced Interview Q&A

141. How do you differentiate between tray columns and packed columns in terms of efficiency?

Tray columns provide a stagewise contact between vapor and liquid, which makes them easier to analyze using McCabe–Thiele methods.

Packed columns, on the other hand, provide continuous contact and are analyzed in terms of HETP (Height Equivalent to a Theoretical

Plate). 

Packed columns generally have lower pressure drops and higher efficiency at low liquid loads, while tray columns handle large throughputs better.

142. What is HETP, and why is it important in packed column design?

HETP stands for Height Equivalent to a Theoretical Plate. It represents the height of packing required to achieve the separation

equivalent to one theoretical tray. 

A lower HETP indicates more efficient packing. For example, structured packing often has an HETP of 0.3–0.6 m, while random packing may be 0.6–0.9 m. 

This metric is crucial for determining column height in design.

143. What operational problems can occur due to liquid maldistribution in packed columns?

Maldistribution causes channeling, where liquid flows unevenly through some paths, leaving other packing surfaces underutilized.

This reduces mass transfer efficiency, lowers product purity, and may cause flooding in certain regions. For instance, in large-diameter

towers, poor distributors can lead to a 20–30% efficiency drop.

144. What is flooding in a packed column, and how can it be detected?

Flooding occurs when vapor flow is too high, preventing liquid from flowing downward properly. 

This results in increased column pressure

drop, reduced separation, and sometimes liquid carryover. 

Indicators include a sudden rise in differential pressure across the packing, poor product separation, and liquid entrainment in the vapor line.

145. How does pressure drop differ in packed columns versus tray columns?

Packed columns usually have much lower pressure drops compared to tray columns because they lack large vapor–liquid interfaces created by trays. 

This makes them suitable for vacuum operations, such as vacuum distillation in refineries. 

For example, structured packing provides a pressure drop as low as 0.1–0.2 mbar per theoretical stage. 

Typical steps include checking the liquid distributor for clogging or poor leveling, verifying packing damage or fouling, monitoring

vapor–liquid load against design values, and ensuring proper reboiler/condenser operation.

In some cases, replacing random

packing with structured packing can restore efficiency. 

For example, replacing 50 mm Raschig rings with Mellapak structured packing may double capacity with the same efficiency.

147. Why are packed columns preferred in vacuum distillation?

Packed columns are ideal for vacuum distillation because theyprovide high efficiency with low pressure drop. 

Trays under vacuum would cause significant pressure loss and poor vapor–liquid contact.

For instance, refinery vacuum distillation units (VDUs) commonly use structured packing to separate heavy vacuum gas oil and residue

under pressures as low as 20–30 mmHg.

148. How do you decide between using random packing or structured packing in column design?

Random packing is cost-effective, easy to install, and suitable for moderate efficiency requirements or corrosive environments.

Structured packing, while more expensive, offers higher efficiency, lower pressure drop, and is used in high-purity separations. 

For example, random saddle packing may be used in absorption towers,

while structured Mellapak is chosen for fine chemical or pharmaceutical distillation.

149. What role does column internals design (distributors, grids, supports) play in overall efficiency?

Column internals ensure that liquid and vapor are evenly distributed and that packing is properly supported. Poorly designed internals can cause maldistribution, increased pressure drop, and loss of capacity.

For instance, even if the best packing is used, a faulty distributor can reduce efficiency by 30–40%. Thus, internals are as critical as the packing itself.

150. How do you evaluate column performance after commissioning? 

Performance is evaluated by comparing actual product purities and recoveries with design specifications. 

Key indicators include reflux ratio, pressure drop, temperature profile, and flow rates. Test runs are conducted to calculate overall column efficiency (OCE) and verify if HETP matches the design. 

For example, if product purity is below

spec, engineers may check reflux adjustments, distributor performance, or column hydraulics.

151. Why do we use reflux in a distillation column?

Reflux is used to improve separation, achieve required product purity, and ensure stable and efficient column operation.

Reflux is the portion of condensed overhead liquid returned back into the distillation column. It is important because:

1. Improves separation efficiency – When reflux is returned, it provides liquid that flows down the column and contacts rising vapors. This repeated contact enhances mass transfer and improves the purity of top and bottom products.

2. Controls product purity – Higher reflux ratio gives better separation and higher product purity, while lower reflux saves energy but reduces separation efficiency.

3. Maintains column stability – Reflux helps control column temperature profile and prevents fluctuations, ensuring steady operation.

4. Reduces number of trays – For a given separation, using reflux allows fewer trays or packing height compared to operating without reflux.

152. How the relative distillation work?

Relative distillation separates two or more liquids based on differences in their boiling points or volatility

The liquid mixture is heated; the more volatile (lower boiling) component vaporizes first

Vapors are cooled in a condenser and collected as distillate, enriched in the volatile component

The less volatile component remains in the boiling flask, raising the boiling point as distillation progresses

Fractional distillation uses a column for repeated vaporization-condensation cycles to improve separation of close-boiling liquids

The process relies on Raoult's and Dalton's laws for vapor-liquid equilibrium

153. Why condenser is at top and reboiler is at bottom of column?

Condenser at top condenses rising vapors, reboiler at bottom vaporizes liquid—both positions use gravity and thermodynamics for efficient separation.

Vapor-Liquid Separation:

Vapors naturally rise to the top of the column → condenser placed at top to cool and condense them.

Liquid naturally flows downward due to gravity → reboiler placed at bottom to supply heat and vaporize.

Energy Efficiency:

Condenser removes heat from rising vapors.

Reboiler adds heat to liquid at bottom, driving continuous vapor generation.

Ease of Operation:

Top: condensed liquid (distillate) can be collected or refluxed back.

Bottom: reboiled vapors rise upward, and bottom liquid (residue) is withdrawn easily.

Process Control:

Placing condenser at top maintains column pressure.

Reboiler at bottom ensures stable temperature gradient from bottom (hot) to top (cool).

154. What is the difference between CDU and VDU?

CDU separates crude oil into basic fractions, while VDU recovers valuable heavy products from CDU residue under vacuum.

CDU (Crude Distillation Unit):

First major unit in a refinery.

Separates crude oil into lighter fractions at atmospheric pressure.

Operates up to ~350–370 °C.

Produces LPG, naphtha, kerosene, diesel, gas oil, and atmospheric residue.

VDU (Vacuum Distillation Unit):

Secondary unit that processes atmospheric residue from CDU.

Works under vacuum conditions to lower boiling points and avoid cracking.

Operates up to ~380–420 °C (under vacuum).

Produces LVGO, HVGO, and vacuum residue used for further processing.

155.What are the safety precautions while handling a distillation column?

1. Pre-operation checks:

Ensure all valves, gauges, and safety devices are in proper working condition.

Verify no leaks in column, piping, and flanges before startup.

Confirm that all manways and openings are properly sealed.

2. Personal safety:

Wear appropriate PPE (gloves, goggles, flame-resistant clothing, safety shoes).

Follow lockout-tagout (LOTO) procedures during maintenance or shutdown.

3. Process safety:

Maintain operating parameters (temperature, pressure, reflux ratio) within design limits.

Never exceed pressure ratings — ensure pressure relief valves are functional.

Avoid sudden changes in feed rate or temperature to prevent thermal shock.

4. Handling flammable materials:

Keep ignition sources away and ensure proper grounding to avoid static discharge.

Ensure proper ventilation and continuous monitoring for gas or vapor leaks.

5. Emergency readiness:

Know the location of emergency shutdown systems, fire extinguishers, and escape routes.

Be trained in handling spills, leaks, and overpressure situations.

Q: What is co-distillation and why is it used in pharmaceutical industries?

Co-distillation is used in pharmaceuticals to safely separate, purify, or dry heat-sensitive compounds at lower temperatures, preventing thermal degradation and improving product quality.

Co-distillation is a distillation technique where a mixture of two immiscible liquids is distilled together, allowing them to vaporize simultaneously at a temperature lower than their individual boiling points.

It is commonly used when a heat-sensitive compound needs to be separated or purified without decomposition.

Uses in Pharmaceutical Industry:

Helps in purifying temperature-sensitive compounds by lowering the effective distillation temperature.

Used to remove impurities or solvents that form azeotropes with water or other liquids.

Facilitates separation of essential oils, natural products, or active pharmaceutical ingredients (APIs) without degradation.

Aids in drying organic solvents by co-distilling with water or other agents.

Q: What is the difference between Fractional Distillation and Rectification?

Fractional distillation separates mixtures by boiling point difference, while rectification is a refined, multi-stage version that provides greater purity and efficiency.

1. Basic Definition:

Fractional Distillation: Separation of a mixture into components (fractions) based on different boiling points using a fractionating column.

Rectification: An advanced or repeated fractional distillation process that involves continuous condensation and vaporization to achieve higher purity of components.

2. Principle:

Fractional Distillation: Single-stage or limited-stage separation.

Rectification: Multi-stage equilibrium process enhancing separation efficiency.

3. Operation Mode:

Fractional Distillation: Can be batch or continuous.

Rectification: Usually continuous for precise and high-purity separation.

4. Purity Level:

Fractional Distillation: Gives moderate purity.

Rectification: Gives high or near-azeotropic purity.

5. Application:

Fractional Distillation: Used in petroleum refining, solvent recovery, and lab separations.

Rectification: Used in alcohol purification, fine chemical and pharmaceutical processes.

Q: What is the effect of energy consumption in case of minimum and maximum reflux ratio?

At Rmin → very high energy use due to infinite stages.

At Rmax → high utility cost due to excess reflux.

Optimum reflux ratio gives minimum total energy consumption and best efficiency.

1️⃣ Reflux Ratio Definition:

Reflux ratio (R) = L/D, where L = liquid returned to the column and D = distillate withdrawn.

It determines separation efficiency and energy requirement in distillation.

2️⃣ At Minimum Reflux Ratio (Rmin):

Energy Consumption: Very high (infinite number of stages required).

Column Size: Very tall column due to more trays/packing needed.

Reboiler and Condenser Duty: Maximum, as more vapor flow is required to achieve separation.

Operation: Theoretically ideal but economically impractical.

3️⃣ At Maximum Reflux Ratio (Rmax):

Energy Consumption: Minimum, because fewer stages are required.

Column Size: Shorter column (fewer trays).

Reboiler and Condenser Duty: Higher reflux → higher condensation load, increasing utility cost.

Operation: Fast separation but energy-inefficient due to excess reflux circulation.

4️⃣ Optimum Reflux Ratio:

Lies between Rmin and Rmax.

Provides minimum total cost (balanced energy and equipment cost).

Q: What are the criteria for using McCabe-Thiele and Ponchon-Savarit methods to find the number of theoretical trays in a distillation column?

McCabe–Thiele → Simple, approximate, assumes CMO.

Ponchon–Savarit → Detailed, accurate, accounts for energy balance.

McCabe–Thiele Method:

Based on graphical approach using x–y equilibrium data.

Assumes constant molal overflow (CMO) — i.e., constant liquid and vapor flow rates throughout column.

Suitable for binary distillation only.

Used when latent heat of vaporization of components is nearly equal.

Feed, distillate, and bottoms compositions are known.

Preferred for quick and approximate design or conceptual understanding.

Ponchon–Savarit Method:

Based on enthalpy–composition (H–x/y) diagram.

Does not assume constant molal overflow.

Suitable for binary systems with significant energy imbalance or non-ideal mixtures.

Requires enthalpy data for liquid and vapor phases.

Used for more accurate design when heat effects (e.g., heats of vaporization or mixing) are significant.

More rigorous and accurate than McCabe–Thiele, but computationally intensive.

Q: What is the location of the feed tray in a distillation column and what are the criteria for its location?

Location of Feed Tray:

The feed tray is the tray where the feed enters the distillation column.

It divides the column into two sections:

Rectifying (above feed tray) – enrichment of more volatile component.

Stripping (below feed tray) – removal of less volatile component.

Criteria for Feed Tray Location:

Should be located where the composition of liquid and vapor most closely matches the feed composition.

Aim is to minimize the number of trays and energy consumption.

Determined graphically by McCabe–Thiele method at the intersection of operating lines (q-line).

If feed is subcooled liquid, feed tray is placed lower in the column.

If feed is superheated vapor, feed tray is placed higher in the column.

Proper feed tray location ensures:

Efficient mass transfer,

Minimum reboiler and condenser duty,

Stable column operation.

Q: Is the feed tray in the rectifying section or stripping section of a distillation column?

Feed tray is the dividing line — it separates the rectifying section above and the stripping section below.

Feed tray acts as the boundary between the rectifying and stripping sections.

Key Points:

The feed tray itself is not part of either section, but divides the column into:

Rectifying section (above feed tray): where vapor is enriched with more volatile component.

Stripping section (below feed tray): where less volatile component is stripped out from the liquid.

Location of feed tray depends on feed condition (q-value) and composition.

Correct placement ensures minimum number of trays and efficient energy use.

Q: What kind of column — packed or tray — would you prefer for vacuum distillation, and why?

Preferred Column:

Packed Column is preferred for vacuum distillation.

Reasons:

Lower Pressure Drop:

Packing offers much less pressure drop compared to trays — crucial under vacuum to maintain low absolute pressure.

Better Efficiency at Low Pressure:

Provides high surface area for vapor–liquid contact, ensuring effective separation even at reduced pressures.

Minimized Liquid Hold-up:

Reduces residence time and thermal degradation of heat-sensitive materials (common in vacuum operations).

Compact Design:

Requires smaller column height for the same separation compared to trays under vacuum.

Smooth Operation:

Avoids issues like weeping, entrainment, and foaming, which are more pronounced in tray columns at low pressures.

Typical Applications:

Used in pharmaceuticals, petrochemicals, and fine chemicals where temperature-sensitive compounds are distilled under vacuum.

Thanks for reading - Distillation Interview Questions

Naitik Patel
Industrial Guide