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

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

Answer: There are three methods

Mccabe thiele method
Ponchon sevrit method
Lewis sorel methodWha

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

Answer: There are three types of tray efficiencies

Local or point efficiencies
Murphee plate efficiencies
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

Flooding
Weeping/Dumping
Entrainment
Foaming

In Packed bed column there are main three problems

Channeling
Loading
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.

Distillation interview questions Tray spacing

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:

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

Distillation interview questions