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

  1. What is normal tray spacing (Distance between two plates) in distillation column?
  2. What are the various graphical methods for the calculation of number of plates in distillation column?
  3. What are the different types of tray efficiencies?
  4. What is relative volatility of fluids? How it impact distillation?
  5. Distinguish between liquid-liquid extraction and leaching?
  6. State operational problems in distillation column
  7. What is flooding?
  8. What is weeping in distillation column?
  9. What is Entrainment in distillation column?
  10. What is Foaming in distillation column?
  11. What is channeling in distillation column?
  12. What is loading in distillation column?
  13. What would be impact if we increase and decrease the size of packing in packed column?
  14. What should be the packing size in packed columns?
  15. When to use absorption factor method to calculate no of plates?
  16. What is the use of plate efficiency in distillation column?
  17. What is effect of reflux ratio on the no. of plate required in distillation column?
  18. Why is Reflux done in distillation column? and Define Reflux Ratio for Increasing product purity.
  19. Define: Distillation, Simple distillation, Steam distillation, vacuum distillation, Azeotropic distillation, Extractive distillation, Fractional distillation. Volatile liquid
  20. What is the Relative Volatility? What is important of Relative Volatility?
  21. What is difference between Distillation and Extraction?

Question 1: 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 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 method


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.

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

Thanks for reading - Distillation interview questions
Naitik Patel
Industrial Guide

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