Question

2. Petroleum refining process involves distillation as the most important separation unit for purification of different products based on their volatility.

a. Discuss various types of distillation processes you may find in petroleum refineries   

b. What can be done to improve purity of a top product in a distillation column ​

  

c. What will be the effect of excessive reflux ratio in a distillation column ​

d. What are the operational factors affecting the efficiency of a distillation column

  

e. Discuss the problems of flooding and weeping in a distillation column ​​

Answer 1

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Solution-

Petroleum refining processes are the chemical engineering processes and other facilities used in petroleum industries/refinaries to transform/change crude oil into useful products like LPG, gasoline or petrol, kerosene, jet fuel, diesel oil and etc

It increases the concentration of a particular component in the mixture or to obtain (almost) pure components from the mixture.

A = Various types of Distillation Processes :

1. Simple Distillation = It causes  heating the liquid mixture to the boiling point and immediately condensing the resulting vapors.

2. Fractional Distillation = It works to separate mixtures of liquids that have similar boiling points.

3. Vacuum Distillation = This distillation is ideal for separating mixtures of liquids with very high boiling points.

B = Distillation helps to gain high purity of the fractions collected. We can improve the purity of the fraction by increasing the surface area of the fractionation column.

Remove the fractionation column from the distillation unit and pack the internal space with steel wool. Mainly use highly porous, non-absorbent material for the column packing materials.

C = Effect of excessive reflux ratio in a distillatolumn :

​​​* Temperature of top tower and bottom tower decreases

* Amount of heavy key product in the distillate decreases, that is, better separation.

* Reboiler duty increases to restore the set point temperature, increasing costs.

* Vapour velocity through the tray increases, hence flooding

* The spray height between the trays increases till incipient flooding point.

* After further increase in reflux ratio, there is backmixing of liquids between the upper tray and lower tray which decreases separation.

D = Various Factors affecting the Efficiency of a distillation column :

* feed conditions- The state of the feed mixture and feed composition affects the operating lines and hence the number of stages required for separation.

* Column diameter - Most of the above factors that affect column operation is due to vapour flow conditions: either excessive or too low. Vapour flow velocity is dependent on column diameter.

* Weather Conditions - Most distillation columns are open to the atmosphere. Although many of the columns are insulated, changing weather conditions can still affect column operation.

E = Problems of Flooding and Weeping in a Distillation column :

Flooding : Excessive liquid buildup inside the column leads to column flooding condition. The nature of flooding depends on the column operating pressure and the liquid to vapor flow ratio.

weeping : Weeping occurs at low vapor/gas flow rates. The upward vapor flow through the plate perforations prevents the liquid from leaking through the tray perforation. At low vapor flow rates, liquid start to leak/rain through the perforation (called weeping). The weeping tendency increases with increasing fractional hole area and liquid flow rates.