Types of Reflux In Distillation Column

All types of reflux in distillation column has there own importance based on which normally we use them. Like if we want to use total reflux it has some advantages and some disadvantage, if we want to use zero reflux it is also has some advantages and some disadvantage.

Hi welcome back to another article, my name is B Kadam, and today I am going to explain about different types of reflux in distillation column.

We are going to start with what is reflux in distillation column? Why we need reflux in distillation column? After that we we cover types of reflux in distillation column, advantages and dis-advantages of them, followed by frequently asked questions and answers and pro trips on it and conclusion. I have used some rough drawings to explain you about each types of reflux.

What Reflux in Distillation column?

Reflux in a distillation column is nothing but sending condensed vapor back to the fractionating column to get the desired purity of the product.

Distillation is unit operation, if you don’t have more Idea about it, please read article ” Unit operation and unit process in chemical engineering”

Importance of Reflux in Distillation Column?

The main use of reflux in a column is to achieve the desired purity of the product by considering the optimum use of energy and batch time.

Types of Reflux in Distillation Column

Normally distillation reflux categories as zero reflux, No reflux, Partial reflux, and optimum Reflux only. Based on my industrial in industry other terms also uses. So I have categorized as below.

Total Reflux

Let’s discuss total reflux in a distillation column. Please see the above image (Figure 1), where I have tried to explain the valve positions in the case of total reflux.

If you don’t have idea bout vertical and horizontal heat shell and tube exchanger Please read “Difference between horizontal and vertical shell and tube heat exchanger“

First, start with what is meant by total reflux in a distillation column?. Total reflux is the condition in fractional distillation where we send all condensate to the fractional column. Normally, we do this until we achieve the desired purity of the product. In fractional distillation, we normally do this at the start of fractional distillation.

For valve positions or a typical system for total reflux, see the above figure. In the above figure, vapors are going to the HE-101 from the top of the fractional distillation column, where it gets condensed, and the total liquid is sent back to the fractional column. The collection valve is 100% closed in this situation, and the reflux valve is 100% open.

Advantages of Total Reflux

• It will hep to increase the purity.

Dis-Advantages of Total Reflux

• It will lead to higher operational costs, because it will consume more energy for heating and cooling.
• It will take more time to complete the distillation.

Partial Reflux

Partial reflux in a distillation column is nothing but some part of the condensate is sent to the distillation column as reflux, and some part is collected as the product. The above figure represents the perfect valve position as per partial reflux, where the reflux valve is partially open, and the collection valve is also partially open. In the above figure (Figure 2), vapor goes to the heat exchanger (HE-101), where it gets condensed. Some part of it is taken as a product in the collection tank, and some part is sent back to the column for maintaining the product’s purity.

In partial reflux, the reflux ratio (R = L/D) may be 1/2 or 2/3, depending on the process and the stage of distillation.

Advantages of Partial Reflux

• The main advantage of this arrangement is that by doing this, we can get the product as well as reflux to maintain the purity.

Dis-Advantages of Partial Reflux

• if we send a maximum quantity or portion of condensed liquid back to the column, then we need to pay a higher energy bill.
• More portion of reflux will lead to higher operation time too

zero Reflux

Zero reflux in a distillation column means the collection of all condensed vapor from the distillation column. This is the condition or stage when we achieve the desired purity by collecting all the condensed liquid. We don’t need reflux to get the desired product purity. When the boiling point difference is very high, we go with this approach.

In the above figure, I have tried to explain the setup or system for zero reflux, no reflux, or only collection condition in a fractional distillation column. If you can see the above figure, the valve for collection is 100% open, and the reflux valve is 100% closed (filled as black). This is the typical valve condition during the zero reflux condition in a fractional distillation column.

Advantages of Zero Reflux

• The main advantage of it is that we can save energy by avoiding the cost of reflux.
• By doing 100% collection, we can save total distillation time as well.

Optimum Reflux

The above figure (Figure 5) is an example of an optimum reflux in distillation column setup. Optimum reflux is the condition where we can collect the product as well as maintain reflux in the column to preserve the purity of the distilled product. We call it optimum because this is the condition where we can get the desired quality of the product in an acceptable time and with manageable energy consumption.

Internal Reflux

Internal reflux in a distillation column is nothing but the reflux within the column. In this case, liquid goes down the column, which helps to improve the purity. In internal reflux, we don’t use a condenser to condense the vapor. This is the condition within the fractional distillation column where vapor in the column gets condensed and flows down to the column.

External Reflux

What is Reflux ratio in Distillation Column

Reflux ratio (R) is the ratio of the amount of condensed liquid sent back to the fractionating column to the amount of product collected.

Reflux Ratio, R = L/D

Where:

• R = Reflux ratio (dimensionless)
• L = Amount of liquid returned to the column
• D = Amount of product withdrawn

Comparison of Reflux Types

Type	Description	Separation Efficiency	Energy Requirement	Product Collection	Main Disadvantage
Total Reflux	Only Reflux n product	help to improve purity	very Low	Zero	Higher Energy Cost
Zero Reflux	Only collection of product	Low	very High	Total	will get low product purity compare to other
Optimum Reflux	We get both reflux and product	Optimum	Optimum	Partial	NA

Frequently Asked Questions and answers

What is the difference between internal reflux and external reflux in a distillation column?

In internal reflux, the reflux is generated by the condensed liquid within the column due to heat exchange occurring inside the column. In the case of external reflux, we use a condenser to condense the vapor, and the condensed liquid is then sent back to the column as external reflux.

What is the main disadvantage of Total reflux?

In total reflux, we don’t collect the product, but we need to pay the energy bill for both vaporization and condensation. So, the higher energy cost is the main disadvantage of total reflux in distillation column.

Pro Tips on Distillation Reflux

Being a process engineer in the chemical industry, refinery, or fertilizer industry, you should know the types of reflux in distillation column. This knowledge will help you design an optimum distillation setup. During line sizing for a distillation column, consider both maximum and minimum flow for different conditions, as discussed above, with respect to reflux. Make a chart for the types of reflux in a distillation column with their uses.

Conclusion

Types of reflux in distillation column are zero reflux, total reflux, partial reflux, optimum reflux, internal reflux (within the column without a condenser), and external reflux. Depending on the process and the required quality of the distilled product, we can use these types of reflux.

Read Also

Line sizing calculation with example

Google Search

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Book Reference

• Distillation Design” by Henry Z. Kister
• RC volume

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