Extractive Distillation

 INTRODUCTION

Extractive distillation is a means of separating two substances with similar boiling points by adding a third substance to change the boiling point of one of compounds, thus making them easier to separate. Extractive distillation is commonly applied in industry, and has become an important separation method in petrochemical engineering. The method is used for mixtures having a low value of relative volatility, nearing unity. In extractive distillation, an additional solvent, i.e. separating agent is used to alter the relative volatility of the components to be separated. In this way, it is possible to obtain one pure component at the top of one column and the other, together with the solvent at the bottom, which may be separated easily in a secondary distillation column, due to a high boiling point of the solvent.

PRINCIPLE

The principle is based on the separation of a two-component mixture.

By using the difference in the volatility of each component in the liquid mixture, the liquid mixture is partially vaporized and the vapor is partially condensed, so as to achieve the separation of its contained components.

An additional solvent is used as a separating agent to alter the relative volatility of the components to be separated.

In this way, it is possible to obtain one pure component at the top of one column and the other.

It is a unit operation that belongs to mass transfer separation.

WORKING

Extractive distillation is an effective separation technique for azeotrope mixtures or close boiling point mixture. 

Main steps involved:

I.   Selection of solvent for the loaded feed. 

II.  Separation of one of the component from solution from extractive distillation column. 

III. Then separating solvent and the component left from solvent recovery column.

SOLVENT SELECTION PROCESS

It is important to select a suitable separation solvent for this type of distillation.

The solvent must alter the relative volatility by a wide enough margin for a successful result. 

The quantity, cost and availability of the solvent should be considered.

The solvent should be easily separable from the bottom product, and should not react chemically with the components or the mixture, or cause corrosion in the equipment.

The two factors that influence the extractive distillation process are separation process and solvents (or separation agents). Assuming that the separation process is determined, the task is to select the basic solvent with high separation ability. When a basic solvent is found, this solvent should be further optimized to improve the separation ability and to decrease the

solvent ratio and liquid load of the extractive distillation column. The solvent is the core of extractive distillation. It is well-known that selection of the most suitable solvent plays an important role in the economical design of extractive distillation. Excellent solvents should at least decrease the solvent ratio and the liquid load of the extractive distillation column, and make the operation easily implemented.

Properties of a solvent required for extractive distillation:

1. Immiscible pair of solvents: water and low polarity organic solvents. 
2. Good solubility to the target compound. 
3. poor solubility of impurities.
4. Volatility of the extraction solvent. 
5. Toxicity and safety properties of the extraction solvent.
6. Boiling point should be fairly more than the components.  

PROCESS DESCRIPTION

Separation of one of the component from solution from extractive distillation column

Let there be mixture of two component (A) and (B)

The feedstock flows into the extractive distillation column . 

The solvent S enters the top sections of extractive distillation column.

The solvent will elevate the boiling point of the target component thus the other component will get separate out from the ED column (here let the target component be (B) )

The mixture of component (B) and solvent will escape out from bottom and the (A) from the top of the column.

Here we will get the maximum purity of component (A) that can be extracted.

Separating solvent and the component from solvent recovery column

Solvent recovery systems use the process of distillation, which separates volatile and nonvolatile solutions in a solvent.

The mixture of component (B) and solvent is next passed to solvent recovery column where due to low boiling point compared to the solvent, the component (B) gets separated out.

The solvent the is recovered from the bottom of the column. 

And the component (B) is obtained from the top of the column in its purest form 

The solvent recovered will be sent again back to the ED column to process the next batch.

Separation of 1,3-butadiene from C4 mixture

Among C4 mixture (butane, butene-1, trans-butylene-2, cis-butylene-2, 1,3-butadiene and vinylacetylene (VAC)), 1,3-butadiene is a basic organic raw material. In industry, 1,3-butadiene is usually separated from C4 mixtures by extractive distillation with the solvent acetonitrile (ACN).

The process designed is in the following sequence: extractive distillation (column 1) – solvent recovery (column 2) – extractive distillation (column 3) – solvent recovery (column 4)

In this case, butane and butene are regarded as one part and used for fuel.

The feedstock flows into the extractive distillation column 

The solvent S enters the top sections of columns 1 

The mixtures of butane and butene are escaped from the top of column 1 while VAC from the top of column 

The product, 1,3-butadiene, is obtained from the top of column 3.

The solvent S is recovered in columns 2 and 4 and recycled.

APPLICATIONS OF EXTRACTIVE DISTILLATION

Extractive distillation is commonly applied in industry, and has become an important separation method in petrochemical engineering.

Separation of butadiene from a mixture of butane, butene and small quantities of other unsaturated hydrocarbon can be separated by using extractive distillation. Low polarity solvents like furfural, N-methyl pyrrolidine (NMP) etc. are good entrainers for butadiene. 

Separation of Benzene from the mixture of benzene-cyclohexane during the production of cyclohexane from benzene via hydrogenation. Here propylene glycol is used as a Solvent/Entrainer.

Examples of extractive distillation include the separation of toluene (BP 110.8 o C) from iso-octane (BP 99.3 o C) using phenol (BP 181.4 o C) as the solvent.

Extractive distillation is used in  separating hydrocarbons with close boiling point, such as C4, C5, C6 mixtures. The other is the separation of mixtures which exhibit an azeotrope, such as alcohol/water, acetic acid/water, acetone/methanol etc.

REFERENCES

(1)Araromi, D.O. (2011). Hybrid Modeling and Fuzzy Control of a Reactive distillation process. PhD Thesis Ladoke Akintola University of Technology Ogbomoso, Nigeria. 

(2)Rahiman, H.F., Taib M.N. And Salleh, Y.M. (2007). Black Box Modeling of Steam Distillation essential Oil Extraction System using ARMAX Structure. Faculty of Electrical Engineering University Technology MARA, Malaysia.

(3)Giwa, A. and Karacan, S. (2012). Nonlinear Black-Box Modeling of a Reactive Distillation Process. International Journal of Engineering Research & Technology (IJERT).

(4)Bipro, R. D. and Kawnish. (2009). Excess Methanol Recovery in Biodiesel Production Process Using Distillation Column : A Simulation Study.

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