CHAPTER 6 Distillation

Part IIntroduction to Distillation

and Flash Distillation

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1. Introduction to Distillation

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Distillation (1)

Distillation is defined as a process in which a liquid or vapour mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat.

Distillation is based on the fact that the vapour of a boiling mixture will be richer in volatile components (i.e. that have lower boiling points).

Therefore, when this vapour is cooled and condensed, the condensate will contain more volatile components while less volatile components will be enriched in the original liquid after distillation.

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Distillation (2) Importance

Distillation is the most common separation technique in chemical industries

Distillation consumes enormous amounts of energy due to cooling and heating requirements

Cost-wise, distillation can contribute to >50% of plant operating costs

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Distillation (3) Industry Examples

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Modified from (Dutta, 2007)

Distillation (4) Basic Equipment

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a reboiler (A) to provide vaporisation for the distillation process

a column (C) which consists of a number of stages and where the separation is carried out

column internals such as trays/plates and/or packings for enhancing component separations

a condenser (D) to cool and condense the vapour

an accumulator (or reflux drum, E) to hold the condensed vapour for recycling the reflux back to the column.

E

Distillation (5) Basic Equipment

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Each stage consists of tray/plate for enhancing component separation.

Distillation (6) How Does It Work?

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The liquid mixture that is to be processed is known as the feed;

Feed is introduced somewhere near the middle of the column to a tray known as the feed tray/plate.

Heat is supplied to the reboiler to generate vapour. The source of heat input can be a suitable fluid (typically, steam).

The vapour raised in the reboiler is introduced into the column at the bottom of the column for separation.

Distillation (7) How Does It Work?

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The liquid removed from the reboiler is known as the bottoms product or conventionally called bottoms.

The vapour moves up the column, exits the top of the unit and then is cooled by a condenser.

The condensed liquid is stored in a accumulator or reflux drum.

Some of this liquid is recycled back to the top of the column and this is called the reflux.

The liquid removed from the system is called distillate or overhead product.

Distillation (8) How Does It Work?

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Feed is fed into the column from a stage (feeding stage):

Stripping (or enriching) section: all stages (trays/plates) above the feed tray/plate

Rectifying section: all stages (trays/plates) below the feed tray/plate (including the feeding tray/plate)

Distillation (9) How Does It Work?

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Stripping section

progressively stripes of the volatile components in the liquid, leaving the liquid be enriched in high-boiling components;

At the bottom, producing a bottom product which is of high purity.

Distillation (10) How Does It Work?

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Rectifying section

The vapour is enriched in volatile components;

Without reflux, no rectification will occur and the concentration of the overhead product would be no greater than that of the vapour rising from the feed plate;

Reflux with a split from overhead product significantly improves the purity of the overhead product, at additional energy costs.

Distillation (11) How Does It Work?

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A single stage is a device where two phases of a different composition come in contact with each other, exchange and leave with new compositions;

Actions at each stay (tray/plate)

The steams coming to the stage are NOT in equilibrium;

The streams leaving the stage are in vapour-liquid equilibrium;

Lin, xin

Lout, xout

Vout, yout

Vin, yin

Therefore, to design a distillation column, the VLE data of the system must be known.

2. Flash Distillation of Binary Mixture

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Flash Distillation of Binary Mixture

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Flash distillation is widely used in petroleum refining. It is essentially a one-stage operation.

A liquid is heated to a high pressure;

The high pressure liquid is then flashed at a lower pressure, causing some vapour to be evolved;

It is essentially a single-stage operation.

Flash Distillation of Binary Mixture

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xF = mole fraction of A in the feed liquid; yD = mole fraction of A in the vapour phase; xB = mole fraction of A in the liquid phase; f = the mole fraction of the feed that is vaporised.

Material balance:

For a binary system, from Eq (5.4), we have

Enthalpy balance

Where HF, Hy and Hx are the enthalpies of feed liquid, the vapour and the liquid product.

Flash Distillation of Binary Mixture

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1 (6.1)

1 1

1 (6.2)

A mixture of 50%mol benzene and 50%mol toluene is subjected to flash distillation at a separator pressure of 1 atm. The incoming liquid is heated to a temperature that will cause 40% the feed to flash. The relative volatility of benzene over toluene = 2.45. The boiling point diagram at 1atm is given as Figure 6.1.

(a) What are the compositions of the vapour and liquid leaving the flash chamber?

(b) What is required feed temperature? For benzene: = 7.36 cal/g mol; cp = 33 cal/mol.C;For toluene: = 7.96 cal/g mol; cp = 40 cal/mol.C;

Flash Distillation Example

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Flash Distillation Example

19Figure 6.1 Boiling point diagram (benzene-toluene system) at 1 atm