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Distillation

DEPARTMENT OF CHEMICAL ENGINEERING

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Weeping in distillation column

Distillation…. Introduction

Unit operation – Separation process

A feed mixture of two or more components is separated into

two or more products whose compositions differ from that of

the feed.

Driving force – Relative volatility

Principle – Phase creation (ESA)

Feed – liquid or vap-liq mixture

Product – Bottom product (liquid) + Overhead product (liq or

vap-liq )5

Distillation… Examples

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Separation of binary mixture

Separation of Multicomponent mixture

Vapor – Liquid Equilibrium

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Equilibrium between vapor and liquid is determinedexperimentally.

Temperature – Composition diagram

Pressure – Composition diagram

T – Composition diagram….

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Bubble point and Dew point

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Bubble point is the point at which first drop of liquid

mixture begin to vaporize.

Dew point is the point at which first drop of gaseous/vapor

mixture begins to condense

T – Composition diagram….

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Partial vaporization and condensation….

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Partial Pressure and Dalton’s Law

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The partial pressure PA of component A in a mixture of

vapours is the pressure that would be exerted by

component A at the same temperature, if present in the

same volumetric concentration as in the mixture.

By Dalton’s law of partial pressures, the total pressure is

equal to the summation of partial pressures.

Raoult’s Law

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In an ideal gas/vapor, the partial pressure of a constituentis proportional to its mole fraction.

For an ideal mixture, the partial pressure of a constituentis related to the liquid phase concentration by Raoult’s law.

Relations from RAOULT’S LAW

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If a mixture follows Raoult’s law, then

And

So

But

Therefore

Hence

Vapor Pressure Relations (1)

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Antoine Equation:

Riedel Equation:

Clapeyron Equation:

Where

Volatility …

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Volatility is defined as the ratio of partial pressure of a

component to the mole fraction of that component in liquid

phase.

For ideal system, volatility of a component is numerically

equal to the vapor pressure of the pure component.

Relative Volatility …

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The ratio of these two volatilities is known as the relative

volatility.

Replacing partial pressures in terms of total pressure gives a

relation for ratio of two components in vapor to the ratio of

two components in liquid.

Relative Volatility …

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For a binary mixture

This gives relations for determination of composition of liquid

or vapor phases, if the composition of the other phase is

known.

Temperature Dependency of Relative Volatility …

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α varies somewhat with temperature, it remains remarkably

steady for many systems.

α increases as the temperature falls, so that it is valuable to

operate at reduced pressure in order to decrease the boiling

point.

An average value of α can be used over whole column if the

relative volatilities at the top and bottom of column vary by less

than 15%.

Otherwise the equilibrium curve must be constructed

incrementally by calculating the relative volatility at several

points along the column.

K-Value … (1)

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Phase-equilibrium ratio is the ratio of mole fractions of a species

in two phases at equilibrium.

For vapor–liquid systems, the constant is referred to as the K-value

or vapor–liquid equilibrium ratio.

For many systems K is constant over an appreciable temperature

range, So it is often much more useful than the simple vapor

pressure.

Using K-values is particularly useful while dealing with

multi‐component systems.

K-Value … (2)

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Relative volatility in terms of K-value:

Methods of Distillation – Binary Mixtures

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Differential Distillation

Flash or Equilibrium distillation

Rectification

Differential Distillation … (1)

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A single‐stage process that starts with a still pot, initially full, and

heated at a constant rate.

The vapor formed by boiling of liquid is removed at once from the

system.

Vapor is richer in more volatile component. Liquid becomes

steadily weaker in more volatile component. Product quality

varies with time.

At any instant, liquid is in equilibrium with the vapor formed on

that instant.

At the end, the remaining liquid is removed as bottom product.

A complete separation is possible only at infinite relative volatility.


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S= number of moles of material in still

x= mole fraction of component A in liquid

dS= amount of material vaporized from still

y= mole fraction of component A in vapor

Material balance of component A


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Integrating

Integral can be solved graphically using equilibrium

relationship between y and x.

Over the range concerned the equilibrium relationship is a

straight line of the form y = mx + c


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or


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If α remains constant over the range of applicability, then

then

Flash or Equilibrium Distillation … (1)

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Frequently carried out as a continuous process.

Feed is usually pumped through a fired heater and enters a

still through a valve that causes a reduction in pressure.

A part of liquid feed is vaporized in such a way that vapor

evolved is in equilibrium with the residual liquid.

The still is a separator vessel that provides sufficient times

for the vapor and liquid to reach equilibrium.

Vapor is removed from top and usually condensed. Liquid is

taken from the bottom.


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F= molar flow rate of feed

V= molar flow rate of vapor

S= molar flow rate of liquid

x= mole fraction of A in liquid product

y= mole fraction of A in vapor product

xf= mole fraction of A in feed

Total material balance

Component A balance


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Rearranging

Equation of straight line that passes through point (xf ,yf) and has a slope

Example 11.6

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An equimolar mixture of benzene and toluene is subjected to

flash distillation at 100 kN/m2 in the separator. Using the

equilibrium data given in Figure 11.9, determine the

composition of the liquid and vapour leaving the separator

when the feed is 25 per cent vaporised. For this condition,

the boiling point diagram in Figure 11.10 may be used to

determine the temperature of the exit liquid stream.

Rectification

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The term rectification is derived from the Latin words

rectefacere, meaning ‘‘to improve’’.

Distillation process that enables successive vaporization and

condensation to be accomplished in one unit.

The essential merit of rectification is that it enables a vapour

to be obtained that is substantially richer in the more volatile

component than is the liquid left in the still.

Such a unit is known as a fractionating column.

Fractionating Column: Schematic

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Fractionating Column: Major Parts

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Rectifying section

Stripping section

Reboiler

Condenser

Reflux drum / Accumulator

Fractionating Column: Streams

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Feed stream

Overhead product

Bottom product

Reflux stream

Boil-up stream

Utilities

Hot

Cold

Fractionating Column: Operation

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Fractionating column is a tall cylindrical vessel that is divided into

several sections by means of perforated trays.

The trays allow the vapor to pass through their perforations in the

upward direction.

The liquid flows across each tray, then over a weir, and then

through a down comer to the next tray in the downward direction.

The vapor from top of the column is condensed and then passed to

a reflux drum (or accumulator or reflux divider).

A fraction of the condensed vapor is drawn as top product

(distillate). The remainder is returned to the top tray as reflux.


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The liquid from the bottom of the column is heated by means

of hot oil or steam in a reboiler.

The bottom product is drawn as liquid that flows over the

weir in the reboiler.

Since the bottom product is hot, it may be used top reheat

the incoming feed.

The vapor generated in the reboiler is returned to the

bottom tray of the column.


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At each tray, vapor is partially condensed to give a vapor

that is richer in more volatile component. Liquid is partially

vaporized and becomes weaker in more volatile component.

This is because of vaporization of some part of more volatile

component from the liquid, and condensation of some part

of less volatile component from the vapor.

The partial condensation of rising vapor and partial

vaporization of reflux liquid occurs at each stage.

Top vapor, distillate product and reflux liquid have same

composition.


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Feed is introduced on some intermediate tray where the

liquid has approximately same composition as the feed.

The part of the column above the feed tray is known as

rectifying section. The lower portion below the feed tray is

known as stripping section.

Vapor and liquid leaving from an ideal tray are in

equilibrium with each other.

Vapor and liquid entering a tray are not in equilibrium.

Assumptions of equimolar counter diffusion and constant

molar overflow are often valid in the system.

Equimolar Counter diffusion

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When the mass transfer rates of the two components are equal

and opposite, the process is said to be one of equimolar counter

diffusion.

It occurs in the distillation column when the molar latent heats of

the two components are the same.

The more volatile component is transferred from liquid to vapor,

and the less volatile components is transferred from vapor to

liquid.

If the molar latent heats of the two components are equal, the

condensation of a given amount of less volatile component

releases exactly the amount of latent heat required to volatilize

the same molar quantity of the more volatile component.

Constant Molar Overflow (CMO)

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Heat losses from a distillation column are usually small and

may be neglected.

For an ideal system, heat of mixing is zero.

With these assumptions, molar heat of vaporization may be

taken as constant and independent of composition.

For such systems, if one mole of vapor condenses, exactly

one mole of vapor is liberated.

With CMO, molar flowrates of liquid and vapor remain

constant in the rectifying section (and in stripping section)

unless material enters (or is withdrawn from) the column.

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