Distill as i

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Distillation



Introduction

Vapor pressure: the pressure produced by fluidin the saturated state.

Vapor pressure depends on the temperature of the system, not on the amount of substance or volume. Unit in mm Hg (Torr)

Boiling point: the temperature at which the vapor pressure of the liquid equal to the external

pressure, the liquid tends to change from liquidphase to vapor phase.

Normal boiling point: boiling point of liquid whichmeasured at 1 atm (air pressure).



Vapor pressure Diagram

Raoult’ law : vapor pressure of a compound is proportionalto the number of moles of compounds contained in the

mixture

X A: mol fraction of A; Pº A=vapor pressure of A in pure condition

XB: mol fraction of B; PºB=vapor pressure of B in pure condition

Number of partial pressure of A and partial pressure of B egual to vapor pressure of mixture

A and B

Vapor pressure above the solution is comparable with mol fraction substance in vapor

phase, composition of A and B in vapor phase are:

B B BP X P .

A A AP X P .

B B A A B AP X P X P P P

..



Vapor pressure in above solution equal to

mol fraction in vapor phase, while thecomposition of substance A and B in vapor

phase can be determine as:

The concentration of each component in

vapor and liquid phase, eg for componentB are:

B A

A

A P P

P X

B A

B

B P P

P

X

A

B

A

A

B

B

B A

B

B

U

A

X P

P X

P

P

P P

P

X

X

1



If Pº A=PºB, so XuB/ XB=1 or mol fraction of

vapor pressure = mol fraction of liquid

phase because XA+XB=1.

If Pº A<PºB, concentration of B in vapor

phase bigger than in liquid phase.

If Pº A

>PºB

, concentration of B in vapor

phase lower than in liquid phase.



Simple Distillation

Principle : mixture of solution was heated at aconstant pressure (atm) would evaporate, themore volatile substances (ex: substances A) willbe in the vapor phase more than in thecomposition of the original solution.

If the vapor phase is cooled then solutioncontaining higher substance of A. Substance B(Td >>) leaves a residue

Distillation is repeated until substance A and Bare more pure



Distillation process to separate ethanol from water

http://www.bbc.co.uk

http://www.bbc.co.uk/

http://www.bbc.co.uk/



Distillation Process

When a mixture AB of a specific

composition is heated, the total

vapor pressure (composed of thecontributions of P A and PB) will rise

until it is equal to the external vapor

pressure. The mixture will begin to

boil.

The vapor which first forms is

enriched in the more volatilecomponent. This behavior is shown

at right,

• Assume a two component mixture with a composition of 30%A:70%B (point W).

The boiling point of this mixture is found by drawing a vertical line from W to where

it intersects the lower curve (point X). A horizontal line drawn from X to where it

intersects the vertical axis (the temperature) gives the bp of composition W. From

the point ( Y) where this horizontal line intersects the upper curve (vapor) drop a

vertical line to intersect the lower axis (the composition). Point Z gives the

composition of the vapor which is in equilibrium with a liquid of composition W at its

boiling point.

Liquid-Vapor Composition Diagram



Condensor

Liebig and west condensor: it has a inner

tube with very thin walls

rapid heattransfer to the fast-flowing cooling water

leading to greater efficiency



Double coil (a) andDavies (b): efficientdouble surface

condensor Friedrich ©: highlyefficient for refluxand distillation

Ice-cooled : usefulfor volatile liquids

Condensor

(a) (b)(c)



Fractional Distillation

Principle: simple distillation which the

distillate (result of distillation) conducted

repeated distillation.

To separate a mixture of two or more

components special condenser

The length and type of the column

depend on boiling point of components to

be separated



The condition necessary for a good

separation are:

Comparatively large amounts of liquid

continually returning through the column

Thorough mixing of liquid and vapour

A large active surface of contact between

liquid and vapor



Fractional DistillationAB at composition of 5% A boils at temperature L1 and the vapors with composition V1 enter the column at

that temperature. The vapor will condense to a liquid with composition V1. The condensate L2 has a

lower boiling point (because it has more of the lower boiling liquid A) and will thus vaporize at a lower

temperature (warmed up by coming in contact with the additional vapors from below) to give vapors of

composition V2. These vapors will condense somewhat farther up the column to give a condensate L3.If the column is long enough or contains sufficient surface area that many successive vaporization-

condensation steps (theoretical plates) can occur, the distillate that comes over the top is nearly pure A.

Distillation yielding pure A continues until all of A is removed, after which the temperature at the

thermometer rises to the boiling point of B.



Distillation Efficiency

•The efficiency of a fractional distillation is determined by the amount of “pure” liquid

components obtained. Keep in mind that if a liquid is “pure” it will have a constant boiling point.

The temperature of vapors in equilibrium with liquid at the boiling point will be constant. A plot of

temperature vs. time for a pure liquid will look like A below.

•The efficiency of a fractional distillation can be demonstrated graphically by plotting the change

in temperature of the distillate over time (or over volume of distillate, as in this experiment). In a

fractional distillation with low efficiency, separation will be poor. There will be little or no “pure”

component as distillate. The composition of the distillate will be constantly changing and the bp

of the vapor in equilibrium with liquid will be constantly changing. It will give a plot such as B.

• An efficient distillation will give pure components which will have constant boiling points. Such

a process is shown below in plot C. The relatively “flat: horizontal regions at the beginning and

end of the plot indicate “pure” components A and B are obtained.

The closer to this ideal sigmoid shape the better the fractional distillation.



Condensor

Vigreux column: to separate mixture of compound withboiling point differences between 15-20ºC

Column consists of a glass tube with a series of

indentattions such that alternate sets of indentations

point downwards at an angle 45º in order to theredistribution of liquid from the walls to the centre of the

column.



Condensor

Packed and

oldershaw column: to

separate mixture of

compound withboiling point

differences closer Packed column

Oldershaw column



Distillation Setups



Fractional Distillation Set-up



Proper Thermometer Depth



Ideal Solution

Distillation process would be working if thesolution is ideal solution

The characteristics of the ideal solution:

1. when mixed, reactions do not occur

2. when mixed, with no change in volume

3. follow Raoult's law:

or

A A AP X P .

B B BP X P .



Azeotrope Mixture

Definition: a mixture of two

solutions at a particular composition

has properties like pure solution.

Compound A Compound B Tb azeotrope %A in azeotrope

Water 100ºC Ethanol 78,3ºC 78,15ºC 4,4

Water 100ºC Isopropanol 82,4ºC 80,40ºC 12,4

Ethanol 78,3ºC Chloroform 61,2ºC 59,40ºC 7,0

• Azeotropic mixture with boiling point minimum:



Azeotropic mixture with boiling point maximum:

Compound A Compound B Tb azeotrope %A in azeotrope

Water 100ºC Formic acid 100,8ºC 107,1ºC 77,5

Water 100ºC HCl 84,0ºC 120ºC 37

Water 100ºC HNO3 86ºC 120,5ºC 68



Separation of azeotrope mixture

Depend on properties of each compound

Add third compound which that compound canchange the vapor pressure of the mixture. Eg:water-ethanol+benzene

Add reagent which can react with one of component in the mixture. Eg: water-ethanol+CaO

Absorb one of component in the mixture with

silica gel or active carbonFractional crystalization

GC or LC



Low Pressure Distillation (vacuum)

Principle: by lowering the pressure above theliquid surface with the aid of a vacuum pump,the fluid distilled will volatile, because the liquidwill boil below its normal boiling point

Many organic compound cannot be distilledunder atm pressure because they undergopartial or complete decomposition before thenormal boliling point is reached

By reducing the external pressure to 0.1-30 mm,the boling point is considerably reduced and thedistillation may ussualy be conducted withoutdanger of decomposition



To separating the mixture which

component of mixture is easily damaged

on their boiling point

Valuable when the substances undergo

decomposition when distilled alone at atm

pressure

To separating highly concentrated mixture



vacuum should be set,

if not set, the

composition of the

vapor similar to liquidcomposition, separation

does not occur

Low Pressure Distillation (vacuum)



Steam Distillation

Is a method to isolating or purifying the organiccompounds

Is used liquid that does not dissolve or mutually miscible

partial pressure of saturated vapour will follow the law

dalton:

If the mixture which is not miscible heated, the boilingpoint are temperature which amount of the vapour pressure equal to atmospheric pressure

Steam distillation used to separated essential oil such asclove oil, rosse oil, etc.

...321 P P P P



if water is used to produce steam, the

boiling point of the mixture will be lower

than 100ºC

the distillate are mixture of all of the

substances which present in the mixture,

while concentration of substance

depending on the vapour pressure of eachsubstance in the mixture



Eg: total pressure of mixture of A and B are:

So the composition of vapour can be formulated:

nA= mol A and nB = mol B

W = mass

M = molecular weight

B AP P P

B

A

P

P

nB

nA

A

A

M

W nA

B

B

M

W nB

B B

A A

B

A

B

A

P M

P M

nB M

nA M

W

W

.

.

.

.

I t i th ti f th d i i



Importance in the separation of the desire organic

compound:

From non-volatile tarry substances which

are formed as by products in manyreactions

From aqueous mixtures containing

dissolved inorganic saltsFrom compound which are not appreciablyvolatile in steam (eg: o-nitrophenol from p-nitrophenol

From certain by products which are steamvolatile

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