DESTILATIONEVEN SEMESTER 2013/2014

Figure 1 Figure 2

What is distillation?

What compounds could be distillated?

How could you distillate those compounds?

Draw the scheme of distillation apparatus !!

Describe types of distillation !!

Define the distillation process

Understanding the principle of Raoult’s Law and Dalton’s Law

Phase diagram

Azeotrop phenomenon

Distillation types and application

Because of solute-solvent intermolecular attraction, higher concentrations of nonvolatile solutes make it harder for solvent to escape to the vapor phase.

Therefore, the vapor pressure of a solution is lower than that of the pure solvent.

• For a liquid, at any temperature, some molecules are evaporating from the surface.

• As the temperature goes up, the number of molecules evaporating in a given time increases.

• If the liquid is in an open container, the molecules escape into the atmosphere.

• In a closed container, some of the vapor molecules strike the walls of the container and return to the liquid.

• Soon a state of equilibrium is reached in which over any time period,

the number of molecules evaporating = number of molecules condensing back to the liquid.

• The pressure of the vapor at this point is known as its vapor pressure.

VAPOUR PRESSURE

THE BOILING POINTTHE BOILING POINT

• The vapor pressure of a pure liquid rises steadily as the temperature is increased .

• The Boiling Point of a liquid is the temperature at which vapor pressure is equal to the external pressure.

• The normal boiling point of a liquid is the temperature at which its vapor pressure is 760 torr (1 atm), normal atmospheric pressure at sea level (applied pressure)

760 torr = 760 mmHg = 1 atm

Liquids with high vapor pressures (Volatile compounds) require relatively little energy (heat) to increase the vapor pressure to match the applied (atmospheric) pressure, and thus, boil, i.e. they have low boiling points.

Liquids with low vapor pressures require considerably more energy to increase the vapor pressure to the point where it matches the applied pressure, thus, they have relatively high boiling points.

The individual compounds in a mixture each use its own pressure: partial pressure.

The sum of the partial pressures equals to the total vapor pressure of the solution.

Raoult’s Law

In a solution : of two miscible liquids (A & B) the partial pressure of component “A” (PA) in the liquid equals the partial pressure of pure “A”

(PAo) times its mole fraction (XA).

Partial Pressure of A in solution = PA = (PAo) x (XA)

Partial Pressure of B in solution = PB = (PBo) x (XB)

Ptotal = PA + PB = PAo XA + PB

o XB

Where XA = nA/(nA + nB) and XB = nB/(nA + nB)

nA and nB are the number of moles of each component in the liquid

When the total pressure (Ptotal) is equal to or greater than the Applied Pressure(760 mm Hg, atmospheric pressure), the solution boils.

If the sum of the two partial pressures of the two compounds in a mixture is less than the applied pressure, the mixture will not boil. The solution must be heated until the combined vapor pressure equals the applied pressure.

Dalton's Law

In the vapor, the mole fraction of a component at a given temperature is equal to the partial pressure of that component

at that temperature divided by the total pressure

XA’ = PA / Ptotal

While XA’ = nA’/(nA’ + nB’) and XB’ = nB’/(nA’ + nB’)

where

nA’ and nB’ are the number of moles of each component in the vapor

BB

AA

B

A

PXPX

XX

'

'

• Combining Raoult’s Law and Dalton’s law, we can obtain the following relationship:

• If AA is more volatile than BB, BPAA < BPBB and P˚P˚AA > P˚P˚BB

thenXXAA

’’/X/XBB’’ > X > XAA/X/XBB

• The ratio of A/B in the vapor is greater than the ratio of A/B in the liquid. The vapor is enriched in the more volatile (lower boiling) component relative to the liquid.

• During the distillation, since the vapor always contains more A than B, the fraction of B in the liquid increases continuously causing the boiling point of the solution to increase

Example

Consider a solution at 100 oC where nA = 0.5 and nB = 0.5

1. What is the Partial Pressure of A in the solution if the Vapor Pressure of Pure A at 100 oC is 1020 mm Hg?

Ans: PA = PoAXA = (1020) * (0.5) = 510 mm Hg

2. What is the Partial Pressure of B in the solution if the Vapor Pressure of Pure B at 100 oC is 500 mm Hg?

Ans: PB = PoBNB = (500) * (0.5) = 250 mm Hg

3. Would the solution boil at atmospheric pressure (760 mm Hg)?

Ans: Yes Ptotal = PA + PB = (510 + 250) = 760 mm Hg4. What is the composition of the Vapor at the Boiling Point?

Ans: The Boiling Point is 100 oC

XA’ (vapor) = PA / Ptotal = 510/760 = 0.67 (mole

fraction)

XB’ (Vapor) = PB / Ptotal = 250/760 = 0.33

PoA > Po

B A is more volatile than B

Solution phase: XA = XB = 0.5 mole fractionVapour phase: X’A = 0.67 mole fraction X’B = 0.33 mole fraction

X’A / X’

B > XA/XB

Senyawa A mempunyai titik didih 700C dan senyawa B mempunyai titik didih 1500C. 10 gram A (BM = 60) dicampur dengan 20 gram B (BM = 40). Campuran ini dipisahkan dengan cara didestilasi pada suhu 1100C. Jika pada 1100C, Po

A = 1500 mmHg dan PoB = 520 mmHg:

◦ Hitunglah fraksi mol A pada fasa cair (liquid) dan pada fasa uap (vapour) !

• When a mixture AB is heated, the total vapor pressure (Ptotal which is composed of the contributions of PA and PB) will rise until it is equal to the external vapor pressure.

• The temperature at which this occurs for various compositions of the liquid is show in the lower curve.

• In this example, the liquid having composition W boils at temperature t.• The vapor in equilibrium with the liquid has composition y. • The vapor condenses to give liquid of composition Z. • After the first drop of liquid distills, the fraction of B in the liquid increases slightly, increasing the boiling point of the solution • The composition of both the liquid and vapor changes continuously.

PHASE DIAGRAM

Distillation Methods

1. Simple

2. Vacuum (at reduced pressure)

3. Fractional

4. Steam

Pure Substance

Temperature remains constant during distillation process so long as both vapor and liquid are present

Miscible Liquid Mixture

Temperature increases throughout process because composition of vapor changes continuously.

Composition of vapor in equilibrium with the heated liquid is different from the composition of the liquid.

Simple Distillation

liquid with a soliddissolved in it

thermometer

condenser

tubedistillingflask

pure liquid

receiving flaskhose connected to

cold water faucet

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 282

-A liquid is heated to vapor in the distilling flask

-The vapor enters the condenser

-The vapor is cooled in a condenser becoming a liquid again

-The cooled liquid is collected in a receiver

- Boiling points of the two liquids must differ by 75°C for effective separations

FRACTIONAL DESTILATION

Fractional distillation can be used to separate liquids with similar boiling points.

On its way up the fractionating column, the vapor condenses and revaporized many times.

At each stage of condensation/ revaporization, the vapor is further enriched in the lower boiling component.

* A fractionating column is placed between the distilling flask and the condenser• The vapors condense in the fractionating column and then the condensed liquids

vaporize again

• These cycles are repeated throughout the length of the fractionating column

• The vapor becomes enriched in the lower boiling compound

Column Efficiency

A common measure of the efficiency of a Fractionation Column is given by its number of Theoretical Plates.

One theoretical plate is equivalent to a Simple Distillation, i.e., one Vaporization / Condensation Cycle.

The smaller the boiling point difference, the greater the number of theoretical plates a fractionating column must have to achieve separation of mixtures

Boiling Point Number ofDifference Theoretical Plates

108 1 54 3 20 10 7 30

4 50 2 100

Temperature-Volume Curves for Simple and Fractional Distillations of Cyclohexane and Toluene Mixture

Fractional Distillation (con’t)As the distillation proceeds, the composition of the liquid and the vapor are continuously changing.

The Horizontal and Vertical Lines represent the processes that occur during a fractional distillation.

Each Horizontal Line (L3V3, L2,V2), etc., represents both the vaporization step of a given vaporization/condensation step and the composition of the vapor in equilibrium with the liquid at a given temperature.

Examples:- At 53oC with a liquid composition of 80% A and 20% B (L4V4 on the diagram), the vapor would have 95% A and 5% B when equilibrium has been established between the liquid and the vapor.

- At 63oC with a 50/50 mixture of A&B (L3V3 on the diagram), the vapor would have a composition of 80% A at equilibrium.

During a distillation, the distillate will be enriched in the more volatile component until the azeotropic ratio is reached. Once this ratio is obtained, the temperature will become constant until one of the components is exhausted.

Wait, how does this apply to a distillation?

What is an Azeotrope?An azeotrope is a mixture of two or more pure chemicals in such a ratio that its composition cannot be changed by simple distillation. This is because when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture of liquids.

• A mixture of liquids of a definite composition that distills at a constant temperature and composition

• Some liquids do not form ideal solutions that conform to Raoult’s law

• Deviations from ideal behavior caused by molecular interactions

Boiling Point-Composition Curve: Ethanol-Water Azeotrope