principles of distillation

7/27/2019 Principles of Distillation

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Theoretical Principles

of Fractional Distillation


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If the temperature of a gas is increased, the moleculesin the gas move faster

The more rapid motion at higher temperatures results inmore violent collisionsof the molecules

As the molecules move faster and the collisionsbecome more violent


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a. In a closed container

Molecules in a closed container strike the walls with

more force. The pressure, therefore, increases

At a lower temperature, the vapor molecules moveslower

b. In an open container

The pressureof a gas in an open container cannot beincreased, since some of the gas would escape to

equalize the pressure with the surroundings

The pressure of a gas in an open container cannot beincreased by increasing the temperature


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The violent motion of the molecules at the surfaceof a liquid causes many of these molecules toescapefrom the liquid

If the temperature of a liquid is increased, themotion of the molecules becomes more violent


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Vapor (gas) Liquid

If an empty container is partially filled with liquid,molecules escape from the surface of the liquid,forming a vaporin the space above the surface

Gas molecules which strike the surface of a liquidtend to stickto the liquid

Thus, molecules may be escaping the liquid and returningto the liquid from the gas at the same time


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When a vessel contains both a liquid and a vapor,molecules are continually going from the liquid tothe vapor or gasandfrom the vapor to the liquid

When the number of molecules leaving the liquid

equals the number entering the liquid, the gas andthe liquid are said to be in equilibriumwith each other

So at equilibrium, equal numbers of molecules pass

between the liquid and the vapor, or gas


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Evaporation

When more molecules are escaping the liquid thanreturning, the liquid is said to be evaporating. Theprocess in which a greater number of molecules leave theliquid than enters it is called evaporation

Condensation

When more molecules are entering the liquid from thevapor than are escaping the liquid, the vapor is

condensing. The process in which a greater number ofmolecules enter the liquid than leave it is calledcondensation


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Temperature Effect

When the temperature of a liquid is increased, thepressure of the vapor in equilibrium with itincreases and maintains equilibrium

On the other hand

When the temperatureof a liquid isdecreased, thepressure of the vapor decreases and maintains

equilibrium


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When a liquid and a vapor are at equilibrium, thepressure of the vapor is called

The vapor pressure of the liquidThe pressure of a gas or vapor above the surface of aliquid in a closed container is not necessarily the same

as the vapor pressure of the liquid

In order for the pressure of the gas to be equal to thevapor pressure of the liquid

The gas and the liquid must be in equilibrium witheach other


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When

A liquid is placed in an open container and heated

Both

its vapor pressure and

its rate of evaporation increase

Until

The vapor pressure equals the pressure of theatmosphere

evaporation becomes very rapid, andthe liquid boils


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Ifthe pressure over a boiling liquid is increased, theboiling temperature is increased and

Ifthe pressure over a boiling liquid is decreased, the

boiling temperature is decreased

Since the pressure of the atmosphere on the top of amountain is lower than at sea level, liquids boil atlower temperatures on top of a mountain


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The vapor pressures fordifferent liquids at the same

temperature are not thesame

Therefore, water and

pentane have differentvapor pressures at 130 F

Pentane has a vaporpressure of is 30 psia

Water has a vapor pressureof 3 psia


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PARTIAL PRESSURE

The pressure that a gas exerts on the walls of acontainer depends on

The number of collisionsbetween the molecules and the container wall

If more molecules are present, there are more collisionsand a greater pressure


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If two containers

of the same volume are filled with gas to the samepressure and temperature,each contains the same number of gas molecules

For the same volume and temperature

the greater the number of molecules in a gas,the greater the pressure of the gas must be


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Whentwo or more gases are mixed, each gas exerts a

pressure on the walls of the container

Although these separate pressures cannot be measured, thetotal pressure can be measured

The total pressure of a gas mixture is equal to the sum ofthe pressures of the different gases in the mixture

The separate pressures of the gases are called

Partial Pressures


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For a mixture of two gases

If the partial pressure of each gas is the same, theremust beequal numbers of each kind of molecule

If a tank contains 5 times as many ethane moleculesas propane molecules

The partial pressure of ethane is

5 times as great as propane


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If a closed container is partially filled with a mixture oftwo liquids, molecules of both liquids escape to the gasfrom the liquid and return to the liquid from the gas

At equilibrium, the number of molecules escaping fromthe liquid is equal to the number returning to the liquid foreach component.

A system ofbutane-hexane

Butane would be thelight molecules and

Hexane the heavymolecules


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At equilibriumthe number of light molecules escaping

from the liquid is equal to the number returning tothe liquid

Also, at equilibrium the number of heavy moleculesescaping from the liquid is equal to the number of

heavy molecules returning to the liquid

However

At equilibrium the number of light molecules escaping andreturning to the liquid is not equal to the number of heavymolecules escaping and returning to the liquid


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Light molecules are able to escape the liquidmorerapidly thanthe heavy ones

Therefore, at equilibrium, there are more lightmolecules escaping and returning to the liquid

Because the light molecules are able to escape the liquidfaster than the heavy ones, the light molecules tend toconcentrate or to be enriched in the vapor

For exampleIf the liquid contains half light and half heavy moleculesAt equilibrium, the gas contains more light moleculesthan heavy molecules


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When the gas and the liquid are at equilibrium

The gas is richer in the light component andThe liquid is richer in the heavy component

Therefore, if a liquid mixture is partially vaporized

The light component in the vapor becomes moreconcentrated than the light component in the liquid

In other words, the vapors are enriched in the lightercomponent


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FUNDAMENTALS

Fractional distillation is an operation which is basedon the differences in volatility or different escapingtendencies of the products which have to be separated

It is obvious that for solving the numerous problems intechnical fractionation a thorough knowledge of thefundamental laws and rules governing the volatilitycharacteristics of systems consisting of two or more

components is a prerequisite


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Applying Henry's law to pure compound

When xi becomes 1, Ci equals Pi

Pi = Pi xi (2)

In this form and restricted to ideal solutions Henry's law is

known as Raoult's law.

Mixtures ofparaffin and naphthene are, in general, verynearly ideal

While mixtures of aromatics with naphthenes orparaffins are further away from ideal solutions


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Dalton's Law

The second important law underlying the theory offractional distillation is Dalton's law regarding partialpressures and their additively. Mathematically, Dalton's law

is expressed as follows:

Pi xi = P Yi and Pi =P

Where: Pi= partial pressure of component i,

P = total pressure of system,

Yi = mole fraction of component i in thevapor phase.


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Pix

i= P Y

i

P

P

x

Yi

i

i

From equation above (known as Raoult-Dalton's law) itfollows that the concentration of any component in avapour which is in equilibrium with a liquid mixture can becalculated by multiplying the concentration of that

component in the liquid phase by the ratio of the vaporpressure of the pure component to the total pressure ofthe system


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K Values

The fact that components in a non-ideal mixture doinfluence the volatility of the other componentsresults in deviations from Raoult Daltons law whichare of the same order of magnitude, a solution of the

problem is found by introducing the concept ofk values

Using k values, Raoult Daltonsequation becomes:

= ki or Yi = ki xi

It is possible to calculate boiling points, dew points,vapour pressures and dew pressures of mixtures

i

i

x

Y


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Boiling Point Calculation

Is the temperature at which, when the temperature of the liquid

mixture is raised, the first infinitesimal amount of vapourescapes as a bubble at a given pressure

This temperature is also known as the "bubble point

The vapour will contain a certain amount of each component,

depending on the volatility of that component, and it is clear that

if the concentration in the vapour phase (Yi) is expressed in mole

fraction,

Yi must equal 1


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principles of distillation

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