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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