# simple and fractional distillation

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• Simple Distillation & Fractional Distillation

(Dr.) Mirza Salman BaigAssistant Professor (Pharmaceutics)

AIKTC, School of Pharmacy,New PanvelAffiliated to University of Mumbai (INDIA)

• "Distillation is defined as the separation of the components of liquid mixture, by a processes involving vaporization and subsequent condensation"

• Two steps1.Converting liquid into vapor phase

(evaporation)2.Recovering liquid from vapor by

condensation at another place.

• Ideal Solution When two liquids are mixed togther it

is known as binary mixture. Ideal Solution is binary mixture of

liquid components and there is no change in properties of other substance.

Heat is nither evolved nor absorbed during mixture formation.

• Raoults Law Raoults law states that, "the partial

vapour pressure of each volatile constituent is equal to vapour pressure of pure constituents multiplied by its mole fraction"

PA = P0A XA

• Contd...Partial Vapour pressure of liquid= Vapour pressure of pure liquid Mole

fraction of the liquidpA = p0A XApB = p0B XBExample Mixture of Ethylene Chloride

and benzene.

• Daltons LawThe total pressure exerted by mixture

of ideal gas may be conciderd as sum of partial vapour prerssure of each gas (if alone were present and occupied same volume)

P= pA + pB

P= p0A XA + p0B XB

• Application of Law Component having higher vapour

pressure (lighter component) will be distilled first....

This principle is used in simple distillation

• Ideal Mixture (obey Raoults Law)

• Non Ideal Liquid mixture(real liquid)

• Deviation from Raoults Law Negative

Positive (Repulsion in two components)Benzene /Ethyl Alcohol

• VolatilityvA= Partial vap pressure of a (pA)/

Mole fraction of A (XA)in solution

• Equilibrium curve

Plot of mole fraction of a more volatile componennt in vapours against its mole fraction in liquid.

• VARIOUS TYPES OF DISTILLATION

Simple Distillation Fractional Distillation Vacuum Distillation

Molecular Distillation Azeotropic and Extractive distillation Steam distillation

• Simple Distillation

• Simple Distillation Single Vaporization/Condensation cycle of a

mixture that produces a distillate which is always impure Therefore, it is impossible to completely

separate the components in a mixture with Simple Distillation Relatively pure substances can be obtained

from a mixture with Simple Distillation if the boiling points of the components differ by a large amount (>25oC)

• Simple Distillation

• Simple Distillation Distillation still is initially filled with a feed

mixture, which evaporates and leaves the still in the vapor form.

This vapor, which is richer in the more volatile component, is collected in the condenser at the top and accumulated in a receiver.

In this operation, no liquid is refluxed back to the still, and no plates or packing materials are present (fractionating column absent) inside the still.

This simple distillation still is an example of a batch operation, often referred to as Rayleigh distillation.

• Simple Distillation It is mostly use to separate volatile liquid

from non/ volatile liquids. Its difficult get pure substance only by

simple distillation. If a small increment of the initial distillate

is separated and redistilled and this process is repeated many times, effectively producing multiple sequential Vaporization/ Condensation Cycles, an increasingly pure solution can be attained.

Solution (feed) is Distilland Vid

• Evaporation vs Distillation Distillation is used when condencate

is required. Evaporation is used when

concentrated liquid resedue is needed as product.

• Applications Sepration of volatile oil Purification of organic solvent Refining of petroleum products Seperation of drug obtained from

plant and animal. Purification of drug from animal

source.

• Fractional Distillation

• Fractional Distillation

The Fractionating Column, containing a variety of packing materials,

With each cycle within the column, the composition of the vapor is progressively enriched in the lower boiling liquid.

This process continues until most of the lower boiling compound is removed from the original mixture and condensed in the receiving flask

Vid

• Fractional Distillation

Fractionating column is important part of this assembly which is absent in simple distillation

• Fractional Distillation (Cont.)

When the lower boiling liquid (more volatile liquid) effectively removed from the original mixture, the temperature of mixture rises and a second fraction containing some of both compounds is produced. As the temperature approaches the near

the higher boiling point compound, the distillate condensing into next part of fractionating column will contain heavy component (higher boiling point compound).

• Fractionating column types Plate column

Sieve plate Bubble cap Valve Plate

Packed column

• 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 one Simple Distillation cycle, 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

• Plate Efficiency

Overall Plate Effic. = Theoretical plate required/ Actual no. of plates

Murphee Plate Effic. = Actual Change in vapour composition/ Change in

composition if perfect composition is achived

• HETP Hieght of packed section required to

give the change in composition that would provided by theoretical plate.

HETP= Hieght of column/ No. of theoretical plate

• Main Components of Distillation Columns

A vertical shell where the separation of liquid components is carried out

Column internals trays/plates and/or packings which are used to enhance component separations

A reboiler to provide the necessary vaporisation for the distillation process

A condenser to cool and condense the vapour leaving the top of the column

A reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column

• Contd...

The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column.

• Basic Terminology

The liquid mixture that is to be processed is known as the feed and this is introduced somewhere near the middle of the column to a tray known as the feed tray.

The feed tray divides the column into a top (enriching or rectification) section and a bottom (stripping) section.

The feed flows down the column where it is collected at the bottom in the reboiler.

• Basic Operation

Heat is supplied to the reboiler to generate vapour.

The source of heat input can be steam.

The vapour raised in the reboiler is re-introduced into the unit at the bottom of the column.

The liquid removed from the reboiler is known as the bottoms product.

• Basic Operation (contd...)The vapour moves up the column, and as it exits the top of the unit, it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The condensed liquid that is removed from the system is known as the distillate or top product.

Thus, there are internal flows of vapour and liquid within the column as well as external flows of feeds and product streams, into and out of the column.

• COLUMN INTERNALS Bubble cap Sieve plate Column packing

• Bubble cap tray

• Bubble cap trays A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapour. Vapour rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray.

• Liquid and Vapour Flows in a Tray ColumnEach tray has 2 conduits, one on each side, called

downcomers. Liquid falls through the downcomers by gravity from one tray to the one below it. The flow across each plate is shown in the diagram on the right.

• A weir on the tray ensures that there is always some liquid (holdup) on the tray and is designed such that the the holdup is at a suitable height, e.g. such that the bubble caps are covered by liquid.Being lighter, vapour flows up the column and is forced to pass through the liquid, via the openings on each tray. The area allowed for the passage of vapour on each tray is called the active tray area.

• Sieve Plate (tray) Sieve trays are simply metal

plates with holes in them. Vapour passes straight

upward through the liquid on the plate.

The arrangement, number and size of the holes are design parameters.

Because of their efficiency, wide operating range, ease of maintenance and cost factors, sieve a