Simple and fractional distillation

HETP, Column design, Packed column, Bubble cap, sieve plate...

1. Simple
Distillation
&
Fractional Distillation
(Dr.) Mirza Salman Baig
Assistant Professor (Pharmaceutics)
AIKTC, School of Pharmacy,New Panvel
Affiliated to University of Mumbai (INDIA)

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

3. Two steps
1.
Converting liquid into vapor phase
(
evaporation
)
2.
Recovering liquid from vapor by
condensation
at another place.

4. 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.

5. 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"
P
A
= P
0
A
X
A

6. Contd...
Partial Vapour pressure of liquid=
Vapour pressure of pure liquid
×
Mole
fraction of the liquid
p
A
= p
0
A
X
A
p
B
= p
0
B
X
B
Example
Mixture of Ethylene Chloride
and benzene.

7. Daltons Law
The 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= p
A
+ p
B
P=
p
0
A
X
A
+ p
0
B
X
B

8. Application of Law
•
Component having higher vapour
pressure (lighter component) will be
distilled first....
This principle is used in simple
distillation

9. Ideal Mixture
(obey Raoults Law)

10. Non Ideal Liquid mixture
(real liquid)

11. Deviation from Raoults Law
•
Negative
–
(Adhesive Interaction)
–
Acetone /chloroform
•
Positive
–
(Repulsion in two components)
–
Benzene /Ethyl Alcohol

12. Volatility
v
A
= Partial vap pressure of a (pA)/
Mole fraction of A (X
A
)in solution

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

14. VARIOUS TYPES OF
DISTILLATION
Ø
Simple Distillation
Ø
Fractional Distillation
Ø
Vacuum Distillation
Molecular Distillation
Ø
Azeotropic and Extractive distillation
Ø
Steam distillation

15. Simple Distillation

16. Simple Distillation
Ø
Single Vaporization/Condensation cycle
of a
mixture that produces a distillate which is
always impure
Ø
Therefore, it is impossible to completely
separate the components in a mixture with
Simple Distillation
Ø
Relatively pure substances can be obtained
from a mixture with Simple Distillation if the
boiling points of the components
differ by a
large amount
(>25
o
C)

17. Simple Distillation

18. 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.

19. 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

20. Evaporation vs Distillation
•
Distillation is used when condencate
is required.
•
Evaporation is used when
concentrated liquid resedue is
needed as product.

21. 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.

22. Fractional Distillation

23. Fractional Distillation
Ø
Fractionating Column inserted between the
Distillation Flask and the Distillation Head.
Ø
The Fractionating Column, containing a variety
of packing materials,
Ø
With each cycle within the column, the
composition of the vapor is progressively
enriched
in the lower boiling liquid.
Ø
This process continues until most of the lower
boiling compound is removed from the original
mixture and condensed in the receiving flask
Ø
Vid

24. Fractional Distillation
Fractionating column
is
important part of this assembly
which is absent in simple
distillation

25. Fractional Distillation (Cont.)
Ø
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.
Ø
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).

26. Fractionating column types
•
Plate column
–
Sieve plate
–
Bubble cap
–
Valve Plate
•
Packed column

27. Column Efficiency
Ø
A common measure of the efficiency of
a Fractionation Column is given by its
number of Theoretical Plates
Ø
One Theoretical Plate is equivalent to
one Simple Distillation cycle, i.e., one
Vaporization / Condensation Cycle.
Ø
The smaller the boiling point
difference, the greater the number
of theoretical plates
a fractionating
column must have to achieve separation
of mixtures

28. 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

29. 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

30. Main Components of Distillation
Columns
•
A

vertical

shell

where

the

separation

of

liquid

components

is

carried

out

•
C
olumn

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

31. Contd...
The

vertical

shell

houses

the

column

internals

and

together

with

the

condenser

and

reboiler
,
constitute

a

distillation

column
.

32. Bas
ic
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
.

33. 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
.

34. 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
.

35. COLUMN INTERNALS
•
Bubble cap
•
Sieve plate
•
Column packing

36. 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
.

37. 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

and

valve

trays

have

replaced

bubble

cap

trays

in

many

applications
.

38. Liquid and
Vapour
Flows in a Tray Column
Each 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.

39. 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
.

40. •
As

the

hotter

vapour

passes

through

the

liquid

on

the

tray

above
,
it

transfers

heat

to

the

liquid
.
•
In

doing

so
,
some

of

the

vapour

condenses

adding

to

the

liquid

on

the

tray
.
•
The

condensate
,
however
,
is

richer

in

the

less

volatile

components

than

is

in

the

vapour
.
•
Additionally
,
because

of

the

heat

input

from

the

vapour
,
the

liquid

on

the

tray

boils
,
generating

more

vapour
.
This

vapour
,
which

moves

up

to

the

next

tray

in

the

column
,
is

richer

in

the

more

volatile

components
.
•
This

continuous

contacting

between

vapour

and

liquid

occurs

on

each

tray

in

the

column

and

brings

about

the

separation

between

low

boiling

point

components

and

those

with

higher

boiling

41. Plate (
Tray
)
Designs
•
A

tray

essentially

acts

as

a

mini
-
column
,
each

accomplishing

a

fraction

of

the

separation

task
.
•
From

this

we

can

deduce

that

the

more

trays

there

are
,
the

better

the

degree

of

separation

and

that

overall

separation

efficiency

will

depend

significantly

on

the

design

of

the

tray
.
•
Trays

are

designed

to

maximise

vapour
-
liquid

contact

by

considering

the

liquid

distribution

and

vapour

distribution

on

the

tray
.
•
This

is

because

better

vapour
/
liquid

contact

means

better

separation

at

each

tray
,
translating

to

better

column

performance
.

42. Packed column (
Packings
)
Packings

are

passive

devices

that

are

designed

to

increase

the

interfacial

area

for

vapour
/
liquid

contact
.
The

following

pictures

show
3
different

types

of

packings

material

pieces

\
..
These

strangely

shaped

pieces

are

supposed

to

impart

good

vapour
/
liquid

contact

when

a

particular

type

is

placed

together

in

numbers
,
without

causing

excessive

pressure
/
drop

across

a

packed

section
.
This

is

important

because

a

high

pressure

drop

would

mean

that

more

energy

is

required

to

drive

the

vapour

up

the

distillation

column
.

43. Packings
versus Trays
•
P
ackings

provide

extra

inter
/
facial

area

for

liquid
/
vapour

contact

•
efficiency

of

separation

is

increased

for

the

same

column

height

•
packed

columns

are

shorter

than

trayed

columns
•
Packed

columns

are

called

continuous
/
contact

columns

while

trayed

columns

are

called

staged
/
contact

columns

because

of

the

manner

in

which

vapour

and

liquid

are

contacted
.