elements of fractional distillation fourth edition

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Title
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book
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the date
last
marked
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Elements of
Fractional
Distillation
BY
CLARK SHOVE
ROBINSON
AND

EDWIN RICHARD
GLLLILAND
Revised
and Rewritten
t>y
EDWIN
RICHARD
GILT.II.ANO
Professor
of
Chemical
Engineering
Afassachusetts Institute
of
Technology
FOURTH
EDITION
SKCOND
McGRAW-HILL BOOK
COMPANY,
INC.
NKW
YORK
TORONTO
LONDON
1950
ELEMENTS
OF
FRACTIONAL
DISTILLATION

Copyright,
1922, 1930,
1939,
1950,
by
the McGraw-Hill
Book
Company,
Inc
Printed
in
the
United States of
America.
All
rights
reserved. This
book,
or
parti
thereof,
may
not
be
reproduced
in
any
form without
permission
of

the
publishers
THE
MAPLE PRESS
COMPANY, YORK,
PA.
PREFACE TO
THE
FOURTH
EDITION
The
firs^^ditipn.of-this
book
and the
early
revisions were the
result
of the
efforts
of
Professor
Robinson,
and he
took
an active
part
in
guiding
the
revision

of the
previous
edition. His death
t
made
it
necessary
to
prepare
this edition
without his
helpful
guidance
and
counsel.
The
present
revision
differs
extensively
from
the
previous
edition.
The
material has been modified
to
bring
it
more

closely
into line
with
the
graduate
instruction
in
distillation at
Massachusetts
Institute of
Technology.
Much
greater
emphasis
has been
placed
on the
measure-
ment,
prediction,
and use
of
vapor-liquid
equilibria
because it
is
believed
that
this
is

one of
the
most serious
limitations in
design
calcu-
lations.
Greater
emphasis
has
also
been
placed upon
the
use
of
enthalpy
balances,
and
the
treatment of batch
distillation
has been
considerably expanded.
Unfortunately,
the
design
calculations for
this
type

of
operation
are still
in
an
unsatisfactory
status.
Azeotropic
and extractive distillation are considered as an
extension of
conven-
tional
multicomponent problems.
The
sections
on column
design
and
column
performance
have
been
completely
rewritten
and
increased
in
scope.
In all
cases

quantitative
examples
have been
given
because
it
has been
found
that
this
greatly
aids the student
in
understanding
descriptive
material.
During
the
last
15
years
a
large
number of
design
methods
have
been
proposed
for

multicomponent
mixtures,
some
of
which
are
reviewed in
Chapter
12.
Most
of
these do
not
appear
to
offer
any
great
advantage
over
the
conventional
Sorel
method,
and it is believed that the
law of
diminishing
returns
has been
applying

in this
field for
some
time.
It is
hoped
that the
present
edition
will
stimulate
some of these
investi-
gators
to
transfer their
efforts
to
more
critical
problems,
such
as
vapor-liquid equilibria,
batch
distillation,
transient
conditions within
the distillation
system,

and
column
performance.
EDWIN
RICHARD
GILLILAND
CAMBRIDGE,
MASS.
July,
1960
PREFACE TO THE FIRST EDITION
The
subject
of
fractional
distillation
has
received
but scant atten-
tion from, writers
in
the
English
language
since
Sidney
Young published
his
book

"
Fractional
Distillation
"
in
1903
(London).
French and
German
authors
have,
on the
other
hand,
produced
a
number of
books
on
the
subject, among
the
more
important
of which are
the
following:
"La
Rectification et les
colonnes rectificatriccs

en
distillerie,"
E.
Barbet, Paris,
1890;
2d
ed.,
1895.
"Der
Wirkungsweise
der
Rcctificir
und Destillir
Apparate,"
E.
Hausbrand,
Berlin,
1893;
3d
ed.,
1910.
"Theorie
der
Verdampfimg
und
Verfliissung
von
gemischcn
und
der

fraktionierten
Destination,"
J. P.
Kuenen,
Leipzig,
1906.
"Theorie
der
Gewinnung
und
Trennung
der atherischen Olc
durch
Destination,"
C. von
Rechenberg, Leipzig,
1910.
"La Distillation
fractione*e et la
rectification,"
Charles
Manlier,
Paris,
1917.
Young's
"Fractional
Distillation,"
although
a
model for its

kind,
has
to do
almost
entirely
with the
aspects
of the
subject
as
viewed
from
the chemical
laboratory,
and there
has
been
literally
no work
in
English
available
for
the
engineer
and
plant
operator
dealing
with the

applications
of
the
laboratory processes
to the
plant.
The use
of
the
modern
types
of
distilling
equipment
is
growing
at
a
very rapid
rate.
Manufacturers
of
chemicals
are
learning
that
they
must
refine
their

products
in
order
to
market
them
successfully,
and
it is often
true
that fractional
distillation
offers
the
most
available
if
not
the
only
way
of
accomplishing
this.
There
has
consequently
arisen
a
wide

demand
among
engineers
and
operators
for a
book which
will
explain
the
principles
involved
in such
a
way
that
these
principles
can be
applied
to the
particular
problem
at
hand.
It has
therefore
been
the
purpose

of the
writer of
this
book to
attempt
to
explain
simply
yet
accurately,
according
to
the
best ideas of
physical
chemistry
and
chemical
engineering,
the
principles
of frac-
tional
distillation,
illustrating
these
principles
with a few
carefully
selected illustrations.

This
book
is
to
be
regarded
neither
as
a
com-
plete
treatise
nor
as an
encyclopedia
on
the
subject
but,
as
the
title
indicates,
as
an
introduction
to
its
study.
Viii

PREFACE
TO
THE FIRST
EDITION
In
general,
it has
been
divided
into
five
parts.
The first
part
deals
with
fractional
distillation
from
the
qualitative
standpoint
of the
phase
rule.
The
second
part
discusses
some

of the
quantitative
aspects
from
the
standpoint
of
the
chemical
engineer.
Part three
discusses
the
factors
involved
in the
design
of
distilling equipment.
Part
four
gives
a few
examples
of
modern
apparatus,
while
the
last

portion
includes
a
number
of
useful
reference
tables which have been
compiled
from
sources
mostly
out
of
print
and unavailable
except
in
large
libraries.
The
writer
has
drawn
at will
on the
several
books
mentioned
above,

some
of
the
tables
being
taken
nearly bodily
from
them,
and has
also
derived
much
help
from
Findlay's
"
Phase
Rule"
(London,
1920)
and
from
"The
General
Principles
of
Chemistry" by Noyes
and Sherrill
(Boston,

1917).
He wishes
especially
to
express
his
gratitude
for
the
inspiration
and
helpful suggestions
from
Dr. W. K. Lewis of
the
Massachusetts
Institute
of
Technology
and
from his
other friends and
associates
at
the
Institute
and
of
the
E. B.

Badger
& Sons
Company.
Finally,
he
wishes
to
express
his
appreciation
of
the assistance
of
Miss
Mildred
B.
McDonald,
without
which
this
book
would never
have
been
written.
CLARK
SHOVE
ROBINSON
CAMBRIDGE,
MASS.

June,
1920,
CONTENTS
PREFACE
TO THE
FOURTH
EDITION .
v
PREFACE
TO
THE FIRST
EDITION
vii
INTRODUCTION
1
1. Determination
of
Vapor-Liquid Equilibria.
.
. .
.3
2.
Presentation of
Vapor-Liquid
Equilibrium
Data
16
3. Calculation
of
Vapor-Liquid

Equilibria
.
.
.26
4.
Calculation
of
Vapor-Liquid Equilibria (Continued)
. .
.79
5.
General Methods
of Fractionation
.
. .
101
6.
Simple
Distillation
and
Condensation. 107
7.
Rectification
of
Binary
Mixtures.
.
118
8.
Special Binary

Mixtures
.
192
/9/
Rectification
of
Multicomponent
Mixtures .
, 214
10.
Extractive and
Azeotropic
Distillation

.
285
i^l.
Rectification of
Complex
Mixtures
.

325
12. Alternate
Design
Methods
for
Multicomp
t
onent Mixtures

,
336
13. Simultaneous
Rectification
and
Chemical Reaction.
.
.
361
14.
Batch
Distillation
370
15. Vacuum Distillation
393
16.
Fractionating
Column
Design
403
17.
Fractionating
Column
Performance
445
18.
Fractionating
Column
Auxiliaries
471

APPENDIX
479
INDEX
481
ix
INTRODUCTION
Definition
of Fractional
Distillation.
By
the
expression
fractional
distillation
was
originally
meant the
process
of
separating
so
far
as
it
may
be feasible
a mixture
of
two

or more volatile
substances into
its
components,
by
causing
the
mixture to
vaporize by
suitable
application
of
heat,
condensing
the
vapors
in such a
way
that fractions of
varying
boiling points
are
obtained,
re
vaporizing
these fractions
and
separating
their
vapors

into
similar
fractions,
combining
fractions
of similar boil-
ing
points,
and
repeating
until
the
desired
degree
of
separation
is
finally
obtained.
Purpose
of
Book. Such
a
process
is
still
occasionally
met
with
in

the
chemical
laboratory,
but
it is
a
laborious
and
time-consuming
operation
which
has its
chief
value as a
problem
for the
student,
for
the
purpose
of
familiarizing
him with some
of
the
characteristic
properties
of
volatile
substances.

It
is
possible
to
carry
on
a
fractional distilla-
tion
by
means
of
certain
mechanical
devices
which
eliminate almost
all
of
this
labor
and
time
and
which
permit
separations
not
only
equal

to
those
obtained
by
this more
tedious
process
but
far
surpassing
it in
quality
and
purity
of
product.
The
purpose
of this
book
is
to
indicate
how
such
devices
may
be
profitably
used

in the
solution
of
distillation
problems.
Origin
of
Fractional
Distillation.
Like all
the
older
industries,
fractional
distillation
is an
art
that
originated
in
past
ages
and
that
developed,
as
did
all
the
arts,

by
the
gradual
accumulation
of
empirical
knowledge.
It
is
probable
that
its
growth
took
place
along
with that
of
the
distilled
alcoholic
beverages,
and
to
the
average
person today
the
word
"still"

is
synonymous
with
apparatus
for
making rum,
brandy,
and
other
distilled
liquors.
To
France,
which has
been the
great producer
of
brandy,
belongs
the
credit
for
the
initial
development
of
the
modern
fractionating
still.

Physical
Chemistry
and
Fractional
Distillation.
Fractional dis-
tillation
has
labored
under
the
same
sort
of
burden that
the other
industrial
arts
have
borne.
Empirical
knowledge
will
carry
an
industry
to
a
certain
point,

and
then
further
advances are
few
and
far
between.
It
has
been the
function
of
the
sciences
to
come to the rescue
1
2
FRACTIONAL DISTILLATION
of
the
arts at such
times
and
thus
permit
advancement to
greater
use-

fulness.
The science
that
has raised
fractional
distillation
from
an
empirical
to
a
theoretical
basis is
physical
chemistry.
By
its
aid
the
study
of
fractionation
problems
becomes
relatively
simple,
and
it is
on
this

account that
the
subject
matter in
this book is based
upon physical
chemistry.
CHAPTER 1
DETERMINATION
OF
VAPOR-LIQUID
EQUILIBRIA
The
separation
of
a
mixture
of
volatile
liquids
by
means
of
fractional
distillation
is
possible
when
the
composition

of
the
vapor
coming
from
the
liquid
mixture
is
different
from
that
of
the
liquid.
The
separation
is
the
easier the
greater
the
difference
between
the
composition
of
the
vapor
and

that of
the
liquid,
but
separation
may
be
practicable
even
when
the
difference
is
small.
The
relation
between
the
vapor
and
liquid
compositions
must be
known in
order
to
compute
fractional
dis-
tillation

relationships.
Usually
this
is
obtained
from
information
concerning
the
composition
of the
vapor
which
is in
equilibrium
with
the
liquid.
On
this
account a
knowledge
of
vapor-liquid
equilibrium
compositions
is
usually
essential for
the

quantitative
design
of
frac-
tional
distillation
apparatus.
In
most
cases the
study
is
made
on
the
basis
of
the
composition
of the
vapor
in
equilibrium
with the
liquid.
However,
this is not a
fundamental
requirement
and

any
method
that
would
allow the
production
of
a
vapor
of a
different
composition
than
that
of the
condensed
phase,
whether
equilibrium
or
not,
could
be used
for
separation.
However,
most of
the
equipment
employed

depends
on
the use of
a
vaporization
type
of
operation,
and
the
equilibrium
vapor
is
a
good
criterion
of the
possibilities
of
obtaining
a
separation.
The
methods for
obtaining
vapor-liquid equilibrium
compositions
can be
considered
in

two main classifications:
(1)
the
experimental
determination of
equilibrium compositions
and
(2)
the
theoretical
relationships.
EXPERIMENTAL
DETERMINATIONS
OF
VAPOR-LIQUID
EQUILIBRIA
The
measurement
of
vapor-liquid
equilibrium
compositions
is
not
simple.
A
highly developed
laboratory
technique
is

therefore
needed
to
obtain
reliable data
in
any
of the
several
methods
described
here.
Circulation
Method. A common
method
for
obtaining
vapor-liquid
equilibrium
(Refs.
11, 13,
16,
23, 27,
35)
is
by circulating
the
vapor
through
a

system
and
bringing
it
into
repeated
contact
with
the
liquid
until
no further
change
in the
composition
of
either
takes
place.
A schematic
diagram
of such
a
system
is
shown
in
Fig.
1-1.
The

vapor
above the
liquid
in
vessel
A
is
removed, passed through
3
FRACTIONAL
DISTILLATION
chamber
5,
and
recirculated
by
pump
C
through
the
liquid
in A.
While the
system appears
simple,
in
actual
practice
it involves a
num-

ber of
complications:
1. The
system
must
be
completely
tigkt;
otherwise
the
total
quan-
tity
of material
will
continually vary
and the
equilibrium compositions
of
the
vapor
and
liquid
will also
change.
2. The
quantities
of
liquid
and

of
vapor
when
equilibrium
is obtained
must
remain
constant
and not
vary
during
the recirculation.
To
keep
Heat-*
exchanger
Equilibrium
I
cell
Vapor
sample
FIG.
1-1.
Circulation
apparatus
for
vapor-liquid
equilibrium
measurements.
them

constant
it is
necessary
for
the
system
to
remain isothermal and
for
the
total volume
to
remain
constant.
The chief
variation
in
the
volume
of the
system
is due to the fact
that
it
is
usually
found
expedient
to
use

a
reciprocating
pump.
The
error due
to
this
variation
is
usually
minimized
by making
the
displacement
volume
of
the
pump
small.
The
pumps
are
generally
of
a
mercury-piston
type; t.e.,
a
mercury
column

is forced
up
and
down in a
steel or
glass cylinder
serving
as the
piston
of
the
pump.
This
makes it
possible
to have
an
essen-
tially leakproof
pump
and allows
the
pumping operation
to
be carried
out with
very
little contamination
of
the

circulating
vapors.
3.
This
type
of
system
has
been used
most
successfully
under
con-
DETERMINATION OF
VAPOR-LIQUID
EQUILIBRIA
5
ditions
where
the
vapor
does
not
condense
at room
temperature.
If
it
were
necessary

to
operate
the
pumping system
at a
high temperature
to
avoid
condensation
of
the
vapor,
difficulties
might
be
encountered
due
to
the
vapor pressure
of the
mercury,
in which
case other lower
vapor
pressure
metallic
liquids
should be
suitable.

4. Another condition that
could
cause
the
relative volumes
of
vapor
and
of
liquid
to
vary
is the
rate of
flow.
The rate
of
recirculation
varies the
pressure
drop through
the
apparatus
and
thereby changes
the
quantity,
of
vapor present.
In

most cases the rate of
recirculation
is
such
that the
pressure
differential for
recirculation
is not
great.
Both
the
volume
variation due to
pumping
and the
pressure
changes
due to recirculation can be made
less detrimental
by
making
the vol-
ume of
the
liquid
in
vessel
A
large.

5.
It is
necessary
to
ensure that there
is no entrainment
of
liquid
with the
vapor
leaving
A.
If
liquid
is carried
over to
vessel
B,
the
vapor
sample
will
be
contaminated.
This
entrainment
is
eliminated
by
the

use
of
low
velocity
and
by
efficient
entrainment
separators
in
the
upper
part
of A.
6.
Another
precaution
is the
necessity
to
prevent
any
condensation
of
the
vapor
during
recirculation.
If
any

vapor
condenses,
the
con-
densate
will
be
of
different
composition
and
the
results
will
be
in
error.
This
type
of
apparatus
has
been used
for a
variety
of
systems.
It
is
particularly

suitable
for
very
low
temperature
studies
such as
those involved
in the
equilibria
associated
with
liquid
air.
In
this
case
vessel
A
is
maintained
in
a
low-temperature
cryostat,
and
the
recycle
vapor
stream

is
heat-exchanged
with
the
exit
vapor;
the
rest
of
the
system
is
maintained
at
essentially
room
temperature.
One of
the
difficulties
with
the
operation
is
the
fact that
the
vapor sample
is
obtained

as
a
vapor
and,
unless
the
pressure
is
high,
the
quantity
of
vapor
obtained
in vessel
B
may
be so
small
as to
offer
difficulties
in
analysis.
The
system
has
the
great
'ad

vantage
that
a
vapor
can
be
repeatedly
bubbled
through
the
liquid
until
equilibrium
is
obtained.
Theoretically
exact
equilibrium
is
not
obtained
because
of
the
fact that
there are
pressure
differentials
in the
system.

Thus the
vapor
entering
at
the
bottom
of
A must
be
under
a
pressure
higher
than
the
vapor
leaving
A,
at
least
by
an
amount
equal
to
the
hydrostatic
head
of the
liquid

in
A.
Since
the
vapor-liquid
equilibria
depend
on
pressure,
it
is
obvious
that
there
cannot
be
exact
equilibrium.
However,
the
change
in the
composition
of
the
equilibrium
vapor
due
to this
small

change
of
pres-
sure
is small
in most
cases.
It
could
be
serious
in
the
critical
region
FRACTIONAL
DISTILLATION
where
the
vapor
is
very compressible.
Basically
this
system
is one
of
the
best
for

obtaining
true
equilibrium.
Bomb
Method.
In
the bomb
method
(Refs. 3, 4, 12, 14,
36)
the
liquid
sample
is
placed
in
a closed evacuated
vessel.
It is then
agitated
by
rocking,
or
by
other
means,
at constant
temperature
until
equilib-

rium is obtained
between
the
vapor
and
the
liquid.
Samples
of
the
vapor
and
the
liquid
are
then withdrawn
and
analyzed.
The
method
appears
simple,
but
it involves
certain
difficulties.
During
sampling
there
are

pressure
changes
due
to the removal
of
material,
and
these
pressure
changes
can
be
large
in
magnitude.
In
order
to
avoid
them,
it is
customary
to
add
some
fluid,
such
as
mercury,
to

the
system
while
the
samples
are
being
removed
in
order
to
prevent
Rocking
mechanism
-
Sampling
line
fjf
-
Constant
temperature
FIG.
1-2.
Bomb
apparatus.
any
vaporization
or
condensation.
Another

difficulty
with
the
method
is
the
fact
that
in
most
cases
it
is
necessary
to
use
sampling
lines
of small
cross sections.
These
may
fill
up
with
liquid
during
the
initial
part

of
the
operation,
and
this
liquid
may
never
come to the true
equilibrium.
It
is
necessary
to
purge
the
sampling
lines
to remove
such
liquid.
This
liquid
holdup
is
particularly
serious
in
the case
of

the
vapor
sample
since
in
quantity
it
may
be
large
in
comparison
to
the
sample.
A
schematic
diagram
of
the
bomb-type
apparatus
is
shown
in
Fig.
1-2.
Dynamic
Flow
Method.

Another
method
that has been
widely
used
(Refs.
10, 21, 25,
37)
for the
determination
of
vapor-liquid
equi-
libria is
one in which
a
vapor
is
passed
through
a
series of
vessels
con-
taining
liquids
of
a
suitable
composition.

The
vapor
entering
the
first vessel
may
be
of a
composition
somewhat
different
from
the
equilibrium
vapor,
but as
it
passes
through
the
system
it
tends
to
approach
equilibrium,
If
all
the
vessels have

approximately
the same
liquid
composition,
the
vapor
will
more
nearly
approach
equilibrium
as
it
passes
through
the
unit,
The
number
of vessels
employed
should
be
suoh that
the
vapor
entering
the
last unit
is of

essentially
equilib-
rium
composition.
DETERMINATION OF
VAPOR-LIQUID
EQUILIBRIA
7
This
system
has
the
advantage
that it is
simple
and,
in
certain
cases,
it
is
possible
to
dispense
with the
analysis
of
the
liquid
sample,

i.e.,
the
liquids
can
be made
of
a
known
composition,
and since the
change
in
the
last
vessel
is
small,
it is
possible
to
assume
that
the
composition
of
the
liquid
in
this
case

is
equal
to
that
originally
charged.
A
schematic
diagram
of
such
a
system
is
shown
in
Fig.
1-3.
It
is
obvious
that it
cannot be
an
exact
equilibrium system
because
of the fact
that
a

pressure drop
is
involved
in
passing
the
vapor
through
the
system; i.e.,
there are
pressure
variations
which will
affect
equi-
librium.
There
is also the
danger
of
entrainment,
although
this
can
be
minimized
by
low
velocities.

In a
great
many
cases,
the
gas
introduced
into
the first vessel
has
.been
carrier
gas
of low
solubility
and not a
component
of
the
system.
FIG. 1-3.
Dynamic
flow method.
Thus,
in
the
determination of
the
vapor-liquid
equilibria

for
systems
such
as
ammonia
and
water,
ammoniacal
solutions
are
placed
in
the
vessels,
and
a
gas
such
as
nitrogen
is bubbled
into
the
first of
these
and
the
resulting
nitrogen-ammonia-water
vapor

mixture
is
passed through
the
succeeding
vessels
obtaining
a
closer
approach
to
equilibrium.
Equilibrium
obtained
in such
a
manner
is
not
the
true
vapor-liquid
equilibria
for the
water
vapor-ammonia
system.
It
closely
approaches

true
equilibrium
for the
binary
system
under
a
total
pressure
equal
to
the
partial
pressure
of
the ammonia
and the
water
vapor
in the
gaseous
mixture.
Even
this
is
not
exact.
The
carrier
gas

has some
solubility
in
the
liquid phase,
and
the
partial
pressure
of
these
added constituents
modifies
the
energy
relations
of
the
liquid
and
vapor
phases.
Usually
for
low-pressure
operation
these
errors
are
not

large
in
magnitude,
but
as
the
pressure
becomes
higher
the
errors
are serious
and
the
method
can
give
erroneous
results
if
the true
vapor-liquid
equilibria
for mix-
tures without
the
carrier
gas
are
desired.

Dew and
Boiling-point
Method.
In
essence
this
technique
consists
in
introducing
a
mixture
of
known
composition
into
an
evacuated
equilibrium
container
of variable
volume
(Refs.
6, 7,
9,
15, 17,
18,
20,
28).
The

system
is
maintained
at
a
constant
temperature,
and
by
varying
the volume
the
pressure
is observed
at
which
condensation
8
FRACTIONAL
DISTILLATION
commences
and
is
completed.
The dew- and
bubble-point
curves
of
pressure
vs.

temperature
for a number
of
these
prepared
samples
are
obtained
and,
by cross-plotting,
conditions
of
phase
equilibrium
may
be
found
by
locating points
at
which
saturated
liquid
and
saturated
vapor
of
different
compositions
exist at the

same
temperature
and
pressure.
The
pressures
are
determined in two
ways.
One
involves the
measuring
and
plotting
of
the
PV
isotherm,
the
dew
point
and bubble
point
being
indicated
by
the discontinuities
in the curve at the
begin-
ning

and
the
end of condensation.
The other
employs
a
glass
or
quartz
equilibrium cell,
and
the
conditions
are
determined
visually.
The
advantages
of
this
method
are that it
allows the
critical
condi-
tions
to
be
determined,
gives

data on
specific
volumes
of
mixtures
at
high
pressures,
and
requires
no
analysis
of the
phases
since
the
total
composition
of the
mixture is
accurately
determined
gravimetrically
upon
charging.
There
are
disadvantages,
however. For
certain

conditions the
dew
and
bubble
points
are not
sharply
defined
;
hence
they
require
measure-
ments to
be
made
with
highly
refined
precision
instruments. The
simpler
units
using
mercury
as a variable volume
confining
fluid
cannot
be

used
below the
freezing point
of
mercury.
In
addition,
the
mate-
rials used
must
be
very pure
and free in
particular
from
traces
of
fixed
gases,
for
in
the
critical
region
the saturation
pressure
is
quite
sensitive

to small
amounts
of
fixed
gases. Further,
a
large
amount
of
experi-
mental
work must
be done in
order to
define
completely
and
accurately
the
phase equilibria
over
all
ranges
of
liquid
and
vapor
composition.
The
major

limitation,
however,
is the
fact
that the
method can
be
used
only
on
binary systems.
As the
phase
rule dictates that
more
complex
systems
are not a
unique
function
of
pressure
and
temperature,
dew
and bubble
points
alone
cannot define the
composition

of two
phases
in
equilibrium.
Dynamic
Distillation
Method. The
four
previous
methods
involved
repeated
contact
of
the
vapor
with the
liquid
and
thus afforded the
time
necessary
for the attainment
of
equilibrium.
The
dynamic
dis-
tillation method
(Refs.

2,
5, 11, 19,
24,
26, 34, 39)
involves
a
different
procedure
(see Fig.
1-4).
In
this
system
a
distilling
vessel
is
connected
to
a
condenser
and
a
receiver.
In
the
simplest
case,
a
small

sample
of
distillate is
taken,
and the
compositions
of this
sample
and the
liquid
in
the
still are
determined.
During
such
a distillation
the
composition
of
the
distillate
and
the
liquid
in
the
still
changes,
and the

samples
represent
average
values.
DETERMINATION
OF
VAPOR-LIQUID
EQUILIBRIA
9
To
reduce
this
composition
variation
the
quantity
of
liquid
in
the
still
is
made
large
in
comparison
to the
quantity
of
distillate.

Frequently
successive
samples
of
condensate are
obtained,
and
these
are
analyzed
and
the
composition plotted
vs. the
quantity
of
liquid
that
has
been
distilled.
An
extrapolation
of
this
curve
back
to
zero
quantity

of
liquid
removed
is taken
as
the
composition
of
the
vapor
in
equilibrium
with the
original
liquid.
Top
heater
Charge
FIG.
1-4.
Dynamic
distillation
apparatus.
The method
involves
a
new
assumption,
namely,
that

the
vapor
obtained
by
boiling
a
liquid
is
in
equilibrium
with
the
liquid.
There
has been
no
adequate
proof
of
this
assumption,
and
theoretical
con-
siderations
would
tend
to
indicate
that

equilibrium
should not be
obtained.
The
few
experimental
data
that
are
available
would indi-
cate
that
the
difference
in
the
composition
between
the
vapor
obtained
in
this manner
and
the
true
equilibrium
is
not

great
in
most
cases,
but
in
a
few
systems
significant
differences
have
been
found.
After
the
vapor
leaves
the
liquid,
any
condensation
in
the
upper part
of the
distilling
vessel
will
change

the
composition
of
the
vapor
and
10
FRACTIONAL
DISTILLATION
therefore
introduce
errors. Such condensation
is
usually
reduced or
eliminated
by
having
the
upper part
of the
system jacketed
and
at
a
higher
temperature
than
the condensation
temperature

of the
vapor.
However,
this
higher
temperature
can
introduce
errors;
for
example,
in such
a
boiling
system
there is a certain amount of
spray
and
splash-
ing.
The
spray
that
lands
on
the
heated
walls
will
tend to

vaporize
totally
and
give
a
vapor
of the
composition
of
the
liquid
rather than
of
the
equilibrium
composition.
The
pressure
involved
in
such a
system
is of course
essentially
that
prevailing
in
the
receiver,
and this

method
can
be
used
either for
nor-
mal
pressures,
high
pressures,
or vacuum.
The
exact
temperature
of
the
operation
is
usually
not known
because
the
liquid
is
generally
superheated.
The
vapor
and the
liquid

therefore
are not
in
thermal
equilibrium,
and it
is doubtful
whether
they
are in
true
composition
equilibrium.
The
apparatus
has
been
extensively
used because of its
simplicity,
and the
results
are
of sufficient
accuracy
to
be
of real value
in distillation
calculations.

In order
to obtain
a closer
approach
to
equilibrium,
various com-
plicating arrangements
have
been
used;
for
example,
Rosanoff
modified
the
system
to obtain
a second contact of
the
vapor
with
the
liquid.
Continuous
Distillation Methods. Continuous
distillation methods
involve
distilling
a

liquid,
condensing
the
vapor
sample,
and
recycling
the
condensate
back to
the
still. A schematic
drawing
of such
an
equilibrium
still
is
given
in
Fig.
1-5. This
system
was
developed by
Yamaguchi
(Ref
.
38)
and

Sameshima
(Ref
.
29)
and
has
been modified
and
improved by
a
number
of
other
investigators (Refs.
1,
8, 22,
30, 31,
32,
33).
This
method
has been
widely
used and
has the
great
advan-
tage
that it is
simple,

and the unit can be
placed
in
operation
and
allowed to
come
to a
steady
state
without
any
great
amount of atten-
tion.
The
same
precautions
relative to
entrainment,
condensation
and
total
vaporization
of
splashed liquid
must
be
observed
in

the
still
as
was
indicated
for the
dynamic
distillation method.
The condensate
collects
until
the
level
is
high
enough
to
flow
over
the
trap
and back
to
the
still.
At the
end of
the
distillation,
this condensate

is removed
and
analyzed
to
determine
the
composition
of
the
vapor,
and
a
sample
is
removed
from
the
still
to
determine
the
still
composition.
This
method
suffers
from the
same difficulties as the
dynamic
dis-

tillation
method
in
that
it is
open
to the
question
of whether
the
vapor
formed
by
boiling
a
liquid
is
in
equilibrium
with
the
liquid.
It is
also
difficult
to
obtain
the true
liquid
temperature

because
of the
super-
heating
effects. The
pressure
is maintained
by
the
pressure
in
the
exit
DETERMINATION
OF
VAPOR-LIQUID
EQUILIBRIA
11
tube,
and
in normal
pressure
determinations
this
is
usually open
to the
atmosphere.
This
theoretically

offers
the
possibility
of errors
in
that
it
allows
Oxygen
and
nitrogen
to
dissolve
in the
condensate
sample,
which
is
then
recycled
back to the
still.
At
low
pressures
the solu-
bility
of
such
gases

is
usually
small
and
the error is
slight,
but
in
high-
pressure
operations
the
use
of
this
gas
system
can lead
to serious
errors.
FIG. 1-5.
Continuous
distillation
equilibrium
still.
The
gas pressuring
system,
however,
is

extremely
desirable
in
that
it
regulates
the
condenser
cooling
capacity
so
that
it
exactly
balances
heat
input
to
the
still.
At
high
pressures
the
errors
become
so
serious
that this
benefit

must
be
foregone.
Figure
1-6 indicates
a
type
of
apparatus
in
which
the
heat
input
and
removal
are
adjusted
so
that
the
pressure
remains
constant
without
the
necessity
of
a
sealing

gas.
Another
source
of
error
in
the
system
is
possible
because
the
con-
densate
returned
to
the
still
is
of a
different
composition
from the
liquid
in
the
still
and
in
general

is
of
lower
boiling
point.
If
this
vaporizes
before
it
is
completely
mixed
with
all
of the
liquid
in the
still,
this
vapor
composition
will not
be
an
equilibrium
vapor.
12
FRACTIONAL
DISTILLATION

Although
the
apparatus
appears
to
be
of
the
recirculation
type
and
it
might
be
supposed
that
the
successive
contacts would tend
to
give
a
closer
approach
to
equilibrium,
this
is
not
the case. If

the
vapor
evolved
from
the
liquid
is
not
an
equilibrium vapor,
this
type
of
recycle
Vent*.
Condenser,
Top
heater
Relay
controlling
heat
supply
to
still
SHU
thermo
couple
St/it
sample
Hot

Bottom
heater
FIG.
1-6.
Continuous
distillation still
for
high-pressure operation.
system
does not
give
a
closer
approach
with
repeated
recycling
since
new
vapor
is formed and the
recycled
material is not
brought
to
equi-
librium
by
successive contacts. The
recycling

does
give
a
steady-state
condition,
but the
approach
to
equilibrium
is
only
that
obtained
by
boiling
the
liquid.
In
order
to
eliminate
some
of
the
sources of error in continuous
dis-
tillation
systems,
various
modifications

^have been
made.
The most