Separation Engineering
Chapter 3 Distillation
Chapter 3 Distillation
Separation Engineering
Chapter 3 Distillation
3.1 Introduction
Principle of distillation operation
The boiling points of components in a miscible liquid mixture are
different. Thus, by evaporating and condensing part of the mixture，
the components can be separated with each other.
Main applications
• Refinery of crude oil
• Purification of products
• Solvent recovery
• Treatment of waste liquids
Separation Engineering
Classification
classified by operating method
simple distillation
equilibrium distillation
fractional distillation
special distillation
extractive distillation
azeotropic distillation
dissolved-salt distillation
classified by operating pressure
classified by number of components
classified by operating procedure
Chapter 3 Distillation
Separation Engineering
Chapter 3 Distillation
In simple distillation, all the hot
vapors produced are immediately
channeled into a condenser which
cools and condenses the vapors.
Therefore, the distillate will not be
pure − its composition will be
identical to the composition of the
vapors at the given temperature
and pressure.
Separation Engineering
Chapter 3 Distillation
Equilibrium distillation (Flash distillation) is a single stage
separation technique. A liquid mixture feed is pumped through a
heater to raise the temperature of the mixture. It then flows
through a valve and the pressure is reduced, causing the liquid to
partially vaporize. Once the mixture enters a big enough volume,
the liquid and vapor separate.
Because the vapor and liquid
are in such close contact up
until the "flash" occurs, the
product liquid and vapor
phases approach equilibrium.
For many cases, the boiling
points of the components in the
mixture will be sufficiently close
that Raoult's law must be taken
into consideration.
Therefore, fractional distillation
must be used in order to separate
the components well by repeated
vaporization-condensation cycles
within a packed fractionating
column.
Continuous fractional distillation tower
separating one feed mixture stream into four
distillate and one bottoms fractions
Separation Engineering
Batch Distillation
In differential distillation a feed
mixture (an initial charge) of a given
composition is placed in a single stage
separator and heated to boiling. The
vapor is collected and condensed to a
distillate. The composition of the
remaining liquid and the distillate are
functions of time.
differential distillation 微分蒸馏
initial charge 初始进料
Chapter 3 Distillation
Separation Engineering
Chapter 3 Distillation
Reasons for running a batch process

1) Small capacity doesn’t warrant continuous operation

2) Separation is to be done only occasionally

3) Separation is preparative to produce a new product

4) Upstream operations are batch-wise or feed-stocks vary
with time or from batch to batch

5) Feed materials are not appropriate for a continuous flow
system
Separation Engineering
Chapter 3 Distillation
Continuous distillation
A mixture is continuously fed into the process and separated
fractions are removed continuously as output streams as
time passes during the operation.
Reasons for running a batch process
In practice when there are multiple distillate fractions, each
of the distillate exit points is located at different heights on a
fractionating column. The bottoms fraction can be taken
from the bottom of the distillation column or unit, but is
often taken from a reboiler connected to the bottom of the
column.
Continuous binary fractional
distillation tower.
Continuous fractional distillation tower
separating one feed mixture stream into four
distillate and one bottoms fractions
Separation Engineering
Chapter 3 Distillation
3.2 Multi-Component Distillation
1. Introduction
Most of the distillation processes deal with multicomponent mixtures
Principle the same as in two-component system.
Calculation similar to that of two-component system, but more
complicated.
Separation Engineering
Chapter 3 Distillation
Multi-Component Distillation– The Problem

While we can graphically solve a binary component distillation
system using the McCabe-Thiele method, it is also possible to do a
complete analytical solution using mass and energy balances with
the equilibrium relationship.

For multi-component systems, C>2, the number of equations
obtained from mass and energy balances with the equilibrium
relationship will always be one less than the number of unknowns.

Consequently, one cannot do a complete analytical solution for
multi-component distillation–it requires a trial-and-error solution
with the additional unknown assumed to be known, as well as
special considerations as to enhancing convergence of the solution.
Separation Engineering
Chapter 3 Distillation
Some additional terms

When dealing with multi-component systems, we introduce some
new terminology in addition to the terms used in binary
distillation:
– Fractional recoveries
– Key components
– Non-key components
– Splits

Note that binary systems can be handled in the same terms.
Separation Engineering
Chapter 3 Distillation
Fractional Recoveries

Fractional recoveries are often specified in MCD.

A fractional recovery, FRi, is the amount or flow rate of
component i in the distillate or bottoms stream with respect to
the amount or flow rate of component i in the feed stream:
 FRi D 

Dxi , D
Fzi
100%
 FRi W

Dxi ,W
Fzi
100%
These are also often specified simply as % recovery.
Separation Engineering
Chapter 3 Distillation
Key Components
In practice we usually choose two components separation of which
serves as an good indication that a desired degree of separation is
achieved.
The components that have their distillate and bottoms composition
specified are known as the key components.
The Light Key component, LK
The most volatile of the key components
The Heavy Key component, HK
The least volatile of the key components
Separation Engineering
Chapter 3 Distillation
Non-Key Components
All other components not specified in the distillate or bottoms are
termed non-key components (NK’s).
The Light Non-Key component, LNK
If a non-key component is more volatile than the light key,
then it is termed a light non-key (LNK).
The Heavy Non-Key component, HNK
If a non-key component is less volatile than the heavy key,
it is a heavy non-key (HNK).
Separation Engineering
Chapter 3 Distillation
Key Components
There are different strategies to select these key components
Choosing two components that are next to each other on the
relative volatility scale often leads to all the components lighter
than the light key components accumulating in the distillate and all
the components heavier than the heavy key component
accumulating in the bottoms product.
Separation Engineering
Chapter 3 Distillation
Non-Key Component Splits

The split of the non–key components is generally defined as to
where the non–key components are obtained with respect to the
distillate or bottoms stream.

One can have two types of situations concerning the split of the
non–key components:
• Sharp split – Non-distribution of non-keys
• Split – Distribution of non-keys
Separation Engineering
Chapter 3 Distillation
Distributed and undistributed components
• Components that are present in both the distillate and the
bottoms product are called distributed components.
- The key components are always distributed components
• Components with negligible concentration (<10-6) in one of the
products are called undistributed components.
A
B
light non-distributed components
(will end up in the overhead product)
C
D
key
components
E
G
heavy non-distributed components
(will end up in bottoms product)
Separation Engineering
Chapter 3 Distillation
Non-distribution of NK’s

Non–distribution of non–keys means that essentially all of the
non–keys are obtained in either the distillate stream or the
bottoms stream.

Non–distribution of non–keys can be assumed when:
• All of the non-keys are either HNK’s or LNK’s
• The fractional recoveries of the LK in the distillate
and HK in the bottoms are relatively large.
Separation Engineering
Chapter 3 Distillation
Distribution of NK’s

Distribution of non–keys means that the non-keys are not
sharply split between the distillate stream or the bottoms
stream.

Distribution of non–keys occurs when:
• Not all of the non-keys are either HNK’s or LNK’s –
we have NK’s.
• The fractional recoveries of the LK in the distillate
and HK in the bottoms are not relatively large.
Separation Engineering
Chapter 3 Distillation
How to determine the keys (LK and HK) and the non–keys
(LNK’s, HNK’s and NK’s) in MCD?

The classification of components in MCD can be determined
from their relative volatilities.

Relative volatility is defined as the ratio of the K values for two
components, which is trivial for a binary system.

In order to use relative volatilities in MCD, we choose a
reference component and define all other component volatilities
with respect to the reference component.
Separation Engineering
Chapter 3 Distillation
How to determine the keys (LK and HK) and the non–keys
(LNK’s, HNK’s and NK’s) in MCD?

The relative volatility for the reference component, of course,
will be 1.

We can then define relative volatilities using equilibrium
coefficient K values for each component, e.g., from the
DePriester charts for hydrocarbon systems.

The choice of the reference component depends upon the
problem, but in general it will be the HK component since it is
less volatile than the LK component.
Separation Engineering
Chapter 3 Distillation
Key and Non-Key Example

Consider a distillation column with the following feed
components:

propane

n–butane

n–pentane

n–hexane

The recoveries for n–butane and n–pentane are specified for
the distillation.

What are the key and non–key designations for this
separation?
Separation Engineering
Chapter 3 Distillation

Component volatilities can be determined from the K values.

From the DePriester charts, the order of volatility is:
propane > n–butane > n–pentane > n–hexane

Since the recoveries of n–butane and n–pentane are
specified…
Separation Engineering

Chapter 3 Distillation
We have:
Volatilities
propane > n–butane > n–pentane > n–hexane
Component
Propane
n–butane
n–pentane
n–hexane
Designation
Light Non–Key
Light Key
Heavy Key
Heavy Non–Key
Separation Engineering

Chapter 3 Distillation
If the recoveries of n-butane and n-hexane are specified:
Volatilities
propane > n-butane > n-pentane > n-hexane
Component
Propane
n-butane
n-pentane
n-hexane
Designation
Light Non-Key
Light Key
Non-Key
Heavy Key
Separation Engineering

Chapter 3 Distillation
If only the recovery of n–butane is specified:
Volatilities
propane > n–butane > n–pentane > n–hexane
Component
Propane
n–butane
n–pentane
n–hexane
Designation
Light Non–Key
Key
Non–Key
Non–Key
Separation Engineering
Chapter 3 Distillation
2. Calculation of the amounts of distillate and bottom
products in sharp splits
Clear distribution: Selected components are neighbors with
large . All components heavier than the heavy key component
are distributed in bottom product and others in top product. The
concentration of other components can be calculated with mass
balance.
Strictly, clear distribution is only a simplification for
convenient and not a real situation.
Separation Engineering
Chapter 3 Distillation
The total variables
F、zi、D、W
c3
xi , D、xi ,W
c2
N v  2c  5
The total equations
Fzi  Dx i , D  Wx i ,W
Material balance
Sum equations
c
z i  1, xi , D  1, xi ,W  1
We have
designing variables
3
Nc  c  3
N i  N v  N c  2c  5  c  3  c  2
Separation Engineering
Chapter 3 Distillation
For LNK
wi  0
d i  f i , 1  i  LK  1
For HNK
di  0
wi  fi , HK  1  i  c
fi
flow rate of i component in feed
di
flow rate of i component in distillate
wi flow rate of i component in bottoms
Separation Engineering
Chapter 3 Distillation
The flow rate of distillate
D
LK 1
LK 1
i 1
i 1
 d i  d LK  d HK   f i  d LK  d HK
The flow rate of bottom
W
c
 wi  wLK  wHK
HK 1

c
 f i  wLK  wHK
HK 1
Separation Engineering
Distillate composition (xD)
x LK , D 
Fz LK  Wx LK ,W
x HK , D 
x LNK , D
D
f LK  wLK

D
Fz HK  Wx HK ,W
D
f HK  wHK

D
Fz LNK
f LNK


D
D
Chapter 3 Distillation
Separation Engineering
Bottom composition (xW)
x HK ,W 
x LK ,W 
x HNK ,W
Fz HK  Dx HK , D
W
Fz LK  Dx LK , D
W
f HK  d HK

W

f LK  d LK
W
Fz HNK
f HNK


W
W
Chapter 3 Distillation
Separation Engineering
Chapter 3 Distillation
⑴ The distillate composition of LK (xLK,D) and bottom composition of
HK (xHK,W) are specified.
LK 1
DF
 zi  z HK  x HK ,W
i 1
1  x LK , D  x HK ,W
c
W F
 zi  z LK  x LK ,D
i  HK 1
1  x LK , D  x HK ,W
⑵ The distillate composition of HK (xHK,D) and bottom composition
of LK (xLK,W ) are specified.
c
LK
DF
 zi  x LK ,W
i 1
1  x HK , D  x LK ,W
W F
 zi  x HK ,D
i  HK
1  x HK , D  x LK ,W
Separation Engineering
Chapter 3 Distillation
⑶ The distillate and bottom compositions of LK (xLK,D , xLK,W ) are
specified.
DF
z LK  x LK ,W
x LK , D  x LK ,W
W F
x LK , D  z LK
x LK , D  x LK ,W
⑷ The distillate composition and recovery of LK (xLK,D, φLK ) are
specified.
 LK 
Dx LK , D
Fz LK
d LK
 100% 
 100%
f LK
DF
z LK
  LK
x LK , D
Separation Engineering
Chapter 3 Distillation
3. Calculation of multi-component distillation
Short-cut method
Method
simplified to two-component distillation
components (pseudo-binary separation).
of
Basic Equations:
Fenske Eq., Underwood Eq. and Gilliland correlation
two
crucial
Separation Engineering
Chapter 3 Distillation
(1) Minimum Number of stages by Fenske’s Equation:
L j  V j 1
Material balance
L j xi , j  V j 1 yi , j 1
xi , j  yi , j 1
Separation Engineering
Chapter 3 Distillation
Phase equilibrium
yi ,1  K i ,1 xi ,1
Operating line
xi ,1  y i , 2
y i ,1  K i ,1 y i , 2
y i , 2  K i , 2 xi , 2
For second equilibrium stage
yi ,1  K i ,1 K i , 2 xi , 2
For Nth stage
yi ,1  K i ,1 K i , 2  K i , N 1 K i , N xi , N
For j component
y j ,1  K j ,1 K j , 2  K j , N 1 K j , N x j , N
yi ,1
y j ,1

K i , N K i , N 1
K j , N K j , N 1

K i ,2 K i ,1 xi , N
K j , 2 K j ,1 x j , N
Separation Engineering
x N  xW
For condenser xD  y1
xi , D
x j,D
Chapter 3 Distillation
  N  N 1  21
xi ,W
x j ,W
 xi , D

x
 i ,W
 x j ,W

 x
 j , D
 N
    ij ,k

 k 1
Average relative volatility
1
N
 N
 ij     ij ,k 
 k 1

 xi , D

x
 i ,W
 xi , D  x j ,W

lg 
 xi ,W  x j , D
Nm 
lg  ij
 x j ,W

 x
 j , D

   ijN m


 x 
 xi


 lg  i 
 xj 
 xj






D


lg  ij
 
 
 
W 
Separation Engineering
Chapter 3 Distillation
i- LK，j- HK
 x LK , D  x HK ,W

lg 
x

 LK ,W  x HK , D
Nm 
lg  LK  HK
 x LK 

 x LK
 lg 


x 

 HK  D  x HK


lg  LK  HK
Geometric mean relative volatility
 平均＝３  D F W
 平均＝  D W
芬斯克公式还可用摩尔、体积或重量之比表示
 d 
d  
lg  
  
 w  LK  w  HK 

Nm 
lg  LK  HK
 
 
W 
Separation Engineering
Chapter 3 Distillation
Check of Non-key distribution
 xi , D

x
 i ,W
 x j ,W

 x
 j , D

   ijN m


 di

 wi

Nm  d r
   ir  
 wr




式中，i为非关键组分；r为关键组分或参考组分；αir为i相对于r的相
对挥发度
wi 
 d HK
1  
 wHK
fi

 i  HK N m

 d HK 
 i  HK N m
f i 
wHK 

di 
d 
N
1   HK  i  HK  m
 wHK 
Separation Engineering
Chapter 3 Distillation
(2) Minimum Reflux Ratio by Underwood’s Equations:
i xiD
    Rm 1
i xiF
    1  q
 HK     LK
Separation Engineering
Chapter 3 Distillation
(3) Erbar-Maddox correlation
Nm
R
vs
R 1
N
with
Rm
Rm  1
as a parameter
Separation Engineering
Chapter 3 Distillation
(4) Fedd stage
For rectifying stages
N R m
For stripping stages
N S m
N R N R m

N S N S m
 x

x
lg ( L ) D ( H ) F 
xH
xL 


 x

x
lg ( L ) F ( H )W 
xL 
 xH
 x

x
lg ( L ) D  ( H ) F 
xH
xL 


lg  LH
 x

x
lg ( L ) F  ( H )W 
xH
xL 


lg  LH
NR  NS  Nm
NR和NS
Separation Engineering
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