Sequencing Separation Trains
CHEN 4460 – Process Synthesis,
Simulation and Optimization
Dr. Mario Richard Eden
Department of Chemical Engineering
Auburn University
Lecture No. 4 – Sequencing Separation Trains
September 11, 2012
Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel
Process Design/Retrofit Steps
Assess Primitive
Problem
Detailed Process
Synthesis Algorithmic
Methods
Development
of Base-case
PART II
Detailed Design,
Equipment sizing, Cap.
Cost Estimation,
Profitability Analysis,
Optimization
Plant-wide
Controllability
Assessment
Algorithmic Methods
Lecture 4 – Introduction
•
Almost all chemical processes require the separation of
chemical species (components), to:
 Purify a reactor feed
 Recover unreacted species for recycle to a reactor
 Separate and purify the products from a reactor
•
Frequently, the major investment and operating costs of a
process will be associated with separation equipment
•
For a binary mixture, it may be possible to select a
separation method that can accomplish the separation task
in just one piece of equipment.
•
More commonly, the feed mixture involves more than two
components, involving more complex separation systems.
Lecture 4 – Objectives
 Be familiar with the more widely used
industrial separation methods and their
basis for separation.
 Understand the concept of the separation
factor and be able to select appropriate
separation methods for liquid mixtures.
Example: Butenes Recovery
Species
b.pt.(C)
Tc (C)
Pc, (MPa)
Propane
A
-42.1
97.7
4.17
1-Butene
B
-6.3
146.4
3.94
n-Butane
C
-0.5
152.0
3.73
trans-2-Butene
D
0.9
155.4
4.12
cis-2-Butene
E
3.7
161.4
4.02
n-Pentane
F
36.1
196.3
3.31
Example: Butenes Recovery
100-tray column
C3 & 1-Butene in
distillate
Pentane withdrawn
as bottoms
2-C4=s withdrawn as
distillate. Furfural is
recovered as bottoms
and recycled to C-4
Propane and
1-Butene recovery
n-C4 and 2-C4=s
cannot be
separated by
ordinary distillation
(=1.03), so 96%
furfural is added as
an extractive agent
(  1.17).
n-C4 withdrawn as
distillate.
Separation is Energy Intensive
•
Unlike the spontaneous mixing of chemical species, the
separation of a mixture of chemicals requires an
expenditure of some form of energy
•
Separation of a feed mixture into streams of differing
chemical composition is achieved by forcing the different
species into different spatial locations, by one or a
combination of four common industrial techniques:
 The creation by heat transfer, shaft work, or pressure reduction
of a second phase that is immiscible with the feed phase (ESA –
energy separating agent)
 Introduction into the system of a second fluid phase (MSA –
mass separating agent). This must be subsequently removed.
 Addition of a solid phase upon which adsorption can occur
 Placement of a membrane barrier
Common Separation Methods
Separation
Method
Phase of
the feed
Separation
agent
Developed or
added phase
Separation
principle
Equilibrium
flash
L and/or V
Pressure
reduction or
heat transfer
V or L
difference
in volatility
Distillation
L and/or V
Heat transfer
or shaft work
V or L
difference
in volatility
Gas
Absorption
V
Liquid
absorbent
L
difference
in volatility
Stripping
L
Vapor stripping
agent
V
difference
in volatility
Extractive
Distillation
L and/or V
Liquid solvent
and heat
transfer
V and L
difference
in volatility
Azeotropic
Distillation
L and/or V
Liquid
entrainer and
heat transfer
V and L
difference
in volatility
Common Separation Methods
Separation
Method
Phase of
the feed
Separation
agent
Developed
or added
phase
Separation
principle
Liquid-liquid
Extraction
L
Liquid
solvent
Second
liquid
Difference in
solubility
Crystallization
L
Heat
transfer
Solid
Difference in
solubility or
m.p.
Gas
adsorption
V
Solid
adsorbent
Solid
difference in
adsorbabililty
Liquid
adsorption
L
Solid
adsorbent
Solid
difference in
adsorbabililty
Membranes
L or V
Membrane
Membrane
difference in
permeability
and/or
solubility
Common Separation Methods
Separation
Method
Phase of
the feed
Separation
agent
Developed
or added
phase
Separation
principle
Supercritical
extraction
L or V
Supercritical
solvent
Supercritical
fluid
Difference
in solubility
Leaching
S
Liquid
solvent
L
Difference
in solubility
Drying
S and L
Heat
transfer
V
Difference
in volatility
Separation Method Selection
•
The development of a separation process requires the
selection of:





•
Separation methods
ESAs and/or MSAs
Separation equipment
Optimal arrangement or sequencing of the equipment
Optimal operating temperature and pressure for the equipment
Selection of separation method depends on feed condition:
•
Vapor
•
Liquid
•
Solid
Partial condensation, distillation, absorption,
adsorption, gas permeation (membranes)
Distillation, stripping, LL extraction, supercritical
extraction, crystallization, adsorption, and dialysis
or reverse osmosis (membranes)
If wet  drying, if dry  leaching
Separation Method Selection
•
The separation factor, SF, defines the degree of separation
achievable between two key components of the feed. This
factor, for separation of component 1 from component 2
between phases I & II, for a single stage of contacting, is:
SF 
•
C 1I / C 2I
II
C1
II
/C2
C = composition variable,
I, II = phases rich in
components 1 and 2.
(8.1)
SF is generally limited by thermodynamic equilibrium. For
example, in the case of distillation, using mole fractions as the
composition variable and letting phase I be the vapor and phase
II be the liquid, the limiting value of SF is given in terms of
vapor-liquid equilibrium ratios (K-values) as:
y 1 / x 1 K1
P1 s
SF 

 1,2  s
y2 / x2 K2
P2
 for ideal L and V 
(8.2), (8.3)
Separation Method Selection
•
For vapor-liquid separation operations that use an MSA that
causes the formation of a non-ideal liquid solution (e.g.
extractive distillation):
SF  1,2 
•
(8.5)
L s
2P2
If the MSA is used to create two liquid phases, such as in liquidliquid extraction, the SF is referred to as the relative selectivity,
β , where:
SF  1,2 
•
1LP1 s
1II / 2II
I
I
1 /  2
(8.6)
In general, MSAs for extractive distillation and liquid-liquid
extraction are selected according to their ease of recovery for
recycle and to achieve relatively large values of SF.
Equal Cost Separators
Liquid-Liquid Extraction
should NOT be used
when α for ordinary
distillation is greater
than 3.2
Extractive distillation
should NOT be used
when α for ordinary
distillation is
greater than 2
Ref: Souders (1964)
Summary – Separation Trains
On completion of this part, you should:

Be familiar with the more widely used industrial separation
methods and their basis for separation.

Understand the concept of the separation factor and be
able to select appropriate separation methods for liquid
mixtures.
Other Business
•
Next Lecture – September 20
–
Sequencing Ordinary Distillation Columns (SSLW p. 216-223)