LECTURE 2
Separation method selection
(Distillation)
Associate Prof Dr Rashmi Walvekar
Lesson outline
❑ Industrial separation process
❑ Factors influencing separation
❑ Selection of feasible separation process
Learning outcomes
At the end of the lecture student are able to:
• explore the issue of how to choose the right separation process, or
narrow down the number of choices for a given separation.
• heuristic approach using simple rules of thumb and direction
guidelines that can be followed to arrive at the separation process
sequence to be adopted in the final plant design.
Introduction
✓ selection of separation process sequence is complicated
✓ no STANDARD procedure
✓ optimum separation scheme depends on time and money allocated in the
development and analysis. Also depends on how skilled is the process engineer.
✓Complexity increases with increase in complexity of feed mixture
Example: binary mixture requires 1 unit
multicomponent mixture requires “separation sequence train”
Industrial separation processes
Fig 1: Schematic flow sheet of a
naphtha steam cracker.
Example of separation train
(series of separation units)
Selecting separation method
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 largely depends on feed condition –
 Vapor: partial condensation, distillation, absorption, adsorption, gas permeation (membranes)
 Liquid: distillation, stripping, LL extraction, supercritical extraction, crystallization, adsorption, and
dialysis or reverse osmosis (membranes)
 Solid: if wet → drying, if dry →leaching
Factors influencing choice of separation process
1. Economics:
✓ economic value of the product and scale of operation
constraints include:
• market strategy and timing, reliability, risks associated with innovation, and capital
allocation
Two extreme cases are:
1. The product is of high unit value with a short market life expectancy
2. The product is a high-volume chemical with many producers in a highly competitive
market
Factors influencing choice of separation process
2. Feasibility:
✓ separation process selected should have potential to produce a desired product.
✓ need for extreme processing conditions (T and P).
✓ each separation unit produces different product (each unit has different working
principle).
Factors influencing choice of separation process
3. Product stability:
✓ avoid damage to the product (thermal damage)
✓ thermal damage - denaturation, formation of unwanted color, polymerization
etc.
✓ example: Distillation (operate under vacuum to reduce reboiler temperature)
✓ minimize holdup time at high temperature
Factors influencing choice of separation process
4. Design reliability:
✓ most important factor – plant should
work properly to product desired
product that is profitable.
✓ only apply well understood processes
that are highly acceptable by
industries -> technologically mature
process for commercial use
Selection of feasible separation processes
1. Classes of processes:
✓ clear distinction between ESA, MSA and rate controlled processes should be done.
✓ energy consumption increases from ESA→MSA→rate controlled.
• multistage rate-governed separation process should only be considered when it gives
a significantly better separation factor than an equilibrium based process.
• a mass-separating-agent process should only be considered when it provides a better
separation factor than an energy-separating-agent process.
Selection of feasible separation processes
2. Initial screening:
✓ define the problem and select most feasible separation process.
✓ depends on feed conditions, mixture properties to be exploited and characteristic of
separation process.
• Feed conditions: flow rate, composition
• Product conditions: purity, recovery
Selection of feasible separation processes
Table 1:Factors that influence the selection of feasible separation operations.
Selection of feasible separation processes
Table 2: Ease of scale-up and staging of the most common separation operations.
Selection of feasible separation processes
3. The separation factor(SF): defines the degree of separation achievable between two key
components of he feed This factor, for the separation of component 1 from component 2
between phases I and II, for a single stage of contacting, is defined as:
SF =
C 1I / C 2I
C 1II / C 2II
Where, C = composition
variable, I, II = phases rich
in components 1 and 2.
✓ The value of the separation factor is limited by thermodynamic equilibrium, except in
the case of membrane separations that are controlled by relative rates of mass transfer
through the membrane.
Selection of feasible separation processes
✓ Operations employing an ESA are
economically feasible at a lower value of
SF than those employing an MSA.
✓ MSA is used based on ease of recovery
for recycle and to achieve large SF.
✓ Exploit the molecular properties in the
most economical manner
Fig 2: Relative separation factors for equal-cost
separation operations. (Adapted from M. Souders,
Chem. Eng. Prog., 60(1964)2 75–82)
Separation of homogeneous liquid mixtures
• Strengths of distillation
✓ small equipment requirement
✓ easy staging
✓ economics of scale
✓ energy costs
✓ design and scale-up reliability
Distillation
is the most frequently
used separation process
in practice
Separation of homogeneous liquid mixtures
• Limitations of distillations:
✓ low relative volatility
✓ Feed composition
✓ extreme conditions
✓ small capacities
✓ product degradation
✓ column fouling
Example 1. Specification for Butenes Recovery
Design for Butenes Recovery System
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.
Sequencing of Ordinary Distillation Columns
21
Use a sequence of ordinary distillation (OD) columns to separate a multicomponent mixture provided:
 in each column is > 1.05.
The reboiler duty is not excessive.
The tower pressure does not cause the mixture to approach the TC of the mixture.
Column pressure drop is tolerable, particularly if operation is under vacuum.
The overhead vapor can be at least partially condensed at the column pressure to provide
reflux without excessive refrigeration requirements.
The bottoms temperature for the tower pressure is not so high that chemical
decomposition occurs.
Azeotropes do not prevent the desired separation.
Algorithm to Select Pressure and Condenser Type
Number of Sequences for Ordinary Distillation
Equation for number of different sequences of P − 1 ordinary distillation (OD) columns, NS, to
produce P products:
[2(P − 1)]!
Eq (2.1)
Ns =
P ! (P − 1)!
P
# of Separators
Ns
2
1
1
3
2
2
4
3
5
5
4
14
6
5
42
7
6
132
8
7
429
Example 2.1 Sequences for 4-component separation
Example 2.1 – Sequences for 4-component separation
Identifying the Best Sequences using Heuristics
26
The following guidelines are often used to reduce the number of OD sequences that need to be
studied in detail:
 Remove thermally unstable, corrosive, or chemically reactive components early in the sequence.
 Remove final products one-by-one as distillates (the direct sequence).
 Sequence separation points to remove, early in the sequence, those components of greatest molar
percentage in the feed.
 Sequence separation points in the order of decreasing relative volatility so that the most difficult splits are
made in the absence of other components.
 Sequence separation points to leave last those separations that give the highest purity products.
 Sequence separation points that favor near equimolar amounts of distillate and bottoms in each column.
The reboiler duty is not excessive.
Class Activity
Example 2.2
Arrange in the order of
increasing boiling point
Design a sequence of
ordinary distillation
columns to meet the
given specifications.
Possible solution
▪ Variation in relative volatility and
molar percentage.
• First column should separate C3,
the most volatile.
• Second column should be for separ
ation of nC4 and iC5 as LK and HK
respectively.
• Two most difficult splits are iC4/nC4
and iC5/nC5, so two separate
columns for these separation
Example 2.3
The table below gives the data for a ternary separation of benzene, toluene
and ethyl benzene. Using the vapour flowrate equation, determine whether
direct or indirect sequence should be used.
solution
Hence, the direct sequenceshould be used. NOTE: High V,
High Capital and Op. Costs!!
Example 2.3 (homework)
In a large chemical plant methylcyclohexane is separated from toluene by
distillation. For the construction of a new plant it is considered whether the
use of extractive distillation or extraction could provide economically more
attractive options. A detailed evaluation requires firstly the characterization
of the current separation between methylcyclohexane and toluene.
a. Calculate from the given data the relative volatility for the separation of
methylcyclohexane and toluene by distillation. Consider the mixture to
behave (almos)t ideal.
b. Which of the given solvents might be feasible to enhance the relative
volatility sufficiently to make extractive distillation economically
attractive?
c. The same question for extraction. Assume that the solvents and the
methylcyclohexane/toluene mixture are (almost) immiscible. Besides
methylcyclohexane (45%) and toluene (45%) the feed also contains 5%
of cyclohexane as well as benzene.
d. What would be the optimal arrangement of separation steps for
distillation, for extractive distillation and for extraction to separate
this mixture into its four pure constituents? What distillative
separation might become problematic and why?
e. Which of the arrangements seems, considering the number of
operations (columns), the most economical solution. f. Why is the
counting of the number of operations sufficient to get a first
impression about the most economical route? What effects are not
taken into account?
Complex Columns for Ternary Mixtures
In some cases, complex rather than simple distillation columns should be considered when
developing a separation sequence.
Ref: Tedder and Rudd (1978)
Regions of optimality
As shown below, optimal regions for the various configurations
depend on the feed composition and the ease-of-separation index:
• ESI = AB/ BC
ESI  1.6
ESI  1.6
Regions of optimality
1. If 40 to 80% is middle product and nearly equal
amounts of overhead and bottoms are present,
then favor design V.
2. If more than 50% is middle product and less
than 5% is bottoms, then favor design VI.
ESI  1.6
3. If more than 50% is middle product and less
than 5% is overheads, then favor design VII.
4. If less than 15% is middle product and nearly
equal amounts of overheads and bottoms are
present, then favor design III.
5. Otherwise, favor design I or II, whichever
removes the most plentiful component first.
Regions of optimality
ESI  1.6
1. If more than 50% is bottoms product, then favor
design II.
2. If more than 50% is middle product and from 5
to 20% is bottoms, favor design V.
3. If more than 50% is middle product and less
than 5% is bottom, favor design VI.
4. If more than 50% is middle product and less
than 5% is overhead then favor design VII.
5. Otherwise, favor design III.
6. Thermally coupled designs I11 and IV should be
considered as alternatives to designs I and 11,
respectively, if less than half the feed is middle
product. In addition, designs 111, IV, VI, and VII
should be considered for separating all mixtures
where a low middle product purity is
acceptable.
Sequencing of V-L Separation Systems
45
When simple distillation is not practical for all separators in a multicomponent mixture
separation system, other types of separators must be employed and the order of volatility or
other separation index may be different for each type.
 If they are all two-product separators and if T equals the number of different types, then
the number of possible sequences is now given by:
T
Ns
=T
P −1
Ns
Eq (2.2)
 For example, if P = 3, and ordinary distillation, extractive distillation with either solvent I
or solvent II, and LL extraction with solvent III are to be considered, then T = 4, and
applying Eqns (2.1) and (2.2) gives 32 possible sequences (for ordinary distillation alone, NS
= 2).