Sequencing Separation Trains
CHEN 4460 – Process Synthesis,
Simulation and Optimization
Dr. Mario Richard Eden
Department of Chemical Engineering
Auburn University
Lab Lecture No. 2 – Sequencing of Separation Trains
September 18, 2012
Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel
Example 1: Four Components
The feed to a separation process consists of the
following species:
Species Number
1
2
3
4
Species
Ethane
Propane
1-Butene
n-Butane
Example 1: Four Components
It is desired to separate this mixture into essentially pure
species. The use of two types of separators is to be
explored:
–
–
•
Ordinary distillation
Extractive distillation with furfural
The separation
orderings are:
Separator Type
Species number
•
I
II
1
1
2
2
3
4
4
3
5
Example 1: Four Components
•
Determine the number
separation sequences.
of
possible
•
What splits would you forbid so as to
reduce greatly the number of possible
sequences?
Example 1: Solution
•
To determine the number of possible sequences , use Eqs.
(8.9) and (A):
(8.9)
(5 . 7 ) N s 
P 1
 N j N P j 
j 1
[ 2( P  1)]!
P!( P  1)!
(5(A)
.8) N sT  T P 1 N s
•
Combining Eqs. (8.9) and (A), the number of possible
sequences is:
[ 2( P  1)]!
N sT  T P 1

P!( P  1)!
[ 2( 4  1)]!
6!
 2 4 1
 23
 8  5  40
4!( 4  1)!
4!3!
Example 1: Solution
•
For ordinary distillation, the approximate relative
volatilities, a between adjacent species are as follows at
150 F:
Adjacent binary pair
a at 150 oF:
C2 /C3
C3 /1-C4=
1-C4= /nC4
2.6
2.45
1.18
Example 1: Solution
Example 1: Solution
From Fig 5.3, forbid use of extractive distillation when a>2
for ordinary distillation.
Thus, forbid the use of extractive distillation for the splits:
C2/C3
C3/1-C4=
C3/nC4 (a=2.45x1.18=2.89)
Thus, the only splits to be considered for extractive
distillation are those involving nC4/1-C4= and 1-C4=/furfural
C2
C3
1-C4=
n-C4
C3
1-C4=
n-C4
1-C4
n-C4=
Example 1: Solution
C2
C3
1-C4=
n-C4
C2
C3
n-C4=
1-C4
Furfural
C3
1-C4=
n-C4
n-C4
1-C4=
Furfural
C2
C3
n-C4
1-C4=
Furfural
1-C4=
Furfural
1-C4=
Furfural
Best Sequence using Heuristics
•
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 should
not be excessive.
Example 2: Three Components
•
A mixture of benzene, toluene and biphenyl needs to be
separated. The boiling points of the three components are:
80.1 C, 110.8 C, and 254.9 C, respectively.
•
Suggest possible separation sequences for this mixture.
Select the most appropriate sequence for each of the two
cases in the table below.
•
Simulate the two sequences you suggest using Aspen Plus,
to produce each of the three components at 99% purity.
Case
Benzene
Toluene
Biphenyl
1
10%
10%
80%
2
10%
60%
30%
Example 2: Solution
•
Case 1: Direct Sequence
Total cooling: 10.7 MM kcal/hr
Total heating: 30.0 MM kcal/hr
Example 2: Solution
•
Case 1: Indirect Sequence
Total cooling: 4.4 MM kcal/hr
(Direct sequence: 10.7 MM kcal/hr)
 Sequence separation points to remove, early in the sequence,
those components of greatest molar percentage in the feed.
 Sequence separation points that favor near equimolar amounts of
distillate and bottoms in each column. The reboiler duty should
not be excessive.
Total heating: 27.0 MM kcal/hr
(Direct sequence: 30 MM kcal/hr)
Example 2: Solution
•
Case 2: Direct Sequence
Total cooling: 1055 MM kcal/hr
Total heating: 1064 MM kcal/hr
Example 2: Solution
•
Case 2: Indirect Sequence
Total cooling: 14.5 MM kcal/hr
(Direct sequence: 1055 MM kcal/hr)
 Sequence separation points to remove, early in the sequence,
those components of greatest molar percentage in the feed.
 Sequence separation points that favor near equimolar amounts of
distillate and bottoms in each column. The reboiler duty should
not be excessive.
Total heating: 25 MM kcal/hr
(Direct sequence: 1064 MM kcal/hr)