Sequencing Distillation Columns
CHEN 4460 – Process Synthesis,
Simulation and Optimization
Dr. Mario Richard Eden
Department of Chemical Engineering
Auburn University
Lecture No. 5 – Sequencing Ordinary Distillation Columns
September 18, 2012
Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel
Lecture 5 – Objectives

Understand how distillation columns are sequenced and
how to apply heuristics to narrow the search for a nearoptimal sequence.

Be able to apply systematic methods to determine an
optimal sequence of distillation-type separations.
Sequencing OD Columns
•
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.
Pressure/Condenser Algorithm
Number of Sequences for OD
•
Number of different sequences of P –1 ordinary distillation
(OD) columns, NS, to produce P products:
Ns 
[2(P  1)]!
P ! (P  1)!
(8.9)
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: 4 Components
Example: 4 Components
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.
Class Exercise
Design a sequence
of ordinary
distillation columns
to meet the given
specifications.
Exercise – Possible Solution
Guided by Heuristic 4, the first
column in position to separate the
key components with the greatest
SF.
 Sequence separation points in the
order of decreasing relative volatility
so that the most difficult splits are
made in the absence of other
components.
Exercise – Possible Solution
 = 1.5
 = 3.6
 = 2.8
 = 1.35
Complex Columns
•
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):
ESI = AB/ BC
ESI  1.6
ESI  1.6
Sequencing V-L Separation
•
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:
NsT  T P 1Ns
•
(A)
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, T = 4, and
applying Eqns (8.9) and (A) gives 32 possible sequences
(for ordinary distillation alone, NS = 2).
Example: Butenes Recovery
Species
•
•
•
•
b.pt.(C)
Tc (C)
Pc, (MPa)
97.7
4.17
Propane
A
-42.1
1-Butene
B
-6.3
n-Butane
C
-0.5
trans-2-Butene
D
0.9
Propane
Butane 3.94
152.0 Butene 3.73
155.4Pentane 4.12
cis-2-Butene
E
3.7
161.4
4.02
n-Pentane
F
36.1
196.3
3.31
146.4
For T = 2 (OD and ED), and P = 4, NS = 40.
However, since 1-Butene must also be separated (why?),
P = 5, and NS = 224.
Clearly,1-Butene
it would
be helpful
to reduce the number of
and 2-Butene
are structurally
sequences
needwhereas,
to be analyzed.
verythat
different,
the optical
are much closer related and are
Need isomers
to difficult
eliminate
infeasible separations, and enforce OD
to separate by distillation
for separations with acceptable volatilities.
Example: Butenes Recovery
Adjacent Binary Pair
ij at 65.5 oC
Propane/1-Butene (A/B)
2.45
1-Butene/n-Butane (B/C)
1.18
n-Butane/trans-2-Butene (C/D)
1.03
cis-2-Butene/n-Pentane (E/F)
2.50
•
Splits A/B and E/F should be by OD only (  2.5)
•
Split C/D is infeasible by OD ( = 1.03). Split B/C is feasible, but an
alternative may be more attractive.
•
Use of 96% furfural as a solvent for ED increases volatilities of
paraffins to olefins, causing a reversal in volatility between 1-Butene
and n-Butane, altering separation order to ACBDEF, and giving C/B =
1.17. Also, split (C/D)II with  = 1.7, should be used instead of OD.
•
Thus, splits to be considered, with all others forbidden, are: (A/B…)I,
(…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
Estimating Annualized Cost
•
For each separation, CA is estimated assuming 99 mol %
recovery of light key in distillate and 99 mol % recovery of
heavy key in bottoms. The following steps are followed:
 Set distillate and bottoms column pressures using
 Estimate number of stages and reflux ratio by WUG method (e.g.,
using Aspen Plus “DSTWU Column”)
 Select tray spacing (typically 2 ft.) and calculate column height, H
 Compute tower diameter, D (using Fair correlation for flooding
velocity, or Aspen Plus Tray Sizing Utility)
 Estimate installed cost of tower (e.g. Peters & Timmerhaus)
 Size and cost ancillary equipment (condenser, reboiler, reflux
drum). Sum total capital investment, CTCI
 Compute annual cost of heating and cooling utilities (COS)
 Compute CA assuming ROI (typically r = 0.2). CA = COS + r *CTCI
Butenes Recovery – 1st Branch
(A/B…)I, (…E/F)I, (…B/C…)I,
(A/C…)I , (…C/B…)II, and (…C/D…)II
Sequence
Cost, $/yr
1-5-16-28
900,200
1-5-17-29
872,400
1-6-18
1-7-19-30
1-7-20
Species
Propane
1-Butene
n-Butane
trans-2-Butene
cis-2-Butene
n-Pentane
1,127,400
878,000
1,095,600
A
B
C
D
E
F
Butenes Recovery – 2nd Branch
(A/B…)I, (…E/F)I, (…B/C…)I,
(A/C…)I , (…C/B…)II, and (…C/D…)II
Sequence
Cost, $/yr
2-(8,9-21)
888,200
2-(8,10-22)
860,400
Species
Propane
1-Butene
n-Butane
trans-2-Butene
cis-2-Butene
n-Pentane
A
B
C
D
E
F
Butenes Recovery – 3rd Branch
(A/B…)I, (…E/F)I, (…B/C…)I,
(A/C…)I , (…C/B…)II, and (…C/D…)II
Sequence
Cost, $/yr
3-11-23-31
878,200
3-11-24
3-12-(25,26)
3-13-27
Species
Propane
1-Butene
n-Butane
trans-2-Butene
cis-2-Butene
n-Pentane
1,095,700
867,400
1,080,100
A
B
C
D
E
F
Butenes Recovery – 4th Branch
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
Sequence
Cost, $/yr
4-14-15
1,115,200
Species
Propane
1-Butene
n-Butane
trans-2-Butene
cis-2-Butene
n-Pentane
A
B
C
D
E
F
Example: Butenes Recovery
•
Lowest Cost Sequence
Sequence
2-(8,10-22)
Cost, $/yr
860,400
Example: Butenes Recovery
Summary – Sequencing
On completion of this part, you should:

Understand how distillation columns are sequenced and
how to apply heuristics to narrow the search for a nearoptimal sequence.

Be able to apply systematic B&B methods to determine an
optimal sequence of distillation-type separations.
Other Business
•
Homework
–
–
•
SSLW: 8.1, 8.2, 8.3
Due Tuesday September 25
Next Lecture – September 25
–
Review of Non-Ideal Thermodynamics (SSLW 223-230)