Transcript Sequencing of Separation Trains - ????????

SEQUENCING OF SEPARATION TRAINS
Ref: Seider, Seader and Lewin (2004), Chapter 7
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Separation Trains - 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 those costs associated with the
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.
However, more commonly, the feed mixture involves
more than two components, involving more complex
separation systems
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Instructional Objectives
When you have finished studying this unit, 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 vapor
and liquid mixtures.
 Understand how distillation columns are sequenced and
how to apply heuristics to narrow the search for a nearoptimal sequence.
 Be able to apply algorithmic methods to determine an
optimal sequence of distillation-type separations.
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Example 1. Specification for Butenes Recovery
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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withdrawn as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2-C4=s withdrawn as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
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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n-C4 withdrawn as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 Trains - 4
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)
 the introduction into the system of a second fluid phase
(MSA – mass separating agent). This must be
subsequently removed.
 the addition of a solid phase upon which adsorption can
occur (MSA)
 the placement of a membrane barrier (ESA)
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Common Industrial Separation Methods
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Separation
Method
Phase of
the feed
Separating
agent(s)
Developed or
added phase
Separation
principle
Flash
L and/or V
Pressure
reduction or
heat transfer
V or L
difference
in volatility
Distillation
(ordinary)
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
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Common Industrial Sep.Methods (Cont’d)
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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
Membrane
L or V
Membrane
Membrane
difference in
permeability
and/or
solubility
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Common Industrial Sep.Methods (Cont’d)
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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
Desublimation
V
Heat
transfer
S
Difference
in volatility
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Selecting Separation Method (1)
 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: partial condensation, cryogenic distillation , absorption,
adsorption, gas permeation (membranes), desublimation
 Liquid: partial vaporization, distillation, stripping, extractive
distillation, azeo-distillation, LL extraction, crystallization ,
adsorption, membrane separation (dialysis, reverse osmosis,
ultrafiltration and pervaporation), supercritical extraction
 Solid: if slurry filtration, if wet  drying, if dry leaching
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Selecting Separation Method (2)
 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
II
C1
II
/C2
C = composition variable,
I, II = phases rich in
components 1 and 2.
(7.1)
 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:
 P1 s

y 1 / x 1 K1

SF 

 1,2   s for ideal L and V 
y2 / x2 K2
 P2

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(7.2,3)
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Selecting Separation Method (3)
 For vapor-liquid separation operations that use an
MSA that causes the formation of a non-ideal liquid
solution (e.g. extractive distillation):
1LP1 s
SF  1,2  L s
(7.5)
2P2
 If the MSA is used to create two liquid phases, such as in
liquid-liquid extraction, the SF is referred to as the
relative selectivity, b , where:
SF  b1,2 
1II / 2II
I
I
1 /  2
(7.6)
 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.
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Relative volatilities for equal cost separators
Ref: Souders (1964)
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Sequencing of Ordinary Distillation 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.
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Algorithm to Select Pressure and Condenser Type
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Number of Sequences for Ordinary Distillation
Equation for number of different sequences of P  1 ordinary
distillation (OD) columns, NS, to produce P products:
Ns 
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[2(P  1)]!
P ! (P  1)!
(7.9)
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
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Example 2 – Sequences for 4-component separation
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Example 2 – Sequences for 4-component separation
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Identifying the Best Sequences 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.
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Class Exercise
Design a sequence of
ordinary distillation
columns to meet the
given specifications.
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Class Exercise – Possible Solution
Guided by Heuristic 4,
the first column in
position to separate the
key components with the
greatest SF.
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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)
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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
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ESI  1.6
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Sequencing of V-L Separation Systems
 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
(7.10)
 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 (7.9) and (7.10) gives 32 possible
sequences (for ordinary distillation alone, NS = 2).
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Example 3 (Example 1 Revisited)
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
 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, it would be helpful to reduce the number of sequences
that need to be analyzed.
 Need to eliminate infeasible separations, and enforce OD for
separations with acceptable volatilities.
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Example 3 (Example 1 Revisited)
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 method 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 1Butene 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
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Estimating Annualized Cost, CA
For each separation, CA is estimated assuming 99 mol %
recovery of light key and heavy key in distillate and bottom,
respectively. The following steps are followed:
1. Set distillate and bottoms column pressures using
2. Estimate number of stages, feed stage and initial guess of reflux
ratio (R = 1.2 Rmin) by using a short-cut distillation method (e.g.,
DSTWU in Aspen plus).
3. Select tray spacing (typically 2 ft.) and calculate column height, H
(assume an overall efficiency equal to 75%).
4. Estimate reboiler duty, condenser duty and column diameter by a
rigorous distillation method (e.g., RadFrac in Aspen plus)
5. Estimate installed cost of tower (see Chapter 16).
6. Size and cost ancillary equipment (condenser, reboiler, reflux
drum). Sum total capital investment, CTCI.
7. Compute annual cost of heating and cooling utilities (CCOS).
8. Compute CA assuming ROI (typically r = 0.33). CA = CCOS + r CTCI
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(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
1st Branch of Sequences
Sequence
Cost, $/yr
1-5-16-28
900,200
1-5-17-29
872,400
1-6-18
1,127,400
1-7-19-30
878,000
1-7-20
Species
Propane
1-Butene
n-Butane
trans-2-Butene
cis-2-Butene
n-Pentane
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1,095,600
A
B
C
D
E
F
Separation Trains - 4
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
2nd Branch of Sequences
Sequence
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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
Separation Trains - 4
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
3rd Branch of Sequences
Sequence
Cost, $/yr
3-11-23-31
878,200
3-11-24
3-12-(25,26)
867,400
3-13-27
1,080,100
Species
Propane
1-Butene
n-Butane
trans-2-Butene
cis-2-Butene
n-Pentane
30
1,095,700
A
B
C
D
E
F
Separation Trains - 4
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
4th Branch of Sequences
Sequence
4-14-15
Species
Propane
1-Butene
n-Butane
trans-2-Butene
cis-2-Butene
n-Pentane
31
Cost, $/yr
1,115,200
A
B
C
D
E
F
Separation Trains - 4
Lowest Cost Sequence
Sequence
2-(8,10-22)
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Cost, $/yr
860,400
Separation Trains - 4
Marginal Vapor Rate Method
When the number of products is more than four, using
the annualized cost method is very difficult and timeconsuming. One of the less rigorous method for OD that
can produce good results is Marginal Vapor Rate (MV)
that proposed by Modi and Westerberg (1992).
 The difference in costs between the separation in the absence
of nonkey components and the separation in the presence of
nonkey components, defined as Marginal Annualized Cost (MAC).
 A good approximation of MAC is the MV, which is the
corresponding difference in molar vapor rate passing up the
column. The sequence with the minimum sum of column MVs is
selected.
 Vapor rate is a good measure of cost because it is a major
factor in determining column diameter, reboiler and condenser
areas, and reboiler and condenser duties.
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Estimating Marginal Vapor Rate, MV
For each separation, MV is estimated assuming feed at
bubble point and 99.9 mol % recovery of light key and
heavy key in distillate and bottom, respectively. The
following steps are followed:
1. Set distillate and bottoms column pressures using
2. Estimate distillate rate (D), by using a short-cut distillation
method (e.g., DSTWU in Aspen plus with R=1.2 Rmin).
3. Calculate the up column vapor rate as V=D(R+1).
4. Calculate the MV (The difference in vapor rate between the
separation in the absence of nonkey components and the
separation in the presence of nonkey components)
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Example 4
Use the marginal vapor rate (MV) method to determine a
sequence for the hydrocarbon specified in the figure,
except:
1.
2.
35
Ignore the given temperature and pressure of the feed
Assume a recovery of 99.9% in each column
Separation Trains - 4
Example 4
A=isobutane, B= n-butane, C=isopentane, D= n-pentane
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Separation Trains - Summary
On completing this unit, 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.
 Understand how distillation columns are sequenced and
how to apply heuristics to narrow the search for a nearoptimal sequence.
 Be able to apply algorithmic methods to determine an
optimal sequence of distillation-type separations.
Next week: Azeotropic Distillation
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Separation Trains - 4