1. SEQUENCING OF SEPARATION TRAINS
LECTURE FOUR
SEQUENCING OF SEPARATION TRAINS
Ref: Seider, Seader and Lewin (2004), Chapter 7
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2. 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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3. 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 near-optimal sequence.
Be able to apply algorithmic methods to determine an optimal sequence of distillation-type separations.
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4. Example 1. Specification for Butenes Recovery
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5. Design for Butenes Recovery System
Propane and Butene recovery
100-tray column C3 & 1-Butene in distillate
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n-C4 withdrawn as distillate.
Pentane withdrawn as bottoms
2-C4=s withdrawn as distillate. Furfural is recovered as bottoms and recycled to C-4
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6. 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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7. Common Industrial Separation Methods
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)
Heat transfer or shaft work
Gas absorption V
Liquid absorbent L
Stripping
Vapor stripping agent
Extractive distillation
Liquid solvent and heat transfer
V and L
Azeotropic distillation
Liquid entrainer and heat transfer
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8. Common Industrial Sep.Methods (Cont’d)
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
Crystalli-zation
Heat transfer
Solid
Difference in solubility or m.p.
Gas adsorption V
Solid adsorbent
difference in adsorbabililty
Liquid adsorption
Membrane
L or V
difference in permeability and/or solubility
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9. Common Industrial Sep.Methods (Cont’d)
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
Drying
S and L
Heat transfer V
Difference in volatility
Desublimation
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10. 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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11. 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:
(7.1)
C = composition variable, I, II = phases rich in components 1 and 2.
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:
(7.2,3)
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12. 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):
(7.5)
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:
(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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13. Relative volatilities for equal cost separators
Ref: Souders (1964)
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14. 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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15. Algorithm to Select Pressure and Condenser Type
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16. Number of Sequences for Ordinary Distillation
Equation for number of different sequences of P  1 ordinary distillation (OD) columns, NS, to produce P products:
(7.9)  P
# of Separators
 Ns 2 1 3 4 5 14 6 42 7
132 8
429
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17. Example 2 – Sequences for 4-component separation
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18. Example 2 – Sequences for 4-component separation
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19. 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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20. Class Exercise
Design a sequence of ordinary distillation columns to meet the given specifications.
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21. 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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22. 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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23. Regions of Optimality ESI  1.6 ESI  1.6
LECTURE FOUR
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
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24. 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.
(7.10)
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:
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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25. 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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26. 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 1-Butene and n-Butane, altering separation order to ACBDEF, and giving C/B = 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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27. 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:
Set distillate and bottoms column pressures using
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).
Select tray spacing (typically 2 ft.) and calculate column height, H (assume an overall efficiency equal to 75%).
Estimate reboiler duty, condenser duty and column diameter by a rigorous distillation method (e.g., RadFrac in Aspen plus)
Estimate installed cost of tower (see Chapter 16).
Size and cost ancillary equipment (condenser, reboiler, reflux drum). Sum total capital investment, CTCI.
Compute annual cost of heating and cooling utilities (CCOS).
Compute CA assuming ROI (typically r = 0.33). CA = CCOS + r CTCI
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28. (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
900,200
872,400
1-6-18
1,127,400
878,000
1-7-20
1,095,600
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
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29. (A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
2nd Branch of Sequences
Sequence
Cost, $/yr
2-(8,9-21)
888,200
2-(8,10-22)
860,400
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
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30. (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
878,200
1,095,700
3-12-(25,26)
867,400
1,080,100
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
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31. (A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
4th Branch of Sequences
Sequence
Cost, $/yr
1,115,200
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
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32. Lowest Cost Sequence Sequence Cost, $/yr 2-(8,10-22) 860,400
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33. Marginal Vapor Rate Method
When the number of products is more than four, using the annualized cost method is very difficult and time- consuming. 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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34. 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:
Set distillate and bottoms column pressures using
Estimate distillate rate (D), by using a short-cut distillation method (e.g., DSTWU in Aspen plus with R=1.2 Rmin).
Calculate the up column vapor rate as V=D(R+1).
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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35. Example 4
Use the marginal vapor rate (MV) method to determine a sequence for the hydrocarbon specified in the figure, except:
Ignore the given temperature and pressure of the feed
Assume a recovery of 99.9% in each column
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36. A=isobutane, B= n-butane, C=isopentane, D= n-pentane
Example 4
A=isobutane, B= n-butane, C=isopentane, D= n-pentane
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37. 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 near-optimal sequence.
Be able to apply algorithmic methods to determine an optimal sequence of distillation-type separations.
Next week: Azeotropic Distillation
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