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CHAPTER 3: Part II
SEPARATION SQUENCES
1
Group Working Session – Previous Example
• Reactor effluent consist of hydrogen, toluene, methane, benzene and diphenyl. Design alternative sequences for 5 component system.
• Assume volatility: Hydrogen>methane>benzene>toluene>diphenyl
2
Number of Alternative Sequences (Ns) for Ordinary Distillation
2
3
4
5
6
7
8
9
10
Number of possible
sequences
Number of
components
1
2
5
14
42
132
429
1,430
4,862
Ns = [2(P-1)]!/P!(P-1)!
3
Characteristics for Sequencing of Distillation Columns
• relative volatility in each column is > 1.05.
• The reboiler duty is not excessive.
• The tower pressure does not cause the mixture to approach the TC (critical temp.) of the mixture.
• Column pressure drop is tolerable/acceptable, 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 (to reducing operating costs).
• The bottoms temperature for the tower pressure is not so high that chemical decomposition occurs.
• Azeotropes (2 components have near similar TC ) do not prevent the desired separation.
Use a sequence of ordinary distillation (OD) columns to separate a multicomponent mixture provided:
4
Ex. 1-Specification for Butens Recovery
C4H8
C4H8
C3H6
C4H8
C5H12
C4H8
5
Design for Butenes Recovery System
100-tray column C3 & 1-Butene in distillate
Propane and 1-Butene recovery
Pentane withdrawn as bottoms
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n-C4 withdrawn as distillate.
2-C4=s withdrawn as distillate. Furfural is recovered as bottoms and recycled to C-4
6
Separation is Energy Intensive/Consumption
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 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.
the placement of a membrane barrier.
7
Common Industrial Separation Methods Separation
Method
Phase of
the feed
Separation
agent
Developed
or added
phase
Separation
principle
Equilibrium
flash
L and/or V Pressure
reduction or
heat transfer
V or L difference in
volatility
Distillation 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
8
Ex. 2 – Sequences for 4-component separation
9
Scoping & Screening Optimal Sequencing of Separator
The number of alternatives may be reduced by way of:
Practical Constraints (e.g. safety, product purity
and operability)
Heuristics (past experiences, observations - to
reduce capital & operating costs)
Vapour Flowrate
10
Practical Constraints
General Heuristics for safety and operability
Remove Corrosive and Hazardous Components Early avoids expensive handling costs - e.g. stainless steel columns throughout) the more columns a corrosive component passes through, the more expensive will be the distillation train/sequence
Remove Reactive or Heat Sensitive Components Early Reactive components change/modification separation problems- e.g. monomers foul reboilers due to polymerization @ high Ts Heat sensitive materials requires costly vacuum columns
Webster’s definition - Heuristic: (from Greek heuriskin to discover) serving to guide, discover or Reveal: Valuable for empirical research but unproved or incapable of prove…
11
Operability and Product Quality Constraints
Remove products or recycle streams as distillates, particularly if they are recycled to a packed bed reactor avoids contamination of the product or recycle stream with heavy materials, rust etc. avoids accumulation of contaminant in the process sometimes to avoid product contamination by additives that may be used to inhibit polymerization
Practical Constraints (cont.)
12
Identifying the Best Sequences Using Heuristics
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.
The following guidelines are often used to reduce the number of ordinary distillation (OD) sequences that need to be studied in detail:
13
Identifying the Best Sequence Using Heuristics
For simple columns (one top, one bottom stream);
Heuristics for column sequencing
Most plentiful first
Lightest first
High-purity separations last
Difficult separations last
Favour Equimolar split
Aim high, rather than low
Use internal species for indirect separation
14
Class Exercise
Design a sequence of ordinary distillation (OD) columns to meet the given specifications.
15
Class Exercise – Possible Solution
16
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 1,095,600
1st Branch of Sequences
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
17
Sequence Cost, $/yr
2-(8,9-21) 888,200
2-(8,10-22) 860,400
2nd Branch of Sequences
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
18
Sequence Cost, $/yr
3-11-23-31 878,200
3-11-24 1,095,700
3-12-(25,26) 867,400
3-13-27 1,080,100
3rd Branch of Sequences
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
19
Sequence Cost, $/yr
4-14-15 1,115,200
4th Branch of Sequences
Species
Propane A
1-Butene B
n-Butane C
trans-2-Butene D
cis-2-Butene E
n-Pentane F
(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
20
Lowest Cost Sequence
Sequence Cost, $/yr
2-(8,10-22) 860,400
21
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.
22
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 = 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
23
Separation Trains - Summary
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 systematic B&B methods to determine an optimal sequence of distillation-type separations..
On completing this unit, you should:
24
Quantities Measure: Most Plentiful First
Separation Sequence Total Load
1 2 3 4
2 3 4
1
3 4
2
4
3 D1 + 2D2 + 3D3 + 3D4
1 2 3 4
2 3 4
1 2 3
4
3
2
1 2 3 4
3 4
1 2
3
4
2
1
D1 + 3D2 + 3D3 + 2D4
2D1 + 2D2 + 2D3 + 2D4
1
2
3
25
Separation Sequence Total Load
1 2 3 4
1 2 3
4 4
3
1 2
3
2
3D1 + 3D2 + 2D3 + D4
1 2 3 4
1 2 3
4 5
1
2 3 2D1 + 3D2 + 3D3 + D4
Sequence Total Load
1 9D
2 9D
3 8D
4 9D
5 9D
Most Plentiful First
2
1
OR, use vapour flowrate approach
(see later)
26
Direct Sequence
Separation Load for Direct Sequence
= A + 2B + 2C
= 60 + 2(60) + 2(180) = 540 units
A (60)
B (60)
C (180) B
C
A (60)
C (180)
B (60)
27
Indirect Sequence
Separation Load for Indirect Sequence
= C + 2A + 2B
= 1(180) + 2(60) + 2(60) = 420 units
A
B
C (180)
B (60) A (60)
B (60)
C (180)
A (60)
Indirect sequence More favourable !
28
Working Session Separation Synthesis Heuristics
• Three hundred kmols of a ternary mixture A, B and C in 0.2:0.2:0.6 proportion needed to be separated into pure components (assume ideal or sharp separation that could recover each of the component in pure form)
• Synthesise every possible separation sequence to recover the pure components A and C. What type of sequences are these?
• Use any of the available heuristics to screen among the sequences synthesised, and identify the better sequence (hint: assume the heat load is directly proportional to the components’ feed rates, and do not use the vapour flowrate approach)
29
Difficult Separation Last
Separation Sequence Total Difficulty
1 2 3 4
2 3 4
1
3 4
2
4
3
1 2 3 4
2 3 4
1 2 3
4
3
2
1 2 3 4
3 4
1 2
3
4
2
1
1
2
3
D1
D2
D3 D
4
12
D
2 D
3 D
4
23
D
3 D
4
34
D1
D2
D3 D
4
12
D
2 D
3
23
D
2 D
3 D
4
34
D1 D
2
12
D
1 D
2 D
3 D
4
23
D
3 D
4
34
load
Sep difficulty
30
Difficult Separation Last
Separation Sequence Total Difficulty
1 2 3 4
1 2 3
4
4
3
1 2
3
2
1 2 3 4
1 2 3
4 5
1
2 3
D1 D
2
12
D
1 D
2 D
3
23
D
1 D
2 D
3 D
4
34
D1 D
2 D
3
12
D
2 D
3
23
D
1 D
2 D
3 D
4
34
• Assume D1=D2=D3=D4=D5=D;
• Assume 12=34=;
and 23=/3
2
1
31
Difficult Separation Last
Sequence Total Difficulty
1
2
3
4
5
D
4 9 2
D
(15)
D
4 6 3
D
(13)
D
2 12 2
D
(16)
D
2 9 4
D
(15)
D
3 6 4
D
(13)
Most Difficult
Least Difficult
Least Difficult
32
Aim to Separate at High T, Rather than Low T
50
100
150
-150 400°F
Rel
ativ
e C
ost
/Btu
Room Temperature
Cooling
Heating
Cost of temperature excursions from [Rudd 72]
All other things being equal, avoid
excursions in T and P, but aim high,
rather than low
33
All other things being equal, during distillation, sequences that remove the components one by one in column overheads should be favored
Eg. Ethylene and Propylene manufacture - use heuristics
1. Difficult separation lasts. BP propane (-420C) and propylene
(-480C) are very close. C3 splitter is for last. The next most
difficult is ethane (-880C) and ethylene (-1040C). C2 splitter also
in a last position
2. Favor overhead removal. To get to last separation, material
more volatile and less volatile than C2 and C3 must first be
removed. First distillation, remove the most volatile component,
hydrogen (BP -2530C) and methane (-1610C).
3. Remove valuable product as distillate. To ensure product purity
and avoid inclusion of colored material, C2 and C3 as top
products.
34
Typical Distillation Sequence for Light Olefin Manufacturing
No Component BP (0C)
1. 18% H2 -253
2. 15% CH4 -161
3. 24% C2H4 -104
4. 15% C2H6 -88
5. 14% C3H6 -48
6. 6% C3H8 -42
7. 8% heavies, C4 -1
1 2 3 4 5 6 7
3 4 5 6 7
3 4
7
5 6
1 2 3
4
5 6 7
6
5
35
Light Olefin Product Separation - Lightest First
H2,C1 C2-,C2
0 C2-,
C3-,C3
0 C3
-
C30 C4
+
C3-,C3
0,C4+
C2-,C2
0,C3-,C3
0,C4+
DM
DP
SP
C20
SP
DM
DM = Demethanizer
DP = Depropanizer
SP = Splitter
18% Hydrogen; H2 [-253]
15% Methane; C1 [-161]
24% Ethylene; C2- [-104]
15% Ethane; C20 [-88]
14% Propylene; C3- [-48]
6% Propane; C30 [-42]
8% Heavies; C4+ [-1]
[BP°C]
Pressurized columns are used to T
36
Limitations
• Conflicts - e.g. When the most plentiful is the heaviest ‡ • When operating variables are altered (e.g. when conversion is altered, the most plentiful component may become the least plentiful) • Applicable to simple columns (single feed stream, 1 top, 1 bottom prods.)
‡ Literature on resolving heuristics conflicts, and detailed sequencing heuristics • Seader and Westerberg, AIChE J. 77 • Nishida and Stephanopolous AIChE J. 81 • Malone et al. AIChE J. 85
37
Case Study - Acrylonitrile Production
The patented reaction path for acrylonitrile (the basis for synthetic rubber) uses a relatively cheap raw material:
2 C3H6 + 3O2 + 2 NH3 2 C3H3N + 6H2O
The reaction occurs in a fluidized bed reactor @ 450°Cand a pressure of 30 to 40 psi. The reactor effluent consists of 40% inerts, 39% propylene, 8%propane, 7% acrylonitrile, 5%water and 1% byproduct impurities. The main concerned is the separation problem to support this reaction commercially.
As a chemical engineer, your job is to first synthesize an economically viable separation scheme for this process
38
SOLUTIONS: Acrylonitrile Separation Synthesis
Separation System
Product Purification
System
Purge Recycle to Reactor
Feed to Separation
System Inerts Propylene Propane
Acrylonitrile Water
Impurities
Acrylonitrile Water
Impurities
Propylene Inerts
Propane
Reactor
39
C3 Splitter - Conventional Distillation
C3=
C30
V200
CW
C3=
200
62
C30
Reflux Drum
R, xRi
High P Column
Reboiler
185 psig Steam
Nstages = 200
Pressure = 300 psia (21 atm)
a C3=:C3
0 = 1.2
Energy and Capital
Intensive!!!
40
Heuristic to avoid an “added stream”
All other things being equal, avoid separation that calls for the use of “external” species. If an external species is used, removed it immediately in seubsequent distillation.
Extractive Solvent A C3:C3= after adding solvent Trichlorobutyronitrile 1.8 Acetonitrile 65% Acrylonitrile 20% 1.7 Water 15% Butyronitrile 1.6 Acrylonitrile 1.5 Acetone 1.4 Benzonitile 1.3
41
Alternative Separation Scheme- Extractive Distillation
Initial Property List Modified Property List
Inerts 40%
C3= 39%
C3 8%
Acrylo 7%
Water 5%
Byprods 1%
C3
Inerts
48%
C3= 39%
Acrylo Water 13% Byprods
Solvent
D1
D2
D3
42
Extractive Distillation - Internal Solvent
Reactor Products
Solvent rich in C3
=, Acrylo., Byproducts
D1 D2 D3
D1 D2
Acrylo from main process
Acrylo rich in C3
=, H2O, Byprods.
Acrylo H2O,
Byprods.
Recycle C3=
Acrylo H2O,
Byprods.
Recycle C3=
Solvent Inerts and C3
0
Inerts and C30
Reactor Products
Scheme 1 - External Solvent
Scheme 2 - Internal Solvent (Acrylonitrile)
43
Vapour Flowrate Approach (1)
1
Favour sequence with the lowest vapour flowrate
Fv Q Oper Cost
Fv Dc Capital Cost
Vapour flowrate estimation
V = D(1 + R)
Define RF = R/Rmin
V = D(1 + RFRmin) (1)
44
Vapour Flowrate Approach (2)
Underwood Equation:
Rmin = 1 - 1
xDLK
xFLK xDHK xFHK
-
2
~ 0 - Assuming sharp separation and LK and LLK o.head
(2) Rmin =
1 - 1
F
D
(Use Underwood Eqn to calculate Rmin)
45
Vapour Flowrate Approach (3)
Combine (1) and (2), (Vapor Flowrate and Rmin Equations):
V = D 1 + RF F - 1 D
V = (FA + FB + ..... + FLK) + (FA + FB .... + FLK + FHK + ...+ FNC) RF - 1
Distillate Feed
46
Example: Vapour Flowrate Approach
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.
Component Flowrate Relative Relative (kmol/h) volatility volatility between adja. comps.
Benzene 269 3.53
Toluene 282 1.80
Ethyl Benzene 57 1.0
1.96
1.80
RF=1.1
47
For the direct sequence S V = 269 + (269 + 282 + 57) 1.1 + 282 + (282 + 57) 1.1 (1.96 -1) (1.8 -1) = 965.7 + 748.1 = 1713.8 kmol/h
269 282 57
269 0 0
0 282 0
0 0 57
Solution: Vapour Flowrate Approach
48
For the indirect sequence
S V = (269 + 282) + (269 + 282 + 57) 1.1 + 269 + (269 + 282) 1.1 (1.8 -1) (1.96 -1) = 1387 + 900.4 = 2287.4 kmol/h
0 282 0
269 0 0
0 0 57
269 282 57
Hence, the direct sequence should be used
49
Assignment 1: Separation Synthesis 1
A stream is to be separated of Methane (bp –161oC), benzene (bp 80oC), toluene (bp 110oC) and orthoxylene (bp 144oC) with a composition 0.50, 0.10, 0.10, 0.30respectively. What sequence of boiling-point exploitations will probably lead to the most economic separation?
(a) Use the heuristic approach (b) Verify your answer in (a) using the vapour flowrate approach Given Relative volatility 3.70, 2.5, 1.7 and 1.0 RF = 1.1
50
Group session: Separation Synthesis 2
Methanolamine can be produced as follows:
O
Ehtylene Ammonia Methanolamine (MEA)
Oxide (EO)
MEA + EO NH(CH2CH2CH2OH)2
(DEA)
DEA + EO N(CH2CH2OH)3
(TEA)
H2C – CH2 + NH3 NH2CH2CH2OH
51
Group session: Separation Synthesis 2 (cont’)
Assuming that
· EO is fully converted
· some NH3 is unconverted
· reactor effluent contains equal amount of NH3, MEA, DEA and TEA
· separation by distillation is desirable
· the boiling point order is according to NH3 << MEA << DEA << TEA
Using only the assumption provided, generate all possible distillation
sequences, and, determine the sequence that would give rise to the
lowest separation load using a short-cut approach. (Hint: You neither
need the Antoine Equation nor the relative volatility)
52
Assignment 3: Separation Synthesis and Simulation
Ethanol-Water system is known to form a homogeneous
azeotrope at a binary composition of 96:4 percent
ethanol:water. Using ethylene glycol as an entrainer,
(a) Synthesise a separation system to recover a minimum
composition of 99 percent ethanol.
(b) Show the separation scheme on a right ternary diagram.
(c) Simulate the separation sequence to obtain the specified
recovery.
(d) Using 3 different (i) reflux ratios; (ii) entrainer flowrate,
observe the product purity variation.
Note: this type of separation is known as homogeneous
azeotropic distillation or better known as extractive distillation.
53
Why distillation?
• Able to handle a large and a wide range of throughputs
• Able to handle a wide range of feed concentrations
• Able to produce a high product purity
Advantages of distillation:
54
Why not distillation?
Separation of low molecular weight materials
Separation of high molecular weight, heat sensitive materials
Separation of components with a low concentration
Separation of different classes of components
Separation of mixtures with low relative volatility or which exhibit
azeotropic behaviour
Separation of volatile liquid from non-volatile components
Separation of mixtures of condensible and non-condensible
components
55
Packed or Plate Column? Factors dependent upon the system:
• If system has a foaming/bubbling tendency, check packed; if not, both • If system contains solid or sludge, check plate; if not, both • If the constituents are corrosive fluids, check packed; if not, both • If system has heat of solution difficulties check plate, if not, both • If operation is irregular/intermittent, check plate; if not both • If the system is small (column < 2 ft) check packed; if not, both • If system is temperature sensitive, check packed; if not, both • If system has close boiling components, check packed; if not, both • If system is viscous/sticky, check packed; if not, both
Packed column
Plate column
56
Packed or Plate Column? - Cont.
Factors dependent upon the flow regime in the column:
• If resistance to mass transfer is controlled by gas phase, check packed; if controlled by liquid phase, check plate; if no phase is controlling, check both • If system requires wide variations in liquid and/or gas rates, check plate; if not, both • If liquid holdup is undesirable, check packed; if not, both • If column DP is to be kept low, check packed; if not, both
57
Packed or Plate Column? - Cont.
Factors dependent upon the physical nature of the column:
• If frequent cleaning is expected, check plate; if not, both • If weight is critical, check plate; if not, both • If side streams are to be employed, check plate; if not both • If diameter of column < 2ft, check packed; if not, both • If overhead clearance is critical, check packed; if not both. • If floor space is critical, check plate; if not both.
58
Alternatives to Distillation
Extraction
Extractive Distillation
Azeotropic Distillation
Reactive Distillation
Crystallization
Adsorption
Reaction
Membrane Separation
Alternatives to Distillation
59
Extraction
B S
C
1
2 3
4
5
6
7
B + C
S
B + S
B
C
B + C
C (+ B)
C + S
(+ B)
1
2 3
4
5
6
7
Refer R.E. Treybal on
the use of liquid extraction
60
Extractive Distillation
C1
C2
A+B
S
A
B
S S
B A
B S
azeotrope
e.g., A = Ethanol
B = H2O
S = Ethylene Glycol
mix split
C1
C2
A+B
61
Azeotropic Distillation
C1 C2
C3 A
B
A + B azeotrope
B
ABS Ternary heterogeneous azeotrope
A
A + B azeotrope
e.g., A = Ethanol
B = H2O
S = Benzene
62
Reactive Distillation
C1
C2
B
C
C + S
S
C (para-xylene)
B(meta-xylene)
Add an entrainer to react with one component in a mixture that is difficult to separate (e.g. xylene isomers) B, C = meta- & para-xylenes: a = 1.03 S = Organometallic, e.g. sodium cumene; C (para) reacts with S; aB:CS ~ 30 Separation greatly simplified, but, problems in handling sodium cumene Important alternative if a simpler to handle entariners can be found
63
THANK YOU
64
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