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Pentane withdrawn as bottoms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n-C4 withdrawn as distillate.

2-C4=s withdrawn as distillate.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