Process Design Lecture5

7/29/2019 Process Design Lecture5

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Multi-component Separations Involving High-Recovery or Sharp Product

Streams

Feed: Species Moles/hr NBP, TC

H2 :Hydrogen(Component A) 18 -253

C1_ :Methane(B) 5 -161

C2o:Ethylene(C) 24 -104C2_:Ethane(D) 15 -88

C3o :Propylene(E) 14 -48

C3+ :Propane(F) 6 -42

C4 :Heavies(G) 8 -1

Products: AB, C, D, E, F, G

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(1) Forsharp product streams, we normally use all-sharp or high-recovery separation sequences to

separate the feed into products. In such sequences,

each component being separated appears almost

completely in one and only one product.

(2) Key component in an all-sharp separation are

commonly defined by:

The light key (LK) is the lightest component in the

bottoms and the heavy key (HK) is the heaviest

component in the overhead.

Definitions:

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DEMETHANI

ZER

C1

SPLITTER

DEPROPANIZER

C2

SPLITT

ER

DEETHANIZ

ER

Feed .

(A, B)

H2 . C1

(C, D)"

2

2

2 .CC

(C)2

2C

(D)"

2C

4

"

1

2

1

"

2

2

2 .... CCCCC

(C, D, E, F, G)

4

"

1

2

1 .. CCC

(E, F, G) "1

2

1 .CC

(E, F)

21

C

(E)

4C

(G)"

1C

(F)

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(sequence a) (sequence b)

A(LK)B(HK)C

DEF

G

A

B(overhead)C

BCDEFG

(bottoms)

CD

EFG

C

D

EF

G

E

F

98.0BLK

dd

(component Bs recovery

fraction in the overhead)98.0 CHK bb

(component Cs recovery

fraction in the bottoms)

ABCDEF

G

A

BCD

E

FG

AB

CD

EF

G

C

D

E

F

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How do you synthesize these two

industrial separation sequences ?

Which sequence is better ?

Does your ranking vary with feed

conditions ?

Questions:

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Possible Sequencesfor a 4-Component Feed

ABCD

BCD

CD

A B C

D

(Direct sequence)

Sequence (a) and (b)

ABCD

BCD

BC

A B

CD

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Possible Sequencesfor a 4-Component Feed

ABCD

AB

CD

Sequence (c)

A C

B D

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Possible Sequences

for a 4-Component Feed

ABCD

ABC

BC

A B

C(Indirect sequence)

Sequence (d) and (e)

ABCD

ABC

AB

A

BCD D

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FOR A FOUR-COMPONENT PROCESS FEEDSUBGROUPS

Process Feed

First Separator

Feeds to Subsequent

Separators Products

A

B

C

D( )decreasing volatility

A A

B B

C

B

B C

C

D C

D

(A)

(B)

(C)

(D)

(

(

)

)

(

(

(

)

)

)

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FOR A FOUR-COMPONENT PROCESS FEEDUNIQUE SPLITS

Splits for

First Separator

A

B

C

D

( )

A

B

C

A

B

C

B

C

D

B

CD

( )

( )

( )

(

(

)

)

Splits for subsequent Separators

(

(

)

)

A

B

BC

C

D

A

B

C

D

A

B

C

D

(( ))10


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Sequences of Two-Product Separators

The Combinatorial Problem:Total number of possible sequences for Ncomponents(products) if only type of separator isused (e. g. ordinary distillation)=

Number of unique feed and product groups =

Number of unique splits =

!1!

!12

NN

N

SN

2

1

NNG

6

11

NNNU

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NUMBER OF SEPARATORS, SEQUENCES,

SUBGROUPS, AND UNIQUE SPLITS FOR SIMPLE

SEQUENCES USIGN ONE SIMPLE METHOD OF

SEPARATION

Number of S, G, U,Number of Separators in Number of Number of Number of

Components a Sequence Sequences Subgroups Unique Splits2 1 1 3 13 2 2 6 44 3 5 10 105 4 14 15 206 5 42 21 35

7 6 132 28 568 7 429 36 849 8 1430 45 120

10 9 4862 55 16511 10 16796 66 220

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n-Butylene Purification by Ordinary Distillation and Extractive Distillation

Feed: Relative Volatility*Species Mole % ()I ()II

A: Propane 1.47

B: 1-Butene 14.75

C: n-Butane 50.29

D: trans-Butene-2 15.62

E: cis-Butene-2 11.96F: n-Pentane 5.90

*()I = adjacent relative volatility at 150 F for separation

method I, ordinary distillation()II = adjacent relative volatility at 150 F for separation

method II, extractive distillation with furfural.

(C4H3OCHO)

Products: A, B, C, DE and F.

1.03

2.451.18

2.50

1.17(nC4/1-C4)

1.17(nC4/T-2-C4)

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How do you get to following industrial separation sequence?

A

BCDEF

Extractive

Distillatio

n

Solvenet

Recovery

CDE II

FEED

C3F

CDE

1-BUTENE COLUMN

AB

DEPROPANIZER

C3A

1-BUTENE B

C+S

DE+ Solvent

DE2-BUTENES

Cn-BUTANE

RECIRCULATEDSOLVENT

DEOILEREXTRACTIVEDISTILLATION

COLUMN

SOLVENT STRIPPER

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A

B

C

DE

F

A

B

C

D

E

F

A

B

CD

E

F

( )II

C

D

E

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Definition:

Extractive distillation is a form of distillation involving the

addition of a solvent which modifies the vapor-liquid equilibria

of the components to be separated such the separation becomes

easier. The added solvent has a volatility lower than the

components to be separated (i.e., the added solvent has a boiling

point higher than those of the components to be separated). Also,

it is usually introduced near the top of a column.

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Examples of Extractive Distillation :

Mixture Solvent

1-Butene(-6.3 C) and 1,3

Buradiene(-4.41C) Acetonitrile(81.6 C)

Nitric Acid(83 C) and Water

(100C) Sulfuric Acid(300 C)

Meth1 Cyclohexane(100 C) and

Benzene(80.1 C) N-Formylmorpholine(243 C)Isooctane(99.2 C) and doiuene

(110.6 C) Phenol(181.75 C)

Monomethyiamine(-6.3 C).

Dimethylamine(7.4 C) and

Trmethylamine(2.87 C) Water(100 C)Methy1 Cyclohexane(100 C) and

Toluene(110.6 C) Phenol(181.75 C)

Acetone(56.2 C) and Methanol

(62.5 C) Water(100 C)

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Example 1 Extractive Distillation

Normal Boiling Points, C

MCH 100.9

Toluene 110.6

Phenol Feed

(Solvent)CH3C6H5: Toluene(T)

CH3C6H11: MCH(M)

181.75 extractive

distillation

MCH(M)

Solvent

R

ecovery

Toluene(T)

Phenol(P)

(Recycled

solvent)

Phenol(P):C6H5OH

(Make-upsolvent)

CT 7.9

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If :1. T type of two-product separators are allowed.2. Any mass-separating agent is recovered for recycle in

the separator following the one into which it is introduced.

Then :

For example :N = 4 components

Ordinary distillation only gives

SN = 5

Ordinary distillation plus extractive distillation with phenol gives

1

!1!

!12

NN T

NN

NS

110

14

24862

14862

10

4025

N

N

N

S

TS

N

S

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HEURISTICS

Heuristics Used in Heuristic and Evolutionary Strategies forSynthesis of Separation Sequences

Type SeparatorsReference Applied Heuristics Used

Lockhart (1947) Distillation 1, 6

Harbert (1957) Distillation 2, 3

Rod & Marek (1959) Distillation 4

Heaven (1969) Distillation 1, 2, 3, 5Rudd and his co- General 1, 2, 3, 6, 8

workers (1971-73) 12, 13

King (1971) and General 1, 2, 3, 7, 11

Thomp son and King

(1972a, b)Stephanopoulos (1974), General 7 plus evol.

and Stephanopoulos rules

and Westerberg (1976)

21

HEURISTICS


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HEURISTICS

Heuristics Used in Heuristic and Evolutionary Strategies forSynthesis of Separation Sequences

Type SeparatorsReference Applied Heuristics Used

Freshwater & Henry (1975) Distillation 1, 2, 3, 5, 6

Mahaec (1976) and General 6, 11 plus evol.

Mahalec & Motard rules

(1977a, b)Seader & Weaterberg General 1, 2, 3, 9, 11, 12,

(1977) 13, 18 plus

evol. rules

Nath & Motard (1978) General 9, 10, 11, 14, 15

17, 19 plus

evol. Rules

Doukas & Luyben (1978) Distillation 1

Hartmann (1979) and General 1, 2, 3, 6, 8, 16

Hartmann and 19

Hacker (1979)22


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Heuristic Rules.

1. Remove components one-by-one as overhead products.2. Save the most difficult separation for last.

3. Favor 50-50 splits.

4. Sequence with the minimum total vapor flow.

5. Make high recovery fractions last.

6. Separate the more plentiful components first.

7. Choose the cheapest as the next separator.

8. Remove the thermally unstable and corrosive material early.

9. Perform least-tight separation first.

10. Favor sequences with the smallest product set.11. Avoid separations using a mass-separating agent (MSA).

12. Remove a MSA from one of the products in another,

subsequent separation product.

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Heuristic Rules

13. A separation method using a MSA cannot be used to isolate.

14. Favor distillation.

15. Separate first the components which might undergo undesirable

reactions.

16. Set splits fractions of the key components to pre-specified values.

17. Avoid extreme processing conditions.

18. Favor ambient operating pressure.

Nishida, Stephanopoulos, Westerberg(1980)

24

H i ti S th i f Hi h R Sh M lti t


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Heuristic Synthesis of High-Recovery or Sharp, Multi-component

Separation Sequences (Nadgir and Liu, 1983; Liu, 1987)

Classification of Heuristics

Method Heuristics (M Heuristics) :Favor the use of certain separation methods under given problem specifications

Design Heuristics (D Heuristics) :Favor specific separation sequences with certain desirable properties

Species Heuristics (S Heuristics) :Based on the property differences between the species to be separated

Composition Heuristics (C Heuristics) :Related to the effects of feed and product composition on separation costs

Nadgir, V. M. and Y. A. Liu, Studies in Chemical Process Design and Synthesis. 5. ASimple Heuristic Method for Systematic Synthesis of Initial Sequences forMulti-component Separation, AIChE Journal, 29, 926-934 (1983).

Liu, Y. A., Process Synthesis: Some Simple and Practical Developments, Chapter 6,in Recent Development in Chemical Process and Plant Design, Y. A. Mebee, Jr. and W.R. Epperly, editor, Wiley, NY (1987), pp. 147-168 and 245-260.

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A Si l R k O d d H i ti M th d


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A Simple, Rank-Ordered, Heuristic Method(Nadgir and Liu, 1983)

a. Decide the separation method to be used :

M1: Favor ordinary distillation

M2: Avoid vacuum and refrigeration

b. Identify the forbidden splits, and essential first and last separations :

D1: Favor smallest product set

S1: Remove corrosive and hazardous components first (also : reactivecomponent, monomer)

S2: Perform difficult separations last

c. Synthesize the initial separation sequences :

C1: Remove most plentiful component first

C2: Favor 50/50 split

d. Remove products and recycle stream as distillates.

If not possible, take vapor from reboiler.26


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Rank-Ordered Heuristics:(1) The heuristics are to be applied one by one in the given order.

Higher-ranked heuristics appear first.

(2) If any heuristic is not important in, or not applicable to, the

synthesis problem, the next one in the method is considered.

(3) If two heuristics give different recommendations regarding thenext split, we should follow the guideline suggested by the

higher-ranked heuristic.For example, heuristic C1 overrules heuristic C2.

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Heuristic M1 (Favor Ordinary Distillation)

a. All other things being equal, favor separation methods using only

energy separating agent (e.g., ordinary distillation), and avoidusing separation methods (e. g., extractive distillation) which

require the use of species not normally present in the processing,

i. e., the mass separating agent (MSA).

b. If the separation factor or relative volatility of the key components< 1.10, then the use of ordinary distillation is not

recommended. or

c. An MSA may be used provided it improves .

d. When an MSA is used, remove it immediately follow the separatorinto which it is used. In other words, always try to remove MSA early.

HKLK,

HKLK,

CT 10

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Heuristic M1 (Favor Ordinary Distillation)

A LLK Light Component

B LK Light Key (e. g. 98% of B appears in overhead)

C HK Heavy Key (e. g., 97% of C appears in bottoms)

D HHK

E HHK

Increasing

Normal

BoilingPoint BC or HKLK,

Heavy

Components

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Minimumrequiredforconsideration

ofextractivedistillation

or

Minimum

requiredfor

considerationofL/Lextraction

Sounders, M., CEP, 60 (2) , 75-82 (1964) 30

Example 2 Extractive distillation


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Add nonvolatile component to modifys

Example 2 Extractive distillation.

B HNO3

S

HNO3BC

H2O

CH2O

SH2SO4

H2OCS

H2SO4

e. g., B = HNO3

C = H2O

S = H2SO4

31

Example 3 Azeotropic Distillation


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Add component that forms an azeotropewith one or more of feed component

Example 3 Azeotropic Distillation.

e. g., B = Ethanol

C = Water

S = Benzene

volatile

S-rich

C-rich

recycle

BCS ternary heterogeneousazeotrope

B+C azeotrope

Cwater

BC

Bethanol

B+Cazeotropa

32

E t ti


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Extraction

B

B+S

B+C

C

C(+B)

B+C

C+S(+B)

S

B S

C

a B/C distillation

33

R ti Di till ti


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Reactive DistillationAdd reactive component to modify s

C

SB

B

C

C+S

e. g., B, C = xylenes: = 1.03S = organometallic: B, CS: 30

B : meta-xyleneC : para-xyleneS : sodium cumene

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Heuristic M2 (Avoid Vacuum and Refrigeration)

a. All other things being equal, avoid excursions in temperature

and pressure, but aim high rather than low.

b. If vacuum operation of ordinary distillation is required, liquid-

liquid extraction with various solvents might be considered.

c. If refrigeration is required, cheaper alternatives to distillationsuch as absorption might be considered.

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Relative Costs of Cooling and Heating at Different

Temperatures (Berthouex and Rudd, 1977)

Roomtemperature

Cooling water

Ammonia refrigerant

Liquid nitrogen

$/Kcal

Temperature

Low pressuresteam

High pressure

steam

Fuel oilBurner

Heating

Cooling

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-100-200 0 100 200 300 400 500 600Atmospheric boiling point, C

Recommended Ranges of Pressure and Temperature

for Separation Operations (Souders, 1964)

Favor: High P and Low T

Low P and High T"

Avoid:

High P and Tor

Low P and T"

Pressureofoperation,

atm

100

10

1.0

0.1

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Heuristic D1 (Favor Smallest Product Set)

When multi-component products are specified, favor sequences

that yield these products directly or with a minimum of blending,

unless separation factors or relative volatilities are appreciably

lower than those for a sequence which requires additional

separators and blending.

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An Example for Heuristic D1

Feed :

Normal BoilingSpecies Mole% Point, TC

A 25 140

B 25 160

C 25 180

D 25 200

20

20

20

T

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An Example for Heuristic D1

Good Sequences for Different Product Sets :

a. Products : A, BC, D

b. Products : A, C, BD

ABCD

A

BC

D

B

D

Product Set for Separation:

(A, BC, D)

A

BCD

A

BCD

B

CD

C

D

BD

Product Set for Separation:

(A, B, C, D)

C

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Heuristic S1

Remove corrosive and hazardous components first.

(Essential first separations)

also reactive components and monomers

Heuristic S2 (Perform Difficult Separations Last)a. All other things being equal, perform the difficult separations

last,

b. Separations where the relative volatility of the key componentsis close to unity should be performed in the absence of non-key

components. 10.1~05.1, HKLK

(Essential last separations)

CT 1041


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Heuristic C1 (Remove Most Plentiful Component First)

A product composing a large fraction of the feed should be

separated first, provided that the separation factor or relative

volatility is reasonable for the separation.

hr

molesb 1

70A

20B

10C

A 70

B 20

C 10

B

C

70A

20B

10C

A 70B 20

C 10

A

B

CT

15

CT 15

42

An Example for Heuristics S1, S2 and C1 :


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p ,

[See Rudd, et al. (1973), pp.197-199, Problem 9]

Industrial Sequence for Separating Chlorination and

Alkylation Products in the Manufacture of Detergent

I. Reactions :

(Chlorination)

(Alkylation)

HClClHCClHC 251222412(kerosene) (chlorine) (keryl (hydrogen

chloride) chloride)

C12H5Cl + C12H25 - + HCl(keryl (benzene) (keryl benzene) (hydrogenchloride) chloride)

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An Example for Heuristics S1, S2 and C1 :

[See Rudd, et al. (1973), pp.197-199, Problem 9]

Industrial Sequence for Separating Chlorination and

Alkylation Products in the Manufacture of Detergent

II. Reaction Products to be Separated into Pure Components

Normal BoilingSpecies Mole/hr Point, TC T

A: HCl 1 -85

B: Benzene 5 80

C: Kerosene 1 214

D: Keryl Benzene 1 250

E: Heavy Ends

Relative

Flow Rate( )

165

134

36

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An Example for Heuristics S1, S2 and C1 :[See Rudd, et al. (1973), pp.197-199, Problem 9]

III. Key Questions :

a. Species: Any corrosive and hazardous components?

Split A/BCD (essential first separation)

b. : Any difficult separations?

Split C/D (essential last separation)

c. C1: Any plentiful components?

Split B/CD (desirable early separation)

T

45

A E l f H i ti S1 S2 d C1


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An Example for Heuristics S1, S2 and C1 :[See Rudd, et al. (1973), pp.197-199, Problem 9]

IV. Initial Separation Sequence

AB

C

D

A(HCl)

B

C

D

B(Benzene)

C

D

C(Kerosene)

D(Keryl Benzene)

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BC D

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Heuristic C2 (Favor 50/50 Split)

If the component compositions do not vary widely, sequences

which give a more nearly 50/50 or equimolal split of the feedbetween the distillate (D) and bottoms (B) products should be

favored, provided that the separation factor or relative volatility

is reasonable for the split.

F(Feed)

100 hrmolesb1

D(overhead) 50, 40, 5

B(Bottoms) 50, 60, 95

D

Bor

B

D

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Coefficient of Ease of Separation (CES)


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Coefficient of Ease of Separation (CES)

If it is difficult to judge which split is closest to 50/50

and with a reasonable separator factor or relative

volatility, then perform the split with the value

of the coefficient of ease of separation (CES)

highest

])()log[(

1

HKLKd

b

b

dfCES

Where orDBf / BD / Such that 1f

T or 100)1( 49

Fractional recoveries of light-key(LK) and heavy-key(HK)


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components in sharp or high-recovery separations :

Overhead Bottoms

LK dLK 0.98 bLK 0.02

HK dHK 0.02 bHK 0.98

As an approximation,

To simplify the calculations of CES in sharp separations, we may

use

02.0

98.0

LKb

d

02.0

98.0

HKd

b

fCES

50

M CES (C ffi i t f E f S ti )


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More on CES (Coefficient of Ease of Separation)

Separator

Feed

100hr

moles

hr

moles

hr

moles

Overhead, D

e. g., 70, 60, 50, 30, 20

Bottoms, B

e. g., 30, 40, 50, 70, 80

Split ratio

D/B or B/D 18020

,70

30,

50

50,

60

40,

70

30

B/D D/B

(Boiling-Point Difference) or 11 , HKLK

CES T

D

Bor

B

D

1

D

Bor

B

D

Close to one, 50/50 split

The larger the or (-1), the easier the separation

T

T

51

Examples of the Synthesis of Sharp or High-Recovery Separation Sequences


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Example 1: Multicomponent Distillation Sequencing for

Thermal Cracking of Hvdrocarbons

1. Feed Normal BoilingSpecies Moles/Hr Point, T C T CES

A: Hydrogen 18 -253B: Methane 5 -161C: Ethylene 24 -104

D: Ethane 15 -88E: Propylene 14 -48F: Propane 6 -42G: Heavies 8 -1

2. Products : C, D, E, F and G.

3. Key Questions :a. Products: Any multi-component products ?

b. Species: Any corrosive and hazardous components ?c. T: Any difficult separations ?d. Moles/hr: Any plentiful components ?e. CES: Any easy and balanced (50/50) splits ?

AB,

72

92 23.0

5714.6

401.1

41 4.0

19.6

18.16

16

Example:

CESA/BCDEFG

= 0.239272

18

T

D

Bor

B

D

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1. M1 (favor ordinary distillation) and M2 (avoid vacuum and refrigeration):

Use ordinary distillation with refrigeration at high pressure.

2. D1 (favor smallest product set):Avoid splitting AB (a single product).

3. S1 (remove corrosive and hazardous components first):

Not applicable.

4. S2 (perform difficult separations last):

Split C/D and E/F last,

5. C1 (remove most plentiful components first):

Not applicable.

6. C2 (favor 50/50 split):

Split AB/CDEFG with the largest CES = 19.6

and AB as a single product.

CT

CT

FE

DC

16

6

/

/

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CES

A

B

C

D

E

F

G

A

B

CD

E

F

G

19.6

18.1

Initial

Separation

Sequence

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For separating CDEFG, splits C/D and E/F are performed last

so that the remaining splits to be chosen are CD/EFG and

CDEF/G. Split CD/EFG is done first since it has a larger CESof 28.7:

CD/EFG CDEF/G

f 28/39 8/59T 40 41

CES 28.7 5.6

55

A


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A

B

CD

E

F

G

A

B

C

D

E

FG

EF

G

C

D

C

D

EF

E

F

G

Difficult Split

Difficult Split

24 C

15 D

14 E6 F

8 G

16C

6C

? CD/EFG

? CDEF/G

This sequence performs splits C/D and E/F last are it is exactly thesame as the one being practiced in the industry.

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The second sequence can be obtained by making the split

ABCD/EFG first, which has the second largest CES of 18.1.

A

B

C

D

E

F

G

ABC

D

E

F

G

Single product?

16C(last)

6C(last)

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Another Initial Separation Sequence :

A

B

C

DE

F

G

AB

C

D

E

F

G

A

B

G

C

D

C

D16C

E

F

E

F6C

Last Separations

CES=19.6(large)

CES=18.1(2nd largest)

[See King, C. J., Separation Processes,

2nd Edition, McGraw Hill (1980), P.718]

58

Example 2: Multi-component Distillation Sequencing for n-Butylene

P ifi ti b O di d E t ti Di till ti


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Purification by Ordinary and Extractive Distillation.

1. Feed:

Relative Volatility*

Species Mole % ()I ()II (CES)I (CES)IIA: Propane 1.47

B: 1-Butene 14.75

C: n-Butane

D: trans-Butene-2 15.62

E: cis-Butene-2 11.96F: n-Pentane 5.90

*()I = adjacent relative volatility at 150 F for separation method I, ordinary

distillation()

II = adjacent relative volatility at 150 F for separation method II, ordinaryextractive distillation with furfural (C4H3OCHO)

2. Products: A, B, C, and F.(Example) (CES)I,A/BCDEF =

(CES)II,ABCDE/F =

163.2100145.253.98

47.11001 2

B

D

404.9100150.250.9490.51001 2

d

DBor

BD

2.45 2.163

1.18 1.17 3.29

1.70 1.510 35.25

2.50

50.30 1.03

3.485

9.404

DE

59


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3. Key Question :

a. Products: Any multi-component products ?

b. Species: Any corrosive and hazardous components ?

c. Mole%: Any plentiful components ?

d. Relative Volatilities: Any difficult separations ?

e. (CES): Any easy and balanced (50/50) splits ?

60


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1. M1 (favor ordinary distillation):

Use extractive distillation for C/D and ordinary distillation for all other splits.

2. M2 (avoid vacuum and refrigeration):

Use refrigeration at high pressure.

3. D1 (favor smallest product set):

Avoid splitting DE (a single product)

4. S1 (remove corrosive and hazardous components first):

Not applicable.

5. S2 (perform difficult separations last):

Split C/DE last (difficult extractive distillation), so that the added solvent

(furfural) can be recovered at the end without having to worry about its

presence as a contaminant in earlier splits.6. C1 (remove most plentiful components first):

C(50.30 mole %) dominates the feed, but it is not separated first due to S2.

Note: Heuristics are ranked in their relative importance according to their given order, e. g.,

Heuristic S2 overrides Heuristic C161

Separation Sequencing for Example 2


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7. C2 (favor 50/50 split):

(a) Split ABCDE/F first with the largest (b)Favor AB/CDE over A/BCDE

(CES)I = 9.404 (Split C/DE last)

(Split C/DE last, avoid splitting DE) A/BCDE AB/CDEf 1.47/92.63 16.22/77.88

(-1)100 145 18

CES 2.301 3.749

A

B

C

D

EF

A 1.47

B 14.75

C 50.30

D 15.62

E 11.95

F

Initial

Separation

Sequence

A

B

C

DE

F

A

B

C

DE

F

A

B

IIE

D

C

A

B

C

D

E

Documents

Process Design Lecture5