CHEN 4470 – Process Design Practice

CHEN 4470 – Process Design Practice

Dr. Mario Richard EdenDepartment of Chemical Engineering

Auburn University

Lecture No. 8 – Synthesis of Mass Exchange Networks I

February 5, 2013

Mass Integration

Mass Exchange Networks 1:7

MassExchange Network

MSA’s (Lean Streams In)

RichStreamsIn

RichStreamsOut

MSA’s (Lean Streams Out)


• What do we know?– Number of rich streams (NR)

– Number of process lean streams or process MSA’s (NSP)

– Number of external MSA’s (NSE)

– Rich stream data• Flowrate (Gi), supply (yi

s) and target compositions (yit)

– Lean stream (MSA) data• Supply (xj

s) and target compositions (xjt)

• Flowrate of each MSA is unknown and is determined as to minimize the network cost


• Synthesis Tasks– Which mass-exchange operations should be used

(e.g., absorption, adsorption, etc.)?

– Which MSA's should be selected (e.g., which solvents, adsorbents, etc.)?

– What is the optimal flowrate of each MSA?

– How should these MSA's be matched with the rich streams (i.e., stream parings)?

– What is the optimal system configuration?


• Classification of Candidate Lean Streams (MSA’s)

– NSP Process MSA’s

– NSE External MSA’s

• Process MSA’s– Already available at plant site– Can be used for pollutant removal virtually for

free– Flowrate is bounded by availability in the plant

• External MSA’s– Must be purchased from market– Flowrates determined according to overall

economics

NS = NSP + NSE


• Target Compositions in the MSA’s– Assigned by the designer based on different

considerations

– Physical• e.g., maximum solubility of the pollutant in the MSA

– Technical• e.g., to avoid excessive corrosion, viscosity or fouling

– Environmental• e.g. to comply with environmental regulations

– Safety• e.g. to stay away from flammability limits

– Economic• e.g., to optimize the cost of subsequent regeneration of

MSA


• The Targeting Approach– Based on identification of performance targets

ahead of design and without prior commitment to the final network configuration

• Minimum Cost of MSA’s– Any design featuring the minimum cost of MSA's

will be referred to as a minimum operating cost "MOC" solution

• Minimum Number of Mass Exchange Units

U = NR + NS – Ni

Number of independent subproblems into which the original synthesis problem can be devided.

USUALLY Ni = 1


• Corresponding Composition Scales

xj

EquilibriumLine

y

j

j

Practical Feasibility Region

Practical Feasibility Line

x*j = (y - bj )/mj

*i j j jy m x b

*j j jx x

*j j jx x

( )i j j j jy m x b

i jj j

j

y bx

m

Two of the most important equations to

remember in mass integration!!

The Pinch Diagram 1:6

• Amount of Mass Transferred by Rich Streams

( ) , 1, 2, ....,s ti i i i RMR G y y i N

MR2

MR1

R2

R1

y1t y2

t y1s y2

s y

Mass Exchanged


• Constructing Rich Composite using Superposition

MR2

MR1 R1

R2

y1t y2

t y1s y2

s y

Mass Exchanged


• Amount of Mass Accepted by Process MSA’s

( ) , 1, 2, ....,C t sj j j j SPMS L x x j N

MS2

MS1

S2

S1

x1s

x2s

x1t

x2t

y

Mass Exchanged

xy b

m11

1

1

xy b

m22

2

2


• Constructing Lean Composite using Superposition

MS2

MS1

S2

S1

x1s

x2s

x1t

x2t

y

Mass Exchanged

xy b

m11

1

1

xy b

m22

2

2


• Constructing the Pinch Diagram– Plot the two composite curves on the same

diagram

y

Mass Exchanged

Excess Capacityof Process MSA’s

Integrated Mass Exchange

Rich CompositeStream

Lean CompositeStream

PinchPoint

Load to beRemovedby ExternalMSA’s

xy b

m11

1

1

xy b

m22

2

2

Pinch Point

Move the lean composite

vertically until the entire stream

exists above the rich composite.

The point closest to the rich

composite is the Pinch.


• Decomposing the Synthesis Problem– Creates two subregions, i.e. a rich end and a

lean end

• Above the Pinch– Mass exchange between rich and lean process

streams– No external MSA’s required

• Below the Pinch– Both process and external MSA’s are used– If mass is transferred across the pinch, the lean

composite moves upward, thus:

DON’T TRANSFER MASS ACROSS THE PINCH!

Example No. 1 1:14

• Benzene Recovery from Polymer Production

Monomers

SolventMakeup

First StageReactor

Second StageReactor Separation

Copolymer (to Coagulation and Finishing)

Catalytic Solution

(S2)

Extending Agent

Recycled Solvent

Unreacted Monomers

GaseousWaste (R 1)

MonomersMixingTank

AdditivesMixing

Column

Inhibitors+ Special Additives

S1

Example No. 1 2:14

• Rich Stream Data

• Candidate MSA’s– Two process MSA’s– One external MSA

Stream

Description

Flowrate

Gi, kgmole/s

Supply Composition

(mole fraction) yi

s

Target Composition (mole fraction)

yit

R1

Off-gas from Product

Separation

0.2

0.0020

0.0001

Example No. 1 3:14

• The Process MSA’s– Additives (S1)

• The additives mixing column can be used as an absorption column by bubbling the gaseous waste into the additives

– Liquid Catalytic Solution (S2)

1 10.25 , 0.001y x

2 20.50 , 0.001y x

Example No. 1 4:14

• The Process MSA’s (Continued)

• The External MSA (S3)– Organic oil, which may be regenerated by flash

sep.– Operating cost is $0.05/kgmol of recirculating

oil

Stream

Description

Upper Bound on Flowrate

L jC

kgmole/s

Supply Composition

of Benzene

(mole fraction) xj

s

Target Composition

of Benzene

(mole fraction)

xjt

S1 Additives 0.08 0.003 0.006

S2 Catalytic Solution 0.05 0.002 0.004

3 30.10 , 0.001y x

Example No. 1 5:14

• The External MSA (S3) (Continued)

Stream

Description

Upper Bound on Flowrate

L jC

kgmole/s

Supply Composition

of Benzene

(mole fraction) xj

s

Target Composition

of Benzene

(mole fraction)

xjt

S3 Organic Oil 0.0008 0.0100

Monomers

SolventMakeup

First StageReactor

Second StageReactor Separation

Copolymer (to Coagulation and Finishing)

Catalytic Solution

Additives (Extending Agent, Inhibitors

and Special Additives)

Recycled Solvent

Unreacted Monomers

GaseousWaste

Mixing

Oil

S3

Rege

neration

OilMakeup

Benzene Recovery MEN R1

S1S2

ToAtmos-phere

Benzene

Example No. 1 6:14

• Constructing the Pinch Diagram– Constructing the rich composite curve

6.0

0.0001 0.0005 0.0010 0.0015 0.0020 0.0025 y0.0

2.0

1.0

3.0

4.0

5.0

Mass Exchanged,10-4 kmole Benzene/s


0.0000

3.8

Example No. 1 7:14

• Constructing the Pinch Diagram (Continued)– Constructing the lean composite curve

0.0001 0.0005 0.0010 0.0015 0.0020 0.0025 y0.0

2.0

1.0

3.0

4.0

5.0

6.0


0.0010 0.0030 0.0050 0.0070 0.0090x1

0.0000 0.0010 0.0020 0.0030 0.0040x2

0.0000

2.4

0.00175

0.006

3.4

S1

S2

Example No. 1 8:14

• Constructing the Pinch Diagram (Continued)– Constructing the lean composite curve

0.0001 0.0005 0.0010 0.0015 0.0020 0.0025 y0.0

2.0

1.0

3.0

4.0

5.0

6.0


0.0010 0.0030 0.0050 0.0070 0.0090x1

0.0000 0.0010 0.0020 0.0030 0.0040x2

0.0000

2.4

0.00175

0.006

3.4

S1

S2

LeanComposite

Stream

Example No. 1 9:14

• Constructing the Pinch Diagram (Continued)– Plot the two composite curves on the same

diagramLean Composite

Stream

0.0001 0.0005 0.0010 0.0015 0.0020 0.0025 y0.0

2.0

1.0

3.0

4.0

5.0

6.0


0.0010 0.0030 0.0050 0.0070 0.0090x1

0.0000 0.0010 0.0020 0.0030 0.0040x2

Load to beRemoved byExternal MSA’s



PinchPoint

0.0000

3.8

0.00175

0.006

1.8

4.2

5.2

Load to BeRemoved By

External MSA’s

IntegratedMass

Exchange

Pinch Point

y = 0.001

x1 = 0.003

x2 = 0.001

Excess Capacity of

Process MSA’s

(5.2 – 3.8)*10-4

=

1.4*10-4 kgmole benzene/s

External MSA Load

1.8*10-4 kgmole benzene/s

Example No. 1 10:14

• Removing Excess Capacity– Infinite combinations of L1 and x1

out capable of removing the excess

– Additives column will be used for absorption, thus all of S1 (0.08 kgmole/s) should be fed to this unit.

1 1 1 1( )out SMS L x x

41 12 10 ( 0.003)outL x

41 12 10 0.08( 0.003) 0.0055out outx x

Example No. 1 11:14

• Removing Excess Capacity (Continued)– Graphical identification of x1

out

0.0001 0.0005 0.0010 0.0015 0.0020 0.0025 y0.0

2.0

1.0

3.0

4.0

5.0

6.0


0.0010 0.0030 0.0050 0.0070 0.0090x1



PinchPoint

0.0000

3.8

0.00175

0.006

1.8

4.2

IntegratedMass

Exchange

0.0055

S1

Example No. 1 12:14

• Identifying the Optimal Value of ε1

– Pinch diagram for ε1 = 0.002

LeanComposite

Stream

0.0001 0.0005 0.0010 0.0015 0.0020 0.0025 y0.0

2.0

1.0

3.0

4.0

5.0

6.0


0.0000 0.0020 0.0040 0.0060 0.0080x1

0.0000 0.0010 0.0020 0.0030 0.0040x2




PinchPoint

0.0000

3.8

2.3

4.7

5.7

Load to BeRemoved By

External MSA’s

IntegratedMass

Exchange

0.0030

0.00125

External MSA Load

Increased from 1.8 to 2.3*10-4 kgmole

benzene/s

Thus optimal value of ε1 is the feasible minimum, i.e. 0.001

Example No. 1 13:14

• Remaining Problem (Below the Pinch)– Optimizing the use of external MSA’s

0.0001 0.0005 0.0010 0.0015 0.0020 0.0025 y0.0

2.0

1.0

3.0

4.0

5.0

6.0


0.0010 0.0030 0.0050 0.0070 0.0090x1



PinchPoint

0.0000

3.8

0.00175

0.006

1.8

4.2

IntegratedMass

Exchange

0.0055

S1

Example No. 1 14:14

• Remaining Problem (Below the Pinch)– Optimizing the use of external MSA’s

Key Results

Optimal flowrate of S3

L3 = 0.0234 kgmol/s

Optimal outlet composition of S3

X3out = 0.0085

Minimum TAC

$41,560/yr

y1t = 0.0001

Gaseous Waste, R1

G1 = 0.2 kgmole/sy1

s = 0.0020

x3out = 0.0085

Regenerated Solvent, S3

L3 = 0.0234 kgmole/sx3

s = 0.0008

Regeneration

Makeup

ypinch = 0.0010Additives Mixture, S1L1 = 0.08 kgmole/sx1

s = 0.0030

x1out = 0.0055

• Next Lecture – February 7– Finalize mass exchange network synthesis– SSLW pp. 297-308

• Progress Report No. 1– Due Friday February 8– Remember to fill out team evaluation forms

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CHEN 4470 – Process Design Practice

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