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Mass Integration. CHEN 4470 – Process Design Practice Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 8 – Synthesis of Mass Exchange Networks I February 5, 2013. Mass Exchange Networks 1:7. Mass Exchange Networks 2:7. What do we know? - PowerPoint PPT Presentation
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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)
Mass Exchange Networks 2:7
• 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
Mass Exchange Networks 3:7
• 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?
Mass Exchange Networks 4:7
• 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
Mass Exchange Networks 5:7
• 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
Mass Exchange Networks 6:7
• 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
Mass Exchange Networks 7:7
• 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
The Pinch Diagram 2:6
• Constructing Rich Composite using Superposition
MR2
MR1 R1
R2
y1t y2
t y1s y2
s y
Mass Exchanged
The Pinch Diagram 3:6
• 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
The Pinch Diagram 4:6
• 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
The Pinch Diagram 5:6
• 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.
The Pinch Diagram 6:6
• 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
Rich CompositeStream
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
Mass Exchanged,10-4 kmole Benzene/s
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
Mass Exchanged,10-4 kmole Benzene/s
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
Mass Exchanged,10-4 kmole Benzene/s
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
Rich CompositeStream
Excess Capacityof Process 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
Mass Exchanged,10-4 kmole Benzene/s
0.0010 0.0030 0.0050 0.0070 0.0090x1
Load to beRemoved byExternal MSA’s
Rich CompositeStream
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
Mass Exchanged,10-4 kmole Benzene/s
0.0000 0.0020 0.0040 0.0060 0.0080x1
0.0000 0.0010 0.0020 0.0030 0.0040x2
Load to beRemoved byExternal MSA’s
Rich CompositeStream
Excess Capacityof Process MSA’s
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
Mass Exchanged,10-4 kmole Benzene/s
0.0010 0.0030 0.0050 0.0070 0.0090x1
Load to beRemoved byExternal MSA’s
Rich CompositeStream
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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