Transcript Slide 1

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
MSA’s (Lean Streams In)
Rich
Streams
In
Mass
Exchange
Network
MSA’s (Lean Streams Out)
Rich
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 (yis) and target compositions (yit)
Lean stream (MSA) data
•
•
Supply (xjs) 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)
–
–
•
Process MSA’s
External MSA’s
NS = NSP + NSE
Process MSA’s
–
–
–
•
NSP
NSE
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
Mass Exchange Networks 5:7
•
Target Compositions in the MSA’s
–
Assigned by
considerations
–
Physical
•
–
on
different
e.g., maximum solubility of the pollutant in the MSA
e.g., to avoid excessive corrosion, viscosity or fouling
e.g. to comply with environmental regulations
Safety
•
–
based
Environmental
•
–
designer
Technical
•
–
the
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
–
•
Minimum Cost of MSA’s
–
•
Based on identification of performance targets ahead of
design and without prior commitment to the final
network configuration
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
yi  m j  x  b j
*
j
y
Practical Feasibility Region
x j  x *j   j
j
x  xj   j
Equilibrium
Line
*
j
x*j = (y - bj )/mj
j
Practical Feasibility
Line
xj
yi  m j  ( x j   j )  b j
xj 
yi  b j
mj
 j
Two of the most
important equations to
remember in mass
integration!!
The Pinch Diagram 1:6
•
Amount of Mass Transferred by Rich Streams
MRi  G i ( y is  y it )
, i  1, 2,...., N R
Mass
Exchanged
R2
MR2
MR1
R1
y1t y2t
y1s
y2s
y
The Pinch Diagram 2:6
•
Constructing Rich Composite using Superposition
Mass
Exchanged
MR2
R2
MR1
R1
y1t y2t
y1s
y2s
y
The Pinch Diagram 3:6
•
Amount of Mass Accepted by Process MSA’s
MS j  LCj ( x tj  x sj )
, j  1, 2,...., N SP
Mass
Exchanged
S2
MS2
MS1
S1
y
x1s
x2s
y  b1
1
m1
y  b2
x2 
2
m2
x1 
x1t
x2t
The Pinch Diagram 4:6
•
Constructing Lean Composite using Superposition
Mass
Exchanged
S2
MS2
MS1
S1
y
x1s
x2s
y  b1
1
m1
y  b2
x2 
2
m2
x1 
x1t
x2t
The Pinch Diagram 5:6
•
Constructing the Pinch Diagram
–
Plot the two composite curves on the same diagram
Mass
Exchanged
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.
Lean Composite
Stream
Pinch
Point
Load to be
Removed
by External
MSA’s
Excess Capacity
of Process MSA’s
Integrated
Mass Exchange
Rich Composite
Stream
y
y  b1
1
m1
y  b2
x2 
2
m2
x1 
The Pinch Diagram 6:6
•
Decomposing the Synthesis Problem
–
•
Above the Pinch
–
–
•
Creates two subregions, i.e. a rich end and a lean end
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
Inhibitors
Extending
+ Special Additives
Agent
S1
Catalytic
Solution
(S2)
Monomers
Monomers
Mixing
Tank
First Stage
Reactor
Additives
Mixing
Column
Second Stage
Reactor
Solvent
Makeup
Recycled Solvent
Unreacted Monomers
Gaseous
Waste (R 1)
Copolymer
(to Coagulation
Separation and Finishing)
Example No. 1 2:14
•
Rich Stream Data
Stream
Description
Flowrate
Gi, kgmole/s
Supply
Composition
(mole fraction)
yis
Target Composition
(mole fraction)
yit
Off-gas from Product
R1
•
Separation
Candidate MSA’s
–
–
Two process MSA’s
One external MSA
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
y  0.25  x1
–
,  1  0.001
Liquid Catalytic Solution (S2)
y  0.50  x 2
,  2  0.001
Example No. 1 4:14
•
The Process MSA’s (Continued)
Stream
•
Description
Upper Bound
on Flowrate
LCj
Supply Composition
Target Composition
of Benzene
of Benzene
(mole fraction)
xjs
(mole fraction)
kgmole/s
xjt
S1
Additives
0.08
0.003
0.006
S2
Catalytic Solution
0.05
0.002
0.004
The External MSA (S3)
–
–
Organic oil, which may be regenerated by flash sep.
Operating cost is $0.05/kgmol of recirculating oil
y  0.10  x3
,  3  0.001
Example No. 1 5:14
•
The External MSA (S3) (Continued)
Stream
Upper Bound
on Flowrate
LCj
Description
Organic Oil
S3
Target Composition
of Benzene
of Benzene
(mole fraction)
xjs
(mole fraction)
kgmole/s

0.0008
0.0100
xjt
Additives
(Extending Agent, Inhibitors
Catalytic Solution and Special Additives)
Oil
Makeup
Oil
Benzene
Supply Composition
S2
S1
S3
Regeneration
Gaseous
Waste
To
Atmosphere
Benzene Recovery MEN
Monomers
Mixing
First Stage
Reactor
Second Stage
Reactor
Solvent
Makeup
Recycled Solvent
Unreacted Monomers
R1
Copolymer
(to Coagulation
Separation and Finishing)
Example No. 1 6:14
•
Constructing the Pinch Diagram
–
Constructing the rich composite curve
6.0
Mass Exchanged,
10-4 kmole Benzene/s
5.0
4.0
3.8
3.0
2.0
Rich Composite
Stream
1.0
0.0
0.0000 0.0001
0.0005
0.0010
0.0015
0.0020
0.0025
y
Example No. 1 7:14
•
Constructing the Pinch Diagram (Continued)
–
Constructing the lean composite curve
6.0
Mass Exchanged,
10-4 kmole Benzene/s
5.0
4.0
3.4
S2
3.0
2.4
2.0
S1
1.0
0.0
0.0000 0.0001
0.0005
0.0010
0.0015 0.00175 0.0020
0.0025
y
0.0010
0.0030
0.0050 0.006 0.0070
0.0090
x1
0.0000
0.0010
0.0020
0.0040
x2
0.0030
Example No. 1 8:14
•
Constructing the Pinch Diagram (Continued)
–
Constructing the lean composite curve
6.0
Mass Exchanged,
10-4 kmole Benzene/s
5.0
4.0
3.4
3.0
S2
2.4
2.0
1.0
Lean
Composite
Stream
S1
0.0
0.0000 0.0001
0.0005
0.0010
0.0015 0.00175 0.0020
0.0025
y
0.0010
0.0030
0.0050 0.006 0.0070
0.0090
x1
0.0000
0.0010
0.0020
0.0040
x2
0.0030
Example No. 1 9:14
•
Constructing the Pinch Diagram (Continued)
–
Plot the two composite curves on the same diagram
6.0
Mass Exchanged,
10-4 kmole Benzene/s
Lean Composite
Stream
5.0
Excess Capacity
of Process MSA’s
4.0
5.2
Excess Capacity
of Process
MSA’s
4.2
(5.2 – 3.8)*10-4
3.8
Pinch Point
y = 0.001
x1 = 0.003
Load to be
3.0
Removed by
External MSA’s
Pinch
Point
Integrated
Mass
Exchange
2.0
1.0
x2 = 0.001
=
1.4*10-4 kgmole
benzene/s
1.8
Rich
Composite
Stream
0.0
0.0000 0.0001
External MSA
Load
Load to Be
Removed By
External MSA’s
0.0005
0.0010
0.0015 0.00175 0.0020
0.0025
y
0.0010
0.0030
0.0050 0.006 0.0070
0.0090
x1
0.0000
0.0010
0.0020
0.0040
x2
0.0030
1.8*10-4 kgmole
benzene/s
Example No. 1 10:14
•
Removing Excess Capacity
–
Infinite combinations of L1 and x1out capable of
removing the excess
MS1  L1 ( x1out  x1S )
2  10  4  L1 ( x1out  0.003)
–
Additives column will be used for absorption, thus all of
S1 (0.08 kgmole/s) should be fed to this unit.
2  10  4  0.08( x1out  0.003)

x1out  0.0055
Example No. 1 11:14
•
Removing Excess Capacity (Continued)
–
Graphical identification of x1out
6.0
Mass Exchanged,
10-4 kmole Benzene/s
5.0
4.2
4.0
3.8
3.0
S1
Pinch
Point
Integrated
Mass
Exchange
2.0
1.0
1.8
Rich
Composite
Stream
Load to be
Removed by
External MSA’s
0.0
0.0000 0.0001
0.0005
0.0010
0.0015 0.00175 0.0020
0.0025
y
0.0010
0.0030
0.0050 0.006 0.0070
0.0090
x1
0.0055
Example No. 1 12:14
•
Identifying the Optimal Value of ε1
–
Pinch diagram for ε1 = 0.002
6.0
Mass Exchanged,
10-4 kmole Benzene/s
5.0
Excess Capacity
of Process MSA’s
Lean
Composite
Stream
5.7
4.7
4.0
External MSA Load
3.8
Integrated
Mass
Exchange
Load to be
3.0
Removed by
External MSA’s
Increased from 1.8
to 2.3*10-4 kgmole
benzene/s
2.3
2.0
1.0
Rich
Composite
Stream
Pinch
Point
0.0
0.0000 0.0001
Thus optimal value
of ε1 is the feasible
minimum, i.e. 0.001
Load to Be
Removed By
External MSA’s
0.0005
0.0010 0.00125 0.0015
0.0020
0.0025
y
0.0000
0.0020 0.0030 0.0040
0.0060
0.0080
x1
0.0000
0.0010
0.0030
0.0040
x2
0.0020
Example No. 1 13:14
•
Remaining Problem (Below the Pinch)
–
Optimizing the use of external MSA’s
6.0
changed,
e Benzene/s
5.0
y iout = 0.0001
4.2
4.0
Regenerated Solvent
Lj ?
x jin = 0.0008
3.8
Cooler
3.0
S1
Pinch
Point
Integrated
Mass
Exchange
2.0
1.0
Absorption
Column
Recovered
Benzene
1.8
Rich
Composite
Stream
Gaseous Waste
G i = 0.2 kgmole/s
y iin = 0.0010
Load to be
Removed by
External MSA’s
0.0
0.0000 0.0001
0.0005
0.0010
0.0015 0.00175 0.0020
0.0025
y
0.0010
0.0030
0.0050 0.006 0.0070
0.0090
x1
0.0055
xj
out
Flash
Column
?
Heater
Fig. 2.12. Recovery of Benzene
from a Gaseous Emission
Example No. 1 14:14
•
Remaining Problem (Below the Pinch)
–
Optimizing the use of external MSA’s
Key Results
Optimal flowrate of S3
y1t = 0.0001
Regenerated Solvent, S3
L3 = 0.0234 kgmole/s
x3s = 0.0008
Makeup
L3 = 0.0234 kgmol/s
Regeneration
Optimal outlet
composition of S3
x3out = 0.0085
ypinch = 0.0010
Additives Mixture, S1
L1 = 0.08 kgmole/s
x1s = 0.0030
X3out = 0.0085
Minimum TAC
$41,560/yr
Gaseous Waste, R1
G1 = 0.2 kgmole/s
y1s = 0.0020
x1out = 0.0055
Other Business
•
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