ch3 synthesis lecture number1
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Chemical Engineering Process Design
PROCESS SYNTHESIS
Keith Marchildon
David Mody
Process synthesis has been defined as the science of arriving in a systematic manner at a flowsheet which is optimized with respect to some objective function.
What objective function?
Any constraints?
Is a “systematic manner” possible?
Process synthesis is more akin to the work of an artist who, while drawing on common principles of technique and using tools that are available to all, uses his or her experience and inner imagination to create an original work.
Combining
Capital Cost with Operating Cost-------
** Depreciation **Raw materials
Energy and other servicesHuman resources
MaintenanceWaste disposal
Typical Optimization Choices
Adding equipment (capital cost) to capture process heat and reduce energy consumption (operating cost)
Using energy to power purification columns that increase yield from raw materials – i.e., increasing one operating
cost to reduce another
Automating to reduce the number of operating personnel
Increasing vessel size and hold-up time to allow a decrease in reactor temperature that lessens waste production.
Ways to Keep the Plant Operating (out of 8766 days per year)
adequate process monitoring and sampling, for early detection and diagnosis of problems
storage capacity for raw materials, product, and intermediate streams, in order to buy time and keep the plant operating if there is a difficulty at one point
redundancy of ancillary equipment such as pumps
ability to handle a range of throughputs, below and above the flowsheet values.
Externally Set Parameters
production rate
product quality
unit cost for raw materials and for services
raw material characteristics
environmental regulations.
CapitalFacility
Raw materials
Energy andother services
Humanresources
Maintenance
Depreciation
Up-time
Product
Usefulco-products
Physical loss ofreactant,product,
intermediates
Chemical loss ofnon-useful products
Disposal
Figure 3.1 – Cash-Carrying Streams in a Chemical Process
CapitalFacility
Raw materials
Energy andother services
Humanresources
Maintenance
Depreciation
Up-time
Product
Usefulco-products
Physical loss ofreactant,product,
intermediates
Chemical loss ofnon-useful products
Disposal
CapitalFacility
Raw materials
Energy andother services
Humanresources
Maintenance
Depreciation
Up-time
Product
Usefulco-products
Physical loss ofreactant,product,
intermediates
Chemical loss ofnon-useful products
Disposal
Figure 3.1 – Cash-Carrying Streams in a Chemical Process
2007 June 2CHEMICAL ENGINEERING PROCESS DESIGNPrefaceIntroductionPart I – Principles of Chemical Process Design1. The Process Design Mandate2. Documentation and Communication3. Synthesis4. Theory and Experiment in Support of Design5. Operating Problems: Solution by Design6. Process Monitoring and Control7. Designing for Health and Safety8. Environmental Protection; Conservation9. Project Economics10. Estimation of Capital and Operating Costs
Part II – Operations and Equipment 11. Bulk Transport and Storage 12. In-Plant Transfer of Solids and Liquids 13. Transfer of gases; Compression and Vacuum 14. Formation and Processing of Solids 15. Heating, Cooling and Change of Phase 16. Mixing and Agitation 17. Mechanical Separations 18. Molecular Separations 19. Chemical Reaction 20. Integrated Reaction and Separation
AppendicesA Estimation of Chemical and Physical PropertiesB Mathematical Support and MethodsC Materials of ConstructionD Services and UtilitiesE Equipment Drives F Six Sigma and ISOG Project ManagementH Process Simplification and Value EngineeringI PatentsJ Plant Location and Lay-Out
The Rate Concept
Rate = Rate Coefficient x zone of action x
driving force
For convective heat transfer this becomes
Rate of heat transfer = Heat transfer coefficient x
area normal to the flow of heat x temperature difference
Two key characteristics:
if any one of the three terms on the right side is increased, the whole rate is increased proportionately,
if any one of the three terms goes to zero, the rate goes to zero.
Temperature
Pellet center
Ambient gas
1
2
Figure 3.2 – Pellet Heating
Temperature
Pellet center
Ambient gas
1
2
Figure 3.2 – Pellet Heating
Look for the Controlling Rate
Figure 3.3 - Reaction and Mass Transfer in a Bubbling Reactor
C
12
0 [C] [C] vle
RATE OFREACTION
RATE OFMASS
TRANSFER
3
Figure 3.3 - Reaction and Mass Transfer in a Bubbling Reactor
CCC
12
0 [C] [C] vle
RATE OFREACTION
RATE OFMASS
TRANSFER
31
2
0 [C] [C] vle
RATE OFREACTION
RATE OFMASS
TRANSFER
31
2
0 [C] [C] vle
RATE OFREACTION
RATE OFMASS
TRANSFER
3
Look for the Controlling Rate
ACHIEVING DRIVING FORCE: SOME PATTERNS IN
SINGLE-STREAM PROCESSES
Batch and continuous
Plug and back-mixed
Multi-stage back-mixed, the stages being similar or stages being dissimilar
Separation and recycle.
Some Advantages of Batch Processing
It is generally simpler, with less vessels or at leastless vessel types
Process development tends to be done by changing operating conditions rather than the design of vessels
There is relatively easy transition between successive product types
Incremental expansion can be low-cost: just add duplicate vessels
Batch Processing Today
Modern-day systems of distributed control incorporate recipe handling and automated addition of raw materials and additives, which relieve many operator functions
Advanced control schemes, particularly model-based control, can track batches and keep them all to an identical process path and/or detect any that stray and require segregation.
Batch-Continuous Hybrids
A continuous processes that has batch operation somewhere along its length, usually for raw material introduction or for product handling
A batch process that has a continuous feed of some component during all or part of its course.(a ‘fed-batch’ process)
Three Continuous Styles
For single-component first-order reaction
Rate of consumption of reactant ‘C’ = k x liquid mass x [C]
In general
Extent = ( [C] no reaction - [C] ) / [C] no reaction
Table 3.1 - Relative Sizes of Various Reactor Types
********* Final Extent of Reaction, Ext final ************0.10 0.20 0.50 0.90 0.99
Plug Flow HUT 0.02 0.05 0.15 0.50 1.00
Back-mixed, one stage 0.02 0.05 0.22 1.95 21.50
2-stages - each fully back-mixed 0.02 0.05 0.18 0.94 3.913-stage reactor 0.02 0.05 0.17 0.75 2.3710-stage reactor 0.02 0.05 0.16 0.56 1.27
Comparisons
Required hold-up time falls off greatly as final extent of reaction drops
All configurations behave about the same at extents up to 0.5
At high (0.99) extent, the single well-mixed reactor requires very large hold-up time
A sequence of well-mixed stages is much more efficient than one stage and, with enough stages, can even approach the performance of plug-flow.
Figure 3.5 – Some Multi Well-Mixed-Stage ConfigurationsFigure 3.5 – Some Multi Well-Mixed-Stage ConfigurationsFigure 3.5 – Some Multi Well-Mixed-Stage Configurations
Wt%water
96 83 20
6
Figure 3.6 – Paper Making
Moving Fourdrinier wire Press felts Heated roll dryers
Wt%water
96 83 20
6
Wt%water
96 83 20
6
Figure 3.6 – Paper Making
Moving Fourdrinier wire Press felts Heated roll dryers
A Vari-Stage Process
REACTOR
SEPARATORFeed
Recycle
Product
78%conversion
99%conversion
Figure 3.7 – Recycle reactor
Separation plus Recycle
The process must be taken to a high final extent of reaction, either for reasons of product purity or because of high cost of the raw material
There is a significant reverse reaction which slows the process and limits the achievable extent
The product is susceptible to a further undesired reaction if it remains at reactor conditions
The product has a poisoning effect on a catalyst.
Situations favoring Separation + recycle
Large-particlefeed
Over-sized recycle
ProductSIZE
REDUCTION UNIT
SCREENINGUNIT
Figure 3.8 – Comminution with Recycle
A Physical example of Sep’n + Recycle
ACHIEVING DRIVING FORCE: SOME PATTERNS IN
TWO-STREAM PROCESSES
Batch and continuous
Plug and back-mixed
Multi-stage back-mixed
Co-current, cross-current, and counter-current
C
H
Figure 3.9 – Two-Liquid Heat Exchange
C
H
Figure 3.9 – Two-Liquid Heat Exchange
A Two-Stream Process
G, A
G, A L, A
L, A
Absorption
G, A
G, A L, A
L, A
Stripping
L1, A L1, A
L2, A L2, A
Extraction
Figure 3.10 – Other Two-Stream Operations
Pneumatic Conveying
G, A
G, A L, A
L, AG, A
G, A L, A
L, AG, A
G, A L, A
L, A
Absorption
G, A
G, A L, A
L, A
Stripping
G, A
G, A L, A
L, A
Stripping
L1, A L1, A
L2, A L2, A
Extraction
L1, A L1, A
L2, A L2, A
Extraction
Figure 3.10 – Other Two-Stream Operations
Pneumatic ConveyingPneumatic Conveying
Counter - Current
Co-Current
Cross-Current
Figure 3.11 – Hot-Air Drying of Solids
Counter - Current
Co-Current
Cross-Current
Counter - CurrentCounter - Current
Co-CurrentCo-Current
Cross-CurrentCross-Current
Figure 3.11 – Hot-Air Drying of Solids
0
50
100
150
200
250
0 5 10 15 20 25
Hot Air
S olids
C ounter-C urrent operation
0
50
100
150
200
250
0 5 10 15 20 25
C o-C urrent O peration
Hot Air
S olids
0
50
100
150
200
250
0 5 10 15 20 25
Hot Air in
Hot Air out
S olids
C ross -C urrent O peration
To Waste
25
CleanSolvent
2426
Figure 3.13 – Single-Flush Batch Cleaning
To Waste
25
CleanSolvent
To Waste
25
CleanSolvent
24242626
Figure 3.13 – Single-Flush Batch Cleaning
A Batch Two-Stream Process
2728
CleanSolvent
To WasteTo Waste
2526
CleanSolvent
To WasteTo Waste
Figure 3.14 – Cross-Current Flushing
2728
CleanSolvent
To WasteTo Waste
2526 2526
CleanSolvent
To WasteTo Waste
Figure 3.14 – Cross-Current Flushing
Batch Cross-Current Analogue
Cleansolvent
To waste
2526
2526
C
CC
C
C C
O O
RINSE
DRAIN
Figure 3.15 – Counter-Current Flushing
2728 Re-FILL
C C
OO
Cleansolvent
To waste
2526
Cleansolvent
To waste
Cleansolvent
To waste
2526 2526
2526 2526 2526
C
CC
C
C C
O O
RINSE
DRAIN
Figure 3.15 – Counter-Current Flushing
2728 2728 2728 Re-FILL
C C
OO
Batch Counter-Current Analogue
2726
1.0D
45kg liq,0.99D
50 kg liq, 0.0D
50 kg liq, 0.1D
5 kg liq, 0.01D
Figure 3.16 – Material Balance for Counter-Current Flushing
2726
1.0D
45kg liq,0.99D
50 kg liq, 0.0D
50 kg liq, 0.1D
5 kg liq, 0.01D
2726 2726 2726
1.0D
45kg liq,0.99D
50 kg liq, 0.0D
50 kg liq, 0.1D
5 kg liq, 0.01D
Figure 3.16 – Material Balance for Counter-Current Flushing
(‘D’ is the amount of fouled material)
20 C
200 C
152 C108 C
Counter-Current
20 C200 C
122 C
128 CCo-Current
20 C200 C
109 C
138 CPlug-Mixed
Mixed-Mixed
20 C200 C
102 C
143 C
Figure 3. – Efficacy of Various Two-Stream Configurations