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