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 services Human resources Maintenance Waste 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.

2007 June 2 CHEMICAL ENGINEERING PROCESS DESIGN Preface Introduction Part I – Principles of Chemical Process Design 1. The Process Design Mandate 2. Documentation and Communication 3. Synthesis 4. Theory and Experiment in Support of Design 5. Operating Problems: Solution by Design 6. Process Monitoring and Control 7. Designing for Health and Safety 8. Environmental Protection; Conservation 9. Project Economics 10. 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

Appendices A Estimation of Chemical and Physical Properties B Mathematical Support and Methods C Materials of Construction D Services and Utilities E Equipment Drives F Six Sigma and ISO G Project Management H Process Simplification and Value Engineering I Patents J 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.

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 least less 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

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.

A Vari-Stage Process

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

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

A Two-Stream Process

A Batch Two-Stream Process

Batch Cross-Current Analogue

Batch Counter-Current Analogue

(‘D’ is the amount of fouled material)

20 C 200 C 152 C 108 C Counter-Current 20 C 200 C 122 C 128 C Co-Current 20 C 200 C 109 C 138 C Plug-Mixed Mixed-Mixed 20 C 200 C 102 C 143 C Figure 3. – Efficacy of Various Two-Stream Configurations