1. Chemical Engineering Process Design PROCESS SYNTHESIS Keith Marchildon David Mody

2. 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. A Definition

3. 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.

4. What objective function? Any constraints? Is a “systematic manner” possible?

5. 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.

6. Process synthesis is the science of arriving in a systematic and creative manner at a flowsheet and initial equipment data sheets and initial piping & instrumentation diagram which is optimized, subject to some constraints, with respect to some objective function. An Expanded Definition

7. 500 kg/h, 1.00 mf A Flowsheet or Process Flow Drawing (PFD) Equipment Data Sheets Piping and Instrumentation Diagram (P&ID) 2-inch, schedule 40

8. Combining Capital Cost with Operating Cost ------- ** Depreciation ** Raw materials Energy and other services Human resources Maintenance Waste disposal

9. 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.

10. 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.

11. Externally Set Parameters production rate product quality unit cost for raw materials and for services raw material characteristics environmental regulations.

13. 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

14. 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

15. 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

16. 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

17. 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.

18. We try to maximize the good rates minimize the bad rates Examples Good: production of desired product necessary heating and cooling flows in the right direction Bad:production of undesired product excessive heat generation heat losses leakages, waste streams frictional pressure drops

19. Look for the Controlling Rate

20. Look for the Controlling Rate-Action [C] is Dissolved concentration

21. DRIVING FORCES differences in pressure, temperature, concentration differences in some function of press, temp, or conc concentration itself

22. 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.

23. 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

24. 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.

25. 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)

26. To analysis Time Feed rate continuous batch

27. Three Continuous Styles

28. 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

30. 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.

32. A Vari-Stage Process

33. Separation plus Recycle

34. 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

35. A Physical example of Sep’n + Recycle

36. 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

37. A Two-Stream Process

38. G, A L, A Absorption G, A L, A Stripping L1, A L2, A Extraction Figure 3.10 – Other Two-Stream Operations Pneumatic Conveying

39. Counter - Current Co-Current Cross-Current Figure 3.11 – Hot-Air Heating of Solids

42. A Batch Two-Stream Process 100 kg (The numbers indicate flush, not vessel. There is only one vessel)

43. Batch Cross-Current Analogue 50 kg

45. (‘D’ is the amount of fouled material removal assumed complete 5 kg liquid left behind each flush)

46. Comparison of Effectiveness Single-flush uses 100 kg, leaves 5% residue Two-flush, cross-flow uses 100 kg, leaves 0.9% Two-flush, counter-flow uses 50 kg, leaves 1%

47. 20 C 200 C 152 C 108 C Counter-Current 20 C 252 C 152C 160 C Co-Current 20 C 265 C 152 C 173 C Plug-Mixed Mixed-Mixed 20 C 310 C 152 C 218 C Figure 3.18– Efficacy of Various Two-Stream Configurations Relative Production of Entropy 1.0 2.1 1.7, 1.8 1.6

48. P P P Fresh steam Dilute feed solution Vapor to condensation Condensate Concentrated Product solution Figure 3.17 – Triple-Effect Evaporation

49. Entropy Production =  dQ / T = Flow x Specific heat x Ln ( T2 / T1 ) for simple temperature change For heat exchange: total entropy production = (Flow x Cp) hot fluid x Ln (TK out / TK in ) + (Flow x Cp) cold fluid x Ln (TK out / TK in )

51. Process Fluid Heating Fluid #1 (200 C ) Heating Fluid #2 (150 C ) Total Area = 100 sq.metres TEXIT TF1 30 C Figure 5. Optimal mixing of two heating fluids 160 140 120 40120160 TEXIT TF1 130 80 150 deg C 200x x x x x x x 110

52. Liquid % B 63 37 22 10 4 2 Vapour % B 89 74 58 36 18 9 654321654321 F = 100 F = 200 40% B 10% B < 2% B, as per spec F = 200 89% B Figure 9. Optimal blending of component streams

53. Summary on Driving Forces prefer plug-flow to back-mixed flow understand that batch process is in ‘plug-flow’ use multi mixed-stages to approximate plug flow sequence different operations with increasing completion-to-capacity ratios prefer counter-current flow to any other avoid mixing unlike streams