1. Structure and Synthesis of the Process Flow Diagram
Chemical Engineering Department
West Virginia University
Copyright - R. Turton and J. Shaeiwitz, 2008

2. Copyright - R. Turton and J. Shaeiwitz, 2008
Outline
Generic Structure of Processes
Process Design Hierarchy
Batch vs. Continuous Processes
Input – Output Structure
Recycle Structure
Copyright - R. Turton and J. Shaeiwitz, 2008

3. Generic Structure of Process Flow Diagrams
Often the environmental controls are centralized (wastewater treatment, sour water stripper, incinerators, etc.) and do not show directly on a process PFD – but they are there. Also some pollution prevention devices are present on process equipment, e.g., NOx reduction on fired heaters, etc.
Copyright - R. Turton and J. Shaeiwitz, 2008

4. Generic Structure of Process Flow Diagrams
Generic PFD may have varying topology but always contains a subset of the 5 basic blocks.
Copyright - R. Turton and J. Shaeiwitz, 2008

5. Generic Structure of Process Flow Diagrams
Identify each equipment with one of the five blocks –
Purple – recycle, grey – reactor prep, flesh – reaction, green – separator prep, and green-grey - separation
C6H5CH3+H2  C6H6 + CH4
Copyright - R. Turton and J. Shaeiwitz, 2008

6. Environmental Control
End of Pipe vs. Green Approach
Most significant changes obtained by changing process chemistry within reactor – eliminate/minimize unwanted by-products
End of Pipe vs. Common Units
Fired Heaters - excess oxygen
- low sulfur fuel
- NOX control
Wastewater - biological/sedimentation/ filtration
Copyright - R. Turton and J. Shaeiwitz, 2008

7. Copyright - R. Turton and J. Shaeiwitz, 2008
Approach of Douglas1
Five step process to tackle a conceptual process design
Batch vs. continuous
Input-output structure
Identify and define recycle structure of process
Identify and design general structure of separation system
Identify and design heat-exchanger network or process energy recovery system
1 – Douglas, J.M., Conceptual Design of Chemical Processes, McGraw-Hill, NY, 1988
Copyright - R. Turton and J. Shaeiwitz, 2008

8. Copyright - R. Turton and J. Shaeiwitz, 2008
Batch vs. Continuous
Variables to Consider:
Size
Batch < 500 tonne/yr ~ 1.5 tonne/day
(< 2 m3 of liquid or solid per day)
Continuous > 5,000 tonne/yr
Copyright - R. Turton and J. Shaeiwitz, 2008

9. Batch vs. Continuous(cont.)
Flexibility
Batch can handle many different feeds and products – more flexible
Continuous is better for smaller product slate and fewer feeds
Copyright - R. Turton and J. Shaeiwitz, 2008

10. Batch vs. Continuous(cont.)
Continuous allows the process to benefit from the “Economy of Scale,” but the price is less flexibility
Batch – scale-up often consists of multiple parallel units
Copyright - R. Turton and J. Shaeiwitz, 2008

11. Batch vs. Continuous(cont.)
Other Issues
Accountability and quality control – FDA requires batch accountability
Safety – batch is more accident prone
Scheduling of equipment – may be most important issue
Seasonal demands – e.g., antifreeze, food products
Pharma companies are often batch because of FDA (Food and Drug Administration) requirements for accountability and ease of recall of faulty products
Scheduling of batch products is covered in Chapter 3
Copyright - R. Turton and J. Shaeiwitz, 2008

12. Input – Output Structure (Process Concept Diagram)
This cloud diagram is even more primitive than the traditional block flow process diagram discussed in Chapter 1.
Copyright - R. Turton and J. Shaeiwitz, 2008

13. Copyright - R. Turton and J. Shaeiwitz, 2008
Input-Output on PFD
This is equivalent to the previous diagram
Copyright - R. Turton and J. Shaeiwitz, 2008

14. Input-Output – Utility Streams
Note that utilities are not considered in the input-output structure since they are not (usually) process streams. Utilities for pumps and compressors are usually electricity and are not shown on the PFD.
Copyright - R. Turton and J. Shaeiwitz, 2008

15. Other Input – Output Issues
Purify Feed ?
Feed purity and trace components
Small quantities and “inerts” – do not separate
Example H2 in feed contains CH4
CH4 does not react
so – do not remove
Copyright - R. Turton and J. Shaeiwitz, 2008

16. Other Input – Output Issues (cont)
If separation of impurities is difficult – Do not separate
Azeotrope – (water and ethanol)
Gases – (requires high P and low T)
How would you remove CH4 from H2?
Copyright - R. Turton and J. Shaeiwitz, 2008

17. Other Input – Output Issues (cont)
If impurities foul or poison catalyst then separate
Sulfur – Group VIII Metals (Pt, Pd, Ru, Rh)
CO in platinum PEM fuel cells
S and CO are big problems for PEM fuel cells (because of platinum)
Note: S and CO may be present in very small amounts (ppm)
Copyright - R. Turton and J. Shaeiwitz, 2008

18. Other Input – Output Issues (cont)
If impurity reacts to form difficult-to-separate material or hazardous product then separate
Phosgene Example
CH4 + H2O CO + 3H2
CO + Cl COCl2
Any H HCl
Note that phosgene is so dangerous that it is no longer transported within the US and is always made on site. In this example, phosgene is used in the the production of polyurethane precursors and any HCL in the phosgene causes problems downstream – therefore great efforts must be made to eliminate its production.
Copyright - R. Turton and J. Shaeiwitz, 2008

19. Other Input – Output Issues (cont)
Impurity in large quantities then purify – why?
A notable exception is air
By allowing large amounts of impurity to flow through the process (even if it is essentially inert) causes all the equipment to be larger and for the utility usages to increase proportionally. Usually cheaper to remove it prior to processing.
Copyright - R. Turton and J. Shaeiwitz, 2008

20. Copyright - R. Turton and J. Shaeiwitz, 2008
Add Materials to Feed
In order to
Stabilize products
Enable separation/minimize side reactions
Anti-oxidants and scavengers
Solvents and catalysts
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21. Copyright - R. Turton and J. Shaeiwitz, 2008
Inert Feeds
Control exothermic reactions
Steam controls oxidation reactions (and may eliminate or modify explosion limits)
Reduces coke formation on catalyst
Control equilibrium
Adding inerts shifts equilibrium to the right
e.g., styrene reaction
C6H5CH2CH3 C6H5CHCH2 + H2
Copyright - R. Turton and J. Shaeiwitz, 2008

22. Copyright - R. Turton and J. Shaeiwitz, 2008
Profit Margin
If $ Products - $ Raw Material < 0,
then do not bother to pursue this process, but start looking for an alternate route
Toluene HDA vs. Toluene Disproportionation
C6H5CH3 + H2 C6H6 + CH4
Toluene benzene
2C6H5CH3 C6H6 +C6H4(CH3)2
Toluene benzene xylene
The actual profitability of the benzene production process also depends on what the benzene is used for. IF it is an intermediate then adverse overall economics for this part of the process may not prohibit production. Nevertheless, the second route via disproportionation makes a lot more sense since all the toluene makes useful products and does not require a second reactant. Fuel gas is generally a low value byproduct.
Toluene used more efficiently
Copyright - R. Turton and J. Shaeiwitz, 2008

23. Copyright - R. Turton and J. Shaeiwitz, 2008
Recycle
Since raw materials make up from 25 to 75% of total operating costs, should recover as much raw material as possible
Exception is when raw materials are very cheap
For example, Air Separation
Copyright - R. Turton and J. Shaeiwitz, 2008

24. 3 Basic Recycle Structures
Separate and purify unreacted feed from products and then recycle, e.g., toluene
Recycle feed and products together and use a purge stream, e.g., hydrogen with purge as fuel gas
Recycle feed and products together but do not use a purge stream - must come to Equilibrium
2C6H6 C12H10 + H2
The diphenyl production is discussed as an example in Chapter 2.
Copyright - R. Turton and J. Shaeiwitz, 2008

25. Recycle Structure in PFD
The pink lines represent raw material recycles while the blue lines are recycles for process control purposes.
Copyright - R. Turton and J. Shaeiwitz, 2008

26. Recycle without separation or purge
Recycle increases and equip. and op. costs increase
When using this option, the intermediate product (diphenyl) must react further – either to destruction or equilibrium.
2C6H6 C12H10 + H2
Copyright - R. Turton and J. Shaeiwitz, 2008

27. Recycle with Separation (and Purge)
2C6H6 C12H10 + H2
Extra tower with associated operating costs
If the diphenyl is not recycled (this would be the case when it either did not react further or the equilibrium was such that the amount in the recycle loop became very large) then separation as a byproduct must be used. The choice of which option is best is purely economic. Note that if diphenyl is not a saleable product then it must be destroyed (possibly by incineration) and this cost should also be included.
Copyright - R. Turton and J. Shaeiwitz, 2008

28. Other Issues on Recycle
Number of recycle streams
Does excess reactant affect structure
Size of Recycle Loop
H2 : Toluene = 5 : 1
Number of Reactors
Separate and recycle to different reactors
The reason for using such a high ratio of hydrogen to toluene in the reactor feed is to suppress coke formation on the catalyst. Otherwise, it would make economic sense to run the reactants at close to stoichiometric ratios.
Copyright - R. Turton and J. Shaeiwitz, 2008

29. Other Issues on Recycle (cont.)
Do we need to purify prior to recycling?
Is recycling of inerts warranted?
Can recycling an unwanted inert material push equilibrium to the right?
Gasification of coal – CO2 recycle
These questions must at least be addressed when considering process alternative. The recycling of CO2 to a gasifier will certainly suppress the shift reaction (CO + H2O  CO2 + H2) but it requires compression and increases the flow through the gasifier loop.
Copyright - R. Turton and J. Shaeiwitz, 2008

30. Other Issues on Recycle (cont.)
Can recycling an unwanted inert control reaction
CO2 in Gasifier
Phase of Recycle Stream?
It is a lot easier and cheaper to recycle a liquid compared to a gas.
Copyright - R. Turton and J. Shaeiwitz, 2008

31. Copyright - R. Turton and J. Shaeiwitz, 2008
Summary
All processes are comprised of the same five basic steps – reactor feed prep, reactor, separation feed prep, separation, recycle, and environmental control blocks
Hierarchy of process design looks at batch vs. continuous, feeds, recycles, and heat and mass integration.
Copyright - R. Turton and J. Shaeiwitz, 2008