1. CHPE308 Engineering Economy
CHEMICAL PROCESS DIAGRAMS

2. Objective
Understand the process flow diagrams of a chemical process
Estimate capital investment.
Estimate manufacturing cost.
Understand engineering economics and perform profitability analysis.

3. PURPOSE OF CHAPTER 2
To show that the evolution of every process follows a similar path.
To provide a framework to generate alternative PFDs for a given process.
for a given process.
Factors determining choice of alternative route Factors :
Cost of raw materials
Value of by-products
Complexity of the synthesis
Environmental impact of waste materials

4. 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 4

5. Batch vs. Continuous
A batch process is one in which a finite quantity (batch) of product is made during a period of a few (batch) of product is made during a period of a few hours or days.
Continuous process is one in which feed is sent continuously to a series of equipment, with each piece usually performing a single unit operation.

6. Batch vs. Continuous (cont)
Variables to Consider:
Size
Batch < 500 tonne/yr ~ 1.5 /yr (< 2 m3 of liquid or solid per day).
Continuous > 5000 tonne/yr
Flexibility
Batch can handle many different feeds and products ––more flexible
Continuous is better for smaller product slate and fewer feeds

7. Batch vs. Continuous (cont)
Continuous will allow you to benefit from the “Economy of Scale” but you will pay the price for less flexibility
Other Issues :
Accountability and quality control––Government should require batch
Safety ––batch is more accident prone
Scheduling of equipment–– may be most important issue.
Seasonal demands––ex. Antifreeze, food ex. Antifreeze, food products

8. Input– Output Structure (Process Concept Diagram)

9. 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.

10. 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

11. Generic Structure of Process Flow Diagrams
Generic PFD may have varying topology but always contains a subset of the 5 basic blocks.

12. Input Output Structure of The PFD
Important factors to consider in analyzing the overall input output structure of a PFD:
Chemicals not consumed are either required to operate a piece of equipment or are inert material.
Any chemical leaving a process must have either entered in one of the feed streams or have been produced by a chemical reaction within the process.
Utility streams are treated differently from process streams. Utility streams rarely directly contact the process streams. They usually provide or remove thermal energy or work.

13. Input – Output Structure (Process Concept Diagram)
This cloud diagram is even more primitive than the traditional block flow process diagram discussed in Chapter 1.

14. Input-Output on PFD
This is equivalent to the previous diagram

15. 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.

16. Other Input – Output Issues (cont)
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

17. 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)

18. 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)

19. Other Considerations for Input Output Structure (cont) –
If impurity reacts to form difficult to separate material or hazardous product then separate
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.
Impurity in large quantities then purify––why?
•A notable exception is air

20. Other Considerations for Input Output Structure (cont) –
Impurity in large quantities then purify––why?
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.

21. Add Materials to Feed
In order to
Stabilize products
Enable separation/minimize side reactions
Anti-oxidants and scavengers
Solvents and catalysts

22. 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

23. Information Obtained From Input Output Diagram Input-
Basic economic analysis on profit margin.
What chemical components must enter with the feed and leave as products.
All the reactions, both desired and undesired, that take place.

24. Profit Margin (PM)
If PM < 0, then don’t bother to pursue this process but start looking for an alternate route
Toluene HDA vs. Toluene Disproportionation
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 sene since all the toluene makes useful products and does not require a second reactant. Fuel gas is generally a low value byproduct.

25. Profit Margin (PM)
Some processes are more sensitive to product and feed prices than others.
Average cost data over a period of several years should be used in evaluating PM.
Cost of raw materials play a role in deciding which chemical path to choose to produce a given product.

26. Example 2.1

28. Consider 1993 prices

29. Example 2.2
Profit?

30. Recycle Structure of the Process
Raw materials are very valuable.
They make up 25 to 75% of total operating costs.
Separation and recycling of unused reactants is very important.
Exception is when raw materials are very cheap.
Extent of recycling of unused reactants depends on ease of separation.

31. Recycle Structure in PFD
The pink lines represent raw material recycles while the blue lines are recycles for process control purposes.

32. Efficiency of Raw Material Usage

33. Efficiency of Raw Material Usage
Single-pass Conversion (SPC):
SPC tells us how much of the reactant that enters the reactor is reacted.
The lower the SPC the greater the amount of recycle.
SPC affects equipment size and utility flows.
Raw material costs are not changed significantly by SPC.
SPC of hydrogen in HDA process is kept low to reduce coking of the catalyst.

34. Efficiency of Raw Material Usage
Overall Conversion (OC)
OC tells us tells us what fraction of the reactant in the feed to the process is converted to product.
High OC (e.g99.3 %) is typical for chemical processes.
High OC shows that unreacted raw materials are not being lost from the process.
Low OC of hydrogen in the HDA process indicates poor raw material usage.

35. Efficiency of Raw Material Usage
Yield
Yield tells us what fraction of the limiting reactant ends up in our desired product.
Competing or side reactions may reduce the yield.
Yields for hydrodealkylation process are generally high (e.g98-99 % for DETOL, Lummus).

36. Basic Recycle Structures
Separate and purify un--reacted 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
A purge stream is one where a portion of a recycle stream is removed from the system in order to avoid accumulation of undesired material in a recycled system.

37. Ease of separation depends on:
Separate and purify
Ease of separation depends on:
What conditions (T and P) are necessary to operate the process?
Are the differences in physical and chemical properties for the species to be separated large or small?

38. Example 2.4

39. Example 2.5

40. When using this option, the intermediate product (diphenyl) must react further – either to destruction or equilibrium.

41. Alternative B
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

44. Summery
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. These is important for cost consideration.