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Design Project 499
Process Design Project 599
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Nature of design work Key documents
Common uncertainties◦ Design methodology
◦ Safety factor
◦ Critical review
Characteristics of better and poorer design
studies
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•
E.g. relationship betweenfactors in distillationcolumn design
• Geometry Operation
• In this unit we’ll distinguishbetween three facets
• Process design
• Operational design
• Mechanical design
• See also Martyn Ray’s viewof design on Blackboard
and the handout from R&S
(Woods, p92-93)
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Memo 1
Volume 1
Memo 2
Memo 3
Volume 2
...
Sets the conditionsand specificationsfor the individualdesigns in Vol 2
Major A
Minor A
Major B
Minor B
Major C
Corrected,revised,expandedbecome
In general, don’trevise Vol 1 based on
the results of Vol 2(but discuss potentialeffects in the Vol 2critical reviews)
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Methodology is the overall approach taken to perform the designwork, expressed as a series of steps◦ Actual equations are not usually mentioned◦ Rather: “Calculate X”, “Decide Y”, “Check Z”, ...
Simple distillation columns, heat exchangers, … have readilyavailable and well-established design methodologies, e.g.◦ Distillation columns: T&S, Ch11; Ludwig, Vol2; GPSA, Sec19◦ Heat exchangers: T&S, Ch12; Ludwig, Vol3; GPSA, Sec8–11
Some aspects of design are covered by Codes and Standards,e.g.◦ AS1210: Australian Standard for Pressure Vessels – covering mechanical
design of unfired vessels and their supports, manufacture, testing, safetysystems
◦ AS4041: Australian Standard for Pressure Piping – covering mechanicaldesign of process piping, manufacture, testing, safety systems
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(T&S, p845)
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The library is your friend◦ Catalogue(s), databases, interlibrary loans (?), document delivery (??), … + Perry’s
Try to get drawings or photos of the equipment◦ Old books, Google images, manufacturer’s brochures, journals perhaps, …
Ask your industry / academic advisor + look at the R&S handout
What are the key issues and main phenomena involved?◦ Tease them apart then think how they are connected…
Go back to the fundamentals: mass and energy balances, rate equations
Don’t wait, make a start!
If some data are very uncertain, pick a value and check sensitivities
If you don’t have much depth in one area of your design, then focus onanother… but get confirmation with your academic advisor
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An amount of “overdesign” / “over-sizing” to allow for◦ Uncertainties in design methods◦ Uncertainties in design data◦ Need for operation above nominal specs (“nominal” = the flows
and conditions in your stream tables)
A balance between◦ Making the equipment a bit bigger to be sure it can handle therequired duty
◦ Making the equipment so big that it becomes too costly
Can be implemented in several ways, e.g.◦ Increase the unit’s feed stream flowrate(s) by x%◦ Increase performance specifications by y%◦ Reduce the efficiency / increase area / increase number of trays /
… by z%
Ask your industry / academic advisor
(P&T, p81-84; T&S, p17-18; Hills, Ch4)
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The safety factor (and its implementation) must beclearly reported
Should check that operation at the nominal flowrateis acceptable, e.g.◦ Nominal heat exchanger flow is 30 kg/s for one stream◦ Use design factor of 10% on the flowrate design the HX
for a flow of 33 kg/s
◦ But, in normal operation the flow will be only 30 kg/s◦ This means the operating OHTC will be lower than design
conditions (since lower v
lower h and higher Rfouling)◦ Will the HX still work with a lower OHTC? Need to check!
Can be 0%, but 10% is typical (and see next slide)
(P&T, p81-84; T&S, p17-18; Hills, Ch4)
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(P&T, p82)
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It’s a self-critique of your design work and decisions
It is not a summary of the design produced ( 0/10)
Needed in both major and minor individual design studies
It’s an instance of “reflective practice”, which is valued byEngineers Australia
What one student said:◦ “Taking a step back and reflecting on the project was an
unfamiliar experience. For the major design project my state of
mind has been ‘down and in the trenches’ focusing on what needsto be done next with no self-reflection. Analysing my work with aself-critique point of view was fairly engaging, I learnt a lot aboutmyself and my approach to problem solving…”
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Re-evaluate your assumptions
Discuss the limitations of the design methods used
Discuss how uncertainties were handled
Compare your design with designs in the literature
Discuss your reservations about the work and also your reasons forhaving confidence in it
Outline any experimental work or further simulation work that should beperformed
Discuss what you would do differently if you had to do it again
Discuss optimisation possibilities
Reflect on the most important issues or key parameters uncovered indesigning that type of unit
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What were the good points of your design work? What are thegood features of your approach you want to tell people about?
What parts need more attention? What are your reservations about the design you’ve come up with?
What extra information would you have liked? How exactly would
you have made use of it? How did you cope with not having allthe information you wanted?
Did you find anything unexpected?
Did you use any innovative design methods or technology? Didyou do some extra work that most other people did not?
Your final design might be different and perform differently fromthe equipment envisaged in Volume 1. How would thesedifferences affect the rest of the process?
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T h e r m o d y n a m i c s
( V L E m e t h o d , …
)
R e a c t i o n s
( s p e c i e s ,
s t o i c h i o m e t r y ,
e q u i l i b r i a ,
k i n e t i c s , …
)
H y d r o d y n a m i c s
( w e l l m i x e d , p l u g
f l o w , …
)
H e a t t r a n s f e r
( m o d e , r a t e
e q u a t i o n ,
l o s s e s ,
… )
M a s s t r a n s f e r
( d r i v i n g f o r c e ,
i n t e r f a c i a l a r e a ,
… )
P e r i p h e r a l s
( p u m p s , p i p e s ,
t a n k s , …
)
I n t e r a c t i o n s
w i t h r e s t o f p l a n t
( u p s t r e a m ,
d o w n s t r e a m )
E t c .
Assumptions, approx-
imations, limitations inmethods used
Alternative approaches
Accuracy of data, andits implications,sensitivities
Time scales (short,long), dynamics, start-up / shutdown
Key issues uncovered,unexpected items
Recommendations
Etc.
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Poorer designs
◦ Forget to mention these things
◦ Focus on mechanical designbecause there are lots of
equations◦ Seem to be mostly qualitative
◦ Ignore the vessel’s pipes,pumps, …
◦ Are presented “as is”
◦
Documents look “homemade”,as if the designer had not seenexamples before
◦ Leave out the critical review orwrite it as a summary
Better designs
◦ Have clear scope, designrequirements, methodology
◦ Focus on process andoperational design
◦ Are strongly quantitative
◦ Include peripherals
◦ Show evidence of checking
◦
Have professional lookingequipment spec sheets,mechanical drawings, …
◦ Have a thoughtful criticalreview
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The major and minor studies need to be decided uponand stated in Memo 3◦ In consultation with fellow group members and your academic
(and possibly industry) advisor
Work on your major and minor studies in parallel◦
Keep Volume 1 revisions ticking over as well Don’t spend forever trying to get that last piece of data:
pick a value and proceed
Look carefully at the scope of work to make sure youdon’t leave anything out
Keep in contact with your academic and industryadvisors – you don’t just need them in Weeks 1–6
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GPSA: Gas Processors Suppliers Association (2004) Engineering Data Book, 12ed, Gas Processors Suppliers Association, Tulsa, OK Ludwig: EE Ludwig (2007) Applied Process Design for Chemical and
Petrochemical Plants , 3ed, Gulf Professional Publishing, Houston, TX Perry: RH Perry and DW Green (eds) (2008) Perry's Chemical Engineers’
Handbook , 8ed, McGraw-Hill, New York P&T: MS Peters, KD Timmerhaus and RE West (2003) Plant Design and
Economics for Chemical Engineers, 5 ed, McGraw-Hill, New York R&S: MS Ray and MG Sneesby (1998) Chemical Engineering Design
Project: A Case Study Approach, 2ed, Gordon and Breach SciencePublishers, Amsterdam
T&S: G Towler and R Sinnott (2008) Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design, Butterworth-Heinemann, Burlington, MA
Woods: DR Woods (2007) Rules of Thumb in Engineering Practice, Wiley-VCH, Weinheim, Germany