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Transcript of Lecture+2+ +Structure+and+Synthesis+of+PFD
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Lecture 2 - Structure andSynthesis of the Process Flow
Diagram
Chemical Engineering
KBTU
Rinat Iskakov, Ph.D., [email protected] +7(701)735-5520 skype: riskakov
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Outlinen Generic Structure of Processes
n Process Design Hierarchy
n Batch vs. Continuous Processesn Input Output Structure
n Recycle Structure
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Generic Structure of Process Flow Diagrams
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Generic Structure of Process Flow Diagrams
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Generic Structure of Process Flow Diagrams
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Environmental Controln End of Pipe vs. Green Approach
n Most significant changes obtained by changingprocess chemistry within reactor
eliminate/minimize unwanted by-productsn End of Pipe vs. Common Units
n Fired Heaters - excess oxygen
- low sulfur fuel
- NOX control
n Wastewater - biological/sedimentation/filtration
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Approach of Douglas1
n Five step process to tackle a conceptual
process designn Batch vs. continuous
n Input-output structuren Identify and define recycle structure of process
n Identify and design general structure ofseparation system
n Identify and design heat-exchanger network or
process energy recovery system1 Douglas, J.M., Conceptual Design of Chemical Processes, McGraw-Hill, NY, 1988
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Batch vs. ContinuousVariables to Consider:
n Size
n Batch < 500 tonne/yr ~ 1.5 tonne/day(< 2 m3 of liquid or solid per day)
n Continuous > 5,000 tonne/yr
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Batch vs. Continuous(cont.)n Flexibility
n Batch can handle many different feeds
and products
more flexiblen Continuous is better for smaller product
slate and fewer feeds
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Batch vs. Continuous(cont.)Continuous allows the process tobenefit from the Economy of Scale,
but the price is less flexibility
Batch scale-up often consists ofmultiple parallel units
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Batch vs. Continuous(cont.)n Other Issues
n Accountability and quality control FDArequires batch accountability
n Safety batch is more accident prone
n Scheduling of equipment may be mostimportant issue
n Seasonal demands e.g., antifreeze, food
products
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Input Output Structure(Process Concept Diagram)
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Input-Output on PFD
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Input-Output Utility Streams
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Other Input Output Issues
Purify Feed ?
n Feed purity and trace components
n Small quantities and inerts do notseparate
Example H2 in feed contains CH4CH4 does not react
so do not remove
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Other Input Output Issues (cont)
n If separation of impurities is difficult Do not separate
n Azeotrope
(water and ethanol)n Gases (requires high P and low T)
How would you remove CH4 from H2?
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Other Input Output Issues (cont)
n If impurities foul or poison catalyst thenseparate
n Sulfur Group VIII Metals (Pt, Pd, Ru,Rh)
n CO in platinum PEM fuel cells
Note: S and CO may be present in verysmall amounts (ppm)
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Other Input Output Issues (cont)
n If impurity reacts to form difficult-to-separate material or hazardous
product then separatePhosgene Example
CH4 +H2O CO + 3H2CO + Cl
2COCl
2Any H2 HCl
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Other Input Output Issues (cont)
n Impurity in large quantities then purify why?
A notable exception is air
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Add Materials to FeedIn order to
n Stabilize products
n Enable separation/minimize sidereactions
n Anti-oxidants and scavengers
n Solvents and catalysts
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Inert Feedsn Control exothermic reactions
n Steam controls oxidation reactions (and mayeliminate or modify explosion limits)
n Reduces coke formation on catalyst
n Control equilibrium
n Adding inerts shifts equilibrium to the right
e.g., styrene reaction
C6H5CH2CH3 C6H5CHCH2 + H2
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Profit Margin
n If $ Products - $ Raw Material < 0,
then do not bother to pursue this process, but startlooking for an alternate route
Toluene HDA vs. Toluene Disproportionation
C6H5CH3 + H2C6H6 + CH4Toluene benzene
2C6H5CH3C6H6 +C6H4(CH3)2Toluene benzene xylene
Toluene used
more efficiently
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Recyclen Since raw materials make up from 25 to
75% of total operating costs, shouldrecover as much raw material aspossible
n Exception is when raw materials arevery cheap
For example, Air Separation
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3 Basic Recycle Structuresn Separate and purify unreacted feed from
products and then recycle, e.g., toluene
n Recycle feed and products together and use a
purge stream, e.g., hydrogen with purge asfuel gas
n Recycle feed and products together but donot use a purge stream - must come toEquilibrium
2C6H6 C12H10 + H2
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Recycle Structure in PFD
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Recycle without separation or purge
Recycle
increases and
equip. and op.
costs increase
2C6H6 C12H10 + H2
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Recycle with Separation (and Purge)
Extra tower
with associated
operating costs
2C6H6 C12H10 + H2
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Other Issues on Recycle
n Number of recycle streams
n Does excess reactant affect structuren Size of Recycle Loop
H2 : Toluene = 5 : 1
n Number of Reactorsn Separate and recycle to different reactors
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Other Issues on Recycle (cont.)
n Do we need to purify prior to recycling?
n Is recycling of inerts warranted?
n Can recycling an unwanted inertmaterial push equilibrium to the right?
n Gasification of coal CO2 recycle
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Other Issues on Recycle (cont.)
n Can recycling an unwanted inert controlreaction
n
CO2 in Gasifiern Phase of Recycle Stream?
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Summary
nAll processes are comprised of the samefive basic steps reactor feed prep,
reactor, separation feed prep,separation, recycle, and environmentalcontrol blocks
n Hierarchy of process design looks at
batch vs. continuous, feeds, recycles,and heat and mass integration.
Rinat Iskakov, Ph.D., [email protected] +7(701)735-5520 skype: riskakov