lecture+2+ +structure+and+synthesis+of+pfd

8/3/2019 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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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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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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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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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.


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