CHEN 4470 – Process Design Practice
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Transcript of CHEN 4470 – Process Design Practice
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CHEN 4470 – Process Design Practice
Dr. Mario Richard EdenDepartment of Chemical Engineering
Auburn University
Lecture No. 12 – Advanced Distillation Column Modeling and Reactive Distillation
February 26, 2013
Material Developed by Dr. Jeffrey R. Seay, University of Kentucky - Paducah
Distillation Modeling
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Distillation Fundamentals
• Understanding the Design Problem– Distillation Column Design vs. Distillation Column
Rating– Requires different tools, i.e. DISTWU and
RADFRAC– Remember you must design the columns for
100% capacity, but rate them for 67% capacity
• DISTWU– This model uses the Winn-Underwood-Gilliland
method to estimate minimum reflux and number of stages
• RADFRAC– Rigorous distillation rating model for a given
number of stages, feed location and reflux ratio
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McCabe-Thiele Diagram
Column Design for a Binary System using McCabe-Thiele Analysis
The graphical approach allows the engineer to visualize how the design problem is bound.
By determining the minimum reflux ratio and the minimum number of theoretical stages, we determine the limits of the design problem.
We must complete the design process before beginning the task of optimization!
How can we determine these values using Aspen Plus?
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Distillation Column Design
Using DISTWU in ASPEN Plus
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Distillation Column Rating
Using RADFRAC in ASPEN Plus
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Tray Sizing and Rating
Using RADFRAC in ASPEN Plus
Tray Sizing Input Form
Tray Rating Input Form
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Reactive Distillation
• Specifying Reactions and Chemistry– Reactive distillation is often employed in
equilibrium systems to drive the reaction forward by removing one of the components:
– In a Reactive Distillation column, each theoretical stage can be considered as an individual CSTR
– From a kinetic standpoint, the design problem can be equated to the “CSTRs in Series” problem from your reactor design course
– In ASPEN Plus “REACTIONS” and “CHEMISTRY” are NOT the same thing!
A + B C + D
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Reactive Distillation
• ASPEN Plus Input Forms
Defining the Reaction
RADFRAC Input Form
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Reactive Distillation
• Balancing Kinetics and Hydraulics
Overflow Weir
Downcomer
Underflow Weir
The volume required for the “CSTR” is independent of the column diameter required based on hydraulics.
Diameter in the column is typically a function of vapor flowrate.
In other words, the tray hold-up requirement and the column diameter requirement are mutually exclusive.
The variable that links these two requirements in the height of the overflow weir.
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Reactive Distillation
• Balancing Kinetics and Hydraulics (Cont’d)
Tray Hold-up Specification
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Troubleshooting RADFRAC
• Common Problems– Over specified column– Not enough iterations– Infeasible design specifications– Operation below minimum reflux– Wrong initialization routine
• Don’t be afraid to use the Help Menu!
• If you get stuck, Bob Kline or I can help!