Distillation Modeling
CHEN 4470 – Process Design Practice
Dr. Mario Richard Eden
Department 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 Fundamentals
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Understanding the Design Problem
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DISTWU
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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
This model uses the Winn-Underwood-Gilliland method
to estimate minimum reflux and number of stages
RADFRAC
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Rigorous distillation rating model for a given number of
stages, feed location and reflux ratio
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?
Distillation Column Design
Using DISTWU in ASPEN Plus
Distillation Column Rating
Using RADFRAC in ASPEN Plus
Tray Sizing and Rating
Using RADFRAC in ASPEN Plus
Tray Sizing Input Form
Tray Rating Input Form
Reactive Distillation
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Specifying Reactions and Chemistry
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Reactive distillation is often employed in equilibrium
systems to drive the reaction forward by removing one
of the components:
A+B  C+D
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In a Reactive Distillation column, each theoretical stage
can be considered as an individual CSTR
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From a kinetic standpoint, the design problem can be
equated to the “CSTRs in Series” problem from your
reactor design course
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In ASPEN Plus “REACTIONS” and “CHEMISTRY” are
NOT the same thing!
Reactive Distillation
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ASPEN Plus Input Forms
Defining the Reaction
RADFRAC Input Form
Reactive Distillation
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Balancing Kinetics and Hydraulics
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.
Overflow
Weir
Underflow
Weir
Downcomer
Reactive Distillation
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Balancing Kinetics and Hydraulics (Cont’d)
Tray Hold-up Specification
Troubleshooting RADFRAC
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Common Problems
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Over specified column
Not enough iterations
Infeasible design specifications
Operation below minimum reflux
Wrong initialization routine
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Don’t be afraid to use the Help Menu!
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If you get stuck, Bob Kline or I can help!