Advanced Distillation Column Modelling and Reactive Distillation

1
Advanced Distillation Column Modelling and
Reactive Distillation
Presented to the Auburn University Dept. of
Chemical Engineering Senior Design Class
Jeffrey R. Seay, P.E. Senior Process
Engineer Evonik Degussa Corporation 7 February
2008
2
Sodium Methylate Physical Property Models
Page 2
7 February 2008 Senior Design Class Lecture
3
Required Property Data for Sodium Methylate

• Not all property parameters are available in
  ASPEN Plus for this simulation.

Page 3
7 February 2008 Senior Design Class Lecture
4
Required Property Data for Sodium Methylate
Page 4
7 February 2008 Senior Design Class Lecture
5
Advanced Distillation Modelling using Aspen Plus
Page 5
7 February 2008 Senior Design Class Lecture
6
Distillation Fundamentals
Understanding the Design Problem Distillation
Column Design, is a different task than
Distillation Column Rating. These two different
tasks require different tools. In Aspen Plus,
the DISTWU and RADFRAC blocks are available for
these tasks. DISTWU This model uses the
Winn-Underwood-Gilliland method to estimate
minimum reflux and number of stages. RADFRAC Rigo
rous distillation rating model for a given number
of stages, feed location and reflux ratio.
Page 6
7 February 2008 Senior Design Class Lecture
7
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?

Page 7
7 February 2008 Senior Design Class Lecture
8
Distillation Design using the DSTWU Model in
Aspen Plus
Page 8
7 February 2008 Senior Design Class Lecture
9
Distillation Column Rating using the RADFRAC Model
Page 9
7 February 2008 Senior Design Class Lecture
10
Tray Sizing and Rating
Tray Sizing Input Form
Tray Rating Input Form
Page 10
7 February 2008 Senior Design Class Lecture
11
Reactive Distillation using the RADFRAC Model
Page 11
7 February 2008 Senior Design Class Lecture
12
Specifying Reactions and Chemistry
Reactive distillation is often employed in
equilibrium systems to drive the reaction to one
side or the other by remove one of the
components. A B ?? C D By removing one of
the products, the reaction is prevented from
going back the other way. In a Reactive
Distillation column, each theoretical stage can
be thought of as an individual CSTR. Therefore,
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!
Page 12
7 February 2008 Senior Design Class Lecture
13
ASPEN Input Forms
Defining the Reaction
RADFRAC Input Form
Page 13
7 February 2008 Senior Design Class Lecture
14
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
Page 14
7 February 2008 Senior Design Class Lecture
Page 14
17 January 2008 Senior Design Class Lecture
15
Balancing Kinetics and Hydraulics Tray Hold-up
Specification
Page 15
7 February 2008 Senior Design Class Lecture
Page 15
17 January 2008 Senior Design Class Lecture
16
Specific Issues regarding the NaOCH3 Process

• The NaOCH3 Column is a Stripping Column NOT a
  Distillation Column. Consider what this means
  with regard to the operating line and the feed
  location.
• Be sure to carefully consider the Minimum Reflux
  Ratio with regard to the Methanol / Water Column.
• You must keep water out of the bottom of the
  column, consider this requirement when
  calculating the required Methanol flow to the
  reaction column.
• Both columns must be designed to operate at 50
  turndown. This means you must Design the column
  for 100 capacity and Rate the column for 50
  capacity.
• This is especially important with regard to your
  Control Concept, which youll be hearing more
  about in a couple of weeks!

Page 16
7 February 2008 Senior Design Class Lecture
17
Troubleshooting RADFRAC Results

• Several problems are often considered with
  RADFRAC Columns
• Over specified column
• Not enough iterations
• Infeasible design specifications
• Operation below minimum reflux
• Wrong initialization routine
• Dont be afraid to use the Help Menu!
• If you get stuck, I can help!

Page 17
7 February 2008 Senior Design Class Lecture
18
Questions and Comments
Thank you!
Page 18
7 February 2008 Senior Design Class Lecture