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

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

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

• 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

Reactive Distillation

• ASPEN Plus Input Forms

Defining the Reaction

RADFRAC Input Form

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.

Reactive Distillation

• Balancing Kinetics and Hydraulics (Cont’d)

Tray Hold-up Specification

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!