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

Sodium Methylate Physical Property Models 7 February 2008 | Senior Design Class Lecture Page | 2

Required Property Data for Sodium Methylate Not all property parameters are available in ASPEN Plus for this simulation. 7 February 2008 | Senior Design Class Lecture Page | 3

Required Property Data for Sodium Methylate 7 February 2008 | Senior Design Class Lecture Page | 4

Advanced Distillation Modelling using Aspen Plus 7 February 2008 | Senior Design Class Lecture Page | 5

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 Rigorous distillation rating model for a given number of stages, feed location and reflux ratio. 7 February 2008 | Senior Design Class Lecture Page | 6

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? 7 February 2008 | Senior Design Class Lecture Page | 7

Distillation Design using the DSTWU Model in Aspen Plus 7 February 2008 | Senior Design Class Lecture Page | 8

Distillation Column Rating using the RADFRAC Model 7 February 2008 | Senior Design Class Lecture Page | 9

Tray Sizing and Rating Tray Sizing Input Form Tray Rating Input Form 7 February 2008 | Senior Design Class Lecture Page | 10

Reactive Distillation using the RADFRAC Model 7 February 2008 | Senior Design Class Lecture Page | 11

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! 7 February 2008 | Senior Design Class Lecture Page | 12

ASPEN Input Forms Defining the Reaction RADFRAC Input Form 7 February 2008 | Senior Design Class Lecture Page | 13

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 17 January 2008 | Senior Design Class Lecture 7 February 2008 | Senior Design Class Lecture Page | 14 Page | 14

Balancing Kinetics and Hydraulics Tray Hold-up Specification 17 January 2008 | Senior Design Class Lecture 7 February 2008 | Senior Design Class Lecture Page | 15 Page | 15

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 you’ll be hearing more about in a couple of weeks! 7 February 2008 | Senior Design Class Lecture Page | 16

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 Don’t be afraid to use the Help Menu! If you get stuck, I can help! 7 February 2008 | Senior Design Class Lecture Page | 17

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