CHEN 4470 – Process Design Practice

1. Distillation Modeling CHEN 4470 – Process Design Practice Dr. Mario Richard EdenDepartment of Chemical EngineeringAuburn 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

2. 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

3. 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?

4. Distillation Column Design Using DISTWU in ASPEN Plus

5. Distillation Column Rating Using RADFRAC in ASPEN Plus

6. Tray Sizing and Rating Using RADFRAC in ASPEN Plus Tray Sizing Input Form Tray Rating Input Form

7. 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

8. Reactive Distillation • ASPEN Plus Input Forms Defining the Reaction RADFRAC Input Form

9. Reactive Distillation • 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

10. Reactive Distillation • Balancing Kinetics and Hydraulics (Cont’d) Tray Hold-up Specification

11. 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!