Modeling of Reactive Distillation John Schell Dr R

Modeling of Reactive Distillation John Schell Dr. R. Bruce Eldridge Dr. Thomas F. Edgar

Outline • Overview of Reactive Distillation • Project Overview – Tower Design – Steady-State Models – Dynamic Models and Control • Individual Work – Column Design and Operation – Validation of Models – Preliminary Dynamics and Control Studies • Future Work

Reactive Distillation • Homogeneous or Heterogeneous/ Catalytic Distillation • First Patents in 1920 s • Applied in 1980 s to Methyl Acetate • Common applications: – Ethylene Glycol – MTBE, TAME, TAA

Favorable Applications Westerterp (1992) • Match between reaction and distillation temperatures • Difference in relative volatility between product and one reactant • Fast reaction not requiring a large amount of catalyst • Others: liquid phase reaction, azeotrope considerations, exothermic reactions

Subawalla Approach (Dissertation) 1. Decide on a Pre-reactor - Rate of reaction - >1/2 of initial reaction rate at 80% of equilibrium conversion 2. Pressure 3. Location of Zone 4. Estimate Catalyst - Isothermal Plug-flow reactor with ideal separators 5. Design Tower - Size reaction zone • Catalyst requirements • Column diameter - Determine reactant feed ratio - Feed location - Reflux ratio • High reflux rate - 2 -3 times non-rxtive column - Diameter • Through-put • Catalyst density

Project Overview • • Design and Construct TAME Column Validate Steady State Models Develop Dynamic Models Test Control Algorithms

TAME Chemistry • Exothermic • Equilibrium Limited – 45 -62% at 50 -80 C • Azeotropes • Catalyst: Amberlyst-15 • Methanol can inhibit rates. • Rihko and Krause (1995)

Pilot Plant (SRP) • 0. 152 -meter diameter column • Finite reflux • 7 meters of packing in 3 sections • Fisher Delta. V Control • Koch’s Katamax packing Unreacted C 5, Me. OH Reactive Distillation Column Recycle Back - Cracking Reactor 3. 7 atm C 5 from Cat Cracker Mixing Tank Pre-Reactor Makeup Me. OH TAME

SRP Pilot Plant • Koch – Spool section, Katamax, Catalyst • SRP - $145 K

Steady-State Multiplicity • Bravo et al. (1993) – Observed multiple steady-states in TAME CD • Hauan et al. (1997) – dynamic simulation provided evidence in MTBE system • Nijuis et al. (1993) – found multiplicity in MTBE system • Jacobs and Krishna (1993) – found multiplicity in MTBE system

Steady-State Distillation Models Trayed Tower: Packed Tower: Equilibrium Model Continuous Model Rate Model

TAME Reaction Rates

TAME Concentration Profile

• Traditionally simulations use intrinsic reaction rate. • Effective rate is a function of intrinsic rate and diffusion limitations. Effective Rate Effective Reaction Rate Molefraction

Control for TAME Tower • Fisher Delta. V – Visual Basic – Matlab, Visual Studio • State Estimation – Temperature Profiles – Online Analyzers • Control Algorithms – PID – Linear MPC – Non-Linear MPC

Individual Work • Design and Construct RD Column for Novel System • Steady State Model Validation • Dynamic Models and Control Study

Novel System • Kinetic Reaction – Not Equilibrium limited – Equilibrium Isomers • Exothermic • Kinetics from CSTR Experiments • Feed is dominated by inerts • Replace hazardous heterogeneous catalyst A + B C 1 C 3 C 2

Novel System Data

Novel System Data

Simulation Validation - 50 psig

Simulation Validation – 35 psi

Effect of Pressure

Effect of Varying Feed Rate

Dynamic Modeling and Control Study • Aspen Custom Modeler/ Aspen Dynamics – Validate Steady State Solution – Validate Dynamic Studies • Develop Control Algorithms – PID – Linear MPC – NLMPC

Aspen Custom Modeler • Formerly Speed-Up and Dyna. Plus • Equation Solver • Aspen Properties Plus • Tear Variables automatically selected • Solves Steady-State and Dynamic • Dynamic Events and Task Automation Equations vs. Variables

Validation of Dynamic Simulator

Feed Disturbance With Manual Control

Control of Reactive Distillation • Configurations – DB – LV – BV, LB… • Goals – Conversion – Product Purity D R F L V Duty B

Control of Reactive Distillation • Bartlett and Wahnschafft (1997) – Simple Feed-Forward/ Feed-Back PI Scheme • Sneesby et al. (1999) – Two point control with linear conversion estimator • Kumar and Daoutidis (1999) – Showed linear controllers unstable for ethylene glycol systems – Demonstrated possible Nonlinear MPC scheme

Dependency of Conversion on Reboiler Duty and Reflux Ratio

Conversion vs Reboiler Duty

Single Tray Conversion Estimation

Single Tray Purity Estimation

Feed Disturbance With Manual Control

Feed Disturbance with Simple PID Control

Conclusion and Future Work • TAME Tower – Collect Data – Validate Models – Developing Advanced Models – Improvements • New chemical system • Adjust for better dynamic studies • Novel System – Validate Dynamic Models – Develop Control Algorithms