Comparison of Advanced Distillation Control Methods

Detailed dynamic simulations of three industrial distillation columns (a propylene/propane splitter, a xylene/toluene column, and a depropanizer) have been used to study the issue of configuration selection for diagonal PI dual composition controls. ATV identification (Astrom and Hagglund, 1988) with on-line detuning was used for tuning the diagonal PI composition controllers. Each configuration was evaluated with respect to steadystate RGA values, sensitivity to feed composition changes, and open loop dynamic performance. Each configuration was tuned using setpoint changes over a wider range of operation for robustness and tested for feed composition upsets. Overall, configuration selection was shown to have a dominant effect upon control performance. Configuration analysis tools (e.g., RGA, condition number, disturbance sensitivity), were found to reject configuration choices that are obviously poor choices, but were unable to critically differentiate between the remaining viable choices. Configuration selection guidelines are given although it is demonstrated that the most reliable configuration selection approach is based upon testing the viable configurations using dynamic column simulators. ACKNOWLEDGMENTS This is a cost-shared project between Texas Tech University and the U.S. Department of Energy, Assistant Secretary for Energy and Renewable Energy, Office of Industrial Technologies, under DOE Albuquerque Field Office Cooperative Agreement DE-FC0494AL98747. Bruce Cranford and Charles Russomanno have been the Program Managers for the DOE Office of Industrial Technologies. Porter Grace and Ken Lucien have been the Project Technical Managers for the DOE Albuquerque Operations Office. Chuck Quinn and Frank Childs, Project Technical Monitor for DOE, were on the staff' of Scientech, Inc., at Albuquerque, NM and Idaho Falls, ID, respectively. Dr. James Eggs Principal Investigator. INTRODUCTION Distillation in the refining and chemical industries consumer 3% of the total U.S. energy usage (hwnphrey it al, 1991) which amounts to approximately 2.4 quad of energy annually. In addition, distillation columns usually determine the quality of fmal products and many time determine the maximum production rates.