Integrated Scheduling and Control of an Air Separation Unit Subject to Time-Varying Electricity Prices

A novel framework for making plant scheduling decisions while considering the plant process dynamics is presented in this thesis. A model of an air separation unit built to supply nitrogen gas and subject to time-varying electricity prices was used to illustrate this framework. The model includes a cryogenic distillation column with an integrated reboiler/condenser, a multi-stream heat exchanger, a compressor, two turbines, and a liquefier. It consisted of 5079 equations and 437 differential variables. The dynamic behavior of the process during operating point transitions was determined using dynamic optimization. This data were used to establish a reduced order dynamic model of the system which captures the transient behavior of relevant process variables. The reduced order model consisted of 525 equations and 67 differential variables. The identified model showed a good fit between the relevant process variables in the simulated transitions and the reduced order model. The air separation unit process schedule was optimized using the reduced order model to minimize electricity cost over a three day time horizon. The optimal result showed a 2.6 % reduction in electricity cost compared to a flat production rate. The optimal schedule was implemented and simulated in the full dynamic model for the first 24 hours to compare the relevant process variables to the reduced model predictions. The result displayed good match between the reduced model and the full dynamic model. This thesis shows that an accurate reduced order dynamic model can be used for quickly finding the optimal schedule of large process systems. This by greatly reducing the size and complexity of the system without sacrificing accuracy of the dynamic behavior. Furthermore, it also shows the economic benefits of the integrating scheduling and control to count for the dynamic behavior of the system. Acknowledgments I was fortunate enough to conduct my research for this thesis at the University of Texas at Austin in the renowned McKetta Department of Chemical Engineering. I worked under the supervision and guidance of Dr. Michael Baldea, who helped me achieve my goals in research and writing. This thesis would not have been possible without his help. I would also like to extend my thanks and gratitude to his research group, who were always welcoming and helpful. Richard Pattison and Cara Touretzky were especially valuable resources, whose knowledge and experience were always readily provided to me. I am proud to present this thesis, a product of a great team of researchers whose help and expertise must be recognized. I would like to also acknowledge the strong foundation in chemical engineering that I built while at KTH. Each of my professors imparted upon me the tools and knowledge I utilized in formulating and carrying out my thesis work. Dr. Per Alvfors, as my examiner, was an invaluable link to my home university and always available for counseling or advice. While I conducted my research abroad, it was made possible by the amazing university that I can now call my alma mater.