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http://hdl.handle.net/11375/25334
Title: | Improvements to the design methodology and control of semicontinuous distillation |
Authors: | Madabhushi, Pranav Bhaswanth |
Advisor: | Adams, Thomas |
Department: | Chemical Engineering |
Keywords: | Semicontinuous Distillation;Design;Control;Dynamic Optimization;Limit Cycle;Shooting Method |
Publication Date: | 2020 |
Abstract: | Distillation technology has been evolving for many decades for a variety of reasons, with the most important ones being energy efficiency and cost. As a part of the evolution, semicontinuous distillation was conceived, which has the advantages of both batch and continuous distillation. The economic benefits of this intensified process compared to batch and continuous distillation were expounded in many of the previous studies. Semicontinuous distillation of ternary mixtures, which is the main focus of this thesis, is carried out in a single distillation column with a tightly integrated external middle vessel and the operation is driven by a control system. The system operation does not include any start-up or shut-down phases of the column and has three periodically repeating operating modes. In the status quo design procedure, called the ‘sequential design methodology,’ an imaginary continuous distillation system design was used to design the semicontinuous distillation system. In this methodology, dynamic simulations of the process were used to find the values of the controller tuning parameters based on the design of the continuous system. Afterwards, black-box optimization was used to find better controller tuning parameter values that minimized cost. However, after analyzing the dynamics of the system for different cases, it was found that the heuristics used in this design methodology yielded suboptimal designs. Therefore, the primary goal of the thesis is to improve these heuristics by incorporating more knowledge of the system and thereby develop a better design methodology. Firstly, the setpoint trajectories generated by the ideal side draw recovery arrangement for side stream flowrate control, which was standard in most semicontinuous distillation studies, was modified. In this thesis, the performance of the status quo as compared to the modified version, based on the criteria, cycle time and cost for different case studies, was presented. Results showed that the modified-ideal side draw recovery arrangement for side stream flowrate control performed better with a 10-20% lower separating cost while maintaining product purities. Furthermore, to reap more cost benefits, dynamic optimization was used to seek the flow rate trajectory that minimized cost. However, it was found that the additional cost savings, which is in addition to the benefits gained by using the modified version, were at the most 2% from different case studies. Subsequently, the impact of changing the imaginary continuous distillation system design on the nature of the semicontinuous distillation limit cycle, specifically, its period was studied. Results revealed the necessity for a new design procedure, and thus the back-stepping design methodology was proposed. This design methodology was used to find better limit cycles of zeotropic ternary semicontinuous distillation using the aspenONE Engineering suite. The proposed methodology was applied to three different case studies using feed mixtures with different chemical components. A comparison with the sequential design methodology for the two case studies indicates that the new method outperforms the state-of-the-art by finding limit cycles that were 4% to 57% lower in terms of cost. Furthermore, the designs obtained from this procedure were guaranteed to have feasible column operation with stable periodic steady-state behaviour. Semicontinuous distillation design using the design methodology with heuristic components involves guessing, checking and then using black-box optimization to find the values of the design variables to meet some performance criteria. Furthermore, mathematical guarantees of either local or global optimality of the designs obtained from the design procedure do not exist. Therefore, to address these issues, in this thesis, the application of using the shooting method for designing the semicontinuous distillation process was demonstrated using two case studies, which involve the separation of hexane, heptane and octane. This method has the potential to be combined with gradient-based optimization algorithms for optimization of the process design in the future. |
URI: | http://hdl.handle.net/11375/25334 |
Appears in Collections: | Open Access Dissertations and Theses |
Files in This Item:
File | Description | Size | Format | |
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Madabhushi_Pranav Bhaswanth_202003_PhD.pdf | 15.31 MB | Adobe PDF | View/Open |
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