An important and challenging problem that Process Engineers frequently encounter is the determination of appropriate control structures that minimize the loss of process performance under the effect of uncertainties. This can be achieved by selecting subsets of controlled and manipulated variables and designing their interconnection (controller synthesis). This is known as the Control Structure Selection Problem (CSSP) In this Thesis, a systematic optimization methodology, based on the back-off concept proposed by Prof J. D. Perkins and co-workers, is presented for the CSSP. The proposed formulation offers the following improvements: a) improves the accuracy of calculations and b) reduces computational time and effort. Specifically, the error involved in the approximation of the nonlinear constraint that defines the magnitude of the back-off vector (needed for control structure selection) is reduced by the introduction of a more accurate linear approximation. In addition, the methodology is able to track the effect of simultaneously occurring disturbances at the same time and estimate their worst impact on process economics. The reduction of computational time is achieved by eliminating the state variables from the final formulation. In this way, the number of equations needed for the CSSP solution is significantly reduced and allows the algorithm to locate the solution faster without affecting the performance. The proposed methodology is firstly applied in a classical distillation column (medium scale) and in a complex and highly nonlinear reactive distillation column (large scale). The results obtained from these case studies made the way for the application of the methodology on the plantwide problem of the benchmark Vinyl Acetate monomer production plant in order to demonstrate the benefits of the proposed algorithm. / Η παρούσα εργασία παρουσιάζει μια συστηματική μεθοδολογία για την εύρεση βέλτιστων δομών ρύθμισης σε συστήματα μεγάλης κλίμακας, όπως για παράδειγμα ολοκληρωμένες μονάδες παραγωγής και βασίζεται στην ελαχιστοποίηση των επιπτώσεων των διαταραχών στην οικονομική απόδοση μιας διεργασίας. Το κύριο στοιχείο της μεθοδολογίας είναι το διάνυσμα υποχώρησης από τους ενεργούς περιορισμούς (μ), το οποίο προτάθηκε από τον Prof. J.D. Perkins και την ερευνητική του ομάδα. Η προτεινόμενη μεθοδολογία ενσωματώνει τα κύρια χαρακτηριστικά της μεθοδολογίας του διανύσματος (μ) και παρουσιάζει μια σειρά βελτιώσεων με στόχο τον περιορισμό των μειονεκτημάτων που απέτρεπαν την εφαρμογή της σε συστήματα μεγάλης κλίμακας. Οι βελτιώσεις επικεντρώνονται σε δύο ζητήματα: α) αύξηση της ακρίβειας των υπολογισμών και β) μείωση του υπολογιστικού φόρτου και χρόνου. Η αύξηση της ακρίβειας των υπολογισμών εντοπίζεται στην κατασκευή ενός βελτιωμένου προσεγγιστικού προβλήματος για την κατασκευή κάτω φραγμάτων για τη βέλτιστη λύση. Η βασική ιδέα πίσω από τη προτεινόμενη βελτίωση βασίζεται στη προσέγγιση των μη γραμμικών όρων που εμφανίζονται με ένα σύνολο γραμμικών εξισώσεων. Επίσης για πιο ακριβείς υπολογισμούς, εξετάζεται κάθε διαταραχή ξεχωριστά σε ένα εύρος συχνοτήτων και στη συνέχεια η μέγιστη επίδραση της καθεμίας χρησιμοποιείται από τον αλγόριθμο έτσι ώστε να υπολογιστεί η μέγιστη δυνατή επίπτωση τους στην οικονομική απόδοση της διεργασίας Η μείωση του υπολογιστικού φόρτου και χρόνου επιτυγχάνεται μέσω της απαλοιφής των μεταβλητών κατάστασης. Με αυτό τον τρόπο, ο αριθμός των εξισώσεων που απαιτείται για την επίλυση του προβλήματος μειώνεται σημαντικά και εντοπίζει τη βέλτιστη λύση σε μικρότερο αριθμό επαναλήψεων της μεθόδου, γεγονός που επιτρέπει την εφαρμογή της σε προβλήματα μεγάλης κλίμακας. Τέλος, η προτεινόμενη μεθοδολογία έχει εφαρμοστεί σε δύο αποστακτικές στήλες και τα αποτελέσματα άνοιξαν το δρόμο για την εφαρμογή της μεθόδου σε μια μονάδα παραγωγής οξικού μεθυλεστέρα, όπου τα πλεονεκτήματα της μεθόδου είναι περισσότερο ορατά και πρακτικά χρήσιμα.

The problem of designing control systems for entire plants is studied. A review of previous works, available techniques and current research challenges is presented, followed by the description of some theoretical tools to improve plantwide control, including the proposal of an augmented lexicographic multi-objective optimization procedure. With these, an augmented hierarchical plantwide control design technique and an optimal multi-objective technique for integrated control structure selection and controller tuning are proposed. The main contributions of these proposed techniques are the inclusion of system identification and optimal control tuning as part of the plantwide design procedure for improved results, support to multi-objective control specifications and support to any type of plant and controllers. Finally, the proposed techniques are applied to industrial benchmarks to demonstrate and validate its applicability. / O problema de projetar sistemas de controle para plantas inteiras é estudado. Uma revisão de trabalhos anteriores, técnicas disponíveis e atuais desafios de pesquisa é apresentada, seguida da descrição de algumas ferramentas teóricas para melhorar o controle plantwide, incluindo a proposta de um procedimento de otimização multi-objetivo lexicográfico aumentado. Com tais elementos, são propostas uma nova técnica hierárquica aumentada de projeto de sistemas de controle plantwide e uma técnica multi-objetivo para seleção de estrutura de controlador integrada à sintonia ótima do controlador. As principais contribuições das técnicas propostas são a inclusão de identificação de sistemas e sintonia ótima de controladores como parte do procedimento de projeto de controle plantwide para melhores resultados, suporte a especificações multi-objetivo e suporte a quaisquer tipos de plantas e controladores. Finalmente, as técnicas propostas são aplicadas a benchmarks industriais para demonstrar e validar sua aplicabilidade.

The problem of designing control systems for entire plants is studied. A review of previous works, available techniques and current research challenges is presented, followed by the description of some theoretical tools to improve plantwide control, including the proposal of an augmented lexicographic multi-objective optimization procedure. With these, an augmented hierarchical plantwide control design technique and an optimal multi-objective technique for integrated control structure selection and controller tuning are proposed. The main contributions of these proposed techniques are the inclusion of system identification and optimal control tuning as part of the plantwide design procedure for improved results, support to multi-objective control specifications and support to any type of plant and controllers. Finally, the proposed techniques are applied to industrial benchmarks to demonstrate and validate its applicability. / O problema de projetar sistemas de controle para plantas inteiras é estudado. Uma revisão de trabalhos anteriores, técnicas disponíveis e atuais desafios de pesquisa é apresentada, seguida da descrição de algumas ferramentas teóricas para melhorar o controle plantwide, incluindo a proposta de um procedimento de otimização multi-objetivo lexicográfico aumentado. Com tais elementos, são propostas uma nova técnica hierárquica aumentada de projeto de sistemas de controle plantwide e uma técnica multi-objetivo para seleção de estrutura de controlador integrada à sintonia ótima do controlador. As principais contribuições das técnicas propostas são a inclusão de identificação de sistemas e sintonia ótima de controladores como parte do procedimento de projeto de controle plantwide para melhores resultados, suporte a especificações multi-objetivo e suporte a quaisquer tipos de plantas e controladores. Finalmente, as técnicas propostas são aplicadas a benchmarks industriais para demonstrar e validar sua aplicabilidade.