The development of a controllability Index based on RGA and Exergy for plantwide control problems

Abstract

Process design is a complex activity and has many levels and stages. The information and knowledge required at every level of the design increases through the process design sequence. As the design process is constrained by control and thermodynamics (eco-efficiency), for efficient and easily controllable process design, an integrated approach involving process design, control and thermodynamics (eco-efficiency) must be used during early design stages. An integrated approach has many advantages over previous approaches in which process control was only considered during the late stages of the process design. For an easily controllable process, the controllability of the process should be determined as early as possible in the process design sequence. Thus there is a need for tools which can evaluate the controllability of the process before detailed design or construction commences. For the selection of the best control configuration, there are many mathematical formulations and systematic strategies, such as; the relative gain array (RGA), the Niederlinski index (NI), singular value decomposition (SVD), the condition number (CN) and Morari’s resiliency index (MRI). In the wake of the energy crises, control system structure selection must focus, not only on controllability, but also on energy cost and environmental impact (eco-efficiency). For the integration of controllability and eco-efficiency, the potential of the thermodynamic property exergy is used. The combination of the 1st and 2nd laws of thermodynamics gives rise to the concept of Exergy. Exergy combines control scheme selection and eco-efficiency into a single domain. Exergy has many valuable uses in process design and control for the facilitation of these complex tasks. There are many exergy based tools and methods, such as; the efficiency concept, the exergy flow diagram, the relative exergy array (REA), the exergy eco-efficiency factor (EEF) and the relative exergy destroyed array (REDA). These tools/methods can be used to describe, analyze, optimize and control the process. In this research project, tools and methods were developed to determine plantwide controllability/control scheme interactions (plantwide RGA/REA), a process exergy calculator, whole process eco-efficiency analysis tools (EEF and REDA), and tools to integrate process design, control and eco-efficiency. These newly-developed tools (the REA, EEF and the REDA) combined with classical controllability tools (the RGA, NI, SVD and the CN) help in deciding on controllable and eco-efficient control schemes for an entire process/plant.