Control Structure Design for Two Distillation Columns in Sequence
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Even for simple distillation systems with only two columns, the control problem can be significant. This project provides an initial outline of a systematic procedure for finding the optimal control structure, through identification of active constraint regions and self-optimizing variables. The procedure is used to study a system of two distillation columns in sequence in order to find the optimal control structure. To achieve optimal control, the active constraints have to be identified and controlled tightly. Which constraints that are active depend on the external conditions. This project has mapped how the active constraints change following variations in the steam price and feed flow. Two of the active constraint regions were studied further. A rigorous steady state analysis was conducted to find variables, which when controlled to constant value, keep the operation close to optimal. Variables with such properties are said to be self-optimizing. Controlled variables consisting of single measurements and measurement combinations have been investigated. In a constraint region with low energy price, controlling a tray temperature in the top part of the second column was found to be optimal. In a second region studied, where only the product specification of the valuable product is active, three tray temperatures that would keep the system close to optimum were identified.