Interpretation of Temperature Control for Multicomponent Distillation
Despite recent advances in on-line composition measurement, temperature control remains as the major control configuration in distillation control. In a control of industrial ternary distillation column, with non-monotonic composition profile for the intermediate boiler, significant different closed-loop composition dynamics were observed when the temperature control tray was above or below the intermediate boiler composition turning point (e.g., above or below the tray where the intermediate exhibits a maximum). In this work, the role of direct temperature control is interpreted in the composition space. First, the temperature isotherm is established in the triangular composition space and the process direction and control direction can be clearly distinguished. Then, a quantitative measure, the traveling distance for all tray compositions under a specific temperature control configuration is defined and it can be computed directly from process and load transfer function matrices. Rigorous distillation column simulation confirms that a temperature control with a large traveling distance results in slow composition dynamics (e.g., considering the tray composition can be changed with a fixed rate) and, consequently, poorer control performance. The situation, the difference in the traveling distance, can become worse when we are controlling two temperatures in the column, dual-end temperature control. The interaction between temperature control and the column composition profile explains the conflict between the process direction and the control direction which leads to a large traveling distance. That is the result of quantitative measure can be visualized directly from the composition space provided with the isotherm of the temperature control tray. Next, the effects of product purities, relative volatilities, feed composition, and types of separation (e.g., direct, indirect, and transition separations) are explored and the results can be interpreted quantitatively with the traveling distance or qualitatively by inspecting the possible conflict between the process/control direction in the composition space. Finally, this concept is extended directly to composition control of multicomponent distillation systems. The results clearly show that improved temperature or composition control can be achieved by avoiding potential conflict in the process/control direction.
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