Interpretation of Temperature Control for Ternary Distillation

Even with recent advances in technology for on-line composition measurement, temperature remains the dominant control configuration in distillation columns for product purity. In controlling industrial ternary distillation columns, with a nonmonotonic composition profile for the intermediate boiler, significantly different closed-loop composition dynamics are observed when the temperature-control tray is above or below the intermediate boiler composition turning point (i.e., 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, called the traveling distance, for all tray compositions under a specific temperature-control configuration is defined. The traveling distance can be computed directly from process and load transfer function matrices. Rigorous nonlinear distillation column simulations confirm that a temperature-control point 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 with the difference in traveling distance can become worse when two temperatures are controlled in the column. Finally, this concept is extended to direct composition control of ternary distillation systems. The results clearly show that improved temperature or composition control can be achieved by avoiding a potential conflict in the process and control directions.