What is the primary reason for using a cascade control loop as opposed to a single-loop control in a distillation column?

The primary reason for using a cascade control loop as opposed to a single-loop control in a distillation column is to better reject disturbances in the heat input to the reboiler, thereby maintaining a more stable column temperature profile and product composition. A distillation column separates components of a liquid mixture based on their boiling points. Precise temperature control is crucial because the vapor-liquid equilibrium, which governs the separation, is highly temperature-dependent. A single-loop control might directly manipulate the steam flow to the reboiler to control the column temperature. However, this approach is susceptible to disturbances like fluctuations in steam supply pressure or changes in the heating value of the steam, which can affect the heat input independently of the steam flow rate. A cascade control loop addresses this by adding an inner loop. The outer loop controller measures the column temperature and sets the setpoint for the inner loop. The inner loop controller then manipulates the steam flow to maintain the reboiler's steam pressure or the temperature of the reboiler itself. By controlling the steam pressure or reboiler temperature, the inner loop effectively isolates the column temperature from steam supply variations. Therefore, even if the steam supply pressure fluctuates, the inner loop adjusts the steam flow to maintain a constant heat input to the reboiler, providing more stable and precise temperature control in the column compared to a single-loop system that only adjusts steam flow based on column temperature, thereby improving separation efficiency and product quality.