Patent ID: 7292899

Claim:
A method of controlling a distillation column having control valves including a reflux flow control valve to manipulate reflux flow rate to a top section of the column and at least one inlet for a feed to be separated situated below a reflux inlet for the reflux flow, the feed having a varying temperature that could potentially affect a first temperature within the top section of the distillation column upon an increase in feed temperature and a varying composition that also could potentially affect the first temperature upon an increase of less volatile components within the feed, said method comprising; repeatedly executing a model predictive controller at a controller frequency; the model predictive controller having a dataset, containing records over a previous time period equal to a prediction horizon that includes valve positions of the control valves, including the reflux flow control valve, as manipulated variables and corresponding actual values of sensed temperatures, including the first temperature, as controlled variables and predicted values for the controlled variables, the model predictive controller also being programmed with step response models relating the manipulated variables to the controlled variables; during each execution of the model predictive controller, updating the dataset with the actual current values of the manipulated variables, utilizing the dataset and the step response models to calculate prediction errors, applying the prediction errors to predictions as an off-set, calculating open and closed loop predictions over the prediction horizon and obtaining a set of move plans for movements of the manipulated variables to minimize the difference between the controlled variables and target values related to the controlled variables, and generating signals referable to initial movements contained in the set of move plans; and transmitting the signals to controllers used in setting the control valves, thereby to implement the initial movements of the control valves; a first of the controlled variables being referable to the first temperature and having a value that is equal to the first temperature, when said first temperature is below a threshold temperature and a transformed value, when said first temperature is above the threshold temperature, the transformed value being calculated from a sum of said threshold temperature, a first tuning factor and a temperature change divided by a second tuning factor used to amplify an effect of the temperature change, the temperature change being computed by subtracting from current first temperature, the first temperature sensed during a previous execution of the model predictive control program.