Control of high purity distillation columns using a nonlinear wave theory

Abstract

High-purity distillation columns have well known to pose a formidable controller design problem, resulting from unique characteristics, such as complex dynamics, high nonlinearity and interaction between the control loops. Better understanding of the nonlinear and distributed nature of distillation columns is indispensable for improving distillation column in general. A high-purity distillation column shows highly nonlinear, distributed characteristics which are indicated by its sharp composition and temperature profiles, because only a small portion of the column is utilized for the major separation while other parts are only responsible for refining the products. The movement of the sharp profiles in distillation columns can be viewed as nonlinear "waves". Therefore, for maintaining product purity, it is necessary to prevent the sharp profile region from propagating to column ends, resulting in dramatic changes in product composition. A new control scheme to deal with the nonlinear and multivariable distillation system has been developed. It is a model-based controller based on a nonlinear wave theory and the Generic Model Control (GMC) to control the profile position of high purity distillation columns. A nonlinear profile position observer has also been developed and proven to estimate the profile position of the column section with sufficient accuracy. The profile position controller (GMC) is used for instantaneous control to prevent the propagation of disturbances to column ends. However, the offset from the setpoint in product composition may occur when profile position of the column section is fixed at the specific point. Therefore, composition controller is employed and cascaded as a primary controller to remove the offset. The control scheme has been applied to the control of high-purity distillation columns ranging from moderate high-purity to very high-purity. It seems that the composition/profile position cascade control scheme can handle significa...