Research On The Curve Control Strategy Based On NMPC For Distillation Process

Distillation process is a typical industrial separation procedure, which is widely used in chemical, petrochemical, petroleum and other industries. The pressure and temperature in distillation process, the composition in top and bottom of the column and other process variables are key factors which will affect the quality of the control and the products quite a lot. Distillation column is a typical production object in industrial process. When the system with constraint encounter the change of the load or the component, it will reflect strong nonlinear characteristics. In this case, the traditional linear model predictive control theory and method can not meet the requirements of the production process. The nonlinear model predictive control algorithm must be used in the prediction and control of the industrial process.In this paper, the distillation column which is described by the DAE differential algebraic equation model are set as the object, and the nonlinear model predictive control strategy (NMPC) which is based on mechanism model is selected as the algorithm of the system. The nonlinear control problem of distillation column is studied under the circumstances of variable condition (the change of load or feed component). In the distillation column control strategy, traditional binary composition control strategy is extended to composition profile curve control strategy, and a centroid control method based on composition profile curve is proposed in this paper. The centroid control strategy based on composition profile curve has obvious advantages both in the convergence speed and the control performance.The main research work and innovation of this paper are as follows:Based on the mechanism model equation of the distillation column, the nonlinear predictive control algorithm based on mechanism model and the solution strategy are studied. The optimization problem of distillation column is analyzed and the constraint of distillation process is discussed in detail. Combined with the mechanism model of the distillation column, the solution strategy of the algorithm is described in detail.Under the circumstance of the change of the load and the product composition, two kinds of the composition profile curve control algorithm is studied:the predictive control algorithm based on the composition curve and the predictive control algorithm based on B spline curve. Under the change of the feed compositon and the load, the system response speed and control quality of the three control algorithm (traditional binary composition control strategy, control algorithm based on composition curve and control algorithm based on B spline curve) are simulated and compared. The results indicate that the performance of the composition profile curve control strategy is superior to the traditional binary composition control strategy, while the control performance and control quality of the predictive control strategy based on composition profile curve and predictive control strategy based on B spline curve does not reflect too much difference.Under the change of operation conditions, considering the problem of the slow convergence speed of the control algorithm above, a centroid control algorithm based on composition profile curve control is proposed in this paper. This algorithm utilize the centroid of the curve to describe the whole composition profile curve. Under the circumstance of the change of the load and the feed composition, the proposed centroid control algorithm is simulated and compared with the other two kinds of profile curve control strategy. The results verify the fast convengence speed and the anti-interference performance of the proposed centroid profile control algorithm.