Design And Simulation Of Control Structure For Methanol Distillation

Distillation is an important link for industrial production of methanol. Distillation column is the main facility of methanol purification process which is a MIMO system. The MIMO system associate with highly nonlinearity and lag, generallyboth features may make the control of distillation process more difficult, so the performance of control has great iMPact on energy saving, improving the quality of product and enterprise’s efficiency.The research object of the thesis is the new three-column double-effect distillation process proposed by domestic scholar in2008. In the paper, the steady mathematical models, dynamic mathematical models, open loop dynamics behaviors and controllability analysis are discussed in this paper to design a set of control scheme. The crude methanol feed flow rate disturbance and the feed component disturbance of methanol in it are set as interference to test the reliability of the basic control scheme. The major studies of the paper are listed below:(l)The Aspen Plus and Aspen Dynamics are taken as simulation tool to finish steady and dynamic simulation respectively.(2)The relay feedback test is used as a model identification tool to obtain low-order plus dead time transfer function. The step response curves of identification models and Aspen Dynamics step response curves of rigorous process models are obtained to verify the reliability of identification model. The result show identification models and Aspen Dynamics step response curves of rigorous process models matched well,so the identification model can be used as mathematical model to design control scheme.(3)The key control schemes R1-XHT、R2-XLT、QRE-XHB、FL-XLB are obtained based on RGA, DRGA and DC analysis which are calculated from low-order plus dead time transfer function, and the reliability of the control scheme is tested.Judging from the dynamic response curves, the basic control scheme cannot adjust the disturbance effectively, especially the control variable-XIt which cannot return to the set value after disturbance happened, and it keeps at a new stable value99.8%(mol) felled short of standard value99.9%and shows overshoot.To solve the problem, dynamic analysesare made. The pressure on sensitive plate in high pressure and low pressure column cannot return to set value, even the temperature on each plate beside the sensitive plate in both columns cannot return to set value still, so the second temperature control loop is installed based on SVD analysis.In addition, the pressure-temperature compensation control scheme is proposed to solve the problem of sensitive plate pressure which cannot return to the set value. To verify the reliability of the modified control scheme, the disturbance which is same as the former one imposed to test the basic control scheme performance is imposed, the result show the modified scheme has better performance than the basic one.