| There are generally slowly-time-varying characteristics in chemical processes,and the changes of slowly-time-varying parameters in an operating cycle of chemical plants gradually decrease chemical plant performance.So the operating point slowly deviates from the steady-state optimum design point such that the process constraints will be violated.In chemical processes there are also various unmeasured disturbances.The slowly-time-varying uncertainties and unmeasured disturbances are the main uncertainties of slowly-time-varying chemical processes,they both go through dynamic processes have different time scales.For this reason,the margin is defined as the value added on the nominal value of the design variable because of process uncertainties.During the design phase of chemical processes,enough margins must be added for design variables so that there are sufficient distances between the operating point and process constraints to prevent process variables from breaking process constraints after the uncertainties become "worse".But the design margins are usually determined according to the worst-case influence of various uncertainties,while the changes of slowly-time-varying parameters from the initial state to the “worst” is a slowly dynamic process,so the process margin consumption is a long dynamic process affected by slowly-time-varying uncertainties.Moreover,unmeasured disturbances also result in the control margin consumption.In this study,considering the dynamic processes of slowly-time-varying uncertainties and unmeasured disturbances under different time scales,the consumed margins involving process margins(for slowly-time-varying uncertainties)and control margins(for unmeasured disturbances)in an operating cycle are estimated through on-line optimization.Operation optimization is an effective method to explore potential economic benefits for existing plants.The maximum potential benefit from operation optimization is determined by the distances between current operating point and process constraints,which is related to the margins of design variables.The maximum potential benefit of steady-state optimization is related to the design margins minus consumed process margins.Because of various disturbances in chemical processes,some distances must be reserved for fluctuations of process variables and the optimum operating point is not on some process constraints.Thus the benefit of steady-state optimization can not be fully achieved,but the benefit of dynamic optimization can be really achieved,which is equal to the achievable benefit and related to the design margins minus consumed process margins(for slowly-time-varying uncertainties)and consumed control margins(for unmeasured disturbances).In this study,the economic benefits of operation optimization(including the maximum potential benefit and achievable benefit)are estimated on-line according to the distances between operating point and active constraints.These methods are applied to the continuous stirred tank reactors in series and acetylene hydrogenation reactors,based on the dynamic changes of slowly-time-varying parameters and unmeasured disturbances in an operating cycle of chemical plants,the consumption of design margins and the economic benefits of operation optimization explored from the surplus margins are estimated online,thus a long period operation optimization plan of chemical plants is given. |
