Research On Bypass Control System Of Flexible Heat Exchanger Networks Based On Frequency Domain Method

The design of heat exchanger networks(HENs)must meet the requirements of flexibility and stability at the process of operation with the existence of disturbance.Some researchers have proposed many methods of flexible analysis and design for HENs,most of which are based on the steady-state characteristics.Therefore,it is necessary to consider the dynamic characteristics and the influence of dynamic characteristics on the design at the beginning of the design of HENs.The dynamic characteristics analysis of process systems includes the time domain analysis and the frequency domain analysis.The former is widely used but pays more attention to instantaneous characteristics of dynamic responses.The latter not only corresponds to instantaneous characteristics of dynamic responses,but also considers the stability of process systems.Stability margins in the frequency domain can quantitatively reflect the stability for process systems and can be applied in the design of control systems.The design of HENs based on flexibility can meet the needs of operation resulting in flexible regions containing unstable part that should be avoided.Therefore,stability should be considered when analyzing flexibility.On the other hand,it is necessary to leave a certain area margins at the beginning of design to meet flexibility and the needs of dynamic control.The design of area margins and stability analysis of HENs will affect the dynamic characteristics and safe operation of system.Therefore,it is necessary to consider economics,flexibility and stability simultaneously in the process of area margins design.In this paper the relationship between the area of heat exchanger,stability margins and flexibility is studied on the basis of the inverse return difference matrix and Nyquist curve.The frequency domain analysis method is used to obtain the stability margins of HENs and a method for optimizing the area margins of HENs based on the frequency domain design method is proposed.On the basis of the area margins design of HENs,the method of bypass control is widely used in order to achieve the aim of continuous energy saving and consumption reduction.In recent years,some researchers have proposed a control system design method in the frequency domain which is extensively used to the study of large time-delay,complex process systems.The HENs are a typical large time-delay and complex process system.The appropriate bypass position is selected based on the frequency domain relative gain array and the parameters of controller are designed to make the control system within a feasible stability margins.Finally two case studies indicated that the design method of area margins and bypass control system can achieve the purpose of meeting stability margins and optimization of control performance and economic.