Study on Integration of Flexible Heat Exchanger Network Synthesis and Advanced Control

Author:Gu Zuo

Supervisor:du jian


Degree Year:2019





With the utilization of process system engineering methodology,optimization and synthesis of heat exchanger networks(HENs)identify opportunities to reduce energy consumption,submitted strong theories of practical significance to the energy recovery and its rational utilization.However,the traditional HEN synthesis methods are faced with numerous challenges because of the industrial demands by guaranteeing smooth and safe operation in spite of dynamic disturbances and process uncertainty.On one hand,the current research is limited to respectively enhance flexibility and controllability of HENs,meanwhile it is also limited to steady-state research.On the other hand,the coupling relationship between flexible synthesis and control is ignored,leading to the increase of control difficulties,the redundancy of HEN structure and heat exchanger areas.Hence,this thesis is focused on the simultaneous synthesis of flexible and controllable HENs,in which an efficient method to dynamic flexible HENs synthesis is developed and then the two-tier control structure design method is proposed,further to propose the method of integration between synthesis of flexible heat exchanger networks and advanced control.The main contents and the results of this thesis are as follows:(1)For avoiding the limitation of the economy and operability of HENs in the plants,this thesis attempts to explore the possibility of coupling the flexibility and controllability by optimizing the initial fractions and locations of bypasses,and then the simultaneous HENs synthesis method is proposed.This work begins with the identification and quantification of disturbance propagations,which are employed to optimize the initial fractions of all the potential bypasses.Then,their sensitivities to the entire HEN are examined for optimizing bypass positions.In this way,a superstructure-based mixed integer non-linear programming(MINLP)model with objective function of minimizing the total annual cost is formulated.With the proposed method,a HEN is not only economically optimal,but also exhibits good flexibility and controllability performances,still as being a basis for the dynamic flexibility synthesis and the control structure design.(2)For avoiding the limitation of multiple disturbances on HENs and operating successfully throughout the whole time horizon even facing the stochastic and the time-varying disturbances,a method for synthesizing the dynamic flexible HENs is proposed.The temperature control ranges are proposed to describe the ranges of the stream output temperature variations deviating from their set points.This insights that the hyperrectangle desired by this thesis merely requires to close to rather than well locating at the boundary of the dynamic feasible region of the HEN,giving additional space for the trade-off between the cost and dynamic flexibility.This work couples the HEN structure synthesis stage with the dynamic flexibility analysis for addressing the problem of optimal design in terms of cost,in which dynamic flexibility consideration is simultaneously accomplished.This work begins with the multiple disturbances and then the bottleneck of dynamic flexibility is identified.For which,the structure synthesis is carried out on the basis of rolling horizon optimization.This work gives the trade-off between the cost and dynamic flexibility,filling in the blank of dynamic flexible synthesis of HENs.This also provides a new idea for the integration of HEN synthesis and control.(3)For enhancing the HEN controllability by cooperating control loops and manipulated variable(MV)pairings,a method for designing the two-tier control structure is proposed.This is developed based on the sequential strategy,coupling an outer-tier with inner-tier control structure design.For a given HEN,the identification and pairing of the MVs are proposed to design an outer-tier control structure;while the inner-tier control structure is determined with the objective of minimizing the control loop interaction.With the proposed method,the heat variation balance is regarded as the breakthrough and the influence of the active pairings on the activity identification is revealed,efficiently distributing sufficient controlled operation space;while the improved quantification strategy can be used to avoid multiple solutions,breaking all the control loops into individuals.(4)In current research,the coupling relationship between dynamic flexible synthesis and control is ingored while the complex controllers are overly depended.To effectively realize the simultaneous optimization of HEN structure,dynamic flexibility and control performance,this thesis proposes a method of simultaneous integration between synthesis of dynamic flexible HENs and advanced control.The temperature control ranges are considered and then divide continuous time into the discrete moments,implementing the dynamic flexible HENs synthesis.In this way,the controllers are introduced to regulate stream output temperatures while optimizing the decision variable of the dynamic flexible synthesis.With the proposed method,the interaction between dynamic flexible synthesis and control is thoroughly investigated,ensuring the capability of accommodating multiple disturbances,energy integration and control performance.This new idea is employed for the guidance to the practical engineering problems.(5)In the last part of this thesis,a large-scale integrated HEN in an ethylene plant is obtained with the proposed methods.The result satisfies the demand for capability of accommodating multiple disturbances,energy integration and control performance.It has demonstrated the application of our methods for the guidance to the practical engineering problems.