OPTIMAL OPERATION OF INTEGRATED PROCESSES Studies on Heat Recovery Systems

Three important parts of an integrated plant are reactors, separators and a heat exchanger network (HEN) for heat recovery. Within the process engineering community, much attention has been paid to design of reactors, separators and HENs. This thesis, however, is devoted to operation and in particular to optimal operation of HENs. The purpose of heat integration is to save energy, but the HEN also introduces new interactions and feedback in the overall plant. A good strategy for operation of HENs should, in addition to controlling the outlet temperatures to target values (setpoints), minimize the external energy consumption when disturbances and setpoint variations are encountered. A prerequisite for optimization is that there are extra degrees of freedom left after regulatory control is implemented. It is shown that extra degrees of freedom may not always be utilized for energy optimization, and a quantitative expression for the degrees of freedom that can be utilized for optimization is presented. A simplified expression that is often valid is also deduced. How to operate a HEN close to optimal may often be found from structural information alone. The thesis presents some improvements and generalizations of a structure based method that has been proposed earlier. Structural information is used to divide possible manipulations into three categories depending on how each manipulation affect the utility consumption. Using these three categories and two heuristic rules for operability, the possible manipulations are ordered in a priority table. This table is used to determine which manipulation should be preferred, and which manipulation should be selected if an active manipulation is saturated. It is shown that the implementation of the method may correspond to split-range control. A method that in addition to structural information, also utilizes parametric information is proposed. This method is heavily inspired by the structural method, however, the optimal control structure is found through solving an integer programming problem. The control structure is found periodically during operation, and as for the structure based method, this can lead to a varying control structure. The basic model does not incorporate any controllability considerations. Two methods to include controllability are proposed. Both methods are based on extending the basic model with logical inference using additional constraints. In both cases, the result is a hierarchical strategy for operation that has been embedded into the model. A third method that combines the use of steady state optimization and optimal selection of measurements is proposed. Steady state optimization is carried out periodically during operation to compute optimal setpoints for the secondary measurements. The setpoints for the primary measurements (outlet temperatures with target values) are not subject to optimization. However, extra manipulations may be used for energy optimization and this may be done by controlling some internal temperatures (secondary measurements). When unknown disturbances and model errors are present, the selection of which secondary measurements that are controlled affects how close to optimum the process can be operated. A procedure for selection of secondary measurements for processes where optimum is located at the intersection of constraints (which is typical for HENs) is presented.

[1]  Efstratios N. Pistikopoulos,et al.  Synthesis and Retrofit Design of Operable Heat Exchanger Networks. 1. Flexibility and Structural Controllability Aspects , 1994 .

[2]  Z. Fonyó,et al.  SYNTHESIS OF HEAT EXCHANGER NETWORKS , 1982 .

[3]  Robin Smith,et al.  Cost optimum heat exchanger networks—2. targets and design for detailed capital cost models , 1990 .

[4]  Ignacio E. Grossmann,et al.  Simultaneous optimization models for heat integration—II. Heat exchanger network synthesis , 1990 .

[5]  James M. Douglas,et al.  Conceptual Design of Chemical Processes , 1988 .

[6]  Ignacio E. Grossmann,et al.  Mixed-integer programming approach for the synthesis of integrated process flowsheets , 1985 .

[7]  Ignacio E. Grossmann,et al.  A global optimization algorithm for linear fractional and bilinear programs , 1995, J. Glob. Optim..

[8]  Ignacio E. Grossmann,et al.  New trends in optimization-based approaches to process synthesis , 1996 .

[9]  Bodo Linnhoff,et al.  A User guide on process integration for the efficient use of energy , 1994 .

[10]  L. Naess,et al.  The synthesis of cost optimal heat exchanger networks. An industrial review of the state of the art , 1988 .

[11]  George Stephanopoulos,et al.  Structural operability analysis of heat exchanger networks , 1986 .

[12]  Ian Postlethwaite,et al.  Multivariable Feedback Control: Analysis and Design , 1996 .

[13]  M. Morari,et al.  Variable selection for decentralized control , 1992 .

[14]  Ignacio E. Grossmann,et al.  A structural optimization approach in process synthesis. II: Heat recovery networks , 1983 .

[15]  Christodoulos A. Floudas,et al.  Automatic synthesis of optimum heat exchanger network configurations , 1986 .

[16]  George Stephanopoulos,et al.  A structural approach to the design of control systems in heat exchanger networks , 1988 .

[17]  T. Gundersen,et al.  Optimal operation of heat exchanger networks based on structural information , 1996 .

[18]  B. Linnhoff,et al.  The design of separators in the context of overall processes , 1988 .

[19]  Ignacio E. Grossmann,et al.  An index for operational flexibility in chemical process design. Part I , 1983 .

[20]  Sigurd Skogestad,et al.  On-line optimization and choice of optimization variables for control of heat exchanger networks , 1997 .

[21]  Sigurd Skogestad,et al.  Bypass selection for control of heat exchanger networks , 1992 .

[22]  Truls Gundersen,et al.  An extended vertical MILP model for Heat Exchanger Network Synthesis , 1996 .

[23]  R. M. Wood,et al.  A new method for heat exchanger network synthesis using area targeting procedures , 1995 .

[24]  Christodoulos A. Floudas,et al.  Synthesis of flexible heat exchanger networks with uncertain flowrates and temperatures , 1987 .

[25]  Christodoulos A. Floudas,et al.  Algorithmic approaches to process synthesis : logic and global optimization , 1994 .

[26]  B. Linnhoff,et al.  Heat-recovery networks: new insights yield big savings , 1981 .

[27]  Manfred Morari,et al.  Design of resilient processing plants—II Design and control of energy management systems , 1982 .

[28]  Ignacio E. Grossmann,et al.  Optimal process design under uncertainty , 1983 .

[29]  Ignacio E. Grossmann,et al.  Improved optimization strategies for automated heat exchanger network synthesis through physical insights , 1990 .

[30]  Manfred Morari,et al.  Design of resilient processing plants—IV: Some new results on heat exchanger network synthesis , 1984 .

[31]  B. Linnhoff,et al.  Sensitivity tables for the design of flexible processes (1) ― How much contingency in heat exchanger networks is cost-effective? , 1986 .

[32]  H. Rosenbrock,et al.  State-space and multivariable theory, , 1970 .

[33]  Robin Smith.  Chemical process design , 1994 .

[34]  Efstratios N. Pistikopoulos,et al.  Synthesis and Retrofit Design of Operable Heat Exchanger Networks. 2. Dynamics and Control Structure Considerations , 1994 .