Study On Nonlinear Characteristics And Global Optimization Of Heat Exchanger

The perfect overall performances of heat exchanger networks (HENs) have crucialimpact on energy conservation and economic benefit in the energy, petroleum andchemical field. Therefore, desiging a HEN with excellent performance is the commongoals for all of the workers in this studing field, more and more attention is paid tooptimization design of HENs. The global optimization of HENs is a Mixed-IntegerNonlinear Programming problem(MINLP) in nature, With the development of networksscale, the nonlinear characteristics become more and more serious for HENsoptimization and the local optimal solutions will increase in the form of factorial. Forachieving global optimization, it should be important to escape from the local optimalsolution effectively. By analysing the nonlinear characteristics of HENs, this articalproposed and improved several HENs optimization methods for further investigatingthe global optimization problems.Firstly, the nonlinear characteristics of HENs was in-depth analysis, and explainingthe difficuly in the sloving process of mixed-integer nonlinear programming (MINLP),including the global optimization of HENs. Base on a specific HEN example, thenonlinear characteristics and the influence of continous variable and integar variable onHENs performance were investigated with performance atlas., it shows that it would beeasily prone to fall into a local optimal solution with sequence optimizing methods. Fordifferent optimizing sequence generates different results, this paper avoided theproblems of local optimal solutions by changing the optimizing sequence and thenimproved the optimization quality.Furthermore, for escaping from a local optimal solution and improving theoptimization quality, this paper proposed three improved optimizing methods: heat loadloop breaking and merging method, improved multi-start method and secondarysuperstructure model. These methods could avoid the problems of optimal solutions andcontinue further optimization by breaking thermal equilibrium effectively.Subsquently, for overcoming trapping in the local optimal solutions, filled functionalgorithm was then introduced into HENs global optimization. With filled function andlocal optimizing methods alternately, the global optimal solution in solving domaincould be obtained by escaping from the local optimal solutions.Afterwards, the function approximation and cutting plane method for the global optimizing of HENs was presented. The typical example of the solving steps based onone-dimensional and two-dimensional nonlinear problems shows the general strategyapplying to the global optimization of HENs. To the inverse problem of confirming thesolving area boundary using cutting plane method, one-dimensional searching methodwas applied to determine the upper and lower bound of solving area boundary. Bycutting the plane repeatedly, the feasible region of performance function would shrinkcontinuously until the global optimal solution was obtained.Finally, the effectively combination of deterministic methods and randomizationmethods was applied to the global optimization of HENs. The initial structure generatedby randomization methods would be further optimized by deterministic methods, whichcould both improve the search efficiency and the solving precision relatively.In a word, for achieving HENs global optimization, the difficulty of easily trappingin optimal solutions in optimizing process was studied and sevel methods wereproposed corresponding to the existing problems...