Synthesis Of Flexible Heat Exchanger Networks Considering Fouling Growth

Flexible heat exchanger network can tolerate the fluctuations of operating conditions within a certain range,and has a great significance to the stability of the industrial production.In the past thirty years,researchers have conducted a great deal of researches on the influence of the fluctuations of conventional uncertain parameters,for instance stream temperatures and heat capacity flow rates,etc,to the flexible heat exchanger network synthesis.However,in real industrial world,the gradual deposition on the heat transfer surface becomes a non-negligible problem,which leads to the continuous decrease of the total heat transfer coefficient and the deviation of the stream outlet temperatures from the target.Therefore,the influence of fluctuations of conventional uncertain parameters and fouling growth on the flexible heat exchanger network synthesis should be considered simultaneously.The main contents of the thesis are as follows:（1）In order to investigate the effect of fouling growth on the optimal heat exchanger network structure,the method of dispersing the fouling growth profile is used to investigate whether the conventional uncertainties and fouling growth in each interval affect the network matches.To identify the critical point in each interval,a flexibility analysis method that takes into account both the conventional uncertainties and fouling growth is proposed.The influence of the conventional uncertain parameters is reflected in the feasibility of temperature difference constraints and minimum heat transfer temperature difference constraints,while the impact of fouling growth is reflected in the initial heat transfer area constraints.Based on the critical points,a flexible heat exchanger network synthesis method is proposed.A mixed integer nonlinear programming model is established on the basis of the initial heat exchanger network structure（under the nominal condition）.By increasing the areas of the initial heat exchangers or the involvement of new heat exchangers,the flexibility of the network is improved so that it is practicable at all operating conditions.For different intervals,the flexibility analysis and the flexible synthesis procedure are repeated in sequence until the resulting heat exchanger network finally meets the flexible requirements.（2）In order to search the global critical points,the continuous time variable is introduced into the flexibility analysis method.The calculation process is divided into two levels:the inner layer considers the fluctuations of the conventional uncertain parameters;and the outer layer considers the effect of fouling growth.At the same time,modify the mathematical formulation,and the resulting network structure can meet the flexibility requirements throughout the operation horizon.（3）In order to reduce the impact of the fouling growth on the heat exchanger network flexibility and the redundancy degree of heat transfer areas,a periodic cleaning method is considered to further optimize the heat exchanger areas and the cleaning schedule simultaneously.The optimization objective consists of two parts:（1）the minimum flexibility index F_min=1 in each cleaning cycle;（2）the minimum annual total cost.