Synthesis Of Heat Exchanger Network Subject To Fouling And Ageing With Cleaning Schedule Optimized

Heat exchangers are widely used in petrochemical and food industries. Some research shows that most equipments subject to fouling. The deposition of unwanted material on heat transfer surface not only increase the resistance to flow, but also reduces a unit’s energy efficiency and changes the outlet temperature of process streams. So it is necessary to consider the effects of fouling in single heat exchanger design or heat exchanger network (HEN) synthesis. In this paper the asymptotic model is used to express the increase of thermal resistant caused by fouling over time. Based on that, the study of flexible HEN design and optimization is carried out in the first two parts and then a new methodology for synthesizing HEN with the cleaning schedule optimized is presented. The main research works are as follows:(1) With the influence of fouling on heat transfer considered, the synthesis of HEN undergoing fouling is studied and a new method is proposed to optimize the HEN structure. First the original HEN is over-synthesized at maximal value of thermal resistant by means of pseudo-temperature enthalpy diagram approach (PTHDA). Then the methods of loop-breaking and small-size units eliminating are used to simplify HEN structures. Although both methods lead the HEN to a more reasonable structure, the process of which is complex and some heat transfer constraints may not be satisfied once the units around pinch are removed from the network. In order to solve those problems and limit the consumption of utilities. Solver in Excel is used to optimize the heat load of each unit based on the simplified structure. The results of two examples show that it leads to a lower total annual cost to perform this method.(2) Since fouling on heat transfer surface increase with time, HEN area at the maximal value of thermal resistant is large for heat transfer at most operation time. In this paper, a strategy of optimizing heat exchanger area is proposed to reduce total annual cost. For precise calculation of operation cost, the operation time is divided into several periods based on the feature of fouling growth. Energy consumption is calculated by simulating the HEN at different time and annual operation cost is the weighted sum of these periods. It is clear that there is a trade-off between capital investment and the utilities consumption. The optimal HEN with minimum total annual cost is obtained by optimizing heat transfer area using parallelized genetic/simulated annealing algorithms (GA/SA). Comparing with former works, this method is cost saving and the computation time is cut down significantly by reducing the calculated amount. Two examples are solved to illustrate the validity and advantages of the method proposed.(3) A new methodology for synthesizing HEN subject to fouling and ageing with the cleaning schedule optimized is presented. The two-layer model is used here to capture fouling and ageing behaviors of individual exchangers. And two cleaning methods with different cleaning efficiency are available in this paper. Comparing with Chemical cleaning, Mechanical cleaning requires more time and costs more. The cleaning mode must be selected along with cleaning schedule. This process includes two steps. First the original HEN at maximal value of thermal resistant is obtained by means of PTHDA, then the structure of which is optimized. Based on that, a mixed integer nonlinear programming (MINLP) is formulated to optimize heat transfer area and the cleaning schedule. Because of the difficulty in solving this model, a stepwise method is used to optimize heat exchanger area and the cleaning schedule first. According to the results of that, the starting points are set and more constraints on integer variable are added in simultaneous optimization to narrow solution space. Both methods are evaluated by using the DICOPT optimizer in the General Algebraic Modeling System (GAMS).