Simultaneous Optimization Of Shell-and-tube Heat-exchanger Design And Heat Exchanger Network Synthesis

Heat exchanger network synthesis (HENS) is an important studied subject in process system engineering, and it is very important for the enterprise to save energy, reduce consumption and save cost, the methods of HENS have made greater development over the past few decades. However, on the one hand, the hypotheses for simplifying problems, such as the assumption of constant stream heat-transfer coefficients, the neglect of the impact of heat-exchanger (HE) fouling and the operating cost for the pressure drop of HE, can lead to a non-optimal heat exchanger network, and make the results not be able to be applied in the industrial condition. On the other hand, considering the complexity and coupling of the problem, HENS and HE design are done sequentially, and it splits the relationship between the heat exchanger network system and the various HE subsystem, which is difficult to achieve the overall optimization. Thus, simultaneous optimization strategy was presented in this paper, the heat transfer area and the overall heat transfer coefficients were set as the relation parameters, a simultaneous optimization model was built for shell-and-tube HE design and HENS, and Genetic/Simulated Annealing algorithm (GA/SA) was used to solve the problem, so that it can realize the overall balance for the design dimensions of HE and the structure parameters of heat exchanger network.Bell-Delaware method was used to describe the pressure drop and heat transfer of the shell-side flow, and internal and external tube diameters, tube length and other structure parameters were selected as main design variables, the minimum total cost of the HE was set as the key objective, thus a mathematical model for optimal design of shell-and-tube HE was established, and the solving progress was based on the GA/SA. Compared with those obtained by literature approaches, our method performed better in balancing the HE area cost and pumping cost, and therefore by searching global optimal solution it can offer options on obtaining main structure parameters of the HE with minimum total cost. Taken into account of the margin requirement of HE design, and imposed relevant constraints with regard to certain industrial project, the process of designing and optimizing HE can be achieved using the proposed model and algorithm. The simulated results of pressure drop and heat-transfer coefficient generated from the proposed model were closed to those given by commercially available HE design software HTRI, suggesting the as-designed HE has reliable feasibility. Furthermore, the proposed method can guarantee economical optimum in HE design, overcoming one major weakness of commercial HTRI software.This work presented a simultaneous optimization model for HENS and detailed design of the HE that cold and hot process streams were not change in phase. Using the stage-wise superstructure of the heat exchanger network, the model combined the heat load and heat capacity flow rate for each match in heat exchanger network with the main geometric constrains of each HE, and GA/SA had been applied to minimize the total annual cost which was based the area of HE and pumping duties, as well as the energy costs for utilities. The heat transfer area and the overall heat transfer coefficients were set as the coupling relation parameters for the HENS and the HE design, and a simultaneous optimization strategy with a nested loop was proposed. Two alternative connection schemes of HE were considered, which were referred to as one-stream series-wound and one-stream parallel in the stage-wise superstructure of heat exchanger network with branch streams, thus, the influence of the pressure drop for branches on the main stream were considered. Two examples were analyzed, the total annual cost of heat exchanger network were reduced for 0.06%,0.01%, and 71.15%, 6.06%, which can be used to illustrate the availability of the proposed model and algorithm.A simultaneous optimization model was presented for HENS and detailed design of HE, which considered the utility HE with phase change. Nusselt correlations, Kern method, as well as Bell-Delaware method were used for heat transfer coefficients and pressure drop of the HE with phase change, and the assumption was cancelled, which the overall heat transfer coefficients were constants. Combining with the improved stage-wise superstructure model of heat exchanger network with non-isothermal mixing of splits, the minimum total cost of the heat exchanger network was set as the key objective, thus a simultaneous optimization model was established. The operating cost was considered, which was caused by the pressure drop in the auxiliary heater and the cooler with process streams and utility streams, and it can be detailed to predict the change of the pressure drop when the process streams flowed through the heat exchanger network. An example in the reference was analyzed, it can reduce the total annual cost for 20.2%, which optimized the heat exchanger network and HE for cold and hot streams, auxiliary heater and the cooler simultaneously, at the same time more detailed design parameters of HE for process streams and utility were provided.