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Design and optimization of plate heat exchanger networks
[Thesis]. Manchester, UK: The University of Manchester; 2020.
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Abstract
With the growth of energy consumption and the increase of greenhouse gas emissions, it is important to improve heat transfer efficiency and save energy. One of the most effective ways is to consider use of heat transfer enhancement for increasing heat recovery. Plate heat exchangers, which allow a small minimum temperature approach, are widely used in the energy-intensive process industries to enhance heat transfer coefficient. A major limitation of applying plate heat exchanges is the lack of reliable design methods to quantify the energy saving effectively. The key objective is to develop a novel method for a single plate heat exchanger design and integrate the plate heat exchangers into conventional heat exchanger network retrofits. A new computer-aided design of two-stream multi-pass plate heat exchangers is proposed, including gasket plate heat exchangers and welded plate heat exchangers, with different plate geometries. To account for multi-pass flow arrangements, the plate heat exchanger is separated into several pure counter-current or co-current one-pass blocks. The correlations of inlet and outlet temperatures of different blocks are obtained in order to apply the logarithmic mean temperature difference (LMTD) method for thermal design in each single-pass block with known temperatures. The selection of the number of passes for streams, plate geometries and plate patterns are considered as integer variables to optimize the total area of plate heat exchanger. An MINLP model is developed in GAMS using ANTIGONE solver in order to derive the optimal solution. A case study is used to demonstrate the capability of proposed method to obtain the optimal solution with required heat load and constraints. The proposed design model can also be further applied to the complex heat exchanger network design. Application of plate heat exchangers into the heat exchanger networks (HENs) retrofit increases the heat recovery due to their small minimum approach temperature. However, the installation cost of plate heat exchangers is relatively high. Thus, the optimization process is based on the trade-off between energy reduction and capital cost. For a fixed structure HEN, the heat recovery is limited. Structure modifications have the possibility of providing more energy saving but bring more cost at the same time. A decision on the best retrofit strategy to apply to a given HEN depends on the given retrofit objective. This thesis presents a methodology for the application of plate heat exchanger in HENs, both with fixed structure and with structure modification. The key point is to find the most beneficial location to apply plate heat exchangers and develop an algorithm to automatically identify the best modifications. The objective of the optimization process of HENs retrofit is to minimize the energy consumption and maximise the retrofit profit at the same time. Case studies highlight the benefits of the new approach. The results are compared with conventional technologies to provide the insight on the potential benefit of integrating plate heat exchanger into HENs retrofit. This work provides an adequate basis on which the decision can be made based on industrial applicability, profit, and energy saving.