Design of a Compact Heat Exchanger Used in a Methanation Plant and Produced by Additive Manufacturing

The aim of this thesis is the design of a high-temperature compact heat exchanger used in a methanation plant and produced by additive manufacturing. The need for such a device arises from the ever-increasing electrical power production by volatile renewable energy sources (RES), which demands for systems allowing to efficiently store the off-peak power for later consumption. Power-to-Gas (P2G) technology exploits the electrical production excess to convert a mixture of carbon dioxide and water steam into methane. Since for such a system to work efficiently it is necessary that the gaseous mixture enters the electrolytic module (SOEC) at a sufficiently high temperature, the present study aims precisely at designing a compact heat exchanger (CHE) able to heat the inlet mixture to the required temperature of 850 °C. The heat exchanger design follows the application of two independent but complimentary design methods, namely (i) an iterative procedure based on the -NTU method and (ii) Computational Fluid Dynamics (CFD) tools. The first lean and cost-effective method, implemented on MATLAB, has allowed to rapidly identify an optimum heat exchanger design and to roughly estimate its size. COMSOL Multiphysics CFD simulations, starting from the geometric model previously produced, have then allowed to optimize the original offset-strip fin geometry, resulting in an elliptical fin shape that drastically cuts the pressure drops. The final result of the optimization procedure is a heat exchanger whose effectiveness , as high as 96 %, whose induced pressure losses lower than 5.5 mbar on each fluid side and whose ultra-compact construction, yielding at a value higher than 3600 , allow to fully meet the very demanding design requirements. Finally, the convenience of functionalizing the heat exchanger, i.e. using special catalytic surfaces to simultaneously transfer heat and catalyze exothermal methanation reactions, has been assessed. In particular, it has been proved that by adopting a two-stages layout, featuring first a low temperature functionalized device followed by a classical heat exchanger, it is possible to boost the cold side outlet temperature of almost 100 °C, therefore improving the P2G plant overall efficiency.