Investigation and optimisation of ammonia decomposition efficiency in a fixed-bed tubular reactor with fractal-shaped fins: A coupled CFD and RSM approach

Abstract

Ammonia is regarded as a suitable hydrogen storage medium due to its high hydrogen content. Ammonia decomposition in fixed-bed reactors is challenged by inefficient heat transfer, primarily due to the low thermal conductivity of the catalytic particles. This numerical investigation aimed at optimising the operating conditions of a typical tubular fixed-bed reactor with a novel fractal-shaped fins approach, employing a combination of numerical simulation and the Response Surface Method. Design parameters included ammonia vapor inlet velocity, temperature, and reactor wall temperature. The design of the experiment methods suggested 15 alternative design points. The objective function was to maximise the H 2 mole fraction at the outlet under a range of moderate pressure drop. Using a finite volume method solver (ANSYS-FLUENT), the flow field was simulated for each design point. By coupling the numerical results with RSM, a sensitivity analysis was conducted, and optimisation was performed using the genetic algorithm. The sensitivity analysis revealed that inlet temperature and velocity have more significant impact on ammonia conversion than wall temperature. Ammonia conversion showed an inverse relationship with inlet velocity while increasing inlet and wall temperature had a proportional relationship. Numerical results indicated that using fractal-shaped fins in the tubular reactor facilitated diffusional heat transfer within ammonia bulk, leading to an increased ammonia conversion rate. Under optimal operating conditions, the H 2 mole fraction at the outlet was 0.44 for a smooth reactor, 0.5 for a reactor with conventional fins, and 0.54 for a reactor with fractal-shaped fins. The fractal-shaped fin enhanced ammonia conversion from 59 % to 72 %; however, it also increased pressure drop by 67 %.

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