A CFD Design Approach for Industrial Size Tubular Reactors for SNG Production from Biogas (CO2 Methanation)

Abstract

A tubular reactor based on the disk and doughnut concept was designed as an engineering solution for biogas upgrading via CO2 methanation. CFD (Computational Fluid Dynamics) benchmarks agreed well with experimental and empirical (correlation) data, giving a maximum error of 8.5% and 20% for the chemical reaction and heat transfer models, respectively. Likewise, hot spot position was accurately predicted, with a 5% error. The methodology was used to investigate the effect of two commercially available coolants (thermal oil and molten salts) on overall reactor performance through a parametric study involving four coolant flow rates. Although molten salts did show higher heat transfer coefficients at lower coolant rates, 82% superior, it also increases, by five times, the pumping power. A critical coolant flow rate (3.5 m3/h) was found, which allows both a stable thermal operation and optimum pumping energy consumption. The adopted coolant flow range remains critical to guarantee thermal design validity in correlation-based studies. Due to the disk and doughnut configuration, coolant flow remains uniform, promoting turbulence (Re ≈ 14,000 at doughnut outlet) and maximizing heat transfer at hot spot. Likewise, baffle positioning was found critical to accommodate and reduce stagnant zones, improving the heat transfer. Finally, a reactor design is presented for SNG (Synthetic Natural Gas) production from a 150 Nm3 h−1 biogas plant.