Title : Investigation of methane conversion and liquid holdup in trickle-bed reactors under different pressures
Abstract:
Biological methanation is a promising approach for converting CO? and H? into CH? by leveraging the metabolic activity of specialized microbial communities. This study investigates the performance of two trickle-bed reactors (TBR1 and TBR2) under different operating pressures, with a particular focus on methane production rate (MPR). Multiple pathways contribute to methane production, with microorganisms and bacteria within the reactor playing a crucial role in the process. Additionally, liquid holdup is a key parameter for enhancing liquid mass transfer efficiency. Two trickle-bed reactors (TBRs) were used for ex-situ biological methanation tests, with carbon dioxide and hydrogen as the inlet gases. The gas was introduced at the bottom of the reactor, while liquid recirculation was distributed through a nozzle positioned at the top. Biological reactions took place on the filling materials within the active zone of the reactor. TBR1 was operated at ambient pressure, whereas TBR2 was tested under elevated pressure. Methane production rate was then evaluated for each reactor. Liquid holdup tests on polyethylene filling materials were conducted using different types of spray nozzles (N, M, and H), which varied in spray angle, respectively 90°, 80° and 70°, to analyze the effect of liquid flow rate on the holdup percentage in the reactor bed. The experimental setup featured a packed bed reactor with structured packing material and a load cell to measure the liquid weight. The MPR analysis (Figures 1 and 2) indicated that TBR1 maintained a stable methane production rate between 6-8 m³ CH?/(m³ bed·day), whereas TBR2 exhibited greater variability but achieved higher peak MPR values. Figure 3 demonstrated superior efficiency in TBR2, compared to TBR1 and literature results, due to higher H? solubility and mass transfer rates under elevated pressure. Figure 4 illustrates the relationship between liquid holdup (%) and liquid flow rate per reactor bed volume. The data show that higher flow rates lead to increased liquid holdup, confirming the expected trend where a greater liquid presence in the system results in higher retention within the reactor bed.Nozzle N (blue circles) exhibits intermediate holdup values, while nozzle M (yellow circles) generally results in the highest holdup levels. In contrast, nozzle H (purple circles), which lower spray angle, demonstrates lower holdup at comparable flow rates. The results represented by blue, yellow, and purple squares correspond to experiments conducted with countercurrent air feeding at a flow rate of 7 L/min. In these cases, liquid holdup clearly decreases compared to conditions without gas feeding. The literature reference (black point) serves as a benchmark value, appearing lower than most of the experimental results, suggesting that the current setup may enhance liquid retention
