Evaluation of microporous hollow fibre membranes for mass transfer of H₂ into anaerobic digesters for biomethanization

Abstract

BACKGROUND: With high surface-to-volume ratios, hollow fibre membranes offer a potential solution to improving gas-liquid mass transfer. This work experimentally determined the mass transfer characteristics of commercially available microporous hollow fibre membranes and compared these with the mass transfer from bubble column reactors. Both mass transfer systems are considered for biological methanization, a process that faces a challenge to enhance the H 2 gas-liquid mass transfer for methanogenic Archaea to combine H 2 and CO 2 into CH 4 . RESULTS:Polypropylene membranes showed the highest mass transfer rate of membranes tested, with a mass transfer coefficient for H 2 measured as k L = 1.2 × 10 −4 ms −1 . These results support the two-film gas-liquid mass transfer theory, with higher mass transfer rates measured with an increase in liquid flow velocity across the membrane. Despite the higher mass transfer rate from polypropylene membranes and with a liquid flow across the membrane, a volumetric surface area of = 10.34 m −1 would be required in a full-scale in situ biological methanization process with much larger values potentially required for high-rate ex situ systems. CONCLUSIONS:The large surface area of hollow fibre membranes required for H 2 mass transfer and issues of fouling and replacement costs of membranes are challenges for hollow fibre membranes in large-scale biological methanization reactors. Provided that the initial bubble size is small enough (d e < 0.5 mm), calculations indicate that microbubbles could offer a simpler means of transferring the required H 2 into the liquid phase at a head typical of that found in commercial-scale anaerobic digesters.

Hyperthermophilic methanogenic archaea act as high-pressure CH4 cell factories

“…The CO 2 -BMP process can be employed in multiple applications such as biogas upgrading, power-to-gas applications, decentralized energy production, and for the conversion of H 2 /CO 2 of process flue gasses in waste to value concepts from, e.g., ethanol, petroleum, steel, and chemical industries 10 . There are two main approaches for CO 2 -BMP 11 : ex situ biomethanation using pure cultures 12,13 or enriched mixed cultures [14][15][16] , and in situ biomethanation 17,18 . In situ biomethanation is examined for upgrading the CH 4 content of biogas by adding H 2 to anaerobic digesters.…”

Potential for Biomethanisation of CO2 from Anaerobic Digestion of Organic Wastes in the United Kingdom

“…Membrane systems (ceramic and HFM) generally achieved good transfer without gas recirculation, while digester mixing also had an effect, although high mixing rates did not always improve methane yields [26,29,31]. Mixing and mass transfer performance are strongly affected by scale: effective mixing is more difficult to achieve in a full-scale plant, while depth and pressure effects may contribute to improved gas transfer [39][40][41]. Understanding of these effects and how they interact with system biology is still in its early stages, and more research at pilot and full scale is clearly needed if industry is to have confidence in adopting these technologies [3,4].…”

Bubble Diameter, Mass Transfer, and Bioreaction of Dynamic Membrane-Stirred Reactors

“…In the fermentation process, bubbles are important carriers of gas–liquid mass transfer, and their movement and mass transfer in the liquid layer directly determine the fermentation efficiency of microorganisms. , When the research object is a single bubble, its radius determines its movement rate, and mass transfer and internal pressure control the radius change. The velocity and size of bubbles will significantly affect the mass transfer rate, , and the main factor affecting these two characteristics is the bubble diameter, which will be affected by the turbulent dissipation rate, liquid medium, and gas–liquid specific surface area. , The larger the bubbles, the smaller the gas–liquid specific surface area, which is not conducive to gas–liquid mass transfer in the reaction process even under high flow ventilation. , Therefore, in order to achieve a better gas–liquid dispersion effect, it is necessary to break the bubbles into smaller bubbles by increasing the stirring speed of the impeller and disperse them into the reactor. However, with the increase of stirring speed, the shearing force generated by the impeller increases, which is not suitable for the fermentation process. , Therefore, it is imperative to develop a unique multifunctional aeration structure to improve the stirred reactor and strengthen the gas–liquid mass transfer.…”

“…The velocity and size of bubbles will significantly affect the mass transfer rate, 6,7 and the main factor affecting these two characteristics is the bubble diameter, which will be affected by the turbulent dissipation rate, liquid medium, and gas−liquid specific surface area. 8,9 The larger the bubbles, the smaller the gas−liquid specific surface area, which is not conducive to gas−liquid mass transfer in the reaction process even under high flow ventilation. 10,11 Therefore, in order to achieve a better gas−liquid dispersion effect, it is necessary to break the bubbles into smaller bubbles by increasing the stirring speed of the impeller and disperse them into the reactor.…”