Water scrubbing is the most widely used technology for removing CO 2 from biogas and landfill gas. This work developed a rate-based mass transfer model of the CO 2 -water system for upgrading biogas in a packed bed absorption column. The simulated results showed good agreement with both a pilot-scale plant operating at 10 bar, and a large-scale biogas upgrading plant operating at atmospheric pressure. The calculated energy requirement for the absorption column to upgrade biogas to 98% CH 4 (0.23 kW h N m 3 , or 4.2 % of the input biogas) is a significantly closer approximation to the measured value (0.26 kW h N m 3 , or 4.8 % of the input biogas) than has previously been reported in the literature. The model allows for improved design of CO 2 capture and biogas upgrading operations, and can also be a useful tool for more detailed cost-benefit analysis of the technology.
To improve the mass transfer efficiency in many industrial applications better understanding of the mass transfer rate is required. High speed images of single CO 2 bubbles rising in tap water were analysed to investigate the relationship between the mass transfer and properties of single bubbles. Transition to a lower mass transfer rate was shown to correspond with the transition from a mobile to an immobile bubble surface. This was indicated by the change in bubble rise velocity, bubble rise path and bubble shape. The presence of surfactants in untreated tap water appear to effect the transition point, particularly for bubbles with a smaller initial diameter and lower rise velocity.
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.
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