Photo-bioreactor design for microalgae: A review from the aspect of CO2 transfer and conversion

Photobioreactor (PBR) is the most critical equipment for microalgal photosynthetic fixation of CO₂. It provides suitable environmental conditions, such as CO₂, light and nutrients, for microalgal growth. As the major carbon source for microalgae, CO₂ gas is pumped into PBR with the formation of bubbles and formed gas-liquid flow. The gas-liquid flow affects CO₂ and nutrients transmission as well as microalgae cells distribution in PBR, thereby affecting the biochemical reaction of microalgae. While the migration and transport of biochemical reaction products affect the two-phase flow, phase distribution and flow resistance in the PBR in return, thus affecting the transport of light and nutrients. Therefore, microalgal photosynthetic rate is determined synthetically by two-phase flow and the transport of CO₂, light and nutrients in PBR. Deep understanding of gas-liquid two-phase flow, energy and mass transfer coupling with microalgal growth in PBR is the cornerstone for the design of an efficient microalgae PBR.

Interactions between gas-liquid mass transfer and bubble behaviours

Interactions between gas-liquid mass transfer and bubble behaviours were investigated to improve the understanding of the relationship between the two sides. The CO₂/N₂-water system was applied to study the bubble behaviours based on the volume-of-fluid (VOF) model. The mass transfer conditions were taken into consideration when the fluid field was analysed. The bubble behaviours were compared with and without mass transfer. The results show that the absolute slopes of the curves for mass fraction inside the single rising bubbles, with diameters from 3 to 6 mm, decrease from 0.09325 to 0.02818. It means that small single bubbles have higher mass transfer efficiency. The daughter bubbles of cutting behaviour and initial side-by-side bubbles of coalescence behaviour also perform better than the initial large bubbles and coalesced bubbles, respectively. The bubble behaviours affect the mass transfer process. However, the latter also reacts upon the former. The critical intervals between the side-by-side bubbles decrease from 2.0 to 0.9 mm when the bubble diameter changes from 3 to 7 mm. For the coalescence behaviour without mass transfer, the critical intervals are larger because there is no influence of concentration around the bubbles on the bubble motion. The coalescence of cut daughter bubbles is also influenced by the concentration. It was suggested that the interaction between the gas-liquid mass transfer and bubble behaviours cannot be ignored.

Experimental and Numerical Study of the Hydrodynamics of a Thin Film Reactor (TFR) for the Decarboxylation of Anacardic Acid

A newly designed laboratory scale thin film reactor (TFR) was tested for the decarboxylation of anacardic acid in Cashew Nut Shell Liquid (CNSL) and to investigate the fluid flow behaviour under the influence of temperature since the fluid properties like viscosity and density have strong dependence on temperature. The CNSL containing 60–65 % anacardic acid was decarboxylated to produce cardanol and CO2at wall temperatures ranging between 393 K and 433 K, respectively. The characteristics of the CNSL, essentially a non-Newtonian fluid, was analysed at different temperatures and its rheological behaviour was studied using the well-known power law model. It was observed that CNSL follows a pseudoplastic behaviour and its viscosity, along with the liquid residence time, was found to decrease till 413 K, while a further increase in temperature resulted in product degradation due to charring, accompanied by an increase in viscosity and residence time. Using measured values for the viscosity, the film thickness was calculated for each wall temperature within the 393–433 K temperature range, showing an increase of the film thickness with temperature and viscosity. Computational Fluid Dynamics (CFD) studies were carried out for the first time for this reactor configuration, using the volume of fluid (VOF) model for the reactive flow. The results obtained from these simulations were in concurrence with the experimental outcomes: velocity profiles along the length of the reactor show its highest values at a wall temperature of 413 K, while lower velocity values were observed when the temperatures were lower or greater than 413 K.

Investigating the effect of sparger configuration on the hydrodynamics of a full-scale membrane bioreactor using computational fluid dynamics

A three-phase computational fluid dynamics (CFD) simulation was carried out in a full-scale membrane bioreactor to investigate the effect of sparger configuration on various hydrodynamic parameters.

Prediction of multiphase flow pattern inside a 3D bubble column reactor using a combination of CFD and ANFIS

This work presents a combination of Computational Fluid Dynamics (CFD) and Adaptive Network-based Fuzzy Inference System (ANFIS) developed for flow characterization inside a cylindrical bubble column reactor.