Design, Commissioning, and Performance Assessment of a Lab-Scale Bubble Column Reactor for Photosynthetic Biogas Upgrading with Spirulina platensis

Abstract

The two-step bubble column-photobioreactor photosynthetic biogas upgrading system can enable simultaneous production of biomethane and value-added products from microalgae. However, due to the influence of a large number of variables, including downstream processes and the presence of microalgae, no unanimity has been reached regarding the performance of bubble column reactors in photosynthetic biogas upgrading. To investigate this further, the present work documents in detail, the design and commissioning of a lab-scale bubble column reactor capable of treating up to 16.3 L/h of biogas while being scalable. The performance of the bubble column was assessed at a pH of 9.35 with different algal densities of Spirulina platensis at 20 °C in the presence of light (3–5 klux or 40.5–67.5 μmol m –2 s –1 ). A liquid/gas flow (L/G) ratio of 0.5 allowed consistent CO 2 removal of over 98% irrespective of the algal density or its photosynthetic activity. For lower concentrations of algae, the volumetric O 2 concentration in the upgraded biomethane varied between 0.05 and 0.52%, thus providing grid quality biomethane. However, for higher algal concentrations, increased oxygen content in the upgraded biomethane due to both enhanced O 2 stripping and the photosynthetic activity of the microalgae as well as clogging and foaming posed severe operational challenges.

A comparative evaluation of design factors on bubble column operation in photosynthetic biogas upgrading

“…Similar global hydrodynamic parameters, such as the gas hold-up (i.e., the ratio of the gassed volume to the static liquid volume in the bubble column), are necessary and sufficient to achieve dynamic similarity upon scale up (Shaikh and Al-Dahhan, 2010). For low superficial gas velocities below 1 cm/s, a minimal gas hold-up together with a negligible impact of pressure on the same allows scale-up of results with minimal errors (Bose et al, 2021). Accordingly, the current results would remain valid for industrial-scale bubble columns operated under similar superficial gas velocities irrespective of the bubble column dimensions.…”

“…Thus, the maximum u G and EBRT were limited to 0.2 cm/s and 15 min, respectively, although their maximum limits lie at 4 cm/s and 90 min, respectively. Additionally, the maximum algal density was fixed at 0.75 g-DW/L, limited by foaming of the algae medium observed during pre-experiments (Bose et al, 2021). However, it is still representative of the algal concentrations obtained in closed and open photobioreactors (Hu et al, 1996;Costa et al, 2003).…”

“…The actual gas flow rate and composition were obtained after a further correction step to the individual oxygen and nitrogen flow rates. This was done because noticeable air (N2 and O2) ingress by diffusion into the gas tubing used in the experiment was recorded during pre-experiments (Bose et al, 2021). To obtain the correction terms needed to remove this interference from the recorded flowrates, N2 was blown into the system at the same rate as that used in the subsequent experiments, and the gas composition and flow rates were measured.…”

Agro-Industrial Wastewaters for Algal Biomass Production, Bio-Based Products, and Biofuels in a Circular Bioeconomy

“…In addition to the above configuration, there is the possibility of using photobioreactors with biogas injection for CO 2 removal, where the culture is intermittently fed with biogas during the active photosynthesis phase [215][216][217]. To optimize biogas upgrading with direct injection into the photobioreactor, it is necessary to have sufficient residence time for CO 2 dissolution, uptake by the microalgae cells, and nutrient removal [203].…”

Energy Microalgae Based on Photobioreactor in the Purification of Heavy Industry Polluted Wastewater

“…Light intensity decreases rapidly in the medium due to shadowing of the cells. Increasing the surface area for absorbing specific light is one of the typical loopholes in the development of high-performance photobioreactors [18][19].…”