Experimental validation of a nonequilibrium model of CO2 fluxes between gas, liquid medium, and algae in a flat-panel photobioreactor

Abstract

Carbon dioxide (CO₂) availability strongly affects the productivity of algal photobioreactors, where it is dynamically exchanged between different compartments, phases, and chemical forms. To understand the underlying processes, we constructed a nonequilibrium mathematical model of CO₂ dynamics in a flat-panel algal photobioreactor. The model includes mass transfer to the algal suspension from a stream of bubbles of CO₂-enriched air and from the photobioreactor headspace. Also included are the hydration of dissolved CO₂ to bicarbonate ion (HCO₃⁻) as well as uptake and/or cycling of these two chemical forms by the cells. The model was validated in experiments using a laboratory-scale flat-panel photobioreactor that controls light, temperature, and pH and where the concentration of dissolved CO₂, and partial pressure of CO₂ in the photobioreactor exhaust are measured. First, the model prediction was compared with measured CO₂ dynamics that occurred in response to a stepwise change in the CO₂ partial pressure in the gas sparger. Furthermore, the model was used to predict CO₂ dynamics in photobioreactors with unicellular, nitrogen-fixing cyanobacterium Cyanothece sp. The metabolism changes dramatically during a day, and the distribution of CO₂ is expected to exhibit a pronounced diurnal modulation that significantly deviates from chemical equilibrium.

Optimization of CO2 fixation by Chlorella kessleri cultivated in a closed raceway photo-bioreactor

“…K r was estimated to be 7.28 lmol photons m À2 s À1 , which is not significantly different from the values of 3 lmol photons m À2 s À1 reported by Takache et al (2012). Also, k L and a B s were estimated to be 0.263 m/h and 0.836 h/m, respectively, which are very close to k L of 1.78 Â 10 À4 m/s (0.641 m/h) and a B s of 0.769 h/m reported by Nedbal et al (2010). Thus, the parameter estimates obtained in our study are consistent with values available in the literature, with some differences existing because of the different media and conditions under which the microalgae were grown.…”

“…In order to explain the dynamics of CO 2 exchange between the liquid and gas phases and its hydration, we have modified a model developed by Nedbal et al (2010). This is coupled with our expression for the uptake of CO 2 by microalgae to explain fully the behavior of CO 2 in the photobioreactor.…”

Monitoring of Microalgal Processes

“…What is the concentration of bicarbonate that is used as the carbon source by some microalgae [253]? Bicarbonate could be estimated online by measuring pH and estimating k L a using the off-gas analytics [90].…”

“…CO 2 control is usually linked to the pH control by adding CO 2 into the inlet gas, where the CO 2 flow is measured using a mass flow meter and is dosed by a pH controller [18,56,61]. Alternatively, measurement of CO 2 in the outlet gas by an IR analyzer is used for CO 2 dosage [18,69,85,90]. Methods and sensors for measurement of dissolved and gaseous CO 2 are reviewed in [91].…”

“…Another alternative to pCO 2 electrodes is the OxyGuard dissolved CO 2 analyzer based on IR absorption, which allows direct and continuous measurement, reported for use in fish aquaculture for the concentration range of 2-50 mg/L CO 2 (0.14-3.4 kPa) [89]. CO 2 in the gas phase can be measured by an IR gas analyzer [18,69,90]. CO 2 control is usually linked to the pH control by adding CO 2 into the inlet gas, where the CO 2 flow is measured using a mass flow meter and is dosed by a pH controller [18,56,61].…”