Abstract
High concentration of dissolved oxygen within microalgae cultures reduces the performance of corresponding microalgae cultivation system (MCS). The main aim of this study is to provide a reliable computational fluid dynamics (CFD)-based methodology enabling to simulate two relevant phenomena governing the distribution of dissolved oxygen within MCS: (i) mass transfer through the liquid–air interface and (ii) oxygen evolution due to microalgae photosynthesis including the inhibition by the same dissolved oxygen. On an open thin-layer cascade (TLC) reactor, a benchmark numerical study to assess the oxygen distribution was conducted. While the mass transfer phenomenon is embedded within CFD code ANSYS Fluent, the oxygen evolution rate has to be implemented via user-defined function (UDF). To validate our methodology, experimental data for dissolved oxygen distribution within the 80 meter long open thin-layer cascade reactor are compared against numerical results. Moreover, the consistency of numerical results with theoretical expectations has been shown on the newly derived differential equation describing the balance of dissolved oxygen along the longitudinal direction of TLC. We argue that employing our methodology, the dissolved oxygen distribution within any MCS can be reliably determined in silico, and eventually optimized or/and controlled.
Highlights
First-generation biofuels were an interesting alternative to provide energy for the automotive and air travel sectors but its reliance on edible feedstocks undermined their viability
A user-defined function (UDF) was implemented for the reaction term including the inhibition by dissolved oxygen (Equation (8))
Even though there would be a possibility to include the spatial dependency of the irradiation according to the Beer–Lambert law, we operated here with constant irradiance getting us close to the maximum value PO2,max mentioned above (380 mg O2 /L h recalculated as 3.3 × 10−6 kmol O2 m−3 s−1 because these are units which must be used in ANSYS Fluent UDF)
Summary
First-generation biofuels were an interesting alternative to provide energy for the automotive and air travel sectors but its reliance on edible feedstocks undermined their viability. It is technically possible to obtain biofuels from microalgae, commercial production is still not possible due to economic and other limiting factors, but the scientific community did not lose interest in working with these simple photosynthetic microorganisms such as cyanobacteria and microalgae [1]. The complexity of mathematical modeling of MCS resides in the fact that we deal with multiple physical and biological (inherently nonlinear) phenomena, e.g., three-dimensional multiphase (gas–liquid–solid) flow dynamics (with mass transfer through the air–liquid interface), irradiance distribution within the illuminated parts of MCS, and multi-level functionality of cellular processes All these parts, which have to be integrated with the corresponding PBR operation mode, interact across different timescales. There exist new measurement technologies and theoretical approaches, most studies of general
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