(191bm) Numerical Evaluation of the Two-Phase Fluid Dynamics in a Bench Scale Bioreactor Applied to Microalgae Cultivation

The cultivation of microalgae is applied in the production of food, biofuels, biochemicals, pharmaceuticals and fertilizers. Through the photosynthesis process, these algae convert water, carbon dioxide and light into oxygen, organic compounds and water. Spirulina is a prokaryote microalgae found in different habitats. Spirulina has a high content of macro and micronutrients, thus it can be used for human consumption, as well as presents pharmaceutical properties, including antioxidant, anti-inflammatory, antiallergic, anticancer and antibacterial activity. In the cultivation of Spirulina microalgae, some factors influence the production of biomass, such as aeration, composition of the culture medium, lighting, pH and temperature. The aeration of the culture medium keeps the cells in suspension, favoring the contact between microalga and substrate, and illumination of the cells, however, with too much air flow, cell disruption and crop compromise can occur. To help in the investigation of these parameters, the computational fluid dynamics (CFD) can be used, as this technique provides detailed and accurate results of the variables that interferes in microalgae production. In this study, the fluid dynamics behavior found in the cultivation of Spirulina sp. LEB 18, due to the air flow used for aeration, was numerically evaluated. The study was carried out for a bench scale photobioreactor with a capacity of 2 L, being 1.96 L of occupied by the liquid phase, with aeration of the medium through a duct in the center of the photobioreactor with air outlet at the base at a flow of 1 L/min. The computational fluid dynamics technique was used to estimate the turbulent two-phase flow in an axissimetric geometry using the OpenFOAM v4.1 code. The simulated phases consisted of air, fed through a central duct to the erlenmeyer, and microalgal medium formed by the Zarrouk medium and Spirulina sp. LEB 18 microalgae. The k-Omega SST model was used to predict the turbulent characteristics of the flow. Meshes with different refinement levels were elaborated, containing 13, 29 and 49.5 thousand control volumes. The simulation of these data provided results for the calculation of the grid convergence index (GCI), whose value indicated that the use of a mesh containing 49.500 control volumes is adequate to capture the variables of interest with precision. The transient simulations were conducted for approximately 0.6 seconds, from which the simulations were continued for a further 14.4 seconds, with calculation of mean values. Process parameters such as velocity, pressure, turbulent kinetic energy, dissipated turbulent kinetic energy, vorticity, shear stress and phase distribution were studied in the present study, verifying its suitability for microalgae cultivation.