Optimal hydrodynamic design of tubular photobioreactors

BACKGROUND: Tubular photobioreactors are being considered for growing microalgae to produce renewable fuel oils via photosynthesis. Such bioreactors need to be highly energy efficient, or the energy captured in the fuel oil through photosynthesis will barely compensate for the fossil energy required to operate the photobioreactor. RESULTS: Computational fluid dynamics (CFD) modeling of the flow in a conventional tubular loop photobioreactor is used to reveal the existence of substantial dead zones in the typically used U-bends and 90 degrees. elbows. Fluid circulation in these dead zones leads to increased energy consumption, formation of deposits, wall growth and potentially reduced productivity. Alternative designs of U-bends and 90 degrees. elbows are evaluated by CFD modeling. Optimal design of a U-bend and a 90 degrees. elbow are identified and shown to substantially reduce the development of dead zones. Use of the proposed new configurations of bends is shown to reduce the energy consumption for fluid circulation in the loop by up to 15% relative to a conventionally designed tubular loop photobioreactor, for turbulent flow of seawater. CONCLUSION: For a typical tubular loop made of a tube of 0.1 m in diameter, the optimal configuration of a U-bend had a radius of curvature of 0.35 m and an angle of the arc of 210 degrees. The optimal configuration of a 90 degrees. elbow was found to have a radius of curvature of 0.3 m. (C) 2012 Society of Chemical Industry