Engineering advances in tubular photobioreactors for microalgae cultivation: Design, scale-up, optimization, and operational challenges

Microalgae cultivation is a complex system where biological reactions and physical operating parameters interact closely. Hence, designing and developing close and controlled cultivation systems (photobioreactors) for maximizing production is crucial. Alongside optimum production, other factors such as handling, operational cost and scalability are also important. Among a wide range of photobioreactor designs, tubular photobioreactors have been considered most suitable owing to their simple design, high surface area to volume ratio and scalability for commercial applications. Nonetheless, some critical drawbacks like accumulation of dissolved oxygen and light attenuation have also been encountered. Over the years, researchers have investigated different strategies to improve the overall performance of tubular photobioreactors. Key efforts have focused on optimizing crucial parameters such as fluid mixing and hydrodynamics, improving light utilization efficiency, enhancing mass transfer, and minimizing biofouling. This paper covers the recent advancements and engineering perspective in tubular photobioreactors design. Key design criteria such as tube dimensions, illumination, mixing and gas exchange systems have been evaluated. Alongside, the associated technical challenges, scale-up strategies and reactor performance equations and models have been discussed. The review will provide a comprehensive understanding of tubular photobioreactors and offer insights for future research directions and industrial applications.