Fig. 4: Impact of pressure requirements on the minimal mass of a photobioreactor’s outer walls.

The assumed photobioreactor is a bubble column with a liquid phase twice as high as wide (except for the bottom-right panel) and of one cubic metre, a gas phase increasing the total height by one tenth, and a maximal gas superficial velocity of 0.08 m s⁻¹. The ambient (outer) pressure is assumed to be 6 hPa. Top left: Minimal total pressure (including gas and hydrostatic pressures), pN₂ and pCO₂ required, as a function of biomass concentration (C_x), to maintain a specific growth rate of 0.2 day⁻¹. Bottom left: Minimal total pressure, pN₂ and pCO₂ required, as a function of C_x, to maintain a biomass productivity of 100 g m^-3 day⁻¹ (assuming a molar mass of 32.965 g mol_x⁻¹ for Anabaena sp., as previously assessed⁴³). Top right: Minimal mass of a photobioreactor’s outer walls required as a function of total pressure, for different materials. Bottom right: Minimal mass of a PMMA photobioreactor’s outer walls required to withstand an inner pressure of 100 hPa, as a function of reactor height (under a constant volume of 1 m³).