Disruption of microalgae with a novel continuous explosive decompression device

Most microalgae cells are very resistant to mechanical and chemical stresses. Because target products are mainly located inside the cells, disruption of the cell wall is an essential step in downstream processing to recover the microalgae ingredients. This study is focused on the development of a mild cell disruption method based on explosive CO2 decompression to allow the recovery of multiple sensitive products. The principle behind cell rupture via explosive decompression is the uptake of a gas under elevated pressure into the microalgae cells, where after a sudden depressurization gas expansion causes rupture of the cells. Conventional explosive decompression uses stirred tank reactors wherein, because of inefficient mixing, the gas/liquid interfacial area is minimal, resulting in long contact times, inefficient gas uptake and inefficient cell rupture. To increase the mixing yield and the formation of a gas-liquid micro-emulsion, a new continuous apparatus and a method were developed to process microalgae biomass by CO2 based explosive decompression. The final design consisted of 2 pumps, a tubing system, a sparger, a hydrodynamic agitation zone and a heated relief valve. Experimental comparison of the conventional batch and the new continuous devices showed that process time was reduced 3.2-9.6 fold, the biomass and biochemical release efficiency increased more than 2 fold, and CO2 consumption reduced 2-4 fold. The method can be considered as mild, since it can operate at room temperature without addition of chemicals. Moreover, the calculated shear rate on the algae cells is only 0.33% of the shear rate recorded during bead milling, while depending on the process parameters releasing a similar amount of biomass components from the cells into the supernatant. The article describes the development of the explosive decompression systems supported by computational fluid dynamics simulations and experimental data on algae cell disruption, complemented with a first techno-economic assessment.