Resumen de Biorefining of microalgae: from harvesting to biofuel production
The study presented in this thesis concerns the application of membrane technology for microalgae biorefining. This complex process requires several improvements due to the relatively high operational costs of each step. The idea of using membranes for this purpose may lead to general cost reduction and simplification of the procedures. The technical improvement and optimization of harvesting, cell disruption, fractionation and transesterification steps was performed.
In the harvesting stage: The production and application of novel polymeric membrane materials together with vibratory technology led to the performance improvement of microalgae Chlorella sorokiniana and Dunaliella tertiolecta dewatering.
It was showed that vibrational membrane filtration improves performance compared to cross-flow filtration resulting in a doubled permeability. Also, when using dynamic filtration, the performance continued to be satisfactory with sludge concentration increment.
Successful production of ABS membranes for the vibratory filtration, knowing that the polymer price is three orders of magnitude lower than the price of commercially available high-grade polymers such as polysulfone and polyacrylonitrile, already gave a huge advantage over existing, commonly used membranes.
It was also proofed that polymeric composition and the temperature of the coagulation bath are important parameters for preparation of ABS membranes with desired mechanical properties.
Further study showed that substantial energy and cost reduction can be achieved when combining pH induced sedimentation with dynamic filtration for microalgae harvesting.
The high concentration factors reached in the pilot scale experiments (CF of 205 and 245 for the studied strains) proofed that this method could lead to concentrations high enough to proceed to cell disruption with no need for further operations.
Regarding the cell disruption and fractionation stage: Satisfactory results were obtained when using the sequence of steam explosion, dynamic membrane filtration, and solvent extraction.
For all the microalgae strains treated (Nannochloropsis gaditana, Chlorella sorokiniana and Dunaliella tertiolecta), the access to organic compounds and carbohydrate hydrolysis into sugars was obtained by acid-catalyzed steam explosion.
The separation of the lipids from the aqueous phase was reached by membrane filtration. Again, dynamic filtration provided better results than conventional technique.
Concerning the transesterification step: The comparison of novel catalytic and inert membrane reactors for biodiesel production with strontium oxide as a heterogeneous catalyst was performed.
Some catalytic activity was detected for self-prepared polymeric membranes with the catalyst immobilized in membrane matrix, but much better performance was observed for the combination of catalyst-filled bag and commercial membrane in the novel IMRCF with the cell heating system.
The transesterification process intensification can be obtained by the application of a CMR/IMRCF using SrO as a heterogeneous catalyst.
Microalgae biorefining in the terms of industrial scale needs modernization leading to final cost reduction of the process. Since this work focuses on the technical improvement of each step of the microalgae treatment for biofuel production, the scope of the future work would be to evaluate economically the impact of the application of the techniques proposed. Further study of harvesting implying sedimentation combined with dynamic filtration of larger volumes of microalgae suspension should be performed to check the maximum concentration possible to be obtained in a pre-industrial test. The possibility of direct processing the concentrate obtained by the proposed steam explosion cell disruption and fractionation techniques should be considered. The investigation of higher SrO catalyst load for the transesterification using IMRCF should be performed.
