w i t h o u t m H T L w i t h m H T L
0
1 5 3 0 4 5 6 0
R e m n a n t G l u c o s e O t h e r s
1 3 . 3 % 9 . 7 %
Product (kg d-1 )
7 7 %
Figure 6.10: Inuence of mild hydrothermal treatment on overall biomass exploitation.
Error bars represent one standard deviation from the estimated average values based on Monte Carlo simulations.
ditions and the use of catalysts, an inexpensive by-product generation seems possible.
Using the current market price of glucose derived from USDA (2016), a gross revenue of 34030±3267 USD a−1 can be achieved for the annual glucose by-production. In contrast, the estimated production costs are calculated to be 4319±909 USD a−1.
However, it needs to be emphasized, that the gross revenue was calculated based on the price of glucose syrup. This assumption is optimistic since the product of the present work is a glucose containing aqueous solution. To achieve syrup consistency the evaporation of water is needed which would further raise the energy demand. Neverthe-less, the results of the cultivation experiments reveal the successfully proven application of the glucose in the form of an aqueous solution without the need of further concen-tration into syrup. With respect to the biorenery concept, Figure 6.10 highlights the high rate of remnant utilization. Here, it becomes obvious that the low valued biomass waste is mainly converted into valuable glucose. According to the results of the modeled β-carotene production process, a total production of 45.5±2.2 t glucose per year can be achieved by liquefying the annual produced biomass remnant of 59.1±2.8 t. Conse-quently, the by-product valorization using mild HTL seems to be highly benecial for the overall process economics. With that, a holistic biorenery concept with a more extensive exploitation of available biomass components appears feasible.
6.5 Conclusion
In the present work the potential of HTL to valorize remnant D. salina biomass was in-vestigated. Mild process conditions of 100◦C and 0 min were found to be sucient for a high biomass conversion of at least 85%. With a yield of more than 70%, glucose was the most abundant hydrolysis product in the aqueous phase. The monomer is the preferred carbon source of a numerous microorganisms, enabling its application for the production
6 VALORIZATION OF D. SALINA REMNANT BIOMASS
of food, feed, pharmaceuticals and fuels. The glucose-rich aqueous phase was success-fully utilized as organic carbon source in mixotrophic and heterotrophic cultivation of three microorganisms of biotechnologically relevance. Accordingly, liquefaction-derived glucose can be considered as valuable co-product of theβ-carotene production process.
The benecial eect of the by-product on the overall process economics was clearly veried by the calculation of energy demand and operating costs. With these ndings initial steps were done to realize a more economic and more sustainable multi-product bio-process based on D. salina.
7
Summary and perspectives
7.1 Concluding remarks
Currently, microalgae are intensively discussed as additional sources of biomass and valuable products that can complement agricultural biomass production. However, to become a viable alternative, the reduction of production costs is an envisaged aim of microalgae process design. Since 50-80% of the overall process costs are caused by the downstream processing (Molina Grima et al., 2003), the present thesis aimed for the investigation of diverse process alternatives in the downstream route. Therefore, microalgal β-carotene production by D. salina was selected as a case study to assess the potential of innovative downstream technologies including occulation, supercritical uid extraction and by-product generation by mild hydrothermal liquefaction. The un-derlining evaluation approach provides the techno-economic analysis of the investigated technologies based on reliable data which have also been obtained in the course of the thesis.
For this purpose, a process model of the industrial β-carotene production by D.
salina was developed in Chapter 3, integrating strain-dependent parameters from lit-erature and industry. Uncertainties of the used parameters were considered by the application of Monte Carlo sampling. As a result, reference values of the operation costs, product yield and energy demand were identied for each processing unit within the production route. The reliability of the results was conrmed by the comparison with the economic data derived from the industrial D. salina production site in Israel.
The annual process costs of 181,603±19,330 USD a−1 as well as the biomass produc-tion cost calculated to be 17.13± 1.59 USD kg−1dw biomass are almost identical to the values reported from there (Ben-Amotz, 2008; Sun et al., 2011). Accordingly, the model was found to be appropriate as reference case and working platform to assess alternative downstream routes regarding their techno-economic properties. In the sub-projects of the thesis this process model was modied or extended by the investigated downstream techniques.
7 SUMMARY AND PERSPECTIVES
Chapter 4 examined occulation as potential preconcentration strategy for D. salina.
Due to the high ionic culture medium, the surface potential of the alga is close to the range which allows self-aggregation. This hypothesis is supported by the results of the surface free energy calculation according to DLVO theory. With that, the preconditions for a gentle occulation of the alga were rated as optimal. In preliminary experiments, the application of 0.3 mM Al2(SO4)3×16H2O, 1 mM FeCl3×6H2O, 20 mM NaOH and 3.4 mA cm−2 Al-electrolysis were selected as potential occulation techniques to pre-concentrate D. salina. The occulation agent NaOH performed most ecient with a harvesting eciency of about 93% which is comparable to that of the reference method centrifugation. Besides harvesting eciencies and concentration factors, also the inu-ence of occulants on the reusability of the separated culture medium, the extractability ofβ-carotene and the biomass contamination were determined for each method. Among all analyzed occulants, NaOH revealed the highest inuence on further process steps.
More precisely, the recycling of the separated culture medium was not feasible as well as the pigment extraction was hampered by the occulant. These ndings clearly demon-strate that a high eciency of a certain method does not automatically result in a good performance within the overall process. Knowledge of the side-eects of the method on up- or downstream ones is essential prior to process installation. The results of the experimental study were incorporated in the extended process model to predict the operating costs and energy requirements for the diverse occulation strategies. The simulation indicated a more energy-saving production of β-carotene from occulated D. salina biomass compared to the use of conventional concentration by centrifuga-tion. With regard to process economics and product yields, the conventional harvesting approach performed better than occulation. Consequently, occulation do not repre-sents the method of choice for an eective and economic preconcentration within the β-carotene production route.
In Chapter 5 the conventional organic solvent extraction was compared to supercrit-ical uid extraction. Therefore, the method of Dean et al. (1995) was used to identify the theoretical potential of scCO2 to solve β-carotene. Pilot scale experiments have shown the strong inuence of the operating parameter T and p on the extraction re-sults. Furthermore, the use of EtOH as polar co-solvent was clearly benecial for the extraction results. By applying 70◦C, 500 bar and a co-solvent addition of 10%, compa-rable results to that of conventional solvent extraction were achieved. The calculation of the techno-economic properties of both extraction techniques revealed that scCO2 is more energy consuming than conventional organic solvent extraction. However, prod-uct yields as well as the prodprod-uction costs reached equivalent values according to the simulation results. Consequently, this sub-project validates the high potential of scCO2
extraction as green and sustainable alternative of conventional used hexane for the re-covery of β-carotene from algal biomass. Based on the results of the present study as well as the less harmful character of scCO2 and its co-solvent EtOH, the application of the extraction approach is highly recommended for installation in new production plants of D. salina.