costs for the solvents including rst ll and make-up as well as the expended energy, hexane extraction was estimated to be more expensive than scCO2. Here, extraction costs of 10.38±0.99 USD kg−1 β-carotene were calculated, whereas the scCO2 extrac-tion only needs 7.7 ±0.83 USD kg−1 β-carotene. Regarding the yield of the pigment, no considerable dierences are visible in the simulation results. Both technologies are expected to achieve an annual production of approximately 2.7 t β-carotene. With 1.6 MUSD, also the annual selling price is comparable. Nevertheless, scCO2 extraction has the advantage to save approximately 5 USD kg−1 β-carotene which is visible in the cumulative production cost. However, the annual production costs which consider the costs of all production steps dier only slightly for both scenarios. Here, values of 409 ±31 kUSD a−1 and 402 ±31 kUSD a−1 were calculated for the hexane or scCO2
extraction.
Nonetheless, it is important to take into account the environmental aspect of product extraction. In recent years, the demand for more sustainable products and goods has been risen constantly. Green products such asβ-carotene should be extracted also with green solvents, especially if they are used in cosmetic, pharmaceutical or food industry.
The toxicity of solvent residues in the nal product after hexane extraction is always a critical issue and controlled by quality constraints (Parliament, 2009). Due to the green character of scCO2 as well as its co-solvent EtOH, the solvent is more appropriate for environmental-friendly and sustainable pigment extraction than hexane. Furthermore, scCO2 has a volatile character at atmospheric pressure, leading to complete solvent removal from the extract which is important for the quality of the product (Chemat &
Abert Vian, 2014).
5.5 Conclusion
Supercritical uid extraction is an already exploited methodology for a wide range of applications. So far, it is not used for industrial scale β-carotene extraction from algal biomass. The present chapter aims for the comparison of conventional and super-critical uid extraction by using the example of β-carotene extraction from D. salina biomass. To nd optimal conditions of conventional solvent extraction, dierent param-eters, namely pigment solubility, time and temperature as well as the inuence of water were investigated by theoretical or experimental eorts. The eects of the dierent parameters are strongly dependent on the individual solvent of choice. Interestingly, hexane the commonly used solvent for β-carotene extraction from algae in industrial scale did not achieved optimal extraction results. In all experiments the performance of acetone led to the best extraction yields, also at low temperatures and short extrac-tion times (room temperature, 30 min). Accordingly, acetone was chosen as reference extraction method to evaluate the extraction eciency of scCO2 extraction.
To assess the feasibility of supercritical uids as environmental friendly extraction method forβ-carotene, pilot scale experiments were done in cooperation with the
Fraun-5 EXTRACTION STRATEGIES OF β-CAROTENE FROM D. SALINA BIOMASS
hofer IGB Leuna, Germany. Theoretical considerations demonstrated a good solubility of the hydrophilic pigment in scCO2. Therefore, the pilot scale experiments were used to nd an optimal parameter set-up to extract β-carotene under consideration of the technical constraints. 10% EtOH was found to be an optimal co-solvent ratio to increase extraction yields. In addition, the extraction conditions of 500 bar and 70◦C led to the most eective extraction of approximately 90% β-carotene.
With the help of energy and operating cost analyses, a nal assessment of conven-tional solvents and supercritical uid extraction was done. It turned out, that scCO2
extraction consumes nearly 2-times more energy than hexane extraction which is mainly attributed to the high compression work needed to achieve the critical pressure. How-ever, the lower solvent to biomass ratio during scCO2 extraction lowers the extraction costs compared to hexane. With that, the economic viability of scCO2 extraction in industrial microalgalβ-carotene extraction was demonstrated for the rst time. Further-more, from today's perspective, the green solvent character of scCO2 and its co-solvent EtOH is more consistent with the rising pursuit of sustainable production processes.
Based on the results of the present study, the use of scCO2 in industrial β-carotene extraction is highly recommended.
6
Valorization of D. salina remnant biomass by mild hydrothermal
treatment
A maximum and eective exploitation of the whole biomass as a multi-product is re-quired to create more sustainable bio-processes. In the case of D. salina, up to 90% of biomass remains unused after extraction of the main productβ-carotene. This remnant is an attractive source of by-product generation. In the present chapter the potential of mild hydrothermal liquefaction (HTL) as an innovative green methodology to release by-products afterβ-carotene production is assessed. Therefore, the theoretical background of thermochemical processes is summarized. In addition, possible by-products are dis-cussed by considering the biochemical and the elemental composition of the biomass remnant. Optimal process conditions are found experimentally by applying dierent HTL reaction times and temperatures. A potential by-product is identied and pos-sible applications are proposed. Finally, an energy consumption and operating costs calculation is done to evaluate the feasibility of hydrothermal by-product generation in the β-carotene process. The chapter incorporates methods and results published in Pirwitz et al. (2016).
6.1 Motivation
The cultivation of algal biomass has become an important alternative to generate valu-able products as well as second-generation biofuels. Nonetheless, the valorization of the complete biomass is still a crucial challenge in microalgal production processes and not yet feasible (t Lam et al., 2018). In most cases, considerable quantities of up to 90%
of the biomass are unused after the extraction of the main product. The residuals can provide additional products or feedstocks to satisfy further industrial needs and increase the process economy and competitiveness of the overall process. But which options are conceivable to realize a benecial use of the remnant?
6 VALORIZATION OF D. SALINA REMNANT BIOMASS
After the algal production of biofuels or other hydrophilic compounds (e.g. func-tional lipids, pigments,...), the remnant biomass consists of carbohydrates, proteins, residual lipids and further organic or inorganic molecules. One possibility is to apply these fractions for aquaculture and animal feed (Mata et al., 2010). Here, proteins and carbohydrates are essential key molecule to promote the animal growth during cultiva-tion. This strategy is supported by the fact that the biomass can be used directly as by-product without the need of further pretreatment. However, a biomass contamina-tion with organic solvent due to main product extraccontamina-tion could hinder this opportunity by inhibitory eects and regulatory restrictions (Parliament, 2009). Furthermore, the economic revenue achieved by this application is considered small.
With respect to the carbohydrate fraction of the residual biomass also microbial fermentation or anaerobic digestion could reveal an alternative pathway of remnant val-orization to produce ethanol or biogas (Harun et al., 2010a; Zhu, 2014). To get high conversion yields, an additional pretreatment of the biomass by enzymes, chemicals or physical methods is usually needed to make the carbohydrates accessible (Jeevan Ku-mar et al., 2017). This is a big drawback especially with respect to the processing cost and time (Kwon et al., 2016).
Another approach to valorize the remnants of microalgae production processes is the use of thermochemical conversion processes. Currently, these methods are investigated for the extraction of bio-crude as well as the generation of biofuels from high-lipid con-taining algal biomass (e.g. Chen et al. (2015); Khoo et al. (2013); López Barreiro et al.
(2013); Orosz & Forney (2008)). Notwithstanding, rst studies demonstrate promising results of thermochemically converted microalgal remnant (Kim et al., 2015; Rihko-Struckmann et al., 2017). The conversion can be done in one step which is easily imple-mentable as a unit extension of an existing production plant. Next to bio-crude, also other combustible (e.g. gas or char) or valuable compounds can be derived from thermo-chemical processes. Consequently, thermothermo-chemical conversion is a potential strategy to fully exploit the complete biomass in the D. salina process (see Figure 6.4) and worthy of detailed investigations.