Continuous production of biodiesel from microalgae by extraction coupling with transesterification under supercritical conditions
Graphical abstract
Introduction
To avoid the crisis for the competition between human’s food and feed stock of biodiesel, traditional biodiesel has a great rely on the edible oil, some researchers have attempted to make nonedible lipids as the feed for biodiesel, which include non-edible seeds from plants, waste oil and microalgae. That is Jatropha, chinaberry seeds, yellow fruit, Sterculia foetida and rubber tree, Mahua oil, castor oil, tobacco, etc. (Karmakar et al., 2010). It realizes the process of waste recovery and reuse that turn waste oil to biodiesel, and reduce the pollution for environment. However, most of them have high acid value and contain too many impurities which are kinds of polymers as well as degradation products, and the supply has a great fluctuation, which is hard to use as the stable raw oil for large-scale industrialization. Microalgae cultivation has low requirement for space and high efficiency of photosynthesis. They have been considered as a bright prospect for biodiesel, as can be produced using saline and waste water with a remarkable growing rate and product yield. Compared with other material, microalgae has obvious advantages which are easy to cultivate, using non drinking water, transition solar to chemicals (Galadima and Muraza, 2014), and short growth period. Moreover, sulfur free feed also reduce the sulfide in the tail gas. Algae have much application value in Pharmaceuticals, Nutraceuticals, Cosmetics and Aquaculture purpose. Besides biodiesel, methane, ethanol and hydrogen can also be generated from microalgae. Due to the high content of polyunsaturated fatty acids such as linolenic acid, EPA and DHA, abundance of vitamins, minerals, and trace elements, they can be made as the feed of health products (Chauton et al., 2015). Recent research works on microalgae have identified this new bio-material as a promising technology for bioenergy production, wastewater treatment, the development of high value added products and CO2 capture. Although the oil content of microalgae is similar to other feeds, the global annual output is larger.
One of the first issues is extracting oil from microalgae in order to transfer the microalgae oil to biodiesel. The most common extraction processes are solvent extraction, ionic liquid extraction, and subcritical water extraction and so on, in which solvent extraction is the most mature technology and have general use in industrial. However, the toxicity of organic solvent and the high energy consumption for recovery make it harmful for the environment. In particular, the residue can be used as the material for pharmaceuticals and nutraceuticals. ScCO2 as a clean and green solvent, easy to separate with extracts and no residue in product, all these characters made it a suitable solvent for the extraction of microalgae (Halim et al., 2012).
Andrich using scCO2 to extract oil from microalgae and obtained an extraction curve (Andrich et al., 2005). As the extraction time prolong, the extraction yield decreased. Within 5000 s, the extraction amount was over 80%, while exceeding 10,000 s, the extraction amount was not increasing a lot, and the trend of increase become slower. That’s because the driving force of extraction is the oil concentration between bulk scCO2 and the inner cell. The initial concentration difference is large, so the extraction speed was fast, extend the extraction time, the concentration difference become smaller and the driving force reduced, so the tendency become flat. Taher studied the extraction conditions for microalgae (Taher et al., 2014), and found that the temperature and pressure has strong impact on the extraction yield, but less effect on the extraction yield. The optimal extraction conditions are temperature at 53 °C, 50 MPa pressure and the flow of CO2 was 1.9 g/min, and the extraction yield reached 7.41 wt% (dry algae). Solana utilized the ethanol as the entrainer to improve the extraction yield (Solana et al., 2014). The results showed that the extraction yield reached the highest at the temperature 60 °C, 30 MPa pressure, and CO2 flow 0.4 kg/h and 5% ethanol. Furthermore, there is a crossover pressure 25 MPa for extraction algae oil. That is when the pressure above the crossover pressure, the key factor for extraction is the vapor pressure of solute rather than the density of CO2. Cheng et al. compare the extraction with organic solvent and scCO2, ethyl acetate and methanol mixed solvent has an extraction yield of 98.7%, much more than the extraction for scCO2, which is 61.6% (Cheng et al., 2011). But ball-milling pretreatment can enhance the scCO2 extraction to 98.7%. Millao et al. investigated the extraction of oil and carotenoids from Nannochloropsis gaditana using supercritical carbon dioxide (Millao and Uquiche, 2016), by response surface methodology, the maximum oil yield 152.2 g/kg dry substrate was gained at 64 °C and 59.3 MPa. They found that temperature had a greater effect than CO2 density.
The extracted oil can be used to prepare biodiesel by supercritical methanol transesterification method, and its yield was affected by the operation conditions, such as temperature, reaction time, ratio of alcohol to oil (ratio of methanol to algae) and pressure. Reddy conducted the transesterification of algae (Nannochloropsis salina) in the condition temperature 265 °C, 20 min, dry algae to ethanol 1:9 (wt/v), and a maximum yield 67% was obtained (Reddy et al., 2014). Patil also used this one-step process for direct liquefaction and conversion of wet biomass containing about 90% of water to biodiesel under supercritical methanol conditions (Patil et al., 2011), in the optimal condition: wet algae to methanol (wt/vol) ratio of around 1:9, reaction temperature and time of about 255 °C, and 25 min lead to yield above 80%. They used the Nannochloropsis salina, too. Due to the deviation of species and culture environment, the ingredients of microalgae are different, but the free fat acid and water content have a great impact on the production process. When algae changed to Nannochloropsis gaditana as feed (Jazzar et al., 2015), only 48% yield was received in the optimized condition at 255–265 °C, 50 min reaction time, and using a methanol to dry algae ratio of 10:1 (vol/wt)
Batch reactions were employed in the above work. Continuous flow process (He et al., 2007, Zhou et al., 2010) can be brought so as to provide much bigger manufacturing scale, and cut the cost. Nan et al. studied the continued production of biodiesel through non-catalytic transesterification of microalgae oil with methanol and ethanol (Nan et al., 2015), and optimization of continuous process by RSM showed that the best condition were 320 °C, 15.2 MPa, 19:1 M ratio, 31 min, 7.5 wt% of water content for methanol transesterification, for ethanol, that is 340 °C, 17 MPa, 33:1 M ratio, 35 min, and also 7.5 wt% of water content. The corresponding yields of fatty acid methyl ester (FAME) and fatty acid ethyl ester (FAEE) were 90.8% and 87.8%, respectively.
To our knowledge, little has been reported in regard to coupling supercritical CO2 (scCO2) extraction and continuous transesterification for the microalgae. Most works are focusing the in-situ transesterification, although the in-situ process can utilize the wet microalgae directly, the residue after the reaction can’t be used again as the raw material of health products. That is a waste for the other ingredients of algae. Therefore, in this work, the continuous supercritical methanol transesterification coupling with scCO2 extraction without catalyst for the production of biodiesel was investigated, and two kinds of microalgae Chrysophyta and Chlorella sp. were adopted as the feed stock. This work aims to expand the feed of biodiesel, and utilize the green solvent CO2 so as to recover and reuse the residue. Through the investigation of the influence parameters of supercritical extraction and continuous transesterification for microalgae, the optimal operation conditions were obtained. We expected to expand and comprehensively utilize the raw material of biodiesel, and realize the continuous production in industry.
Section snippets
Materials and instruments
Chrysophyta and Chlorella sp. were provided by Dalian institutions of Chemical Physics, with fatty acid contents of 10.5 wt% and 19.8 wt% respectively (analyzed by in-situ transesterification method due to the reason that the lipids extracted by organic solvents for microalgal case are very complicated, including the components without the fatty acid chains such as lipid-soluble pigments, sterol and so on, which cannot be converted to FAME, and the lipid content obtained by extraction method is
The optimal operation parameters of scCO2 extraction
The extraction mechanism was provided by studying the extraction curve against the time with the condition: 20 g Chrysophyta in 40 mesh size, 60 °C, 30 MPa and 300 min. As is shown in Fig. 2, the extraction yield was increasing along with time, and the increasing tendency became gently after 240 min. Microalgae possess a cell wall, which is a thick and rigid layer composed of complex carbohydrates and glycoproteins with high mechanical strength and chemical resistance (Kim et al., 2013). Due to the
Conclusion
A continuous coupling process of supercritical extraction and non-catalytic supercritical methanol transesterification method for preparing biodiesel was put forward. Using scCO2 as the extraction solvent avoiding the damage of high-value compositions in high temperature, and the residue can be recycled. The optimum reaction condition was that 40 mesh algae, the extraction and reaction temperature 60 °C and 340 °C, 18–20 MPa, molar ration of methanol to oil 84:1 and the flow rate of CO2 and
Acknowledgements
The Author would like to thank the financial support from the National Natural Science Foundation of China (21376045, 21506027), Petrochemicals Joint Fund of National Natural Science Foundation of China and China National Petroleum Corporation (U1662130), Chinese Postdoctoral Science Foundation (2015M571307) and the Open Project Program of State Key Laboratory of Catalysis (N-15-01).
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