This thesis work explored the development of effective means of disrupting microalgae biomass and increasing lipid yield, thereby leading to effectual and cost-effective biodiesel production. First of all, hydrothermal acid pretreatment, a combined treatment of nitric acid and autoclaving, was applied to wet biomass of Nannochloropsis salina for the purpose of extracting lipid. Optimal conditions for this pretreatment were determined using a statistical approach, and mechanistic modes of action of nitric acid were also speculated. Through response surface methodology, the maximum lipid yield (predicted: 24.6%; experimental: 24.4%) was obtained using 0.57% nitric acid at 120 °C for 30 min. A relatively lower lipid yield (18.4%) was obtained using 2% nitric acid; however, chlorophyll and unsaturated fatty acids, both of which adversely affect the refinery and oxidative stability of biodiesel, were found to be not co-extracted. Considering its comparable extractability even from wet biomass and ability to reduce chlorophyll and unsaturated fatty acids, the hydrothermal nitric acid pretreatment can serve as one direct and promising route of extracting microalgae oil. In addition, hydrothermal acid treatment was adopted to extract eicosapentaenoic acid (EPA) from wet biomass of N. salina. It was found that sulfuric acid-based treatment increased EPA yield from 11.8 to 58.1 mg/g cell in a way that was nearly proportional to its concentration. Nitric acid exhibited the same pattern at low concentrations, but unlike sulfuric acid its effectiveness unexpectedly dropped from 0.5% to 2.0%. The optimal and minimal conditions for hydrothermal acid pretreatment were determined using a statistical approach; its maximum EPA yield (predicted: 43.69 mg/g cell; experimental: 43.93 mg/g cell) was established at a condition of 1.27% of sulfuric acid, 113.34 °C of temperature, and 36.71 min of reaction time. Our work demonstrated that the acid-catalyzed cell disruption, accompanied by heat, can be one potentially promising option for ω-3 fatty acids extraction.
In the next study, simultaneous treatment (combining with cell disruption and lipid extraction) using hydrodynamic cavitation (HC) was applied to N. salina to demonstrate a simple and integrated way to produce oil from wet microalgae. Higher lipid yields with HC (25.9-99.0%) were observed than those with autoclaving (16.2-66.5%) and ultrasonication (5.4-26.9%); this improvement became distinct when the specific energy input (500-10,000 kJ/kg) was considered. Optimal conditions for the HC-based simultaneous treatment were determined using a statistical approach. The efficiency of the simultaneous method was also demonstrated by comparing each separate treatment. Via response surface methodology, the maximum lipid yield (predicted: 45.9% and experimental: 45.5%) was obtained using 0.89% sulfuric acid with a cavitation number of 1.17 for a reaction time of 25.05 min. Considering its comparable extractability, energy-efficiency and potential for scale-up, HC is indeed an effective physical means that can make oil extraction more competitive.
In the following study, the Fenton-like reaction, a potent oxidation based on H2O2 with FeCl3, was employed to induce a sonocatalytic effect in a synergetic way with HC for the efficient extraction of lipids from wet microalgae biomass. The expected synergetic effect was in fact observed, and a high lipid was yielded when pH was adjusted at pH 3. The maximum lipid yield was obtained at 1% H2O2. What is better, 97% of chlorophyll, which must be removed when lipids are converted to biodiesel, was removed at the same condition. These results clearly supported that the sono-Fenton-like reaction is indeed a robust and feasible way for lipid extraction.
Lastly, the HC-based disintegration of microalgal cells was enhanced via design optimization to take full advantage of cavitation effects, in terms of different types of orifice plate (α = 2.5-10%; β = 1-10%). Correlation of hydraulic parameters, such as flux, velocity, and RPM, has been studied. Flow geometry of the orifice plate (α = 7.5%; β = 5%) led to the lowest cavitation number; however, cavity generation was disturbed at large orifice holes (α ≥ 7.5%). The optimal design of orifice plate to maximize lipid extraction was established at 5% of both α and β. The cavitation effect of the optimized HC was interpreted and supported by way of a computational fluid dynamics. This particular study of HC optimization is expected to serve as a guideline for industrial scaling-up with respect to the lipid extraction from microalgae.
바이오에너지 생산을 위한 공급원료로 빛에너지로부터 광합성을 하고 지구온난화를 야기하는 온실가스 중 이산화탄소를 흡수하여 빠르게 성장하는 미세조류가 주목받고 있다. 미세조류 종마다 다양한 성분을 함유하고있고, 그 중에서 지질성분을 이용하여 수송연료인 바이오디젤의 생산이 가능하다. 미세조류로부터 바이오디젤을 생산하기 위한 공정은 크게 미세조류 종의 선별, 배양, 수확, 건조, 추출, 전환 공정으로 나뉜다. 모든 공정이 연료생산을 위한 필수 공정이지만, 에너지효율과 공정의 간소화를 위해 건조과정을 생략하여 수확한 미세조류로부터 바로 지질을 추출하는 접근을 하였다. 하지만 미세조류 세포벽은 쉽게 분해되지 않는 셀룰로오스 구조로 이루어져있어서 지질추출율을 높이기 위해서는 미세조류 셀을 분해하는 전처리 공정이 필수적이다. 문헌에 의하면 셀룰로오스는 물리화학적 처리가 병행이 되었을 때 분해가 잘 이루어진다. 따라서 수력학적 공동현상(hydrodynamic cavitation)의 물리적 공법과 질산촉매, 펜톤촉매를 이용한 화학적 공법을 선택하여 고효율 저비용의 공정을 개발하였다.
수력학적 공동현상공법을 먼저 적용하기에 앞서 셀룰로오스의 기본전처리법인 열수처리공정을 통해 질산촉매의 미세조류 세포분해 효과를 알아보았다. 질산은 고온고압조건에서 라디컬 형태의 불안정하고 반응성이 높은 이산화질소로 전환이 되고, 이 물질은 불포화지방산을 산화시키는 지질과산화작용을 야기한다. 고농도의 질산은 지질과산화작용에 의해 지질추출율을 감소시켰지만 불포화지방산의 산화로 인해 산화안정성이 높은 연료의 생산을 가능하게 했다. 더욱이 바이오디젤 생산에 불순물인 클로로필이 제거되는 부수적인 효과가 있었다.
일반적으로는 전처리공정과 지질추출공정이 별개로 진행되지만 수력학적 공동현상 공법을 이용하여 반응기 내부에서 두 공정이 동시에 이루어질 수 있는 공법을 개발하였다. 동시처리공법은 반응기 내부에서 미세조류 셀이 분해되자마자 지질추출용매가 지질을 회수하는 원리로 분리공정보다 높은 지질추출율과 2배이상의 짧은반응시간을 보였다.
수력학적 공동현상을 이용하여 전처리와 지질추출의 동시처리가 가능하게 했고, 펜톤시료를 이용하여 수확공정까지 연계하는 공법을 개발하였다. FeCl3의 3가철을 미세조류 수확을 위한 응집제로 사용했을 때 pH 3에서 높은 수확효율을 보였고, 이 pH조건에서 유사펜톤반응 또한 활발하게 진행되었다. 즉 FeCl3을 이용해 수확한 미세조류와 과산화수소를 수력학적 공동현상 반응기에 주입하여 동시처리공법을 진행하였다. 그렇게 함으로써 수확부터 추출공정까지의 연계공정을 가능하게 했고 지질추출율을 높이는 시너지 효과를 확인하였다.
마지막으로 캐비테이션 효과를 최대한으로 이끌어내기 위한 오리피스의 최적형태를 디자인했고, 최적화 조건에서의 미세조류 높은 세포 파쇄 효과를 확인하였다. 유체 시뮬레이션을 통해 미세공기방울의 형성과 파괴 구간을 알아보고 캐비테이션 효과를 입증하였다.
