Abstract
Supercritical CO2 extraction was employed to separate simulated and real bio-oils. Effects of extraction pressure, temperature and adsorbents on distribution coefficient (or enrichment coefficient) of five representative compounds were investigated using a simulated bio-oil, which was composed of acetic acid (AC), propanoic acid (PA), furfural (FR), acetylacetone (AA) and 2-methoxyphenol (MP). The distribution coefficients of AA, FR and MP between super-critical CO2 phase and liquid phase were bigger than 1.5, while those of AC and PA characteristic of relatively strong polarity were less than 1. Temperature and pressure also had impacts on the distribution coefficients of AA, FR and MP, especially remarkable for AA. The extraction of simulated bio-oil spiked on three adsorbents shows that adsorbents influence extraction efficiency and selectivity by changing intermolecular forces. High extraction pressure and relative low temperature are beneficial to reduce the water content in the extract. In addition, the feasibility of supercritical CO2 extraction of real bio-oil was examined. After extraction in the extraction fraction total ketones increased from 14.1% to 21.15~25.40%, phenols from 10.74% to 31.32~41.25%, and aldehydes from 1.92% to 3.95~8.46%, while the acids significantly dropped from 28.15% to 6.92~12.32%, and water from 35.90% to 6.64~4.90%. In view of extraction efficiency, the optimal extraction temperature was determined to be 55℃. Extraction efficiency of the real bio-oil increased with rising pressure. The maximal extraction efficiency of real bio-oil on water-free basis could reach to 88.6%. After scCO2 extraction, the calorific value and stability of the extract fraction evidently increased and the acidity slight decreased with nearly 100% volatility below 140℃, suggesting potentially applicable as substitute for engine fuel.
Highlights
Facing upcoming depletion of fossil fuels and increasing environmental concerns, great effort has been devoted in exploration of biomass energy in the past few decades all over the world, because it is recognized as one of the most attractive alternative energy resources in the current century and it is available in abundance, renewable and environmentally friendly [1]
The effects of CO2 pressure on the components distribution coefficients between supercritical CO2 phase and liquid phase was investigated at 45°C in a pressure range of 7~17 MPa (Figure 2). 24.0 g simulated bio-oil was used in each experiment
MP is weak in polarity and readily extracted by nonpolar CO2, where as AC, propanoic acid (PA), FR and AA relatively difficult to be extracted as they are polar compounds
Summary
Facing upcoming depletion of fossil fuels and increasing environmental concerns, great effort has been devoted in exploration of biomass energy in the past few decades all over the world, because it is recognized as one of the most attractive alternative energy resources in the current century and it is available in abundance, renewable and environmentally friendly [1]. Biomass as an energy source is considered sustainable since it is CO2 neutral in the life cycle, causing almost zero net emissions of CO2 It contains negligible contents of sulfur, nitrogen, and ash, and gives much lower emission of SO2, NOx, and soot, by combustion, than the conventional fossil fuels [2]. Being a non-thermodynamic equilibrium mixture, it is highly viscous, non-volatile, poor in heat value, and corrosive [7]. It is such a complicated mixture, containing almost all kinds of oxygenated organic compounds, including alcohols, aldehydes, carboxylic acids, ketones, esters, saccharides, phenols, guaiacols, syringols, furans and multifunctional compounds [8], that severely restrains its direct application in vehicle engines as fuels.
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