Research On Stepwise Separation Of Bio-oil Based On Water Extraction And Molecular Distillation

Bio-oil, produced from biomass fast pyrolysis, has many inferior properties, including high oxygen content, low heating value, strong corrosiveness, etc. Thereore, it should be upgraded before being used as high grade fuels. However, the composition of bio-oil is so complicated that single upgrading technology cannot convert its overall compounds with high efficiency. Meanwhile, the large molecular compounds are easy to cause the fast deactivation of catalyst and the blockage of reactor. The stepwise separation of bio-oil can enrich compounds suitable for the same upgrading technology into individual fractions, which will effectively solve these problems. Moreover, valuable chemicals can be extracted from these fractions. Currently, there are two main graded upgrading schemes based on water extraction and molecular distillation separation respectively. This thesis focuses on the optimization of the primary bio-oil separation for these two schemes, in order to combine the reasonable enrichment of chemical families with the high-efficiency extraction of valuable compounds.Small molecular compounds in bio-oil aqueous phase often have high reactivities, while the sugars are easy to form coke during their upgrading. Five different solvents with distinct polarities were employed to elute the bio-oil aqueous phase. Gas chromatography technology was introduced to off-line monitor the composition of eluents. The eluents with similar peaks were combined as one fraction, and finally 11 fractions rich in different chemical families were obtained. The fractions rich in benzenediols and sugars were further purified respectively. A high catechol content of 62.81% was achieved through pH control method, and the majority of impurities like phenolic compounds were eliminated from the sugar-rich fraction by column chromatography with active carbon and diatomite as adsorbents. Acid and alkaline solutions combined with organic solvent were used to separate the water-insoluble phase, which contained monophenols and phenolic polymers (pyrolytic lignin). A phenolic fraction, obtained through reactive extraction, concentrated the most abundant monophenols with a relative content of 94.35%, in which the relative content of guaiacols reached 48.27%. The pyrolytic lignins with different reactivities consisted of similar basic guaiacyl and syringyl units. Polymers with molecular weight over 1000 dominated in the high-molecular-weight pyrolytic lignin, while the low-molecular-weight pyrolytic lignin contained more reactive phenolic hydroxyls.Molecular distillation technology realized the primary separation of various chemical families into different bio-oil fractions. Aiming at the different properties of these fractions, appropriate methods were adopted for their further separation. Solvents with weak water solubility were used to extract the light fraction, in which water and small molecular compounds with high reactivities were the most abundant. It is found that butanol had good extraction efficiency for the organic compounds in the light fraction, and produced a solvent phase suitable for subsequent esterification. The composition of middle fraction was similar to that of crude bio-oil but free of pyrolytic lignin. Solvents with various polarities were adopted to elute the middle fraction. With the aid of gas chromatography, a fraction rich in monophenols with a relative content of 88.49% was obtained, and cyclic ketones were concentrated in other fractions. Heavy fraction was difficult to dispose of, since it involved monophenols, sugars and pyrolytic lignin. Based on the properties of these chemical families, a new stepwise separation method including several technologies, such as extraction, heating treatment and column chromatography, was developed. Pyrolytic lignins with different reactivities were separated preferentially from the heavy fraction with methanol-water extraction method. The characterization indicates that pyrolytic lignins acquired from this method and water extraction method had similarities in elemental composition, and their basic structures both consisted of etherified and non-etherified syringyl and guaiacyl units. Nevertheless, they had significant differences in molecular weight distribution, side-chain functional groups, interunit linkages, etc. Since the low-molecular-weight pyrolytic lignin obtained with methanol-water extraction method was composed of tri- to pentamers and 0.14/Ar carbonyls, it had high reactivity. Interunit linkages of the three pyrolytic lignins contained β-β’ resinol moieties, whereas the low-molecular-weight pyrolytic lignin contained the most abundant alkyl ether linkages. The pyrolytic products from three pyrolytic lignins all involved phenolic compounds with contents of 73-78%, while the final char residue of low-molecular-weight pyrolytic lignin was the lowest. After the removal of pyrolytic lignin from heavy fraction, a fraction, with monophenols concentration of 70.90%, was separated from the residual aqueous phase by solvent extraction. Heating treatment, silica gel and neutral aluminium oxide could further eliminate the residual impurities. Eventually, the sugar fractions were obtained with a recovery rate of 29.65wt%. The combined sugar-rich fraction was further separated into an ethanol-soluble fraction and an ethanol-insoluble fraction by alcohol sedimentation method. Both of these fractions contained some monosaccharides, such as levoglucosan, xylose, and glucose, whereas the ethanol-insoluble fraction still had a small amount of cellobiose. The ultimate recovery rates for several identified monosaccharides were all in the range of 75-86%.