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Multi-step separation of different chemical groups from the heavy fraction in biomass fast pyrolysis oil
Fuel Processing Technology 202 (2020) 106366
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Multi-step separation of different chemical groups from the heavy fraction in biomass fast pyrolysis oil
T
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Junyu Taoa, Chen Lia, Jian Lia, Beibei Yana, , Guanyi Chenb,c, Zhanjun Chenga,c, Wanqing Lia, Fawei Lina, Lian Houa a
School of Environmental Science and Engineering, Tianjin University, Tianjin 300350, China School of Science, Tibet University, Lhasa 850012, China c Tianjin Key Lab of Biomass Wastes Utilization/Tianjin Engineering Research Center of Bio Gas/Oil Technology, Tianjin 300072, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Biomass Fast pyrolysis oil Heavy bio-oil Separation Chemical group
The heterogeneity of the heavy fraction in bio-oil (HBO) causes serious problem for its downstream utilization. A multi-step method is thus proposed to separate different chemical groups from HBO. In this study, HBO from fast pyrolysis of poplar wood was collected and the characteristics of the HBO were investigated. A series of separation experiments was conducted on HBO, whose chemical groups were analyzed by chromatography coupled with mass spectrometry. The results show that through primary pH adjustment and CH2Cl2 extraction, HBO is divided into two fractions with fewer types of chemicals. After further distillation, HBO is separated into four fractions, whose primary chemical groups and their peak area percentages are ketones (89.5%), esters (49.9%), aromatic hydrocarbons (63.8%) and phenols (39.2%), respectively. These fractions can be used efficiently for either upgrading towards fuels or purification towards valuable chemicals. The findings in this paper is hoped to shed a light on solving the heterogeneity problem of bio-oil.
1. Introduction Fast pyrolysis is one of the most efficient approaches to dispose and utilize biomass. Pyrolysis oil is the main product of fast pyrolysis, and it can be used as a liquid fuel [1]. There are up to hundreds of chemicals in biomass pyrolysis oil (bio-oil), which include acids, alcohols, ketones, phenols, etc. [2]. Because of the high water content, low heating value and strong corrosiveness of bio-oil, its direct combustion is usually less satisfactory than fossil fuels [3]. Consequently, various upgrading methods (e.g., hydrodeoxygenation [4], catalytic reforming [5] and catalytic cracking [6]) have been developed to enhance the fuel property of bio-oil. There are also a number of studies focusing on separating value-added chemicals from bio-oil [7]. However, regardless of the downstream treating methods, the complex chemical composition is a most serious problem for bio-oil utilization. Bio-oil can be easily separated to a light fraction and a heavy fraction by density difference [8]. The light fraction primarily contains water-soluble contents such as low-molecular-weight acids and ketones. These contents have high reactivity towards both catalytic reforming and hydrodeoxygenation reactions; thus they are rather easy to be utilized. In comparison, heavy fraction of bio-oil (HBO) primarily contains water-insoluble contents which have more complex ⁎
composition [9]. The various components in HBO also have different chemical reactivities. Hardly any technique can simultaneously upgrade all components in HBO. This makes it more important to study the separation of HBO. Common separation methods of HBO include solvent extraction, distillation, column chromatography and membrane filtration. In respect of solvent extraction, water is a frequently used solvent for primary separation of water-soluble contents [10–12]. Various organic solvents (hexane, toluene, chloroform, methanol, methylene chloride, acetone, etc.) can separate chemicals in HBO by their polarity differences [13–15]. The pH of extraction solvents significantly affects the extracted species, and eventually affects the overall extraction performance [16]. In respect of distillation, it is a commonly used method to separate chemicals with different boiling points in bio-oil [17,18]. Selection of both distillation temperature and pressure is essential to the performance of bio-oil distillation [19]. There are also studies shedding light on separation of raw bio-oil [20] and HBO [21] by molecular distillation, which solves the polymerization and coking problems associated with traditional distillation [22]. While due to the extremely complex chemical composition in HBO, it could be rather difficult to obtain products with high purity solely by one-step extraction or distillation.
Corresponding author. E-mail address: [email protected] (B. Yan).
https://doi.org/10.1016/j.fuproc.2020.106366 Received 29 December 2019; Received in revised form 25 January 2020; Accepted 8 February 2020 0378-3820/ © 2020 Published by Elsevier B.V.
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solution was added to 5 mL HBO, making its pH ≈ 14. The alkaline solution adjusted HBO was divided into a light fraction and a heavy fraction. The light fraction (NaOH solution was not included) took a proportion of 25.51 wt% of the HBO, while the heavy fraction took 74.49 wt% of the HBO. The light fraction was named FA, and the heavy fraction was named FB. On the one hand, 1 mol/L HCl solution was added dropwise to FA, making its pH ≈ 7. This process consumed approximately 10 mL of HCl solution. Then 20 mL CH2Cl2 was added to acid solution adjusted FA, dividing it into a light fraction and a heavy fraction. The water-soluble light fraction was abandoned, and the heavy fraction was named FA1 for further distillation. Distillation of FA1 was conducted in 160 °C together with the solvent. The distillate was named FA2, and the distillation residual was named FA3. On the other hand, 20 mL CH2Cl2 was added to FB, dividing it into a light fraction and a heavy fraction. The water-soluble light fraction was abandoned, and the heavy fraction was named FB1 for further distillation. Distillation of FB1 was conducted in 150 °C together with the solvent. The distillate was named FB2, and the distillation residual was named FB3. All extraction processes were conducted by magnetically stirring for 5 min at room temperature. After extraction, the fractions were separated by centrifugation with a rotating speed of 800 r/min for 10 min. All distillation processes were conducted by a rotary evaporator. The rotating speed was set at 20 r/min, and the distillation pressure was 0.1 MPa. The distillation time for each bath of sample was 12 h.
Comparatively speaking, column chromatography and membrane filtration have better separation capability of specific chemicals. Welldesigned column chromatography system can precisely separate compounds in bio-oil even when they are from the same chemical group [23]. Consequently, column chromatography has been reported capable to separate value-added chemicals such as syringol, acetosyringone [24] and phthalate ester [25]. The separation property of membrane filtration mainly depends on the membrane material. Previous literatures have reported application of different membranes to separate sugars [26,27] and acetic acid [28] from bio-oil. However, considering the high cost of chromatographic column and efficient membrane, these methods requires relatively homogeneous feedstock. Compared with the reported works mentioned above, this work aims to provide a multi-step method for enriching different chemical groups from HBO, instead of separating specific compounds. This multistep method is hoped to serve as an effective pre-process for downstream upgrading (by hydrodeoxygenation, catalytic reforming, etc.) and purification (by column chromatography, membrane filtration, etc.), thus to enhance their efficiency and economy. Accordingly, a series of separation experiments involving pH adjusting, solvent extraction and distillation were conducted, and the separated fractions were carefully characterized. The results of this work are promising to contribute to better utilization of bio-oil. 2. Material and methods
2.3. Analysis of HBO and extracted fractions 2.1. Materials and reagents The compounds in HBO and extracted fractions were analyzed using a gas chromatography coupled with a mass spectrometer (QP2010 PLUS, Shimadzu Co., Ltd.). A crossbond diphenyl dimethyl polysiloxane capillary column (Rtx-5MS, Restek Co., Ltd.) was used. For each test, 0.15 mL of separated fraction and 1.35 mL of methanol solvent were prepared, and 10 μL of the diluted sample was injected for GC–MS analysis. The initial column temperature was 50 °C, and the column oven temperature was heated to 300 °C with a heating rate of 10 °C/ min. The column oven temperature was kept for 10 min after reaching 300 °C. The relative contents of different compounds in the sample were calculated by peak area normalization. The calculation of relative contents is shown in Eq. (1).
Bio-oil sample was obtained by fast pyrolysis of poplar wood (one of the most common fast pyrolysis feedstocks) in 550 °C. 100 g of poplar wood powder with particle size of 20 mesh was used for every batch of fast pyrolysis. A fixed-bed reacting system with 1 L/min N2 flow as carrier gas was used, and the poplar wood feedstock was injected by a feeder after the fixed-bed reactor had been stable at 550 °C. The obtained raw bio-oil sample contained approximately 19.8 wt% (20.0 vol %) HBO. The HBO was obtained by centrifugation of the bio-oil, where the rotating speed was set at 2000 r/min and the centrifugation time was 10 min. After centrifugation, the HBO was obtained by removing the upper light fraction with a pipette. Ultimate analysis of the HBO sample was conducted using an elemental analyzer (Thermo Scientific Flash 2000, Thermo Fisher Co., Ltd.), and the results are shown in Table 1. Methanol (CH3OH, guaranteed reagent, Rionlon Co., Ltd.) was used as solvent for gas chromatography - mass spectrometry (GC–MS) test. Dichloromethane (CH2Cl2, guaranteed reagent, Rionlon Co., Ltd.) was used as extraction solvent. As mentioned above, dichloromethane has an intermediate polarity, and was widely used as extraction solvent in the field of bio-oil extraction [9,14]. Hydrogen chloride (HCl, guaranteed reagent, Rionlon Co., Ltd.) and sodium hydroxide (NaOH, guaranteed reagent, Chemreagent Co., Ltd.) were used to adjust pH during the separation process. The reagents were used as received from manufacturer.
C x = Ax /A s × 100%
where Cx is relative content of compound x (%); Ax is the peak area of compound x (mV·s); As is the sum of peak areas of all concerned compounds (mV·s). 3. Results and discussion 3.1. Single-step separation of HBO The main compounds in HBO are listed in Table 2. It can be found that furfural is the most abundant compound in HBO, whose relative content is 22.52%. Furfural primarily comes from hydrolysis of hemicellulose in biomass, and is reported as a major component in raw biooil [17]. While the second most abundant compound only accounts for 5.28% in HBO. Among all the 30 main compounds listed in Table 2, most of them have relevant contents of 1–5%. It implies that HBO has rather complex chemical composition and strong and strong heterogeneity. The distribution of chemical groups in HBO is shown in Fig. 2(a). It can be found that the main chemical groups in HBO include aldehydes, phenols, ketones, aromatic hydrocarbons, oxygenated aromatic compounds (except phenols) and carboxylic acids. Aldehydes are the most abundant chemical group in HBO, whose relevant content reaches 28.5%. This can be attributed to the extremely high furfural content in HBO. In order to compare the performance of the multi-step separation method with single-step methods, the single CH2Cl2 extraction and
2.2. Multi-step separation of HBO The procedures for multi-step separation of different chemicals from HBO are shown in Fig. 1. At room temperature, 10 mL of 1 mol/L NaOH Table 1 Characteristics of the HBO sample. Density
C content
H content
N content
S content
O contenta
C/H ratio
1.05 g/mL
61.00 wt%
9.43 wt%
0.74 wt%
0.61 wt%
28.22 wt%
9.43
a
(1)
Calculated by difference. 2
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Fig. 1. Procedures for multi-step separation of different chemicals from HBO. Different colors are used to show different fractions, but the colors are not necessarily the same with the real fractions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in Fig. 2(b)–(d). It can be observed that distillation can separate compounds in HBO to some extent. Aromatic hydrocarbons are enriched in 150 °C distillate, while phenols and other oxygenated aromatic compounds are enriched in the distillation residual. The phenols primarily come from decomposition of lignin in biomass feedstock [29]. However, according to the chemical composition of these three distillation products, they still have relatively complex chemical group compositions, which make it difficult to directly and efficiently use these distillation products for subsequent use. Therefore, multi-step separation of HBO concerning pH adjustment, CH2Cl2 extraction and distillation was conducted as shown in Fig. 1 in the following work.
Table 2 Main compounds in HBO. No.
Compound
Relative content (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Furfural 1,2,4-Trimethoxybenzene 2,6-Dimethoxy-phenol 1,2,3-Trimethoxy-5-methyl-benzene 5-Methyl-2-furancarboxaldehyde 4-Ethyl-2-methoxy-phenol Toluene p-Xylene 2,6-Dimethoxy-4-(2-propenyl)-phenol 3,5-Dimethoxy-4-hydroxyphenylacetic acid Creosol 2,6-Dimethoxy-4-(2-propenyl)-phenol 2-Ethyl-4-methyl-4,6-dipropyl-4H-1,3,2-dioxaborin 2-Methyl-2-cyclopenten-1-one 2-Hydroxy-3-methyl-2-cyclopenten-1-one 3,4,5-Trimethyl-phenol ( ± )-2-Hydroxyoctanoic acid, acetate 3-Methyl-2-butanone 2,3-Dimethyl-2-cyclopenten-1-one 1-Methyl-naphthalene 3-Phenylpropyl ester cyclopropanecarboxylic acid 2-Pentanone 1-Chloro-2,3-dihydro-1H-indene 2,3-Pentanedione Cyclopentanone 1-(4-Hydroxy-3,5-dimethoxyphenyl)-ethanone 4,7-Dimethyl-benzofuran 3-Methyl-3-buten-2-one 4-Hydroxy-3,5-dimethoxy-benzaldehyde 1,1a,6,6a-Tetrahydro-cycloprop[a]indene
22.52 5.28 5.08 5.03 4.90 3.89 3.81 3.55 3.33 3.05 2.66 2.59 2.42 2.39 2.36 2.18 2.07 2.02 1.68 1.67 1.65 1.64 1.47 1.44 1.29 1.24 1.21 1.18 1.09 1.09
3.2. Primary pH adjustment and solvent extraction of HBO As shown in Fig. 1, the multi-step separation method first used NaOH solution to divide HBO into two phases, which are named FA and FB. Then the two phases were extracted separately. The weighing results show that the mass ratio of FA to FB is 1:2.92, ignoring the mass of the NaOH solution. FA mainly contains components that can be dissolved in NaOH solution, and its pH is strongly alkaline. Therefore, it was first neutralized by HCl solution and then extracted with CH2Cl2 in the subsequent step. The extracted FA is separated into a lighter watersoluble fraction and a heavier FA1 fraction. The water-soluble fraction has very high water content and Cl− concentration, which are introduced during pH adjustment process. So the water-soluble fraction was not further treated and utilized in this study. It was observed that most component in FB can be dissolved in CH2Cl2. The dissolved fraction has higher density than the undissolved fraction. The dissolved fraction was named FB1 for further study, while the little amount of undissolved fraction was abandoned. The main compounds in FA1 and FB1 are shown in Table 3. According to Table 3, furfural is still the major component in FA1 and FB1, which indicates that the primary separation process cannot well enrich furfural in any fraction. Compared with the original HBO, the relative contents of 2,6-dimethoxy-phenol and creosol in FA1 increase by 105% and 24%, respectively. Esters are found in FA1, while hardly any esters have been found in original HBO. According to Fig. 2, esters are neither detected during distillation of HBO. This implies that the esters are likely to be generated by esterification reactions during pH adjustment process. Esterification is a common bio-oil upgrading method which can reduce the water content and oxygen content in biooil [30]. Esterification reactions consume carboxylic acids and alcohols
distillation of HBO were investigated in this study, respectively. When extracting HBO with CH2Cl2, almost all HBO can be dissolved in CH2Cl2, leaving only a small amount of light yellow fraction. The CH2Cl2 soluble fraction has very similar composition with HBO. Thus, it can be concluded that the direct extraction of HBO with CH2Cl2 cannot achieve an ideal separation performance. Through analyzing the boiling points of main compounds in HBO, it is found that various compounds have boiling points either below 150 °C or between 150 and 200 °C. Therefore, a two stage distillation of HBO was carried out with distillation temperatures of 150 °C and 200 °C, respectively. The distribution of chemical groups of 150 °C distillate, 150–200 °C distillate and 200 °C distillation residual are listed 3
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Fig. 2. Chemical group composition of (a) HBO, (b) 150 °C distillate of HBO, (c) 150–200 °C distillate of HBO and (d) 200 °C distillation residual of HBO. Phenols are excluded from oxygenated aromatic compounds, and so do other figures in this paper.
compounds are hydrocarbons with weak polarity. These substances can exist in original HBO with low relative content [32], and they are enriched in FB1 due to the extraction effect. The only phenol in main compounds of FB1 is 2,6-dimethoxy-4-(2-propenyl)-phenol, whose relative content is 2.4%. Compared with original HBO, the relative contents of toluene, 1,2,3-trimethoxy-5-methyl-benzene and 1,2,3-trimethoxybenzene in FB2 are increased by 191%, 90% and 64%,
to generate esters, and bio-oil esterification can occur under temperature below 100 °C in acid mediums [31]. Among the listed 20 main compounds in FA1 whose relative contents exceed 1%, only 11 are in the original HBO. This implies that during pH adjustment of HBO, a series of reactions may occur, enriching the products in FA1. In contrast, 12 of the 18 main compounds in FB1 are found in the original HBO. Most of the 6 newly discovered
Table 3 Main compounds in FA1 and FB1. No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
FA1
FB1
Compound
Relative content (%)
Compound
Relative content (%)
Furfural Dibutyl phthalate 2,6-Dimethoxy-phenol 1,2,4-Trimethoxybenzene Desaspidinol 3-Methyl-1,2-cyclopentanedione 1-(4-Hydroxy-3,5-dimethoxyphenyl)-ethanone Bis(2-ethylhexyl) phthalate 1,2,3-Trimethoxy-5-methyl-benzene Creosol 3-Methyl-2-butanone 5-(4-Morpholyl)-furan-2-carboxaldehyde 2-Methyl-2-cyclopenten-1-one Cyclopentanone 2,3-Dimethyl-2-cyclopenten-1-one 5-Methyl-2-furancarboxaldehyde 1-(2-Methyl-1-cyclopenten-1-yl)-ethanone 10-Bromo-undecanoic acid 2-(Hydroxymethyl)-2-nitro-1,3-propanediol 3,5-Dimethoxy-4-hydroxycinnamaldehyde
17.78 15.86 10.41 6.94 6.51 5.49 5.41 4.91 3.93 3.29 3.13 2.30 2.30 2.12 1.77 1.67 1.54 1.49 1.38 1.34
Furfural Toluene 1,2,3-Trimethoxy-5-methyl-benzene 1,2,3-Trimethoxybenzene Azulene o-Xylene 1-Ethyl-3-(phenylmethyl)-benzene 3-Methyl-2-butanone Benzene p-Xylene 1-Chloro-2,3-dihydro-1H-indene 2,3-Dimethyl-2-cyclopenten-1-one 2,6-Dimethoxy-4-(2-propenyl)-phenol Cyclopentanone 3-Methyl-3-buten-2-one Methyl ester hexadecanoic acid Diisooctyl phthalate 4,5-Dihydro-2-methylimidazole-4-one
21.64 11.10 9.56 8.67 5.77 5.06 4.94 4.68 3.67 3.54 2.87 2.55 2.37 2.34 2.31 1.65 1.34 1.03
4
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Fig. 3. Chemical group composition of (a) FA1 and (b) FB1.
Other ketones are mainly 2-pentanone and 3-methyl-3-buten-2-one. The main compounds and chemical group composition of FA1 distillation residual (FA3) are shown in Table 4 and Fig. 4(b), respectively. The most abundant chemical group in FA3 are esters, whose relative content reaches 49.9%. However, the esters contained in FA3 are not exactly the same with FA1. Meanwhile, phenols and furfural which are abundant in FA1 are neither detected in FA2 nor FA3. This may be attributed to esterification and polycondensation reactions of these two types of highly reactive substances under the distillation temperature. Esterification is the reaction for making esters from carboxylic acids and phenols or alcohols. The polycondensation reactions here could be phenolic aldehyde condensation which makes furfural resin from phenols and furfural. Similar results are also obtained by previous studies [9]. A large number of heterocyclic compounds were detected in FA3, which may also be caused by the chemical reactions occurring under distillation temperature. Besides, slight sintering phenomenon was observed during the distillation of FA1. The distillation temperature of FB1 was set to 150 °C, aiming to separate aromatic hydrocarbons whose boiling points are usually below 150 °C. The main compounds and chemical group composition of the 150 °C distillate (FB2) are shown in Table 5 and Fig. 4(c), respectively. It can be found that a large amount of aromatic hydrocarbons are enriched in FB2, whose relative content reaches 63.8%. The aromatic hydrocarbons may not only source from original HBO, but also from interactions among phenols, acids and ketones [34]. Meanwhile, there are also ketones and aldehydes in FB2, and their relative contents are 17.9% and 8.6%, respectively. Aldehydes are also commonly used as model compounds to investigate the upgrading characteristics of bio-oil [36]. The main compounds and chemical group composition of FB2
respectively. The chemical group compositions of FA1 and FB1 are shown in Fig. 3. It can be observed that the phenols in original HBO and the esters produced during the pH adjustment process are mainly enriched in FA1, while the aromatic hydrocarbons in original HBO are mainly enriched in FB1. As mentioned above, there is no significant difference between the distributions of aldehydes (which are primarily furfural) in FA1 and FB1. Besides, there are also ketones and oxygenated aromatic compounds with different species and contents in FA1 and FB1, respectively. Comparatively speaking, FB1 contains more aromatic compounds except phenols than FA1. This may because that the alkaline solution is beneficial for the extraction of smaller molecules with stronger polarity [22], while most of these aromatic compounds have relatively weak polarity and large molecular size. 3.3. Secondary distillation of extracted fractions Different chemical groups in FA1 and FB1 were further separated by distillation. FA1 contains large concentrations of phenols, aldehydes, ketones and esters. Among these contents, the boiling points of ketones are usually lower than 160 °C, while the other compounds commonly have higher boiling points [33]. Therefore, 160 °C was adopted as distillation temperature for FA1 separation. The main compounds and chemical group composition of 160 °C distillate (FA2) are shown in Table 4 and Fig. 4(a), respectively. The main chemical group in FA2 is ketones, accounting for 89.5% of all chemical groups. Ketones are important components in bio-oil, and there are comprehensive researches using ketones as model compounds to investigate upgrading methods especially hydrodeoxygenation of bio-oil [34,35]. Among various ketones, the relative content of 3-methyl-2-butanone reaches 47.64%. Table 4 Main compounds in FA2 and FA3. No.
1 2 3 4 5 6 7 8 9 10 11 12
FA2
FA3
Compound
Relative content (%)
Compound
Relative content (%)
3-Methyl-2-butanone 2-Pentanone 3-Methyl-3-buten-2-one 2,3,3-Trimethyl-hexane Butanoic acid methyl ester
47.64 27.20 14.66 5.30 2.46
3-Aminopyrazine 1-oxide Hexanoic acid methyl ester Hexadecanoic acid methyl ester Octanoic acid methyl ester 4-Methoxy-1-oxide-pyridine Methyl stearate Butanoic acid methyl ester Nonanedioic acid dimethyl ester Docosanoic acid methyl ester 2,2-Dimethoxy-propane Methyl 18-methylnonadecanoate 2,2-Dimethoxybutane
27.49 12.76 12.67 8.47 8.02 7.55 5.99 3.91 3.28 3.13 2.85 1.27
5
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Fig. 4. Chemical group composition of (a) FA2, (b) FA3, (c) FB2 and (d) FB3.
distillation of FB1.
distillation residual are shown in Table 5 and Fig. 4(d), respectively. A large number of phenols are enriched in FB3, and their relative content reaches 39.2%. Phenols are regarded as valuable chemicals which can be derived from bio-oil [37]. The phenols in bio-oil primarily come from thermal degradation of lignin in biomass [38]. Although a large amount of aromatic hydrocarbons are enriched in FB2, the relative content of aromatic hydrocarbons in FB3 is still as high as 23.5%. Similar to composition results of FA3, FB3 also contains plenty of compounds which have not been detected during previous separation process. They may be generated from chemical reactions occurring under the distillation temperature as well. While different with circumstance of distilling FA1, no obvious sintering phenomenon occurred during the
3.4. Overview of HBO composition change during multi-step separation In order to demonstrate the composition changes of HBO during the multi-step separation process, a scheme is shown in Fig. 5. It is worth noting that the data in Fig. 5 are based on relative content of each component, so the mass ratio of same substances in FA2, FA3, FB2 and FB3 cannot be reflected in this graph. According to Fig. 5, almost all carboxylic acids and phenols in HBO are enriched in FB3, while almost all aldehydes in HBO are enriched in FB2. The rich phenols content in FB3 make it rather potential to derive value added chemicals from it.
Table 5 Main compounds in FB2 and FB3. No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
FB2
FB3
Compound
Relative content (%)
Compound
Relative content (%)
Toluene o-Xylene Furfural Benzene 3-Methyl-2-butanone 1,3-Dimethyl-benzene Ethylbenzene 3-Methyl-3-buten-2-one 2-Pentanone 2,5-Dimethyl-furan 1,2,4-Trimethyl-benzene 1-Ethyl-4-methyl-benzene 1-Ethyl-2-methyl-benzene Cyclopentanone 1-Ethyl-2-methyl-benzene Heptane
24.10 11.71 8.57 8.38 6.82 6.27 4.74 4.58 4.56 3.20 2.80 2.18 2.04 1.93 1.53 1.07
1,2,3-Trimethoxybenzene 5-tert-Butylpyrogallol 4-Ethyl-2-methoxy-phenol Naphthalene 1-Methyl-naphthalene Methyl ester hexadecanoic acid 2-Methyl-naphthalene 4-Amino-2,5-dimethyl-phenol Methyl stearate
27.49 12.76 12.67 8.47 8.02 7.55 5.99 3.91 1.27
6
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Fig. 5. Overview of HBO composition change during multi-step separation. The yellow streams on the left indicate chemical group composition of HBO, and the width of the streams is proportional to the absolute content. Streams in other colors show the sources of chemical groups in the separated fractions, while the width of the streams is not proportional to the absolute content. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
HBO is divided into two fractions with fewer types of chemicals. After further distillation, these two fractions are separated into fractions FA2, FA3, FB2 and FB3, whose primary chemical groups are ketones (89.5%), esters (49.9%), aromatic hydrocarbons (63.8%) and phenols (39.2%), respectively. The homogeneity of these separated fractions is significantly enhanced, and thus more suitable for further upgrading and purification.
However, FB2 and FB3 also contain almost all the aromatic hydrocarbons in HBO, and the aromatic hydrocarbons have relatively high contents in FB2 and FB3. This caused more serious heterogeneity problem for their further purification. The ketones in HBO are partly distributed in FB2, and the others are distributed in FA2. The relative content of ketones in FA2 is as high as 89.5%, making it easier to further upgrade FA2. Although there are little esters existed in original HBO, a few esters are generated during pH adjustment and distillation process. All these esters are distributed in FA2 and FA3, among which the relative content of esters is only 2.5%, while the relative content of esters in FA3 reaches 49.9%. Esters are major components in biodiesel [39], and the high esters content in FA3 makes it rather suitable to be refined to a liquid fuel. Through the multi-step separation process proposed in this study, HBO is separated into fractions FA2, FA3, FB2 and FB3, whose primary chemical groups are ketones (89.5%), esters (49.9%), aromatic hydrocarbons (63.8%) and phenols (39.2%), respectively. On the one hand, the homogeneity of the four fractions is significantly enhanced, which can make the further upgrading process more efficient. In the field of bio-oil conversion and upgrading, many researches tend to optimize reaction parameters such as catalyst, temperature and pressure on specific chemical groups or model compounds. Therefore, the optimized parameters are usually especially effective on homogeneous feedstocks. On the other hand, the chemical groups in HBO are enriched in specific separated fractions, making it more efficient to purify valuable products from HBO. This is especially important when the further purification process uses expensive consumables to get products with high purity. According to reported publications for bio-oil upgrading and purification, FA2, FA3 and FB2 are promising to be upgraded efficiently, while FB3 is potential to be a qualified source for phenols purification.
CRediT authorship contribution statement Junyu Tao: Conceptualization, Writing - original draft, Methodology, Investigation, Visualization. Chen Li: Conceptualization, Investigation, Visualization. Jian Li: Validation, Investigation, Writing - review & editing. Beibei Yan: Resources, Writing - original draft, Writing - review & editing. Guanyi Chen: Funding acquisition, Resources, Writing - review & editing, Supervision. Zhanjun Cheng: Methodology, Writing - review & editing. Wanqing Li: Methodology, Validation. Fawei Lin: Validation, Investigation. Lian Hou: Funding acquisition, Supervision. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was financially supported by the National Key Research and Development Program of China (2016YFE0201800), the National Natural Science Foundation of China (51676138, 51878557), and the Tianjin Science and Technology Project (18YFJLCG00090).
4. Conclusions
Appendix A. Supplementary data
The HBO collected in this study primarily contains carboxylic acids, phenols, ketones, aldehydes and aromatic hydrocarbons. The multi-step separation method is effective to generate more homogenous fractions from HBO. Through primary pH adjustment and CH2Cl2 extraction,
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuproc.2020.106366. 7
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Multi-step separation of different chemical groups from the heavy fraction in biomass fast pyrolysis oil
T
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Junyu Taoa, Chen Lia, Jian Lia, Beibei Yana, , Guanyi Chenb,c, Zhanjun Chenga,c, Wanqing Lia, Fawei Lina, Lian Houa a
School of Environmental Science and Engineering, Tianjin University, Tianjin 300350, China School of Science, Tibet University, Lhasa 850012, China c Tianjin Key Lab of Biomass Wastes Utilization/Tianjin Engineering Research Center of Bio Gas/Oil Technology, Tianjin 300072, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Biomass Fast pyrolysis oil Heavy bio-oil Separation Chemical group
The heterogeneity of the heavy fraction in bio-oil (HBO) causes serious problem for its downstream utilization. A multi-step method is thus proposed to separate different chemical groups from HBO. In this study, HBO from fast pyrolysis of poplar wood was collected and the characteristics of the HBO were investigated. A series of separation experiments was conducted on HBO, whose chemical groups were analyzed by chromatography coupled with mass spectrometry. The results show that through primary pH adjustment and CH2Cl2 extraction, HBO is divided into two fractions with fewer types of chemicals. After further distillation, HBO is separated into four fractions, whose primary chemical groups and their peak area percentages are ketones (89.5%), esters (49.9%), aromatic hydrocarbons (63.8%) and phenols (39.2%), respectively. These fractions can be used efficiently for either upgrading towards fuels or purification towards valuable chemicals. The findings in this paper is hoped to shed a light on solving the heterogeneity problem of bio-oil.
1. Introduction Fast pyrolysis is one of the most efficient approaches to dispose and utilize biomass. Pyrolysis oil is the main product of fast pyrolysis, and it can be used as a liquid fuel [1]. There are up to hundreds of chemicals in biomass pyrolysis oil (bio-oil), which include acids, alcohols, ketones, phenols, etc. [2]. Because of the high water content, low heating value and strong corrosiveness of bio-oil, its direct combustion is usually less satisfactory than fossil fuels [3]. Consequently, various upgrading methods (e.g., hydrodeoxygenation [4], catalytic reforming [5] and catalytic cracking [6]) have been developed to enhance the fuel property of bio-oil. There are also a number of studies focusing on separating value-added chemicals from bio-oil [7]. However, regardless of the downstream treating methods, the complex chemical composition is a most serious problem for bio-oil utilization. Bio-oil can be easily separated to a light fraction and a heavy fraction by density difference [8]. The light fraction primarily contains water-soluble contents such as low-molecular-weight acids and ketones. These contents have high reactivity towards both catalytic reforming and hydrodeoxygenation reactions; thus they are rather easy to be utilized. In comparison, heavy fraction of bio-oil (HBO) primarily contains water-insoluble contents which have more complex ⁎
composition [9]. The various components in HBO also have different chemical reactivities. Hardly any technique can simultaneously upgrade all components in HBO. This makes it more important to study the separation of HBO. Common separation methods of HBO include solvent extraction, distillation, column chromatography and membrane filtration. In respect of solvent extraction, water is a frequently used solvent for primary separation of water-soluble contents [10–12]. Various organic solvents (hexane, toluene, chloroform, methanol, methylene chloride, acetone, etc.) can separate chemicals in HBO by their polarity differences [13–15]. The pH of extraction solvents significantly affects the extracted species, and eventually affects the overall extraction performance [16]. In respect of distillation, it is a commonly used method to separate chemicals with different boiling points in bio-oil [17,18]. Selection of both distillation temperature and pressure is essential to the performance of bio-oil distillation [19]. There are also studies shedding light on separation of raw bio-oil [20] and HBO [21] by molecular distillation, which solves the polymerization and coking problems associated with traditional distillation [22]. While due to the extremely complex chemical composition in HBO, it could be rather difficult to obtain products with high purity solely by one-step extraction or distillation.
Corresponding author. E-mail address: [email protected] (B. Yan).
https://doi.org/10.1016/j.fuproc.2020.106366 Received 29 December 2019; Received in revised form 25 January 2020; Accepted 8 February 2020 0378-3820/ © 2020 Published by Elsevier B.V.
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solution was added to 5 mL HBO, making its pH ≈ 14. The alkaline solution adjusted HBO was divided into a light fraction and a heavy fraction. The light fraction (NaOH solution was not included) took a proportion of 25.51 wt% of the HBO, while the heavy fraction took 74.49 wt% of the HBO. The light fraction was named FA, and the heavy fraction was named FB. On the one hand, 1 mol/L HCl solution was added dropwise to FA, making its pH ≈ 7. This process consumed approximately 10 mL of HCl solution. Then 20 mL CH2Cl2 was added to acid solution adjusted FA, dividing it into a light fraction and a heavy fraction. The water-soluble light fraction was abandoned, and the heavy fraction was named FA1 for further distillation. Distillation of FA1 was conducted in 160 °C together with the solvent. The distillate was named FA2, and the distillation residual was named FA3. On the other hand, 20 mL CH2Cl2 was added to FB, dividing it into a light fraction and a heavy fraction. The water-soluble light fraction was abandoned, and the heavy fraction was named FB1 for further distillation. Distillation of FB1 was conducted in 150 °C together with the solvent. The distillate was named FB2, and the distillation residual was named FB3. All extraction processes were conducted by magnetically stirring for 5 min at room temperature. After extraction, the fractions were separated by centrifugation with a rotating speed of 800 r/min for 10 min. All distillation processes were conducted by a rotary evaporator. The rotating speed was set at 20 r/min, and the distillation pressure was 0.1 MPa. The distillation time for each bath of sample was 12 h.
Comparatively speaking, column chromatography and membrane filtration have better separation capability of specific chemicals. Welldesigned column chromatography system can precisely separate compounds in bio-oil even when they are from the same chemical group [23]. Consequently, column chromatography has been reported capable to separate value-added chemicals such as syringol, acetosyringone [24] and phthalate ester [25]. The separation property of membrane filtration mainly depends on the membrane material. Previous literatures have reported application of different membranes to separate sugars [26,27] and acetic acid [28] from bio-oil. However, considering the high cost of chromatographic column and efficient membrane, these methods requires relatively homogeneous feedstock. Compared with the reported works mentioned above, this work aims to provide a multi-step method for enriching different chemical groups from HBO, instead of separating specific compounds. This multistep method is hoped to serve as an effective pre-process for downstream upgrading (by hydrodeoxygenation, catalytic reforming, etc.) and purification (by column chromatography, membrane filtration, etc.), thus to enhance their efficiency and economy. Accordingly, a series of separation experiments involving pH adjusting, solvent extraction and distillation were conducted, and the separated fractions were carefully characterized. The results of this work are promising to contribute to better utilization of bio-oil. 2. Material and methods
2.3. Analysis of HBO and extracted fractions 2.1. Materials and reagents The compounds in HBO and extracted fractions were analyzed using a gas chromatography coupled with a mass spectrometer (QP2010 PLUS, Shimadzu Co., Ltd.). A crossbond diphenyl dimethyl polysiloxane capillary column (Rtx-5MS, Restek Co., Ltd.) was used. For each test, 0.15 mL of separated fraction and 1.35 mL of methanol solvent were prepared, and 10 μL of the diluted sample was injected for GC–MS analysis. The initial column temperature was 50 °C, and the column oven temperature was heated to 300 °C with a heating rate of 10 °C/ min. The column oven temperature was kept for 10 min after reaching 300 °C. The relative contents of different compounds in the sample were calculated by peak area normalization. The calculation of relative contents is shown in Eq. (1).
Bio-oil sample was obtained by fast pyrolysis of poplar wood (one of the most common fast pyrolysis feedstocks) in 550 °C. 100 g of poplar wood powder with particle size of 20 mesh was used for every batch of fast pyrolysis. A fixed-bed reacting system with 1 L/min N2 flow as carrier gas was used, and the poplar wood feedstock was injected by a feeder after the fixed-bed reactor had been stable at 550 °C. The obtained raw bio-oil sample contained approximately 19.8 wt% (20.0 vol %) HBO. The HBO was obtained by centrifugation of the bio-oil, where the rotating speed was set at 2000 r/min and the centrifugation time was 10 min. After centrifugation, the HBO was obtained by removing the upper light fraction with a pipette. Ultimate analysis of the HBO sample was conducted using an elemental analyzer (Thermo Scientific Flash 2000, Thermo Fisher Co., Ltd.), and the results are shown in Table 1. Methanol (CH3OH, guaranteed reagent, Rionlon Co., Ltd.) was used as solvent for gas chromatography - mass spectrometry (GC–MS) test. Dichloromethane (CH2Cl2, guaranteed reagent, Rionlon Co., Ltd.) was used as extraction solvent. As mentioned above, dichloromethane has an intermediate polarity, and was widely used as extraction solvent in the field of bio-oil extraction [9,14]. Hydrogen chloride (HCl, guaranteed reagent, Rionlon Co., Ltd.) and sodium hydroxide (NaOH, guaranteed reagent, Chemreagent Co., Ltd.) were used to adjust pH during the separation process. The reagents were used as received from manufacturer.
C x = Ax /A s × 100%
where Cx is relative content of compound x (%); Ax is the peak area of compound x (mV·s); As is the sum of peak areas of all concerned compounds (mV·s). 3. Results and discussion 3.1. Single-step separation of HBO The main compounds in HBO are listed in Table 2. It can be found that furfural is the most abundant compound in HBO, whose relative content is 22.52%. Furfural primarily comes from hydrolysis of hemicellulose in biomass, and is reported as a major component in raw biooil [17]. While the second most abundant compound only accounts for 5.28% in HBO. Among all the 30 main compounds listed in Table 2, most of them have relevant contents of 1–5%. It implies that HBO has rather complex chemical composition and strong and strong heterogeneity. The distribution of chemical groups in HBO is shown in Fig. 2(a). It can be found that the main chemical groups in HBO include aldehydes, phenols, ketones, aromatic hydrocarbons, oxygenated aromatic compounds (except phenols) and carboxylic acids. Aldehydes are the most abundant chemical group in HBO, whose relevant content reaches 28.5%. This can be attributed to the extremely high furfural content in HBO. In order to compare the performance of the multi-step separation method with single-step methods, the single CH2Cl2 extraction and
2.2. Multi-step separation of HBO The procedures for multi-step separation of different chemicals from HBO are shown in Fig. 1. At room temperature, 10 mL of 1 mol/L NaOH Table 1 Characteristics of the HBO sample. Density
C content
H content
N content
S content
O contenta
C/H ratio
1.05 g/mL
61.00 wt%
9.43 wt%
0.74 wt%
0.61 wt%
28.22 wt%
9.43
a
(1)
Calculated by difference. 2
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Fig. 1. Procedures for multi-step separation of different chemicals from HBO. Different colors are used to show different fractions, but the colors are not necessarily the same with the real fractions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in Fig. 2(b)–(d). It can be observed that distillation can separate compounds in HBO to some extent. Aromatic hydrocarbons are enriched in 150 °C distillate, while phenols and other oxygenated aromatic compounds are enriched in the distillation residual. The phenols primarily come from decomposition of lignin in biomass feedstock [29]. However, according to the chemical composition of these three distillation products, they still have relatively complex chemical group compositions, which make it difficult to directly and efficiently use these distillation products for subsequent use. Therefore, multi-step separation of HBO concerning pH adjustment, CH2Cl2 extraction and distillation was conducted as shown in Fig. 1 in the following work.
Table 2 Main compounds in HBO. No.
Compound
Relative content (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Furfural 1,2,4-Trimethoxybenzene 2,6-Dimethoxy-phenol 1,2,3-Trimethoxy-5-methyl-benzene 5-Methyl-2-furancarboxaldehyde 4-Ethyl-2-methoxy-phenol Toluene p-Xylene 2,6-Dimethoxy-4-(2-propenyl)-phenol 3,5-Dimethoxy-4-hydroxyphenylacetic acid Creosol 2,6-Dimethoxy-4-(2-propenyl)-phenol 2-Ethyl-4-methyl-4,6-dipropyl-4H-1,3,2-dioxaborin 2-Methyl-2-cyclopenten-1-one 2-Hydroxy-3-methyl-2-cyclopenten-1-one 3,4,5-Trimethyl-phenol ( ± )-2-Hydroxyoctanoic acid, acetate 3-Methyl-2-butanone 2,3-Dimethyl-2-cyclopenten-1-one 1-Methyl-naphthalene 3-Phenylpropyl ester cyclopropanecarboxylic acid 2-Pentanone 1-Chloro-2,3-dihydro-1H-indene 2,3-Pentanedione Cyclopentanone 1-(4-Hydroxy-3,5-dimethoxyphenyl)-ethanone 4,7-Dimethyl-benzofuran 3-Methyl-3-buten-2-one 4-Hydroxy-3,5-dimethoxy-benzaldehyde 1,1a,6,6a-Tetrahydro-cycloprop[a]indene
22.52 5.28 5.08 5.03 4.90 3.89 3.81 3.55 3.33 3.05 2.66 2.59 2.42 2.39 2.36 2.18 2.07 2.02 1.68 1.67 1.65 1.64 1.47 1.44 1.29 1.24 1.21 1.18 1.09 1.09
3.2. Primary pH adjustment and solvent extraction of HBO As shown in Fig. 1, the multi-step separation method first used NaOH solution to divide HBO into two phases, which are named FA and FB. Then the two phases were extracted separately. The weighing results show that the mass ratio of FA to FB is 1:2.92, ignoring the mass of the NaOH solution. FA mainly contains components that can be dissolved in NaOH solution, and its pH is strongly alkaline. Therefore, it was first neutralized by HCl solution and then extracted with CH2Cl2 in the subsequent step. The extracted FA is separated into a lighter watersoluble fraction and a heavier FA1 fraction. The water-soluble fraction has very high water content and Cl− concentration, which are introduced during pH adjustment process. So the water-soluble fraction was not further treated and utilized in this study. It was observed that most component in FB can be dissolved in CH2Cl2. The dissolved fraction has higher density than the undissolved fraction. The dissolved fraction was named FB1 for further study, while the little amount of undissolved fraction was abandoned. The main compounds in FA1 and FB1 are shown in Table 3. According to Table 3, furfural is still the major component in FA1 and FB1, which indicates that the primary separation process cannot well enrich furfural in any fraction. Compared with the original HBO, the relative contents of 2,6-dimethoxy-phenol and creosol in FA1 increase by 105% and 24%, respectively. Esters are found in FA1, while hardly any esters have been found in original HBO. According to Fig. 2, esters are neither detected during distillation of HBO. This implies that the esters are likely to be generated by esterification reactions during pH adjustment process. Esterification is a common bio-oil upgrading method which can reduce the water content and oxygen content in biooil [30]. Esterification reactions consume carboxylic acids and alcohols
distillation of HBO were investigated in this study, respectively. When extracting HBO with CH2Cl2, almost all HBO can be dissolved in CH2Cl2, leaving only a small amount of light yellow fraction. The CH2Cl2 soluble fraction has very similar composition with HBO. Thus, it can be concluded that the direct extraction of HBO with CH2Cl2 cannot achieve an ideal separation performance. Through analyzing the boiling points of main compounds in HBO, it is found that various compounds have boiling points either below 150 °C or between 150 and 200 °C. Therefore, a two stage distillation of HBO was carried out with distillation temperatures of 150 °C and 200 °C, respectively. The distribution of chemical groups of 150 °C distillate, 150–200 °C distillate and 200 °C distillation residual are listed 3
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Fig. 2. Chemical group composition of (a) HBO, (b) 150 °C distillate of HBO, (c) 150–200 °C distillate of HBO and (d) 200 °C distillation residual of HBO. Phenols are excluded from oxygenated aromatic compounds, and so do other figures in this paper.
compounds are hydrocarbons with weak polarity. These substances can exist in original HBO with low relative content [32], and they are enriched in FB1 due to the extraction effect. The only phenol in main compounds of FB1 is 2,6-dimethoxy-4-(2-propenyl)-phenol, whose relative content is 2.4%. Compared with original HBO, the relative contents of toluene, 1,2,3-trimethoxy-5-methyl-benzene and 1,2,3-trimethoxybenzene in FB2 are increased by 191%, 90% and 64%,
to generate esters, and bio-oil esterification can occur under temperature below 100 °C in acid mediums [31]. Among the listed 20 main compounds in FA1 whose relative contents exceed 1%, only 11 are in the original HBO. This implies that during pH adjustment of HBO, a series of reactions may occur, enriching the products in FA1. In contrast, 12 of the 18 main compounds in FB1 are found in the original HBO. Most of the 6 newly discovered
Table 3 Main compounds in FA1 and FB1. No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
FA1
FB1
Compound
Relative content (%)
Compound
Relative content (%)
Furfural Dibutyl phthalate 2,6-Dimethoxy-phenol 1,2,4-Trimethoxybenzene Desaspidinol 3-Methyl-1,2-cyclopentanedione 1-(4-Hydroxy-3,5-dimethoxyphenyl)-ethanone Bis(2-ethylhexyl) phthalate 1,2,3-Trimethoxy-5-methyl-benzene Creosol 3-Methyl-2-butanone 5-(4-Morpholyl)-furan-2-carboxaldehyde 2-Methyl-2-cyclopenten-1-one Cyclopentanone 2,3-Dimethyl-2-cyclopenten-1-one 5-Methyl-2-furancarboxaldehyde 1-(2-Methyl-1-cyclopenten-1-yl)-ethanone 10-Bromo-undecanoic acid 2-(Hydroxymethyl)-2-nitro-1,3-propanediol 3,5-Dimethoxy-4-hydroxycinnamaldehyde
17.78 15.86 10.41 6.94 6.51 5.49 5.41 4.91 3.93 3.29 3.13 2.30 2.30 2.12 1.77 1.67 1.54 1.49 1.38 1.34
Furfural Toluene 1,2,3-Trimethoxy-5-methyl-benzene 1,2,3-Trimethoxybenzene Azulene o-Xylene 1-Ethyl-3-(phenylmethyl)-benzene 3-Methyl-2-butanone Benzene p-Xylene 1-Chloro-2,3-dihydro-1H-indene 2,3-Dimethyl-2-cyclopenten-1-one 2,6-Dimethoxy-4-(2-propenyl)-phenol Cyclopentanone 3-Methyl-3-buten-2-one Methyl ester hexadecanoic acid Diisooctyl phthalate 4,5-Dihydro-2-methylimidazole-4-one
21.64 11.10 9.56 8.67 5.77 5.06 4.94 4.68 3.67 3.54 2.87 2.55 2.37 2.34 2.31 1.65 1.34 1.03
4
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Fig. 3. Chemical group composition of (a) FA1 and (b) FB1.
Other ketones are mainly 2-pentanone and 3-methyl-3-buten-2-one. The main compounds and chemical group composition of FA1 distillation residual (FA3) are shown in Table 4 and Fig. 4(b), respectively. The most abundant chemical group in FA3 are esters, whose relative content reaches 49.9%. However, the esters contained in FA3 are not exactly the same with FA1. Meanwhile, phenols and furfural which are abundant in FA1 are neither detected in FA2 nor FA3. This may be attributed to esterification and polycondensation reactions of these two types of highly reactive substances under the distillation temperature. Esterification is the reaction for making esters from carboxylic acids and phenols or alcohols. The polycondensation reactions here could be phenolic aldehyde condensation which makes furfural resin from phenols and furfural. Similar results are also obtained by previous studies [9]. A large number of heterocyclic compounds were detected in FA3, which may also be caused by the chemical reactions occurring under distillation temperature. Besides, slight sintering phenomenon was observed during the distillation of FA1. The distillation temperature of FB1 was set to 150 °C, aiming to separate aromatic hydrocarbons whose boiling points are usually below 150 °C. The main compounds and chemical group composition of the 150 °C distillate (FB2) are shown in Table 5 and Fig. 4(c), respectively. It can be found that a large amount of aromatic hydrocarbons are enriched in FB2, whose relative content reaches 63.8%. The aromatic hydrocarbons may not only source from original HBO, but also from interactions among phenols, acids and ketones [34]. Meanwhile, there are also ketones and aldehydes in FB2, and their relative contents are 17.9% and 8.6%, respectively. Aldehydes are also commonly used as model compounds to investigate the upgrading characteristics of bio-oil [36]. The main compounds and chemical group composition of FB2
respectively. The chemical group compositions of FA1 and FB1 are shown in Fig. 3. It can be observed that the phenols in original HBO and the esters produced during the pH adjustment process are mainly enriched in FA1, while the aromatic hydrocarbons in original HBO are mainly enriched in FB1. As mentioned above, there is no significant difference between the distributions of aldehydes (which are primarily furfural) in FA1 and FB1. Besides, there are also ketones and oxygenated aromatic compounds with different species and contents in FA1 and FB1, respectively. Comparatively speaking, FB1 contains more aromatic compounds except phenols than FA1. This may because that the alkaline solution is beneficial for the extraction of smaller molecules with stronger polarity [22], while most of these aromatic compounds have relatively weak polarity and large molecular size. 3.3. Secondary distillation of extracted fractions Different chemical groups in FA1 and FB1 were further separated by distillation. FA1 contains large concentrations of phenols, aldehydes, ketones and esters. Among these contents, the boiling points of ketones are usually lower than 160 °C, while the other compounds commonly have higher boiling points [33]. Therefore, 160 °C was adopted as distillation temperature for FA1 separation. The main compounds and chemical group composition of 160 °C distillate (FA2) are shown in Table 4 and Fig. 4(a), respectively. The main chemical group in FA2 is ketones, accounting for 89.5% of all chemical groups. Ketones are important components in bio-oil, and there are comprehensive researches using ketones as model compounds to investigate upgrading methods especially hydrodeoxygenation of bio-oil [34,35]. Among various ketones, the relative content of 3-methyl-2-butanone reaches 47.64%. Table 4 Main compounds in FA2 and FA3. No.
1 2 3 4 5 6 7 8 9 10 11 12
FA2
FA3
Compound
Relative content (%)
Compound
Relative content (%)
3-Methyl-2-butanone 2-Pentanone 3-Methyl-3-buten-2-one 2,3,3-Trimethyl-hexane Butanoic acid methyl ester
47.64 27.20 14.66 5.30 2.46
3-Aminopyrazine 1-oxide Hexanoic acid methyl ester Hexadecanoic acid methyl ester Octanoic acid methyl ester 4-Methoxy-1-oxide-pyridine Methyl stearate Butanoic acid methyl ester Nonanedioic acid dimethyl ester Docosanoic acid methyl ester 2,2-Dimethoxy-propane Methyl 18-methylnonadecanoate 2,2-Dimethoxybutane
27.49 12.76 12.67 8.47 8.02 7.55 5.99 3.91 3.28 3.13 2.85 1.27
5
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Fig. 4. Chemical group composition of (a) FA2, (b) FA3, (c) FB2 and (d) FB3.
distillation of FB1.
distillation residual are shown in Table 5 and Fig. 4(d), respectively. A large number of phenols are enriched in FB3, and their relative content reaches 39.2%. Phenols are regarded as valuable chemicals which can be derived from bio-oil [37]. The phenols in bio-oil primarily come from thermal degradation of lignin in biomass [38]. Although a large amount of aromatic hydrocarbons are enriched in FB2, the relative content of aromatic hydrocarbons in FB3 is still as high as 23.5%. Similar to composition results of FA3, FB3 also contains plenty of compounds which have not been detected during previous separation process. They may be generated from chemical reactions occurring under the distillation temperature as well. While different with circumstance of distilling FA1, no obvious sintering phenomenon occurred during the
3.4. Overview of HBO composition change during multi-step separation In order to demonstrate the composition changes of HBO during the multi-step separation process, a scheme is shown in Fig. 5. It is worth noting that the data in Fig. 5 are based on relative content of each component, so the mass ratio of same substances in FA2, FA3, FB2 and FB3 cannot be reflected in this graph. According to Fig. 5, almost all carboxylic acids and phenols in HBO are enriched in FB3, while almost all aldehydes in HBO are enriched in FB2. The rich phenols content in FB3 make it rather potential to derive value added chemicals from it.
Table 5 Main compounds in FB2 and FB3. No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
FB2
FB3
Compound
Relative content (%)
Compound
Relative content (%)
Toluene o-Xylene Furfural Benzene 3-Methyl-2-butanone 1,3-Dimethyl-benzene Ethylbenzene 3-Methyl-3-buten-2-one 2-Pentanone 2,5-Dimethyl-furan 1,2,4-Trimethyl-benzene 1-Ethyl-4-methyl-benzene 1-Ethyl-2-methyl-benzene Cyclopentanone 1-Ethyl-2-methyl-benzene Heptane
24.10 11.71 8.57 8.38 6.82 6.27 4.74 4.58 4.56 3.20 2.80 2.18 2.04 1.93 1.53 1.07
1,2,3-Trimethoxybenzene 5-tert-Butylpyrogallol 4-Ethyl-2-methoxy-phenol Naphthalene 1-Methyl-naphthalene Methyl ester hexadecanoic acid 2-Methyl-naphthalene 4-Amino-2,5-dimethyl-phenol Methyl stearate
27.49 12.76 12.67 8.47 8.02 7.55 5.99 3.91 1.27
6
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Fig. 5. Overview of HBO composition change during multi-step separation. The yellow streams on the left indicate chemical group composition of HBO, and the width of the streams is proportional to the absolute content. Streams in other colors show the sources of chemical groups in the separated fractions, while the width of the streams is not proportional to the absolute content. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
HBO is divided into two fractions with fewer types of chemicals. After further distillation, these two fractions are separated into fractions FA2, FA3, FB2 and FB3, whose primary chemical groups are ketones (89.5%), esters (49.9%), aromatic hydrocarbons (63.8%) and phenols (39.2%), respectively. The homogeneity of these separated fractions is significantly enhanced, and thus more suitable for further upgrading and purification.
However, FB2 and FB3 also contain almost all the aromatic hydrocarbons in HBO, and the aromatic hydrocarbons have relatively high contents in FB2 and FB3. This caused more serious heterogeneity problem for their further purification. The ketones in HBO are partly distributed in FB2, and the others are distributed in FA2. The relative content of ketones in FA2 is as high as 89.5%, making it easier to further upgrade FA2. Although there are little esters existed in original HBO, a few esters are generated during pH adjustment and distillation process. All these esters are distributed in FA2 and FA3, among which the relative content of esters is only 2.5%, while the relative content of esters in FA3 reaches 49.9%. Esters are major components in biodiesel [39], and the high esters content in FA3 makes it rather suitable to be refined to a liquid fuel. Through the multi-step separation process proposed in this study, HBO is separated into fractions FA2, FA3, FB2 and FB3, whose primary chemical groups are ketones (89.5%), esters (49.9%), aromatic hydrocarbons (63.8%) and phenols (39.2%), respectively. On the one hand, the homogeneity of the four fractions is significantly enhanced, which can make the further upgrading process more efficient. In the field of bio-oil conversion and upgrading, many researches tend to optimize reaction parameters such as catalyst, temperature and pressure on specific chemical groups or model compounds. Therefore, the optimized parameters are usually especially effective on homogeneous feedstocks. On the other hand, the chemical groups in HBO are enriched in specific separated fractions, making it more efficient to purify valuable products from HBO. This is especially important when the further purification process uses expensive consumables to get products with high purity. According to reported publications for bio-oil upgrading and purification, FA2, FA3 and FB2 are promising to be upgraded efficiently, while FB3 is potential to be a qualified source for phenols purification.
CRediT authorship contribution statement Junyu Tao: Conceptualization, Writing - original draft, Methodology, Investigation, Visualization. Chen Li: Conceptualization, Investigation, Visualization. Jian Li: Validation, Investigation, Writing - review & editing. Beibei Yan: Resources, Writing - original draft, Writing - review & editing. Guanyi Chen: Funding acquisition, Resources, Writing - review & editing, Supervision. Zhanjun Cheng: Methodology, Writing - review & editing. Wanqing Li: Methodology, Validation. Fawei Lin: Validation, Investigation. Lian Hou: Funding acquisition, Supervision. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was financially supported by the National Key Research and Development Program of China (2016YFE0201800), the National Natural Science Foundation of China (51676138, 51878557), and the Tianjin Science and Technology Project (18YFJLCG00090).
4. Conclusions
Appendix A. Supplementary data
The HBO collected in this study primarily contains carboxylic acids, phenols, ketones, aldehydes and aromatic hydrocarbons. The multi-step separation method is effective to generate more homogenous fractions from HBO. Through primary pH adjustment and CH2Cl2 extraction,
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuproc.2020.106366. 7
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