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Separation of Bio-oil by Hydrophilic Surfactants Mingming Zhang, and Hongwei Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b04007 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018
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Energy & Fuels
Separation of Bio-oil by Hydrophilic Surfactants
Mingming Zhang, Hongwei Wu*
Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth WA 6845, Australia
* Corresponding Author. E-mail:
[email protected]; Tel: +61-8-92667592; Fax: +61-8-92662681
A manuscript submitted to Energy Fuels for consideration of publication
February 2018
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Abstract. This study reports the effectiveness of bio-oil separation using several hydrophilic surfactants, including IGEPAL CO520, Tween 80, IGEPAL CO890 and sodium oleate which have hydrophiliclipophilic balance (HLB) values of 10, 15, 17 and 18 respectively. The results show that hydrophobic groups of surfactants (rather than HLB value) and surfactant concentration play significant roles in bio-oil separation. Permanent separation can be achieved at high surfactant loading levels (e.g. 5%) whereas a low surfactant concentration (e.g. 0.5%) leads to temporary separation. At 5% addition, IGEPAL surfactants (CO520 and CO890) require shorter separation time than Tween 80 and sodium oleate. Except the case with sodium oleate, the separation results in two layers (a clear layer and a dark layer). Compared to the dark layer, the clear layer has a lower carbon content and higher oxygen content, leading to a lower aromaticity and a higher polarity. Water is more distributed in the clear layer, while aromatic compounds with fused rings are more concentrated in the dark layer. It is interesting to note that separation of bio-oil by sodium oleate results in an additional clear layer with distinct characteristics from other normal clear layers. This additional layer contains extractives in bio-oil and is at least composed of fatty acid esters.
Keywords: bio-oil; separation; hydrophilic surfactant; hydrophobic groups
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1. Introduction Bio-oil produced from biomass fast pyrolysis is considered to be a promising feedstock for producing transport fuel and/or chemicals.1 However, bio-oil has very complex chemical compositions that consist of acids, aldehydes, alcohols, and phenolic compounds etc.2 As a result, separation is usually a crucial step before value-added utilization of bio-oil.1,3 Table 1 summarizes some existing bio-oil separation methods (reported in the literature3-17). Broadly, bio-oil separation methods can be categorized based on differences in volatility (e.g. staged fractionation/condensation4 or vacuum distillation6) or polarity (e.g. water addition,11 salt solution addition,3,13 solvent extraction18 and column chromatography15). The separation methods based on differences in volatility generally require high operational temperature and bear the disadvantages induced by thermal instability of bio-oil. Those based on differences in polarity are of relative simplicity and have insignificant effect on the properties of bio-oil. Among these methods, water addition has been the widely used and extensively studied,12,17,19 focusing on the utilization of the resultant water-soluble and waterinsoluble fractions. The water-insoluble fraction contains mainly phenolic compounds and is widely recognized as feedstock to replace phenolic resins.17,19 However, the water-soluble fraction contains a large quantity of water (mainly due to the extra water addition into bio-oil for phase separation), which poses adverse effect on its use as a fuel because of its low energy content and tendency of causing catalyst deactivation during thermochemical processing (e.g. aqueous reforming/gasification).12 Surfactants with high hydrophilic-lipophilic balance (HLB) values, possessing hydrophilic nature and hydrocarbon groups, may be a substitution of water for bio-oil separation targeting value-added utilization of bio-oil fractions. This was implicitly shown in our recent studies on the important roles of surfactants in emulsification of fast pyrolysis bio-oil (or its water-soluble fraction) and glycerol (a biodiesel by-product) to produce emulsion fuels under high energy stirring.20,21 It appears to be possible to directly emulsify one fraction of bio-oil separated by hydrophilic surfactant with glycerol or other fuels to produce an emulsion fuel without extra surfactant addition or intensive energy input while other fraction(s) may find similar applications as water-insoluble fraction. Therefore, the objective of this study is to investigate the possibility Page 3 of 22 ACS Paragon Plus Environment
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of using hydrophilic surfactant for initial separation of bio-oil. Several hydrophilic surfactants with different HLB values and different hydrophobic groups are evaluated for performance and efficiency in bio-oil separation.
2. Experimental Section 2.1 Bio-oil separation. The bio-oil (produced from pine wood fast pyrolysis at 500 °C) was provided by a commercial supplier. Four surfactants were sourced from Sigma, including IGEPA CO520 (C25H44O6), Tween 80 (C64H124O26), IGEPAL CO890 (C95H184O41) and sodium oleate (C18H34O2Na, hereafter referred to as “soap”). Each surfactant was mixed with bio-oil in a plastic container at concentrations of 0.5, 2 and 5%, respectively. The mixture was firstly placed in an ultrasonic water bath for 30 minutes to ensure good mixing, then transferred into a centrifuge tube for repeated centrifugations at 4700 rpm at 2 minutes interval until phase separation is observed under torch light. The separated phases were then transferred into plastic containers by syringe and kept in fridge before characterization. In separated experiments, bio-oil without addition of surfactants was also subjected to the same process as a control group, but no phase separation was observed under the experiment conditions.
2.2 Sample Characterization. Elemental analyses of the samples were carried out using an elemental analyser (model: PerkinElmer CHN/O 2400 series II). Water content and acidity were determined using a Karl−Fisher titrator (model: Mettler V30) and acid-base titrator (model: MEP Oil Titrino plus 848), respectively. UV−fluorescence analysis was performed on a PerkinElmer LS55 spectrometer for understanding aromatic ring systems of samples. The analysis used a synchronous scan mode at constant energy difference of -2800 cm-1, following a previous method.22 At least four scans for each sample were conducted to minimize the noise. Gel permeation chromatography (GPC) analysis was conducted using a Varian 380 liquid chromatography (LC) system for obtaining molecule weight distribution according to a previous method.23 Briefly, a PLgel column and an UV detector (at wavelength of 280 nm) were equipped with the LC system, and tetrahydrofuran at a flow rate of 1.0 mL/min was used as the eluent. Fourier Page 4 of 22 ACS Paragon Plus Environment
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transform infrared (FTIR) spectroscopic analysis was carried out for probing the functional groups of samples using a PerkinElmer Spectrum100 FTIR spectrometer at scan wave numbers in the range of 4000－ 600 cm-1. Gas chromatography–mass spectrometry (GC/MS) analysis was performed for understanding the chemical composition of samples using an Agilent GC/MS equipped with a FAMEWAX column. The sample was diluted in methanol at a concentration of ~1% and injected of 1 µL with a split ratio of 1:20. The temperature programme used for the analysis was heating from 50 °C to 120 °C at 10 °C/min, holding at 120 °C for 2 mins before rising to 230 °C at a rate of 5 °C/min, and holding at 230 °C for another 10 mins.
3. Results and Discussion 3.1 Separation of bio-oil by several hydrophilic surfactants Figure 1 shows the time required for bio-oil separation by several surfactants and the distribution of each layer after separation. Experiments were carried out for surfactants including IGEPAL CO520 (C25H44O6), Tween 80 (C64H124O26), IGEPAL CO890 (C95H184O41) and soap (C18H34O2Na) with HLB values being 10, 15, 17 and 18, respectively. Except soap (sodium oleate), the other three surfactants are non-ionic with the same type of hydrophilic group (polyoxyethylene) but different hydrophobic groups (i.e. nonylphenyl for two IGEPAL surfactants CO520 and CO890, and glycol sorbitan monooleate for Tween 80, respectively). The results presented in Figure 1 show that surfactant type and concentration have significant influence on both separation time and layer distribution. In general, addition of surfactant aids the separation of bio-oil into two layers, i.e. a clear layer and a dark layer, and an increase in surfactant concentration decreases the time required for separation and the percentage of the clear layer. For IGEPAL surfactants, the reductions in separation times (both first appearance of clear layer and complete separation time) increase with an increase in concentration. However, in the cases of Tween 80 and soap, the concentration of surfactant affects more on the time for the appearance of clear or additional clear layer, whereas, the required time for achieving complete separation is independent of the surfactant concentration. For each concentration, the time required for achieving complete separation with Tween 80 is longest (~1 hour), being about 3 times of that with soap. At high surfactant concentrations (5% and 2%), Page 5 of 22 ACS Paragon Plus Environment
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CO890 requires the shortest time (4 and 10 minutes respectively) for separation, followed by CO520 (10 and 16 minutes respectively) and soap (16 and 20 minutes respectively). It should be pointed out that separation at a surfactant concentration of 0.5% can be easily reversed into a single phase within about one hour. Bio-oil separation with Tween 80 results in the highest proportion of the clear layer, ~80%, independent of the surfactant concentration (2 – 5%). Comparatively, the proportions of clear layers separated by IGEPAL surfactants decrease from ~70 to 37% for CO890, and from ~72 to 47% for CO520, respectively, when the surfactant concentration increases from 0.5% to 5%. It is noteworthy that separation with 5% or 2% soap leads to an interesting observation of an additional clear layer appearing on top. This layer accounts for ~5−7% of the bio-oil and requires separation time (2 and 10 minutes respectively for cases of 5% and 2% soap addition) similar to separation by CO890. The above observations suggest that hydrophobic groups likely play a more important role in bio-oil separation than the hydrophilic portions of surfactant do. The branched structure of Tween 80 appears to have posed steric hindrance in the separation process, resulting in the longest separation time. Additionally, the special molecule structure of Tween 80 has led to a special observation where dark layer situated on top of clear layer at a surfactant concentration of 5%. Although the exact reason is unknown, this may be attributed to the transformation into a special aggregate shape formed between Tween 80 and bio-oil components. It has been reported that transformation of aggregate shape is related to surfactant concentration.24 The appearance of the additional clear layer in separation by soap can be attributed to the hydrophobic group (oleate) of soap (see subsequent discussion in section 3.2).
3.2 Characterization of the clear and dark layers Table 2 lists the elemental compositions, water content and total acid number of each layer separated from the bio-oil by different surfactants. It can be seen that the clear layers have lower C content (36−42%) and higher O content (50−57%), whereas the content of C and O in the dark layers are in a range of 51−59% and 35−43% respectively. As a result, the clear layers have a lower molar ratio of H/C and higher molar ratio of O/C than the dark layers. H/C and O/C can be used to estimate aromaticity and polarity Page 6 of 22 ACS Paragon Plus Environment
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respectively.18,25,26 Therefore, the results show that the clear layers separated by surfactants have lower aromaticities and higher polarities than the dark layers. This implies that the polar compounds such as alcohols, aldehydes or ketones in the bio-oil are more likely distributed in the clear layers after separation by the surfactants. In addition, the water contents of the clear layers are in a range of 27−32%, which are also much higher than those of the dark layers (11−20%). This is in good agreement with the higher polarities of the clear layers. However, the total acid numbers of clear layers are only slightly higher than those of the dark layers, indicating a relatively even distribution of acidic compounds during bio-oil separation by surfactant. UV−fluorescence spectra normalised to unit sample mass in Figure 2 (a−f) show that both clear and dark layers have two peaks. The dark layers have much higher peak intensities at 290–400 nm (representing the abundance of two or more condensed aromatic ring structures22,27) compared to the clear layers. This is also in consistency with the aforementioned lower aromaticities of the clear layers. However, the peak intensities at 270－290 nm (representing the abundance of single aromatic ring structures22,27) are similar for all the samples including the bio-oil. This indicates that the aromatic compounds with a single ring structure in bio-oil have distributed evenly in the two layers, and the fused aromatic compounds are more concentrated in the dark layer. Consequently, as shown by the molecule weight distribution curves in Figure 3 (a−f) from GPC analyses, the proportions of heavy organic compounds (with molecule weight in the range of 400 to 3000 g/mol) in the dark layers are significantly higher than those in the clear layers. A close comparison in the characteristics of the clear layers separated by different surfactants shows that at a low surfactant concentration of 0.5% there is little difference among the clear layers. However, as the surfactant concentration increases, the differences become more significant. For example, at 5% or 2% surfactant addition, the clear layers separated by IGEPAL surfactants (CO520 and CO890) have lower C contents, higher O contents, higher water contents, slightly higher acidities, higher polarities and lower aromaticities than those separated by the other two surfactants.
3.3 Characterization of the additional clear layer separated by soap Page 7 of 22 ACS Paragon Plus Environment
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As seen from Table 2, the additional clear layer separated by soap has distinctly different characteristics, i.e. significantly high C content (74%) and low O content (14%), considerably low water content (
