Liquefaction of Lignocellulose - American Chemical Society

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Liquefaction of Lignocellulose: Process Parameter Study To Minimize Heavy Ends Shushil Kumar,† Jean-Paul Lange,*,†,‡ Guus Van Rossum,†,‡ and Sascha R. A. Kersten† †

Sustainable Process Technology, Faculty of Science and Technology, University of Twente, Drienerlolaan 5, 7522 NB, Enschede, The Netherlands ‡ Shell Global Solutions International B.V., Shell Technology Centre Amsterdam, Grasweg 31, 1031 HW, Amsterdam, The Netherlands S Supporting Information *

ABSTRACT: Lignocellulosic feedstock can be converted to bio-oil by direct liquefaction in a phenolic solvent such as guaiacol with an oil yield of >90 C% at 300−350 °C without the assistance of catalyst or reactive atmosphere. Despite good initial performance, the liquefaction was rapidly hindered by the formation of heavy components (molecular weight > 1000 Da), which increased the viscosity of the bio-oil upon recycling the bio-oil or a fraction of it as a liquefaction solvent. This paper explores the possibility to minimize the production of this undesirably heavy fraction by optimizing the process parameters such as temperature, heating rate, reaction time, and concentration of water. This study allowed us to find a compromise between maximizing the bio-oil yield and minimizing its heavy fraction. It also provides insight onto the reaction network of the liquefaction reaction, showing for instance that all product fractions, including the heaviest products and the char, are mainly direct liquefaction products rather than secondary reaction products, e.g. from bio-oil recondensation. However, the resulting heavy fraction is still too high to allow effective recycling of the bio-oil. Complementary approaches need to be investigated.

1. INTRODUCTION A variation of the pyrolysis process for the production of bio-oil is the direct thermal liquefaction of lignocellulosic biomass under liquid phase conditions.1,2 Bio-oil, produced by direct thermal liquefaction of lignocellulosic biomass, was identified as a potential biobased alternative to crude oil for the production of transportation fuels during the first oil crisis in the early 1970s. Work on high-pressure liquefaction of biomass began in the early 1970s at the Pittsburgh Energy Research Centre (PERC).3,4 In 1975, a pilot plant was constructed based on the PERC work which was known as the PERC process.5 However, the pilot unit was stopped after 1981 due to serious technical problems caused by solids and an increase in medium viscosity.2 After PERC, further research was carried out at many laboratories,6−8 but the high processing costs and the lack of basic scientific understanding of the process led to the failure of these projects as well. Another approach, which was explored by Shell and further developed by Biofuel B.V., was the Hydro-Thermal Upgrading (HTU) process9,10 which uses water as the liquefaction medium. The economics of the process suffered from extreme process conditions (pressures 10−18 MPa at 300−350 °C), low oil quality (heavy organic liquid with 10−15 wt % oxygen and a heating value of 30−35 MJ/kg), and light organics (yield ∼12 wt %) dissolved in the aqueous phase formed. More recently, van Rossum et al.11 showed the possibility to liquefy wood with high bio-oil yields (>90 C%) using a phenolic solvent, guaiacol. In their work, it was found that guaiacol performs better than water and other organic solvents. Their studies were mainly focused on liquid yields, which depended on wood loading, temperature and reaction time. They also proposed to use the bio-oil as liquefaction medium by recycling it to the liquefaction reactor. The recycling of the bio-oil initially © 2014 American Chemical Society

succeeded in achieving high oil yield but readily lost its effectiveness as the liquid medium became very viscous because of accumulation of heavy product. The idea of recycling the biooil is not new and was also applied in the older PERC process in which anthracene oil was used as a starting solvent that had to be gradually replaced by the liquefaction product. 2,7 The information on literature data on biomass direct liquefaction is provided by van Rossum et al.11 The purpose of the present work is, therefore, to revisit the work of van Rossum et al.11 and investigate the possibility to minimize the formation of heavy components by optimizing the operating conditions. Particular attention will be given to optimize the reaction temperature, the heating rate, the reaction time and the water concentration. Investigation of reaction is also expected to provide more insight on the eventual role of secondary reactions in the liquefaction process and the formation of heavy components.

2. MATERIALS AND METHODS 2.1. Materials. Pine wood was obtained from Rettenmaier Söhne GmbH (Germany). It was crushed to a particle size of 1000 Da. The vacuum residue fraction was defined as the fraction of the bio-oil (excluding solvent) that is found in the heavy product (MW,GPC > 1000 Da) based on eq 4. It assumes comparable response factor for the light and heavy bio-oil components. vacuum residue fraction area corresponds to MW,GPC > 1000 Da = area correspond to MW,GPC > 180 Da

(4)

The molar mass cutoff of the bio-oil was set at 180 Da to exclude the solvent (guaiacol, MW = 124 Da) and its eventual primary degradation products (e.g., 1,2-dimethoxybenzene). Selectivity of gas, liquid (bio-oil), VR, and distillates were defined as eqs 5−8. selectivitygas =

yield gas conversion

selectivityliquid = Figure 2. Overall procedure for products (solid, liquid, and gas) collection after liquefaction experiments. In the brackets is shown the analytical equipment used for analysis of the product.

yield gas(C%) =

Macetone_insoluble M wood_intake(dry)

Mgas_formed M wood_intake(dry)

yield liquid conversion

(6)

selectivityVR = selectivityliquid × VR fraction

(7)

selectivitydistillates = selectivityliquid − selectivityVR

(8)

It should be mentioned that the apparent molecular weight of the bio-oil corresponds to that of refinery distillate + vacuum gasoil while that of vacuum residue corresponds to that of refinery vacuum residue.11 Refinery terminology is used here to facilitate the translation into the refinery operation to be selected for upgrading the bio-oil into final biofuel. The yield calculation using eqs 1−3 could suffer from inaccuracies due the reaction of guaiacol. However, blank experiment (with guaiacol and water only) showed that this contribution is very marginal as no solid formation and a very small amount of gas formation were observed. Hence it can be safely neglected. Guaiacol was partly converted to bio-oil without measurable VR formation. Nevertheless it will not affect the liquid yield as the yield was calculated by difference.

% and, more than often, even exceeded 95 wt %. Gas, liquid, and solid yields were calculated as carbon-fraction of the wood intake (eq 1−3; excluding guaiacol) rather than more common weight fraction to avoid counting the oxygen content or water as valuable product for subsequent conversion to biofuel. Gas yield was calculated using composition analyzed by the off-line GC, available gas volume and, end pressure, and temperature after cooling, using the ideal gas law and defining the available gas volume as being the total volume of the reactor minus the volume of the liquid product. The solid yield was determined based on the weight fraction of solid residue and its carbon content. The liquid yield was obtained by difference for convenience. yield solid(C%) =

(5)

3. RESULTS Results of the study of process parameters are presented and discussed in detail here. Temperature and reaction time are generally the most important factors in chemical processes, and the direct liquefaction process is no exception here. Effect of temperature was studied first considering its large effect on conversion of wood as reported in the literature.11,13,14 The second parameter of the study was reaction time as it can influence the primary and secondary reactions. Higher reaction time may provide ample time for secondary reactions such as polymerization, leading to the formation of high molecular weight compounds. A shorter reaction time may help reducing the amount of heavy compounds. The third parameter studied was: water concentration in the feed. Beyond being a cosolvent for the most polar products, water can also help in hydrolyzing the glucosidic bonds which are present in the wood and, hence, may accelerate the conversion of wood. The last parameter was the heating rate. A high heating rate will favor reactions with high activation energy over those with a lower one, as reported in the case of pyrolysis where more char was produced at low heating rate.15,16 However, heating rate studied in this work is much

× 100 (1)

× 100 (2)

yield liquid(C%) = 100 − yield solid(C%) − yield gas(C%) (3)

Where M stands for total mass of the carbon present. It should be noted that by doing so all the losses are attributed to the liquid and hence the liquid yield may be over-reported. However, the validity of the definition of liquid yield (eq 3) was checked and confirmed by recalculating liquid yield of selected samples using their GPC analysis and a calibration line developed with a biocrude sample that was freed from the guaiacol solvent by means of vacuum distillation (see the Supporting Information). The liquid product was further divided into two fractions, based on apparent molecular weight (as determined by GPC), namely solvent/guaiacol with MW,GPC < 180 Da and bio-oil with MW,GPC > 180 Da, from which the distillates is defined as MW,GPC 11670

dx.doi.org/10.1021/ie501579v | Ind. Eng. Chem. Res. 2014, 53, 11668−11676

Industrial & Engineering Chemistry Research

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lower compare to the pyrolysis. This is an in depth study and an extension of the work of van Rossum et al.11 with a focus on reducing the heavies fraction. Effect of particle size and solvent− wood ratio were not investigated at this stage, but they were both chosen as low end of the practical range ( 1000 Da) and leads to a shift of the GPC curve toward lower molecular weight. However, end points of the GPC curves are not affected by temperature. Also shown in Figure 3, vacuum residue fraction decreases with the temperature increasing from 250 to 400 °C. 11671



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Accurate monitoring of the reaction pressure and temperature revealed a highly endothermic reaction, which is likely caused by the dehydration of the biomass into unsaturated compounds. A calculation based on the higher heating value (HHV) of products and feed shows that liquefaction is endothermic and the heat of liquefaction reaction is around 3 MJ/kg of dry pine wood (see the Supporting Information). This is discussed in detail in the Supporting Information. 3.3. Effect of Water. Water plays an important role in the decomposition of biomass,11,19 possibly due to its easy penetration into the solid wood particle and its assistance (also as reactant) in the hydrolysis of the glucosidic bonds. In Elliott’s work,19 water was reported to be beneficial mainly due to presence of alkali catalyst. Such explanation cannot apply here since no alkali is used. As shown in Table 1, the addition of 10 wt % water reduces the solid yield and enhances the liquid yield at short reaction time (≤700 s). The gas yield is not affected by water. Increasing water concentration from 10 to 20 wt % does not have any significant impact on product yields. Moreover, the addition of 20 wt % water does not affect the product yields significantly for the long reaction time (3h or 10800 s). The vacuum residue fraction is mainly influenced by the wood conversion, i.e. by the combination of time and water concentration. Hence, the water addition seems to influence the product yield and oil heaviness only indirectly by affecting the rate and extent of liquefaction. Hence, water functions mainly as catalyst here. However, water addition has its downside, namely a significant increase in operating pressure (e.g., from 5 MPa without water to 9.2 MPa with 10 wt % water for reaction time of 700 s). The water is acting as a catalyst which accelerates the wood conversion and therefore shortens the residence time. From a process point of view, this is beneficial as shorter residence time means a smaller reactor. Also complete drying of biomass will not be required, saving extra cost of putting a drying equipment. However, penalty of using wet biomass will be higher reactor pressure coming from the water, that will require expensive reactor as well as feeding the biomass in a high pressure reactor. Nevertheless a part of the gas and water vapor produced during reaction can be vented off and hence a low pressure operation can be achieved. 3.4. Effect of Heating Rate. High heating rates have been reported to benefit the bulk fragmentation of biomass and inhibit char formation in the pyrolysis.20 In this section, the heating rate is investigated to check whether it helps in reducing the vacuum residue. Experiments were carried out in the 9, 45, and 560 mL autoclave. As shown in Table 2, heating rate has no significant effect on solid, liquid and gas yields. GPC analysis of the liquids revealed no significant change in the apparent molecular weight distribution and vacuum residue fraction (Table 2). This shows ineffectiveness of the heating rate in improving the liquid quality. Heating rate studied here is much lower compare to pyrolysis, instead it produces much less solid. It should be noted that from a process point of view, heating rates studied here will be representative of the actual process if biomass is fed as a liquid slurry. And if the biomass is fed as dry solids into hot liquid solvent, a much higher heating rate can be achieved (∼300 °C/s). However, such a high heating rate is redundant as the feed has to stay in the reactor for 100−200 s in order to obtain complete conversion of the wood at 320 °C (Figure 5). It should be recognized here that the effect of heating rate may have been partly contaminated by other effects. For instance, the

Figure 5. Products distribution and vacuum residue (VR) fraction obtained after liquefaction of pine wood at different reaction times. Autoclave 9 mL, T 320 °C, feed (wt %); guaiacol:water: wood = 60:20:20, heating rate 300 °C/min, mass closure 92.8, 73.9, 95.8, 89.9, 93.3, 88, 94.8, and 97.3 with time.

This shows that liquid obtained is quite stable and not converted to gas and solid to a significant extent at longer reaction time and at the studied temperature. It should also be mentioned that the low solid yield achieved beyond 300 s did not allow to perform elemental analysis of the solid. The solid yields were therefore determined by assuming the carbon content measured for the solid obtained at 350 °C (Table 3). The gas phase consists mainly of CO2, CO, and H2. Methane becomes significant at longer reaction time of 3 h (see the SI). The bio-oils show a decreasing fraction in vacuum residue during the initial 300 s (Figure 5). The GPC profiles (Figure 6)

Figure 6. Apparent molecular weight distribution of the liquid products obtained after liquefaction of pine wood at different reaction times. Autoclave 9 mL, T 320 °C, heating rate 300 °C/min, feed (wt %); guaiacol:water:wood = 60:20:20.

show indeed appearance of very heavy components (MW,GPC > 10 kDa) in the earliest stage of the reaction. As the time proceed, the amount of very heavy product decreases and end points shifts from 20 to 10 kDa until stabilization beyond 300 s. Hence, the vacuum residue seems to be a primary reaction product (within the time scale of the experiment) that undergoes further depolymerization with time. Confirmation of this hypothesis could be sought in eventual changes in selectivity of the various products as a function of the conversion of the wood and it is discussed in detail later in the Discussion section. 11672



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Table 1. Product Yields of Liquefaction Experiments with Different Water Concentrationsa water

τ

wt %

s

liquid

solidb

gas

wt %

0 0 10 20 20 0

400 700 700 500 10800 10800

73.5 (70.4) 85.7 (85.7) 94.0 (91.2) 93.9 (91.6) 91.9 (88.4) 91.4 (88.1)

22.4 (23.3) 8.2 (5.6) 0.2 (0.2) 0.7 (0.5) 0.5 (0.3) 1.2 (0.8)

4.1 (6.3) 6.1 (8.7) 5.8c (8.6) 5.4 (7.9) 7.6 (11.3) 7.4d (11.1)

101 99.6 90.9 93.9 97.3 95.0

yield, C% (wt %)

mass balance

VR fraction

max P MPa

0.39 0.34 0.30 0.32 0.27 0.25

3.2 5.0 9.2 9.3 6.8 5.4

Autoclave 9 mL, T 320 °C, feed (wt %); guaiacol:water:wood = (80 − X):X:20, τ reaction time, heating rate 300 °C/min, VR vacuum residue. Solid yield is calculated using elemental analysis of char obtained at 350 °C except 0 wt % and 400 s. cAssuming final gas pressure and composition are the same as in case of 20 wt % water. dBased on gas composition in the case of 20 wt % water. a b

Table 2. Product Yields of Liquefaction Experiments with Different Heating Ratesa autoclave 9 mL 45 mL 560 mL a

τ

heating rate

min

°C/min

liquid

solid

8.4 8.4 130

300 75 2.7

93.9 (91.6) 92.1 (89.8) 93.1 (90.1)

0.7 (0.5) 2.1 (1.5) 1.0 (0.7)

yield C%, (wt %) b

mass balance gas

wt %

5.4 (7.9) 5.8 (8.8) 5.9 (9.2)

93.3 97.7 98.5

VR fraction 0.32 0.28 0.29

T 320 °C, τ reaction time, VR vacuum residue. bSolid yield is calculated using elemental analysis of char obtained at 350 °C.

band at 1250−1000 cm−1. This is possibly due to the removal/ depolymerization of hemicellulose that frees cellulose and lignin. As the temperature increases, the carbohydrate bands decrease, the aromatic bands first increase and eventually decreases and the carbonyl bands increase. The broad residual band that is observed around 1200 cm−1 at >350 °C cannot be clearly assigned at this stage. It may however be indicative of oxygenated aromatic structures based e.g. on fused phenolic or furanic rings. Hence, the solid residue obtained upon increasing the liquefaction temperature appears to change from wood-type (polysaccharide and lignin at 250 °C) to lignin-type (at 350 °C) to oxygenated char (at 400 °C). These phases are consistent with the steady increase in C-content and decrease in O-content determined by elemental analysis of the solid (Table 3). The solid residue obtained after various liquefaction time showed no pronounced transformation in FTIR (Figure 7b). The absorbance bands at 3330, 2900, and 1000 cm−1 are present in all of the residue. The strength of absorption bands ascribed to aromatic skeletal vibration (1592 and 1506 cm−1) slightly enhanced in the residue obtained at 3 h, suggesting an increase of lignin fraction in the residue. However, the CO band at 1735 cm−1 reported above is not observed at 320 °C. This may indicate that dehydration to carbonyl is not favored at 320 °C but favored at 350 °C or higher temperatures. As shown in Table 3, the elemental composition of the solid at 250 °C is similar to the elemental composition of the wood while the elemental composition of solid at 350 and 370 °C are similar to the typical elemental composition of lignin.25 Further increasing the temperature to 400 °C resulted in the solid, having an elemental composition similar to char.25

feed to the 9 and 45 mL reactor consisted of 20 wt % wood, 20 wt % water and 60 wt % guaiacol while the feed composition for the 560 mL reactor was 10, 20 and 70 wt %. Moreover, the 9 mL autoclave was shaken while the other two were stirred, the 9 and 45 mL autoclave were operated at 320 °C and quenched with water while the 560 mL autoclave was operated at 300 °C and cooled in air. Reaction time used in the 560 mL reactor was 130 min, significantly higher than reaction time used in other two reactors (8.4 min). These variations cannot be ignored here. However, the limited effect of time, temperature and water amount reported above suggest that these influences should have been marginal here. 3.5. Solid Characterization. FTIR analysis was performed to determine the structure of the solid residue obtained after the liquefaction. Figure 7a and b shows the FTIR spectra of residue derived from the liquefaction experiments at different temperatures and at different reaction times, respectively. Because of analytical artifacts such as sample packing, absolute differences in band intensities between different spectra should be ignored. However, relative difference between bands heights can be taken as indication for relative differences in concentration of the corresponding functional group between the samples. The band around 3330 cm−1 is due to the −OH groups either from wood carbohydrates or lignin.21,22 The band at 2900 cm−1 corresponds to C−H stretch in methyl and methylene groups.23 Absorption bands at 1710−1740 cm−1 are mainly due to the carbonyl (C O) stretch in unconjugated ester, ketone or carboxylic groups in carbohydrates and not from lignin.22 This band is possibly coming from dehydration of sugars to carbonyls such as furfural or HMF like products. However, it can also indicate the presence of carboxylic acids. The absorption bands at 1590 and 1505 cm−1 represent two aromatic skeletal vibrations e.g. from lignin.22,24 The band at 1030 cm−1 is attributed to the aliphatic C−O stretch in alcohol and/or ethers, such as in sugars. As shown in Figure 7a, wood structure is dominated by carbohydrates (C−O at 1030 cm−1, C−H at 2900 cm−1 and O− H at 3330 cm−1) and some aromatic structure of lignin (CC at 1590 and 1505 cm−1). The solid obtained at low liquefaction temperature of 250 °C shows fine structure in the broad C−O

4. DISCUSSION Different process conditions were investigated for the liquefaction of lignocellulosic biomass in order to reduce vacuum residue in the liquid. Formation of vacuum residue is undesired as it results in a highly viscous liquid product and was one of the major cause of the failure of earlier liquefaction processes.2 The time profile discussed above also shed some lights onto the reaction network of the liquefaction process. This seen most clearly after recalculating the yield data into selectivity, and 11673



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Figure 8. Selectivity versus wood conversion obtained in the liquefaction of pine wood at different reaction times. Autoclave 9 mL, T 320 °C, feed (wt %); guaiacol:water:wood = 60:20:20 (open symbols), 80:0:20 (solid symbols). Trend lines are only fitted data with water. Arrow shows the trend of distillates and VR after complete conversion of wood.

(58%). However, the selectivity profiles also reveal that the VR undergoes secondary degradation while some additional distillate is formed via secondary reactions. Interestingly, the selectivity profile is not affected by the water concentration as the few data at zero water concentration fall on the same selectivity line as those with 20% water. Further inspection of the GPC traces reveals that the decrease in vacuum residue fraction and its selectivity (Figures 5 and 8) is accompanied by an increase in its “aromaticity” for the ratio between the RI and UV signal of the vacuum residue is steadily decreasing with time (Figure 9). It should be recalled here that

Figure 7. FTIR spectra of residue constituent after liquefaction at (a) different temperatures, reaction time 20 min, and (b) different reaction times, T 320 °C. In the brackets, the wavelength (cm−1) of the corresponding peak is shown. Autoclave 9 mL, heating rate 300 °C/min, feed (wt %); guaiacol:water:wood = 60:20:20.

Table 3. Elemental Composition (wt %) of Solids Obtained at Different Reaction Temperaturesa temperature

C

°C wood 250 350 370 400 lignin25

H

Ob

wt % 46.6 49.0 67.7 66.7 77.0 67.6

6.3 6.2 4.1 4.7 4.4 5.8

H/Ceffc at/at

47.1 44.8 28.2 28.6 18.6 26.6

0.11 0.15 0.10 0.20 0.32 0.44

Figure 9. RI/UV signal for the molar mass of 2000 Da in the liquid obtained after liquefaction of pine wood at different reaction times. T 320 °C, feed (wt %); guaiacol:water:wood = 60:20:20, heating rate 300 °C/min (9 mL); 180 °C/min (45 mL).

a

Autoclave 9 mL; reaction time 20 min, feed (wt %); guaiacol:water:wood = 60:20:20, heating rate 300°C/min. bBy difference. cH/Ceff = (H − 2O)/C in atomic ratio.

the RI response of heavy molecules is not very sensitive to their molecular structure whereas their UV response increases very significantly with their degree of unsaturation/aromaticity and the degree of conjugation of these unsaturations. These observations allows the proposition of the following reaction network for the liquefaction process at 300−350 °C (Figure 10). Accordingly, the lignocellulose would undergo a primary and random depolymerization or cracking of its main

redrawing them as a function of the “wood conversion” (Figure 8). Accordingly, all product fraction behaves like a primary product as their selectivity extrapolates a significant positive values at zero wood conversion, namely 3% for the gas and 97% for the liquid, which can be split into distillate (39%) and VR 11674



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liquefaction of the fresh load of wood as a result of deteriorating solvent properties e.g. due to a steady and unfavorable dilution of light components by heavy bio-oil.

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CONCLUSIONS The present optimization of process conditions achieved the best compromise between maximizing the bio-oil yield and minimizing the fraction of vacuum residue under the following conditions: (1) Temperature in the range of 320−350 °C. (2) Reaction time higher than 200 s (excluding heating time). (3) Addition of up to 10 wt % water. This study further revealed that the formation of undesired vacuum residue is not due to secondary condensation of the biooil but rather to incomplete primary depolymerization of the biomass with simultaneous dehydration of fragments. Water acts as a catalyst and accelerates liquefaction reaction but it does not seem to affect the final product yields. Regrettably, the resulting fraction of vacuum residue (∼30%) is still too high to allow effective recycling of the bio-oil as liquefaction medium. Alternative approaches are needed such as applying catalysts and/or removing or cracking the vacuum residue prior to recycling.

Figure 10. Reaction scheme of conversion of lignocellulose in various products.

constituents, namely cellulose, hemicellulose and lignin. As reported in the literature and confirmed by the FTIR spectra of the solid (Figure 7), the cellulose is expectedly the slowest constituent to depolymerize. This initial depolymerization is proposed to proceed randomly to produce a large range of molecular weight, varying from gas to vacuum residues. However, the primary oligosaccharides, which could not be detected within the experimental timescale, would be unstable and would directly undergo secondary dehydration and degradation to form the colored unsaturated derivatives observed here. These secondary degradation reactions would compete with further depolymerization, rapidly leading to a stable product state. This is supported by the decrease in VR selectivity and the simultaneous increase in its unsaturation (RI/ UV ratio). A similar competition between depolymerization and degradation likely takes place for the distillates (which leads to lights or distillate oil, respectively) as well as the solid residue (which leads to VR or “primary” char, respectively). However, the variation in reaction temperature (Figure 3) provides some additional information on this network. Indeed, the steep increase in solid residue above 350 °C cannot be explained by simple primary depolymerization. It rather indicates the presence of secondary recondensation and charring reactions at elevated temperature, possibly, from the VR, as suggested by the simultaneous steep decrease in VR fraction (Figure 3). Such repolymerization reactions were indeed confirmed by a liquefaction experiment which was carried out at 325 °C for 20 min and after that reactor temperature was raised to 370 °C and liquefaction was carried out for 20 more minutes, which produced 19 C% of solid. After all, no operating parameter appears to reduce the formation of vacuum residue to a sufficiently low level. Raising the liquefaction temperature beyond 350 °C is the most effective but also leads to severe drop in bio-oil yield. The vacuum residue appears to be a primary depolymerization product unlike the case of pyrolysis where products reported to be formed via primary as well as secondary reactions.20,26,27 The observation that the VR fraction and selectivity stabilizes with time at high conversion (Figures 5 and 8) suggests that the increased vacuum residue reported by van Rossum et al.11 for their refill experiments is not due to repolymerization of the bio-oil but rather to poorer

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ASSOCIATED CONTENT

* Supporting Information S

Wood characterization in Table S1. Liquid yield calculated using GPC versus the liquid yield calculated 100-gas−solid yield (C%) in the liquefaction experiments at different temperatures in Figure S1. GPC graphs of the liquid product obtained with different heating rates (Figure S2), with different water concentrations (Figure S3). RI/UV signal ratio of the liquids obtained after liquefaction of pine wood at different reaction times (Figure S4). Product yields at different temperatures and at different reaction times in the 9 mL autoclave (Table S2), at different reaction times in the 45 mL autoclave (Table S3), yields of produced gases at different temperatures (Figure S5), and at different reaction times (Figure S6). Temperature and pressure profiles (Figures S7−S8). This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Shell Global Solutions International B.V. for funding this research and Benno Knaken for the technical support.

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NOMENCLATURE τ = reaction time Da = Dalton GPC = gel permeable chromatography Mi = mass carbon of component i MW,GPC = molecular weight defined by GPC RID = refractive index detector VR = vacuum residue UV = ultraviolet dx.doi.org/10.1021/ie501579v | Ind. Eng. Chem. Res. 2014, 53, 11668−11676


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Liquefaction of Lignocellulose - American Chemical Society

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