Water Prehydrolysis of Birch Wood Chips and Meal ... - ACS Publications

Article pubs.acs.org/IECR

Water Prehydrolysis of Birch Wood Chips and Meal in Batch and Flow-through Systems: A Comparative Evaluation Marc Borrega* and Herbert Sixta Department of Forest Products Technology, Aalto University, PO Box 16300, FI-00076 Aalto, Finland S Supporting Information *

ABSTRACT: Water prehydrolysis can be used as a pretreatment to extract hemicelluloses and lignin from biomass prior to its conversion into value-added products. In this study, the effects of operational conditions such as reactor system, flow, particle size, and solids content during prehydrolysis of birch wood are compared, using the wood yield as indicator of pretreatment intensity. The results show that both batch and flow-through (FT) systems are equally effective in removing the carbohydrates from the wood. Increasing flow and decreasing particle size and solids content, however, facilitate the removal of lignin. This increased delignification is partly related to a lower extent of condensation reactions. A FT system is also advantageous for the recovery of the extracted sugars because degradation reactions are minimized. Furthermore, by applying elevated temperatures and short retention times, the sugars concentration in the hydrolysate might be only somewhat higher than that in a batch system.

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INTRODUCTION In recent years, many research efforts have been directed toward developing pretreatment technologies for the fractionation and conversion of lignocellulosic materials into valueadded products. Among the various pretreatments, water prehydrolysis, also commonly referred to as autohydrolysis, hydrothermolysis, hot water treatment, or pressurized liquid water extraction, has received a great deal of attention mainly because no chemicals are required, as pure liquid water is the only solvent used. Prehydrolysis can be applied to extract the hemicelluloses and some of the lignin prior to wood pulping for the production of paper- and dissolving-grade pulps,1−3 or alternatively, prior to enzymatic hydrolysis of cellulose in herbaceous feedstock for the production of biofuels.4,5 During water prehydrolysis of wood, hydronium ions from water autoionization catalyze the cleavage of the acetyl groups bound to the hemicelluloses, with the consequent formation of acetic acid. Under these acidic conditions, the hemicelluloses are easily depolymerized by cleavage of the glycosidic bonds, while the lignin is partly removed by cleavage of the aryl-ether bonds. The chemical structure of the lignin remaining in the wood residue is also altered as a result of competing fragmentation and condensation reactions.6,7 The degree of polymerization in the cellulose chains is somewhat reduced, but the cellulose content remains mostly unchanged unless severe prehydrolysis conditions are applied.8−10 The formation of organic acids, other than acetic acid, through degradation of the carbohydrates further contributes to the overall degradation of wood components.11 Under mild prehydrolysis conditions, high-molar-mass oligosaccharides derived from the hemicelluloses are found in the hydrolysate, while increasing prehydrolysis intensity favors the recovery of low-molar-mass oligosaccharides, monosaccharides, and sugar dehydration products like furfural and 5hydroxymethylfurfural (HMF).12−14 The possibility of recovering valuable sugars, furans, and organic acids (mainly acetic © 2015 American Chemical Society

acid) from the hydrolysates is one of the most interesting features of water prehydrolysis of wood. In addition to sugarderived products, a variety of low-molar-mass lignin compounds are also found in the hydrolysates.11,15 During cooling, some of these lignin compounds may condense into highmolar-mass fragments and form sticky precipitates, particularly when intense prehydrolysis conditions are applied.16 The formation of lignin precipitates is undesirable because it may cause operational problems. The removal of precipitates, however, can be accomplished by adsorption technologies and by flocculation with lignin-specific adsorption polymers.17,18 In general, hardwood species are more suitable to water prehydrolysis than softwoods, mostly because of the presence of syringyl lignin units, with lesser tendency for condensation reactions.19 In a batch system, increasing the solids content during prehydrolysis tends to hinder delignification but has little effect on the removal of carbohydrates.20 On the other hand, decreasing wood particle size enhances the extraction of wood components, mainly the hemicelluloses.21,22 The extraction of hemicelluloses also appears to be enhanced in flow-through (FT) systems compared to batch systems.23 In a FT system, the total yield of sugars in the hydrolysate, and particularly the yield of oligosaccharides, is higher than in a batch reactor because the dissolved sugars are continuously removed from the reaction chamber before they can degrade further.23,24 Higher amounts of dissolved lignin can also be found in hydrolysates from FT experiments, probably because of increased water consumption in this type of reactor and thus lower product concentration, which leads to improved lignin solubility.25 Received: Revised: Accepted: Published: 6075

March 9, 2015 May 15, 2015 May 21, 2015 May 21, 2015 DOI: 10.1021/acs.iecr.5b00908 Ind. Eng. Chem. Res. 2015, 54, 6075−6084

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Industrial & Engineering Chemistry Research Despite a large number of studies on water prehydrolysis of lignocellulosic biomass, a direct comparison of prehydrolysis efficiency in batch and FT systems is rather scarce in the literature. Moreover, results from different research laboratories are often difficult to compare because, in addition to different experimental equipment, prehydrolysis trials are conducted using different raw materials and process conditions such as temperature, reaction time, wood particle size, or solids content (liquid-to-wood ratio). Therefore, the objective of this study is to provide a comparative evaluation of the effects of birch wood prehydrolysis in batch and FT systems under a wide range of operational conditions. Previously, we have reported the effects of water prehydrolysis on the delignification and on the degradation of carbohydrates from birch wood meal treated in a batch reactor.10,26 We have also reported the effects of prehydrolysis intensity on the conversion of birch wood chips to dissolving-grade pulps.3,27 Here, we have conducted water prehydrolysis of birch wood meal in a FT system, and the results are compared to those obtained in our previous work in which batch reactors were used. The composition of the wood residue and the hydrolysate is investigated as a function of wood yield, and the efficiency of reactor system, particle size, and solids content is discussed. So far, the wood yield as indicator of prehydrolysis intensity has been used only to compare water prehydrolysis and acid-catalyzed treatments in a batch reactor.28 The results of this study are expected to provide a deeper understanding on the fractionation of hardwoods by prehydrolysis under different process conditions and may help designing future experiments based on the composition of solid and liquid fractions at a given prehydrolysis intensity or wood yield.

Table 1. Operational Parameters for Prehydrolysis Experiments of Birch Wood wood material birch chips

birch meal

birch meal

reactor system batch (10 L)

batch (300 mL)

flowthrough (190 mL)

temperature (°C)

reaction timea (min)

180

0−120

−

3

200 220 180

0−60 0−25 0−180

− − −

3 3 40

200 220 240 200

0−180 0−60 0−30 5−60

− − − 50

40 40 40 8.3−100

200

5−60

100

240 280

2−30 2−10

100 100

200

5−60

400

240

2−30

400

280

2−15

400

16.7− 200 6.7−100 6.7− 33.3 66.7− 800 26.7− 400 26.7− 200

L:W ratio (g/g)

flow rate (mL/min)

a Time at temperature in batch experiments; flow time in flow-through experiments.

extractives-free wood meal were placed in a 300 mL batch reactor (Autoclave Engineers, United States) equipped with a mechanical stirrer, and 200 mL of deionized water were added to reach a L:W ratio of 40:1 g/g. The reactor was heated to the setup temperature, and after an isothermal reaction time, the hydrolysate was discharged (at the setup temperature) through a cooling coil and collected. After the reactor was cooled, the wood residue was recovered, washed, and stored for subsequent analyses. The washing water was collected and mixed with the crude hydrolysate. More information on these prehydrolysis experiments can be found in Borrega et al.10,26 For the prehydrolysis experiments in the FT reactor, about 30 g (oven-dried mass) of wood meal (without preextraction in acetone) were placed in a 190 mL percolation reactor (Unipress Equipment, Poland) equipped with a high-pressure pump, a back-pressure regulator, a preheater, and auxiliary band heaters to minimize heat losses along the reaction chamber. Regular tap water was preheated to a predetermined temperature before entering the reactor, and the outgoing hydrolysate was collected after passing through a heat exchanger. After the reactor was cooled, the wood residue was recovered, washed, and stored for subsequent analyses. More information on the FT reactor and its operation can be found in Borrega and Sixta.30 The intensity of any prehydrolysis experiment was determined by a modified P-factor. The P-factor is an indicator used to account for the combined time−temperature effect during water prehydrolysis of wood, and is calculated with an Arrhenius-type expression that uses an activation energy of 125.6 kJ/mol, corresponding to the removal of fast-reacting xylan.19 In this study, because of the intense prehydrolysis conditions applied, an activation energy of 180 kJ/mol, corresponding to the removal of the recalcitrant xylan fraction in birch wood,10 was used instead to compute the modified P-

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MATERIALS AND METHODS Wood Material. Birch wood chips and meal were used for the prehydrolysis experiments. The chips, delivered by a pulp mill in Finland, were a mixture of silver birch (Betula pendula) and European white birch (Betula pubescens) wood. The chips were screened upon delivery according to the SCAN-CM 40:0129 method. The birch meal was obtained by milling coarse sawdust from silver birch wood felled in southern Finland and provided by Metla (Finnish Forest Research Institute). The sawdust was milled in a Wiley mill to a particle size smaller than 0.6 mm (30 mesh). Water Prehydrolysis. Prehydrolysis experiments were conducted with wood chips in a batch reactor and with wood meal in batch and FT reactors. The main operational parameters are summarized in Table 1. For the prehydrolysis of wood chips, about 1 kg (oven-dried mass) of chips were placed in a 10 L batch reactor (Tankki, Finland), and a measured amount of deionized water was added to reach a liquid-to-wood (L:W) ratio of 3:1 g/g. A high-pressure pump was used to continuously recirculate the water through the chips bed. The reactor was heated to the setup temperature, and after an isothermal reaction time, the hydrolysate was discharged (at the setup temperature) through a cooling coil and collected. After the reactor was cooled, the wood residue was recovered, washed, and stored for subsequent analyses. More information on these prehydrolysis experiments can be found in Borrega et al.3 Prior to prehydrolysis of wood meal in a batch reactor, the meal was screened to reject particles smaller than 0.25 mm (60 mesh) and extracted with acetone in a Soxhlet apparatus to remove the extractives. About 5 g (oven-dried mass) of 6076

DOI: 10.1021/acs.iecr.5b00908 Ind. Eng. Chem. Res. 2015, 54, 6075−6084

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Industrial & Engineering Chemistry Research

of 280 nm, using Milli-Q water and acetonitrile as eluents, with a flow rate of 0.6 mL/min at 30 °C. Soluble lignin in the hydrolysates was determined in a Shimadzu UV-2550 spectrophotometer at a wavelength of 205 nm and using an adsorption coefficient of 110 L/(g cm).33

factor, termed PXs. An increase in PXs corresponds to an increase in prehydrolysis intensity. Chemical Composition Analyses. The identified chemical composition of the initial wood is shown in Table 2. The

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Table 2. Identified Chemical Composition of the Birch Wood

RESULTS AND DISCUSSION Wood Yield and pH of Hydrolysates. The wood yield as a function of prehydrolysis intensity, determined by the PXs, is shown in Figure 1. The logarithmic value of PXs (log PXs) is

% on oven-dried wood

a

component

chips

meal

extractivesa glucose xylose mannose other sugars klason lignin acid-soluble lignin acetyl groups

2.0 37.8 21.4 1.5 0.6 21.7 4.4 3.6

2.0 42.6 20.9 1.7 0.7 18.4 4.1 3.5

Amount of extractives soluble in acetone.

amount of acetone-soluble extractives was quantified according to the SCAN-CM 49:0331 method. The extractives quantification was done only on the initial wood material because, after water prehydrolysis, some of the lignin in the wood residue is removed by the acetone. The carbohydrates, acetyl groups, and lignin content in wood and wood residues was determined after a two-stage acid hydrolysis, according to the analytical method NREL/TP-510-4261832 issued by the U.S. National Renewable Energy Laboratory (NREL). Monosaccharides were determined by high-performance anion exchange chromatography with pulse amperometric detection (HPAEC-PAD) in a Dionex ICS-3000 (Sunnyvale, CA) system, equipped with a CarboPac PA20 (3.0 × 150 mm) analytical column. Milli-Q water was the eluent used, with a flow rate of 0.4 mL/min at 30 °C. The acetyl groups in wood were calculated from the acetic acid content, determined by high-performance liquid chromatography (HPLC) in a Dionex Ultimate 3000 (Sunnyvale, CA) system, equipped with an Acclaim Organic Acid (4.0 × 250 mm) analytical column and a DAD-3000 ultraviolet−visible (UV−vis) diode array detector. The acetic acid was measured at a wavelength of 210 nm, using 2.5 mmol/L methanesulfonic acid and acetonitrile as eluents, with a flow rate of 0.6 mL/min at 30 °C. The amount of acid-insoluble (Klason) lignin was quantified gravimetrically, and the amount of acid-soluble lignin (ASL) was determined in a Shimadzu UV-2550 spectrophotometer at a wavelength of 205 nm, using an adsorption coefficient of 110 L/(g cm).33 To study the behavior of the main polysaccharides in birch wood during water prehydrolysis, the amounts of neutral monosaccharides given by HPAEC analyses were converted to cellulose and xylan fractions using the formulas published by Janson.34 With these formulas, cellulose is defined as the content of anhydroglucose in the sample after subtracting the contribution of glucose to glucomannan, and xylan is defined as the content of anhydroxylose including uronic acid substituents. The carbohydrate and furanic composition in the hydrolysates was determined according to the analytical method NREL/TP-510−4262335 issued by the U.S. NREL. Monosaccharides were quantified by HPAEC-PAD by direct injection and after total hydrolysis in an autoclave at 121 °C for 60 min. Oligosaccharides were calculated by difference in the monosaccharide content before and after total hydrolysis. Furfural and HMF were determined by HPLC at a wavelength

Figure 1. Wood yield after water prehydrolysis of birch wood meal and chips, plotted as a function of treatment intensity (Log PXs). The prehydrolysis experiments were conducted in batch and in flowthrough reactors, with flow rates of 50, 100, and 400 mL/min.

used to have an even distribution of data along the x-axis. For each particular set of experiments, the decrease in wood yield was well explained by an increase in prehydrolysis intensity, although some scatter at the highest intensities was observed for wood meal treated in the FT system. At any intensity, however, the wood yield was the highest for chips prehydrolyzed in a batch reactor with high solids content, and it was the lowest for wood meal prehydrolyzed in the FT system. The difference in wood yield between these two sets of experiments was at least 20%. Obviously, the results from Figure 1 indicate that in addition to reaction temperature and time, other process parameters like reactor system, wood particle size, and/or solids content had a significant influence on the removal of wood components during prehydrolysis. From here on, in order to compare the results from different experiments, the wood yield will be used as an indicator of prehydrolysis intensity. The wood yield has been utilized already as an indicator to allow comparison between water prehydrolysis and acid-catalyzed treatments.28 The degradation and dissolution of wood components during water prehydrolysis is catalyzed by the acetic acid formed through cleavage of the acetyl groups bound to the hemicelluloses.8 Additionally, a variety of other organic acids are formed by degradation of the carbohydrates, particularly under intense prehydrolysis conditions.11 The acidity of the hydrolysates was evident by the pH values measured after the experiments; the pH was predominantly between 2.5 and 4 (Figure 2). In the batch reactor, those experiments with wood chips had the lowest pH, most likely because of the higher solids content and thus higher concentration of acids. On the other hand, the large volumes of water used in the FT system 6077


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Figure 2. Acidity (pH) of hydrolysates after water prehydrolysis of birch wood meal and chips, plotted as a function of wood yield. The prehydrolysis experiments were conducted in batch and in flowthrough reactors, with flow rates of 50, 100, and 400 mL/min.

diluted the acids formed and consequently resulted in higher pH levels. Composition of Wood Residue. The chemical composition of the wood residue was analyzed in terms of cellulose, xylan (polymeric xylose), and lignin (Klason and ASL), the main components in birch wood. The residual cellulose content decreased to 80% with decreasing wood yield to about 45%, and thereafter the cellulose content decreased rapidly (Figure 3a). No major differences in cellulose removal were observed between the different operational conditions. Interestingly, the rapid removal of cellulose beyond a residual content of 80% was associated with the quantitative removal of xylan (Figure 3b). This observation seems to indicate that xylan had a protective effect toward the hydrolytic degradation of cellulose. Once all the xylan was removed, the cellulose chains were then exposed to attack by the acids (acetic, formic, others) formed through degradation of carbohydrates. In kraft pulp, the protective effect of xylan toward acidic hydrolysis of cellulose has been suggested to be due to the close association between both polymers, with the xylan laying along the cellulose chains and protecting the more accessible amorphous regions.36 At any residual xylan, the cellulose content after prehydrolysis of wood chips appeared to be somewhat higher than after prehydrolysis of wood meal (Figure 3b). The residual xylan in wood meal decreased steadily with decreasing yield, with quantitative xylan removal occurring at wood yields of about 45% (Figure 4). The xylan removal from the meal was the same regardless of the system used, i.e., batch or FT. Moreover, the use of different flow rates in the FT system did not seem to affect the removal of xylan. Liu and Wyman reported that the solubilization of xylan from corn stover increased with flow, and especially when higher temperatures were applied.23 However, the yield of corn stover also decreased with increasing flow and temperature; thus, the higher xylan solubilization was probably a direct consequence of higher substrate solubilization. In birch chips, the residual xylan at any wood yield was lower than that in birch meal, with quantitative xylan removal occurring at yields of about 60% (Figure 4). The superior xylan removal in chips may be partly related to the higher acidity of the hydrolysates (see Figure 2). The lower xylan content in wood chips after prehydrolysis obviously translated into a higher lignin content, given that the residual cellulose content was the same for all operational

Figure 3. Residual cellulose after water prehydrolysis of birch wood meal and chips, plotted as a function of (a) wood yield and (b) residual xylan. The prehydrolysis experiments were conducted in batch and in flow-through reactors, with flow rates of 50, 100, and 400 mL/ min.

Figure 4. Residual xylan after water prehydrolysis of birch wood meal and chips, plotted as a function of wood yield. The prehydrolysis experiments were conducted in batch and in flow-through reactors, with flow rates of 50, 100, and 400 mL/min.

conditions. As shown in Figure 5, the residual lignin content in chips did not go below 80% even at wood yields below 60%. In wood meal, the residual lignin decreased rapidly down to about 20%, corresponding to a wood yield of 40%, and thereafter the delignification rate slowed. Previous studies have suggested that the extent of delignification during water prehydrolysis of wood 6078


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batch system, where delignification efficiency during prehydrolysis decreases dramatically with increasing wood particle size and solids content. Increasing particle size limits the diffusion of hydronium ions into the wood cell wall and hinders the transport and removal of depolymerized lignin fragments. Increasing solids content lowers the solubility of dissolved lignin and increases the acidity of the hydrolysate, thus favoring the condensation of lignin and its precipitation back onto the wood surface. The increase in residual lignin, some of it in condensed form, is likely to negatively affect the subsequent utilization of the wood residue. Composition of Hydrolysates. Water prehydrolysis of wood is conducted not only to extract the hemicelluloses but also to recover them for their utilization in the manufacture of value-added products like films, fuels, or food additives.38−40 In a batch reactor, the maximum amount of xylose found in the hydrolysates was about 65% for meal but only 25% for chips, corresponding to wood yields of about 60% and 80%, respectively (Figure 6a). The low recovery of xylose from

Figure 5. Residual lignin after water prehydrolysis of birch wood meal and chips, plotted as a function of wood yield. The prehydrolysis experiments were conducted in batch and in flow-through reactors, with flow rates of 50, 100, and 400 mL/min.

is affected by mass transport and solubility limitations.20,25 In wood meal, owing to the small particle size and high surface area, depolymerized lignin fragments can readily solubilize in the hydrolysate. In wood chips, however, the dissolved lignin fragments must be first transported through the pore network of the wood cellular structure. During transport, some of these fragments may condense and repolymerize with the lignin still remaining within the cell walls. The diffusion of hydronium ions into the cell wall may also be hindered, thus limiting the extent of delignification. Furthermore, prehydrolysis experiments with wood chips were conducted at high solids content, which increased the concentration of low-molar-mass lignin fragments dissolved in the hydrolysate. This high concentration of lignin lowered its solubility and promoted its condensation and precipitation onto the wood surface.6,16 The condensation of dissolved lignin was also likely promoted by the low pH, derived from the high concentration of organic acids at high solids content.37 In Figure 5, the data for residual lignin after prehydrolysis of wood meal in the batch reactor showed some scatter. Such scatter was due to increased condensation and reprecipitation of lignin with increasing prehydrolysis temperature and time, resulting in higher residual lignin contents.26 In the FT system, depolymerized lignin fragments were rapidly removed from the reaction chamber and thus had less time to condense, which facilitated delignification, particularly at the highest flow rate (400 mL/min). The condensation and precipitation of lignin is not desired because it negatively affects the subsequent utilization of the wood residue. Condensed lignin is more difficult to remove from the fibers during pulping,16,37 and higher lignin content in the substrate may hinder the enzymatic hydrolysis of cellulose.4,5 To summarize, the degradation of cellulose during water prehydrolysis of birch wood appeared to be little affected by reactor system, wood particle size, and solids content. The removal of hemicelluloses, however, was facilitated in chips prehydrolyzed at high solids content, partly because of the higher concentration of organic acids formed. Therefore, if the goal of prehydrolysis is to remove the hemicelluloses, one might utilize wood chips and a batch reactor with high solids content, allowing for considerable savings in water and energy consumption. Unfortunately, in terms of lignin removal, the use of a FT system offers performance that is superior to that of a

Figure 6. (a) Total xylose content and (b) xylose monomers in hydrolysates from birch wood meal and chips, plotted as a function of wood yield. The prehydrolysis experiments were conducted in batch and in flow-through reactors, with flow rates of 50, 100, and 400 mL/ min.

chips was likely affected by the high solids content of the prehydrolysis process, which resulted in high acidic conditions in the hydrolysate and probably increased degradation of dissolved sugars. However, the large difference in the amount of xylose recovered from meal and chips is mainly explained by the experimental procedure; after the wood residue was washed, the washing filtrate from the meal was collected and mixed with the undiluted hydrolysate, while the washing filtrate from the chips was not recovered. Testova et al. found that 6079


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sugars without affecting the total sugars yield. In the prehydrolysis experiments on chips, where the L:W ratio was only 3:1 g/g, the xylose concentration was as high as 17.8 g/L, but the total xylose content in the hydrolysate was the lowest. One should keep in mind though, that the xylose content (and concentration) from the chips experiments is underestimated because the dissolved solids entrapped within the wood residue were not recovered. More data are needed in order to optimize the flow, particle size, and solids content on the total xylose yield and concentration in batch and FT systems. The recovery of glucose from the hydrolysates, in contrast to the recovery of xylose, did not appear to depend on the operational conditions (Figure 7a). The total glucose content in

about 40% of the total dissolved matter after prehydrolysis of birch chips (L:W ratio of 4:1 g/g) remained entrapped within the wood pores.14 Therefore, the total amount of xylose recovery from the chips experiments, if the washing water is collected, could be closer to the 50% found by Testova et al. in their combined hydrolysate and washing filtrate.14 In the FT system, about 85−90% of the xylan in wood meal was found in the hydrolysate at wood yields of 45% (Figure 6a). This is the yield at which xylan was quantitatively removed from the meal (see Figure 4); thus, decreasing wood yield did not obviously result in any further increase in xylose recovery. The higher xylose recovery in hydrolysates from the FT reactor, compared to that of the batch reactor, is due to the continuous removal of xylan-derived products from the system before they can react and degrade further. Liu and Wyman found similar xylose recovery during autohydrolysis of corn stover meal in a batch reactor, but their total xylose content in hydrolysates from FT experiments was somewhat higher, reaching nearly quantitative recovery.23 In our experiments, the washing filtrate from the wood residue was not collected; thus, the total amount of xylose recovered might be somewhat underestimated. Nonetheless, in a FT system, a high exchange of the hydrolysate entrapped within the wood pores is usually achieved. In Figure 6a, some data from the FT system showed a rather low xylose recovery (60−70%) at wood yields below 45%. These data points corresponded to prehydrolysis experiments conducted at 280 °C with a flow rate of 100 mL/min; thus, the lower xylose content in the hydrolysate might be due to increased formation of degradation products, favored by the combination of low flow rate and elevated temperatures. The amount of xylose monomers in the hydrolysates from FT experiments increased linearly with decreasing wood yield (Figure 6b). The maximum amount of xylose monomers increased from about 4% to 12% with decreasing flow, probably because of increased conversion (degradation) of xylooligosaccharides (XOS) into xylose. Overall, the relatively low amount of xylose monomers indicates that most of the xylose in the hydrolysates from the FT experiments was dissolved as XOS. In a batch reactor, up to 35−40% of the xylan from wood meal and 12% from chips was found in the hydrolysate as xylose monomers (Figure 6b). These amounts were found at wood yields slightly lower than those corresponding to the maximum xylose content (see Figure 6a). Therefore, less than 50% of the total xylose recovered from prehydrolysis of birch wood in a batch reactor was found as XOS. The superior performance of FT systems in the recovery of sugars from wood is generally offset by the higher water consumption and thus lower product concentration in the hydrolysate. In this study, the maximum xylose content (about 85−90%) recovered from the FT experiments was obtained at different operational conditions of temperature, time, and flow. The maximum xylose concentration, corresponding to an experiment at 240 °C for 12 min and 100 mL/min flow, was 4.4 g/L. In the batch reactor, however, the maximum xylose content (about 65%) from wood meal was given at a concentration of 3.4 g/L. Furthermore, for a similar xylose content of about 65%, the sugar concentration in hydrolysates from the FT system was in some cases 6.8 g/L, twice as much as that from the batch reactor. Nonetheless, it should be noted that the L:W ratio (40:1 g/g) of the batch experiments with wood meal was very high; thus, it is likely that increasing the solids content would result in an increased concentration of

Figure 7. (a) Total glucose content and (b) glucose monomers in hydrolysates from birch wood meal and chips, plotted as a function of wood yield. The prehydrolysis experiments were conducted in batch and in flow-through reactors, with flow rates of 50, 100, and 400 mL/ min.

the hydrolysates, based on the initial amount of oven-dried wood, slightly increased up to 2% at wood yields of about 40%, and thereafter it increased up to 15% with decreasing yield. This sharp increase in dissolved glucose was a direct consequence of the cellulose content in wood decreasing abruptly below a yield of 40−45% (see Figure 3a). Some of the glucose dissolved at higher wood yields may have originated from the degradation of cellulose, but most of it probably corresponded to the glucose fraction of the glucomannan present in birch wood.10 The glucose in the hydrolysates consisted of both monomers and cello-oligomers; the formation of glucose monomers was favored in a batch reactor 6080


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Industrial & Engineering Chemistry Research and by lower flow rates in the FT reactor (Figure 7b). In the case of chips, some glucose may not have been accounted for because the washing filtrates were not collected. However, the degradation of cellulose in those experiments did not exceed 20%; thus, the amount of unaccounted glucose was expected to be minor compared to that of xylose. The operational optimum of water prehydrolysis, determined as the highest ratio of xylose to glucose in the hydrolysates, was shifted to lower wood yields by using wood meal instead of chips in a batch system (Figure 8). However, the xylose-to-

Figure 8. Xylose-to-glucose ratio in hydrolysates from birch wood meal and chips, plotted as a function of wood yield. The prehydrolysis experiments were conducted in batch and in flow-through reactors, with flow rates of 50, 100, and 400 mL/min.

Figure 9. (a) Furfural and (b) HMF content in hydrolysates from birch wood meal and chips, plotted as a function of wood yield. The prehydrolysis experiments were conducted in batch and in flowthrough reactors, with flow rates of 50, 100, and 400 mL/min.

glucose ratio was higher in the case of chips, even when large amounts of xylose possibly remained in the hydrolysate within the chips. For wood meal, the operational optimum was attained at similar xylose-to-glucose ratios in both batch and FT systems, although this optimum was slightly shifted to lower yields in the latter. Gütsch et al. reported that in a batch reactor, the operational optimum in prehydrolysis of eucalyptus wood meal was attained at wood yields of 70% and xylose-to-glucose ratios of about 20.28 These values are higher than the wood yields and ratios corresponding to prehydrolysis of birch meal in the batch reactor (see Figure 8). Because those authors used a lower L:W ratio in their experiments (5:1 versus 40:1 g/g), it appears that not only the particle size but also the solids content affects the operational optimum during water prehydrolysis of wood. In the batch experiments, the decrease in xylose-to-glucose ratio was mostly due to increased degradation of xylose in solution, whereas in the flow-through experiments the decrease in xylose-to-glucose ratio was due to increased cellulose degradation because the xylose content in the hydrolysates remained constant. Furanic compounds and low-molar-mass carboxylic acids, including acetic and formic acid, are typically the main degradation products from carbohydrates found in the hydrolysates.11 Furfural and HMF, formed by dehydration of C5 and C6 sugars like xylose and glucose, are of interest because they are considered valuable platform chemicals for production of levulinic and formic acid as well as biofuels.41,42 In both batch and FT experiments, the furfural content in the hydrolysates increased with decreasing wood yield (Figure 9a). For the wood meal prehydrolyzed in the batch system, a maximum of 9% of the initial dry wood, corresponding to about

one-third of the birch xylan, was converted into furfural. For wood chips, the conversion to furfural was somewhat lower, about 7%. The maximum furfural conversion was reached at wood yields of about 60% for chips and 45% for meal, when all xylan in wood had been removed (see Figure 4). The formation of furfural in FT experiments was only minor, with less than 1% conversion on oven-dried wood, corresponding to about 5% of the xylan. This low furfural conversion was expected because FT systems minimize the degradation of sugars in solution as they are continuously removed from the reaction chamber. In general, the mass balances for xylan were rather closed in the FT experiments (see Table S1 in Supporting Information). However, in the case of batch experiments, and particularly at low wood yields, large amounts of the initial xylan were not recovered as xylose or furfural in solution, possibly because of the formation of other degradation products like carboxylic acids and humins.11,43,44 The amount of HMF in hydrolysates from batch experiments increased rapidly at a wood yield of about 65% for chips and 50% for meal (Figure 9b). The maximum amount of HMF found in hydrolysates from chips was lower than from meal because of the milder prehydrolysis intensities applied, which resulted in a higher preservation of the cellulose fraction in wood and a lower content of monomeric glucose in solution (see Figures 3 and 7b). In FT experiments, the amount of HMF started increasing at wood yields between 40 and 50%, with its formation seemingly favored by lower flow rates. In both batch and FT systems, the amount of HMF found in the hydrolysates was rather small and predominantly less than 3% 6081


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completed because of lack of data for lignin precipitates. However, a visual observation of the hydrolysates after the prehydrolysis experiments revealed a higher amount of precipitates in hydrolysates from the batch experiments than from the FT experiments. In summary, compared to a batch reactor, the use of a FT system for water prehydrolysis of birch wood significantly favored the recovery of carbohydrates because sugar degradation reactions were minimized, most notably the formation of furfural from xylose monomers. About 85−90% of the original xylan in birch wood meal was recovered after prehydrolysis in the FT reactor, with about 80% of it being recovered as XOS. The total glucose content was similar in hydrolysates from both batch and FT systems, but the latter appeared to minimize the depolymerization of dissolved cello-oligosaccharides to glucose monomers. The operational efficiency of prehydrolysis was affected by the reaction system, wood particle size, and solids content. The operational optimum was shifted to lower wood yields when the FT reactor was used, and it was the highest in a batch reactor with wood chips. Finally, the amount of soluble lignin was the highest in hydrolysates from FT experiments, probably because of the combination of higher pH and lower solids content, which improved lignin solubility and decreased its tendency to condensation reactions.

of the initial wood material. Overall, up to 15% of the initial glucose in wood could not be accounted for at yields higher than 40% (see Table S1 in Supporting Information). At lower wood yields, however, the mass balances were considerably open, with only small amounts of the initial glucose being recovered in the hydrolysate as glucose monomers or as HMF. In addition to xylose- and glucose-derived products, some of the wood lignin was also solubilized in the hydrolysates. For wood meal, the amount of soluble lignin in hydrolysates from the FT system was typically higher than in hydrolysates from the batch system, despite some scatter of data (Figure 10a).

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CONCLUSIONS Operational conditions for wood prehydrolysis should be selected according to the main target product. If the main goal is to produce a substrate with low hemicellulosic content, one might utilize wood chips and a batch reactor with high solids content, thus reducing water and energy consumption. However, in a FT system, the extraction of lignin is higher than in a batch system, where delignification is hindered by increasing the wood particle size and solids content. If the aim of prehydrolysis is to recover the extracted sugars for their conversion to value-added products, a FT system is advantageous over a batch system because sugar degradation reactions are minimized. More data is still necessary to optimize the flow, particle size, and solids content on the total xylose yield and concentration in batch and FT reactors. Furthermore, degradation kinetics of carbohydrates and lignin will differ when softwoods and other plant materials are used, primarily because of the different chemical nature of hemicelluloses and lignin.

Figure 10. Soluble lignin in hydrolysates from birch wood meal, plotted as a function of (a) wood yield and (b) liquid-to-wood (L:W) ratio. The prehydrolysis experiments were conducted in batch and in flow-through reactors, with flow rates of 50, 100, and 400 mL/min.

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ASSOCIATED CONTENT

S Supporting Information *

Unfortunately, data for soluble lignin from prehydrolysis of wood chips is not available. It should also be pointed out that in the case of FT experiments the wood meal was not preextracted with acetone; thus, some aromatic extractive compounds dissolved in the hydrolysate might have absorbed light at the same wavelength (205 nm) as lignin, overestimating the amount of soluble lignin determined. Nonetheless, the higher content of soluble lignin in the FT experiments may be partly related to the higher pH of the hydrolysate45 (see Figure 2) and to the higher water consumption, which decreased lignin concentration and improved its solubility.25 The increase in lignin solubility with decreasing solids content can be clearly seen in Figure 10b. In the FT system, differences in the solids content at a given wood yield are likely to explain the scatter of lignin data shown in Figure 10a. The mass balances for lignin (see Table S1 in Supporting Information) could not be

Mass balances for cellulose, xylan, and lignin for each prehydrolysis experiment. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b00908.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: marc.borrega@aalto.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experimental data here presented was obtained during the Future Biorefinery (FuBio) research program (2009−2014), funded by the Finnish Bioeconomy Cluster (FIBIC) and the 6082


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Finnish Agency for Technology and Innovation (TEKES). The help of Ms. Lei Wang in conducting water prehydrolysis experiments in the flow-through reactor is gratefully acknowledged.

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