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Extraction of galactoglucomannan from thermomechanical pulp mill process water by microfiltration and ultrafiltration—Influence of microfiltration membrane pore size on ultrafiltration performance
chemical engineering research and design 1 0 5 ( 2 0 1 6 ) 171–176
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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Extraction of galactoglucomannan from thermomechanical pulp mill process water by microfiltration and ultrafiltration—Influence of microfiltration membrane pore size on ultrafiltration performance Johan Thuvander, Ann-Sofi Jönsson ∗ Department of Chemical Engineering, Lund University, P.O. Box 124 SE-221 00 Lund, Sweden
a r t i c l e
i n f o
a b s t r a c t
Article history:
Galactoglucomannan is the major hemicellulose in process water from thermomechani-
Received 16 September 2015
cal pulping of spruce. The recovery of hemicelluloses from this process water would both
Received in revised form 12
reduce the load on pulp mill water treatment plants and yield a biopolymer suitable as
November 2015
a raw material for renewable products. One method of recovering these hemicelluloses is
Accepted 5 December 2015
two-stage filtration employing microfiltration to remove large contaminants, such as sus-
Available online 14 December 2015
pended matter and colloidal extractives, followed by ultrafiltration to concentrate and purify
Keywords:
a microfiltration membrane with a pore size of 0.1 m removes colloidal extractives while
Galactoglucomannan
still maintaining a high amount of high molecular mass hemicelluloses in the microfiltra-
Microfiltration
tion permeate. The removal of extractives resulted in an increase in the initial flux during
Ultrafiltration
the subsequent ultrafiltration step from 90 L/m2 h to over 200 L/m2 h.
Thermomechanical pulp
© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
the hemicelluloses from low molecular mass contaminants. This investigation shows that
SEC
1.
Introduction
The abundance of hemicellulose in biomass makes it an interesting raw material for biobased products. Galactoglucomannan (GGM) is the main hemicellulose in softwood and constitutes about 15–20% of the wood biomass of spruce (Sjöström, 1993; Willför et al., 2005). During the production of thermomechanical pulp (TMP) from spruce, about 10% of the GGM in the wood is dissolved in the process water (Thornton et al., 1994). Upgrading GGM to high-value-added products has attracted significant interest (Willför et al., 2008). Potential uses of GGM include barrier films (Hartman et al., 2006; Kisonen et al., 2014; Mikkonen et al., 2010), hydrogels (Lindblad et al., 2005; Voepel et al., 2009) and substrates for probiotic bacteria (Polari et al., 2012). Furthermore, extraction of GGM from the process water of TMP mills would also reduce
∗
the load on the water treatment plant of the mill. Another method of extracting GGM from wood is by hot-water extraction (Desharnais et al., 2011; Örså et al., 1997), which closely resembles the conditions during the production of TMP, but in this case the conditions for hemicellulose extraction can be optimized. To obtain a pure GGM fraction, other components present in the process water must be separated from the GGM. Contaminants that need to be removed include large components such as suspended solids and colloidal extractives, and low molecular mass contaminants such as lignin and salts. Suspended solids and colloidal extractives can be removed by coagulation with a cationic surfactant and deposition on TMP (Willför et al., 2003), or by microfiltration (MF) (Persson et al., 2010). The GGM can then be simultaneously concentrated and purified from low molecular lignin and salts using
Corresponding author. Tel.: +46 46 2228291. E-mail address: ann-sofi[email protected] (A.-S. Jönsson). http://dx.doi.org/10.1016/j.cherd.2015.12.003 0263-8762/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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chemical engineering research and design 1 0 5 ( 2 0 1 6 ) 171–176
ultrafiltration (UF) (Persson et al., 2010). Polymeric adsorbents can be used to remove lignans and lignin (Koivula et al., 2013; Westerberg et al., 2012; Willför et al., 2003) and when used before the UF step, they have been shown to improve UF performance (Koivula et al., 2013). Further purification can be achieved by diafiltration, size exclusion chromatography (SEC) (Andersson et al., 2007) or ethanol precipitation (Willför et al., 2003). A surprisingly high retention of GGM during MF was found in a previous study on the recovery of GGM from TMP mill process water by membrane filtration (Persson et al., 2010). It has been shown that the retention of hemicelluloses, and hence the loss of hemicelluloses, increases with decreasing MF pore size (Krawczyk and Jönsson, 2011). It was also shown that MF membranes with larger pores are more susceptible to fouling and exhibit lower fluxes than MF membranes with smaller pores. The higher flux but lower permeability of denser MF membranes to GGM means that the choice of MF membrane pore size is a trade-off between hemicellulose recovery and MF capacity. The MF pore size may also affect the performance of the subsequent UF concentration step, as both dissolved and colloidal extractives have been shown to foul UF membranes (Puro et al., 2002, 2011). To the best of our knowledge, the influence of MF pore size on UF performance has not been studied previously. The aim of this work was to investigate how the flux and recovery of GGM during UF of MF permeates are affected by MF membranes with pore sizes from 0.04 m to 0.5 m by determining the purity, recovery and molecular mass distribution of GGM after UF of MF permeates.
2.
Materials and methods
2.1.
Process water
Process water from a Swedish thermomechanical pulp mill with spruce as primary raw material was used in the investigation. Suspended solids and colloidal extractives were removed by MF at the mill. The nominal pore size of the ceramic MF membranes from LiqTech International A/S, Denmark, was 0.04 m, 0.1 m and 0.5 m. The volume of MF permeate produced with each MF membrane was about 1000 L for the 0.04 m and 0.1 m membranes and 300 L for the 0.5 m membrane. The MF permeates from the 0.04 m, 0.1 m and 0.5 m membranes are hereafter referred to as MF0.04, MF0.1 and MF0.5, respectively.
2.2.
stepwise increase in the transmembrane pressure from 1.5 bar to 4 bar. Permeate was withdrawn in the experiment with MF0.04 during the increase in pressure, while both retentate and permeate were recirculated to the feed tank in the experiments with MF0.1 and MF0.5. When 4 bar was reached, concentration also started in the MF0.1 and MF0.5 experiments. Concentration was performed in semi-batch mode by recirculating retentate to the feed tank while withdrawing permeate. The volume in the feed tank was kept constant by replenishing withdrawn UF permeate with fresh MF permeate until all the MF permeate had been transferred to the feed tank. Concentration then continued in batch mode until a final retentate volume of about 20 L was reached. This resulted in a final volume reduction (VR) during UF of MF0.04 and MF0.1 of 98%, and during UF of MF0.5 of 94%, corresponding to volume reduction factors of 50 and 17, respectively.
2.3.
Analytical methods
2.3.1.
Total solids and ash
Samples were dried for 24 h in an oven at 105 ◦ C. The content of total solids was determined from the weight of the residue after cooling to room temperature in a desiccator. The dry sample was further heated to 575 ◦ C, and this temperature was maintained for 3 h. The ash content was calculated from the weight of the residue after cooling to room temperature in a desiccator.
2.3.2.
Hemicelluloses
The concentration of hemicelluloses was determined by hydrolyzing the polysaccharide to monomeric sugars by acid hydrolysis using a standardized method for acid hydrolysis (Ruiz and Ehrman, 1996). Monomeric sugars were measured using high-performance anion-exchange chromatography coupled with pulsed amperometric detection in an ICS-3000 chromatography system (Dionex Corp., USA). The system was equipped with a Carbo Pac PA1 analytical column. A solution of 200 mM NaOH dissolved in 170 mM sodium acetate was used to clean the column. The sample injection volume was 10 L, and deionized water was used as eluent, at a flow rate of 1 mL/min. The monomeric sugars d-glucose, d-galactose, d-mannose, d-xylose and l-arabinose (Fluka Chemie AG, Switzerland) were used as standards. The concentration of hemicelluloses was defined as the sum of the monomeric sugars after anhydro corrections of 0.88 and 0.90 for pentoses and hexoses, respectively.
Experimental procedure during ultrafiltration 2.3.3.
The MF permeate was concentrated by UF using a spiralwound UF membrane (UFX5pHt, Alfa Laval Nakskov A/S, Denmark) with a nominal molecular mass cut-off of 5 kDa. The spiral-wound element was equipped with a 48-mil spacer (1.2 mm), and had an outer diameter of 64 mm and a length of 0.43 m. Persson and Jönsson (2010) have previously shown that this membrane gives a high flux and good GGM yield during UF of TMP mill process water. The temperature was 60 ◦ C and the cross-flow was 1.3 m3 /h during all UF experiments. The crossflow was chosen so that the maximum frictional pressure drop in the spiral-wound element would not exceed 0.6 bar. Before the experiment, the membrane was cleaned with a 0.4 wt% solution of the alkaline cleaning agent Ultrasil 10 (Ecolab AB, Sweden) at 50 ◦ C and 1 bar transmembrane pressure for 1 h. A 200 L feed tank was temperature controlled using a steam-heated copper coil. The experiment started with
Lignin
The total lignin content was determined by measuring the light absorption at a wavelength of 280 nm, using a UV-160 spectrophotometer (Shimadzu, Japan) and an absorption coefficient of 17.8 L/(g cm) (Örså et al., 1997).
2.3.4. lignin
Molecular mass distribution of hemicelluloses and
The molecular mass distribution of hemicelluloses and lignin was determined by SEC using a Waters 600E chromatography system (Waters, USA) equipped with a refractive index (RI) detector (model 2414, Waters) and a UV detector (model 486, Waters). The analytical column was packed with 30 cm Superdex 30 and 30 cm Superdex 200 (GE Healthcare, Sweden). The injection volume was 500 L. A 125 mM NaOH solution was used as eluent at a flow rate of 1 mL/min. The system was calibrated with polyethylene glycol standards with
173
Table 1 – Characteristics of MF permeates.
Total solids (g/L) Ash (g/L) Turbidity (NTU) Lignin (g/L) Hemicelluloses (g/L) - Arabinan (g/L) - Galactan (g/L) - Glucan (g/L) - Xylan (g/L) - Mannan (g/L) Hemicellulose purity (%)
MF0.04
MF0.1
MF0.5
4.7 2.1 11 0.89 0.85 0.07 0.11 0.17 0.01 0.48 18
4.9 1.5 14 1.00 1.3 0.08 0.16 0.26 0.02 0.78 27
4.7 1.2 105 0.88 1.5 0.11 0.21 0.30 0.02 0.88 32
a)
500
Refractive index (mV)
chemical engineering research and design 1 0 5 ( 2 0 1 6 ) 171–176
400
UV absorbance (AU)
Turbidity
The turbidity was determined at room temperature using a turbidimeter (2100P ISO, HACH Co., USA). Formazin standards (also from HACH) of 0, 20, 100 and 800 nephelometric turbidity units (NTU) were used for calibration.
Microfiltration permeates
The total solids content and the lignin concentration were similar in MF0.04 and MF0.5, and slightly higher in MF0.1, as can be seen from Table 1. The concentration of hemicelluloses increased with increasing membrane pore size. MF0.5 is thus the permeate with the highest hemicellulose purity when purity is defined as the ratio between the amount of hemicelluloses and the content of total solids. However, the turbidity of MF0.5 was higher than for the other permeates. Turbidity is a good indicator of colloidal extractives in process water from the production of TMP (Sundberg et al., 1993). The high turbidity of MF0.5 thus indicates that MF0.5 was contaminated with colloidal extractives.
3.2.
Molecular mass distribution of MF permeates
A high recovery of high molecular mass GGM when isolating GGM from TMP mill process water is desirable as a high molecular mass improves the properties of GGM films (Kisonen et al., 2014). The molecular mass distribution of the MF permeates is shown in Fig. 1. All three permeates had RI peaks at about 0.2 and 0.8 kDa, and in the interval 10–20 kDa. MF0.5 showed an additional peak above 100 kDa, which was also visible in the UV absorption chromatogram. The RI and UV peaks below 100 kDa corresponded well with molecular mass distributions in TMP mill process water reported by Krawczyk and Jönsson (2011), whereas the peak above 100 kDa has not been reported previously. Colloidal extractives that were not retained by the 0.5 m MF membrane (seen as an elevated turbidity in Table 1) are probably responsible for the mass fraction above 100 kDa, as hemicelluloses have been found to adsorb onto the surface of colloidal extractives (Stack et al., 2014). It should be noted that the SEC column was not calibrated for substances with molecular mass higher than 35 kDa PEG equivalents, and thus sizes above this value are uncertain.
10.0
100.0
1,000.0
0.8 0.6 0.4 0.2
1.0
10.0
100.0
1,000.0
Molecular mass (kDa) Fig. 1 – Molecular mass distribution of (a) sugars (measured as refractive index) and (b) lignin (measured as UV absorbance at 280 nm) in MF0.04 (unbroken line), MF0.1 (dashed line) and MF 0.5 (dotted line).
3.3.
Flux during UF
The flux during concentration of MF0.04 and MF0.1 was almost identical, whereas the flux of MF0.5 was markedly lower, as shown in Fig. 2. Permeate was withdrawn already during the initial stepwise increase in pressure in the experiment with MF0.04, which is why the first flux was recorded at VR = 20% for this permeate. The flux when starting concentration at 4 bar with MF0.1 and MF0.5 was 226 L/m2 h and 92 L/m2 h, respectively. The decline in flux when filtering MF0.04 and MF0.1 was quite similar. When concentration was complete at VR = 98% the flux was 30 L/m2 h for MF0.04 and 20 L/m2 h for MF0.1. The final flux for MF0.5 at a VR of 94% was 30 L/m2 h. The low flux during UF of MF0.5, compared to MF0.04 and MF0.1, is probably due to the high amount of colloidal 250 200 2
3.1.
1.0
1.0
0.0 0.1
Flux (l/m h)
Results and discussion
100
Molecular mass (kDa)
peak molecular masses of 0.4, 4, 10 and 35 kg/mol (Merck Schuchardt OHG, Germany). Samples were filtered through a 0.2 m filter (Schleicher & Schuell, Germany) before determination of molecular mass distribution.
3.
200
0 0.1
b)
2.3.5.
300
150 100 50 0
0
20
40
60
80
100
Volume reduction (%) Fig. 2 – Influence of volume reduction on flux during concentration of MF0.04 (), MF0.1 () and MF0.5 (). The temperature was 60 ◦ C, the transmembrane pressure was 4 bar and the cross-flow was 1.3 m3 /h.
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Hemicellulose retention (%)
a) 100 90 80 70 60
0
20
40
60
80
100
Volume reduction (%)
b) Hemicellulose retention (%)
100 90 80
3.5.
70 60
1
10
100
Hemicellulose concentration (g/l) Fig. 3 – Influence of (a) volume reduction and (b) concentration on hemicellulose retention during UF of MF0.04 (), MF0.1 () and MF0.5 (). extractives in MF0.5. Fluxes in this investigation can be compared with those reported previously by Persson et al. (2010), where a 0.2 m MF membrane was used to remove colloidal and suspended matter from TMP mill process water before concentration of hemicelluloses by UF. In the study of Persson et al. the initial hemicellulose concentration was 0.83 g/L and UF was performed at 80 ◦ C. The initial flux was lower, about 130 L/m2 h, in spite of the lower concentration and higher temperature, than during UF of MF0.1. This suggests that by using an MF membrane with a pore size smaller than 0.2 m, compounds that severely limit the UF flux are retained and not found in the MF permeate.
3.4.
distribution of hemicellulose molecules in TMP mill process water, as shown in Fig. 1. Low molecular mass molecules have a lower retention and are withdrawn with the permeate, leading to an increase in the mean molecular size in the retentate as concentration proceeds. This results in an increase in retention during concentration. MF0.5 contained a higher amount of high molecular mass hemicelluloses than the other MF permeates, as well as colloidal extractives with adsorbed hemicelluloses. These colloids are completely retained by the UF membrane, contributing to a high initial retention of hemicelluloses. MF0.04 had the highest amount of low molecular mass hemicelluloses, and consequently the lowest hemicellulose retention at the beginning of concentration. However, the retention of hemicelluloses was similar during UF of MF0.04 and MF0.1 at similar concentrations of hemicelluloses, as shown in Fig. 3b and the retention was >95% at the final VR during UF of all three MF permeates.
Retention of hemicelluloses during UF
The retention of hemicelluloses increased with increasing VR and concentration of hemicelluloses, as shown in Fig. 3. Retention increases during concentration because of the wide size
Retentate and permeate composition
The concentration of hemicelluloses in the retentate after UF of MF0.04, MF0.1 and MF0.5 was 33 g/L, 52 g/L and 25 g/L, respectively. The concentration of hemicelluloses in the collected permeate was about 0.3 g/L in all three permeates, as can be seen from Table 2. The concentration of lignin in the retentate was 4 g/L (MF0.04 and MF0.1) and 3 g/L (MF0.5). The highest hemicellulose purity was obtained after UF of MF0.1. The hemicellulose purity was then 68%. This was a consequence of MF0.1 having a higher starting purity than MF0.04 and reaching a higher VR than MF0.5. Higher purities can be reached by using additional purification steps. Using diafiltration or SEC Andersson et al. (2007) obtained a hemicellulose purity of 80% and Willför et al. (2003) reported a GGM purity of 95 mole% when using polymeric adsorbents and ethanol precipitation. The flux was still relatively high at the end of the UF concentration when the dead volume of the membrane equipment was reached. An estimate of how much the hemicelluloses could have been concentrated if the UF concentration had been allowed to proceed further can be obtained by applying the film model (Blatt et al., 1970). The theoretical, maximal hemicellulose concentrations obtained using the film model are 111 g/L (MF0.04), 110 g/L (MF0.1) and 107 g/L (MF0.5). Other techniques, such as, ethanol precipitation and spray drying, can be used to increase the hemicellulose concentration further (Xu et al., 2007).
Table 2 – Characteristics of retentate and permeate after UF of MF0.04, MF0.1 and MF0.5. MF0.04
Total solids (g/L) Ash (g/L) Lignin (g/L) Hemicelluloses (g/L) - Arabinan (g/L) - Galactan (g/L) - Glucan (g/L) - Xylan (g/L) - Mannan (g/L) Hemicellulose purity (%)
MF0.1
MF0.5
Retentate VR 98%
Total perm.
Retentate VR 98%
Total perm.
Retentate VR 94%
Total perm.
52 3.2 4.0 33 0.8 4.2 6.0 0.2 22 63
4.0 2.0 0.87 0.33 0.06 0.04 0.08 0.01 0.15
77 2.3 4.1 52 1.2 6.6 9.2 0.5 35 68
3.4 1.4 0.97 0.35 0.06 0.04 0.08 0.01 0.17
40 1.8 3.1 25 0.8 3.6 4.8 0.2 15 61
2.8 1.2 0.78 0.25 0.07 0.03 0.05 0.01 0.10
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a)
400
Recovery of GGM (%)
Refractive index (mV)
a)
300 200 100 0 0.1
1.0
10.0
100.0
100 90 80 70 60 50
1,000.0
0
0.25 0.20
0.10 0.05
1.0
10.0
100.0
1,000.0
Molecular mass (kDa) Fig. 4 – Molecular mass distribution of (a) sugars (measured as refractive index) and (b) lignin (measured as UV absorbance) in the retentate after UF of MF0.04 (unbroken line), MF0.1 (dashed line) and MF 0.5 (dotted line). Samples were diluted 10 times before analysis.
3.6.
Molecular mass distribution
The molecular mass distribution of hemicelluloses and lignin in the UF retentate is shown in Fig. 4. Comparison with Fig. 1 shows that the amount of low molecular mass hemicelluloses is markedly reduced and the high molecular mass fraction of hemicelluloses with a peak at about 10 kDa is now the dominating mass fraction (as can be seen in Fig. 4a). The colloidal extractives present in MF0.5 were also concentrated in the retentate during UF. As the amount of small lignin molecules is lower after UF, a small peak in the UV absorption chromatogram at around 20 kDa becomes visible in Fig. 4b. This peak is also present in Fig. 1, but is not visible because of the relatively high amount of low molecular mass lignin in the samples at 0% VR. In the native lignocellulosic matrix, hemicelluloses are crosslinked by lignin (Lawoko et al., 2006). It is possible that the UV response at 20 kDa is the result of a small amount of aromatic residues attached to the larger hemicelluloses originating from these lignin-hemicellulose bonds.
3.7.
40
60
80
100
80
100
b)
0.15
0.00 0.1
20
Volume reduction (%)
Recovery of hemicelluloses (%)
b) UV absorbance (AU)
Molecular mass (kDa)
Fractionation of sugars
The main hemicelluloses in softwood consist of a glucomannan backbone with acetyl groups and galactose residues attached. Other hemicelluloses in softwood include arabinoglucuronoxylan and arabinogalactan (Sjöström, 1993). Krawczyk and Jönsson (2011) showed that different molecular mass fractions of the components in process water from the production of TMP consist of different types of polysaccharides. GGM was found to be the major polysaccharide in the mass fraction above 7.7 kDa. The relative amount of GGM was found to decrease to 60–80% in the fraction between 1
100 90 80 70 60 50
0
20
40
60
Volume reduction (%) Fig. 5 – Influence of volume reduction on recovery of (a) GGM and b) hemicelluloses during UF of MF0.04 (), MF0.1 () and MF0.5 (). and 7.7 kDa, while the fraction below 1 kDa consisted almost entirely of other polysaccharides. As a consequence, the retention of mannan will be higher than of other sugars, as found by Persson et al. (2010). This was also seen in the present study as the relative amount of mannan increased in the retentate while the relative amount of arabinan and xylan was higher in permeates, as seen in Table 2. During UF, the GGM was thus not only purified from lignin and salts, but also from other low molecular mass hemicelluloses.
3.8.
Recovery of hemicelluloses during UF
The recovery of hemicelluloses decreased with increasing VR during UF, as shown in Fig. 5. As the retention of mannan is higher than that of other sugars, the recovery of GGM will be higher than that of hemicelluloses as a group. The recovery of GGM, based on the content of mannan, was 70%, 79% and 90% (Fig. 5a) while the recovery of hemicelluloses was 62%, 74% and 84% (Fig. 5b). The higher recovery of hemicelluloses during UF of MF0.5 is partly due to the lower VR used in this experiment, and partly due to the higher amount of high molecular mass hemicelluloses, and hence, higher retention. The higher recovery of hemicelluloses during UF of MF0.1, compared with MF0.04, is due to the greater amount of large hemicelluloses in MF0.1. The lower recovery arising from operating UF at high VR may appear discouraging. However, as VR increases, the large molecular mass fraction of hemicelluloses with an average size of about 10 kDa increases compared with the mass fraction of low molecular mass hemicelluloses, as shown in Fig. 6. It should be noted that the samples were diluted so that all the results would be accommodated in Fig. 6. The ratio between the dilutions at VR = 0%:VR = 85%:VR = 98% in Fig. 6 was 1:2:10. The benefits of operating the UF at a high VR is thus a purer
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Refractive index (mV)
500 400 300 200 100 0 0.1
1.0
10.0
100.0
1,000.0
Molecular mass (kDa) Fig. 6 – Molecular mass distribution of sugar (measured as refractive index) in the retentate during UF of MF0.1 at VR 0% (unbroken line), 85% (dashed line) and 98% (dotted line). The retentate solutions at VR 85% and 98% were diluted 2 and 10 times, respectively, prior to analysis. and more concentrated product, but at the cost of the mean flux and yield.
4.
Conclusions
The content of colloidal extractives in the MF permeate could be greatly reduced by using an MF membrane with a pore size no larger than 0.1 m. This results in a substantial increase in the UF flux, from 90 L/m2 h to over 200 L/m2 h at 4 bar and 60 ◦ C. Smaller pore size of the MF membrane reduces the fraction of large molecules in the MF permeate, resulting in lower retention and recovery of hemicelluloses during UF. The pore size of the MF membrane must thus be just small enough to retain colloidal extractives to optimize the recovery of hemicelluloses.
References Andersson, A., Persson, T., Zacchi, G., Stålbrand, H., Jönsson, A.-S., 2007. Comparison of diafiltration and size-exclusion chromatography to recover hemicelluloses from process water from thermomechanical pulping of spruce. Appl. Biochem. Biotechnol. 137, 971–983. Blatt, W.F., Dravid, A., Michaels, A.S., Nelsen, L., 1970. Solute polarization and cake formation in membrane ultrafiltration: causes, consequences, and control techniques. In: Flinn, J.E. (Ed.), Membrane Science Technology. Plenum Press, New York, pp. 47–97. Desharnais, L., Du, F., Brosse, N., 2011. Optimization of Galactoglucomannans and acidic arabinans recovery in softwood. Ind. Eng. Chem. Res. 50 (24), 14217–14220. Hartman, J., Albertsson, A.-C., Sjöberg, J., 2006. Surface- and bulk-modified galactoglucomannan hemicellulose films and film laminates for versatile oxygen barriers. Biomacromolecules 7 (6), 1983–1989. Kisonen, V., Xu, C., Bollström, R., Hartman, J., Rautkoski, H., Nurmi, M., Hemming, J., Eklund, P., Willför, S., 2014. O-acetyl galactoglucomannan esters for barrier coatings. Cellulose 21 (6), 4497–4509. Koivula, E., Kallioinen, M., Sainio, T., Antón, E., Luque, S., Mänttäri, M., 2013. Enhanced membrane filtration of wood hydrolysates for hemicelluloses recovery by pretreatment with polymeric adsorbents. Bioresour. Technol. 143, 275–281. Krawczyk, H., Jönsson, A.-S., 2011. Separation of dispersed substances and galactoglucomannan in thermomechanical pulp process water by microfiltration. Sep. Purif. Technol. 79 (1), 43–49. Lawoko, M., Henriksson, G., Gellerstedt, G., 2006. Characterisation of lignin-carbohydrate complexes (LCCs) of spruce wood (Picea
abies L.) isolated with two methods. Holzforschung 60 (2), 156–161. Lindblad, M.S., Albertsson, A.-C., Ranucci, E., Laus, M., Giani, E., 2005. Biodegradable polymers from renewable sources: rheological characterization of hemicellulose-based hydrogels. Biomacromolecules 6 (2), 684–690. Mikkonen, K.S., Heikkilä, M.I., Helén, H., Hyvönen, L., Tenkanen, M., 2010. Spruce galactoglucomannan films show promising barrier properties. Carbohydr. Polym. 79 (4), 1107–1112. Persson, T., Jönsson, A.-S., 2010. Isolation of hemicelluloses by ultrafiltration of thermomechanical pulp mill process water—influence of operating conditions. Chem. Eng. Res. Des. 88 (12), 1548–1554. Persson, T., Krawczyk, H., Nordin, A.-K., Jönsson, A.-S., 2010. Fractionation of process water in thermomechanical pulp mills. Bioresour. Technol. 101 (11), 3884–3892. Polari, L., Ojansivu, P., Mäkelä, S., Eckerman, C., Holmbom, B., Salminen, S., 2012. Galactoglucomannan extracted from spruce (Picea abies) as a carbohydrate source for probiotic bacteria. J. Agric. Food Chem. 60 (44), 11037–11043. Puro, L., Tanninen, J., Nyström, M., 2002. Analyses of organic foulants in membranes fouled by pulp and paper mill effluent using solid–liquid extraction. Desalination 143 (1), 1–9. Puro, L., Kallioinen, M., Mänttäri, M., Nyström, M., 2011. Evaluation of behavior and fouling potential of wood extractives in ultrafiltration of pulp and paper mill process water. J. Membr. Sci. 368, 150–158. Ruiz, R., Ehrman, T., 1996. Laboratory Analytical Procedure LAP-014: Dilute Acid Hydrolysis Procedure for Determination of Total Sugars in the Liquid Fraction of Process Samples. NREL, Midwest Research Institute for the Department of Energy, USA. Sjöström, E., 1993. Wood Chemistry—Fundamentals and Applications. Academic Press, San Diego, USA. Stack, K., Lee, R., Richardson, D., Lewis, T., Garnier, G., 2014. Complex formation and stability of colloidal wood resin pitch suspensions with hemicellulose polymers. Colloids Surf. A 441, 101–108. Sundberg, A., Ekman, R., Holmbom, B., Sundberg, K., Thornton, J., 1993. Interactions between dissolved and colloidal substances and a cationic fixing agent in mechanical pulp suspensions. Nord Pulp Pap. Res. J. 8 (1), 226–231. Thornton, J., Ekman, R., Holmbom, B., Örså, F., 1994. Polysaccharides dissolved from Norway spruce in thermomechanical pulping and peroxide bleaching. J. Wood Chem. Technol. 14 (2), 159–175. Voepel, J., Sjöberg, J., Reif, M., Albertsson, A.-C., Hultin, U.-K., Gasslander, U., 2009. Drug diffusion on neutral and ionic hydrogels assembled from acetylated galactoglucomannan. J. Appl. Polym. Sci. 112 (4), 2401–2412. Westerberg, N., Sunner, H., Helander, M., Henriksson, G., 2012. Separation of galactoglucomannans, lignin, and lignin-carbohydrate complexes from hot-water-extracted Norway spruce by cross-flow filtration and adsorption chromatography. Bioresources 7 (4), 4501–4516. Willför, S., Rehn, P., Sundberg, A., Sundberg, K., Holmbom, B., 2003. Recovery of water-soluble acetylgalactoglucomannans from mechanical pulp of spruce. Tappi J. 2 (11), 27–32. Willför, S., Sundberg, A., Hemming, J., Holmbom, B., 2005. Polysaccharides in some industrially important softwood species. Wood Sci. Technol. 39 (4), 245–258. Willför, S., Sundberg, K., Tenkanen, M., Holmbom, B., 2008. Spruce-derived mannans—a potential raw material for hydrocolloids and novel advanced natural materials. Carbohydr. Polym. 72 (2), 197–210. Xu, C., Willför, S., Sundberg, K., Petterson, C., Holmbom, B., 2007. Physico-chemical characterization of spruce galactoglucomannan solutions: stability, surface activity and rheology. Cellulose Chem. Technol. 41 (1), 51–62. Örså, F., Holmbom, B., Thornton, J., 1997. Dissolution and dispersion of spruce wood components into hot water. Wood Sci. Technol. 31 (4), 279–290.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Extraction of galactoglucomannan from thermomechanical pulp mill process water by microfiltration and ultrafiltration—Influence of microfiltration membrane pore size on ultrafiltration performance Johan Thuvander, Ann-Sofi Jönsson ∗ Department of Chemical Engineering, Lund University, P.O. Box 124 SE-221 00 Lund, Sweden
a r t i c l e
i n f o
a b s t r a c t
Article history:
Galactoglucomannan is the major hemicellulose in process water from thermomechani-
Received 16 September 2015
cal pulping of spruce. The recovery of hemicelluloses from this process water would both
Received in revised form 12
reduce the load on pulp mill water treatment plants and yield a biopolymer suitable as
November 2015
a raw material for renewable products. One method of recovering these hemicelluloses is
Accepted 5 December 2015
two-stage filtration employing microfiltration to remove large contaminants, such as sus-
Available online 14 December 2015
pended matter and colloidal extractives, followed by ultrafiltration to concentrate and purify
Keywords:
a microfiltration membrane with a pore size of 0.1 m removes colloidal extractives while
Galactoglucomannan
still maintaining a high amount of high molecular mass hemicelluloses in the microfiltra-
Microfiltration
tion permeate. The removal of extractives resulted in an increase in the initial flux during
Ultrafiltration
the subsequent ultrafiltration step from 90 L/m2 h to over 200 L/m2 h.
Thermomechanical pulp
© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
the hemicelluloses from low molecular mass contaminants. This investigation shows that
SEC
1.
Introduction
The abundance of hemicellulose in biomass makes it an interesting raw material for biobased products. Galactoglucomannan (GGM) is the main hemicellulose in softwood and constitutes about 15–20% of the wood biomass of spruce (Sjöström, 1993; Willför et al., 2005). During the production of thermomechanical pulp (TMP) from spruce, about 10% of the GGM in the wood is dissolved in the process water (Thornton et al., 1994). Upgrading GGM to high-value-added products has attracted significant interest (Willför et al., 2008). Potential uses of GGM include barrier films (Hartman et al., 2006; Kisonen et al., 2014; Mikkonen et al., 2010), hydrogels (Lindblad et al., 2005; Voepel et al., 2009) and substrates for probiotic bacteria (Polari et al., 2012). Furthermore, extraction of GGM from the process water of TMP mills would also reduce
∗
the load on the water treatment plant of the mill. Another method of extracting GGM from wood is by hot-water extraction (Desharnais et al., 2011; Örså et al., 1997), which closely resembles the conditions during the production of TMP, but in this case the conditions for hemicellulose extraction can be optimized. To obtain a pure GGM fraction, other components present in the process water must be separated from the GGM. Contaminants that need to be removed include large components such as suspended solids and colloidal extractives, and low molecular mass contaminants such as lignin and salts. Suspended solids and colloidal extractives can be removed by coagulation with a cationic surfactant and deposition on TMP (Willför et al., 2003), or by microfiltration (MF) (Persson et al., 2010). The GGM can then be simultaneously concentrated and purified from low molecular lignin and salts using
Corresponding author. Tel.: +46 46 2228291. E-mail address: ann-sofi[email protected] (A.-S. Jönsson). http://dx.doi.org/10.1016/j.cherd.2015.12.003 0263-8762/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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ultrafiltration (UF) (Persson et al., 2010). Polymeric adsorbents can be used to remove lignans and lignin (Koivula et al., 2013; Westerberg et al., 2012; Willför et al., 2003) and when used before the UF step, they have been shown to improve UF performance (Koivula et al., 2013). Further purification can be achieved by diafiltration, size exclusion chromatography (SEC) (Andersson et al., 2007) or ethanol precipitation (Willför et al., 2003). A surprisingly high retention of GGM during MF was found in a previous study on the recovery of GGM from TMP mill process water by membrane filtration (Persson et al., 2010). It has been shown that the retention of hemicelluloses, and hence the loss of hemicelluloses, increases with decreasing MF pore size (Krawczyk and Jönsson, 2011). It was also shown that MF membranes with larger pores are more susceptible to fouling and exhibit lower fluxes than MF membranes with smaller pores. The higher flux but lower permeability of denser MF membranes to GGM means that the choice of MF membrane pore size is a trade-off between hemicellulose recovery and MF capacity. The MF pore size may also affect the performance of the subsequent UF concentration step, as both dissolved and colloidal extractives have been shown to foul UF membranes (Puro et al., 2002, 2011). To the best of our knowledge, the influence of MF pore size on UF performance has not been studied previously. The aim of this work was to investigate how the flux and recovery of GGM during UF of MF permeates are affected by MF membranes with pore sizes from 0.04 m to 0.5 m by determining the purity, recovery and molecular mass distribution of GGM after UF of MF permeates.
2.
Materials and methods
2.1.
Process water
Process water from a Swedish thermomechanical pulp mill with spruce as primary raw material was used in the investigation. Suspended solids and colloidal extractives were removed by MF at the mill. The nominal pore size of the ceramic MF membranes from LiqTech International A/S, Denmark, was 0.04 m, 0.1 m and 0.5 m. The volume of MF permeate produced with each MF membrane was about 1000 L for the 0.04 m and 0.1 m membranes and 300 L for the 0.5 m membrane. The MF permeates from the 0.04 m, 0.1 m and 0.5 m membranes are hereafter referred to as MF0.04, MF0.1 and MF0.5, respectively.
2.2.
stepwise increase in the transmembrane pressure from 1.5 bar to 4 bar. Permeate was withdrawn in the experiment with MF0.04 during the increase in pressure, while both retentate and permeate were recirculated to the feed tank in the experiments with MF0.1 and MF0.5. When 4 bar was reached, concentration also started in the MF0.1 and MF0.5 experiments. Concentration was performed in semi-batch mode by recirculating retentate to the feed tank while withdrawing permeate. The volume in the feed tank was kept constant by replenishing withdrawn UF permeate with fresh MF permeate until all the MF permeate had been transferred to the feed tank. Concentration then continued in batch mode until a final retentate volume of about 20 L was reached. This resulted in a final volume reduction (VR) during UF of MF0.04 and MF0.1 of 98%, and during UF of MF0.5 of 94%, corresponding to volume reduction factors of 50 and 17, respectively.
2.3.
Analytical methods
2.3.1.
Total solids and ash
Samples were dried for 24 h in an oven at 105 ◦ C. The content of total solids was determined from the weight of the residue after cooling to room temperature in a desiccator. The dry sample was further heated to 575 ◦ C, and this temperature was maintained for 3 h. The ash content was calculated from the weight of the residue after cooling to room temperature in a desiccator.
2.3.2.
Hemicelluloses
The concentration of hemicelluloses was determined by hydrolyzing the polysaccharide to monomeric sugars by acid hydrolysis using a standardized method for acid hydrolysis (Ruiz and Ehrman, 1996). Monomeric sugars were measured using high-performance anion-exchange chromatography coupled with pulsed amperometric detection in an ICS-3000 chromatography system (Dionex Corp., USA). The system was equipped with a Carbo Pac PA1 analytical column. A solution of 200 mM NaOH dissolved in 170 mM sodium acetate was used to clean the column. The sample injection volume was 10 L, and deionized water was used as eluent, at a flow rate of 1 mL/min. The monomeric sugars d-glucose, d-galactose, d-mannose, d-xylose and l-arabinose (Fluka Chemie AG, Switzerland) were used as standards. The concentration of hemicelluloses was defined as the sum of the monomeric sugars after anhydro corrections of 0.88 and 0.90 for pentoses and hexoses, respectively.
Experimental procedure during ultrafiltration 2.3.3.
The MF permeate was concentrated by UF using a spiralwound UF membrane (UFX5pHt, Alfa Laval Nakskov A/S, Denmark) with a nominal molecular mass cut-off of 5 kDa. The spiral-wound element was equipped with a 48-mil spacer (1.2 mm), and had an outer diameter of 64 mm and a length of 0.43 m. Persson and Jönsson (2010) have previously shown that this membrane gives a high flux and good GGM yield during UF of TMP mill process water. The temperature was 60 ◦ C and the cross-flow was 1.3 m3 /h during all UF experiments. The crossflow was chosen so that the maximum frictional pressure drop in the spiral-wound element would not exceed 0.6 bar. Before the experiment, the membrane was cleaned with a 0.4 wt% solution of the alkaline cleaning agent Ultrasil 10 (Ecolab AB, Sweden) at 50 ◦ C and 1 bar transmembrane pressure for 1 h. A 200 L feed tank was temperature controlled using a steam-heated copper coil. The experiment started with
Lignin
The total lignin content was determined by measuring the light absorption at a wavelength of 280 nm, using a UV-160 spectrophotometer (Shimadzu, Japan) and an absorption coefficient of 17.8 L/(g cm) (Örså et al., 1997).
2.3.4. lignin
Molecular mass distribution of hemicelluloses and
The molecular mass distribution of hemicelluloses and lignin was determined by SEC using a Waters 600E chromatography system (Waters, USA) equipped with a refractive index (RI) detector (model 2414, Waters) and a UV detector (model 486, Waters). The analytical column was packed with 30 cm Superdex 30 and 30 cm Superdex 200 (GE Healthcare, Sweden). The injection volume was 500 L. A 125 mM NaOH solution was used as eluent at a flow rate of 1 mL/min. The system was calibrated with polyethylene glycol standards with
173
Table 1 – Characteristics of MF permeates.
Total solids (g/L) Ash (g/L) Turbidity (NTU) Lignin (g/L) Hemicelluloses (g/L) - Arabinan (g/L) - Galactan (g/L) - Glucan (g/L) - Xylan (g/L) - Mannan (g/L) Hemicellulose purity (%)
MF0.04
MF0.1
MF0.5
4.7 2.1 11 0.89 0.85 0.07 0.11 0.17 0.01 0.48 18
4.9 1.5 14 1.00 1.3 0.08 0.16 0.26 0.02 0.78 27
4.7 1.2 105 0.88 1.5 0.11 0.21 0.30 0.02 0.88 32
a)
500
Refractive index (mV)
chemical engineering research and design 1 0 5 ( 2 0 1 6 ) 171–176
400
UV absorbance (AU)
Turbidity
The turbidity was determined at room temperature using a turbidimeter (2100P ISO, HACH Co., USA). Formazin standards (also from HACH) of 0, 20, 100 and 800 nephelometric turbidity units (NTU) were used for calibration.
Microfiltration permeates
The total solids content and the lignin concentration were similar in MF0.04 and MF0.5, and slightly higher in MF0.1, as can be seen from Table 1. The concentration of hemicelluloses increased with increasing membrane pore size. MF0.5 is thus the permeate with the highest hemicellulose purity when purity is defined as the ratio between the amount of hemicelluloses and the content of total solids. However, the turbidity of MF0.5 was higher than for the other permeates. Turbidity is a good indicator of colloidal extractives in process water from the production of TMP (Sundberg et al., 1993). The high turbidity of MF0.5 thus indicates that MF0.5 was contaminated with colloidal extractives.
3.2.
Molecular mass distribution of MF permeates
A high recovery of high molecular mass GGM when isolating GGM from TMP mill process water is desirable as a high molecular mass improves the properties of GGM films (Kisonen et al., 2014). The molecular mass distribution of the MF permeates is shown in Fig. 1. All three permeates had RI peaks at about 0.2 and 0.8 kDa, and in the interval 10–20 kDa. MF0.5 showed an additional peak above 100 kDa, which was also visible in the UV absorption chromatogram. The RI and UV peaks below 100 kDa corresponded well with molecular mass distributions in TMP mill process water reported by Krawczyk and Jönsson (2011), whereas the peak above 100 kDa has not been reported previously. Colloidal extractives that were not retained by the 0.5 m MF membrane (seen as an elevated turbidity in Table 1) are probably responsible for the mass fraction above 100 kDa, as hemicelluloses have been found to adsorb onto the surface of colloidal extractives (Stack et al., 2014). It should be noted that the SEC column was not calibrated for substances with molecular mass higher than 35 kDa PEG equivalents, and thus sizes above this value are uncertain.
10.0
100.0
1,000.0
0.8 0.6 0.4 0.2
1.0
10.0
100.0
1,000.0
Molecular mass (kDa) Fig. 1 – Molecular mass distribution of (a) sugars (measured as refractive index) and (b) lignin (measured as UV absorbance at 280 nm) in MF0.04 (unbroken line), MF0.1 (dashed line) and MF 0.5 (dotted line).
3.3.
Flux during UF
The flux during concentration of MF0.04 and MF0.1 was almost identical, whereas the flux of MF0.5 was markedly lower, as shown in Fig. 2. Permeate was withdrawn already during the initial stepwise increase in pressure in the experiment with MF0.04, which is why the first flux was recorded at VR = 20% for this permeate. The flux when starting concentration at 4 bar with MF0.1 and MF0.5 was 226 L/m2 h and 92 L/m2 h, respectively. The decline in flux when filtering MF0.04 and MF0.1 was quite similar. When concentration was complete at VR = 98% the flux was 30 L/m2 h for MF0.04 and 20 L/m2 h for MF0.1. The final flux for MF0.5 at a VR of 94% was 30 L/m2 h. The low flux during UF of MF0.5, compared to MF0.04 and MF0.1, is probably due to the high amount of colloidal 250 200 2
3.1.
1.0
1.0
0.0 0.1
Flux (l/m h)
Results and discussion
100
Molecular mass (kDa)
peak molecular masses of 0.4, 4, 10 and 35 kg/mol (Merck Schuchardt OHG, Germany). Samples were filtered through a 0.2 m filter (Schleicher & Schuell, Germany) before determination of molecular mass distribution.
3.
200
0 0.1
b)
2.3.5.
300
150 100 50 0
0
20
40
60
80
100
Volume reduction (%) Fig. 2 – Influence of volume reduction on flux during concentration of MF0.04 (), MF0.1 () and MF0.5 (). The temperature was 60 ◦ C, the transmembrane pressure was 4 bar and the cross-flow was 1.3 m3 /h.
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Hemicellulose retention (%)
a) 100 90 80 70 60
0
20
40
60
80
100
Volume reduction (%)
b) Hemicellulose retention (%)
100 90 80
3.5.
70 60
1
10
100
Hemicellulose concentration (g/l) Fig. 3 – Influence of (a) volume reduction and (b) concentration on hemicellulose retention during UF of MF0.04 (), MF0.1 () and MF0.5 (). extractives in MF0.5. Fluxes in this investigation can be compared with those reported previously by Persson et al. (2010), where a 0.2 m MF membrane was used to remove colloidal and suspended matter from TMP mill process water before concentration of hemicelluloses by UF. In the study of Persson et al. the initial hemicellulose concentration was 0.83 g/L and UF was performed at 80 ◦ C. The initial flux was lower, about 130 L/m2 h, in spite of the lower concentration and higher temperature, than during UF of MF0.1. This suggests that by using an MF membrane with a pore size smaller than 0.2 m, compounds that severely limit the UF flux are retained and not found in the MF permeate.
3.4.
distribution of hemicellulose molecules in TMP mill process water, as shown in Fig. 1. Low molecular mass molecules have a lower retention and are withdrawn with the permeate, leading to an increase in the mean molecular size in the retentate as concentration proceeds. This results in an increase in retention during concentration. MF0.5 contained a higher amount of high molecular mass hemicelluloses than the other MF permeates, as well as colloidal extractives with adsorbed hemicelluloses. These colloids are completely retained by the UF membrane, contributing to a high initial retention of hemicelluloses. MF0.04 had the highest amount of low molecular mass hemicelluloses, and consequently the lowest hemicellulose retention at the beginning of concentration. However, the retention of hemicelluloses was similar during UF of MF0.04 and MF0.1 at similar concentrations of hemicelluloses, as shown in Fig. 3b and the retention was >95% at the final VR during UF of all three MF permeates.
Retention of hemicelluloses during UF
The retention of hemicelluloses increased with increasing VR and concentration of hemicelluloses, as shown in Fig. 3. Retention increases during concentration because of the wide size
Retentate and permeate composition
The concentration of hemicelluloses in the retentate after UF of MF0.04, MF0.1 and MF0.5 was 33 g/L, 52 g/L and 25 g/L, respectively. The concentration of hemicelluloses in the collected permeate was about 0.3 g/L in all three permeates, as can be seen from Table 2. The concentration of lignin in the retentate was 4 g/L (MF0.04 and MF0.1) and 3 g/L (MF0.5). The highest hemicellulose purity was obtained after UF of MF0.1. The hemicellulose purity was then 68%. This was a consequence of MF0.1 having a higher starting purity than MF0.04 and reaching a higher VR than MF0.5. Higher purities can be reached by using additional purification steps. Using diafiltration or SEC Andersson et al. (2007) obtained a hemicellulose purity of 80% and Willför et al. (2003) reported a GGM purity of 95 mole% when using polymeric adsorbents and ethanol precipitation. The flux was still relatively high at the end of the UF concentration when the dead volume of the membrane equipment was reached. An estimate of how much the hemicelluloses could have been concentrated if the UF concentration had been allowed to proceed further can be obtained by applying the film model (Blatt et al., 1970). The theoretical, maximal hemicellulose concentrations obtained using the film model are 111 g/L (MF0.04), 110 g/L (MF0.1) and 107 g/L (MF0.5). Other techniques, such as, ethanol precipitation and spray drying, can be used to increase the hemicellulose concentration further (Xu et al., 2007).
Table 2 – Characteristics of retentate and permeate after UF of MF0.04, MF0.1 and MF0.5. MF0.04
Total solids (g/L) Ash (g/L) Lignin (g/L) Hemicelluloses (g/L) - Arabinan (g/L) - Galactan (g/L) - Glucan (g/L) - Xylan (g/L) - Mannan (g/L) Hemicellulose purity (%)
MF0.1
MF0.5
Retentate VR 98%
Total perm.
Retentate VR 98%
Total perm.
Retentate VR 94%
Total perm.
52 3.2 4.0 33 0.8 4.2 6.0 0.2 22 63
4.0 2.0 0.87 0.33 0.06 0.04 0.08 0.01 0.15
77 2.3 4.1 52 1.2 6.6 9.2 0.5 35 68
3.4 1.4 0.97 0.35 0.06 0.04 0.08 0.01 0.17
40 1.8 3.1 25 0.8 3.6 4.8 0.2 15 61
2.8 1.2 0.78 0.25 0.07 0.03 0.05 0.01 0.10
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a)
400
Recovery of GGM (%)
Refractive index (mV)
a)
300 200 100 0 0.1
1.0
10.0
100.0
100 90 80 70 60 50
1,000.0
0
0.25 0.20
0.10 0.05
1.0
10.0
100.0
1,000.0
Molecular mass (kDa) Fig. 4 – Molecular mass distribution of (a) sugars (measured as refractive index) and (b) lignin (measured as UV absorbance) in the retentate after UF of MF0.04 (unbroken line), MF0.1 (dashed line) and MF 0.5 (dotted line). Samples were diluted 10 times before analysis.
3.6.
Molecular mass distribution
The molecular mass distribution of hemicelluloses and lignin in the UF retentate is shown in Fig. 4. Comparison with Fig. 1 shows that the amount of low molecular mass hemicelluloses is markedly reduced and the high molecular mass fraction of hemicelluloses with a peak at about 10 kDa is now the dominating mass fraction (as can be seen in Fig. 4a). The colloidal extractives present in MF0.5 were also concentrated in the retentate during UF. As the amount of small lignin molecules is lower after UF, a small peak in the UV absorption chromatogram at around 20 kDa becomes visible in Fig. 4b. This peak is also present in Fig. 1, but is not visible because of the relatively high amount of low molecular mass lignin in the samples at 0% VR. In the native lignocellulosic matrix, hemicelluloses are crosslinked by lignin (Lawoko et al., 2006). It is possible that the UV response at 20 kDa is the result of a small amount of aromatic residues attached to the larger hemicelluloses originating from these lignin-hemicellulose bonds.
3.7.
40
60
80
100
80
100
b)
0.15
0.00 0.1
20
Volume reduction (%)
Recovery of hemicelluloses (%)
b) UV absorbance (AU)
Molecular mass (kDa)
Fractionation of sugars
The main hemicelluloses in softwood consist of a glucomannan backbone with acetyl groups and galactose residues attached. Other hemicelluloses in softwood include arabinoglucuronoxylan and arabinogalactan (Sjöström, 1993). Krawczyk and Jönsson (2011) showed that different molecular mass fractions of the components in process water from the production of TMP consist of different types of polysaccharides. GGM was found to be the major polysaccharide in the mass fraction above 7.7 kDa. The relative amount of GGM was found to decrease to 60–80% in the fraction between 1
100 90 80 70 60 50
0
20
40
60
Volume reduction (%) Fig. 5 – Influence of volume reduction on recovery of (a) GGM and b) hemicelluloses during UF of MF0.04 (), MF0.1 () and MF0.5 (). and 7.7 kDa, while the fraction below 1 kDa consisted almost entirely of other polysaccharides. As a consequence, the retention of mannan will be higher than of other sugars, as found by Persson et al. (2010). This was also seen in the present study as the relative amount of mannan increased in the retentate while the relative amount of arabinan and xylan was higher in permeates, as seen in Table 2. During UF, the GGM was thus not only purified from lignin and salts, but also from other low molecular mass hemicelluloses.
3.8.
Recovery of hemicelluloses during UF
The recovery of hemicelluloses decreased with increasing VR during UF, as shown in Fig. 5. As the retention of mannan is higher than that of other sugars, the recovery of GGM will be higher than that of hemicelluloses as a group. The recovery of GGM, based on the content of mannan, was 70%, 79% and 90% (Fig. 5a) while the recovery of hemicelluloses was 62%, 74% and 84% (Fig. 5b). The higher recovery of hemicelluloses during UF of MF0.5 is partly due to the lower VR used in this experiment, and partly due to the higher amount of high molecular mass hemicelluloses, and hence, higher retention. The higher recovery of hemicelluloses during UF of MF0.1, compared with MF0.04, is due to the greater amount of large hemicelluloses in MF0.1. The lower recovery arising from operating UF at high VR may appear discouraging. However, as VR increases, the large molecular mass fraction of hemicelluloses with an average size of about 10 kDa increases compared with the mass fraction of low molecular mass hemicelluloses, as shown in Fig. 6. It should be noted that the samples were diluted so that all the results would be accommodated in Fig. 6. The ratio between the dilutions at VR = 0%:VR = 85%:VR = 98% in Fig. 6 was 1:2:10. The benefits of operating the UF at a high VR is thus a purer
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Refractive index (mV)
500 400 300 200 100 0 0.1
1.0
10.0
100.0
1,000.0
Molecular mass (kDa) Fig. 6 – Molecular mass distribution of sugar (measured as refractive index) in the retentate during UF of MF0.1 at VR 0% (unbroken line), 85% (dashed line) and 98% (dotted line). The retentate solutions at VR 85% and 98% were diluted 2 and 10 times, respectively, prior to analysis. and more concentrated product, but at the cost of the mean flux and yield.
4.
Conclusions
The content of colloidal extractives in the MF permeate could be greatly reduced by using an MF membrane with a pore size no larger than 0.1 m. This results in a substantial increase in the UF flux, from 90 L/m2 h to over 200 L/m2 h at 4 bar and 60 ◦ C. Smaller pore size of the MF membrane reduces the fraction of large molecules in the MF permeate, resulting in lower retention and recovery of hemicelluloses during UF. The pore size of the MF membrane must thus be just small enough to retain colloidal extractives to optimize the recovery of hemicelluloses.
References Andersson, A., Persson, T., Zacchi, G., Stålbrand, H., Jönsson, A.-S., 2007. Comparison of diafiltration and size-exclusion chromatography to recover hemicelluloses from process water from thermomechanical pulping of spruce. Appl. Biochem. Biotechnol. 137, 971–983. Blatt, W.F., Dravid, A., Michaels, A.S., Nelsen, L., 1970. Solute polarization and cake formation in membrane ultrafiltration: causes, consequences, and control techniques. In: Flinn, J.E. (Ed.), Membrane Science Technology. Plenum Press, New York, pp. 47–97. Desharnais, L., Du, F., Brosse, N., 2011. Optimization of Galactoglucomannans and acidic arabinans recovery in softwood. Ind. Eng. Chem. Res. 50 (24), 14217–14220. Hartman, J., Albertsson, A.-C., Sjöberg, J., 2006. Surface- and bulk-modified galactoglucomannan hemicellulose films and film laminates for versatile oxygen barriers. Biomacromolecules 7 (6), 1983–1989. Kisonen, V., Xu, C., Bollström, R., Hartman, J., Rautkoski, H., Nurmi, M., Hemming, J., Eklund, P., Willför, S., 2014. O-acetyl galactoglucomannan esters for barrier coatings. Cellulose 21 (6), 4497–4509. Koivula, E., Kallioinen, M., Sainio, T., Antón, E., Luque, S., Mänttäri, M., 2013. Enhanced membrane filtration of wood hydrolysates for hemicelluloses recovery by pretreatment with polymeric adsorbents. Bioresour. Technol. 143, 275–281. Krawczyk, H., Jönsson, A.-S., 2011. Separation of dispersed substances and galactoglucomannan in thermomechanical pulp process water by microfiltration. Sep. Purif. Technol. 79 (1), 43–49. Lawoko, M., Henriksson, G., Gellerstedt, G., 2006. Characterisation of lignin-carbohydrate complexes (LCCs) of spruce wood (Picea
abies L.) isolated with two methods. Holzforschung 60 (2), 156–161. Lindblad, M.S., Albertsson, A.-C., Ranucci, E., Laus, M., Giani, E., 2005. Biodegradable polymers from renewable sources: rheological characterization of hemicellulose-based hydrogels. Biomacromolecules 6 (2), 684–690. Mikkonen, K.S., Heikkilä, M.I., Helén, H., Hyvönen, L., Tenkanen, M., 2010. Spruce galactoglucomannan films show promising barrier properties. Carbohydr. Polym. 79 (4), 1107–1112. Persson, T., Jönsson, A.-S., 2010. Isolation of hemicelluloses by ultrafiltration of thermomechanical pulp mill process water—influence of operating conditions. Chem. Eng. Res. Des. 88 (12), 1548–1554. Persson, T., Krawczyk, H., Nordin, A.-K., Jönsson, A.-S., 2010. Fractionation of process water in thermomechanical pulp mills. Bioresour. Technol. 101 (11), 3884–3892. Polari, L., Ojansivu, P., Mäkelä, S., Eckerman, C., Holmbom, B., Salminen, S., 2012. Galactoglucomannan extracted from spruce (Picea abies) as a carbohydrate source for probiotic bacteria. J. Agric. Food Chem. 60 (44), 11037–11043. Puro, L., Tanninen, J., Nyström, M., 2002. Analyses of organic foulants in membranes fouled by pulp and paper mill effluent using solid–liquid extraction. Desalination 143 (1), 1–9. Puro, L., Kallioinen, M., Mänttäri, M., Nyström, M., 2011. Evaluation of behavior and fouling potential of wood extractives in ultrafiltration of pulp and paper mill process water. J. Membr. Sci. 368, 150–158. Ruiz, R., Ehrman, T., 1996. Laboratory Analytical Procedure LAP-014: Dilute Acid Hydrolysis Procedure for Determination of Total Sugars in the Liquid Fraction of Process Samples. NREL, Midwest Research Institute for the Department of Energy, USA. Sjöström, E., 1993. Wood Chemistry—Fundamentals and Applications. Academic Press, San Diego, USA. Stack, K., Lee, R., Richardson, D., Lewis, T., Garnier, G., 2014. Complex formation and stability of colloidal wood resin pitch suspensions with hemicellulose polymers. Colloids Surf. A 441, 101–108. Sundberg, A., Ekman, R., Holmbom, B., Sundberg, K., Thornton, J., 1993. Interactions between dissolved and colloidal substances and a cationic fixing agent in mechanical pulp suspensions. Nord Pulp Pap. Res. J. 8 (1), 226–231. Thornton, J., Ekman, R., Holmbom, B., Örså, F., 1994. Polysaccharides dissolved from Norway spruce in thermomechanical pulping and peroxide bleaching. J. Wood Chem. Technol. 14 (2), 159–175. Voepel, J., Sjöberg, J., Reif, M., Albertsson, A.-C., Hultin, U.-K., Gasslander, U., 2009. Drug diffusion on neutral and ionic hydrogels assembled from acetylated galactoglucomannan. J. Appl. Polym. Sci. 112 (4), 2401–2412. Westerberg, N., Sunner, H., Helander, M., Henriksson, G., 2012. Separation of galactoglucomannans, lignin, and lignin-carbohydrate complexes from hot-water-extracted Norway spruce by cross-flow filtration and adsorption chromatography. Bioresources 7 (4), 4501–4516. Willför, S., Rehn, P., Sundberg, A., Sundberg, K., Holmbom, B., 2003. Recovery of water-soluble acetylgalactoglucomannans from mechanical pulp of spruce. Tappi J. 2 (11), 27–32. Willför, S., Sundberg, A., Hemming, J., Holmbom, B., 2005. Polysaccharides in some industrially important softwood species. Wood Sci. Technol. 39 (4), 245–258. Willför, S., Sundberg, K., Tenkanen, M., Holmbom, B., 2008. Spruce-derived mannans—a potential raw material for hydrocolloids and novel advanced natural materials. Carbohydr. Polym. 72 (2), 197–210. Xu, C., Willför, S., Sundberg, K., Petterson, C., Holmbom, B., 2007. Physico-chemical characterization of spruce galactoglucomannan solutions: stability, surface activity and rheology. Cellulose Chem. Technol. 41 (1), 51–62. Örså, F., Holmbom, B., Thornton, J., 1997. Dissolution and dispersion of spruce wood components into hot water. Wood Sci. Technol. 31 (4), 279–290.