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Centrifugation as pretreatment before ultrafiltration of hemicelluloses extracted from wheat bran
Separation and Purification Technology 138 (2014) 1–6
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Centrifugation as pretreatment before ultrafiltration of hemicelluloses extracted from wheat bran J. Thuvander, A. Arkell, A.-S. 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
Article history: Received 26 June 2014 Received in revised form 7 October 2014 Accepted 8 October 2014
Keywords: Prefiltration Centrifugation Ultrafiltration Viscosity Hemicelluloses
a b s t r a c t Hemicelluloses recovered by alkaline extraction of wheat bran can be isolated and purified by ultrafiltration and used to produce high-value products such as barrier films for food packaging. The flux during ultrafiltration depends to a large extent on the pretreatment of the solution. It has been shown previously that dead-end filtration with kieselguhr can increase the flux during ultrafiltration. In the present study, pretreatment by centrifugation and a combination of centrifugation and dead-end filtration was compared with dead-end filtration alone. Centrifugation reduced the turbidity and viscosity of the solution but did not significantly improve the flux during ultrafiltration. The flux of the untreated and the centrifuged solutions was about 65 l/m2 h at 1 bar transmembrane pressure. The flux after dead-end filtration was 150 l/m2 h. After combining centrifugation and dead-end filtration the flux was 200 l/ m2 h. In addition to the higher UF flux, filtration resistance during dead-end filtration was markedly reduced after pre-centrifugation of the solution. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Hemicelluloses from lignocellulosic materials can be used to produce polymer films. These renewable films have properties that make them interesting as barrier materials for food packaging [1–5]. One promising raw material for barrier films is wheat bran, an agricultural residue that can contain up to 30% of the hemicellulose arabinoxylan [6]. An effective and cost-efficient extraction and purification process is necessary when using hemicelluloses as raw material in the production of barrier films. Examples of extraction methods that can be used are alkali extraction and hot water extraction [7–10]. High yield and low polymer degradation have been achieved with alkali extraction [7]. The extract obtained contains not only arabinoxylan, but a mixture of compounds that makes further purification of the arabinoxylan solution necessary, before it can be used to produce a high-value product. One way of purifying the hemicelluloses in the alkaline extract is ultrafiltration (UF) [11–15]. Membrane filtration is an attractive separation method because of its high energy efficiency and selectivity. However, to ensure high flux during UF, as well as high product quality, the solution must be pretreated prior to UF to remove particulate matter and other compounds that may foul the UF membrane. ⇑ Corresponding author. Tel.: +46 46 222 8291; fax: +46 46 222 4526. E-mail address: ann-sofi[email protected] (A.-S. Jönsson). http://dx.doi.org/10.1016/j.seppur.2014.10.003 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
High viscosity can severely reduce the flux in UF [16–18]. It is therefore advantageous if the pretreatment method lowers the viscosity of the solution. It has been shown that heat treatment of the alkali extract lowers the viscosity and increases the flux during UF [18]. However, the degradation of arabinoxylan during heat treatment was considered too great to motivate the increase in flux. The viscosity can also be reduced by dead-end filtration (DEF), which simultaneously removes suspended solids from the solution. In a previous study we found that DEF with kieselguhr as a filter aid increased the flux during UF threefold [17]. However, a large amount of filter aid is needed because of the high filter cake resistance. The aim of this study was to investigate whether centrifugation can be used to remove suspended solids from the solution and thus improve the UF flux. The influence of three pretreatment methods on UF flux was studied: (1) increasing the residence time in the nozzle separator already used for pretreatment of the hemicellulose solution, (2) removing dense components using high-speed centrifugation, and (3) centrifugation prior to DEF in order to lower the filter cake resistance during DEF. 2. Materials and methods Two batches of a hemicellulose-rich solution were obtained by alkali extraction of wheat bran. The composition of the solutions is found in Table 1. After alkali extraction, the cellulose-rich solid fraction was removed by centrifugation in a continuous Clara
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20 low-flow nozzle separator (Alfa Laval, Tumba, Sweden) at a flow rate of 300 l/h. The fixed gravitational force of the Clara 20 separator is 11 130 g. A flow rate of 300 l/h corresponds to solid flush every 10 min. The solution from the first batch is hereafter referred to as reference solution 1 and the solution from the second batch is referred to as reference solution 2. 2.1. Pretreatment methods 2.1.1. High-residence time in the existing nozzle separator When producing reference solution 1, another solution from the same batch was produced using a higher residence time in the nozzle separator (called the HRT solution) by decreasing the flow rate to 65 l/h (solid flush every 15 min). 2.1.2. High-speed centrifugation High-speed centrifugation of reference solution 2 was carried out at ambient temperature in a Sorval Lynx 4000 centrifuge (Thermo Scientific, Waltham, USA). The influence of gravitational force and centrifugation time was investigated by adding the solution to 300 ml plastic bottles so that the total weight of the bottle and solution was 250 g. The samples were centrifuged at 10 000g, 20 000g and 30 000g for 5 min, 10 min and 15 min. After centrifugation, 150 ml of the supernatant was transferred to new bottles for later analysis. High-speed centrifuged (HSC) solution, to be used for UF studies and pretreatment before DEF, was prepared in 1-l plastic bottles. Solution was added to the bottles so that the total weight of the bottle and solution was 800 g. After centrifugation for 15 min at 20 000 g, the supernatant was carefully recovered, and the pellet was discarded. 2.1.3. Combination of centrifugation and dead-end filtration DEF was carried out using a PF 0.1 H2 filter press (Larox Corp., Lappeenranta, Finland). This filter press has a 100-l feed tank with an agitator and an air-driven slurry pump (Wilden M2-P Champ, Wilden Pump & Engineering Co., Colton, USA). A 10 lm filter cloth (Hydrotech AB, Vellinge, Sweden) was used, with a filtration area of 0.1 m2. The feed tank was equipped with a Pt-100 temperature sensor (Pentronic, Gunnebo, Sweden) and electrical heaters connected to a temperature regulator (Shinko MCM, Shinko Europe BV, Haarsteeg, the Netherlands) for temperature control. Kieselguhr (2 wt%) (Dicalite, Burney, USA) was added to the solution as a filter aid before heating the solution to the filtration temperature of 50 °C. Filtration was carried out at 4 bar. The filter cake resistance was calculated using the constant-pressure filtration equation [19]:
t l Rm lac ¼ þ V V A DP 2 A2 DP
ð1Þ
where t (s) is the filtration time, V (m3) the filtrate volume, l (Pa s) the viscosity of the filtrate, Rm (m1) the hydraulic resistance of the filter cloth, A (m2) the area of the filter, DP (Pa) the pressure drop across the filter cake, a (m/kg) the specific filter cake resistance, and c (kg/m3) the mass of filter-cake-forming particles per unit volume of the filtrate. 2.2. Ultrafiltration 2.2.1. Membrane The UF membrane used was a seven-channel tubular ceramic membrane (Atech Innovations GmbH, Gladbeck, Germany) made of a-Al2O3, with an active layer ofTiO2. The molecular mass cutoff of the membrane was 10 kDa. The diameter of the channels was 6 mm and the membrane was 1 m long, giving a total membrane area of 0.132 m2. The end-cap sealing material was
Table 1 Characteristics of the solutions treated in the nozzle separator at 300 l/h (reference solution 1 and 2) and the high-residence time (HRT) solution treated at a flow rate of 65 l/h (same batch as reference solution 1). The viscosity was measured at 80 °C and a shear rate of 100 s1.
Total solids (g/l) NaOH (g/l) Hemicelluloses (g/l) -Arabinan (g/l) -Galactan (g/l) -Glucan (g/l) -Xylan (g/l) Acid-insoluble solids (g/l) Acid-soluble lignin (g/l) Turbidity (NTU) Viscosity (mPa s)
Reference solution 1
HRT solution
Reference solution 2
42 34 6.9 1.7 0.2 1.3 3.7 2.6 0.20 340 2.7
40 31 6.2 1.6 0.1 1.2 3.3 2.3 0.20 270 1.9
48 38 7.9 1.9 0.1 1.5 4.5 2.8 0.25 550 3.6
poly(tetrafluoroethene) (PTFE) in order to withstand the pH and temperature of the solution. 2.2.2. UF set-up A schematic illustration of the experimental set-up is shown in Fig. 1. The set-up consisted of two 200-l tanks. Tank 1 was used as the feed tank during the experiments, and tank 2 was used during start-up and membrane cleaning. Both tanks were equipped with a Pt-100 temperature sensor for temperature measurement. The contents of the tanks were heated with 5-bar steam condensing in heat exchanger coils inside the tanks. The membrane was fitted in an M1 module (Atech innovations GmbH, Gladbeck, Germany). The set-up included a feed pump (Hydra-cell D25XL, Wanner, Minneapolis, USA) and a circulation pump (NB32/25-20, ABS Pump Production, Mölndal, Sweden). The feed flow was measured with a volume flow meter (Fischer&Porter Co. Ltd., Göttningen, Germany), and the permeate flow was measured with an electronic balance (PL6001-S, Mettler-Toledo Inc., Columbus, USA). The feed pressure (PF), the retentate pressure (PR) and the permeate pressure (PP) were measured with pressure transmitters (dTrans p02, Jumo AB, Helsingborg, Sweden). The transmembrane pressure (TMP) was calculated from the relation:
TMP ¼
PF þ PR PP 2
ð2Þ
2.2.3. Membrane cleaning The membrane was cleaned before and after each experiment. The membrane was cleaned with a 0.5 wt% solution of the alkaline cleaning agent Ultrasil 11 (Ecolab AB, Älvsjö, Sweden) at a TMP of 0.8 bar and a cross-flow velocity (CFV) of 4 m/s for 1 h. The membrane was then rinsed with deionized water to remove any residual cleaning agent. After the cleaning cycle the pure water flux of the membrane was measured. A 0.1 M NaOH solution was circulated in the system prior to the experiment to equilibrate the membrane to the pH of the process solution. After the experiment, the solution was rinsed out of the system with a 0.1 M NaOH solution, before the membrane was cleaned as described above. 2.2.4. Ultrafiltration UF experiments were performed on 75-l of the studied solution at a temperature of 80 °C and a cross-flow velocity of 4 m/s. Both permeate and retentate were returned to the feed tank during the experiments to keep the feed concentration constant. Flux was studied as a function of TMP, starting at a TMP of 0.4 bar. The flux was recorded for 15 min before feed and permeate samples were withdrawn for analysis. The TMP was increased in steps of 0.2 bar to 1.8 bar, or until the flux became independent of pressure.
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P P
Tank 2
Membrane module
Tank 1
Balance
T
T P
F Feed pump
Circulaon pump
Fig. 1. Schematic illustration of the experimental set-up used in the UF experiments.
2.3. Analytical methods 2.3.1. Total solids, ash and sodium hydroxide Samples were dried in porcelain crucibles at 105 °C for 24 h. The dry weight of the sample residue was determined as the difference in weight before and after drying. The ash content of the sample was determined by weighing the residue again after it had been heated to 575 °C and maintained at this temperature for 3 h. The sodium hydroxide content was determined by multiplying the ash content by a factor of 1.29 [16]. 2.3.2. Hemicelluloses The polysaccharides in the samples were degraded using a standardized method for acid hydrolysis [20]. The concentration of monomeric sugars was measured using high-performance anionexchange chromatography coupled with pulsed amperometric detection in an ICS-3000 chromatography system (Dionex Corp., Sunnyvale, USA). Deionized water was used as eluent at a flow rate of 1 ml/min. D-galactose, D-glucose, D-mannose, D-xylose and Larabinose (Fluka Chemie AG, Buchs, Switzerland) were used as standards. The concentration of hemicelluloses was determined from the sum of the monomeric sugars after anhydro corrections of 0.88 for pentoses and 0.90 for hexoses. 2.3.3. Acid-insoluble solids and acid-soluble lignin The samples were subjected to acid hydrolysis [20]. The content of acid-insoluble solids in the samples was determined using a filter crucible with a maximal pore size of 16 lm to capture the precipitate, which was dried in an oven at 105 °C for 24 h and then weighed. The content of acid-soluble lignin was determined by measuring the UV absorbance of the filtrate at 320 nm with a spectrophotometer (UV-160, Shimadzu Corp., Kyoto, Japan). An extinction coefficient of 30 l/g cm was used [21]. 2.3.4. Turbidity The turbidity was measured in a turbidimeter (2100P ISO, HACH Co., Loveland, USA). Formazin standards from HACH of 0, 20, 100 and 800 nephelometric turbidity units (NTU), were used to calibrate the instrument. 2.3.5. Viscosity Viscosity measurements were performed using a Malvern Kinexus Pro rotational rheometer (Malvern Instruments Ltd.,
Worcestershire, UK). A cup-and-bob geometry was used, and the apparent viscosity was measured at a temperature of 80 °C at a shear rate of 100 s1.
3. Results and discussion 3.1. Increased residence time in the nozzle separator The composition of the two process solutions resulting from different residence times in the nozzle separator is given in Table 1. The main part of the dry solids in the hemicellulose solution was sodium hydroxide, which originates from the extraction liquid. The dominating hemicellulose component was xylan, followed by arabinan. The main components in the acid-insoluble solids fraction are lignin, proteins and fats, which account for 3–9%, 14– 18% and 4% of the mass of the wheat bran, respectively [8,9,13]. Increasing the residence time in the nozzle separator slightly reduced the concentration of all components. The turbidity was reduced by 20% and the viscosity by 30% when prolonging the residence time. Increasing the residence time seemed thus to offer a means of reducing the amount of suspended solids in the process solution. The flux during UF of reference solution 1 and the HRT solution was similar, as can be seen in Fig. 2. The flux levelled off at about 65 l/m2 h, which is in accordance with the results obtained when treating an alkaline extract of wheat bran at corresponding operating conditions in our previous study [17]. From the results of this experiment it can be concluded that insufficient suspended solids were removed by increasing the residence time, or the suspended solids that were removed were not responsible for the flux-reducing effect during UF. 3.2. High-speed centrifugation A parametric study was conducted to study the efficiency of suspended solids removal by high-speed centrifugation. A new batch of reference solution (reference solution 2) was produced as described above. This had a turbidity of 550 NTU and a viscosity of 3.6 mPa s, and was thus more turbid and viscous than reference solution 1 (340 NTU and 2.7 mPa s). A comparable solution used in our previous investigation had a turbidity of 690 NTU and viscosity of 4.8 mPa s [17], showing that there is considerable variation in both turbidity and viscosity between batches.
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Fig. 2. Flux during UF of reference solution 1 (s) and the high-residence time (HRT) solution (j) at 80 °C and a CFV of 4 m/s.
Neither gravitational force (10 000–30 000g), nor centrifugation time (5 min to 15 min) did significantly affect the total solids content or the concentration of hemicelluloses and lignin, whereas the turbidity (see Fig. 3) decreased with increasing gravitational force and centrifugation time. After 15 min of centrifugation at 30 000g, the turbidity was 100 NTU. It is thus possible to separate part of the suspended solids from the solution by high-speed centrifugation. The viscosity of the supernatant was only influenced to a minor extent by centrifugation; decreasing from 3.6 mPa s to 2.8 mPa s, after 5 min of centrifugation at 10 000g. Prolonged centrifugation and/or higher gravitational force had only a minor additional effect on viscosity, as can be seen in Fig. 4. After 15 min of centrifugation at 30 000g the solution had a viscosity of 2.6 mPa s, which is equivalent to a 27% decrease in viscosity compared to the viscosity of reference solution 2 before centrifugation. The decrease in viscosity obtained by high-speed centrifugation is significant but smaller than when using other pretreatment methods. Preheating of the solution reduced the viscosity by 37%, from 4.6 mPa s to 2.9 mPa s [18], and DEF with kieselguhr by 63%, from 4.8 mPa s to 1.8 mPa s [17]. Heat pretreatment and DEF thus appear to degrade or remove one or more compounds responsible for high viscosity from the solution that cannot be removed by centrifugation. The influence of high-speed centrifugation on flux during UF was studied. A sample of reference solution 2 was centrifuged at 20 000g for 15 min, and the supernatant was then ultrafiltered. The UF flux of the high-speed centrifuged solution was 72 l/m2 h at 1 bar TMP, see Fig. 5. This corresponds to an increase in flux of 14% and 9% compared to UF of reference solution 1 and the HRT solution, respectively.
Fig. 4. Viscosity of hemicellulose reference solution 2 after high-speed centrifugation at 10 000g (j), 20 000g (s) and 30 000g (N) for 5, 10 and 15 min.
Fig. 5. Flux during UF of the supernatant at 80 °C and a CFV of 4 m/s, after centrifugation at 20 000g for 15 min.
Fig. 6. Filtration curves during DEF of the high-speed centrifugation (HSC) solution (h), the high-residence time (HRT) solution (N) and reference solution 1 (j) with 2 wt% kieselguhr as filter aid.
3.3. Combination of centrifugation and dead-end filtration
Fig. 3. Turbidity of the hemicellulose solution after high-speed centrifugation at 10 000g (j), 20 000g (s) and 30 000g (N) for 5, 10 and 15 min.
The UF flux was still considerably lower than after pretreatment by DEF with kieselguhr, even when the sample had been subjected to high-speed centrifugation. The drawbacks of filtration with kieselguhr is the generation of waste (used kieselguhr), and the relatively high filtration resistance, which increases the cost of filtration. Centrifugation combined with DEF with kieselguhr was carried out to study the influence of centrifugation on filtration resistance during DEF. The same amount of filtrate (50 kg) was obtained after
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Table 2 Characteristics of reference solution 1, the high-residence time (HRT) solution and the high-speed centrifugation (HSC) solution, before and after dead-end filtration (DEF) with 2 wt% kieselguhr. Reference solution 1
Total solids (mg/g) NaOH (mg/g) Hemicelluloses (mg/g) Acid-insoluble solids (mg/g) Acid-soluble lignin (mg/g) Turbidity (NTU) Viscosity (mPa s)
HRT solution
HSC solution
Unfiltered
DEF
Unfiltered
DEF
Unfiltered
DEF
42 34 6.9 2.6 0.20 340 2.7
41 32 6.4 2.3 0.20 140 1.9
40 31 6.2 2.3 0.20 270 1.9
38 30 6.0 2.1 0.20 120 1.5
46 36 7.4 2.3 0.30 170 1.3
43 34 7.2 2.1 0.29 100 0.9
DEF with kieselguhr is a more efficient way of reducing the turbidity and viscosity of the hemicellulose solution. The UF flux of a DEF solution was more than twice as high as the flux of an untreated solution. Using centrifugation prior to DEF reduces the filtration resistance and potentially also the need of a filter aid. The UF flux after combined pretreatment with centrifugation and DEF was nearly three times higher than during UF of an untreated reference solution.
Acknowledgements The Swedish Energy Agency and the Swedish Foundation for Strategic Environmental Research (MISTRA) are gratefully acknowledged for their financial support. Fig. 7. Flux during UF at 80 °C and a CFV of 4 m/s of dead-end filtered high-speed centrifugation (HSC) solution (N), high-residence time (HRT) solution (s) and reference solution 1 (j).
360 s DEF of reference solution 1, 180 s DEF of the HRT solution, and 110 s DEF of the HSC solution, as shown in Fig. 6. Because of the low filtration resistance of the HSC solution, the filtration pressure of 4 bar was not reached until the very end of filtration of this solution. The filtration resistance was calculated using Eq. (1), and found to be 1.5 1010 m/kg for reference solution 1 and 5.6 109 m/kg for the HRT solution. As it was not possible to maintain a constant pressure during filtration of the HSC solution, it was not possible to calculate the filtration resistance of this solution. However, it is evident from Fig. 6 that this solution had the lowest filtration resistance. The composition of the solutions before and after DEF is given in Table 2. A slight overall decrease in concentration can be seen after filtration. The most significant difference is in the reduction of turbidity and viscosity after filtration. This shows that suspended solids remaining after centrifugation can be removed by DEF. Furthermore, filtration also removes viscous compounds. The UF flux after centrifugation and DEF was significantly higher than after centrifugation alone, as shown in Fig. 7. This is in accordance with the results of our previous study [17]. The DEF treated HSC solution had the highest flux (230 l/m2 h at 1.8 bar) and the non-centrifuged reference solution had the lowest flux (180 l/m2 h). This shows that centrifugation of the solution before DEF has a positive effect, not only on filtration resistance during DEF, but also on UF flux. 4. Conclusions In this study, it was found that the turbidity and viscosity of an alkaline hemicellulose solution could be reduced by centrifugation. However, it was not possible to significantly increase the flux during UF by pretreating the solution with centrifugation alone.
References [1] M. Gröndahl, L. Eriksson, P. Gatenholm, Material properties of plasticized hardwood xylans for potential application as oxygen barrier films, Biomacromolecules 5 (2004) 1528–1535. [2] N.M.L. Hansen, D. Plackett, Sustainable films and coatings from hemicelluloses: a review, Biomacromolecules 9 (2008) 1493–1505. [3] J. Hartman, A.-C. Albertsson, M. Söderqvist Lindblad, J. Sjöberg, Oxygen barrier materials from renewable sources: material properties of softwood hemicellulose-based films, J. Appl. Polym. Sci. 100 (2006) 2985–2991. [4] J. Hartman, A.-C. Albertsson, J. Sjöberg, Surface- and bulk-modified galactoglucomannan hemicellulose films and film laminates from versatile oxygen barriers, Biomacromolecules 7 (2006) 1983–1989. [5] A.I. Yaich, U. Edlund, A.-C. Albertsson, Wood hydrolysate barriers: performance controlled via selective recovery, Biomacromolecules 13 (2012) 466–473. [6] A. Ebringerová, T. Heinze, Xylan and xylan derivatives – biopolymers with valuable properties 1, Macromol. Rapid Commun. 21 (2000) 542–556. [7] G. Annison, M. Choct, N.W. Cheetham, Analysis of wheat arabinoxylans from a large-scale isolation, Carbohydr. Polym. 19 (1992) 151–159. [8] J. Hollmann, M.G. Lindhauer, Pilot-scale isolation of glucuronoarabinoxylans from wheat bran, Carbohydr. Polym. 59 (2005) 225–230. [9] S. Zhou, X. Liu, Y. Guo, Q. Wang, D. Peng, L. Cao, Comparison of the immunological activity of arabinoxylans from wheat bran with alkali and xylanase-aided extraction, Carbohydr. Polym. 81 (2010) 784–789. [10] J. Krogell, E. Korotkova, A. Pranovich, T. Salmi, D. Murzin, S. Willför, Intensification of hemicellulose hot-water extraction from spruce wood in a batch extractor – effects of wood particle size, Bioresour. Technol. 143 (2013) 212–220. [11] K. Swennen, C.M. Courtin, B. van der Bruggen, C. Vandecasteele, J.A. Delcour, Ultrafiltration and ethanol precipitation for isolation of arabinoxylooligosaccharides with different structures, Carbohydr. Polym. 62 (2005) 283–292. [12] H. Krawczyk, T. Persson, A. Andersson, A.-S. Jönsson, Isolation of hemicelluloses from barley husks, Food Bioprod. Process. 86 (2008) 31–36. [13] R. Zeitoun, P.Y. Pontalier, P. Marechal, L. Rigal, Twin-screw extrusion for hemicellulose recovery: influence on extract purity and purification performance, Bioresour. Technol. 101 (2010) 9348–9354. [14] L. Jacquemin, R. Zeitoun, C. Sablayrolles, P.-Y. Pontalier, L. Rigal, Evaluation of the technical and environmental performances of extraction and purification processes of arabinoxylans from wheat straw and bran, Process Biochem. 47 (2012) 373–380. [15] J. Jorda, P. Marechal, L. Rigal, P.Y. Pontalier, Biopolymer purification by ultrafiltration, Desalination 148 (2002) 187–191. [16] H. Krawczyk, A. Arkell, A.-S. Jönsson, Membrane performance during ultrafiltration of a high-viscosity solution containing hemicelluloses from wheat bran, Sep. Purif. Technol. 83 (2011) 144–150.
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[17] H. Krawczyk, A. Arkell, A.-S. Jönsson, Impact of prefiltration on membrane performance during isolation of hemicelluloses extracted from wheat bran, Sep. Purf. Technol. 116 (2013) 192–198. [18] A. Arkell, H. Krawczyk, A.-S. Jönsson, Influence of heat pretreatment on ultrafiltration of a solution containing hemicelluloses extracted from wheat bran, Sep. Purf. Technol. 119 (2013) 46–50. [19] W.L. McCabe, J.C. Smith, P. Harriott, Unit Operations of Chemical Engineering, Seventh ed., McGraw Hill, Boston, 2005.
[20] R. Ruiz, T. Ehrman, 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, 1996. [21] A. Sluiter, B. Hames, R. Ruiz, C. Scarlata, J. Sluiter, D. Templeton, D. Crocker, Laboratory analytical procedure (LAP): determination of structural carbohydrates and lignin in biomass, NREL, Midwest Research Institute for the Department of Energy, USA, 2008.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Centrifugation as pretreatment before ultrafiltration of hemicelluloses extracted from wheat bran J. Thuvander, A. Arkell, A.-S. 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
Article history: Received 26 June 2014 Received in revised form 7 October 2014 Accepted 8 October 2014
Keywords: Prefiltration Centrifugation Ultrafiltration Viscosity Hemicelluloses
a b s t r a c t Hemicelluloses recovered by alkaline extraction of wheat bran can be isolated and purified by ultrafiltration and used to produce high-value products such as barrier films for food packaging. The flux during ultrafiltration depends to a large extent on the pretreatment of the solution. It has been shown previously that dead-end filtration with kieselguhr can increase the flux during ultrafiltration. In the present study, pretreatment by centrifugation and a combination of centrifugation and dead-end filtration was compared with dead-end filtration alone. Centrifugation reduced the turbidity and viscosity of the solution but did not significantly improve the flux during ultrafiltration. The flux of the untreated and the centrifuged solutions was about 65 l/m2 h at 1 bar transmembrane pressure. The flux after dead-end filtration was 150 l/m2 h. After combining centrifugation and dead-end filtration the flux was 200 l/ m2 h. In addition to the higher UF flux, filtration resistance during dead-end filtration was markedly reduced after pre-centrifugation of the solution. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Hemicelluloses from lignocellulosic materials can be used to produce polymer films. These renewable films have properties that make them interesting as barrier materials for food packaging [1–5]. One promising raw material for barrier films is wheat bran, an agricultural residue that can contain up to 30% of the hemicellulose arabinoxylan [6]. An effective and cost-efficient extraction and purification process is necessary when using hemicelluloses as raw material in the production of barrier films. Examples of extraction methods that can be used are alkali extraction and hot water extraction [7–10]. High yield and low polymer degradation have been achieved with alkali extraction [7]. The extract obtained contains not only arabinoxylan, but a mixture of compounds that makes further purification of the arabinoxylan solution necessary, before it can be used to produce a high-value product. One way of purifying the hemicelluloses in the alkaline extract is ultrafiltration (UF) [11–15]. Membrane filtration is an attractive separation method because of its high energy efficiency and selectivity. However, to ensure high flux during UF, as well as high product quality, the solution must be pretreated prior to UF to remove particulate matter and other compounds that may foul the UF membrane. ⇑ Corresponding author. Tel.: +46 46 222 8291; fax: +46 46 222 4526. E-mail address: ann-sofi[email protected] (A.-S. Jönsson). http://dx.doi.org/10.1016/j.seppur.2014.10.003 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
High viscosity can severely reduce the flux in UF [16–18]. It is therefore advantageous if the pretreatment method lowers the viscosity of the solution. It has been shown that heat treatment of the alkali extract lowers the viscosity and increases the flux during UF [18]. However, the degradation of arabinoxylan during heat treatment was considered too great to motivate the increase in flux. The viscosity can also be reduced by dead-end filtration (DEF), which simultaneously removes suspended solids from the solution. In a previous study we found that DEF with kieselguhr as a filter aid increased the flux during UF threefold [17]. However, a large amount of filter aid is needed because of the high filter cake resistance. The aim of this study was to investigate whether centrifugation can be used to remove suspended solids from the solution and thus improve the UF flux. The influence of three pretreatment methods on UF flux was studied: (1) increasing the residence time in the nozzle separator already used for pretreatment of the hemicellulose solution, (2) removing dense components using high-speed centrifugation, and (3) centrifugation prior to DEF in order to lower the filter cake resistance during DEF. 2. Materials and methods Two batches of a hemicellulose-rich solution were obtained by alkali extraction of wheat bran. The composition of the solutions is found in Table 1. After alkali extraction, the cellulose-rich solid fraction was removed by centrifugation in a continuous Clara
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20 low-flow nozzle separator (Alfa Laval, Tumba, Sweden) at a flow rate of 300 l/h. The fixed gravitational force of the Clara 20 separator is 11 130 g. A flow rate of 300 l/h corresponds to solid flush every 10 min. The solution from the first batch is hereafter referred to as reference solution 1 and the solution from the second batch is referred to as reference solution 2. 2.1. Pretreatment methods 2.1.1. High-residence time in the existing nozzle separator When producing reference solution 1, another solution from the same batch was produced using a higher residence time in the nozzle separator (called the HRT solution) by decreasing the flow rate to 65 l/h (solid flush every 15 min). 2.1.2. High-speed centrifugation High-speed centrifugation of reference solution 2 was carried out at ambient temperature in a Sorval Lynx 4000 centrifuge (Thermo Scientific, Waltham, USA). The influence of gravitational force and centrifugation time was investigated by adding the solution to 300 ml plastic bottles so that the total weight of the bottle and solution was 250 g. The samples were centrifuged at 10 000g, 20 000g and 30 000g for 5 min, 10 min and 15 min. After centrifugation, 150 ml of the supernatant was transferred to new bottles for later analysis. High-speed centrifuged (HSC) solution, to be used for UF studies and pretreatment before DEF, was prepared in 1-l plastic bottles. Solution was added to the bottles so that the total weight of the bottle and solution was 800 g. After centrifugation for 15 min at 20 000 g, the supernatant was carefully recovered, and the pellet was discarded. 2.1.3. Combination of centrifugation and dead-end filtration DEF was carried out using a PF 0.1 H2 filter press (Larox Corp., Lappeenranta, Finland). This filter press has a 100-l feed tank with an agitator and an air-driven slurry pump (Wilden M2-P Champ, Wilden Pump & Engineering Co., Colton, USA). A 10 lm filter cloth (Hydrotech AB, Vellinge, Sweden) was used, with a filtration area of 0.1 m2. The feed tank was equipped with a Pt-100 temperature sensor (Pentronic, Gunnebo, Sweden) and electrical heaters connected to a temperature regulator (Shinko MCM, Shinko Europe BV, Haarsteeg, the Netherlands) for temperature control. Kieselguhr (2 wt%) (Dicalite, Burney, USA) was added to the solution as a filter aid before heating the solution to the filtration temperature of 50 °C. Filtration was carried out at 4 bar. The filter cake resistance was calculated using the constant-pressure filtration equation [19]:
t l Rm lac ¼ þ V V A DP 2 A2 DP
ð1Þ
where t (s) is the filtration time, V (m3) the filtrate volume, l (Pa s) the viscosity of the filtrate, Rm (m1) the hydraulic resistance of the filter cloth, A (m2) the area of the filter, DP (Pa) the pressure drop across the filter cake, a (m/kg) the specific filter cake resistance, and c (kg/m3) the mass of filter-cake-forming particles per unit volume of the filtrate. 2.2. Ultrafiltration 2.2.1. Membrane The UF membrane used was a seven-channel tubular ceramic membrane (Atech Innovations GmbH, Gladbeck, Germany) made of a-Al2O3, with an active layer ofTiO2. The molecular mass cutoff of the membrane was 10 kDa. The diameter of the channels was 6 mm and the membrane was 1 m long, giving a total membrane area of 0.132 m2. The end-cap sealing material was
Table 1 Characteristics of the solutions treated in the nozzle separator at 300 l/h (reference solution 1 and 2) and the high-residence time (HRT) solution treated at a flow rate of 65 l/h (same batch as reference solution 1). The viscosity was measured at 80 °C and a shear rate of 100 s1.
Total solids (g/l) NaOH (g/l) Hemicelluloses (g/l) -Arabinan (g/l) -Galactan (g/l) -Glucan (g/l) -Xylan (g/l) Acid-insoluble solids (g/l) Acid-soluble lignin (g/l) Turbidity (NTU) Viscosity (mPa s)
Reference solution 1
HRT solution
Reference solution 2
42 34 6.9 1.7 0.2 1.3 3.7 2.6 0.20 340 2.7
40 31 6.2 1.6 0.1 1.2 3.3 2.3 0.20 270 1.9
48 38 7.9 1.9 0.1 1.5 4.5 2.8 0.25 550 3.6
poly(tetrafluoroethene) (PTFE) in order to withstand the pH and temperature of the solution. 2.2.2. UF set-up A schematic illustration of the experimental set-up is shown in Fig. 1. The set-up consisted of two 200-l tanks. Tank 1 was used as the feed tank during the experiments, and tank 2 was used during start-up and membrane cleaning. Both tanks were equipped with a Pt-100 temperature sensor for temperature measurement. The contents of the tanks were heated with 5-bar steam condensing in heat exchanger coils inside the tanks. The membrane was fitted in an M1 module (Atech innovations GmbH, Gladbeck, Germany). The set-up included a feed pump (Hydra-cell D25XL, Wanner, Minneapolis, USA) and a circulation pump (NB32/25-20, ABS Pump Production, Mölndal, Sweden). The feed flow was measured with a volume flow meter (Fischer&Porter Co. Ltd., Göttningen, Germany), and the permeate flow was measured with an electronic balance (PL6001-S, Mettler-Toledo Inc., Columbus, USA). The feed pressure (PF), the retentate pressure (PR) and the permeate pressure (PP) were measured with pressure transmitters (dTrans p02, Jumo AB, Helsingborg, Sweden). The transmembrane pressure (TMP) was calculated from the relation:
TMP ¼
PF þ PR PP 2
ð2Þ
2.2.3. Membrane cleaning The membrane was cleaned before and after each experiment. The membrane was cleaned with a 0.5 wt% solution of the alkaline cleaning agent Ultrasil 11 (Ecolab AB, Älvsjö, Sweden) at a TMP of 0.8 bar and a cross-flow velocity (CFV) of 4 m/s for 1 h. The membrane was then rinsed with deionized water to remove any residual cleaning agent. After the cleaning cycle the pure water flux of the membrane was measured. A 0.1 M NaOH solution was circulated in the system prior to the experiment to equilibrate the membrane to the pH of the process solution. After the experiment, the solution was rinsed out of the system with a 0.1 M NaOH solution, before the membrane was cleaned as described above. 2.2.4. Ultrafiltration UF experiments were performed on 75-l of the studied solution at a temperature of 80 °C and a cross-flow velocity of 4 m/s. Both permeate and retentate were returned to the feed tank during the experiments to keep the feed concentration constant. Flux was studied as a function of TMP, starting at a TMP of 0.4 bar. The flux was recorded for 15 min before feed and permeate samples were withdrawn for analysis. The TMP was increased in steps of 0.2 bar to 1.8 bar, or until the flux became independent of pressure.
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P P
Tank 2
Membrane module
Tank 1
Balance
T
T P
F Feed pump
Circulaon pump
Fig. 1. Schematic illustration of the experimental set-up used in the UF experiments.
2.3. Analytical methods 2.3.1. Total solids, ash and sodium hydroxide Samples were dried in porcelain crucibles at 105 °C for 24 h. The dry weight of the sample residue was determined as the difference in weight before and after drying. The ash content of the sample was determined by weighing the residue again after it had been heated to 575 °C and maintained at this temperature for 3 h. The sodium hydroxide content was determined by multiplying the ash content by a factor of 1.29 [16]. 2.3.2. Hemicelluloses The polysaccharides in the samples were degraded using a standardized method for acid hydrolysis [20]. The concentration of monomeric sugars was measured using high-performance anionexchange chromatography coupled with pulsed amperometric detection in an ICS-3000 chromatography system (Dionex Corp., Sunnyvale, USA). Deionized water was used as eluent at a flow rate of 1 ml/min. D-galactose, D-glucose, D-mannose, D-xylose and Larabinose (Fluka Chemie AG, Buchs, Switzerland) were used as standards. The concentration of hemicelluloses was determined from the sum of the monomeric sugars after anhydro corrections of 0.88 for pentoses and 0.90 for hexoses. 2.3.3. Acid-insoluble solids and acid-soluble lignin The samples were subjected to acid hydrolysis [20]. The content of acid-insoluble solids in the samples was determined using a filter crucible with a maximal pore size of 16 lm to capture the precipitate, which was dried in an oven at 105 °C for 24 h and then weighed. The content of acid-soluble lignin was determined by measuring the UV absorbance of the filtrate at 320 nm with a spectrophotometer (UV-160, Shimadzu Corp., Kyoto, Japan). An extinction coefficient of 30 l/g cm was used [21]. 2.3.4. Turbidity The turbidity was measured in a turbidimeter (2100P ISO, HACH Co., Loveland, USA). Formazin standards from HACH of 0, 20, 100 and 800 nephelometric turbidity units (NTU), were used to calibrate the instrument. 2.3.5. Viscosity Viscosity measurements were performed using a Malvern Kinexus Pro rotational rheometer (Malvern Instruments Ltd.,
Worcestershire, UK). A cup-and-bob geometry was used, and the apparent viscosity was measured at a temperature of 80 °C at a shear rate of 100 s1.
3. Results and discussion 3.1. Increased residence time in the nozzle separator The composition of the two process solutions resulting from different residence times in the nozzle separator is given in Table 1. The main part of the dry solids in the hemicellulose solution was sodium hydroxide, which originates from the extraction liquid. The dominating hemicellulose component was xylan, followed by arabinan. The main components in the acid-insoluble solids fraction are lignin, proteins and fats, which account for 3–9%, 14– 18% and 4% of the mass of the wheat bran, respectively [8,9,13]. Increasing the residence time in the nozzle separator slightly reduced the concentration of all components. The turbidity was reduced by 20% and the viscosity by 30% when prolonging the residence time. Increasing the residence time seemed thus to offer a means of reducing the amount of suspended solids in the process solution. The flux during UF of reference solution 1 and the HRT solution was similar, as can be seen in Fig. 2. The flux levelled off at about 65 l/m2 h, which is in accordance with the results obtained when treating an alkaline extract of wheat bran at corresponding operating conditions in our previous study [17]. From the results of this experiment it can be concluded that insufficient suspended solids were removed by increasing the residence time, or the suspended solids that were removed were not responsible for the flux-reducing effect during UF. 3.2. High-speed centrifugation A parametric study was conducted to study the efficiency of suspended solids removal by high-speed centrifugation. A new batch of reference solution (reference solution 2) was produced as described above. This had a turbidity of 550 NTU and a viscosity of 3.6 mPa s, and was thus more turbid and viscous than reference solution 1 (340 NTU and 2.7 mPa s). A comparable solution used in our previous investigation had a turbidity of 690 NTU and viscosity of 4.8 mPa s [17], showing that there is considerable variation in both turbidity and viscosity between batches.
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Fig. 2. Flux during UF of reference solution 1 (s) and the high-residence time (HRT) solution (j) at 80 °C and a CFV of 4 m/s.
Neither gravitational force (10 000–30 000g), nor centrifugation time (5 min to 15 min) did significantly affect the total solids content or the concentration of hemicelluloses and lignin, whereas the turbidity (see Fig. 3) decreased with increasing gravitational force and centrifugation time. After 15 min of centrifugation at 30 000g, the turbidity was 100 NTU. It is thus possible to separate part of the suspended solids from the solution by high-speed centrifugation. The viscosity of the supernatant was only influenced to a minor extent by centrifugation; decreasing from 3.6 mPa s to 2.8 mPa s, after 5 min of centrifugation at 10 000g. Prolonged centrifugation and/or higher gravitational force had only a minor additional effect on viscosity, as can be seen in Fig. 4. After 15 min of centrifugation at 30 000g the solution had a viscosity of 2.6 mPa s, which is equivalent to a 27% decrease in viscosity compared to the viscosity of reference solution 2 before centrifugation. The decrease in viscosity obtained by high-speed centrifugation is significant but smaller than when using other pretreatment methods. Preheating of the solution reduced the viscosity by 37%, from 4.6 mPa s to 2.9 mPa s [18], and DEF with kieselguhr by 63%, from 4.8 mPa s to 1.8 mPa s [17]. Heat pretreatment and DEF thus appear to degrade or remove one or more compounds responsible for high viscosity from the solution that cannot be removed by centrifugation. The influence of high-speed centrifugation on flux during UF was studied. A sample of reference solution 2 was centrifuged at 20 000g for 15 min, and the supernatant was then ultrafiltered. The UF flux of the high-speed centrifuged solution was 72 l/m2 h at 1 bar TMP, see Fig. 5. This corresponds to an increase in flux of 14% and 9% compared to UF of reference solution 1 and the HRT solution, respectively.
Fig. 4. Viscosity of hemicellulose reference solution 2 after high-speed centrifugation at 10 000g (j), 20 000g (s) and 30 000g (N) for 5, 10 and 15 min.
Fig. 5. Flux during UF of the supernatant at 80 °C and a CFV of 4 m/s, after centrifugation at 20 000g for 15 min.
Fig. 6. Filtration curves during DEF of the high-speed centrifugation (HSC) solution (h), the high-residence time (HRT) solution (N) and reference solution 1 (j) with 2 wt% kieselguhr as filter aid.
3.3. Combination of centrifugation and dead-end filtration
Fig. 3. Turbidity of the hemicellulose solution after high-speed centrifugation at 10 000g (j), 20 000g (s) and 30 000g (N) for 5, 10 and 15 min.
The UF flux was still considerably lower than after pretreatment by DEF with kieselguhr, even when the sample had been subjected to high-speed centrifugation. The drawbacks of filtration with kieselguhr is the generation of waste (used kieselguhr), and the relatively high filtration resistance, which increases the cost of filtration. Centrifugation combined with DEF with kieselguhr was carried out to study the influence of centrifugation on filtration resistance during DEF. The same amount of filtrate (50 kg) was obtained after
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Table 2 Characteristics of reference solution 1, the high-residence time (HRT) solution and the high-speed centrifugation (HSC) solution, before and after dead-end filtration (DEF) with 2 wt% kieselguhr. Reference solution 1
Total solids (mg/g) NaOH (mg/g) Hemicelluloses (mg/g) Acid-insoluble solids (mg/g) Acid-soluble lignin (mg/g) Turbidity (NTU) Viscosity (mPa s)
HRT solution
HSC solution
Unfiltered
DEF
Unfiltered
DEF
Unfiltered
DEF
42 34 6.9 2.6 0.20 340 2.7
41 32 6.4 2.3 0.20 140 1.9
40 31 6.2 2.3 0.20 270 1.9
38 30 6.0 2.1 0.20 120 1.5
46 36 7.4 2.3 0.30 170 1.3
43 34 7.2 2.1 0.29 100 0.9
DEF with kieselguhr is a more efficient way of reducing the turbidity and viscosity of the hemicellulose solution. The UF flux of a DEF solution was more than twice as high as the flux of an untreated solution. Using centrifugation prior to DEF reduces the filtration resistance and potentially also the need of a filter aid. The UF flux after combined pretreatment with centrifugation and DEF was nearly three times higher than during UF of an untreated reference solution.
Acknowledgements The Swedish Energy Agency and the Swedish Foundation for Strategic Environmental Research (MISTRA) are gratefully acknowledged for their financial support. Fig. 7. Flux during UF at 80 °C and a CFV of 4 m/s of dead-end filtered high-speed centrifugation (HSC) solution (N), high-residence time (HRT) solution (s) and reference solution 1 (j).
360 s DEF of reference solution 1, 180 s DEF of the HRT solution, and 110 s DEF of the HSC solution, as shown in Fig. 6. Because of the low filtration resistance of the HSC solution, the filtration pressure of 4 bar was not reached until the very end of filtration of this solution. The filtration resistance was calculated using Eq. (1), and found to be 1.5 1010 m/kg for reference solution 1 and 5.6 109 m/kg for the HRT solution. As it was not possible to maintain a constant pressure during filtration of the HSC solution, it was not possible to calculate the filtration resistance of this solution. However, it is evident from Fig. 6 that this solution had the lowest filtration resistance. The composition of the solutions before and after DEF is given in Table 2. A slight overall decrease in concentration can be seen after filtration. The most significant difference is in the reduction of turbidity and viscosity after filtration. This shows that suspended solids remaining after centrifugation can be removed by DEF. Furthermore, filtration also removes viscous compounds. The UF flux after centrifugation and DEF was significantly higher than after centrifugation alone, as shown in Fig. 7. This is in accordance with the results of our previous study [17]. The DEF treated HSC solution had the highest flux (230 l/m2 h at 1.8 bar) and the non-centrifuged reference solution had the lowest flux (180 l/m2 h). This shows that centrifugation of the solution before DEF has a positive effect, not only on filtration resistance during DEF, but also on UF flux. 4. Conclusions In this study, it was found that the turbidity and viscosity of an alkaline hemicellulose solution could be reduced by centrifugation. However, it was not possible to significantly increase the flux during UF by pretreating the solution with centrifugation alone.
References [1] M. Gröndahl, L. Eriksson, P. Gatenholm, Material properties of plasticized hardwood xylans for potential application as oxygen barrier films, Biomacromolecules 5 (2004) 1528–1535. [2] N.M.L. Hansen, D. Plackett, Sustainable films and coatings from hemicelluloses: a review, Biomacromolecules 9 (2008) 1493–1505. [3] J. Hartman, A.-C. Albertsson, M. Söderqvist Lindblad, J. Sjöberg, Oxygen barrier materials from renewable sources: material properties of softwood hemicellulose-based films, J. Appl. Polym. Sci. 100 (2006) 2985–2991. [4] J. Hartman, A.-C. Albertsson, J. Sjöberg, Surface- and bulk-modified galactoglucomannan hemicellulose films and film laminates from versatile oxygen barriers, Biomacromolecules 7 (2006) 1983–1989. [5] A.I. Yaich, U. Edlund, A.-C. Albertsson, Wood hydrolysate barriers: performance controlled via selective recovery, Biomacromolecules 13 (2012) 466–473. [6] A. Ebringerová, T. Heinze, Xylan and xylan derivatives – biopolymers with valuable properties 1, Macromol. Rapid Commun. 21 (2000) 542–556. [7] G. Annison, M. Choct, N.W. Cheetham, Analysis of wheat arabinoxylans from a large-scale isolation, Carbohydr. Polym. 19 (1992) 151–159. [8] J. Hollmann, M.G. Lindhauer, Pilot-scale isolation of glucuronoarabinoxylans from wheat bran, Carbohydr. Polym. 59 (2005) 225–230. [9] S. Zhou, X. Liu, Y. Guo, Q. Wang, D. Peng, L. Cao, Comparison of the immunological activity of arabinoxylans from wheat bran with alkali and xylanase-aided extraction, Carbohydr. Polym. 81 (2010) 784–789. [10] J. Krogell, E. Korotkova, A. Pranovich, T. Salmi, D. Murzin, S. Willför, Intensification of hemicellulose hot-water extraction from spruce wood in a batch extractor – effects of wood particle size, Bioresour. Technol. 143 (2013) 212–220. [11] K. Swennen, C.M. Courtin, B. van der Bruggen, C. Vandecasteele, J.A. Delcour, Ultrafiltration and ethanol precipitation for isolation of arabinoxylooligosaccharides with different structures, Carbohydr. Polym. 62 (2005) 283–292. [12] H. Krawczyk, T. Persson, A. Andersson, A.-S. Jönsson, Isolation of hemicelluloses from barley husks, Food Bioprod. Process. 86 (2008) 31–36. [13] R. Zeitoun, P.Y. Pontalier, P. Marechal, L. Rigal, Twin-screw extrusion for hemicellulose recovery: influence on extract purity and purification performance, Bioresour. Technol. 101 (2010) 9348–9354. [14] L. Jacquemin, R. Zeitoun, C. Sablayrolles, P.-Y. Pontalier, L. Rigal, Evaluation of the technical and environmental performances of extraction and purification processes of arabinoxylans from wheat straw and bran, Process Biochem. 47 (2012) 373–380. [15] J. Jorda, P. Marechal, L. Rigal, P.Y. Pontalier, Biopolymer purification by ultrafiltration, Desalination 148 (2002) 187–191. [16] H. Krawczyk, A. Arkell, A.-S. Jönsson, Membrane performance during ultrafiltration of a high-viscosity solution containing hemicelluloses from wheat bran, Sep. Purif. Technol. 83 (2011) 144–150.
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[17] H. Krawczyk, A. Arkell, A.-S. Jönsson, Impact of prefiltration on membrane performance during isolation of hemicelluloses extracted from wheat bran, Sep. Purf. Technol. 116 (2013) 192–198. [18] A. Arkell, H. Krawczyk, A.-S. Jönsson, Influence of heat pretreatment on ultrafiltration of a solution containing hemicelluloses extracted from wheat bran, Sep. Purf. Technol. 119 (2013) 46–50. [19] W.L. McCabe, J.C. Smith, P. Harriott, Unit Operations of Chemical Engineering, Seventh ed., McGraw Hill, Boston, 2005.
[20] R. Ruiz, T. Ehrman, 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, 1996. [21] A. Sluiter, B. Hames, R. Ruiz, C. Scarlata, J. Sluiter, D. Templeton, D. Crocker, Laboratory analytical procedure (LAP): determination of structural carbohydrates and lignin in biomass, NREL, Midwest Research Institute for the Department of Energy, USA, 2008.