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Isolation of hemicelluloses by ultrafiltration of thermomechanical pulp mill process water—Influence of operating conditions
chemical engineering research and design 8 8 ( 2 0 1 0 ) 1548–1554
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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Isolation of hemicelluloses by ultrafiltration of thermomechanical pulp mill process water—Influence of operating conditions T. Persson, A.-S. Jönsson ∗ Department of Chemical Engineering, Lund University, PO Box 124, SE-221 00 Lund, Sweden
a b s t r a c t Hemicelluloses could be used to replace fossil-based materials in several high-value-added products. Today, vast amounts of hemicelluloses are discharged from pulp mills all over the world as waste, but these could be isolated by membrane filtration, and utilized in various applications. In this study, the hemicellulose galactoglucomannan was isolated from process water from a thermomechanical pulp mill using ultrafiltration. The retention of hemicelluloses and lignin, and the flux and fouling of three ultrafiltration membranes (ETNA01, ETNA10 and UFX5) were studied at various operating conditions. One of the membranes (UFX5) was found to have a high hemicellulose retention (above 90%) independent of flux and pressure. With the ETNA01 membrane it was impossible to combine a high flux with high hemicellulose retention, while with the ETNA10 membrane the hemicellulose retention could be increased above 90% by running at transmembrane pressures above the critical flux. The UFX5 membrane could be used at the temperature of the process water in the pulp mill (75–85 ◦ C), while the ETNA10 membrane could only withstand temperatures below 60 ◦ C, increasing the cost due to the need to cool the process water. However, the susceptibility of UFX5 to fouling was much greater than for ETNA10, which would increase the cleaning cost of the UFX5 membrane. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Galactoglucomannan; Ultrafiltration; Spiral-wound membrane; ETNA10; UFX5
1.
Introduction
In thermomechanical pulp mills, 95% of the wood is used in the manufacture of paper, while the remaining 5% is discharged as waste. Galactoglucomannans (GGM) are the main hemicelluloses in softwood. The amount of GGM dissolved in the effluent process water corresponds to 1–2% of the wood (Thornton et al., 1994), and the amount of dissolved lignin to 0.5–1% of the wood (Pranovich et al., 2005). If the hemicelluloses could be isolated from the waste stream, they could be used in high-value-added products such as barrier films, coatings, hydrogels or paper additives (Lima et al., 2003; Hansen and Plackett, 2008). The characteristics of the process water in a thermomechanical pulp mill have been studied in detail (Thornton et al., 1994; Willför et al., 2003, 2008; Pranovich et al., 2005), but the isolation of valuable compounds such as hemicelluloses from the process water has not been studied to the same extent
∗
(Persson et al., 2010). When isolating a substance at a very low concentration, such as hemicelluloses in pulp mill process water, the cost of the separation process will determine whether it is economically feasible, or not. Ultrafiltration is a suitable separation technique for this kind of application due to its low energy requirement and the fact that separation is achieved without the need for chemicals. However, the choice of membrane and operating conditions has considerable impact on the yield and problems such as fouling and, hence, on the economics of the isolation process. Previous studies have shown that hydrophobic molecules, such as lignin and resins, fouled hydrophobic membranes during ultrafiltration of pulp mill process streams, whereas almost no fouling was observed when hydrophilic membranes were used (Maartens et al., 2002; Persson et al., 2005). A plate-and-frame module was used in a previous investigation of ultrafiltration of process water from a thermomechanical pulp mill (Persson et al., 2007). In the present
Corresponding author. Tel.: +46 46 222 8291; fax: +46 46 222 4526. E-mail address: ann-sofi[email protected] (A.-S. Jönsson). Received 20 May 2009; Received in revised form 24 March 2010; Accepted 6 April 2010 0263-8762/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2010.04.002
chemical engineering research and design 8 8 ( 2 0 1 0 ) 1548–1554
study, spiral-wound modules were used. The maximum operating temperature of the ETNA membranes (Alfa Laval, Nakskov, Denmark), used successfully in the previous investigation, is only 60 ◦ C whereas the temperature of the process water in the pulp mill is 75–85 ◦ C. It would thus be beneficial to use a membrane with similar hydrophilic properties and cutoff but higher temperature tolerance. A membrane that fulfils these requirements is the newly developed UFX5 membrane (Alfa Laval), made of permanently hydrophilic polysulphone which has a maximum operating temperature of 90 ◦ C. The influence of transmembrane pressure and the concentration of hemicelluloses on (i) flux, (ii) the retention of hemicelluloses and lignin, and (iii) fouling was studied to identify the most suitable membrane and operating conditions.
membrane surface can be described by (Blatt et al., 1970):
J = k ln
Cg − Cp Cb − Cp
(6)
Theory
Due to the low concentration of hemicelluloses in the process water, the initial flux will be high during ultrafiltration, which is favourable for the productivity. However, when the flux is high, the pressure drop in the permeate spacer may be significant (Schock and Miquel, 1987), which would decrease the productivity. Furthermore, a high flux increases the concentration polarisation of the retained molecules, which may decrease retention and increase fouling. The concentration at the membrane surface increases with increasing flux as (Cheryan, 1998) Cm = Cp + (Cb − Cp ) · exp
J
(1)
k
where Cm , Cp and Cb are the concentration at the membrane surface, in the permeate and in the bulk, respectively, J is the flux and k is the mass transfer coefficient. The true retention is constant and equal to the reflection coefficient for a specific membrane and a specific molecule at high fluxes. However, the observed retention will decrease with increasing flux until a boundary layer is formed on the membrane (Jonsson, 1986; de Balmann and Nobrega, 1989). The true and observed retention are defined as (Cheryan, 1998): Rtrue = 1 −
Cp Cm
(2)
Robs = 1 −
Cp Cb
(3)
As it is easy to measure Cb , the observed retention is usually used in practice. A correlation between the true and the observed retention is obtained by combining Eqs. (1)–(3) (Jonsson and Boesen, 1977; Nakao and Kimura, 1981). ln
(5)
The yield of the product during ultrafiltration depends on its concentration in the feed solution and its retention by the membrane. The low concentration of hemicelluloses in thermomechanical pulp process water means that a high concentration factor is needed if the hemicelluloses are to be recovered at a reasonable cost. When the observed retention is constant during concentration, the yield is given by (Cheryan, 1998): Yield = VRF(Robs −1)
2.
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1 − R
obs
Robs
= ln
1 − R
true
Rtrue
J +
k
(4)
The observed retention increases at fluxes above the critical flux due to the formation of an additional permeation barrier on the membrane. This acts as a secondary membrane and is referred to as the “gel” in this paper, although it is not a true gel. When the concentration at the membrane surface approaches a limiting concentration, often called the gel concentration, Cg , the flux is independent of transmembrane pressure and the fluid flow in the boundary layer at the
where VRF is the volume reduction factor (initial feed volume/volume of retentate). The concentration in the retentate, Cr , is given by: Cr = C0 · VRFRobs
(7)
where C0 is the initial concentration in the feed.
3.
Materials and methods
A disc filter filtrate from a Swedish thermomechanical pulp mill producing pulp from spruce wood was used as raw material in this study. The hemicelluloses were isolated from the process water in two steps: (i) microfiltration to remove suspended matter and (ii) ultrafiltration to concentrate and purify the hemicelluloses.
3.1.
Microfiltration
The process water was filtered through a 1 mm gauge mesh prior to microfiltration to remove aggregated fibres which would otherwise damage the pump. The pre-filtered process water was microfiltrated in a Vibratory Shear-Enhanced Processing (VSEP) unit (series L/P, New Logic, Emeryville, CA, USA) to remove the solids. The membrane stack consisted of 19 double-sided polytetrafluoroethylene membrane discs with a pore diameter of 0.2 m (Donaldson membranes, Newton le Willows, UK) and a total membrane area of 1.6 m2 . The pump was a displacement pump (G-03, Wanner Engineering Inc., Minneapolis, MN, USA). Microfiltration was carried out at room temperature. The feed flow was 8.5 l/min, the vibration frequency 50 Hz and the transmembrane pressure 4 bar. The transmembrane pressure is the difference between the average pressure on the feed side and the permeate pressure.
3.2.
Ultrafiltration
The performance of three ultrafiltration membranes, manufactured by Alfa Laval, was investigated. A new, permanently hydrophilic polysulphone membrane, UFX5 (cut-off of 5 kDa), was compared with two surface-modified polyvinylidene fluoride membranes, ETNA10 (cut-off 10 kDa) and ETNA01 (cut-off 1 kDa). The UFX5 and ETNA10 membranes consisted of spiralwound elements with 48 mil (1.2 mm) spacers and the ETNA01 had 30 mil (0.75 mm) spacers. All the elements had an outer diameter of 2.5 in. (0.06 m) and a length of 17 in. (0.43 m). The maximum operating temperature for the ETNA membranes is 60 ◦ C and for the UFX5 membrane 90 ◦ C.
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The bench-scale setup included a displacement pump (D-25, Wanner Engineering Inc.) and pressure transmitters (dTrans p02, JUMO AB, Helsingborg, Sweden) measuring the pressure at the inlet and outlet of the membrane module. The transmembrane pressure is the average pressure between the inlet and outlet, and the pressure drop is the pressure difference between the inlet and outlet. The permeate flow was measured gravimetrically with a balance. Pressure, temperature and permeate flow rate were recorded by a PC equipped with LabView 6.0 software (National Instruments Co., Austin, TX, USA). The microfiltrated process water was concentrated by withdrawing the permeate, while the retentate was recirculated to the feed tank. Concentration was performed at 0.5 bar with the ETNA10 membrane and at 6 bar with the ETNA01 membrane. A higher transmembrane pressure was needed for ETNA01 due to the higher filtration resistance of this membrane. The influence of transmembrane pressure on membrane performance was studied at 0.5, 1.0, 1.5, 2.0 and 2.5 bar, after concentration to VRFs of 4, 14, 50 and 100 with ETNA10, and at 4, 6, 8 and 10 bar after concentration to VRFs of 1, 2, 4, 7, 10 and 20 with ETNA01. Both retentate and permeate were recirculated to the feed tank during the parametric studies. The flux was stabilised for 15 min at each pressure level before samples were withdrawn. Due to the higher fouling susceptibility of the UFX5 membrane, no parametric study was performed during concentration. Instead, three concentration experiments were performed at constant pressures of 2, 4 and 6 bar. The experiments with the ETNA membranes were performed at 60 ◦ C and with the UFX5 membrane at 75 ◦ C. The crossflow in the experiments was initially 1.2 m3 /h for the elements with 48 mil spacers (UFX5 and ETNA10) and 0.6 m3 /h for the element with 30 mil spacers (ETNA01). The crossflow was then continuously adjusted so that the frictional pressure drop in the spiral-wound element did not exceed 0.6 bar, which is the maximum pressure drop recommended by the manufacturer. The membranes were cleaned with an alkaline cleaning agent, 0.5 wt% Ultrasil 10 (Henkel Chemicals Ltd., Düsseldorf, Germany), for about 45 min at 60 ◦ C before and after each experiment. The system was thoroughly rinsed with deionized water after cleaning.
Fig. 1 – Mw of galactoglucomannan and lignin in various batches of microfiltrated process water used in the ultrafiltration experiments. (pore size 1.0 m) and diluted with deionized water. It must be pointed out that other materials in the samples, for example, extractives and carbohydrate-derived compounds, also absorb UV light at 280 nm (Song et al., 2008). This may lead to overestimation of the lignin concentration. The molecular mass distribution of the hemicelluloses and lignin was determined using size-exclusion chromatography. The chromatograph was equipped with a pump and a system controller (600, Waters, Milford, MA, USA), an autosampler (717plus, Waters), a refractive index detector (410, Waters), an ultraviolet detector operating at 280 nm (486, Waters), and a column (16 mm inner diameter) packed with 30 cm of Superdex 30 and 30 cm of Superdex 200 (both from GE Healthcare, Uppsala, Sweden). The injection volume was 2000 l. The flow rate of the 0.5 wt% NaOH solution was set to 1 ml/min. The system was calibrated with polyethylene glycol standards with peak molecular masses of 400, 4000, 10,000 and 40,000 Da (Merck, Darmstadt, Germany). Nine fractions, each of 10 ml, were collected between 39 and 119 min. The mannan concentration in the fractions was used to determine the molecular mass distribution of the hemicellulose GGM.
4. 3.3.
Results
Analysis
The concentration of hemicelluloses was analysed by hydrolysing the polysaccharide to monomeric sugars by acid hydrolysis, according to the standardised NREL method (Ruiz and Ehrman, 1996). The concentration of monomeric sugars was then analysed using high-performance anionexchange chromatography and a pulsed amperometric ED40 electrochemical detector (Dionex, Sunnyvale, CA, USA). The chromatograph was equipped with a GP40 gradient pump, an AS50 autosampler and a Carbo Pac PA10 column (all from Dionex). The injection volume was 10 l, and deionized water with 2 mM NaOH was used as eluent, at a flow rate of 1 ml/min. d-Mannose, d-glucose, d-galactose, d-xylose and l-arabinose (Fluka Chemie AG, Buchs, Switzerland) were used as standards. The lignin content was determined by measuring the light absorption at a wavelength of 280 nm, using a Shimadzu UV-160 spectrophotometer (Kyoto, Japan) and an extinction coefficient of 17.8 l/(g cm) (Örså et al., 1997). Before measurement, the samples were filtered through a glass fibre filter
The concentration of hemicelluloses in the process water was around 1–1.5 g/l. The hemicelluloses were partly retained during microfiltration and the concentration of hemicelluloses after microfiltration varied between 0.5 and 0.9 g/l. The ratio between mannose, glucose and galactose residues was similar to that in GGM except for a minor content of arabinan (4 wt% of the hemicelluloses), probably originating from arabinogalactan (Pranovich et al., 2005). The lignin concentration in the microfiltration permeates varied between 0.6 and 0.7 g/l. Fig. 1 shows the molecular mass of GGM in three batches of microfiltrated process water, with an average molecular mass (Mw ) of hemicelluloses between 5 and 10 kDa. The molecular mass distribution of lignin was identical in all batches, with a Mw of about 1 kDa, and is represented by the dotted line in Fig. 1. The molecular mass of the hemicelluloses and the lignin, shown in Fig. 1, suggests that an ultrafiltration membrane with a cut-off between 1 and 10 kDa should be used to retain the hemicelluloses and allow at least part of the lignin to pass through. The performance of the ETNA10 (evaluated with
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Fig. 2 – Influence of concentration of hemicelluloses in the feed on membrane flux at various transmembrane pressures for the ETNA10 membrane at 60 ◦ C. batch 1), UFX5 (batch 2) and ETNA01 (batch 3) was therefore investigated.
4.1.
Flux
The flux of the ETNA10 membrane was very high at low hemicellulose concentration, even at very low pressure, which is advantageous for both the investment cost and energy requirement. The flux was almost independent of the pressure at transmembrane pressures above 1 bar, as can be seen in Fig. 2. At these pressures, a straight line can be drawn between the flux and the logarithmic concentration, indicating limiting flux conditions, according to Eq. (5). The maximum operating pressure of the ETNA01 membrane is 10 bar, but even at this high pressure the flux was only half of what could be achieved with the ETNA10 membrane at the same hemicellulose concentration. At 10 bar and a hemicellulose concentration of 1 g/l, the flux was no higher than 105 l/m2 h. With the UFX5 membrane it was possible to achieve a flux of the same order of magnitude as that with the ETNA10 membrane, but a higher transmembrane pressure was needed, as can be seen in Fig. 3. A straight line can be drawn between the flux and the logarithmic concentration at a transmembrane pressure of 6 bar, but not at 2 or 4 bar. Thus, according to Eq. (5), limiting flux was reached at 6 bar.
4.2.
Retention and selectivity
Not only a high concentration and a high yield, but also a high purity is required for a separation process to be successful.
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Fig. 3 – Influence of hemicellulose concentration on flux at various transmembrane pressures for the UFX5 membrane at 75 ◦ C. Thus, the retention of hemicelluloses should be high, whereas the retention of lignin and other compounds dissolved in the process water should be low. Transmembrane pressure and the concentration of hemicelluloses in the feed not only affect the flux, but also the retention, as shown in Fig. 4. Flux and retention increase with increasing transmembrane pressure, whereas flux decreases and retention increases with increasing concentration. The retention of hemicelluloses by the ETNA10 membrane was much higher than the lignin retention, as can be seen in Fig. 4, which is necessary to obtain a pure hemicellulose solution as a product. High retention of hemicelluloses is also necessary to ensure a satisfactory yield. The lowest retention of hemicelluloses (72%) was obtained at 2 g/l and 0.5 bar, and the highest at 25 g/l and 2.5 bar (94%). In an effort to improve the retention of hemicelluloses, the performance of the ETNA01 membrane with a cut-off of 1 kDa was investigated. The flux increased with increasing pressure but, in contrast to ETNA10, the retention of hemicelluloses decreased with increasing pressure, as shown in Fig. 5. Despite the fact that the cut-off was 10 times lower than that of the ETNA10 membrane, the retention of hemicelluloses did not exceed 90% under any of the conditions investigated. Concentrations above 4 g/l were not investigated as both the flux and retention were inadequate. A marked increase in flux, but no significant effect on retention, with increasing transmembrane pressure was noted when evaluating the performance of the UFX5 membrane in the pressure interval 2–6 bar, as shown in Fig. 6. The retention of both hemicelluloses and lignin was higher than for
Fig. 4 – Influence of transmembrane pressure on flux and the retention of hemicelluloses and lignin with the ETNA10 membrane when the concentration of hemicelluloses was 2 g/l (results shown in the figure to the left) and 25 g/l (results shown in the figure to the right).
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Fig. 5 – Influence of transmembrane pressure on flux and retention of hemicelluloses and lignin by the ETNA01 membrane when the hemicellulose concentration was 2 g/l. both ETNA membranes. The retention of hemicelluloses was above 90% in the concentration interval 2–15 g/l hemicelluloses at all pressures. The retention of lignin was between 30 and 50% in the same concentration interval. Although the retention of lignin may seem high, it is still possible to separate hemicelluloses and lignin due to the large volume reduction achieved during ultrafiltration. The hemicellulose concentration was increased from 0.7 to 10–15 g/l, whereas the lignin concentration was only increased from 0.6 to 2 g/l in the three experiments with the UFX5 membrane.
4.3.
Fouling
The fouling tendencies of the ETNA10 and the UFX5 membrane were studied by measuring the flux of microfiltered process water continuously for several days while recirculating both the retentate and the permeate to the feed tank. The concentration of hemicelluloses was about 1 g/l. The ETNA10 membrane was operated at a transmembrane pressure of 1 bar and a temperature of 60 ◦ C for 7 days, and the UFX5 membrane at 2 bar and 75 ◦ C for 3 days. In Fig. 7, it can be seen that the flux decline was less than 20% after 7 days for the ETNA10 membrane and around 60% after 3 days for the UFX5 membrane, which indicates that the UFX5 membrane will probably have to be cleaned more frequently in a full-scale process. The pure water flux of both membranes was completely recovered after cleaning. The pure water flux of the ETNA10 membrane
Fig. 7 – Flux decline (fouling) of the ETNA10 and UFX5 membranes during ultrafiltration of process water without concentration. was 210 l/m2 h at 1 bar and 105 l/m2 h of the UFX5 membrane at 2 bar.
5.
Discussion
The process water used in the experiments needs to be fresh. The properties of the process water vary due to variations in the refining process. It is therefore not possible to repeat experiments at identical conditions, and hence, difficult to reproduce experiments. However, results from the investigation are in good agreement with results from other investigations with process water from a thermomecanical pulp mill (Persson et al., 2007, 2010). The results discussed below are thus characteristic of this application.
5.1.
Retention and purity
A large VRF is required to concentrate the hemicelluloses from less than 1 g/l to more than 30 g/l, which is necessary if the hemicelluloses are to be used in high-value-added products. For the process to be cost efficient, both the flux and hemicellulose retention must be high. A low flux increases the investment cost, and a large VRF in combination with a low retention decreases the yield (see Eq. (6)). To maintain the yield of hemicelluloses above 70%, while concentrating the hemicelluloses from 1 to 30 g/l, the hemicellulose retention must be above 90%, according to Eqs. (6) and (7). A large VRF is beneficial when increasing the purity of the hemicelluloses by removing the lignin from the process water. A VRF of about 45 is required to increase the concentration of the hemicelluloses from 1 to 30 g/l at a hemicellulose retention of 90%. At this VRF more than 85% of the lignin is removed, despite the fact that the lignin retention is as high as 50%. Thus, a high retention of hemicelluloses is more important than a low retention of lignin.
5.2. Decreased retention due to concentration polarisation
Fig. 6 – Influence of transmembrane pressure on flux and retention of hemicelluloses and lignin by the UFX5 membrane when the hemicellulose concentration was 2 g/l.
According to Eq. (4), a high flux decreases the observed retention due to increased concentration polarisation, as long as the critical flux is not reached and the true retention is constant. The hemicellulose retention decreased with increasing flux in the experiments with both the ETNA membranes, but not with the UFX5 membrane, probably due to the high true retention
chemical engineering research and design 8 8 ( 2 0 1 0 ) 1548–1554
Fig. 8 – Influence of flux on observed retention of hemicelluloses at various hemicellulose concentrations using the ETNA01 membrane at 60 ◦ C. of this membrane. The influence of flux on the retention of hemicelluloses by ETNA01 is shown in Fig. 8. The dashed line in Fig. 8 was calculated from Eqs. (1)–(3) using a mass transfer coefficient of 1.05 × 10−5 m/s and a true retention of 94%. The mass transfer coefficient and the true retention were calculated using the experimental values and plotting ln((1 − Robs )/Robs ) against the flux and using the slope to calculate the mass transfer coefficient, and the intercept on the y-axis to give the true retention, in accordance with Eq. (4). A tendency towards increased retention with increasing hemicellulose concentration can be observed in Fig. 8. This is probably due to an increase in Mw during concentration. It can be seen in Fig. 8 that the observed retention of hemicelluloses decreases from about 90% to 50% as the flux increases from 25 to 100 l/m2 h. A combination of a high flux and high retention is a prerequisite for the process to be cost efficient. Thus, ETNA01 is not a feasible alternative for the current application.
5.3.
Increased retention due to gel formation
Retention decreases with increasing flux below the critical flux, but increases when a gel is formed on the membrane. The critical flux of the ETNA01 membrane was not reached in the experiments described here due to the high filtration resistance and the limitation on the transmembrane pressure of this membrane. However, ETNA10 can be operated at transmembrane pressures above the critical flux where the formation of a gel can increase the retention. The retention of the ETNA10 membrane decreased initially with increasing flux. However, when the pressure was increased beyond the critical flux, a small increase in flux was seen, while the retention increased significantly, as can be seen in Fig. 9. The increase in the retention above the critical flux is due to the formation of a gel that acts as an additional permeation barrier. The results shown in Fig. 9 imply that in order to achieve an acceptable retention at low hemicellulose concentrations, the ETNA10 membrane must be operated above the critical flux. By creating a gel on the surface of the membrane the hemicellulose retention can be increased above 90% even at low concentrations. The properties of the additional retention barrier formed on the membrane depend on the transmembrane pressure, the crossflow velocity used and the molecular mass of the hemicelluloses.
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Fig. 9 – Influence of flux on the observed retention of hemicelluloses at various hemicellulose concentrations using the ETNA10 membrane at 60 ◦ C. The data points for each hemicellulose concentration represent the transmembrane pressures 0.5, 1.0, 1.5, 2.0 and 2.5 bar (except for 26 g/l where the point representing 2.0 bar is missing).
5.4.
Increased Mw during ultrafiltration
The hemicelluloses in the process water are polydisperse with molecular masses mainly between 1 and 10 kDa, as shown in Fig. 1. If hemicelluloses are not totally retained during ultrafiltration (i.e., if R < 100%), the Mw of the hemicelluloses will increase during concentration. The yield of GGM with Mw below 4 kDa was only 14%, while it was 53% for GGM above 4 kDa, when the hemicelluloses were concentrated from 0.9 to 5 g/l at 0.5 bar during ultrafiltration with the ETNA10 membrane. The corresponding values for the UFX5 membrane were 45% and 96% during concentration from 0.7 to 5 g/l at 2 bar. Hence, the molecular mass distribution of the product will depend on the retention characteristics of the membrane used during ultrafiltration.
5.5.
Pressure drop due to high flux
Permeate fluxes between 150 and 200 l/m2 h could be obtained at low hemicellulose concentrations with both the ETNA10 and UFX5 membranes. These fluxes are higher than in most applications employing spiral-wound membranes. Thus, possible negative effects arising from pressure losses in the permeate spacer and permeate collector must be considered when scaling up the process. The length of the permeate flow channel equals the length of the membrane sheets. The membrane sheets in the 2.5 in. spiral-wound membrane elements used in this study were 1.4 m in length. In 8 in. elements, commonly used in larger full-scale applications, the length of the membrane sheets is 1.5 m. Thus, the pressure drop in the permeate spacer is similar, regardless of the diameter of the membrane element, and should not be important when scaling up the process from bench scale to full scale. In a full-scale housing, three membrane elements are connected in series and the permeate from all three membranes is removed from the housing through the same tubular permeate collector. The magnitude of the frictional pressure drop in the permeate collector was calculated by assuming a flux of 200 l/m2 h, a permeate temperature of 75 ◦ C and
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using data from the membrane supplier (40 m2 /element, 1 m length/element, 31 mm inner diameter of permeate collector). The frictional pressure drop is given by (Bird et al., 2007): Pf = 4f
L v2 dh
2
(8)
where f is a friction factor (estimated to be 0.005–0.01 in the Reynolds number interval in question), L is the length of the flow path in the collector, dh is the inner diameter of the collector, is the density and v is the velocity of the permeate in the permeate collector. The pressure drop in the permeate collector was found to be less than 1 bar under the prevailing conditions. The pressure drop allowed on the feed side is 1 bar/element, and in comparison with this the pressure drop in the permeate collector is of minor importance, even when the flux is very high. Furthermore, the pressure drop in the permeate collector is in the same direction as the pressure drop on the feed side, and should therefore be beneficial for the process since it evens out the variation in transmembrane pressure between the elements in the housing.
6.
Conclusions
Flux, retention and fouling have a considerable impact on the cost of an ultrafiltration process. The performance of three membranes for the separation of hemicelluloses from thermomechanical pulp mill process water was studied in this investigation. Two membranes (ETNA10 and UFX5) could be operated at rather high fluxes. Furthermore, the UFX5 membrane had a high hemicellulose retention, independent of the operating conditions, while ultrafiltration with the ETNA10 membrane had to be run above the critical flux to create a gel layer that increased the hemicellulose retention. An additional benefit of the UFX5 membrane is its high temperature tolerance (90 ◦ C). The temperature of the process water in the pulp mill is 75–85 ◦ C and would have to be cooled to 60 ◦ C to use the ETNA membrane, which would be costly. The lower fouling susceptibility of the ETNA10 membrane is, however, an important advantage. The cleaning frequency must be further evaluated in pilot trials at a pulp mill, but according to the results shown in Fig. 7, UFX5 must be cleaned more often than ETNA10, which is costly. However, an important observation made in this investigation is that it was possible to restore the pure water flux of both membranes after cleaning in five subsequent experiments. The Mw of the hemicelluloses increased during concentration with both the UFX5 and ETNA10 membranes, but to different degrees. Thus, the Mw of the hemicelluloses could perhaps be altered by changing the membrane and the operating conditions. This may be useful to adjust the properties of the hemicelluloses for specific applications. The ETNA01 membrane was found to be unsuitable for this application, since both the flux and the retention were too low for a commercial hemicellulose isolation process, regardless of whether it was operated at a high or low transmembrane pressure.
Acknowledgements Alfa Laval is gratefully acknowledged for donating the membranes, and MISTRA, the Swedish Foundation for Strategic Environmental Research, for financial support.
References Bird, R.B., Stewart, W.E. and Lightfoot, E.N., (2007). Transport Phenomena Second Edition. (John Wiley & Sons, Inc, New York, USA), pp. 177–196 Blatt, W.F., Dravid, A., Michaels, A.S. and Nelsen, L., (1970). Solute Polarization and Cake Formation in Membrane Ultrafiltration: Causes, Consequences, and Control Techniques. (Memb. Sci. Tech. Plenum Press, New York, NY, USA). Cheryan, M., (1998). Ultrafiltration and Microfiltration Handbook. (Technomic Publishing Co, Lancaster, USA). de Balmann, H. and Nobrega, R., 1989, The deformation of dextran molecules—causes and consequences in ultrafiltration. Journal of Membrane Science, 40: 311–327. Hansen, N.M.L. and Plackett, D., 2008, Sustainable films and coatings from hemicelluloses: a review. Biomacromolecules, 9(6): 1493–1505. Jonsson, G., 1986, Transport phenomena in ultrafiltration: membrane selectivity and boundary layer phenomena. Pure and Applied Chemistry, 58: 1647–1656. Jonsson, G. and Boesen, C.E., 1977, Concentration polarization in a reverse osmosis test cell. Desalination, 21: 1–10. Lima, D.U., Oliveira, R.C. and Buckeridge, M.S., 2003, Seed storage hemicelluloses as wet-end additives in papermaking. Carbohydrate Polymers, 52: 367–373. Maartens, A., Jacobs, E.P. and Swart, P., 2002, UF of pulp and paper effluent: membrane fouling-prevention and cleaning. Journal of Membrane Science, 209: 81–92. Nakao, S.-I. and Kimura, S., 1981, Analysis of solutes rejection in ultrafiltration. Journal of Chemical Engineering of Japan, 32: 32.37. Örså, F., Holmbom, B. and Thornton, J., 1997, Dissolution and dispersion of spruce wood components into hot water. Wood Science and Technology, 31: 279–290. Persson, T., Krawczyk, H., Nordin, A.-K. and Jönsson, A.-S., 2010, Fractionation of process water in thermomechanical pulp mills. Bioresource Technology, 101: 3884–3892. Persson, T., Jönsson, A.-S. and Zacchi, G., 2005, Fractionation of hemicelluloses by membrane filtration, In 14th European Biomass Conference and Exhibition Paris, France, October 17–21, Persson, T., Nordin, A.-K., Zacchi, G. and Jönsson, A.-S., 2007, Economic evaluation of isolation of hemicelluloses from process streams from thermo-mechanical pulping of spruce. Applied Biochemistry and Biotechnology, 136–140: 741–752. Pranovich, A.V., Reunanen, M., Sjöholm, R. and Holmbom, B., 2005, Dissolved lignin and other aromatic substances in thermomechanical pulp waters. Journal of Wood Chemistry and Technology, 25: 109–132. Ruiz, R. and Ehrman, T., 1996, Dilute acid hydrolysis procedure for determination of total sugars in the liquid fraction of process samples. Laboratory Analytical Procedure-014, Schock, G. and Miquel, A., 1987, Mass transfer and pressure loss in spiral wound modules. Desalination, 64: 339–352. Song, T., Pranovich, A., Sumerskiy, I. and Holmbom, B., 2008, Extraction of galactoglucomannan from spruce wood with pressurised hot water. Holzforschung, 62: 659–666. Thornton, J., Ekman, R., Holmbom, B. and Örså, F., 1994, Polysaccharides dissolved from Norway spruce in thermomechanical pulping and peroxide bleaching. Journal of Wood Chemistry and Technology, 14: 159–175. Willför, S., Sjöholm, R., Laine, C., Roslund, M., Hemming, J. and Holmbom, B., 2003, Characterisation of water soluble galactoglucomannans from Norway spruce wood and thermomechanical pulp. Carbohydrate Polymers, 52: 175–187. Willför, S., Sundberg, K., Tenkanen, M. and Holmbom, B., 2008, Spruce-derived mannans—a potential raw material for hydrocolloids and novel advance natural materials. Carbohydrate Polymers, 72: 197–210.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Isolation of hemicelluloses by ultrafiltration of thermomechanical pulp mill process water—Influence of operating conditions T. Persson, A.-S. Jönsson ∗ Department of Chemical Engineering, Lund University, PO Box 124, SE-221 00 Lund, Sweden
a b s t r a c t Hemicelluloses could be used to replace fossil-based materials in several high-value-added products. Today, vast amounts of hemicelluloses are discharged from pulp mills all over the world as waste, but these could be isolated by membrane filtration, and utilized in various applications. In this study, the hemicellulose galactoglucomannan was isolated from process water from a thermomechanical pulp mill using ultrafiltration. The retention of hemicelluloses and lignin, and the flux and fouling of three ultrafiltration membranes (ETNA01, ETNA10 and UFX5) were studied at various operating conditions. One of the membranes (UFX5) was found to have a high hemicellulose retention (above 90%) independent of flux and pressure. With the ETNA01 membrane it was impossible to combine a high flux with high hemicellulose retention, while with the ETNA10 membrane the hemicellulose retention could be increased above 90% by running at transmembrane pressures above the critical flux. The UFX5 membrane could be used at the temperature of the process water in the pulp mill (75–85 ◦ C), while the ETNA10 membrane could only withstand temperatures below 60 ◦ C, increasing the cost due to the need to cool the process water. However, the susceptibility of UFX5 to fouling was much greater than for ETNA10, which would increase the cleaning cost of the UFX5 membrane. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Galactoglucomannan; Ultrafiltration; Spiral-wound membrane; ETNA10; UFX5
1.
Introduction
In thermomechanical pulp mills, 95% of the wood is used in the manufacture of paper, while the remaining 5% is discharged as waste. Galactoglucomannans (GGM) are the main hemicelluloses in softwood. The amount of GGM dissolved in the effluent process water corresponds to 1–2% of the wood (Thornton et al., 1994), and the amount of dissolved lignin to 0.5–1% of the wood (Pranovich et al., 2005). If the hemicelluloses could be isolated from the waste stream, they could be used in high-value-added products such as barrier films, coatings, hydrogels or paper additives (Lima et al., 2003; Hansen and Plackett, 2008). The characteristics of the process water in a thermomechanical pulp mill have been studied in detail (Thornton et al., 1994; Willför et al., 2003, 2008; Pranovich et al., 2005), but the isolation of valuable compounds such as hemicelluloses from the process water has not been studied to the same extent
∗
(Persson et al., 2010). When isolating a substance at a very low concentration, such as hemicelluloses in pulp mill process water, the cost of the separation process will determine whether it is economically feasible, or not. Ultrafiltration is a suitable separation technique for this kind of application due to its low energy requirement and the fact that separation is achieved without the need for chemicals. However, the choice of membrane and operating conditions has considerable impact on the yield and problems such as fouling and, hence, on the economics of the isolation process. Previous studies have shown that hydrophobic molecules, such as lignin and resins, fouled hydrophobic membranes during ultrafiltration of pulp mill process streams, whereas almost no fouling was observed when hydrophilic membranes were used (Maartens et al., 2002; Persson et al., 2005). A plate-and-frame module was used in a previous investigation of ultrafiltration of process water from a thermomechanical pulp mill (Persson et al., 2007). In the present
Corresponding author. Tel.: +46 46 222 8291; fax: +46 46 222 4526. E-mail address: ann-sofi[email protected] (A.-S. Jönsson). Received 20 May 2009; Received in revised form 24 March 2010; Accepted 6 April 2010 0263-8762/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2010.04.002
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study, spiral-wound modules were used. The maximum operating temperature of the ETNA membranes (Alfa Laval, Nakskov, Denmark), used successfully in the previous investigation, is only 60 ◦ C whereas the temperature of the process water in the pulp mill is 75–85 ◦ C. It would thus be beneficial to use a membrane with similar hydrophilic properties and cutoff but higher temperature tolerance. A membrane that fulfils these requirements is the newly developed UFX5 membrane (Alfa Laval), made of permanently hydrophilic polysulphone which has a maximum operating temperature of 90 ◦ C. The influence of transmembrane pressure and the concentration of hemicelluloses on (i) flux, (ii) the retention of hemicelluloses and lignin, and (iii) fouling was studied to identify the most suitable membrane and operating conditions.
membrane surface can be described by (Blatt et al., 1970):
J = k ln
Cg − Cp Cb − Cp
(6)
Theory
Due to the low concentration of hemicelluloses in the process water, the initial flux will be high during ultrafiltration, which is favourable for the productivity. However, when the flux is high, the pressure drop in the permeate spacer may be significant (Schock and Miquel, 1987), which would decrease the productivity. Furthermore, a high flux increases the concentration polarisation of the retained molecules, which may decrease retention and increase fouling. The concentration at the membrane surface increases with increasing flux as (Cheryan, 1998) Cm = Cp + (Cb − Cp ) · exp
J
(1)
k
where Cm , Cp and Cb are the concentration at the membrane surface, in the permeate and in the bulk, respectively, J is the flux and k is the mass transfer coefficient. The true retention is constant and equal to the reflection coefficient for a specific membrane and a specific molecule at high fluxes. However, the observed retention will decrease with increasing flux until a boundary layer is formed on the membrane (Jonsson, 1986; de Balmann and Nobrega, 1989). The true and observed retention are defined as (Cheryan, 1998): Rtrue = 1 −
Cp Cm
(2)
Robs = 1 −
Cp Cb
(3)
As it is easy to measure Cb , the observed retention is usually used in practice. A correlation between the true and the observed retention is obtained by combining Eqs. (1)–(3) (Jonsson and Boesen, 1977; Nakao and Kimura, 1981). ln
(5)
The yield of the product during ultrafiltration depends on its concentration in the feed solution and its retention by the membrane. The low concentration of hemicelluloses in thermomechanical pulp process water means that a high concentration factor is needed if the hemicelluloses are to be recovered at a reasonable cost. When the observed retention is constant during concentration, the yield is given by (Cheryan, 1998): Yield = VRF(Robs −1)
2.
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1 − R
obs
Robs
= ln
1 − R
true
Rtrue
J +
k
(4)
The observed retention increases at fluxes above the critical flux due to the formation of an additional permeation barrier on the membrane. This acts as a secondary membrane and is referred to as the “gel” in this paper, although it is not a true gel. When the concentration at the membrane surface approaches a limiting concentration, often called the gel concentration, Cg , the flux is independent of transmembrane pressure and the fluid flow in the boundary layer at the
where VRF is the volume reduction factor (initial feed volume/volume of retentate). The concentration in the retentate, Cr , is given by: Cr = C0 · VRFRobs
(7)
where C0 is the initial concentration in the feed.
3.
Materials and methods
A disc filter filtrate from a Swedish thermomechanical pulp mill producing pulp from spruce wood was used as raw material in this study. The hemicelluloses were isolated from the process water in two steps: (i) microfiltration to remove suspended matter and (ii) ultrafiltration to concentrate and purify the hemicelluloses.
3.1.
Microfiltration
The process water was filtered through a 1 mm gauge mesh prior to microfiltration to remove aggregated fibres which would otherwise damage the pump. The pre-filtered process water was microfiltrated in a Vibratory Shear-Enhanced Processing (VSEP) unit (series L/P, New Logic, Emeryville, CA, USA) to remove the solids. The membrane stack consisted of 19 double-sided polytetrafluoroethylene membrane discs with a pore diameter of 0.2 m (Donaldson membranes, Newton le Willows, UK) and a total membrane area of 1.6 m2 . The pump was a displacement pump (G-03, Wanner Engineering Inc., Minneapolis, MN, USA). Microfiltration was carried out at room temperature. The feed flow was 8.5 l/min, the vibration frequency 50 Hz and the transmembrane pressure 4 bar. The transmembrane pressure is the difference between the average pressure on the feed side and the permeate pressure.
3.2.
Ultrafiltration
The performance of three ultrafiltration membranes, manufactured by Alfa Laval, was investigated. A new, permanently hydrophilic polysulphone membrane, UFX5 (cut-off of 5 kDa), was compared with two surface-modified polyvinylidene fluoride membranes, ETNA10 (cut-off 10 kDa) and ETNA01 (cut-off 1 kDa). The UFX5 and ETNA10 membranes consisted of spiralwound elements with 48 mil (1.2 mm) spacers and the ETNA01 had 30 mil (0.75 mm) spacers. All the elements had an outer diameter of 2.5 in. (0.06 m) and a length of 17 in. (0.43 m). The maximum operating temperature for the ETNA membranes is 60 ◦ C and for the UFX5 membrane 90 ◦ C.
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The bench-scale setup included a displacement pump (D-25, Wanner Engineering Inc.) and pressure transmitters (dTrans p02, JUMO AB, Helsingborg, Sweden) measuring the pressure at the inlet and outlet of the membrane module. The transmembrane pressure is the average pressure between the inlet and outlet, and the pressure drop is the pressure difference between the inlet and outlet. The permeate flow was measured gravimetrically with a balance. Pressure, temperature and permeate flow rate were recorded by a PC equipped with LabView 6.0 software (National Instruments Co., Austin, TX, USA). The microfiltrated process water was concentrated by withdrawing the permeate, while the retentate was recirculated to the feed tank. Concentration was performed at 0.5 bar with the ETNA10 membrane and at 6 bar with the ETNA01 membrane. A higher transmembrane pressure was needed for ETNA01 due to the higher filtration resistance of this membrane. The influence of transmembrane pressure on membrane performance was studied at 0.5, 1.0, 1.5, 2.0 and 2.5 bar, after concentration to VRFs of 4, 14, 50 and 100 with ETNA10, and at 4, 6, 8 and 10 bar after concentration to VRFs of 1, 2, 4, 7, 10 and 20 with ETNA01. Both retentate and permeate were recirculated to the feed tank during the parametric studies. The flux was stabilised for 15 min at each pressure level before samples were withdrawn. Due to the higher fouling susceptibility of the UFX5 membrane, no parametric study was performed during concentration. Instead, three concentration experiments were performed at constant pressures of 2, 4 and 6 bar. The experiments with the ETNA membranes were performed at 60 ◦ C and with the UFX5 membrane at 75 ◦ C. The crossflow in the experiments was initially 1.2 m3 /h for the elements with 48 mil spacers (UFX5 and ETNA10) and 0.6 m3 /h for the element with 30 mil spacers (ETNA01). The crossflow was then continuously adjusted so that the frictional pressure drop in the spiral-wound element did not exceed 0.6 bar, which is the maximum pressure drop recommended by the manufacturer. The membranes were cleaned with an alkaline cleaning agent, 0.5 wt% Ultrasil 10 (Henkel Chemicals Ltd., Düsseldorf, Germany), for about 45 min at 60 ◦ C before and after each experiment. The system was thoroughly rinsed with deionized water after cleaning.
Fig. 1 – Mw of galactoglucomannan and lignin in various batches of microfiltrated process water used in the ultrafiltration experiments. (pore size 1.0 m) and diluted with deionized water. It must be pointed out that other materials in the samples, for example, extractives and carbohydrate-derived compounds, also absorb UV light at 280 nm (Song et al., 2008). This may lead to overestimation of the lignin concentration. The molecular mass distribution of the hemicelluloses and lignin was determined using size-exclusion chromatography. The chromatograph was equipped with a pump and a system controller (600, Waters, Milford, MA, USA), an autosampler (717plus, Waters), a refractive index detector (410, Waters), an ultraviolet detector operating at 280 nm (486, Waters), and a column (16 mm inner diameter) packed with 30 cm of Superdex 30 and 30 cm of Superdex 200 (both from GE Healthcare, Uppsala, Sweden). The injection volume was 2000 l. The flow rate of the 0.5 wt% NaOH solution was set to 1 ml/min. The system was calibrated with polyethylene glycol standards with peak molecular masses of 400, 4000, 10,000 and 40,000 Da (Merck, Darmstadt, Germany). Nine fractions, each of 10 ml, were collected between 39 and 119 min. The mannan concentration in the fractions was used to determine the molecular mass distribution of the hemicellulose GGM.
4. 3.3.
Results
Analysis
The concentration of hemicelluloses was analysed by hydrolysing the polysaccharide to monomeric sugars by acid hydrolysis, according to the standardised NREL method (Ruiz and Ehrman, 1996). The concentration of monomeric sugars was then analysed using high-performance anionexchange chromatography and a pulsed amperometric ED40 electrochemical detector (Dionex, Sunnyvale, CA, USA). The chromatograph was equipped with a GP40 gradient pump, an AS50 autosampler and a Carbo Pac PA10 column (all from Dionex). The injection volume was 10 l, and deionized water with 2 mM NaOH was used as eluent, at a flow rate of 1 ml/min. d-Mannose, d-glucose, d-galactose, d-xylose and l-arabinose (Fluka Chemie AG, Buchs, Switzerland) were used as standards. The lignin content was determined by measuring the light absorption at a wavelength of 280 nm, using a Shimadzu UV-160 spectrophotometer (Kyoto, Japan) and an extinction coefficient of 17.8 l/(g cm) (Örså et al., 1997). Before measurement, the samples were filtered through a glass fibre filter
The concentration of hemicelluloses in the process water was around 1–1.5 g/l. The hemicelluloses were partly retained during microfiltration and the concentration of hemicelluloses after microfiltration varied between 0.5 and 0.9 g/l. The ratio between mannose, glucose and galactose residues was similar to that in GGM except for a minor content of arabinan (4 wt% of the hemicelluloses), probably originating from arabinogalactan (Pranovich et al., 2005). The lignin concentration in the microfiltration permeates varied between 0.6 and 0.7 g/l. Fig. 1 shows the molecular mass of GGM in three batches of microfiltrated process water, with an average molecular mass (Mw ) of hemicelluloses between 5 and 10 kDa. The molecular mass distribution of lignin was identical in all batches, with a Mw of about 1 kDa, and is represented by the dotted line in Fig. 1. The molecular mass of the hemicelluloses and the lignin, shown in Fig. 1, suggests that an ultrafiltration membrane with a cut-off between 1 and 10 kDa should be used to retain the hemicelluloses and allow at least part of the lignin to pass through. The performance of the ETNA10 (evaluated with
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Fig. 2 – Influence of concentration of hemicelluloses in the feed on membrane flux at various transmembrane pressures for the ETNA10 membrane at 60 ◦ C. batch 1), UFX5 (batch 2) and ETNA01 (batch 3) was therefore investigated.
4.1.
Flux
The flux of the ETNA10 membrane was very high at low hemicellulose concentration, even at very low pressure, which is advantageous for both the investment cost and energy requirement. The flux was almost independent of the pressure at transmembrane pressures above 1 bar, as can be seen in Fig. 2. At these pressures, a straight line can be drawn between the flux and the logarithmic concentration, indicating limiting flux conditions, according to Eq. (5). The maximum operating pressure of the ETNA01 membrane is 10 bar, but even at this high pressure the flux was only half of what could be achieved with the ETNA10 membrane at the same hemicellulose concentration. At 10 bar and a hemicellulose concentration of 1 g/l, the flux was no higher than 105 l/m2 h. With the UFX5 membrane it was possible to achieve a flux of the same order of magnitude as that with the ETNA10 membrane, but a higher transmembrane pressure was needed, as can be seen in Fig. 3. A straight line can be drawn between the flux and the logarithmic concentration at a transmembrane pressure of 6 bar, but not at 2 or 4 bar. Thus, according to Eq. (5), limiting flux was reached at 6 bar.
4.2.
Retention and selectivity
Not only a high concentration and a high yield, but also a high purity is required for a separation process to be successful.
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Fig. 3 – Influence of hemicellulose concentration on flux at various transmembrane pressures for the UFX5 membrane at 75 ◦ C. Thus, the retention of hemicelluloses should be high, whereas the retention of lignin and other compounds dissolved in the process water should be low. Transmembrane pressure and the concentration of hemicelluloses in the feed not only affect the flux, but also the retention, as shown in Fig. 4. Flux and retention increase with increasing transmembrane pressure, whereas flux decreases and retention increases with increasing concentration. The retention of hemicelluloses by the ETNA10 membrane was much higher than the lignin retention, as can be seen in Fig. 4, which is necessary to obtain a pure hemicellulose solution as a product. High retention of hemicelluloses is also necessary to ensure a satisfactory yield. The lowest retention of hemicelluloses (72%) was obtained at 2 g/l and 0.5 bar, and the highest at 25 g/l and 2.5 bar (94%). In an effort to improve the retention of hemicelluloses, the performance of the ETNA01 membrane with a cut-off of 1 kDa was investigated. The flux increased with increasing pressure but, in contrast to ETNA10, the retention of hemicelluloses decreased with increasing pressure, as shown in Fig. 5. Despite the fact that the cut-off was 10 times lower than that of the ETNA10 membrane, the retention of hemicelluloses did not exceed 90% under any of the conditions investigated. Concentrations above 4 g/l were not investigated as both the flux and retention were inadequate. A marked increase in flux, but no significant effect on retention, with increasing transmembrane pressure was noted when evaluating the performance of the UFX5 membrane in the pressure interval 2–6 bar, as shown in Fig. 6. The retention of both hemicelluloses and lignin was higher than for
Fig. 4 – Influence of transmembrane pressure on flux and the retention of hemicelluloses and lignin with the ETNA10 membrane when the concentration of hemicelluloses was 2 g/l (results shown in the figure to the left) and 25 g/l (results shown in the figure to the right).
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Fig. 5 – Influence of transmembrane pressure on flux and retention of hemicelluloses and lignin by the ETNA01 membrane when the hemicellulose concentration was 2 g/l. both ETNA membranes. The retention of hemicelluloses was above 90% in the concentration interval 2–15 g/l hemicelluloses at all pressures. The retention of lignin was between 30 and 50% in the same concentration interval. Although the retention of lignin may seem high, it is still possible to separate hemicelluloses and lignin due to the large volume reduction achieved during ultrafiltration. The hemicellulose concentration was increased from 0.7 to 10–15 g/l, whereas the lignin concentration was only increased from 0.6 to 2 g/l in the three experiments with the UFX5 membrane.
4.3.
Fouling
The fouling tendencies of the ETNA10 and the UFX5 membrane were studied by measuring the flux of microfiltered process water continuously for several days while recirculating both the retentate and the permeate to the feed tank. The concentration of hemicelluloses was about 1 g/l. The ETNA10 membrane was operated at a transmembrane pressure of 1 bar and a temperature of 60 ◦ C for 7 days, and the UFX5 membrane at 2 bar and 75 ◦ C for 3 days. In Fig. 7, it can be seen that the flux decline was less than 20% after 7 days for the ETNA10 membrane and around 60% after 3 days for the UFX5 membrane, which indicates that the UFX5 membrane will probably have to be cleaned more frequently in a full-scale process. The pure water flux of both membranes was completely recovered after cleaning. The pure water flux of the ETNA10 membrane
Fig. 7 – Flux decline (fouling) of the ETNA10 and UFX5 membranes during ultrafiltration of process water without concentration. was 210 l/m2 h at 1 bar and 105 l/m2 h of the UFX5 membrane at 2 bar.
5.
Discussion
The process water used in the experiments needs to be fresh. The properties of the process water vary due to variations in the refining process. It is therefore not possible to repeat experiments at identical conditions, and hence, difficult to reproduce experiments. However, results from the investigation are in good agreement with results from other investigations with process water from a thermomecanical pulp mill (Persson et al., 2007, 2010). The results discussed below are thus characteristic of this application.
5.1.
Retention and purity
A large VRF is required to concentrate the hemicelluloses from less than 1 g/l to more than 30 g/l, which is necessary if the hemicelluloses are to be used in high-value-added products. For the process to be cost efficient, both the flux and hemicellulose retention must be high. A low flux increases the investment cost, and a large VRF in combination with a low retention decreases the yield (see Eq. (6)). To maintain the yield of hemicelluloses above 70%, while concentrating the hemicelluloses from 1 to 30 g/l, the hemicellulose retention must be above 90%, according to Eqs. (6) and (7). A large VRF is beneficial when increasing the purity of the hemicelluloses by removing the lignin from the process water. A VRF of about 45 is required to increase the concentration of the hemicelluloses from 1 to 30 g/l at a hemicellulose retention of 90%. At this VRF more than 85% of the lignin is removed, despite the fact that the lignin retention is as high as 50%. Thus, a high retention of hemicelluloses is more important than a low retention of lignin.
5.2. Decreased retention due to concentration polarisation
Fig. 6 – Influence of transmembrane pressure on flux and retention of hemicelluloses and lignin by the UFX5 membrane when the hemicellulose concentration was 2 g/l.
According to Eq. (4), a high flux decreases the observed retention due to increased concentration polarisation, as long as the critical flux is not reached and the true retention is constant. The hemicellulose retention decreased with increasing flux in the experiments with both the ETNA membranes, but not with the UFX5 membrane, probably due to the high true retention
chemical engineering research and design 8 8 ( 2 0 1 0 ) 1548–1554
Fig. 8 – Influence of flux on observed retention of hemicelluloses at various hemicellulose concentrations using the ETNA01 membrane at 60 ◦ C. of this membrane. The influence of flux on the retention of hemicelluloses by ETNA01 is shown in Fig. 8. The dashed line in Fig. 8 was calculated from Eqs. (1)–(3) using a mass transfer coefficient of 1.05 × 10−5 m/s and a true retention of 94%. The mass transfer coefficient and the true retention were calculated using the experimental values and plotting ln((1 − Robs )/Robs ) against the flux and using the slope to calculate the mass transfer coefficient, and the intercept on the y-axis to give the true retention, in accordance with Eq. (4). A tendency towards increased retention with increasing hemicellulose concentration can be observed in Fig. 8. This is probably due to an increase in Mw during concentration. It can be seen in Fig. 8 that the observed retention of hemicelluloses decreases from about 90% to 50% as the flux increases from 25 to 100 l/m2 h. A combination of a high flux and high retention is a prerequisite for the process to be cost efficient. Thus, ETNA01 is not a feasible alternative for the current application.
5.3.
Increased retention due to gel formation
Retention decreases with increasing flux below the critical flux, but increases when a gel is formed on the membrane. The critical flux of the ETNA01 membrane was not reached in the experiments described here due to the high filtration resistance and the limitation on the transmembrane pressure of this membrane. However, ETNA10 can be operated at transmembrane pressures above the critical flux where the formation of a gel can increase the retention. The retention of the ETNA10 membrane decreased initially with increasing flux. However, when the pressure was increased beyond the critical flux, a small increase in flux was seen, while the retention increased significantly, as can be seen in Fig. 9. The increase in the retention above the critical flux is due to the formation of a gel that acts as an additional permeation barrier. The results shown in Fig. 9 imply that in order to achieve an acceptable retention at low hemicellulose concentrations, the ETNA10 membrane must be operated above the critical flux. By creating a gel on the surface of the membrane the hemicellulose retention can be increased above 90% even at low concentrations. The properties of the additional retention barrier formed on the membrane depend on the transmembrane pressure, the crossflow velocity used and the molecular mass of the hemicelluloses.
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Fig. 9 – Influence of flux on the observed retention of hemicelluloses at various hemicellulose concentrations using the ETNA10 membrane at 60 ◦ C. The data points for each hemicellulose concentration represent the transmembrane pressures 0.5, 1.0, 1.5, 2.0 and 2.5 bar (except for 26 g/l where the point representing 2.0 bar is missing).
5.4.
Increased Mw during ultrafiltration
The hemicelluloses in the process water are polydisperse with molecular masses mainly between 1 and 10 kDa, as shown in Fig. 1. If hemicelluloses are not totally retained during ultrafiltration (i.e., if R < 100%), the Mw of the hemicelluloses will increase during concentration. The yield of GGM with Mw below 4 kDa was only 14%, while it was 53% for GGM above 4 kDa, when the hemicelluloses were concentrated from 0.9 to 5 g/l at 0.5 bar during ultrafiltration with the ETNA10 membrane. The corresponding values for the UFX5 membrane were 45% and 96% during concentration from 0.7 to 5 g/l at 2 bar. Hence, the molecular mass distribution of the product will depend on the retention characteristics of the membrane used during ultrafiltration.
5.5.
Pressure drop due to high flux
Permeate fluxes between 150 and 200 l/m2 h could be obtained at low hemicellulose concentrations with both the ETNA10 and UFX5 membranes. These fluxes are higher than in most applications employing spiral-wound membranes. Thus, possible negative effects arising from pressure losses in the permeate spacer and permeate collector must be considered when scaling up the process. The length of the permeate flow channel equals the length of the membrane sheets. The membrane sheets in the 2.5 in. spiral-wound membrane elements used in this study were 1.4 m in length. In 8 in. elements, commonly used in larger full-scale applications, the length of the membrane sheets is 1.5 m. Thus, the pressure drop in the permeate spacer is similar, regardless of the diameter of the membrane element, and should not be important when scaling up the process from bench scale to full scale. In a full-scale housing, three membrane elements are connected in series and the permeate from all three membranes is removed from the housing through the same tubular permeate collector. The magnitude of the frictional pressure drop in the permeate collector was calculated by assuming a flux of 200 l/m2 h, a permeate temperature of 75 ◦ C and
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using data from the membrane supplier (40 m2 /element, 1 m length/element, 31 mm inner diameter of permeate collector). The frictional pressure drop is given by (Bird et al., 2007): Pf = 4f
L v2 dh
2
(8)
where f is a friction factor (estimated to be 0.005–0.01 in the Reynolds number interval in question), L is the length of the flow path in the collector, dh is the inner diameter of the collector, is the density and v is the velocity of the permeate in the permeate collector. The pressure drop in the permeate collector was found to be less than 1 bar under the prevailing conditions. The pressure drop allowed on the feed side is 1 bar/element, and in comparison with this the pressure drop in the permeate collector is of minor importance, even when the flux is very high. Furthermore, the pressure drop in the permeate collector is in the same direction as the pressure drop on the feed side, and should therefore be beneficial for the process since it evens out the variation in transmembrane pressure between the elements in the housing.
6.
Conclusions
Flux, retention and fouling have a considerable impact on the cost of an ultrafiltration process. The performance of three membranes for the separation of hemicelluloses from thermomechanical pulp mill process water was studied in this investigation. Two membranes (ETNA10 and UFX5) could be operated at rather high fluxes. Furthermore, the UFX5 membrane had a high hemicellulose retention, independent of the operating conditions, while ultrafiltration with the ETNA10 membrane had to be run above the critical flux to create a gel layer that increased the hemicellulose retention. An additional benefit of the UFX5 membrane is its high temperature tolerance (90 ◦ C). The temperature of the process water in the pulp mill is 75–85 ◦ C and would have to be cooled to 60 ◦ C to use the ETNA membrane, which would be costly. The lower fouling susceptibility of the ETNA10 membrane is, however, an important advantage. The cleaning frequency must be further evaluated in pilot trials at a pulp mill, but according to the results shown in Fig. 7, UFX5 must be cleaned more often than ETNA10, which is costly. However, an important observation made in this investigation is that it was possible to restore the pure water flux of both membranes after cleaning in five subsequent experiments. The Mw of the hemicelluloses increased during concentration with both the UFX5 and ETNA10 membranes, but to different degrees. Thus, the Mw of the hemicelluloses could perhaps be altered by changing the membrane and the operating conditions. This may be useful to adjust the properties of the hemicelluloses for specific applications. The ETNA01 membrane was found to be unsuitable for this application, since both the flux and the retention were too low for a commercial hemicellulose isolation process, regardless of whether it was operated at a high or low transmembrane pressure.
Acknowledgements Alfa Laval is gratefully acknowledged for donating the membranes, and MISTRA, the Swedish Foundation for Strategic Environmental Research, for financial support.
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