Vincent Oriez

Jérôme Peydecastaing

Pierre-Yves Pontalier

Keywords:

Membrane filtration

Mild alkaline treatment (1.5% NaOH (w/v), solid:li qu id ratio of 1:20, 60 •c, 6 h) of sugarcane bagasse (SCB) produoed an extract composed of hemioelluloses, lignin, phenolic monomers and aoetic acid. The purification of this extract, usually considered a by-product in lignooellulosic biorefineries, is of major importance to give value to the whole mild alkaline fractionation process. Ultrafiltration was assessed to separate the components of the SCB alkaline extract which is a prerequisite for their further valorization. The permeate fl ux and the retention of the extract components were studied on seven membranes (polysulfone hollow fi ber and ceramic tubular) with different molecular weight eut-offs, under various operating conditions. On ail the membranes tested, oligomers of lignin and hemicelluloses were separated from salts, phenolic monomers and acetic acid. The l0kDa poly sulfone hollow fiber membrane presented the highest lignin and hemicelluloses retention, exceeding 85 and 90%, respectively, regardless of shear rate and with a limited influence of transmembrane pressure. For salts, acetic acid and phenolic monomers, retention levels of about 0-10% were recon:led for this membrane. At 2.8 bar and at 20 •c, the permeate flux reached 16 L/h/m 2 and the critical fl ux was not reached.

Introduction lignocellulosic or second generation biorefineries transform agri cultural by products or forest biomass into energy, various chemicals and materials

 [START_REF] Huang | A revlew of separ ation technologies ln current and future biore8neries[END_REF]. Most lignocellulosic biorefineries currently subject the raw materials to pretreatment with a minerai acid, usually sulfuric acid, to produce monomeric sugars from the cellulose and the hemi celluloses, with lignin mostly recovered in the solid residue of this ex traction. Another process gaining importance is the dissolution of the lignin and the hemicelluloses in mildly alkaline conditions, with the recovery of cellulose in the solid residue [START_REF] Aj | Llgnin valorization: improving Ugnùt prooessing in the biorefinery[END_REF]. Mild alkali based pre treatments originated in the pulp and paper industry, but use Jess hars h conditions. They yield higher delignification rates and higher total monomeric sugar rates after enzymatic saccharification of the cellulose than acid pretreatments [START_REF] Saha | Dilute acid pretreabnent, enzymatie sacdtar iflcation, and fermen tation of rice hulls to ethanol[END_REF][START_REF] Xu | Revlew of alkall-based pretreabnent to enhance enzymatie sacdtariflcation for Ugnocellulooic biomass conversion[END_REF].

The separation/purification steps are of crucial importance in lig nocellulosic biorefineries as they account for 60 800/ 4 of the production cost of the end products [START_REF] Ragauskas | The path forward for biofuels and bioma terials[END_REF]. For lignocellulosic acid extracts, the monomeric sugars are purified by two chromatographie steps, the first involving the separation of sulfuric acid from the monomeric sugars [START_REF] Heinonen | Chromatographie recovery of monosaccharides for the production of bioethanol from wood, lnd. I!ng[END_REF], and the second involving separation of the various sugars from each other [START_REF] Chen | Chromatographie separation of glucose, xylose and arablnoee from Ugnooellulooie hydrolysates uslng cation ex change resin[END_REF]. This process is already used industrially by several companies, including BlueFire Renewables (https://bfreinc. com).

For mild alkaline pretreatments, the cellulose remains in the solid residues and the alkaline extracts are considered to be the by products of the biorefinery. lignocellulosic alkaline extracts contain lignin and hemicelluloses in an oligomeric form, inorganic salts, acetic acid and phenolic monomers [START_REF] Kim | A revlew on alkatine pretreatment tecltnology for bioconversion of Hgnocellulosie biomass[END_REF]. The separation of these compounds and their further valorization would provide added value for the biorefineries using mild alkaline pretreatment processes. The purification of some of these compounds has been studied with the use of several different methods including acid precipitation [START_REF] Garcia | Llgnin as natural radical sca venger. I!ffect of the obtaining and purification processes on the antioxidant be haviour of lignin[END_REF], ethanol precipitation [START_REF] Zeitoun | Twin-screw extrusion for hemi cellulose recovery: influence on extract purity and purification performance[END_REF], adsorption [START_REF] Ou | Seperation and purification of ferutic acid in alkallne-hydrolysate from sugarcane bagasse by actlvated dtarcoal adsorption/anion macroporous resin exchange dtromatography[END_REF] and membrane filtration [START_REF] Qi | Separation and concentr ation of hydro xycinnanûc acids ùt alkatine hydrolp.ate from rice straw by nanofiltration[END_REF], but none of these methods has yet been adopted by industry.

Membrane filtration is of particular interest because of its low levels of chemical and energy consumption [START_REF] He | Recent advances ln membrane technologies for biore8nlng and bioenergy production[END_REF]. lt can be performed on various streams of the lignocellulosic biorefinery: suspended solids can be retained by microfiltration (MF), or macromolecules, such as hemicelluloses and lignin, can be concentrated by ultrafiltration (UF) [START_REF] Jonsson | Microfiltr ation, ultr afiltration and diafiltration[END_REF]. Monosaccharides, low molar mass lignin and phenolic monomers can be concentrated by nanofiltration (NF) and salts can be removed by reverse osmosis (RO) for water recycling.

Membrane filtration has been studied for the purification of hemi celluloses and lignin, mostly from the black liquors obtained in the strongly alkaline conditions used in the pulp and paper industry. performances of PS hollow fiber and ceramic tubular membranes for separating all the components of a model lignocellulosic mild alkaline extract. Sugarcane bagasse was used for this study, as it is one of the most produced and studied lignocellulosic biomasses for second gen eration biorefineries worldwide.

Materials and methods

Chemicals

Sodium hydroxide (≥98.5% purity) for the alkaline extraction, sulfuric acid 72% for NREL hydrolysis, 95% sulfuric acid and acetoni trile (≥99.9%) for HPLC eluents were purchased from VWR, and cal cium carbonate (NREL protocol) was obtained from Merck. The fol lowing HPLC standards were purchased from Sigma Aldrich: D (+) cellobiose (≥98%), D (+) glucose (≥99.5%), D (+) galactose (≥99%), L (+) arabinose (≥99%), D (+) xylose (≥99%), D (-) fructose (≥99%), acetic acid (≥99%), furfural (99%), 5 hydro xymethyl 2 furfuraldehyde (99%), gallic acid (97%), 4 hydroxybenzoic acid (≥99%), caffeic acid (≥98%), vanillic acid (97%), syringic acid (≥95%), 4 hydroxybenzaldehyde (98%), vanillin (99%), p coumaric acid (≥98%), syringaldehyde (99%), trans ferulic acid (≥99%), sinapic acid (≥98%), trans 3 hydroxycinnamic acid (99%).

Alkaline extraction

The alkaline extraction was carried out on 3 kg of SCB with 60 L of 1.5% NaOH (w/v) in a stainless steel lined vessel (De Dietrich), with a solid:liquid ratio of 1:20 (w/v) and a NaOH:SCB ratio of 1:3 (w/w), with continuous mechanical stirring (200 rpm) for 6 h at 60 °C. These are optimized conditions reported by [START_REF] Sun | Influence of alkaline pre-treatments on the cell wall components of wheat straw[END_REF], for maximizing the recovery of hemicelluloses and lignin by the mild alkaline pre treatment of wheat straw [START_REF] Sun | Influence of alkaline pre-treatments on the cell wall components of wheat straw[END_REF]. The solid residue was removed from the alkaline extract using a top discharge vertical basket centrifuge (RC 50 PX R, Rousselet) equipped with a 5 μm polypropylene bag. This residue was rinsed with distilled water, dried at 50 °C for 48 h and ground by microfine grinder (IKA MF 10 basic) on a 1 mm sieve before analysis. The filtered SCB alkaline extract and the filtered solution used to rinse the solid residue were mixed, analyzed and used as the feed for the membrane filtration experiments (mixture referred to hereafter as the SCB alkaline extract).

Membrane filtration

Set up

Membrane filtration was carried out on the filtered SCB alkaline extract. Five new PS hollow fiber membranes (GE Healthcare) and two Kerasep ceramic tubular membranes (Novasep Process) were tested (Table 1). Reproducibility was assessed with another new 10 kDa PS hollow fiber membrane and a new 3 kDa PS hollow fiber membrane. The feed tank contained 5 L of water or filtered SCB alkaline extract.

Table 1

Characteristics of the membranes used for the filtration of sugarcane bagasse mild alkaline extract. ultrafiltration of black liquor on inorganic membranes [START_REF] Wallberg | Separation of Ugnin ln !craf t coolclng Uquor from a continuous digester by ultrafiltration at ternperatures above 1 oo•c[END_REF], and hemicelluloses retention of 70% was obtained with a spiral wound membrane following the pulp steeping of viscose [START_REF] Sùtgh | Hernicelluloses separation from caustic-containing pro cess str eam by ultrafiltr ation[END_REF]. High hemi celluloses retention (over 90%) and intermediate lignin retention (30 50%) have been reported for pulp mill process water [START_REF] Persson | Practionation of process water ln thermomecltanical pulp nûlls[END_REF][START_REF] Persson | Isolation of hernicellul oses by ultrafiltr ation of thermomechanical pulp mill process water-influence of operating conditions[END_REF]. Some studies have reported lignin retention of up to 75% [START_REF] Liu | Application of inorganic membranes in the alkali recovery process[END_REF] or 80% [START_REF] Wallberg | Fractionation and concentration of kraft black liquor lignin with ultrafiltration[END_REF] with total recovery in the permeate of black liquor salts. Compare to the severe alkaline conditions employed in the pulp and paper industry, the mild alkaline conditions studied in the second generation biorefinery produce a different extract, notably with longer hemicelluloses and lignin oligomers [START_REF] Sun | Fractional extraction and structural characterization of sugarcane bagasse hemicelluloses[END_REF][START_REF] Mansouri | Structural characterization of technical lignins for the production of adhesives: application to lignosulfonate, kraft, soda-anthraquinone, organosolv and ethanol process lignins[END_REF] or the absence of lignosulphonates and sodium hydrosulfide salts. Only a few studies have investigated the membrane filtration of lignocellulosic mild alkaline extracts [START_REF] Zeitoun | Twin-screw extrusion for hemi cellulose recovery: influence on extract purity and purification performance[END_REF][START_REF] Toledano | Lignin separation and fractionation by ultrafiltration[END_REF]. However, these studies did not report retention values for all the components typically found in lignocellulosic mild alkaline extracts and flux beha vior was not discussed. Retention of molecules and permeate flux (reflecting productivity) depend on the molecular weight cut off (MWCO) and type of mem brane, and the hydrodynamic conditions used. In studies on various hemicelluloses lignin strong alkaline extracts, the best MWCOs for high retention of hemicelluloses and lignin associated with low retention of smaller molecules were found to be in the range of 1 50 kDa [START_REF] Wallberg | Separation of Ugnin ln !craf t coolclng Uquor from a continuous digester by ultrafiltration at ternperatures above 1 oo•c[END_REF][START_REF] Sùtgh | Hernicelluloses separation from caustic-containing pro cess str eam by ultrafiltr ation[END_REF][START_REF] Persson | Isolation of hernicellul oses by ultrafiltr ation of thermomechanical pulp mill process water-influence of operating conditions[END_REF][START_REF] Wallberg | Fractionation and concentration of kraft black liquor lignin with ultrafiltration[END_REF][START_REF] Jacquemin | Performance evaluation of a semi-industrial production process of arabinoxylans from wheat bran[END_REF]. Among the different membrane configurations, hollow fiber membranes present the best filtration area:volume ratio (m 2 /m 3 ) [START_REF] Ladisch | Bioseparations Engineering: Principles, Practice, and Economics[END_REF]. Polysulfone (PS) is a common organic polymer used in hollow fiber membranes, with good mechanical, chemical and thermal stability [START_REF] Scott | Handbook of Industrial Membranes[END_REF]. Conversely, ceramic membranes have a longer life span than organic membranes [START_REF] Owen | Economic assessment of membrane processes for water and waste water treatment[END_REF], are more resistant to temperature and chemicals, and back washing is possible to remove fouling (pore blocking and cake formation). PS hollow fiber and ceramic tubular membranes are both suitable for use with alkaline extracts at pH values of up to 13. In terms of hydrodynamic conditions, shear rate can be increased by raising the flow rate (or cross flow velocity or Reynolds number) and, thus, the energy consumption of the filtration unit. But the shear rate must be high enough to minimize the polarization layer and guarantee efficient cake removal, thereby maintaining a high permeate flux [START_REF] Rossi | Arthrospira platensis harvesting with membranes: fouling phenomenon with limiting and critical flux[END_REF]. The transmembrane pressure (TMP) applied to the membrane must also be optimized. Increasing the TMP increases flux, thus increasing the productivity of the filtration unit, linearly at first, but the slope then becomes more shallow and a plateau known as the limiting flux is eventually reached, due to pore blocking, cake formation and/or in creases in the polarization layer. The inflection point of the curve after its linear region is known as the critical flux, and increasing the TMP above this point is generally not economically favorable [START_REF] Bacchin | Critical and sustainable fluxes: theory, experiments and applications[END_REF]. Besides, increasing the TMP can also affect the retention of molecules in various ways, either reducing retention due to higher solution diffusion through the membrane below the critical flux, or increasing retention due to membrane fouling above the critical flux [START_REF] Persson | Isolation of hernicellul oses by ultrafiltr ation of thermomechanical pulp mill process water-influence of operating conditions[END_REF].

The aim of this work was to investigate in detail the influence of operating conditions (TMP, shear rate, temperature) on the The feed was circulated in the membrane with a gear pump (Johnson Pump, model 10/0005). Feed flow was measured with a flowmeter (Rosemount, Mexico). Permeate flux was assessed by collecting permeate over a given time period and weighing the sample collected. TMP was set with a valve on the retentate stream and checked with two manometers (Tecsis), one on either side of the membrane. The tem perature was maintained at 20 °C during the experiments, with a monotube heat exchanger located in the retentate flow. When the effect of temperature was studied, the feed tank was heated to 40 °C with a hot plate (Heidolph). Experiments were run in recycling mode, with both the retentate and the permeate recirculated to the feed tank (Fig. 1).

Before each experiment, the new membrane (PS hollow fiber membranes were stored in glycerol), was washed several times with an ethanol/water solution (1:1, v/v), rinsed with water and the initial water flux (IWF) was measured at 20 °C and various TMP values. Water was drained from the installation and the SCB alkaline extract was loaded into the feed tank and recirculated at a TMP of 0.8 bar until the flux was stable over time (about 15 min) and a quasi stationary state was reached. Permeate flux was measured at different TMP values, from 0.8 to 2.8 bar, and different shear rates: 1966, 3408 and 4587 s -1 for ceramic tubular membranes (corresponding to cross flow velocities of 1.5, 2.6 and 3.4 m/s, respectively) and 3396, 6791 and 10,187 s -1 for PS hollow fiber membranes (corresponding to cross flow velocities of 0.4, 0.8 and 1.3 m/s, respectively). Nine permeate samples were col lected for analysis at three different TMP values (0.8, 1.6 and 2.4 bar) and at the three shear rates tested. The feed volume was large enough relative to the total volume of permeates collected for analysis to as sume that the composition of the SCB alkaline extract remained con stant throughout each experiment. The final retentate was collected for analysis, to confirm that there had been no change in the composition of the SCB alkaline extract during the filtration process. At the end of the experiment, the SCB alkaline extract was drained and the mem brane was washed several times with water.

Theoretical notions

The transmembrane pressure TMP is given by Eq. ( 1):

= + - TMP P P 2 P inlet outlet permeate (1)
where P inlet and P outlet are the pressures (bar) at the inlet and the outlet of the membrane, and P permeate the pressure on the permeate stream. No back pressure was applied on the permeate side, so P permeate = 0.

The shear rate ɣ (s -1 ) is given by Eq. ( 2):

= ɣ 4Q nr 3 ( 2
)
where Q is the flow rate (m 3 /s), n is the number of channels or fibers and r is the radius (m) of a channel or fiber.

The retention R is given by Eq. ( 3):

= R 1- C C P R (3)
where C P and C R are the solute concentrations (g/L) in the permeate and the retentate streams, respectively.

Analytical methods

The following analytical methods were applied to the initial SCB, the extract obtained following mild alkaline pretreatment (to determine its composition and extraction yields), and the various permeates ob tained during the filtration experiments (to assess the retention of the various components of the SCB alkaline extract).

Dry solid and ash

Dry solid (DS) content was determined gravimetrically by heating at 103 °C for 12 h and ash content was determined at 500 °C for 12 h: 1 g was used for solid samples, 1 mL was used for alkaline extract and re tentate samples, and 5 mL was used for permeate samples.

Carbohydrates and lignin

Based on Laboratory Analytical Procedure of the National Renewable Energy Laboratory [START_REF] Sluiter | Determination of Structural Carbohydrates and Lignin in Biomass[END_REF], acid insoluble lignin (AIL) was quantified gravimetrically and acid soluble lignin (ASL) was de termined spectrophotometrically, at a wavelength of 240 nm using an absorptivity constant of 25 L/g/cm. High performance liquid chroma tography (HPLC) on a Rezex RPM Monosaccharide Pb +2 300 x 7.8 mm column (Phenomenex), used in conjunction with a Rezex RPM Mono saccharide Pb +2 50 x 7.8 mm guard column (Phenomenex) was per formed to quantify the cellobiose, glucose, xylose, galactose, arabinose, mannose and fructose released by the acidic hydrolysis of cellulose, hemicelluloses or residual sucrose. Before injection, the samples were filtered on an ABW solid phase extraction (SPE) cartridge (Phenom enex) to remove salts and prevent interference with the sugar peaks. Isocratic conditions were used with Milli Q water at a flow rate of 0.6 mL/min; the injection volume was 20 μL, the column was main tained at 80 °C and the RI detector was maintained at 50 °C. For the alkaline extract and purified samples, HPLC on a Rezex RHM Mono saccharide H + 300 x 7.8 mm column (Phenomenex), used in conjunc tion with a Rezex RHM Monosaccharide H + 50 x 7.8 mm guard column (Phenomenex) was performed to quantify glucose, xylose, arabinose, acetic acid, furfural and hydroxymethylfurfural (HMF) [START_REF] Sluiter | Determination of Sugars, Byproducts, and Degradation Products in Liquid Fraction Process Samples[END_REF]. The salts did not interfere with the sugar peaks on the RHM column, so, by contrast to the RPM column, no desalting of the samples was required before their injection. Isocratic conditions were applied, with 5 mmol/L H 2 SO 4 at a flow rate of 0.6 mL/min; the injection volume was 50 μL, the column was maintained at 65 °C and the RI detector was maintained at 50 °C. The SCB alkaline extract and permeates collected were diluted by four fold with distilled water before the NREL protocol.

Monomeric sugars and hemicelluloses acetyl groups

Sulfuric acid was added to the alkaline extract to adjust its pH to 2, corresponding to the pH of the RHM column HPLC eluent. The extract was then analyzed on the RHM column without running the NREL protocol. The direct injection of pH adjusted samples onto the HPLC column made it possible to quantify monomeric sugars and free acetic acid, whereas the analysis of samples with the NREL protocol provided data for total sugars (monomeric and oligomeric forms) and acetic acid (free and bound to hemicelluloses).

Phenolic monomers

Twelve phenolic monomeric compounds potentially present in SCB alkaline extract [START_REF] Xu | Determination of cell wall ferulic and p-coumaric acids in sugarcane bagasse[END_REF][START_REF] Capriotti | Chromatographic methods coupled to mass spectrometry detection for the determination of phenolic acids in plants and fruits[END_REF] gallic acid, 4 hydroxybenzoic acid, caffeic acid, vanillic acid (VA), syringic acid, 4 hydroxybenzaldehyde (4HBA), vanillin, p coumaric acid (p CA), syringaldehyde, ferulic acid (FA), si napic acid and trans 3 hydroxycinnamic acid were quantified by The composition of the SCB alkaline extract and the yields of the various components of SCB in titis fraction are detailed in Table 2. The yields of dry solid and ash included the sodium hydroxide added in the process as detailed in Eqs. ( 4) and ( 5):

Yieldos = mos in a ll:aline extract mdrySCB + mNaoH ( 4
)
Where mdrySCB is the mass of dry SCB introduced in the process, m,;00H

is the mass of sodium hydroxide used for the extraction and mos i11 a/kali ne extract the mass of dry solid in the alkaline extract.

Yield a .sh = mas11 i11 alkalinttxtract ma.sh in SCB + l'nf.laoH [START_REF] Ragauskas | The path forward for biofuels and bioma terials[END_REF] Where ma.sh in ses is the mass of ash in the dry SCB introduced in the process, mNaOH is the mass of sodium hydroxide used for the extraction and mAsh in all:aline txtraei is the mass of ash in the alkaline extract. Only very small amounts of glucan (3%) were recovered in the extract. Xylan, arabinan, AIL and ASL were reoovered at levels of 29,

Table 2

Initial sugarcane bagasse (SCB) composition, SCB mild alkaline extract com position and yield of the various components in the SCB mild alkaline extract. 59, 49 and 96%, respectively. Most of the salts (85%) were reoovered in the alkaline extract, sodium hydroxide being the main one regarding the quantity introduced in the process compared to the ash content of SCB. Specific analyses for the identification of the inorganic salts of SCB were not carried out since these compounds constitute a minor fraction of SCB in terms of quantity and in terms of potential valorization.

SCB

AllcaHne extract

Components

The SCB alkaline extract used for ail the membrane experiments consisted of six major pools of molecules: 19.4 g/L inorganic salts, 6.2 g/L AIL, 3.1 g/L ASL, 5.3 g/L oligomeric sugars (3.8 g/L xylan, 0.9 g/L arabinan, 0.6 g/L glucan), 1.5 g/L acetic acid and 1.3 g/L phe nolic monomers.

Membrane filtratin

The effects of MWCO, TMP, shear rate, temperature and the nature of the membrane on permeate flux and the retention of the various compounds present in the SCB alkaline extract were evaluated in re cycling mode at quasi steady state. In most of the permeates analyzed, glucan and arabinan were barely detectable by HPLC. Xylan was, therefore, the only sugar oligomer studied in analyses of the permeate composition. Mass closure for each permeate composition was between 89 and 101%, so we can assume that the main compounds of the col lected permeates were analyzed.

Effect of âme: quasi stalionmy stall! study

The effect of the experiment duration on the flux was investigated by recirculating the SCB mild alkaline extract at the lowest TMP (0.8 bar) and shear rate (3396 s -i) (Fig. 2). On ail three membranes tested, flux slowly decreased with a small change in magnitude, for instance, from 6 to 5 L/h/m 2 after 1 h for the 5 kDa PS hollow fiber membrane. Zeitoun et al. (2010) also observed a very small decrease in flux over time during the filtration of a wheat bran mild alkaline extract with a 30 kDa PS hollow fiber membrane from the same supplier but with a 0. 5mm fiber lumen (10).

Effect of MWCO

IWF increased with MWCO, from 21 to 113 l./h/m 2 for the PS hollow fiber membranes, confirming that, for a given type of mem brane, higher MWCO is associated with a higher water flux (Table 1). A similar trend was observed when SCB alkaline extract was recirculated at low TMP and low shear rate (Fig. 3A). At 0.8 bar and 3396 s-1, the flux gradually increased with increasing MWCO for the PS hollow fiber membranes, from 3 l./h/m 2 for the 1 kDa membrane to 28 l./h/m 2 for the 50 kDa membrane. Only the 5 kDa membrane behaved differently with a flux similar to that for the l0kDa membrane.

On PS hollow fiber membranes, at 0.8bar and 3396 s-1, the re tention of xylan, AIL and the ASL increased from 1 kDa to 10 kDa then decreased from l0 kDa to 50 kDa (Fig. 4). The highest retention rates were reoorded for the 10 kDa PS hollow fiber membrane with values of 85%, 86% and 36% obtained for xylan, AIL and ASL, respectively. The evolution of the retention of ASL with MWCO was less marked than those of xylan and AIL, probably because phenolic monomers, which were accounted for in ASL (and also followed via HPLC on C18 10 ! �-. •S kDa With the MWCO values tested, it was possible to separate xylan and AIL from phenolic monomers, acetic acid and ash, whereas ASL was moderately retained by the membranes. The retention of both xylan and AIL decreased with increasing MWCO (Fig. 4A&B), from 85 and 86% on the 10 kDa PS hollow fi ber membrane to 64 and 46% on the 50 kDa PS hollow fiber membrane, respectively. Xylan and AIL from SCB alkaline extract could not be separated by filt ration through PS hollow fiber membranes under the conditions tested.

0 0 • • • - -: 1 • • • • . ! = 1 • • • • • -. • • . . . . . . • • • ♦ • TMP(bv) • • • • • • • • • • • • • • • • • • • • • • ThfP (ba,) 40
i 10 0 0 B • • SOl<Da -301d>a � • • • 1 • • • ♦IOkDI 1 • i • • • • • ■5U>a • • . • • • • • • • ♦ l ld>a • . • • • • 2 TMP (bar) D • • • • • • • • • • • ■JS k.Da • • • • • • • ♦51cDa • • • • TMl'(bar)

Effect of TMP

On the 1 and 10 kDa PS hollow fiber membranes, permeate flux increased linearly with TMP (Fig . 3 An increase in TMP from 0.8 to 2.4 bar at a shear rate of 3396 s -1, resulted in a slight increase in the xylan retention for ail membranes tested, for instance, from 85 to 87% for the 10 kDa PS hollow fiber membrane and from 64 to 68% for the 50 kDa PS hollow fiber mem brane (Fig . 4A). Persson and Jêinsson (2010) observed that the retention of hemicelluloses decreased with increasing TMP below the critical flux, but increased with increasing TMP above the critical flux, during the filtration of pulp mill process water with 1 and l0 kDa polyvinylidene fluoride spiral wound membranes [START_REF] Persson | Isolation of hernicellul oses by ultrafiltr ation of thermomechanical pulp mill process water-influence of operating conditions[END_REF]. In this study, the retention of xylan increased with increasing TMP independently of critical flux.

Increase in TMP led to a moderate increase in AIL retention for ail PS hollow fiber membranes except for the 50 kDa membrane, where AIL retention varied more significantly from 46% at 0.8 bar to 67% at 2.4 bar (Fig. 4B). The increase in ASL retention with TMP was clearly observed for ail the PS hollow fiber membranes (Fig . 4C). Slightly higher retentions for small molecules (inorganic salts, acetic acid, phenolic monomers) were a1so observed, but their retentions did not exoeeded 200/4.

OveraII, the change in retention with TMP was lower for PS hollow fiber membranes with smaller MWCOs (1, 5 and 10 kDa) than for those with larger MWCOs (30 and 50 kDa). As for the lowest TMP (0.8 bar), the 10 kDa PS hollow fiber membrane presented the highest retention for xylan (87%) and AIL (88%) at the highest TMP (2.4 bar).

Effect of shear rate

For the three shear rates tested, fluxes increased on ail the PS hollow fiber membranes, but only to a limited extent (Fig. 3A&B, only the two extreme shear rates are presented). On the 1 kDa and 10 kDa PS hollow fiber membranes, a three fold increase in shear rate led to an increase of about 100/4 in flux on the TMP range tested. Behavior was different on the other PS hollow fiber membranes. On the 5 kDa PS hollow fiber membrane, the highest shear rate had a positive impact on flux but only below the critical point at 1.2 bar. On this membrane, for the three shear rates tested (Fig. 3A&B), the limiting flux was 17 L/h/m 2 , and the flux began to decrease at 2 bar. On the 30 kDa PS hollow fiber mem brane, the opposite effect was observed. Shear rate variations had no impact below the critical point, but above it, the lowest shear rate was associated with a lower flux. On the 50 kDa PS hollow fiber membrane, the highest shear rate increased the flux from 23 to 28 L/h/m 2 at the beginning of the TMP range, but at 2.6 bar, flux had fallen to 13 L/h/m 2 for ail shear rates. Overall, a large increase in shear rate, from 3396 to 10,187 s-1 , triggered a limited increase in flux on ail the PS hollow fiber membranes tested. A low shear rate may be sufficient to guarantee slow cake formation and/or minimization of polarization layer during the filtration. Shear rate had no significant effect on the retention of the molecules on any of the PS hollow fiber membranes.

Effect of temperature

Two temperatures (20 and 40•c) were tested on the l0 kDa PS hollow fiber membrane at a shear rate of 10,187 s-1 (Fig. 6). At both temperatures, the retention of molecules passing through the mem brane (inorganic salts, aoetic acid, phenolic monomers) remained close to 00/4. The increase in temperature ( + 20 •c) led to a doubling of the flux, for instance, from 5 to 10 L/h/m 2 at 0.8 bar and from 15 to 28 L/ h/m 2 at 2.4 bar. In parallel, the retention of xylan and AIL decreased with increasing temperature, from 87 to 81 % and from 88 to 79%, respectively. The decrease in the viscosity of the SCB alkaline extract and the dilation of the pores of the membrane may account for the increase in flux and the lower retention of large molecules at high temperature.

Effect of the nature of the membrane

Membranes made of different materials and with different config urations presented similar IWFs for the same MWCO. IWF was 38 L/h/ m 2 /bar for the 5 kDa oeramic tubular membrane and 41 L/h/m 2 /bar for the 5 kDa PS hollow fiber membrane (Table 1). The IWF for the 15 kDa oeramic tubular membrane, 57 L/h/m 2 /bar, was intermediate between the IWF values for the 10 and 30kDa PS hollow fiber membranes (48 and 76L/h/m 2 /bar, respectively).

During filtration of the SCB alkaline extract, change in flux with 1MP for the ceramie tubular membranes (Fig . 3C&D) was similar to that for PS hollow fiber membranes (Fig. 3A&B). The 5 kDa oeramie tubular membrane behaved similarly to the 5 kDa PS hollow fiber membrane, with a linear increase in flux with TMP up to a critical flux merging with the limiting flux, although the critical flux was lower (10 L/h/m 2 ) and was reached at a higher TMP (2.0 bar) for the 5kDa oeramie tubular membrane. The shear rates tested did not affect flux on this membrane. The 15 kDa oeramic tubular membrane had a similar flux versus TMP profile to the 50 kDa PS hollow fiber membrane. At the lowest shear rate (1966 s-1 ) and 1MP (0.8 bar), the critical flux had already been reached and the limiting flux (28 L/h/m 2 ) was reached at 1.4 bar. On this membrane, an increase in shear rate had a significant impact on the flux before the limiting flux, as limiting fluxes were 28, 36 and 48L/h/m 2 for shear rates of 1966, 3408 and 4587 s-1 , re spectively. For TMP values above that at which the limiting flux was achieved, shear rate had no influence on flux. Membranes of different composition and configuration yielded similar results in terms of the effects of MWCO, 1MP and shear rate on flux. The retention of xylan was similar on the 5 kDa oeramic tubular membrane and 5 kDa PS hollow fiber membrane, ranging from 79 to 84% at 0.8 and 2.4 bar, respectively (Fig. 4A). On the 15 kDa ceramie tubular membrane, xylan retention (71 77%) was lower and close to the values ob served for the 30 kDa PS hollow fi ber membrane (Fig. 4A). As for PS hollow fiber membranes, xylan retention on the ceramie tubular membranes increased significantly with MWCO and 1MP, but was not influenced by shear rate.

AIL retention was significantly lower on the ceramic tubular mem branes than on the PS hollow fiber membranes. For instance, on the 5 kDa ceramic tubular membrane, the AIL retention was between 66 and 73% depending on TMP, whereas retention values of 82 84% were obtained with the 5 kDa PS hollow fiber membrane. Ceramie materials are Jess hydrophobie than PS, potentially accounting for the lower re tention of the hydrophobie AIL by oeramic membranes and the poten tial adsorption of AIL on the PS membranes. Inorganic salts, acetic acid and phenolic monomers were not retained on the ceramic tubular membranes either (Fig . 5).

As observed for PS hollow fi ber membranes, an increase in TMP led to an increase in retentions for xylan, AIL and ASL on the oeramie tubular membranes, and shear rate had no significant effect on the retention of SCB alkaline extract components.

Repeatabilit;y study

Repeatability is often assessed on the same membrane, with suc oessive filtration runs. The flux usually decrease over the filtration ex periments, whereas the retention of the compounds gradually increase [START_REF] Sùtgh | Hernicelluloses separation from caustic-containing pro cess str eam by ultrafiltr ation[END_REF]. In this study, the repeatability test was run on a new 10 kDa PS hollow fiber membrane, to determine whether the selectivity and flux of the membrane presenting the best separation performances could be extrapolated to another new membrane. The two 10 kDa PS hollow fiber membranes presented similar linear flux behavior, with a slightly lower flux on the new membrane, with increases from 3 L/h/m 2 at 0.8 bar to 16 L/h/m 2 at 2.8 bar at 10,187 s-1 (Fig. 7). Retention of the various compounds were also similar in the different conditions tested, with values of 91 % for xylan and 82% for AIL at 2.8 bar on the second membrane tested (Fig. 7).

Concerning the different flux versus TMP profiles of the 5 kDa PS hollow fiber membrane, and the 1 and l0 kDa PS hollow fiber mem branes, changes in flux with TMP were assessed on a new 3 kDa PS hollow fiber membrane from the same supplier. The same flux behavior as reported for the 5 kDa membrane was observed, with merging cri tical and limiting fluxes (18 L/h/m 2 at low shear rate and 19 L/h/m 2 at high shear rate) reached at 1.4 bar, regardless of shear rate (Fig. 7). The difference in flux behavior on these two membranes relative to the 1 and l0kDa PS hollow fiber membranes showed that the results ob tained for a given MWCO cannot be extrapolated to other membranes of the same type with different MWCO values for the filtration of SCB alkaline extract. Diafiltration could be investigated as a way of eliminating additional inorganic salts, aoetic acid and phenolic monomers, if a higher purity of retained compounds is required.

• • • • • • • • • • � • • � • • • • • • • • • . •

  Hemicelluloses retention of 69 to 81 % have been achieved by the • Corresponding authors at: Laboratoire de Chimie Agro-industrielle (LCA), Université de Toulouse, INRA, IN PT, Toulouse, France. E-mail addresses: vincent.orie2@ensiacet.fr 0/. Orie2), pierreyves.pontalier@ensiacet.fr (P .-Y. Pontalier). https://doi.org/10.1016/j.procbio.2019.01 .006

Fig. 1 . 3 . Results and discussion 3 . 1 .

 1331 Fig. 1. Set-up of the filtration system used for the screening of membranes for the filtration of sugarcane bagasse mild alkaline extract.

  were calculated as a percentage of dry solid. Ail analyses were run in duplicate.

•

  � • _...,lOkDI � � _,.,,._,.,,.

Fig. 2 .

 2 Fig. 2. Changes in permeate flux over time during the filtration of the su garcane bagasse mild alkaline extract at 0.8 bar and 3396 s-1 on three poly sulfone hollow fiber membranes.

Fig. 3 .

 3 Fig. 3. Influenoe of the molecular weight eut-off, transmembrane pressure (TMP) and shear rate on the permeate flux during the filtration of sugarcane bagasse mild alkaline extract (A) Polysulfone hollow fiber membranes with a shear rate of 3396 s-1 , (B) polysulfone hollow fiber membranes with a shear rate of 10,187 s-1 , (C) oeramic tubular membranes with a shear rate of 1966 s-1 , (D) oeramic tubular membranes with a shear rate of 3408 s-1 •

  ). On the 1 kDa PS hollow fiber membrane with a shear rate of 3396 s -1, flux increased from 3 L/h/m 2 at 0.8 bar to 8 L/h/m 2 at 2.8 bar. With 10 kDa PS hollow fiber mem branes at the same shear rate, flux displayed the same linear behavior but with a steeper slope, increasing from 5 L/h/m 2 at 0.8 bar to 15 L/h/ m 2 at 2.8 bar. The linear increase in flux with TMP for these two membranes showed that the critical flux was not reached in the TMP range tested. On the 5 kDa PS hollow fiber membrane, flux increased linearly with TMP up to 1.4 bar, at which the critical flux (16L/h/m2) was reached. Critical flux and limiting flux merged into the same point on this membrane. For filtration with 30 kDa PS hollow fiber membrane, the critical flux (25 L/h/m 2 ) was reached at 1.8 bar and the limiting flux (28 L/h/mÎ at 22 bar. With the 50 kDa PS hollow fiber membrane, flux steadily decreased with increasing TMP. Rossi et al. (2008) also re ported a decrease in flux with increasing TMP during the ultrafiltration of microalgae, probably due to polysaccharides retention, resulting in membrane fouling (28 ). Here, the retentions of xylan or AIL were most likely accounted for the decrease in flux with increasing TMP for the PS hollow fiber membranes with higher MWCO. The limiting flux was reached within the range of TMPs used in the experiment, and the critical and limiting fluxes must therefore have occurred at TMP values below 0.8 bar. With the filtration system used, it was not possible to have a TMP lower than 0.8 bar at the shear rates tested, so it was not possible to check the critical point on this membrane. During this study, critical fluxes appeared at different TMP values for PS hollow fiber membranes differing only in terms of their MWCO values: under critical flux at 2.8 bar for 1 and 10 kDa membranes, cri tical flux at 1.4 bar for the 5 kDa membrane critical flux, at 1.8 bar for the 30 kDa membrane, and over critical flux and even limiting flux at 0.8 bar for the 50 kDa membrane. TMP critical flux decreased with in creasing MWCO for ail the membranes other than the 5 kDa PS hollow fiber membrane. In another study, Wu et al (1999) observed a similar phenomenon when a colloidal silica suspension was filtered on poly ethersulfone fiat sheet membranes with an MWCO of 50 and 100 kDa, and a PS fiat sheet membrane with an MWCO of 0.2 µm (36).

Fig. 4 .■ 0 Fig. 5 .

 405 Fig. 4. Effect, during the filtration of sugarcane bagasse mild alkaline extract, of the molecular weight eut-off, the transmembrane pressure (1MP) and the nature of the membrane on the retentions of (A) xylan, (B) acid insoluble lignin (AIL) and (C) acid soluble lignin (ASL). Shear rate of 3396 s-1 for PS hollow fiber membranes and 3408 s-1 for ceramic tubular membranes.

"Fig. 6 .

 6 Fig. 6. Change in permeate flux evolution with transmembrane pressure (TMP) during the filtration of sugarcane bagasse mild alkaline extract on 10 kDa polysulfone hollow fiber membrane, with a shear rate of 10,187 s-', and temperatures of 20 •c and 40 •c

  rafiltration of SCB alkaline extract, xylan and AIL were retained in the retentate to different extents, depending on the

Fig. 7 .

 7 Fig. 7. Repeatabllity study on new 3 and 10 kDa polysulfone hollow fiber membranes for the filtration of sug an:ane bagasse mild alkaline extract. (A) Flux vs transmembrane pressu re (TMP) profile at a shear rate of 3396 s -1 , (B) flux vs. 1MP profile at a shear rate ofl0,187 s -1 , (C) xylan retenti on on the two new 10 kDa polysulfone (PS) hollow fiber membranes, (D) acid-insoluble lignin (AIL) retention on the two new l0kDa polysulfone hollow fiber membranes.

Acknowledgments

We would like to thank the ANR (Agence Nationale de la Recherche) for financial support for this research in the framework of the LigNov project (ANR 14 CE06 0025 01) and Novasep for providing the ceramic tubular membranes and expertise.