Separation of lignin and sugars from biomass pre-treatment liquors

Abstract

The invention relates to an improved process for separating lignin and monomeric sugars from a liquor comprising lignin and monomeric sugars in a solvent mixture of water and at least one organic solvent, which employs membrane filtration techniques such as nanofiltration and selective water removal, preferably by permeation through a membrane which is selective for water molecules. The invention further relates to a modular system for executing the process according to the invention. The process and system according to the invention are particularly suitable to be incorporated with pre-treatment of lignocellulosic biomass, in particular by organosolv fractionation or solvolysis.

Claims

1. A process for separating lignin and monomeric sugars from a biomass pre-treatment liquor comprising lignin and monomeric sugars in a solvent mixture comprising water and an organic solvent, the process comprising: (a) subjecting the liquor to nanofiltration over a membrane capable of retaining lignin and permeating monomeric sugars; and (b) subjecting a permeate originating from step (a) to selective water removal to obtain a suspension comprising precipitated monomeric sugars; (c) precipitating lignin from a retentate originating from step (a), and (d) isolating precipitated monomeric sugars from the suspension of step (b).

2. The process according to claim 1, wherein the liquor is an organosolv liquor or a solvolysis liquor.

3. The process according to claim 1, wherein the liquor is an organosolv liquor.

4. The process according to claim 2, wherein the organosolv liquor is obtained by subjecting a lignocellulosic biomass to organosolv prior to step (a).

5. The process according to claim 4, wherein the organosolv is performed using a ketone organic solvent.

6. The process according to claim 5, wherein the ketone organic solvent is acetone.

7. The process according to claim 1, wherein the liquor has a temperature of 100 - 280 C. at the beginning of step (a).

8. The process according to claim 1, wherein the nanofiltration membrane has a molecular weight cut-off of 200 - 2500 Da.

9. The process according to claim 8, wherein the nanofiltration membrane has a molecular weight cut-off of 300 - 2000 Da.

10. The process according to claim 1, wherein the selective water removal is accomplished by selective permeation using a membrane selective for permeation of water molecules.

11. The process according to claim 10, wherein the selective permeation is accomplished by vapour permeation.

12. The process according to claim 10, wherein the membrane selective for permeation of water molecules comprises organosilane moieties represented by Si.sub.pO.sub.q((CH.sub.2).sub.n).sub.r, wherein n=17, p=1.6 2.4, q =2.53.5, and r =0.6 1.4.

13. The process according to claim 1, which does not comprise active cooling and/or heating a solution or suspension.

14. The process according to claim 1, which does not comprise a distillation step.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 depicts a preferred system according to the invention, comprising an organosolv reactor (e), a nanofiltration module (a), a module for selective water permeation (b), a lignin precipitation module (c) and a sugar isolation module (d). Reference numbers are in conformity with the description of the system above. The system according to this preferred embodiment is designed to receive biomass via inlet (e1) and discharge pulp via outlet (e3), lignin via outlet (c3) and monomeric sugars via outlet (d3).

(2) FIG. 2 shows the gradual change in water and acetone content at the retentate side of the pervaporation membrane as function of time during example 2.

(3) FIG. 3 shows the changes in molecular weight distribution (measured by HPSEC) of the organosolv liquor before and after nano filtration during example 3A.

(4) FIG. 4 shows the composition of the retentate and the water flux as function of time, during pervaporation of a nano filtration retentate (example 3B).

EXAMPLES

Example 1: Solubility Test

(5) The solubility of xylose (>99% pure, VWR) in different solvent systems, that are suitable to be used as treatment liquid during organosolv, is tested. Xylose was used as representative of all monomeric sugars, as it is the most abundant carbohydrate found in hemicellulose present in hardwoods and herbaceous biomass and thus in corresponding organosolv liquors. The indicated amount of xylose was added to each solvent system at 20 C. and the suspension was mechanically stirred for 24 h. Hereafter, the saturated solution containing remaining xylose was allowed to settle for 3 h and the liquid was removed. The residue was dried and the amount of undissolved xylose was determined. The results are displayed in table 2.

(6) TABLE-US-00002 TABLE 2 Xylose solubility xylose added xylose not solvent system (wt %).sup.[a] dissolved (wt %).sup.[b] water/acetone 9.9/90.1 (w/w) 2.3 80.2 water/acetone 8.4/91.6 (w/w) 2.0 79.1 water/acetone 4.9/95.1 (w/w) 2.7 93.0 water/acetone 1.9/98.1 (w/w) 2.7 99.1 acetone (100%) 2.7 99.1 water/ethanol 9.7/90.3 (w/w) 2.3 0.0 water/ethanol 7.8/92.2 (w/w) 2.3 16.6 water/ethanol 4.7/95.3 (w/w) 2.3 46.2 water/ethanol 4.3/95.7 (w/w) 2.2 49.7 ethanol (100%) 2.5 82.2 .sup.[a]based on total weight of the solvent system. .sup.[b]based on total weight of the xylose added.

(7) Indeed, the amount of xylose dissolved decreased upon increasing the organic solvent content, both for ethanol and acetone as organic solvent. Using acetone as organic solvent led to a much decreased xylose solubility, compared to ethanol as organic solvent, indicating that xylose will precipitate more effectively when the process according to the invention employs acetone as organic solvent.

Example 2: Lignin and Sugar Isolation from an Acetone Organosolv Liquor

(8) An organosolv liquor obtained by subjecting biomass (wheat straw) to organosolv at 140 C. during 60 min using a solvent system of acetone/water 50/50 (w/w) (10 L/kg dry biomass) and 60 mM H.sub.2SO.sub.4. The liquor was subjected to nanofiltration at 60 C. over a sulfonated polyether ether ketone membrane on a ceramic support, having a molecular weight cut-off of 500 g/mol. Nanofiltration was performed with a VCF of 3 and a flux of 0.2 kg/m.sup.2.Math.h.Math.bar, which remained stable for at least 7 days. Upon nano filtration a retentate (NFR) and a permeate (NFP) were obtained.

(9) The NFP was subjected to pervaporation at 58 C. using a hybrid silica tubular membrane (length 5.4 cm) consisting of a macroporous -alumina support and a top layer prepared from bis(triethoxysilyl)ethane (BTESE) by sol-gel deposition as described in WO2007/081212. The liquor was placed in direct contact with one side of the membrane (the feed side), while the other side of the membrane (the permeate side) was placed under vacuum. Water permeated through the membrane, evaporated and was then condensed and collected. The water and acetone concentrations at the feed side of the membrane were monitored and are shown in FIG. 2.

(10) Permeation occurred with an initial flux of 4.5 kg/m.sup.2.Math.h, which decreased to <0.5 kg/m.sup.2h after six days, mainly due to the loss of driving force as the water content on the feed side of the pervaporation membrane decreased from 44 wt % to <5 wt % and only for a small part due to membrane fouling by deposited sugar particles that had formed a layer on the membrane. Membrane fouling by crystallisation was only observed after several days, when the water concentration had dropped to <10 wt %. It is believed that in view of water removal through the membrane, the water concentration close to the feed side of the membrane is locally lower than further away from the membrane in the body of the nanofiltration permeate being subjected to pervaporation, causing sugar crystallisation. Despite the presence of minor amounts of such deposits, the water concentration of the permeate was 99 wt % over the complete 10 days of the experiment and the water concentration at the feed side was decreased to 1.5 wt % at the end of the experiment (see FIG. 2). It is expected that such membrane fouling is completely avoided when selective water permeation is achieved by vapour permeation, as dissolved sugar molecules are then not in direct contact with the membrane. Additionally or alternatively, the water removal could be stopped at a slightly higher remaining water concentration at the retentate side to further reduce membrane fouling.

(11) At the end of the pervaporation experiment, the retentate was cooled to room temperature to stimulate further sugar crystallisation. The suspension was decanted and a sample from the precipitated sugars in the pervaporation vessel and on the membrane was taken. This sample was dissolved in water and analyzed by High Performance Anion Exchange Chromatography with Pulsed Amperometric Detection (HPAEC-PAD) for sugars, confirming the presence all sugars present in the organosolv liquor and nanofiltration permeate in the precipitate. The liquid streams were analyzed by HPAEC-PAD (monomeric sugars) and HPLC (sugar derivatives and organic acids). Results are given in table 3.

(12) TABLE-US-00003 TABLE 3 stream compositions (in mg/kg liquid) Component OS Liquor NFR.sup.[a] NFP.sup.[a] PVP.sup.[a] PVR.sup.[a,b] Monomeric sugars arabinose 2145 3586 923 <13 432 fructose <25 <25 <25 <25 <25 galactose 560 1070 141 <13 <13 glucose 2004 3859 500 <13 54 mannose <63 <63 <63 <63 <63 rhamnose 129 210 60 <13 99 xylose 23.6 10.sup.3 39.7 10.sup.3 10.2 10.sup.3 <13 5208 Sugar derivatives furfural 1732 1342 1712 587 HMF.sup.[d] 266 216 245 628 MMF.sup.[d] <20 <20 <20 <20 Organic acids acetic acid 3847 4578 3410 6947 formic acid <300 383 <200 <200 levulinic acid <60 84 <60 157 Lignin.sup.[e] 4000 20000 <200 <200 .sup.[a]NFR = nanofiltration retentate; NFP = nanofiltration permeate; PVP = pervaporation permeate, PVR = pervaporation retentate. .sup.[b]PVR decant .sup.[d]HMF = 5-(hydroxymethyl)-furfural; MMF = 5-(methoxymethyl)-furfural .sup.[e]Estimates based on peak integral molecular weight measurement by alkaline size exclusion chromatography (SEC).

(13) Nano filtration effectively fractioned the organosolv liquor in a retentate containing substantially all of the lignin and a lignin-depleted permeate. Although some monomeric sugars were retained, the majority ended up in the permeate. Isolation of monomeric sugars from the NFP was readily achieved using pervaporation resulting in a very clean permeate and a retentate comprising precipitated sugars. These precipitated sugars were readily isolated by decantation and rinsing of the pervaporation vessel and membrane with water. Thus, the present process provides an efficient process for isolating both lignin and monomeric sugars from an organosolv liquor.

Example 3: Lignin and Sugar Isolation from an Ethanol Organosolv Liquor

(14) Step A: Nanofiltration separation

(15) A nano filtration membrane was prepared by coating a boehmite sol with a coating velocity of 5 mm/s on a macroporous -alumina tubular support. This layer was air-dried overnight and was exposed to a heat treatment of 200 C. for 1 hour. After cooling, the membrane was sealed by welding stainless steel caps with a carbon seal inside on the membrane. The membrane had a length of 48 cm and an outer diameter of 14 mm. The molecular weight cut-off of the membrane was 1700 g/mol. To characterize the membrane, first a retention test was executed by dissolving 1 wt % polyethyleneglycol (PEG 2000 gr/mol) in water and using a nano filtration cross flow set up operated at 20 C. with a feed flow of 1 m.sup.3/h. During 38 days of operation, the flux was higher than 1.2 kg/h.Math.m.sup.2.Math.bar and the retention was >95%. Flux and retention were more or less constant over time.

(16) An organosolv experiment was performed with wheat straw that was treated using a 20 L batch autoclave reactor under the following conditions: 190 C., 60 min, 60/40 ethanol/water (w/w), liquid to solid ratio=10 kg/kg dw, 30 mM H.sub.2SO.sub.4. The organosolv liquor was neutralized with concentrated NaOH solution to a pH value of 6.5 to protect the installation and pump against corrosion. After neutralization the organosolv liquor was transferred to the feed vessel of the nanofiltration cross flow set-up and subjected to nanofiltration over the membrane. The nanofiltration test was done at about 20 C. A pressure of 8 bar was applied on the fluid by the pump and a long term measurement was executed by measuring the flux through the membrane in time. The feed was pumped over the membrane module with a flow of 1 m.sup.3/h. The pressure drop over the membrane module was always smaller than 1 bar. The permeate stream was collected and the weight of permeate was followed in time. The starting permeance was around 0.085 kg/bar.Math.h.Math.m.sup.2, but declined during time especially during the first 3 days of operation. After a period of approximately 5 days, the flux stabilised at about 0.053 kg/bar.Math.h.Math.m.sup.2.

(17) The molecular weight distribution of all compounds (lignin, carbohydrates, etc) in the feed (organosolv liquor) and in the nanofiltration permeate was analyzed by High Pressure Size Exclusion Chromatography (HPSEC). The results are presented in FIG. 3. The sugar content of the nano filtration retentate and permeate streams was measured by HPAEC-PAD and is given in Table 4.

(18) TABLE-US-00004 TABLE 4 Composition of nanofiltration retentate and permeate. Compound (mg/kg liquid) NF Retentate NF Permeate Arabinose 397 411 Cellobiose 70 59 Fructose 0 0 Galactose 197 196 Glucose 359 369 Mannose 0 0 Rhamnose 40 37 Xylose 3272 3363 Ethyl--D-glucopyranoside 17 15 Ethyl--D-xylopyranoside 93 82 Ethyl--glucoside + Ethyl-- 78 70 xyloside Sum 4523 4602

(19) From the HPSEC measurement, it is clear that the molecular weight distribution in the organosolv liquor was between 100 and 30000 g/mol. The feed stream had a peak between 2000 and 3000 g/mol, typical for organosolv lignin. For the nanofiltration permeate, the peak is around 1100 gr/mol. The nanofiltration membrane retains about 80% of the total amount of lignin in the organosolv stream including all components with a weight above 15000 g/mol. As can be seen in table 4, the total concentration of sugars is around 4.6 wt % in both the permeate and the retentate, which shows that these compounds are not retained by the membrane. These results demonstrate that the membrane effectively separates lignin and monomeric sugars present in the organosolv liquor.

(20) Step B: Sugar Isolation by Pervaporation

(21) Hybsi membranes resistant in aggressive media were prepared, having a tubular macroporous -alumina support and a BTESE top layer, and no mesoporous -alumina layer (see WO 2014/025259). These membranes are known to be selective towards water, meaning that water permeates faster than ethanol or other organic liquids. The supports were made by coating a dispersion of very well dispersed 0.4 m alumina particles (AKP-20, Sumitomo) on a commercial macroporous alumina support (TAMI). After drying at room temperature, the -alumina layer was sintered at 1200 C. The final porosity of these support layers is about 30% and the average pore diameter is about 0.17 micrometer. Three individual Hybsi membranes of approximately 0.5 m thickness were prepared by coating of the BTESE sol on the above mentioned porous -alumina tubular supports, followed by drying and calcination at 240 C. for 1 h under nitrogen (heating/cooling rates were 0.5 C./min, respectively). The water concentrations of the feed and permeate were determined by refractive index at ambient conditions (Mettler Toledo RA510M) and a volumetric Karl Fischer titration.

(22) The prepared membranes were first subjected to a model pervaporation experiment to determine the selectivity of the individual membranes (length about 4.7 cm and outer diameter 1.4 cm) for a feed mixture of 2 wt % xylose in water/ethanol 40/60 (w/w) at a temperature of 75 C. After 9 days, the water concentration at the retentate side was reduced to <20 wt % (>80 wt % ethanol). An initial flux of 3.0 kg/m.sup.2.Math.h was observed, which declined to about 0.5 kg/m.sup.2.Math.h at day 3 and remained constant afterwards until day 9. Fluxes for xylose (0.0 kg/m.sup.2.Math.h) and ethanol (0.1 kg/m.sup.2.Math.h at day 0; 0.0 kg/m.sup.2.Math.h at day 3-9) were negligible. Initially, the reduction in H.sub.2O flux was mainly due to the loss of driving force as the water content on the feed side of the pervaporation membrane decreased. At lower water concentrations deposited sugar particles that had formed a layer on the membrane also partly caused flux decline. Brownish xylose crystals were observed on the membrane, which were easily removed after the pervaporation experiment by placing the membrane in demineralized water for 48 h. On day 9, the membrane was replaced by a fresh one, after which the flux increased back to 2.5 kg/m.sup.2.Math.h and the water content further reduced to about 3 wt % (at day 11). During the entire measurement, the xylose content at the retentate side remained constant at 2 wt %. The water concentration in the permeate was >90 wt % during the entire process. At day 14, the pervaporation experiment was stopped and the retentate was allowed to cool to room temperature, after which sedimentation of xylose crystals was observed.

(23) Secondly, the nano filtration permeate of step A was subjected to pervaporation over the Hybsi membrane (length about 6 cm). The composition of the retentate and the water flux was measured as function of time over a total of 6 days, see FIG. 4. Water removal from the nanofiltration permeate went fast during the first 3 days of measuring. When the water concentration in the organosolv liquor was reduced to below 7.5 wt %, the water flux had decreased from 3.0 kg/m.sup.2.Math.h to 0.3 kg/m.sup.2.Math.h. About half of this flux decline can be explained by the reduction of the driving force for water transport as less water is present at the retentate side. When still about 7.5 wt % of water was present in the feed mixture a colour change of the retentate was observed from light transparent brown to cloudy dark brown. The pervaporation process was stopped when the water concentration at the retentate side was 3.5 wt %. The feed mixture was allowed to cool to room temperature and xylose crystals precipitated at the bottom of the feed vessel. The water concentration in the permeate was >90 wt % during the entire process. These results show that separation of carbohydrates by removing the solvent by pervaporation over a Hybsi membrane is successful, and that a separation/precipitation of xylose/carbohydrates from an organosolv mixture was achieved by means of selectively removing water from the mixture via membrane pervaporation and temperature decrease.