PROCESS FOR PRODUCING LIGNIN PARTICLES

Abstract

Described is a process for producing lignin particles in the context of a continuous process, including a particle-free lignin-containing solution and a precipitation agent are combined in a mixing apparatus and subsequently passed out of the mixing apparatus again, wherein a mixing efficiency of the lignin-containing solution with the precipitation agent of at least 90% and a precipitation of lignin particles are achieved to form a suspension of lignin particles, and the residence time in the mixing apparatus does not exceed a duration of 30 seconds.

Claims

1. A process for producing lignin particles in the context of a continuous process, comprising: a particle-free lignin-containing solution and a precipitation agent are combined in a mixer and then discharged from the mixer again; a mixing quality of the lignin-containing solution with the precipitation agent of at least 90% and a precipitation of lignin particles being achieved, resulting in a suspension of lignin particles; and the residence time in the mixer does not exceed a period of 5 seconds.

2. A process for producing lignin particles in the context of a continuous process, comprising: a particle-free lignin-containing solution and a precipitation agent are combined in a mixing device and are subsequently discharged from the mixing device again, a mixing quality of the lignin-containing solution with the precipitation agent of at least 90% and a precipitation of lignin particles being achieved, resulting a suspension of lignin particles; the mixing device comprising at least one mixer and the line leading out thereof with a diameter of 10 mm or less, and the residence time in the mixing device does not exceed a period of 30 seconds.

3. The process according to claim 1, characterised in that the residence time in the mixer does not exceed a period of 4 seconds, preferably 3 seconds, even more preferably 2 seconds, in particular 1 second.

4. The process according to claim 2, characterised in that the residence time in the mixing device does not exceed a period of 25 seconds, preferably 20 seconds, in particular 15 seconds.

5. The process according to claim 1, characterised in that the mixer is selected from a static mixer, a dynamic mixer or combinations thereof.

6. The process according to one claim 1, characterised in that the particle-free lignin-containing solution comprises at least one organic solvent and water or at least one organic solvent.

7. The process according to claim 1, characterised in that the particle-free lignin-containing solution is obtained by a kraft lignin (KL) process, a soda-lignin process, a lignosulfonate (LS) process, an organosolv-lignin (OS) process, a steam explosion lignin process, a hydrothermal process, an ammonia explosion process, a supercritical CO.sub.2 process, an acid process, an ionic-liquid process, a biological process or an enzymatic hydrolysis lignin (EHL) process.

8. The process according to claim 1, characterised in that the precipitation agent is water or a diluted acid, preferably sulphuric acid, phosphoric acid, nitric acid or an organic acid, in particular formic acid, acetic acid, propionic acid or butyric acid, or CO.sub.2, or a diluted lye, preferably caustic soda or potassium hydroxide, with water being particularly preferred as precipitation agent.

9. The process according to claim 1, characterised in that the precipitation agent is a solution and the volume of the precipitation agent is at least 0.5 times, preferably at least twice, in particular at least 5 times the volume of the lignin-containing solution.

10. The process according to claim 1, characterised in that the precipitation agent is a solution and the volume of the precipitation agent is 1 to 20 times, preferably 1.5 to 10 times, in particular 2 to 10 times the volume of the lignin-containing solution.

11. The process according to claim 1, characterised in that the pH of the precipitation agent is in the range of 2 to 12, preferably 3 to 11, in particular 4 to 8.

12. The process according to claim 1, characterised in that the pH of the suspension of lignin particles is in the range of 2 to 12, preferably 3 to 11, in particular 4 to 8.

13. The process according to claim 1, characterised in that a mixing quality of the lignin-containing solution with the precipitation agent of at least 95% is achieved in the mixing device.

14. The process according to claim 1, characterised in that the particle-free lignin-containing solution contains an organic solvent, preferably an alcohol, a ketone or THE, with ethanol being particularly preferred, in particular in a mixture with water.

15. The process according to claim 1, characterised in that the particle-free lignin-containing solution contains an organic solvent, preferably a C.sub.1 to C.sub.5 alcohol, in particular selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, ethane-1,2-diol, propane-1,2-diol, propane-1,2,3-triol, butane-1,2,3,4-tetraol and pentane-1,2,3,4,5-pentol; or a ketone selected from acetone and 2-butanone.

16. The process according to claim 1, characterised in that the precipitation is carried out at a temperature of 0 to 100° C., preferably of 5 to 80° C., even more preferably of 10 to 60° C., even more preferably of 15 to 50° C., even more preferably of 20 to 30° C.

17. The process according to claim 1, characterised in that the particle-free lignin-containing solution contains lignin in an amount of 0.1 to 50 g lignin/L, preferably 0.5 to 40 g/L, even more preferably 1 to 30 g/L, even more preferably 2 to 20 g/L.

18. The process according to claim 1, characterised in that the suspension of lignin particles from the mixer or mixing device is introduced into a suspension container.

19. The process according to claim 1, characterised in that the particle-free lignin-containing solution comprises an organic solvent in an amount of 10 to 90 wt. %, preferably 20 to 80 wt. %, even more preferably 30 to 70 wt. %, even more preferably 40 to 60 wt. %, even more preferably 50 to 65 wt. %.

20. The process according to claim 1, characterised in that the particle-free lignin-containing solution is obtained by extraction of lignin-containing starting material at a temperature of 100 to 230° C., preferably of 120 to 230° C., even more preferably of 140 to 210° C., even more preferably of 150 to 200° C., even more preferably of 160 to 200° C., even more preferably of 170 to 200° C., even more preferably of 170 to 195° C., even more preferably of 175 to 190° C.

21. The process according to claim 1, characterised in that the particle-free lignin-containing solution is obtained by extraction of lignin-containing starting material at a pressure of 1 to 100 bar, preferably 1.1 to 90 bar, even more preferably 1.2 to 80 bar, even more preferably 1.3 to 70 bar, even more preferably 1.4 to 60 bar.

22. The process according to claim 1, characterised in that the particle-free lignin-containing solution is obtained by extraction of lignin-containing starting material selected from material of multi-year plants, preferably wood, wood waste or shrub cuttings, or material of single-year plants, preferably straw, or biogenic waste.

23. The process according to claim 1, characterised in that the particle-free lignin-containing solution is obtained by extraction of lignin-containing starting material having an average size of 0.5 to 50 mm, preferably of 0.5 to 40 mm, even more preferably of 0.5 to 30 mm, even more preferably of 1 to 25 mm, even more preferably of 1 to 20 mm, even more preferably of 5 to 10 mm.

24. The process according to claim 1, characterised in that the particle-free lignin-containing solution is obtained by extraction of lignin-containing starting material and subsequent removal of solid particles still present in the extraction mixture.

25. The process according to claim 1, characterised in that the lignin particles in the suspension have an average diameter of less than 400 nm, preferably less than 250 nm, even more preferably less than 200 nm, even more preferably less than 150 nm, in particular less than 100 nm.

26. The process according to claim 1, characterised in that at least 50 or more of the lignin particles in the suspension have a size, measured as hydrodynamic diameter (11D), in particular measured with dynamic light scattering (DIS), of less than 400 nm, preferably of less than 300 nm, even more preferably of less than 250 nm, in particular of less than 150 nm, even more preferably of less than 100 nm.

27. The process according to claim 1, characterised in that at least 60% or more, preferably at least 70% or more, even more preferably at least 80% or more, in particular at least 90% or more; of the lignin particles in the suspension have a size, measured as hydrodynamic diameter (HD), in particular measured with dynamic light scattering (DLS), of less than 500 nm, preferably less than 300 nm, even more preferably less than 250 nm, even more preferably less than 200 nm, in particular less than 100 nm.

28. The process according to claim 1, characterised in that the precipitation agent is a liquid precipitation agent and is added in such a way that the concentration of a solvent in the lignin-containing solution is reduced in the range of 1 to 10,000 wt. %/s, preferably 10 to 5,000 wt. %/s, preferably 10 to 1,000 wt. %/s, preferably 10 to 100 wt. %/s, in particular 50 to 90 wt. %/s, in the mixer or in the mixing device.

Description

[0062] The present invention is explained in more detail by means of the following examples and the figures in the drawing, but without being limited to them.

[0063] In the drawing:

[0064] FIG. 1 shows: (a) turbidity against ethanol concentration in solution/suspension. The ethanol concentration was gradually reduced by adding precipitation agent at different pH values to the organosolv extract in a stirred tank; (b) The images of the particle suspensions and supernatants after centrifugation, obtained from precipitates in the static mixer with pH 5 precipitation agent and a flow rate of 112.5 ml/min.

[0065] FIG. 2 shows: the effect of the interaction of the independent variables on the hydrodynamic diameter of the resulting particles and SEM images of selected precipitation parameters.

[0066] FIG. 3 shows: distributions of hydrodynamic diameter of and SEM images of lignin particles precipitated directly from organosolv extract or from a solution of purified lignin. The parameters used were pH 7, precipitation agent to extract ratio of 5, and a flow rate of 112.5 ml/min in the static mixer.

[0067] FIG. 4 shows: (a) Boxplot diagrams of the relative carbohydrate content found in the 34 individual experiments; (b) Boxplot diagram of the total carbohydrate content in the direct precipitation from organosolv extracts and in the purified lignin.

[0068] FIG. 5 shows: the effect of the interaction of the independent variables on the total carbohydrate content of the resulting dry precipitate.

EXAMPLES: DIRECT PRECIPITATION OF LIGNIN NANOPARTICLES

[0069] Summary:

[0070] Micro- and nano-sized lignin shows improved properties compared to standard lignin available today and has gained interest in recent years. Lignin is the largest renewable resource on earth with an aromatic skeleton, but is used for relatively low-value applications. However, the use of lignin on the micro to nano scale could lead to valuable applications. Current production processes consume large quantities of solvents for purification and precipitation. The process investigated in this paper applies the direct precipitation of lignin nanoparticles from organosolv pre-treatment extract in a static mixer and can drastically reduce solvent consumption. pH value, precipitation agent to organosolv extract ratio, and flow rate in the mixer were investigated as precipitation parameters in relation to the resulting particle properties. Particles in size ranges from 97.3 nm to 219.3 nm could be produced, and with certain precipitation parameters the carbohydrate contamination reaches values as low as those for purified lignin particles. Yields were 48.2±4.99% regardless of the precipitation parameters. The presented results can be used to optimise the precipitation parameters with regard to particle size, carbohydrate impurities or solvent consumption.

[0071] Introduction

[0072] This paper focuses on the direct precipitation of lignin nanoparticles from organosolv pre-treatment extracts (OSE) in a wheat straw biorefinery, potentially reducing the solvent consumption of the whole process. Precipitation is performed in a static mixer, resulting in smaller particles compared to batch precipitation (Beisl et al., Molecules 23 (2018), 633-646). It combines the most commonly used precipitation methods of solvent shifting and pH shifting and reduces lignin solubility by lowering the solvent concentration and lowering the pH (Lewis et al., Industrial Crystallization; Cambridge University Press: Cambridge, 2015; pp. 234-260). The degree of lignin supersaturation, the hydrodynamic conditions prevailing during the process and the pH of the fluid surrounding the particles are important parameters that influence the final particle size and behaviour. These mentioned process conditions are investigated by varying the precipitation parameters of pH value, ratio of precipitation agent to OSE, and the flow rate in the static mixer. The resulting particles were investigated with respect to particle size, stability, carbohydrate contamination and yield of the process. The best precipitation parameters were identified and a comparison was made with the precipitation of the previously purified and redissolved lignin.

Experimental Part

[0073] Materials

[0074] The wheat straw used was harvested in 2015 in the province of Lower Austria and stored under dry conditions until use. The particle size was crushed in a cutting mill equipped with a 5 mm sieve, before the pre-treatment. The composition of the dry straw was 16.1 wt. % lignin and 63.1 wt. % carbohydrates, consisting of arabinose, glucose, mannose, xylose and galactose. Ultrapure water (18 MΩ/cm) and ethanol (Merck, Darmstadt, Germany, 96 vol. %, undenatured) were used in the organosolv treatment, and sulphuric acid (Merck, 98%) was additionally used in the precipitation steps.

[0075] Organosolv Pre-Treatment

[0076] The organosolv pre-treatment was carried out as previously described in Beisl et al (Molecules 23 (2018), 633-646). In brief, wheat straw was treated at a maximum temperature of 180° C. for 1 h in 60 wt. % aqueous ethanol. Residual particles were separated by centrifugation. The composition of the extract can be found in Table 1.

[0077] Precipitation

[0078] The applied precipitation arrangement is generally described in Beisl et al. (Molecules 23 (2018), 633-646). However, in comparison to Beisl et al., the time spent in the mixing device (consisting of the T-connector, a 20.4 cm long tube with an inner diameter of 3.7 mm containing the static mixing elements, and the 1 m long rubber hose (diameter 4 mm)) was considerably shorter for the present invention. Whereas Beisl et al. spent more than 36 s in the static mixing device (volume: about 15 ml at a flow rate of about 24 ml/min) and more than 5 s in the static mixer itself (volume: about 2.2 ml at a flow rate of about 24 ml/min), shorter mixing times (30 s or less) are used in the process according to the present invention. The time in the mixing device in the present examples ranges from about 23 s to 3 s and the time in the mixer in the present examples ranges from about 5 s to 0.6 s.

[0079] The assembly consists of two syringe pumps, a static mixer and a stirred collection vessel. The stirrer speed in the collection vessel was set to 375 rpm. The acidified precipitation agent with a pH value of 3 and 5 was set using sulphuric acid, and the pH 7 precipitation agent was pure water. The particles were separated from the suspension after precipitation in a ThermoWX-80+ ultracentrifuge (Thermo Scientific, Waltham, Mass., USA) at 288,000 g for 60 min. The supernatant was decanted and the precipitated substance was freeze-dried. For the purified lignin, lignin was precipitated from the same extraction process and purified by repeated ultrasonic treatment, centrifugation and replacement of the supernatant. The purified lignin (“purified lignin”; PL) was freeze-dried and then dissolved in an ethanol/water mixture at equal ethanol concentrations compared to undiluted OSE. This artificial extract was used for the comparison with direct precipitation.

[0080] Design of the Experiments

[0081] The experimental design and statistical analysis of the results were carried out using Statgraphics Centurion XVII software (Statpoint Technologies, Inc., USA). A face-centered central composite design comprising three central points with a full repetition (34 individual experiments) was applied for the precipitation parameters of flow rate in the static mixer, pH value of the precipitation agent, and volume ratio of precipitation agent to OSE. The flow rates in the static mixer were set to 37.5 ml/min, 112.5 ml/min and 187.5 ml/min. The precipitation agent to extract volume ratios were set to 2, 5 and 8, while the pH of the precipitation agent was 3, 5 and 7. The significance level was set at α=0.05 in all statistical tests.

[0082] The results from the face-centered central composite design were used to describe the effects of the independent variables using a cubic model approach. High coefficients of determination were achieved for the carbohydrate content (R.sup.2 0.89/Adj. R.sup.2 0.87) and particle size (0.92/0.88). Non-significant factors were gradually removed from the model and were not included in the results.

[0083] Characterisation

[0084] The ethanol concentration-dependent turbidity of the particle suspension was determined with a Hach 2100Qis (Hach, CO, USA). To stay within the calibration range, the extract was diluted 1:6 by volume with ethanol/water to maintain the undiluted ethanol concentration of the extract. Water or sulphuric acid/water mixtures were gradually added to a stirring vessel filled with the diluted extract and measured after each addition.

[0085] The hydrodynamic diameter (HD) of the particles was measured with dynamic light scattering (DLS) (ZetaPALS, Brookhaven Instruments, Holtsville, N.Y., USA). The measurements were performed in the particle suspension directly after precipitation—both undiluted and in a 1:100 dilution with pure water. Undiluted measurements were corrected for their viscosity and the refractive index of the obtained supernatant after centrifugation. For long-term stability tests, the particles were stored at 8° C. but measured at 25° C.

[0086] The ζ-potential was investigated with a ZetaPALS (Brookhaven Instruments, Holtsville, N.Y., USA). Dried particles were dispersed in water at an appropriate concentration of 20 mg/L and stored for 24 h before the measurement. Each measurement consisted of five runs, each with 30 sub-runs, and was performed at 25° C.

[0087] Freeze-dried particles were dispersed in hexane, spread on a sample holder and examined under a scanning electron microscope (SEM) (Fei, Quanta 200 FEGSEM). The samples were sputter-coated with 4 nm Au/Pd (60 wt. %/40 wt. %) before analysis.

[0088] The carbohydrate content was determined using sample preparation in accordance with the laboratory analytical procedure (LAP) of the National Renewable Energy Laboratory (NREL): “Determination of Structural Carbohydrates and Lignin in Biomass” (Sluiter et al., Determination of Structural Carbohydrates and Lignin in Biomass; Denver, 2008), but the samples were not neutralised after hydrolysis. A Thermo Scientific ICS-5000 HPAEC-PAD system (Thermo Scientific, Waltham, Mass., USA) with deionised water as eluent was used to determine arabinose, glucose, mannose, xylose and galactose.

[0089] The yield was determined by the difference in dry matter content of the particle suspension directly after precipitation and the supernatant of the particle suspension after centrifugation.

[0090] Results and Discussion

[0091] Ratio of Precipitation Agent/Organosolv Extract

[0092] The solubility of lignin depends strongly on the concentration of ethanol in ethanol/water solvent mixtures and the type of lignin (Buranov et al. Bioresour. Technol. 101 (2010), 7446-7455). To determine the required final ethanol concentration in the precipitation process and thus the ratio of precipitation agent to OSE, the turbidity was measured as a function of the ethanol concentration (see FIG. 1). Pure water and water/sulphuric acid mixtures were gradually added to the OSE in a stirred flask at an initial ethanol concentration of 56.7 wt. %. To remain within the measuring range of the turbidimeter, the initial OSE was diluted by a factor of 1:6 by mass, maintaining the initial ethanol concentration. The undiluted lignin concentration of 7.35 g/kg was therefore reduced to 1.23 g/kg. This could lead to a slight shift of the turbidity maxima towards lower ethanol concentrations, as the solubility limit is reached at lower ethanol concentrations. The maxima of the turbidity curves were used to determine the minimum precipitation agent/OSE ratios required for the precipitation. The turbidity maxima were reached at 19.9 wt. %, 18.1 wt. % and 17.9 wt. % for the addition of precipitation agent with a pH of 2, 5 and 7 respectively. The lowest precipitation agent/OSE ratio for the precipitation experiments was therefore set at 2, resulting in a final ethanol concentration in the suspension of 17.6 wt. %. Further investigated ratios were set to 5 and 8, resulting in a final ethanol concentration of 8.7 wt. % and 5.7 wt. % respectively, in order to increase the lignin supersaturation. The shift in the maxima of turbidity towards higher ethanol concentrations for decreasing pH values indicates a decreasing solubility of the lignin with decreasing pH values. However, the lowest pH of the precipitation agent used for the precipitation experiments in the static mixer was fixed at 3 instead of 2 due to an isoelectric point at a pH of around 2.5 identified in the ζ-potential measurements.

[0093] Particle Size

[0094] The independent variables of pH value of the precipitation agent, flow rate in the static mixer and precipitation agent/OSE ratio were investigated in relation to the resulting particle HD. The resulting particle suspensions were measured by dynamic light scattering (DLS) directly after precipitation in two variants: undiluted and in a 1:100 dilution with water. After correcting the viscosity and refractive index for the undiluted samples, the HDs for both dilutions were compared with a paired t-test and showed significantly equal results for both conditions. The results shown in FIG. 2 are based on the HDs obtained by diluted measurements.

[0095] The resulting HDs range from 97.3 nm to 219.3 nm. The smallest HD is achieved in precipitates with a precipitation agent/OSE ratio of 6.29, pH 7 and a flow rate of 132.06 ml/min. The particles with the highest HD result from a precipitation agent/OSE ratio of 2, pH 4.93 and a flow rate of 187.5 ml/min.

[0096] The HD of the particles shows a strong dependence on the flow rate with minima of between 107.25 ml/min and 138.0 ml/min depending on pH and ratio. This behaviour could result from changing flow conditions that influence the equilibrium of primary nucleation and agglomeration by changing the supersaturation of lignin and the collision rate of the resulting particles. At low flow rates the supersaturation is comparatively low and larger particles are formed. With increasing flow rates, the supersaturation of lignin increases, resulting in smaller particles. However, further increased supersaturation leads to higher collision and agglomeration rates (Lewis et al., Industrial Crystallization; Cambridge University Press: Cambridge, 2015; pp. 234-260).

[0097] A similar behaviour can be observed for the precipitation agent/OSE ratio. HDs decrease with increasing ratios due to higher supersaturation and coherently increasing nucleation rates. For example, at a constant pH of 5 and a flow rate of 112.5 ml/min, the HD of the particles decreases from 172.9 nm to 117.3 nm and 101.7 nm for ratios of 2, 5 and 8, respectively. However, the mechanical energy supply does not increase due to the constant flow rate. Therefore the particle collision rates depend only on the particle concentrations. Consequently, higher precipitation agent/OSE ratios coherently lead to lower agglomeration (Lewis et al., Industrial Crystallization; Cambridge University Press: Cambridge, 2018; pp. 130-150).

[0098] The pH value shows the least influence of the variables examined on the HD. The HD increases from 104.0 nm to 131.2 nm by raising the pH of the precipitation agent from 3 to 7 at a constant precipitation agent/OSE ratio of 5 and a flow rate of 112.5 ml/min. The increased HD at low pH could be explained by the ζ-potential of the particles, which decreases to pH 3 and reaches the isoelectric point at pH values around 2.5.

[0099] The OSE contains not only lignin, but also components such as carbohydrates, acetic acid and various degradation products, which must be considered as impurities during the precipitation process. In order to investigate the influence of these impurities, lignin was purified from used OSE and dissolved in an aqueous ethanol solution with an ethanol concentration of 56.7 wt. %, equal to undiluted OSE. The solubility of PL reached its limit at a concentration of 6.65 g/kg, which is lower than the lignin concentration of 7.35 g/kg in the OSE. Therefore, the OSE was diluted to the same concentration of lignin at constant ethanol concentration. The precipitation parameters were set at pH 7, ratio 5, and a flow rate of 112.5 ml/min, which is the closest experimental point to the calculated parameters for the smallest particles. The HD distributions and REM images of the precipitation directly from OSE and the dissolved PL are shown in FIG. 3. The PL precipitation results in an HD of 77.62±2.74 nm, whereas the precipitation directly from OSE leads to a higher HD of 102.7±7.75 nm. A comparable result was achieved by Richter et al. (Langmuir 2016, 32 (25), 6468-6477) with organosolv lignin dissolved in acetone and a precipitation leading to particles of about 80 nm in diameter. The SEM images show only minor differences and in both cases separate particles. However, based on the DLS results, a negative influence of the impurities can be observed with regard to particle size.

[0100] Yields

[0101] The precipitation yields were found to be independent of the precipitation parameters and had an average value of 48.2±4.99%. The standard deviation is quite high, but the values are normally distributed. For comparison, Tian et al. (ACS Sustain. Chem. Eng. 2017, 5 (3), 2702-2710) were able to achieve values between 41.0% and 90.9% using a dialysis procedure using dimethyl sulfoxide as a solvent for poplar, coastal pine and corn straw lignin and water as a precipitation agent. Moreover, this paper represents the most comparable process found in the literature, as it considers a complete process chain from raw material to finished lignin particles, including impurities. Yearla et al (J. Exp. Nanosci. 2016, 11 (4), 289-302) showed a process that produced 33% to 63% yield by rapidly adding lignin/acetone/water mixtures to water.

[0102] Carbohydrate Impurities

[0103] In addition to lignin, the OSE also contains carbohydrates as a major source of impurities during precipitation. In terms of concentration, the total carbohydrate content in the extract is 10.2% of the lignin content. Therefore, the resulting precipitated substance was analysed for its carbohydrate content after centrifugation and freeze drying.

[0104] The relative proportion of carbohydrates is shown in FIG. 4a. Glucose, with a relative proportion of 47.2±3.36%, is the predominant carbohydrate compound in the precipitated substance. FIG. 4b compares the carbohydrate concentrations found in the precipitated substance of the direct OSE experiments with the PL precipitates. The total carbohydrate content in the PL is 2.41±0.25 wt. % and appears to be covalently bound to the lignin. The lowest carbohydrate content found within all direct OSE precipitates was 2.39 wt. %, which is within the concentration range of the PL. This shows that certain precipitation parameters allow precipitation of almost pure lignin relative to the carbohydrates dissolved in the OSE that remain on the particles. FIG. 5 shows the dependencies of the carbohydrate contents on pH value, flow rate and precipitation agent/OSE ratio. The results are in a comparable range to the results of Huijgen et al. (Ind. Crops Prod. 2014, 59, 85-95), which achieved carbohydrate contents in precipitated wheat straw organosolv lignins of 0.4 wt. % to 4.9 wt. % with treatment temperatures between 190° C. and 210° C. However, the higher temperatures compared to the 180° C. used in this paper favour carbohydrate cleavage and lead to lower concentrations.

[0105] Contrary to the conclusion that a higher dilution factor would reduce the carbohydrate content, the carbohydrate concentration increases with an increase in the precipitation agent to extract ratio. The carbohydrate concentrations for a ratio of 2 are between 2.35 wt. % and 2.80 wt. % for precipitations with pH 3 and a flow rate of 187.5 ml/min or pH 4.79 and a flow rate of 37.5 ml/min. For a ratio of 8, a minimum concentration of 3.47 wt. % and a maximum of 6.10 wt. % can be found, both at a flow rate of 187.5 ml/min and a precipitation agent pH of 3 and 7 respectively.

[0106] A contrary behaviour is observed with increasing flow rates, which leads to either a decreasing or increasing carbohydrate content in the precipitated substance, depending on the pH and the ratio of precipitation agent/OSE. For a combination of pH 3, precipitation agent and a ratio of 2, the carbohydrate concentration decreases from 2.72 wt. % to 2.35 wt. % by increasing the flow rate from 37.5 to 187.5 ml/min. On the other hand, by increasing the flow rate by 150.0 ml/min at a pH of 5 and a precipitation agent/OSE ratio of 8, the carbohydrate content increases from 4.18 wt. % to 5.21 wt. %.

[0107] The pH value shows an increasing influence on increasing precipitation agent/OSE ratios and flow rates. The carbohydrate concentration at otherwise constant precipitation parameters can be reduced by up to 43% by changing the pH value of the precipitation agent. This maximum reduction is achieved at a precipitation agent/OSE ratio of 8 and a flow rate of 187.5 ml/min, and the carbohydrate content can be reduced from 6.09 wt. % to 3.47 wt. % by changing the pH from 7 to 3.

CONCLUSION

[0108] The influence of the precipitation parameters of pH-value, ratio of precipitation agent to organosolv extract, and flow rate in the mixer was investigated with regard to the resulting particle properties. The direct precipitation of lignin nanoparticles from wheat straw organosolv extracts can drastically reduce the solvent consumption in a production process for lignin nanoparticles. Particles with size ranges from 97.3 nm to 219.3 nm could be produced, and the carbohydrate impurities reached as low values at certain precipitation parameters as in purified lignin particles. The results found in this paper can be used to optimise the precipitation parameters in terms of particle size, carbohydrate impurities or solvent consumption in an uncomplicated process design.

TABLE-US-00001 TABLE 1 Composition of the organosolv extract used in the precipitation experiments Compound/property Value Unit Ethanol 511 g/l Total carbohydrates.sup.1 0.677 g/l Monomer carbohydrates.sup.1 0.201 g/l Acetic acid 1.43 g/l Acid-insoluble lignin 5.53 g/l Acid-soluble lignin 1.09 g/l Density.sup.2 0.901 g/ml Dry mass.sup.3 1.57 wt. % .sup.1Sum of the arabinose, galactose, glucose, xylose and mannose concentrations; .sup.2at 25° C.; .sup.3determined at 105° C.