METHOD FOR PRODUCING ULTRAFINE LIGNIN PARTICLES

Abstract

A method for producing ultrafine lignin particles by means of spray-drying at a dual-fluid nozzle (2), which has a first nozzle opening (31) and a second nozzle opening (41), wherein a lignin-containing solution or suspension is fed to the first nozzle opening (31) of the dual-fluid nozzle (2) and an atomizer gas is fed to the second nozzle opening (41) of the dual-fluid nozzle (2), and wherein: a) the flow rate at which the lignin-containing solution or suspension is fed to the first nozzle opening (31) of the dual-fluid nozzle (2) is 60 to 65 ml/min; b) the drying temperature is 150 to 175° C.; and c) the pressure of the atomizing gas at the second nozzle opening (41) of the dual-fluid nozzle (2) is 3 to 6 bar.

Claims

1. A method for the production of ultrafine lignin particles (6) by means of spray drying at a dual fluid nozzle (2) with a first nozzle opening (31) and a second nozzle opening (41), wherein a lignin-containing solution or suspension is supplied to the first nozzle opening (31) of the dual fluid nozzle (2) and an atomizing gas is supplied to the second nozzle opening (41) of the dual fluid nozzle (2), and wherein: a) the flow rate at which the lignin-containing solution or suspension is supplied to the first nozzle opening (31) of the dual fluid nozzle (2) is 60 to 65 mL/min; b) the drying temperature is 150° C. to 175° C.; and c) the pressure of the atomizing gas at the second nozzle opening (41) of the dual fluid nozzle (2) is 3 to 6 bar.

2. The method as claimed in claim 1, wherein the diameter of the first nozzle opening (31) of the dual fluid nozzle (2) is 1 to 2 mm, preferably 1.5 to 2 mm.

3. The method as claimed in claim 1, wherein the lignin-containing solution or suspension is a solution or suspension of lignin which has undergone a hot water extraction.

4. The method as claimed in claim 3, wherein the lignin-containing solution or suspension is a solution or suspension of lignin, which has undergone an enzymatic hydrolysis using cellulase following the hot water extraction.

5. The method as claimed in claim 1, wherein the solids content of the lignin solution or suspension is 5% to 20% by weight.

6. The method as claimed in claim 1, wherein the lignin-containing solution or suspension is an aqueous solution or suspension, preferably an aqueous suspension.

7. The method as claimed in claim 1, wherein the lignin-containing solution or suspension is injected through the first nozzle opening (31) of the dual fluid nozzle (2) into a drying chamber (1) which contains a hot drying gas, preferably air, CO.sub.2 or nitrogen gas.

8. The method as claimed in claim 7, wherein the hot drying gas is fed into the drying chamber (1) as a co-current with the lignin-containing solution or suspension.

9. The method as claimed in claim 1, wherein the first nozzle opening (31) of the dual fluid nozzle (2) is centrally disposed and the second nozzle opening (41) is annular and concentrically surrounds the central first nozzle opening (31), and wherein the lignin-containing solution or suspension is supplied to the central first nozzle opening (31) of the dual fluid nozzle (2) and the second nozzle opening (41) is supplied with a pressurized atomizing gas, preferably air, CO.sub.2 or nitrogen.

10. The method as claimed in claim 1, wherein the pressure of the atomizing gas at the second nozzle opening (41) of the dual fluid nozzle (2) is 3 to 5.5 bar, preferably 3 to 5 bar or 3 to 4.5 bar, particularly preferably 3 to 4 bar.

11. Lignin-containing microbeads, comprising a) a plurality of ultrafine lignin-containing particles which are produced by means of the method as claimed claim 1, and b) at least one binder.

12. The lignin-containing microbeads as claimed in claim 11, wherein the ultrafine lignin-containing particles are produced from AS lignin.

13. The lignin-containing microbeads as claimed in claim 11, wherein the binder is a gel-forming biopolymer, preferably alginate, cellulose, pectin, chitosan, polylactide or starch, or silicate or protein.

14. The lignin-containing microbeads as claimed in claim 11, wherein the proportion of lignin in the microbeads is 10-90% by weight, preferably 30-90% by weight.

15. The lignin-containing microbeads as claimed in claim 11, wherein the mean particle diameter of the microbeads is 300 μm to 5 mm, preferably 300 μm to 1.5 mm.

Description

[0047] The invention will now be described in more detail with reference to the accompanying figures and exemplary embodiments, for the purposes of illustration only.

[0048] FIG. 1. Schematic of part of a device with which a preferred embodiment of the method in accordance with the invention can be carried out.

[0049] FIG. 2. A simplified flowchart for a device for carrying out an embodiment of the method in accordance with the invention.

[0050] FIG. 3. Particle size distribution of AS lignin particles obtained with two different nozzles (1.5 and 2 mm opening diameter).

[0051] FIG. 4. Cytotoxicity of spray dried lignin (fine fraction=SDL fine; coarse fraction=SDL coarse) compared with Organosolv and alkali lignin.

[0052] FIG. 5. IC50 values for different lignins for the inhibition of α-glucosidase and α-amylase. Fine fraction=SDL fine; coarse fraction=SDL coarse.

[0053] FIG. 6. Hardness of tablets obtained by direct compression of AS lignin with different excipients: F1 (alginate) F2 (starch), F3 (direct compression-excipient, DCE), F4 (microcrystalline cellulose, MCC), F5 (lactose), in various concentrations: 10%, 20% and 30% by weight.

[0054] FIG. 7. Radical scavenger capacity (% inhibition) of spray dried lignin and lignins in accordance with the invention which have been obtained by other biorefining processes. EtOH=ethanol; Organosolv-lignin=lignin extracted with ethanol (produced by Fraunhofer CBP Leuna); EtOH CO.sub.2=lignin particles which have undergone a solvent exchange (exchange of water to ethanol), wherein the ethanol was then extracted with supercritical CO.sub.2.

[0055] FIG. 8. DPPH radical scavenger activity for two antioxidants: spray dried lignin particles and Tesa antioxidant compound from a brand manufacturer of adhesive film.

[0056] FIG. 9. Composition of nine different lignin fillers at different excipient concentrations.

[0057] AS lignin particles with appropriate properties were produced by means of spray drying. Regarding the particle size distribution, homogeneous lignin powder with the desired properties could be produced in a single step using the method in accordance with the invention. The multiple comminution and milling steps which had been required until now can be dispensed with.

[0058] FIGS. 1 and 2 concern diagrammatic representations of a device which was used for the production of lignin particles 6 from AS lignin suspensions. FIG. 1 diagrammatically shows the spray drying process using a dual fluid nozzle 2 as part of the unit 100 employed for the test. FIG. 2 shows a simplified flow diagram for the unit 100 for the production of AS lignin particles.

[0059] The spray drying method in accordance with the invention encompasses the production of a lignin powder by drying a liquid solution or suspension with a hot drying gas. In the tests described here, nitrogen (N.sub.2) was used. AS lignin suspended in water was used as the lignin material; it had undergone a cellulase treatment following a hot water extraction. During the spray drying process, the liquid lignin-containing material (solution or suspension) was atomized and brought into contact with a stream of hot gas. To this end, a dual fluid nozzle 2 was used; this is shown in longitudinal section. The dual fluid nozzle 2 comprises a central bore 3 with a central first nozzle opening 31 to a drying chamber 1 and a bore 4 which concentrically surrounds the central bore 3 with a second nozzle opening 41 to the drying chamber 1 which concentrically surrounds the first nozzle opening 31. The liquid AS lignin-containing material was fed via a first inlet opening 32 through the central bore 3 to the central first nozzle opening 31 (arrows with solid lines), the atomizing gas, in this case N.sub.2, was fed via a second inlet opening 42 through the concentric bore 4 to the second nozzle opening 41 (arrow with dotted lines). The hot gas, in this case also N.sub.2, was thus fed into the drying chamber 1 in a manner such that the hot gas stream 7 ran as a co-current to the stream of particles which had been formed. In this manner, the hot gas was introduced into the drying chamber 1 in the immediate vicinity of the dual fluid nozzle 2. The hot gas stream 7 and the stream of particles ran essentially in the gravitational direction. The contact between the material to be dried and the hot gas in the drying chamber 1 is brief but sufficient to carry out evaporation of the water within the atomized droplets 5. The atomization was obtained pneumatically by means of a high speed of the compressed atomizing gas in contact with the liquid lignin material (droplet formation), as can be seen in simplified manner in FIG. 1.

[0060] The morphology and particle size of the final product can be controlled by varying the ratio of the flow rates between the starting material (lignin suspension or solution) and the pressurized atomizing gas at the respective nozzle openings 31, 41 as well as the inlet and outlet temperatures in the drying chamber 1. The mode of obtaining the dried lignin particles 6 may vary as a function of the configuration of the spray drying unit 100. In the tests described here, a spray drying unit with a drying chamber 1 was used which was equipped with two extraction points 11, 12 (see FIG. 2). Coarse (larger and heavier) particles 6 (coarse fraction) could be collected through a bottom first extraction point 12; finer particles 6 (fine fraction) could be collected through the laterally disposed second extraction point 11. A fraction with finer lignin particles 6 could be transferred via the lateral extraction point 11 to a cyclone 101 for further separation. The AS lignin suspension was held in a storage container 104 and supplied by means of a pump 105 to the first (central) bore 3 of the dual fluid nozzle 2. The atomizing gas, in this case N.sub.2, was held in a tank 102 and supplied to the nozzle 2 by means of a compressor unit 103. The hot gas, in this case also N.sub.2, was heated to the desired temperature by means of a heater 106 and fed into the drying chamber 1 close to the nozzle 2.

[0061] In order to spray dry AS lignin, in the configuration shown here with a dual fluid nozzle, the following three parameters were varied:

1. The inlet temperature of the hot gas, which corresponded to the temperature of the hot gas (N.sub.2) at the dual fluid nozzle 2 because the hot gas was introduced in the immediate vicinity of the dual fluid nozzle 2.
2. The pressure of the atomizing gas at the second nozzle opening 31 of the dual fluid nozzle 2 (also referred to as the “atomization pressure”).
3. The flow rate at which the lignin-containing solution or suspension was supplied to the first nozzle opening 31 of the dual fluid nozzle 2.

[0062] An AS lignin suspension with a solid content of 5% to 20% by weight was used. The AS lignin was produced by thermal hydrolysis and subsequent enzymatic hydrolysis, in which liquid water at approximately 200° C. and under pressure was forced through straw and the suspension produced was supplemented with cellulases. The diameter of the nozzle opening 31 was 1-2 mm. In this configuration, the inlet temperature for the hot gas controlled the temperature in the drying chamber 1.

[0063] The formation of ultrafine lignin particles and of hollow ultrafine lignin particles was obtained at an inlet temperature for the hot gas of 150-175° C., 3-6 bar atomization pressure and a flow rate for the AS lignin suspension of 60-65 mL/min. Ultrafine particles with a size range of 3-15 μm could be formed using the method in accordance with the invention.

[0064] Surprisingly, tests have shown that spray dried lignin particles 6 produced in accordance with the invention exhibited a substantially smaller agglomeration compared with milled particles. The reason for this could be the disposition of hydrophobic lignin sites on the outer particle layer which prevents binding or interaction of particle surfaces with each other. These results are of great significance for applications in adhesive compounds for adhesive tapes, because the agglomeration can lead to dark spots in the adhesive tape, which is not desirable.

[0065] As an example, hollow particles can be used for the controlled release of drugs, chemical reagents and cosmetics from the interior of the hollow particles via the surface. For many pharmaceutical applications, the low density of the hollow lignin particles is advantageous.

[0066] Regarding the particle sizes obtained with the method in accordance with the invention, in the case of nozzle openings of 1.5-2 mm, a D50 value of <10 μm and a D10 value of <5 μm was obtained (see FIG. 3). It is of particular note that furthermore, a D90 value of <25 μm (see FIG. 3) could be obtained, which means that all the particles added to adhesive compounds have sizes below 30 The water content of the lignin powder obtained was 1.5% to 5% by weight.

Cytotoxicity

[0067] The viability of cells of a CaCo-2 cell culture which had been incubated at various concentrations with lignin from a variety of origins (particles in accordance with the invention (SDL) of the coarse (SDL coarse) and fine fractions (SDL fine), Organosolv lignin and alkali lignin) was measured (see FIG. 4). It can be seen that in the case of Organosolv and alkali lignin, a concentration of approximately 2.5 mg/mL was sufficient to kill 50% of the cells, while spray dried lignin required a concentration which was almost 10 times higher to obtain the same effect.

Anti-Diabetic Action

[0068] The activity of the enzymes α-glucosidase and α-amylase at different lignin concentrations was investigated. These enzymes are responsible for the degradation of complex high molecular weight carbohydrates and produce sugar monomers which are readily available for absorption in the human body. It was assumed that compounds which could inhibit the activity of these enzymes have an anti-diabetic action when consumed. FIG. 5 shows IC50 values (effective concentration for inhibition of the activity of the enzyme by 50%) for lignin particles produced in accordance with the invention (fine fraction and coarse fraction) compared with those for Organosolv and alkali lignin particles. In this case, lower values indicated a larger expected anti-diabetic action. Lignin particles produced in accordance with the invention exhibit a comparatively high anti-diabetic action.

Tabletting Lignin Particles

[0069] Lignin is a known alternative to activated carbon. The behaviour of the pharmaceutical forms, however, is strongly dependent on the particle properties and the compression behaviour. Thus, direct compression of the lignin particles obtained by the method in accordance with the invention was compared with known pharmaceutical excipients.

[0070] The tested formulations are shown in FIG. 9: alginate, starch, microcrystalline cellulose (MCC), lactose and a direct compression excipient (DCE) were used in various concentrations. The spray dried AS lignin in accordance with the invention was compared with a medical lignin (“softwood lignin”) which was used for the production of a lignin-based medical tablet.

[0071] FIG. 6 shows the hardness of the various compositions based on AS lignin given in FIG. 9. In general, the hardness of the tablets increased with the addition of excipients. Despite this, the desired hardness depends on the release behaviour of the tablet. According to the USP Pharmacopoeia, tablet dosage forms should have a friability (fracture behaviour) of less than 5%. In the case of the spray dried AS lignin in accordance with the invention, they all complied with this specification.

DPPH Measurement

[0072] The antioxidant potential of the spray dried lignin was measured using the DPPH radical method. DPPH (2,2-diphenyl-1-picrylhydrazyl) is violet in its active radical form and discolours to yellow when it is stabilized by radical scavengers/antioxidants. In order to compare the radical scavenging capacity of spray dried lignin in accordance with the invention, various lignins were used (see FIG. 7). In general, a high molecular weight lignin is linked to a low antioxidizing potential compared with other biorefining methods. The spray dried lignin in accordance with the invention, however, exhibited similar results to that of low molecular weight lignins, possibly because of a large contact surface area which is formed by employing this method.

Incorporation into Adhesive Compounds

[0073] Spray dried fine lignin produced in accordance with the invention was incorporated into adhesive compounds. The appearance of kneaded samples was evaluated. The samples contained lignin which had been obtained by a variety of methods, and lignin from different biomass sources. The adhesive films, which contained ultrafine spray dried lignin particles produced in accordance with the invention, exhibited the most similar behaviour to calcium carbonate, which is a frequently used standard filler.

Application of Spray Dried Lignin in Accordance with the Invention to Adhesive Films

[0074] Two concentrations of antioxidants (standard agents from a brand manufacturer of adhesive films and fine lignin particles produced in accordance with the invention) were tested at two different reaction times: 5 mg/mL and 10 mg/mL at 30 and 60 min. the results are shown in FIG. 8. It can be seen that lignin particles produced in accordance with the invention at both concentrations have a clear advantage over the antioxidants used by Tesa at least at these reaction times.

Production of AS Lignin-Based Microbeads (“Microbeads”)

[0075] Lignin-based microbeads (“microbeads”) in accordance with the invention were produced using lignin-containing particles produced using the method in accordance with the invention. To this end, stock solutions or stock suspensions were produced which contained AS lignin particles in combination with gel-forming biopolymers (alginate, cellulose, pectin, chitosan, starch, polylactide). The gel-forming biopolymers were mixed in water (deionized or distilled, occasionally heated), acidic or alkaline media to a proportion by weight of 1% by weight, 2% by weight, 3% by weight, 4% by weight and 5% by weight. The mixture of biopolymer solution and AS lignin particles could contain a proportion of 10-90% by weight of lignin. The mixture was carefully and thoroughly mixed until a homogeneous solution/suspension was obtained.

[0076] Microparticles were produced from the stock solutions/suspensions by means of a Type S jet cutter (geniaLab GmbH, Braunschweig, Germany). To this end, particles were produced from a stream of fluid which exited from a nozzle under pressure, wherein the full stream exiting the nozzle is cut by means of a rotating cutter tool produced from radially disposed cutting wires into identically shaped cylindrical segments. Because of surface tension, while they fall under gravity, the fluid segments form spherical droplets with a uniform size. The size of the droplets may, for example, be adjusted via the speed of rotation of the cutter tool, the diameter and the volumetric flow rate of the stream of liquid. The droplets produced in this manner fall into a crosslinking/curing solution. The nozzle was driven by compressed air (1-3 bar) which was adjusted by means of a pressure regulation valve or via pumps.

[0077] The production of microparticles (“microbeads”) was, for example, carried out using the following parameters: flow rate of stock solution/suspension in the range 0.5 to 10 g/s, nozzle diameter from 200 μm to 5 mm. The separating disks could, for example, contain 16 to 180 wires with a wire thickness of 100 μm to 500 μm.

[0078] The crosslinking solution could comprise calcium chloride, ethanol, acetic acid, aqueous acidic solutions or aqueous basic solutions, for example. The distance between the nozzle and the gelling bath was maintained in the range from approximately 50 to 100 cm. The volume in the crosslinking bath was at least four times the total volume of the processed biopolymer solution/suspension, in order to prevent agglomeration of the microbeads. After jet cutting was complete, the contents of the collecting baths were stirred until the particles had been removed. Gelled particles were separated from the gelling/crosslinking bath by screening and/or filtration. The separated microparticles could be dried at ambient temperature, oven dried or supercritically dried.