Cross Reference to Related Applications

This application claims priority from Finnish Application Serial No. 20041467, filed November 16, 2004.

BACKGROUND

Field of Invention

This invention relates to improved methods and devices for the fabrication of cellulose from wood and other plant materials. The method utilizes ultrasonic vibration to promote heterogeneous reactions for the solubilization of lignin.

Prior art and overall description

Paper is made from pulp that can be produced from several plants either by chemical, mechanical methods, or their combination.

Cellulose is renewable material, and its annual production in the nature might be more than the production of any other natural or man-made material. Especially wood, grass, hey, and straw contain significant amount of cellulose. One problem is that cellulose is intertwined with lignin that is an aromatic polyether. Lignin is insoluble and chemically very stable. It can be broken down at fairly high temperatures using strong nucleophiles, such as hydroxide, and sulfide ions. Two main methods for the fabrication of pulp are sulfate and sulfite methods. The sulphate method is currently favored, because almost any kind of wood can be used as raw material, including birch, eucalyptus, balsa, fir, and pine. Actual nucleophiles in these processes are hydroxide, and most importantly sulfide ions. The process is energy extensive, and the use of sulfide generates methyl mercaptan, and other smelly compounds. More details can be found in "Puumassan valmistus", Suomen Paperi-insinoόrien Yhdistyksen oppi- ja kasikirja II, Osa 1, Editor Nils-Erik Virkola, Abo 1983.

Cellulose fibers have rectangular cross-section. They have hollow interior that has also rectangular cross-section. The walls of cellulose fibers contain

also significant amount of lignin. The fibers are clued together by thin layer of lignin. The wetting starts by the filling the hollow spaces by cooking liquid. That process can be speeded up by hot steam. In the present method vacuum that is followed by steam is preferred.

Simplest way to produce pulp is grinding. The efficacy of grinding can be increased by steam, producing thermomechanical pulp. The quality and efficacy of mechanical pulp can be further increased by chemical means. Then the product is chemithermo-mechanical pulp.

Ultrasound is very powerful. Local heating can be about 5000 K, pressure about 1000 atm, and liquid jet streams can have velocities up to about 400 km/h. Ultrasound accelerates chemical reactions, enhances depolymerization by breaking chemical bonds, improves diffusion, and increases emulsification rates (Hielscher T., Ultrasonic production of nano- size dispersions and emulsions, ENS'05, Paris, France, 14-16 December 2005).

The methods and devices of the present invention reduce the energy requirement and speed up the fabrication of the cellulose and pulp.

FIGURE CAPTIONS

Fig. 1 The schematic depiction of one embodiment of the present invention. A tubular reactor 101 that has sonotrodes 103, 104, 105, and an external vibration source 102. B. Cross-section of the sonotrodes.

Fig. 2 The schematic depiction of another embodiment of the present invention. A tubular reactor that has vibrating plates, and an internal vibration source. A. Side view that shows one plate. B. Perpendicular side view, where all plates can be seen. C. Cross-section that depicts also the connection to power source.

Fig. 3 Schematics of one embodiment of the present invention that depicts the connection of the reactor to the chip and chemical feeding tower.

SUMMARY OF THE PRESENT INVENTION

The present invention provides methods and devices for the efficient, fast, and economical fabrication of cellulose from wood and other cellulose containing natural materials. The significant improvement as compared with the current technology is obtained by the use of ultrasonic vibration during the cooking of the wood chips.

It is a further aspect of the present invention to improve chemithermomechanical process for the production of pulp by the use of ultrasonic vibration during the grinding of wood.

In another embodiment of the present invention nanoparticles are added into the reaction mixture so that ultrasonic vibration will be more efficient for the detachment of cellulose fibers.

In still another aspect of the present invention wetting is facilitated with sequential use of partial vacuum and ultrasonic vibration.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Mixing of chemicals and reactions rely on diffusion on a molecular scale. Mechanical mixing is ineffective at very short distances because the molecules move together about the same speed. The ultrasonic vibration agitates molecules and particles by accelerating them back and forth in a rapid succession. Thus, the diffusion gradients of the reagents are diminished. This greatly increases the rate of the diffusion dependent reactions. The separation of large molecules and nanoparticles benefits even more from the ultrasonic agitation. The particle that is partly or totally detached from the aggregate will have different acceleration in the same oscillating medium than the remaining aggregate. Thus, they are pulled apart. If the process relays on the diffusion alone, it would take much longer time.

Accordingly, when wood chips are cooked in a chemical mixture that is able to degrade lignin, but leave cellulose virtually intact, the degradation reaction is faster in the presence of ultrasonic vibration. Equally importantly, the detachment of cellulose fibers is faster than without ultrasonic vibration. The fast removal of the cellulose fibers will expose new lignin surface that will be fast degraded under these conditions.

The shorter time will allow the performance of the whole cooking process in tubular reactors. The chip slurry is pumped into one end, and the pulp will come out from the other. The whole tubular reactor may be vibrated with an ultrasonic power source. Alternatively, the vibration is mediated into the tube by rods or plates that have either external or internal ultrasonic vibrators.

The process can be further accelerated by the addition of nanoparticles that are locally accelerated by ultrasonic vibration. When these accelerated particles hit the solid wood surface they transfer their kinetic energy to the cellulose and lignin. This will further promote the chemical reaction and separation of these components. Nanoparticles must be tolerant against the harsh chemical milieu of the cooking process. For example, hydoxyapatite nanoparticles tolerate basic conditions. Hydroxyl group can be substituted by fluorine, and during cooking the sulfide will at least partially replace hydroxyl on the surface of the particle. Thus, these particles will also induce chemical transformation of the lignin in addition to the physical effect. Earth alkali sulfates are suitable in the sulfate process, because the excess of the sulfate ions prevents the dissolution of these particles. Some carbonates, such as calcium or barium carbonates, may be used especially, if the solution contains soluble carbonate, such as sodium carbonate.

Wetting is important part of any heterogeneous chemical reaction. It is advantageous to wet the whole wood mass before or at the beginning of the cooking process. Removal of the air from the cellulose fibers is important for the wetting. In one implementation of the current process the air is first largely removed by vacuum. Steam and hot cooking liquid are let in so that they will fill the cavities inside the cellulose fibers. This mixture is then added into the reactor. Alternatively, chips are treated with steam, and then they are added into the cooking liquid. In Fig. 3 is depicted a device, in which the chips 107 are compressed in portions inside a tubular chamber 310 with a piston 312. The perpendicular compressing piston 311 closes the opening in the side- wall. The piston inside the tubular chamber 310 is pulled outward so that a partial vacuum is created. Overheated steam is let in so that the cavities inside the wood chips are filled with steam and condensed water. Cooking liquid is added and the piston 312 is used to compress the mixture so that the cavities are further filled with the cooking liquid. The mixture is added into the reactor tower 301, 306. Several wetting units may operate so that the supply of the chips is virtually continuous.

Ultrasonic vibration will also help during wetting process. Even, if some air is left in the cavities, it is broken into smaller bubbles that are more easily replaced with water. The same kind of ultrasonic devices and methods that are applicable in the cooking can be used for the wetting.

Ultrasonic vibrators are made of piezoelectric materials. The most commonly used material is lead zirconium titanate. The piezoelectric material is sandwiched between two electrodes. The frequency, and amplitude of the ultrasonic vibration can be adjusted the electric potential and frequency of the AC field between the electrodes. The amplitude of the vibration is limited also by the thickness of the piezoelectric layer. The frequencies vary typically between 10 kHz and 1 Mhz, although frequencies outside these ranges can be used. Currently, the frequencies between 20 and 30 kHz are preferred. The vibration amplitude is preferably between 5 and 200 μm, most advantageously between 20 and 120 μm.

The power of one sonotrode can be between 0.1 and 50 kW, most advantageously between 1 and 20 kW. When multiple sonotrodes are used, the power may vary with time so that interference pattern will change all the time. Thus, the whole reaction mixture can be more evenly agitated. Ultrasonic processes can be easily scaled up and optimized (Hielscher T., Ultrasonic production of nano-size dispersions and emulsions, ENS '05, Paris, France, 14-16 December 2005).

The reactor tube of this invention may have circular, elliptic, rectangular, or some other practical cross-section. The diameter of the reactor may be variable, and also it may be divided into smaller compartments that will combine again. The purpose is to get the maximum power into the reaction mixture. The reactor walls or some plate inside the reactor may vibrate. Currently, a vibrating sonotrode inside the reaction mixture is preferred. The sonotrode may be made of stainless steel, titanium or some other metal. In order to obtain chemical resistance the sonotrode may be coated with chemically more resistant material, for example with platinum. The main orientation may horizontal, vertical, or some intermediate orientation. The objective is to keep the chips evenly distributed. This is often achieved, when the tubular reactor is vertical, and the raw material is added from the bottom.

The reactor may contain a mechanical mixer. The mixer may have blades, propel, or screw. The motor is advantageously outside the reactor and on the top, and the shaft comes into the reactor.

The mixture may be heated to a temperature of 100 - 180 ⁰ C before it enters the reactor. Thus, no heating is mandatory in the reactor. The reactor should only be thermally isolated so that the temperature does not change much, while the reaction mixture transverses through the reactor. The ultrasonic vibrator generates heat, and the temperature can actually increase slightly in the reactor. External or internal heating is also possible in the reactor. The heating can be accomplished by the methods that are well known in the art. Also electromagnetic radiation can be used, especially IR-, or microwave radiation. The wavelength is preferably chosen so that cellulose, lignin, or both absorb it effectively. This radiation may be advantageously be directed into the reaction chamber.

Although the continuous reactor (Fig. 3) is currently preferred, it is obvious that the ultrasonic method of this invention is applicable also in bath type reactors. Even, when the temperature is 100 ⁰ C, it is advantageous to apply increased pressure. The ultrasonic vibration is more efficient at higher pressures. The pressure can be increased in continuous reactors by hydrostatic pressure. About 10 m high column of water based cooking liquid in a tube or pipe corresponds to one bar (megapascal) of pressure. The pressure can between 1 and 50 bar and most advantageously between 2 and 5 bars. Another method to increase the pressure is to pump the chip slurry into the reactor tube and constrict the outlet of the pulp from the other end. This methods avoids the high tower-like reactors.

The present invention is applicable also for the fabrication of the mechanical pulp. Especially, chemithermomechanical process will benefit from the ultrasonic vibration.

EXPERIMENTAL DETAILS

While this invention has been described in detail with reference to certain examples and illustrations of the invention, it should be appreciated that the present invention is not limited to the precise examples. Rather, in view of the present disclosure, many modifications and variations would present themselves to those skilled in the art without departing from the scope and

spirit of this invention. The examples provided are set forth to aid in an understanding of the invention but are not intended to, and should not be construed to limit in any way the present invention.

Example 1.

In Fig. 2 is depicted a rectangular reactor that contains four vibrating plates 203. Plates have bent so that their cross-section is U-shaped. The ultrasonic vibrators 204 are inside the plates in this embodiment. Trie density of the vibrators is 25 per square meter, and the power about 500 W per each vibrator. The location of this reactor relative to the feeding tower 301 and heating elements is depicted in Fig. 3. The tower is a very long tube, about 40 m in this specific embodiment. The long tower allows continuous open feed of the raw materials 107. The heating of the feed is performed by heat exchange from the processed pulp to the beginning of the reactor and the lower end of the feeding tower. Additional heating is provided by heating coils 302. The temperature of the mixture is about 150 ⁰ C, and the pressure about 5 bar in the reactor. The product goes up about 40 m, where it comes out of an open tube 303.

Example 2.

In Fig. 1 is depicted a tubular reactor that contains three 1 kW sonotrods 103, one 2 kW sonotrod 104, one 4 kW sonotrodl05, and one 8 kW sonotrod 106 in series. In this specific example four different types of sonotrods are used. The location of this reactor relative to the feeding tower 301 and heating elements 302 is analogous to the location of the plate vibrator 202 and 203 (Fig. 2) in Fig. 3. The tower 301 is a long tube, about 20 m in this specific embodiment. The long tower allows continuous open feed of the raw materials. The wood chips 107 are pretreated in a bath mode reactor 310 immediately before the addition into the enlargened part 306 of the reactor 301 by a piston 311. First, the air is removed from the chips by suction in partial vacuum that is created by pulling the piston 311 away from the . Hot steam is let in, and immediately after that hot cooking liquid is added. The chip slurry is moved downward with a screw 305 that also takes care of mixing. The heating of the feed is performed by heat exchange from the processed pulp to the beginning of the reactor and the lower end of the feeding tower (not shown in the figure). Additional heating is provided by heating coils 302. The temperature of the mixture is about 120 ⁰ C, and the

pressure about 2 bar in the reactor. The product goes up about 15 m, where it comes out of an open tube 303.

Additional modifications and advantages will readily occur to those skilled in the art. Therefore the invention in its broader aspects is not limited to the specific details, and representative materials and devices shown and described. Accordingly, various modifications may be made without departing from the spirit and scope of the general inventive concept as described in the disclosure and defined by the claims and their equivalents.