EP0054015B1 - Process and apparatus for producing cellulose - Google Patents

Description

The invention relates to a process for the production of cellulose from cellulose-containing starting materials by digestion under elevated temperature and pressure. The starting materials can be e.g. Wood, straw, grass or waste. Another object of the invention is a device for performing this method, in which two pistons located opposite one another are arranged in a press jacket.

Impregnation of the pre-shredded raw materials, e.g. Wood chips are made in a conventional way by mixing the chips with a basic or acidic digestion liquid and slowly bringing them to a temperature of 80 ° to 150 ° C in large pressure vessels. The slow heating to a residence time of 0.5 to 2 hours at this temperature and the resulting pressure of up to 10 bar are necessary to ensure sufficient penetration (impregnation) of the wood chips with digestion solution. Sulfate or sulfite compounds with a chemical concentration of approximately 5 to 10% are currently preferably used as the digestion solution. The ratio of the mass of the digested liquid taken up to the mass of the dry wood chips in the cooker is referred to as hydromodule H and is about 3 to 5 in the current cooking processes. The impregnation can also be carried out outside the cooker in a separate apparatus.

DE-B-2 448 547 relates to a process for improving the heat economy when cooking lignocellulosic material in batch cookers. This process actually concerns pretreatment, i.e. impregnation of wood chips in the cooker with digestion liquid using a vapor pressure or gas pressure of 1 to 5 bar, preferably 1.5 to 8 bar for a duration of 3 to 70 min, if necessary with heating to a temperature of 50 to 165 ° C, with subsequent measurement and adjustment of the liquid level in the cooker to the minimum amount of digestion liquid required for the subsequent cooking process. The heating takes place either indirectly by means of a heat exchanger outside the cooker, or directly by introducing steam into the cooker. This process works in the liquid phase. The pressure does not vent the pore volume, but rather compresses the air in the pores so that the impregnation is incomplete. A hydromodule of 0.5 to 2 cannot be achieved with this method.

US-A-32 15588 relates to a process for producing pulp from wood chips in a continuously operating cooker. Here already impregnated wood chips are introduced into the upper part of the stove via a sluice, where they are subjected to high-pressure steam digestion at a temperature of 180 to 190 ° C over a period of 5 to 25 minutes. The lower part of the cooker is filled approximately to the middle with digestion liquid, which enters at a temperature of 70 to 120 ° C in the lower quarter of the cooker and is partially left out at the bottom or partly in the middle of the cooker. After the digestion phase, the continuously moving wood chips dip into the digestion liquid and are cooled there, where they shrink and release the highly concentrated digestion liquid with which they have been impregnated, after which they remain for 5 to 30 minutes via a lock at the bottom of the Kochers are carried out. Qualitative information is given about the type of impregnation and ventilation of the wood chips. Various types of impregnation should result in an essentially uniform distribution of the digestion liquid within each chip, at temperatures of 100 to 150 ° C. Venting is preferably carried out by steam treatment before impregnation, whereupon the impregnation takes place under relatively high hydrostatic pressure without air being able to enter. According to another method, the wood chips are first treated with steam at atmospheric pressure, whereupon they are exposed to a steam with excess pressure and then the impregnation with digestion liquid takes place by rapidly reducing the excess pressure without air being added. The unnecessary digestion liquid is drained off in order to separate the impregnated wood chips from the digestion liquid. However, by letting the digestion liquid run off, a hydromodule as low as can be achieved according to the invention by mechanical pressing can never be achieved. The 2: 1 ratio of digestion liquid to wood given in Example 3 of this US patent specification (it is not stated whether mass or volume ratio is meant) after the impregnation is therefore a question. By draining the digestion liquid, practically only a hydromodule> 2.5 can be achieved.

The steps washing, sorting, bleaching and drying are usually connected to the steps of impregnation, pressing and digestion. Despite strong efforts in the environmental sector, modern pulp plants are still characterized by high energy and water consumption. The high energy consumption essentially results from the heating of large quantities of liquid, especially in the case of digestion and bleaching, the long residence times, and from the poor efficiency of the heat recovery systems.

When using the organic substances contained in the waste liquor to cover the energy requirements of a pulp mill, the thin waste liquor caused by the given processes must be thickened to a dry matter content of 55 to 65% for the subsequent combustion, which in turn requires a considerable expenditure of energy. The energy expenditure A pulp mill results approximately in equal parts from pulping, bleaching, lye evaporation and cellulose drying. After all, in the most modern factories, burning the thickened waste liquor can currently achieve amounts of energy that make a pulp mill almost self-sufficient in terms of energy.

The invention has for its object to provide a method which is characterized by minimal energy, water and chemical consumption. The methods used can be seen as the maximum possible internal measures of a pulp mill to solve their environmental problems, in addition to the energy saving primarily of the wastewater problems. Since wastewater and pulp mills are currently the most difficult to treat, minimizing the amount of wastewater is essential. Furthermore, the method according to the invention is intended to make it possible to reduce the volume of the apparatus, so that even smaller systems which can be adapted to the amount of wood involved can be operated in an economical manner.

An additional object of the invention is to increase the quality of the pulp produced.

In the method mentioned at the outset, this is achieved according to the invention in that, by pressing impregnated starting materials, a mass ratio of the absorbed digestion liquid to the dry starting materials is set to 0.5 to 2, and that the digestion in a press device at 160 ° to 300 ° C. a time of less than 10 minutes is carried out, the heating being carried out by the direct introduction of heat. For example, a hydraulic module of 1 can be selected. The direct introduction of heat can preferably by electrical heating of the impregnated starting materials at a digestion temperature of z. B. 180 ° to 220 ° C, the digestion can take place for example in 10 to 60 s. Electrical heating is advantageously carried out by resistance heating.

The main advantages of the process according to the invention are: short digestion times, reduced energy consumption, reduced chemical consumption, small amounts of liquid, and, as tests have shown, improved pulp quality and increased wood utilization, which means that the otherwise usual grinding process, which requires additional energy expenditure, may be dispensed with can. The improved pulp quality results primarily from the favorable location of the residual lignins and hemicelluloses, which results in a particular ability to bind and bleach.

Due to the short reaction times in general and the rapid separation of the fibers from one another at the time of lignin softening by the shear forces mechanically applied by the pressing device (breakdown of the structural resistance) and the possibility of rapid interruption of reactions, e.g. by interrupting the energy supply, the cellulose chains (important for the basic strength of the fibers) and the hemicelluloses (important for the binding ability in the finished paper sheet) are largely preserved. In this way there is a deliberate accumulation or concentration of the hemicelluloses on the fiber surface.

In the method according to the invention, the impregnation in the same press device with impregnation liquid is advantageously carried out in chemical concentrations necessary for the digestion of preferably 5 to 50%, for example 10 to 20%, and at constant pressure or in the form of pressure surges, preferably above 1 bar, for example 10 to 50 bar and 2 to 10 pressure surges, to reduce the unwetted capillary length of the starting material, the impregnation time being kept short by the applied pressure, preferably less than 10 min, expediently less than 1 min, so that it is cold and without prior venting can be impregnated. This impregnation process can also be used advantageously in the case of mixtures of dry and wet starting materials. It is essential that when impregnating the entire inner and outer surface of the wood chips is adequately and evenly wetted with less concentrated digestion solutions than the pore volume of the wood chips. This is achieved by initially filling the entire pore volume of the wood chips as in conventional plants, but with more concentrated digestion solutions, by applying pressure in the press device mentioned (definition of the impregnation).

However, since for filling the pore volume and for wetting the surfaces, e.g. B. from 1 kg of spruce wood chips about 2.5 I, corresponds to about 3 kg, digestion solution is necessary, an adjustment of the low hydro module used according to the invention must be made. According to the invention, the impregnated starting materials are pressed in order to adjust the hydromodule H in the same press device mentioned.

However, the impregnation itself can optionally be carried out by conventional methods in a separate plant.

In extreme cases (hydromodule H = 0.5 corresponding lye concentration 61%), the evaporation system can be omitted or the lye can be burned directly. There are known chemical cycle closing process, come in de- _'nen spent liquors having a solids content of 35% in boilers for combustion, but the use of energy is correspondingly lower.

The measure of lowering the hydromodule H in the same way also reduces the energy consumption when digesting and evaporating the waste liquor. In the process according to the invention, in comparison to conventional processes, corresponding to the reduced hydromodule H, with more concentrated digestion solutions and a smaller amount per Batch worked, which means that a smaller amount of concentrated fresh or waste liquor has to be processed, so that the total number of auxiliary systems is reduced in number and extent, and the theoretical chemical consumption per kg of pulp is reduced to around half of the usual consumption due to the increased concentration can.

The pressure when adjusting the hydromodule H can be lower if more time is available or if the temperature of the wood chips is increased so that they become more elastic and the structural resistance decreases. Temperatures up to 130 ° C have proven to be advantageous, at which no appreciable digestion reactions take place and a hydromodule H of approx. 1 is reached in approx. 30 s with mechanical pressures around 50 bar. A filling density of z. B. 160 to 220 kg / m ³ for softwood stands at a mechanical pressure of, for example, 50 bar and a temperature of 130 ° C after the adjustment of the hydromodule, an increased fill density of 800 kg / m ³ . With this measure, the volume of the pressing device can be reduced to a third to a fifth of the previous volume of the pressure vessels. The hydromodule changes continuously during the process and can be determined at the end of the process as a function of the reaction parameters.

The hydromodule setting outside of the pressing device is also possible, but it is complicated because the hydromodule has to be determined in a separate process step. A further embodiment of the method according to the invention provides that, in the case of wet starting materials, the moisture (water content) is brought to a value of below 50%, for example 20 to 40%, preferably 30%, by pressing off the wet starting materials in the same pressing device becomes, whereby the majority of the capillaries of the starting material for the impregnating liquid is removable.

The direct introduction of heat into the impregnated starting materials can also take place, for example, by capacitive or inductive high-frequency heating or by microwave heating.

According to the invention, the digestion can also take place as a continuous process step. The invention is basically suitable for all digestion processes (acidic to alkaline). A pressing device according to the invention for carrying out the method is characterized in that two pistons lying opposite one another are arranged in a pressing jacket, between which the compact is located as an electrical resistance load, both the pistons relative to one another and the pressing jacket being relatively movable relative to the pistons.

Devices for carrying out the new method are described below.

1 to 5 each show different exemplary embodiments of discontinuously operating pressing devices according to the invention in cross section, FIG. 6 shows the schematic structure of a continuously working screw press in cross section according to the invention and FIG. 7 shows a largely simplified vertical section through a further embodiment of the pressing device .

In Fig. 1 a pressing device of a basic type is shown. A cylindrically shaped press jacket 1 is lined with an electrical insulation 2, which also extends over the lower edge of the press jacket 1 and thus also insulates the bottom cover 3. A piston 4 is pressurized in the press jacket 1, e.g. hydraulically moved up and down and is provided with a seal 5. In the lower part of the press jacket 1 there is a drain opening 6 for the waste liquor, which continues in a pipeline with a valve 7. The bottom cover 3 and the piston 4 are provided with electrically conductive contacts which are connected to the poles of a power source. AC sources are preferably used. In this arrangement, the mechanical pressure of the piston is superimposed on the vapor pressure which arises during the development of heat when the current passes through the compact 8.

Fig. 2 shows a second embodiment, similar to that of FIG. 1, but with the piston 4a is offset, and the seal 5a is laid in the upper part of the press casing 1a. A measuring opening 9 for the connection of a steam pressure meter 10 is laid in the upper third of the press casing 1a. The gap formed between the piston 4a and the press jacket 1a enables the steam produced to escape upwards, as a result of which steam pressure and mechanically applied pressure can be metered separately and also measured. Power is supplied as shown in Fig. 1.

Fig. 3 shows a third embodiment in which the electrical insulation can be made simpler. A central opening is provided in the floor cover 3a, in which an electrode 11 projects into the press jacket 1. Insulation 2a insulates the bottom cover 3a from the press jacket 1 and the electrode 11. A cover 12 closes the press jacket 1 at the top. In the cover 12, a linkage 13 leads via the seals 14 into the interior of the press casing and is connected to a piston ring 4b. The mechanical pressure is transmitted via the linkage 13 to the piston ring 4b, and from there to the compact 8. Also in this example, a gap is provided between the piston ring 4b and the press jacket 1, and between the electrode 11 and the piston ring 4b, so that A vapor space is created above the compact 8.

FIG. 4 shows a fourth exemplary embodiment in which the electrode 11a has been enlarged in diameter and at the same time functions as a piston for the mechanical pressure. This version proves to be advantageous due to the omission of seals; only the insulation 2a assumes a sealing function at the same time.

According to the invention, the compact is heated under mechanical pressure by heat, preferably electrical heat, i.e. H. the electrical current is conducted through the compact. The electrodes, as already described in the examples, are the pistons and the press jacket and their possible combinations, the electrical insulation being attached at those points in the area of the press chamber which would otherwise bridge the current flow through the compact. This measure is also used for heating by inductive or capacitive HF heating, or microwave heating, and is a problem that can be solved by a person skilled in the art when specifying this criterion.

5 shows a pressing device according to the invention as an exemplary embodiment in cross section. In FIG. 5, 1 b denotes a hydraulically liftable and lowerable press jacket, in which a movable, likewise hydraulically liftable and lowerable upper piston 4 and a lower piston 4 ′ rigidly connected to a base plate 3 b are arranged. The cloth cake or pellet 8 is located between the two pistons 4 and 4 ', which are each equipped with seals 5 b. The two pistons are electrically connected to the poles of a power source via contacts. Insulation 2b is provided in the press jacket 1b in order to prevent current flow through the press jacket. The press jacket 1 b is attached to the base plate 3 b by auxiliary devices, e.g. Hydraulic or pneumatic cylinder 15 raised and lowered, whereby an easy discharge of the compact is accomplished or the introduction of the wood chips is made possible via a funnel 16 arranged around the upper piston 2, which is carried by a rod 17 mounted on the press jacket 1 b. The auxiliary device can also be of a mechanical type, e.g. B. be a spindle drive. The upper piston 4 is provided with a bore 18, which serves to supply liquid or gaseous treatment media, and ends in many small bores on the end face of the piston 4, as a result of which the liquid can be fed to the compact 8. Likewise, a bore 19 is provided in the lower piston 4, which is used to discharge the treatment media from the press chamber and opens into many small bores on the end face of the piston 4 '. Treatment media can also be fed in and out via bores in the press jacket 1 b (not shown).

The treatment media can also be distributed within the press room using sieve or perforated plates, etc. (not shown).

Furthermore, an insulation 20 is provided between each cylinder 15 and a flange 21 rigidly arranged on the press jacket 1 b, so that the entire current flow is conducted over the compact 8 and heats it directly. In this example, resistance heating is used, but, as mentioned, high-frequency heating of an inductive or capacitive type and microwave heating can also be used for the press jacket 1 b and the two pistons 4 and 4 'with a suitable choice of material. The press jacket 1 b can, for. B. be made of high-strength ceramic insulating material.

Numerous variants in terms of structural design can be derived from the exemplary embodiment shown. So would be B. also a movable lower piston 4 'executable to expose the compact to a pressure on both sides, which would also make it easier to remove the compact from the pressing space by simultaneously lowering the pistons 4 and 4'.

In a further embodiment of the invention, it would be possible to arrange a plurality of pressing devices according to the invention next to one another or in a circular form, with either the same working process taking place in each of the pressing devices or the processes in the sequence impregnation in the first press, disintegration in the second press ... .etc. expire.

For the continuous production of pulp by means of the method according to the invention, a screw press 22 according to FIG. 6 is proposed, in which the impregnated wood chips are broken down optimally in a continuous form.

A feed screw 23 conveys the pressed material (pre-impregnated wood chips) into the cooking area, which is formed between a screw electrode 24 as the first electrode and the cooker housing 25 as the second electrode.

The lye escaping from the compression of the wood chips on a particular hydromodule can escape from the screw press 22 via a sieve 26 and is returned to a separate impregnation plant (not shown). The feed screw 23 is connected to the screw electrode 24 via an insulating coupling 27. The cooker housing 25 and the screw electrode 24 are connected to a power source via contacts. The current flows via the screw electrode 24 to the cooker housing 25 and heats the wood chip mass lying in between. If, for example, the consistency (solid concentration) of the wood chips mass can be assumed to be 50% in the inlet with a hydromodule of 1, this consistency drops to around 30% due to the increasing dissolution of the lignin against the outlet. The pulp is again concentrated to about 55% in the discharge screw 28, as a result of which a large part of the primary liquor is separated off in a hot state (low viscosity) at a concentration of about 60% via a sieve 29.

The dwell time in the cooking area and the mechanical pressure on the wood chips can be set as desired by varying the speed between the feed and discharge screws.

7 consists essentially of the press jacket 1, the lower piston 4 'and the upper piston 4. Der Press jacket 1 is provided with three or more arms 21, each of which rests on a working cylinder 15. The rod 36 of each piston 37 which is displaceable in the cylinder 15 protrudes downward from the cylinder 15 and is anchored to the foundation 3b. The two connections of the double-acting cylinders are labeled 49 and 30. The press jacket 1 can be raised and lowered by appropriately loading the cylinder 15 with pressure oil or the like pressure medium. In its highest position, the press jacket 1 is at a sufficient distance from the piston 4 ', so that the previously pressed material can be removed from the press jacket 1 laterally between the cylinders 15. The cylinders 15 stand on a circle concentric to the press jacket 1 and have a lateral distance from one another which is greater than the inside diameter of the press jacket 1.

In the embodiment shown, the lower piston 4 'is attached to the foundation 3 b. It is penetrated in the axial direction by a tube 31 which is connected to a steam line 34 via a valve 32 and a valve 33. An additional valve 63 may be present. The tube 31 opens at the piston head 35. A filter plate or perforated plate 46 is held at a distance from the piston head 35 and has a multiplicity of small bores 47. These holes can be tapered towards the top to avoid clogging and to facilitate cleaning. The sieve plate can be supported on the piston head by strips or similar stiffeners (not shown). Between the perforated plate 46 and the piston head 35, a distribution and collection space 48 is created. Between the piston 4 'and the valve 32, a branch 59 with a valve 40 is provided on the pipe 31, which leads to a liquid supply and discharge pipe 41.

To seal the piston 4 'with respect to the press jacket 1, an annular seal 5b is provided, which is inserted into a circumferential groove near the piston head 35 or into the press jacket 1 near its end.

The upper piston 4 can be moved up and down by a hydraulic or mechanical device, not shown. It is penetrated by an axial line 43, which opens at the piston crown 44 and with its other end in a manner not shown further out of the piston and z. B. is connected via a hose 45 and a valve 42 to a liquid inlet and outlet 57. The upper piston 4, like the lower one, is provided with a filter plate 46 with optionally conical bores 47, so that a distributor and collecting space 48 is also formed on the upper piston 4 between the piston crown 44 and the filter plate. A circumferential groove provided in a piston crown 44 or in the press jacket 1 near its end has an annular seal 5b effecting the seal between the press jacket 1 and the upper piston 4.

In its upper part, the press jacket 1 is provided with a number of radial passages 52, which are closed at their inner mouth with sieve plates 38. In order to avoid damage to the ring seal 5 b by the sieve plate 38, the sieve plates are set back somewhat in relation to the inner wall of the press jacket 1. On the outside of the press casing 1, a ring line 51 is provided, into which all radial passages 52 open.

A circulation line 53 extends from the ring line 51 via a valve 62 and contains a flexible or length-variable connection (here a telescopic tube 54) and a blower 55. The other end of the circulation line 53 is connected via the valve 32 to the pipe 31 leading into the lower piston 4 '.

The upper end face of the press casing 1 can carry a filling funnel 56 which concentrically surrounds the upper piston 4. Supports 67 serve to hold the funnel. The upper piston 4 can be lifted out of the press jacket 1, so that the press jacket 1 can be filled with solid material through the annular gap between the upper piston 4 and the press jacket edge.

The mode of operation of the press according to the invention is described below in connection with the production of cellulose from wood chips.

For filling the press, the upper piston 4 is moved into its highest position _'and the press belt 1 is brought by appropriately energizing the operating cylinder 15 to its lowermost position. This creates a sufficiently large space between the perforated plates 46 of the piston 4 and the upper edge of the press casing 1, through which wood chips, which could have already been introduced into the hopper 56, can fall into the interior of the press cylinder. After the desired filling quantity has been reached (if desired, with intermediate compression by the piston 4), the upper piston 4 is brought into the position shown, ie it penetrates into the press jacket 1 to such an extent that the ring seal 5 b effects the sealing of the press jacket 1, which radial passages 52 but remain free.

By opening the valves 32 and 33, but with the valves 40 and 62 and possibly 63 closed, steam can be brought into the interior of the press casing 1 in order to pre-dampen the wood chips.

If the wood chips filled in the press are very moist, they can be pressed to a desired degree of dryness. After opening the valve 40 and closing all other valves, the upper piston 4 can be lowered. Due to the resulting high pressure (e.g. 100 to 200 bar), the liquid squeezed out of them penetrates through the bore 47 in the lower perforated plate 46 and flows out via the line 41.

After pretreatment, the valves are closed again. Via the line 41, which has meanwhile been connected to storage containers, 40 different impregnation liquids can and can be added to the press jacket 1 after opening the valve

brought to the wood chips. By using hot impregnation liquids or by subsequent heating with steam through line 34 via valves 33 and 32, a high percentage of the air contained in the pores of the wood chips can be expelled via open valve 42 and line 57.

If a further removal of the air from the wood chips is required, then steam of more than 100 ° temperature is introduced into the interior of the press via the valves 32, 33 (all other valves closed). This heats the impregnation liquid (also in the wood chips) above its boiling point. After a short time, the valves 32, 33 are closed and by opening the valve 46 (the line 57 can now lead to the outside, for example), the excess pressure is released and the impregnation liquid evaporates. This evaporation also takes place inside the wood chips, the air contained therein being displaced by steam. This procedure can be repeated several times; it is possible to completely remove the air from the wood chips, which corresponds to a «perfect impregnation».

On the other hand, the high pressure that can be achieved with the press (e.g. 100 to 200 bar) can be used for the impregnation by simply pressing the upper piston 4 (all valves closed) mechanically into the pores of the wood (the wood chips) ) is pressed.

However, it goes without saying that this purely mechanical penetration impregnation of the wood chips can be combined with one or with both of the above-described types of operation, since if the air is not sufficiently removed from the wood chips, these expand again when the pressure is removed or reduced and the impregnation liquid, can at least partially displace.

After the desired degree of impregnation, if necessary perfect impregnation, can be pressed onto any hydromodule. In general, a hydromodule of about 0.5 to 2, but preferably about 1, is pressed, i. H. the ratio between the mass of the dry wood chips and the mass of the impregnation liquid they absorb is then approximately equal to one. The pressing takes place by lowering the upper piston 4, the resulting, pressed liquid, as already mentioned, being able to flow out through the valve 40 into the line 41.

To achieve the final digestion of the wood chips, the upper piston 4 is raised again to the position shown in the drawing. The press jacket 1 can e.g. about half filled with already pressed wood chips, above which there is a steam-air mixture. By opening the valves 32, 33, steam is again blown into the interior of the press. The temperature of this steam is, for example, 10 to 50 ° above the desired final digestion temperature, which can be approximately 170 ° to 220 °. The pressure of the steam preferably corresponds to the equilibrium pressure at the selected maximum digestion temperature. So it is superheated steam; when the digestion temperature is reached, the steam supply stops automatically.

The steam supplied condenses on the relatively cold wood chips and heats them both by the heat of condensation and by conduction.

The excess steam can be circulated by opening the valve 62 and starting the blower 55: sieves 38, passages 52, ring line 51, valve 62, circulation line 53, valves 63 and 32, pipe 31. By appropriately dimensioning the bores 47 in the Sieve plate 46 of the lower piston 4 'and by means of a correspondingly high speed of the steam or steam-air mixture, a swirling or intensive mixing and flow around the impregnated wood chip particles - now to be referred to as "pulping material" - is achieved. This swirling of the material does not have to correspond to the ideal fluidized bed process, but only turbulent, so that the heat transfer takes place quickly and homogeneously from a static point of view. With a low filling quantity, the vortex will be strong, with a larger filling quantity it may be slight or only a certain movement.

As soon as the desired maximum digestion temperature has been reached, the circulation of the steam or steam-air mixture is interrupted. The digestion material remains at the maximum temperature for a short time - for example about 10 seconds to about six minutes. Then, after opening the valves 40 and 42 and closing all the other valves, the upper piston 4 is lowered, the digestion material is pressed off by exerting the highest possible pressure. The original liquor that flows off flows through the correspondingly switched lines 41 and 57. The more this pressing takes place, the lower the water consumption in the subsequent washing processes.

At this stage, the pressure is generally not abruptly reduced, since the strong steam development which occurs when the entire digestion mass is suddenly released can, under certain circumstances, lead to fiber damage. However, it may sometimes be desirable to further concentrate the waste liquor.

The pressing takes place approximately on hydromodule 1, if possible lower than 1, based on the cellulose produced. Since approximately 50% yield is achieved, half of the wood is dissolved, half of the original impregnation liquid is pressed off and the press in this example is therefore a quarter full. The shear forces applied at the digestion temperature support the rapid separation of the individual fibers.

In continuation of the treatment described so far, the upper piston can be shown in the te position are raised. Any amount of washing or bleaching liquid can now be introduced into the press jacket via the valve 42 and or the valve 40, so that, for example, liquid is above the compact digestion mass and displacement washing can be carried out. For this purpose, the liquid is pressed through the digestion material by the action of, for example, the upper piston 4, the primary liquor being replaced by the fresh liquid. The so-called dilution wash can also be carried out by z. B. the piston 4 is lowered from its drawn position until it covers the sieves 38, the valve 42 is opened and air is introduced intermittently through the line 34 via the valves 32, 33, which after the penetration of the digestion mass and the liquid and thus their mixture can escape through line 57. It can then be pressed repeatedly.

In order to dispense the extruded digestion mass, the piston 4 is lowered to the digestion mass with the valve 40 and / or the valve 42 open, without exerting any particular pressure. The press jacket is then brought into its uppermost position with the aid of the working cylinders and the digestion mass is discharged laterally through the space between the lower piston 4 '(its screen plate 46) and the lower edge of the press jacket 1.

Various modifications of the described embodiments are possible without leaving the scope of the invention. Thus, the upper piston 4 can also be provided with an axial tube corresponding to the tube 31 in the lower piston 4 ', which, via a valve in the valve 62, merges directly into the circulation line 53. The passages 52 together with sieves 38 and ring line 51 are then omitted. Furthermore, the lower piston 4 'can also be vertically movable, the press jacket 1 being held stationary. The operation of the press takes place as described above, i. H. the upper piston makes the working movement, the lower piston remains at rest. To discharge the digestion mass, both pistons, holding the digestion mass between them, are moved downwards until the upper piston with its perforated plate is flush with the lower edge of the press casing 1. The blower 55 can be reversible in its direction of action, i.e. after switching, it is able to blow air from outside through the valve 62 and the ring line 51 through the sieves 38 or through the axial tube of the modified embodiment and thus through the bores 47 in order to clean them. In the area of the sieves 38, the inside diameter of the press casing 1 can be somewhat enlarged in the case of a continuous transition in order to allow the ring seal 5b to slide more easily over the sieve region. Furthermore, a cyclone 39 or the like can be provided in the direction of circulation in order to remove any entrained material from the steam or steam-air mixture flowing in the circulation pipe 53. A heat exchanger for reheating the circulating steam or the steam-air mixture can also be arranged in the circulation pipe 53 (not shown). The device can be filled in other ways, e.g. through a corresponding tube in the upper piston or in the press jacket, for example by gravity or pneumatically, or in another way.

A particular advantage of the press according to the invention is that all common types of wood chips or cellulose-containing materials can be processed. For example, for wood chips typical in discontinuous sulfate pulp plants, average values are around 16-18 mm in length, 10-17 mm in width and about 3 mm in thickness. In modern continuous sulfate pulp plants, mean values of 38 mm in length and width and 6 mm in thickness are more common, although the distribution around these values can be very wide in both cases.

Attempts to use a more precisely defined chip size have recently increased. New wood chip sorters that effectively remove both oversize and undersize are known. Ideal would be wood chips, the thickness (radial, based on the trunk) of 2 to 4 mm, the length is 15 - 30 mm (fiber longitudinal direction) and the width is arbitrary, but largely homogeneous, and is, for example, 10 - 20 mm. A very large percentage of this can be achieved with an effective sorter. What is more important than the absolute size of the chips is a relatively sharp distribution and a largely homogeneous thickness.

A homogeneous wood chip material offers advantages in terms of uniform impregnation and heating and thus a higher-quality pulp with a lower proportion of chips and undigested. In the case of turbulent heating of the digestion mass with steam or a steam-air mixture, it is easier to achieve through a narrow spectrum that the flow rate is not greater than the speed of the small particles and less than the loosening rate of the largest particles.

The shape of the bores 47 in the sieve plates 46 is arbitrary, but is generally round. The hole size must meet two conditions. Firstly, optimal squeezing of liquids, secondly, optimal introduction of gases for whirling or heating must be ensured. Both are preferably carried out through the same holes, but can in principle also be separated. The gaseous heating medium could be blown in through several larger, separately closable openings. The hole diameter will be in the range of 0.2 to 10 mm, preferably 0.5 to 6 mm, and in particular 1 to 3 mm. This can also be achieved through larger holes that are covered with a sieve of the appropriate hole size.

The hole spacing (the division) is preferably selected such that free surfaces of 1 to 90%, preferably 3 to 80%, more preferably 10 to 70% and particularly 20 to 50% are achieved with the hole sizes indicated. For example, for a 2 mm hole size (round), a pitch of 4 mm could be used, which results in a free surface of about 23%.

The sieve plates 46 can both be self-supporting with a thickness corresponding to the pressure occurring in the press, but they can also consist of perforated plates which rest on webs on a thicker, load-bearing plate provided with larger bores. The holes which have the above-described diameter on the inside of the press of the pistons can increase in cross section in the piston towards the outside of the press in order to achieve an easier drainage of liquids and to avoid possible blockages.

All or some of the bores can be inclined with respect to the press axis in order to achieve additional movement and mixing of the digestion material. Likewise, one or more, optionally also separately switchable, openings of larger diameter than the normal bores in the end face of the pistons, possibly arranged eccentrically, may be present in order to improve the mixing when gaseous media are introduced.

The type of steam or steam / air mixture supply can offer possibilities for a different inflow speed for different sections of the sieve plate 46 of the lower piston. For this purpose, e.g. As the Moeller-Scherström method, the Polysius or the Fuller-Peters quadrant method are known. In the latter, the inflow floor (the sieve plate) is divided into four equally large circular sectors (quadrants), with one quadrant alternately being ventilated more strongly than the other. This requires four connections and a corresponding control. Immediately after impregnation, i.e. after pressing onto the chosen hydromodule, a high throughput of a gaseous medium through parts of the piston cross section (e.g. individual quadrants) will generally be required in order to loosen the material.

It is possible to blow in the steam, the air or the steam-air mixture intermittently or pulsatingly, so that the material is thrown up irregularly and drops back mixed thoroughly.

The speeds of the gaseous medium, measured in the free cross section of the press jacket (in the holes depending on the free surface of the screen plate), will depend on the digestion material, its size and moisture (hydromodule), the layer thickness, the temperature and the desired speed of the temperature increase and generally range from 0.5 to 20 m / s, preferably from 1 to 15 m / s, in particular from 3 to 10 m / s. With intermittent or pulsating operation, these speeds can also be exceeded for a short time.

It may be desirable to inject steam into the closed press intermittently. This condenses on the relatively cool digestion material, whereupon it is blown in again, and accordingly swiftly again for swirling. Under certain circumstances, the ring line 53 (between 38 and 63) can even be dispensed with and blown in and heated intermittently until the digestion temperature is reached. Alternatively, with the circulation line 53 open, the steam-air mixture is constantly pumped through the blower 55 and steam is blown in intermittently or continuously. A sequence of the two methods is also conceivable.

Claims (8)

1. A process of recovering cellulose from cellulose-containig starting materials by a digestion at elevated temperature and elevated pressure, characterized in that impregnated starting material are pressed to adjust a mass ratio of 0.5 to 2 of the absorbed digesting liquor to dry starting materials, the digestion is effected in a pressing apparatus at 160 to 300°C for less than 10 minutes, and heating is effected by a direct suply of heat.

2. A process according to claim 1, characterized in that the direct heating is effected by electric resistance heating.

3. A prosess according to claim 1, characterized in that heat is directly supplied to the impregnated starting materials by a supply of steam in the pressing apparatus.

4. A process according to any of the preceding claims, characterized in that the impregnating is effected in the same pressing apparatus with impregnating liquor in a chemical concentration which is required for the digestion and under constant pressure orwith pressure surges above 1 bar in order to reduce the unwetted capillary length of the starting material.

5. Pressing apparatus for carrying out the process according to at least one of claims 1, 2 and 4, consisting of a pressing shell (1; 1a; 1b), which contains two mutually opposite pistons (4; 4'; 4a; 4b), which are liquid-permeable, if desired, characterized in that in case of electric resistance heating the inside surface of the pressing shell (1; 1a; 1b) is lined entirely or in part with electrical insulation (2; 2a; 2b), contacts for a supply of electrical energy are attached to the top and bottom pistons (4, 4') and are adapted to be connected to a power source, and one of the two pistons is stationary, if desired.

6. Pressing apparatus according to claim 5, characterized in that the pressing shell (1b) is adapted to be lifted and lowered by auxiliary means, which preferably consist of hydraulically or pneumatically actuated cylinders (15) or of screw drive, and electrical insulation (20) is provided between the pressing shell (1b) and the auxiliary means.

7. Pressing apparatus for carrying out the process according to any of claims 1, and 4, consisting of a pressing shell (1; 1a; 1 b), which contains two mutually opposite pistons (4; 4'; 4a; 4b), which are liquidpermeable, if desired, characterized in that in case of a direct heating by a supply of steam the pressing shell (1) has lateral passages (52), which are closed by sieves (38), a curculating line (53) is provided, which incorporates a blower (55) and is connected at one end to the tube (31) of one piston (4') and is connected at the other end to the passages (52) or to the tube of the other piston or end and is preferably provided at each end with a valve (32, 62), and one of the pistons is stationary, if desired.

8. Pressing apparatus according to claim 5 or 7, characterized in that the pressing shell (1) is provided, if desired, with bores (6) for supplying or draining treating fluids.

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