FI80732B - APPARAT FOER MINSKNING AV LIGNING I VEDMASSA. - Google Patents

Description

1 80732

Apparatus and method for reducing lignin in wood pulp

The present invention relates to a gas reaction of a substance, and more particularly to an apparatus and methods particularly suitable for gas gas treatment of a fibrous material, such as, according to a particular example, bleaching or lignin removal of a lignin-containing cellulose pulp.

The lignin content or other non-cellulosic content of the wood pulp may vary. The amount of lignin in a given pulp can be determined by suitable conventional tests and by the number of pieces given to the pulp, which indicates the amount of lignin. The higher the number of pieces, the more lignin the pulp contains.

• “When the wood pulp is bleached, the reduction in the number of pieces indicates the amount of lignin removal. The larger the number reduction, V the higher the lignin removal rate.

· * '-. The reaction between lignin and oxygen is exothermic, and the heat generated now is proportional to the amount of lignin removed. In a reactor without significant ventilation or other pulp temperature control means, the accumulated heat results in a significant temperature rise from the bottom to the top of the pulp bed. In general, it is desirable to maintain the temperature at the top of the bed at 95-100 ° C to obtain a reaction rate sufficient to complete lignin removal in 20-30 minutes. On the other hand, there is a risk of deterioration in the quality of the pulp if the temperature of the pulp exceeds 120-125 ° C. Therefore, it is important that the pulp lignin. The gas reacting with the mixture is adjusted so that the temperature at the top of the pulp bed is maintained at 95 to 100 ° C, but at no point in the bed does it exceed 120 to 125 ° C.

In currently known reactor systems in which a gas substance is allowed to react with a gas permeable substance, 1) the reaction gas is allowed to flow parallel to the mass flow and 2) the gas substance is allowed to flow countercurrent to the mass flow. Both systems operate satisfactorily with low lignin removal rates with a kappa number reduction of 20 or less. When the number of reductions is more than 20, special arrangements must be used in the reactor system for cooling or temperature control of the pulp bed. In both unidirectional and countercurrent, the recirculated refrigerated gas operates satisfactorily with lignin removal corresponding to a lump reduction of up to 30. However, neither system performs well enough with lignin removal corresponding to a lump reduction of more than 30.

Thus, if it is desired to obtain a lignin removal corresponding to a kappa number reduction of 20 or less, this can be done efficiently with currently known parallel and countercurrent systems. If it is desired to obtain a lignin removal of 20-30 kappa, this is achieved by adding special arrangements to the conventional reactor. Special arrangements would comprise reactor-gas recirculation means, whereby the gas is cooled in this means. However, neither known system works effectively if a reduction in lignin removal greater than kappa 30 is desired.

The aim of the pulp bleaching industry is to expand oxygen bleaching to an ever-increasing number of pieces. Thus, a pulp bleaching system that allows pulp lignin removal corresponding to a reduction in bulk number of 30 or more is highly desirable. The present invention provides the industry with such a system. The essential features of the invention are set out in the appended claims.

In short, the new reactor consists mainly of a vertical vessel. The substance supply parts are connected to the upper end of the container. The substance to be treated flows through the vessel from the top to the bottom in the form of a gas-permeable layer and leaves the lower end of the vessel.

At the top of the vessel is the upper gas inlet. Gas is supplied through this inlet and flows parallel to the current of the gas permeable layer 3 80732 but at a much higher rate. At the lower end of the vessel is a lower gas inlet portion through which gas is supplied which flows countercurrent to the mass bed flow. The vessel has degassing sections between the upper and lower gas inlet. The upper gas recirculation line connects the degassing portions to the top of the vessel to recycle the gas back to the top of the vessel. The lower gas recirculation line connects the degassing portions to the lower end of the vessel to recycle the gas back to the lower end of the vessel.

Briefly, the novel method of carrying out an exothermic gas reaction with a suitable agent according to the invention comprises the steps of feeding gas to the top of the retention vessel at a controlled rate and passing the gas down through the gas permeable layer at a rate greater than the gas permeable layer. The unreacted gas is removed from the vessel at a point above the bottom of the gas permeable layer. The gas is simultaneously fed to the bottom of the retention vessel at a controlled rate at a temperature lower than the temperature of the gas fed to the top of the retention vessel. The gas flows upwards through the gas permeable layer. The unreacted gas is removed from the vessel at a point above the bottom of the gas permeable layer. The temperature and velocity of the gas fed to the top of the retention vessel are such that the gas permeable layer rapidly heats to the mineaction temperature. The temperature and velocity of the gas fed to the bottom of the retention vessel are such that the temperature of the gas permeable layer does not exceed the maximum allowable temperature.

The invention and many of its advantages will become more apparent from the following detailed description and drawings, in which Figure 1 is a diagram showing calculated temperature profiles for particle number reduction 50 in a pulp bed using 4,80732 parallel recirculation gas cooled to 100 ° C , Fig. 2 is a diagram illustrating temperature profiles for block number reduction 50 in a pulp bed using a known countercurrent circulation with recirculating gas cooled to 100 ° C, Fig. 3 is a diagram showing a preferred embodiment constructed in accordance with the present invention, and Fig. 4 is a diagram , which shows the calculated temperature profile with a number reduction of 50 in the pulp layer using the embodiment of Figure 3.

Looking at the drawings, and in particular Figure 1, its diagram shows the calculated temperature profiles at different gas flow rates with a mass bed height of 6 meters.

The pulp enters the retention vessel at 90 ° C, the parallel circulating gas is cooled to 100 ° C, and the lignin removal number is 50. The diagram in Figure 1 is a typical diagram of the known parallel circulating oxygen bleaching of the pulp system. In such a known system, the gas is allowed to flow downwards through the mass bed at a higher rate than the moving mass. Near the bottom of the reactor, the unreacted gas is separated from the pulp and recycled back to the top of the reactor. The temperature in the vicinity of the top of the pulp bed should rapidly rise above 95 ° C to provide a sufficient reaction rate to complete lignin removal in a retention time of 20-30 minutes. However, the temperature at no point in the pulp layer must exceed approx. 120 ° C. If the temperature exceeds 120 ° C, the quality of the pulp is likely to deteriorate.

The diagram in Figure 1 shows the temperature profiles in a parallel circulating gas cooled system with gas flow rates of 0.5 kg oxygen / kg mass, 1.0 kg oxygen / kg mass and 2.0 kg oxygen / kg mass. A recirculation rate of 1 kg or more of oxygen per kilogram of pulp provides the desired temperature control. However, this corresponds to a relative gas velocity of 5,80732 that is 5.4 m / min or greater. Such large gas flows cannot be handled in a parallel system without excessive pressure drop and compression of the pulp in a porous, 6 m high pulp bed. For effective pulp bleaching, excessive compression of the pulp in the porous pulp layer must be prevented. The mass of the pulp layer must be fluffy and sufficiently gas permeable to perform the bleaching step. The parallel back-recirculation gas system is therefore not efficient in lignin removal, where the reduction in kappa number is large, even if the gas is cooled before being recycled back to the top of the retention vessel.

Fig. 2 is a diagram showing the calculated temperature profiles at different gas flow rates with a mass layer height of 6 meters. The pulp enters the retention vessel at 90 ° C, the countercurrent gas is cooled to 100 ° C, and the lignin removal number is 50. In a countercurrent system, the gas flows countercurrent to the flow of the pulp bed. The circulating gas is removed from the top of the reactor and returns to the bottom after flowing through the condenser. The diagram in Figure 2 shows the gas flow rates of 0.5 kg oxygen / kg mass. 1.0 kg oxygen / kg mass, 2.0 kg oxygen / kg mass and 3.0 kg oxygen / kg mass. It is clear from the diagram in Figure 2 that a gas recirculation flow rate of 3 kg or more of oxygen per kilogram of pulp must be maintained in the reactor to ensure that the temperature of the pulp at no point in the reactor exceeds 120 ° C. Such a high gas recirculation rate compared to the mass flow is not at all desirable. High gas recirculation rates compared to the mass flow can prevent the mass from flowing downwards in the reactor and cause a "stop" especially at the top of the bed where the mass is only slightly compressed. The mass entering the top of the reactor is also fluffy and composed of fibers. Degassing at this point is not desirable due to the large amounts of fine fibers floating in the gas stream. Conventional t 6 80732 countercurrent circulation systems therefore cannot be used effectively for good lignin removal either.

A preferred embodiment of the invention shown in Figure 3 comprises a reaction vessel 10. The reaction vessel 10 has an upper part 12, an associated central part 14 and an adjacent bottom or lower part 16.

The horizontal cross-section or cross-sectional area of the top 12 of the container 10 is significantly smaller than the cross-sectional area of the associated central portion 14. The lower portion 20 of the upper portion 12 extends partially within the central portion 14 to form a first gas receiving chamber 18 defined by the lower portion 20 of the upper portion 12, the upper circumference 21 of the central portion 14, and an upwardly tapering annular barrier wall 22 connecting the central portion 14 and the upper portion 12.

The upper end of the chamber 18 is enclosed by an annular barrier wall 22, but below the barrier wall 22, however, a degassing portion 24 is connected to the chamber 18 for degassing the chamber.

The horizontal cross-section or cross-sectional area of the central portion of the container 10 is significantly smaller than the cross-section of the associated lower portion 16. The lower portion 26 of the central portion 14 extends partially within the lower portion 16 to form a second gas receiving chamber 28 bounded by the lower portion 26, an upwardly tapering annular barrier wall 30, and an upper outer periphery 31 of the lower portion 16.

The upper end of the chamber 28 is enclosed by an upwardly tapering annular closure wall 30. However, below the barrier wall 30, the gas inlet part 32 is connected to a chamber 30 which receives the gas supplied to the lower part 16.

The upper end of the container top 12 is provided with at least one gas inlet 34 connecting this top 12 to the gas filling pipe 36 controlled by the valve 37. The top of the container top 12 is further provided with a substance inlet or inlet pipe 38, 7 80732 connected to this upper end mainly in the container. 10 in the middle. The inlet pipe 38 includes a feed screw 40 mounted on a support shaft 42 rotatably driven by a drive motor (not shown) for feeding the substance down to the top of the container 12. The inlet pipe 38 is connected to the substance supply pipe 44.

The upper end of the top 12 of the container has means specially adapted to disintegrate, grind and fluff the material fed to the inlet pipe 38. This means also spreads the disintegrated, comminuted and fluffed substance over the cross-section 12 of the top of the container. As shown, this device is of the type described in U.S. Patent 3,785,577 and comprises an inner ring 46 of arcuate spaced pins connected to a shaft 42 for rotation therewith and a coaxial spindle of spaced apart fixed pins. a radially spaced outer ring 48. During use of the device, the ring 46 formed by the rotating pins is continuously rotated relative to the ring 48 formed by the stationary pins. The material fed through the inlet pipe 38 is disintegrated or fluffed by the mutual movement of the pins and then removed outwards through the spaces between the adjacent pins, after which it is spread fluffed, comminuted or dispersed over the cross-section of the vessel 10.

The lower end of the container has outlet portions for removing the substance from the container. These discharge portions comprise a pulp discharge pipe 50 connected to the lower end of the vessel and a scraper 52 located at the lower end of the vessel near the discharge pipe 50 and rotatably driven by a drive motor (not shown) via a drive shaft 54. The dilution supply line 56 supplies the diluent to the lower end of the vessel. However, it is clear that the outlet parts of the device may have some other suitable design.

The apparatus further includes means for redistributing heat in the reaction vessel 10 during the reaction process for adjusting its temperature to 8,80732. More specifically, a recirculation line 60 outside the vessel 10 is connected at opposite ends to the degassing portion 24 and the gas inlet portion 34 to circulate the portion of the gas removed through the gas pipe 62 back through the gas inlet portion 34 to the upper end of the vessel. The recirculation pipe 60 controlled by the valve 61 is further provided with a conventional centrifugal fan 64 adapted to blow back the recirculated gas through the recirculation pipe 60 to the gas inlet 34. As shown, the gas filling pipe 36 is connected to the gas inlet section 34 by the recirculation pipe 60, the supplied gas is fed mixed together through the inlet section 34.

A portion of the gas flowing from the outlet portion 24 through the tube 62 is recirculated through a recirculation tube 70 controlled by a valve 72. A recirculation tube 70 outside the vessel 10 is connected to the gas inlet portion 32 to recirculate the gas discharged through the tube 70 to the lower end of the vessel. The recirculation pipe 70 is provided with a conventional centrifugal fan 74 adapted to blow back the recycled gas through the recirculation pipe 70 to the gas inlet 32. The gas circulating through the recirculation pipe 70 is passed through a gas cooler 76 where it is cooled before being recirculated to the gas inlet.

The operation of the apparatus described above and the oxygen lignin removal of the cellulosic pulp will be described below, it being clear that the maximum temperature of the vessel 10 must be kept below about 120-125 ° C during such a reaction to prevent deterioration of the pulp quality.

Throughout operation, the shafts 42 and 54 are rotatably driven so that the screws 40, the ring 46 formed by the rotating pins, and the bottom scraper 52 are rotated continuously. The reactive mass 2 is kept in a vessel 10 at a reactor pressure of about 7 kp / cm for 20-30 minutes, which is suitable for the reaction or desirable, so as to obtain a porous, gas-permeable layer of fluffy mass, the upper end of which is mainly at the level indicated by reference numeral 78. and a gas space 80 separated below the top of the top of the vessel. The mass gradually moves downward as a porous layer as the reacted mass is removed from the bottom of the layer through an outlet pipe 50.

The feed screw 40 continuously receives the pulp from the tube 44 and feeds the pulp down to the lower rings 46, 48 formed by rotating pins. By their mutual rotation, these pins disintegrate and fluff the pulp to a state suitable for the reaction and the porous gas permeable mass layer described above. 10. The rings 46, 48 formed by the pins further remove the pulp in a radially outward flow between the adjacent pins, the pulp flowing down through the space 80 onto the upper end 78 of the pulp layer below.

The reaction gas or oxygen is simultaneously fed continuously at a controlled rate and temperature through the gas inlet 34 to the space 80 into the mass flowing downwards therethrough. As it flows downwards, the pulp is thus exposed to the effect of the feed reaction gas, and a small amount of reaction gas reacts with the pulp at this time. The unreacted gas continues to flow downward through the porous mass bed at a rate substantially greater than the rate of downward movement of the mass. An additional amount of recirculation gas reacts with the pulp during the downward flow, and the remaining gas enters upward through the gas outlet chamber 24 and the outlet pipe 62. A portion of the gas flowing through the outlet pipe 62 is fed through the valve 61 and gas recirculation pipe 60 by a blower 64 and a vessel top 12. The recirculated gas is fed to the gas space 80 through the gas inlet 34 mixed with the gas from the gas supply pipe 36. Most of the heat contained in the recycled gas is in the form of water vapor due to the evaporation of water from 10,80732 hot masses. The recycled gas entering the gas space 80 has a higher temperature than the incoming pulp, and due to the large vortex in the gas space 80 and the large surface area exposed to the gas in the gas space 80, heat transfer . Therefore, substantially the same temperature of the recycled gas and the fluffed pulp is reached before the pulp settles to the upper level 78 of the pulp bed. The amount of this heat transfer from the recycled gas to the pulp depends, of course, on the amount and temperature of said gas.

A portion of the gas from the tube 62 is simultaneously fed through a recirculation line 70 and a gas cooler 76 controlled by the valve 72, after which it is recirculated through the lower gas inlet 32 to the gas chamber 28 and upward through the center 10 and gas chamber 18 of the vessel 10 and out through the degassing tube 62.

The temperature and amount of gas fed to the top 12 of the retention vessel 10 must be such that the gas permeable layer rapidly heats to the minimum reaction temperature. The temperature and amount of gas fed to the bottom of the retention vessel must also be such that the temperature of the gas-permeable layer does not exceed the maximum permissible temperature of 120 to 125 ° C. This is especially important when large amounts of lignin are removed from the pulp, so that the reduction in kappa number is e.g. 30 or more.

If the height of the pulp bed is about 6 m and a reduction in mass number of 30 or more is desired, less than 1 kg oxygen / kg pulp must be recycled back to the top of the retention vessel and at least 2 kg oxygen / kg pulp cooled to 100 ° C recirculated countercurrent to the bottom of the retention vessel. .

n 80732

The lignin removal of the pulp, which corresponds to a reduction in the number of chunks 50, is shown in Figure 4, which is a diagram of bottom recirculation. The hot gas is cooled to 100 ° C before recirculation.

This prevents the pulp from exceeding the maximum permissible temperature. As can be seen from the specific example of the 50-figure reduction shown in Figure 4, the amount of gas recycled to the top of the retention vessel is 0.5 kg oxygen / kg mass and the amount of oxygen recycled to the bottom of the retention vessel is 2 kg oxygen / kg mass.

Parallel recirculation at the top of the reactor has the following advantages: 1. The pulp is only slightly compressed in the upper layers. Resistance to gas flow is very low.

2. Parallel flow in the upper layers prevents the mass from stopping.

3. The hot gas from the reactor is used to heat the incoming pulp, which saves steam.

4. The pulp heats up rapidly to the minimum reaction temperature due to the rapid heat transfer between the hot gas and the fluffed pulp.

5. Large amounts of fine fibers float in the gas stream surrounding the pulverizer. A parallel gas flow forces these fibers to settle in the pulp bed.

Countercurrent recirculation in the bottom layers of the pulp layer has the following advantages: 1. Countercurrent flow prevents excessive compression in the bottom layers and thus keeps the flow resistance to a minimum.

2. The heat is removed efficiently, which prevents the pulp from exceeding the maximum permissible temperature and prevents the pulp from deteriorating.

Claims (3)

12 80732 Patent Requirements An apparatus for reducing lignin in wood pulp by means of oxygen gas, the apparatus comprising a substantially unobstructed hollow vertical vessel (10), a pulp feed member (44) connected to the upper end of the vessel for feeding pulp to said upper end, means (50) for removing pulp from the lower end of the vessel, oxygen supply connected to the upper end of the vessel (34) for supplying oxygen gas to this upper end and the mass therein, an oxygen gas outlet (24) connected to the vessel, and an oxygen gas recirculation line (62, 60) connecting the oxygen gas outlet (24) to the upper end of the vessel for circulating oxygen back to the upper end of the vessel; that the device comprises a lower oxygen gas supply member (32) connected to the lower end of the vessel for supplying oxygen gas to this lower end and the mass therein, that the oxygen gas outlet (24) is connected to the vessel between the upper and lower oxygen gas supply members (34, 32); comprising a lower oxygen gas recirculation line (62, 70) connecting the oxygen gas outlet (24) to the lower end of the vessel to recycle the oxygen gas back to the lower end of the vessel, the lower oxygen gas recirculation line (70) having an oxygen gas cooler (76) for flowing oxygen through the recycle line the radiator (76) is the only gas cooler in the device n. A method for reducing lignin in wood pulp with a kappa count reduction of more than 30, wherein the wood pulp is continuously fed to the top of a retention vessel (10) having a pulp inlet (44) at its upper end, a discharge member (50) at its lower end and an unobstructed pulp flow. a background path between the pulp inlet and the discharge member, the wood pulp moving downwards through the vessel in a single gas-permeable layer and leaving the lower part of the retention vessel; oxygen gas is fed to the top of the retention vessel and allowed to flow downward in the same direction as and through the gas permeable layer at a rate greater than the rate of downward movement of the gas permeable layer i3 80732; and the unreacted oxygen gas is removed from the vessel and recycled to the vessel, characterized in that the unreacted oxygen gas is removed from the vessel in the region with the highest gas temperature located at a point above the bottom of the gas permeable layer; a portion of the unreacted gas removed from the retention vessel is recycled to the top of the retention vessel without cooling; the remainder of the unreacted gas removed from the retention vessel is cooled and recycled to the bottom of the retention vessel; this oxygen gas is allowed to flow upwards in the opposite direction to and through the gas permeable layer and unreacted oxygen gas is removed from the vessel at said point above the bottom of the gas permeable layer, the total amount of unreacted oxygen gas removed and the amount and temperature of oxygen gas fed to the top of the retention vessel and the amount and temperature of oxygen gas fed to the bottom of the retention vessel are such that the gas permeable layer heats rapidly to the nominal reaction temperature, but the temperature of the gas permeable layer does not exceed the maximum allowable temperature. A method according to claim 2, characterized in that the amount of oxygen gas fed to the upper part of the retention vessel is less than 1 kg / kg of mass and the amount of oxygen gas fed to the lower part of the retention vessel is at least 2 kg / kg of mass. I-. 80732