FI58522C - OIL ANCHORING FOR BLEKNING AV ETT FIBROEST MATERIAL SPECIELLT CELLULOSA - Google Patents

Description

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Patent · och regiltarttyralsan v 'AnaMun utlagd oeh utLkrtfwn publfcend 31.10.80 (32) (33) (31) ^ T4 «tty · ο · οΙΙμμ · —Begird priorkat 2k. 07.70 17.02.71 Sweden-Sweden (SE) 10238/70, 2023/71 (71) Sunds Aktiebolag, Sundsvall, Sweden-Sweden (SE) (72) Hans-Erik Rye Engström, Sundsvall, Bengt Edvard Pettersson,

Sundsvall, Sweden-Sverige (SE) (7 * +) Berggren Oy Ab (5 * 0 Method and apparatus for bleaching a fibrous substance, in particular cellulose - Method and apparatus for bleaching a fibrous material, in particular cellulose for bleaching a substance, especially cellulose, with a gaseous bleach, and is specifically intended for use in systems in which the fibrous material is introduced in a comminuted state into a gas chamber located at the top of a vertical bleaching tower with at least substantially smooth inner walls The invention also relates to a system for carrying out the method.

When cellulose is bleached with a gaseous bleach in a tower of the type described, the desired reaction time is normally longer than the time taken for the cellulose to fall freely through the tower and generally longer than five minutes. Therefore, a cellulose pulp column is formed in the tower, and due to the weight of the pulp layers above, the weight of the pulp per unit volume is greater at the bottom of the column than at the top. If the mass at the bottom of the statue becomes compressed in too large an amount, it will be difficult to supply a sufficient amount of gas to this portion of the mass, 2,58522, and it will be impossible to complete the reaction within a reasonable time. The mass in the lower part of the statue is subjected to such high pressure that water is squeezed out of the mass, which makes the mass only weakly permeable to gas, since the only way in which gas can enter the mass is by diffusion in the liquid phase. This problem can, of course, be solved by using blasting towers with a larger diameter. However, such towers require a lot of space and require more expensive structures in the equipment needed to remove the pulp from the tower. Attempts have been made to solve the problem by equipping the tower with inwardly projecting shelves which prevent the mass from passing downwards and from which the mass is scraped off after a suitable reaction period. This incurs additional costs due to the mechanical equipment required, which tends to reduce the economic benefits achieved by the gas phase bleaching process.

It has now been found that a continuous gas-permeable pulp column with a height such that the tower can have an economic diameter can be maintained in the tower, provided that the cellulose pulp has a fluffy structure and a high fiber content.

When bleaching cellulosic pulps with a gaseous bleach according to the invention, the pulp must be fluffy and have a fiber content of between 18-40%. A suitable concentration is between 25-35%, with the most preferred concentration being 30%. It will be readily appreciated that in order to obtain the best bleaching results, the interface between the pulp and the gaseous bleach must be large enough and the pulp fluffy enough for the fresh gaseous bleach to mix with the pulp and replace the bleach consumed. Most of the bleaching steps applied during the bleaching process can be preferably carried out in the above-mentioned concentration range.

The lower limit of the fiber concentration range is determined by structural reasons, as for concentrations below 18% towers with too large a diameter must be used, while the upper limit should not be exceeded due to process reasons. When chlorine or its compounds are used for gas phase bleaching, the best pulp quality and the highest lignin dissolution rate are achieved with pulp fiber contents in the range of 30-40%. If the gaseous bleach is oxygen, then relatively dry pulps with a content of more than 35% pose a fire hazard and are highly flammable if the pulp overheats for some reason or if sparking occurs. Masses with a pi, 58522 content of more than 35% are highly flammable and, upon ignition, acquire explosive properties quickly, while masses with a lower concentration are less susceptible to ignition and, due to their high moisture content, are self-extinguishing when ignited. A large number of experiments have been carried out in a tower equipped with puncture plates and pressure and temperature gauges, and it has been found that the dangerous limit for the dry matter content of pulp sludge is between 33-35%, depending somewhat on the partial pressure of oxygen gas. The limit values that can be applied in practice for bleaching cellulose with oxygen-gas in vertical reaction towers, taking into account the economical diameters of the tower and the elimination of the risk of fire, are 25% and 35 S. A suitable concentration is 30%.

The above experiments have also shown that in order to achieve a satisfactory mixing of the gas with the lowest layer of the pulp column, the maximum height to which the pulp column is allowed to rise must be approximately in line with the pulp fiber content in the range of 18-40%. This dependence is illustrated in Figure 3, which shows the maximum height of the pulp column in meters as a function of the fiber content of the pulp.

It is therefore an object of the present invention to provide a method and system by which cellulose can be bleached uniformly and rapidly with a gaseous bleach using relatively inexpensive equipment. The process according to the invention is characterized in that the cellulose is fed to the top of a reaction tower with a smooth or substantially smooth inner surface, i.e. no shelves or similar protrusions are placed inside the tower to collect and retain the pulp for reaction periods of the desired length. and the tower maintains a continuous column of comminuted cellulose pulp with a fiber content of between 18 and 40% and a maximum height in meters of approximately five tenths to seven tenths, preferably six tenths of the weight of the pulp in the uniform column.

The invention will now be described with reference to the accompanying drawings, in which Figure 1 shows a closed bleaching tower, Figure 2 shows the top of the tower in vertical section on an enlarged scale, Figure 3 shows the relationship between maximum pulp height and pulp fiber content, Figure 4 Fig. 5 shows the density of the fluffed pulp as a function of the distance measured from the top surface of the pulp column, Fig. 6 shows the relative gas volume in the pulp column as a function of the distance from the top surface, Fig. 7 shows as a function of time from the start of combustion, Fig. 11 is a flow chart for a system for recovering and recovering solids dissolved in oxygen bleaching; o 12 shows the dilution zone on an enlarged scale.

The bleaching device described is a downstream tower, to the upper end of which is connected a supply line 2, which, as shown in Figure 2, may be provided with a transport screw 3 to ensure a uniform feed of cellulose down to the device 4 which grinds the cellulose. The screw conveyor 3 and the grinding device 4 can suitably be mounted on the same shaft, whereby they will rotate at the same speed.

They can also be mounted on different shafts, and the screw 3 can be rotated at a lower speed than the device 4. The pulp is fed into the line 2 by means of a screw conveyor 5 with an inlet opening 6 suitably arranged in a horizontal position. The screw conveyor 5 is constructed in such a way that a pulp plug is formed inside its casing, which prevents the gaseous bleach from escaping against the pulp feed direction. Both the screw 5 and its casing 7 are conical in shape and have a cross-sectional area decreasing in the direction of mass feeding. The screw conveyor system terminates in a part 8 located close to the line 2. This part of the screw conveyor system may be cylindrical or have an increasing cross-section.

The cellulosic pulp is comminuted by means of cylindrical rotating pins 9 or similar rod-like members, at least some of which members cooperate with solid members 10 of approximately the same shape. The plate 11 and possibly also the displacing or pushing member 12 are adapted to guide the mass and to distribute it radially towards the members 9 and 10. It may also be appropriate to deflect the radial movement of the mass in an oblique downward direction. In the embodiment shown, this is achieved by means of a fixed conical dome 13. It is clear that the system according to the invention is not limited to the cellulose pulp disintegration and flocculation device described above, but other known devices suitable for this purpose can also be used.

The cellulose can be treated either immediately upon entering the tower or as it passes through the tower. A device for comminuting the cellulose can also be placed outside the tower, but in this case, additional means are required to ensure that the cellulose is introduced into the tower without significantly losing its fluff. If desired, certain safety devices may be located at the top of the tower, such as spray nozzles 14 for water or other suitable fire extinguishing fluids and puncture plates 15. If punch plates are used, there should be at least two symmetrically placed to avoid adverse reaction forces in case of plate rupture. The supply line for the spray nozzles is indicated by reference numeral 52.

The cellulosic pulp at the bottom of the tower can be diluted with a liquid discharged from the dilution nozzles on different sides of the tower and the pulp can be mixed by one or more vane mixers to achieve a uniform reduction in fiber content to achieve the desired pumpable consistency. The equipment required to accomplish this need not be described in detail as they are known in conventional downstream bleaching towers. The embodiment shown in Figure 1 has a pulp removal system which has the advantage that the risk of the pulp dilution zone moving up the tower is eliminated. In the exemplary embodiment, the tower is divided into a bleaching zone 16 comprising a gas space 30 and a dilution zone 17 separated by a gas zone 18. The pulp in the bleaching zone rests on a rotating loose bottom plate 18 separating the gas zone and the bleaching zone. comprising discharge pushing means 20 for discharging the pulp through the gap 21 between the loose base plate 19 and the walls of the bleaching tower. The pulp then falls immediately through the gas zone 18 maintained below the loose base plate 19 into the dilution zone 17. The pulp is diluted with water (return water) fed from one or more pipes 22 and mixed with vane mixers 23, after which the pulp slurry is subjected to a suitable pumping consistency. about 4%, having been removed through pipe 2 * 1. In the embodiment shown, the loose plate 19 is driven by a hydraulic motor 25 and is supported by a shaft 26 supported on bearings 27 and 28.

The dilution zone may extend near the loose plate 19 or at the same level as the plate 19.

The gaseous bleaching agent is introduced into the bleaching zone via one or more lines 29, suitably to the lower part of the zone or simultaneously in several planes below the surface of the pulp in the bleaching zone. The supply of gases to the spaces 30 and 18 can also be carried out either simultaneously or alternatively to one or 6 58522 to both gas spaces. In the latter case, direct introduction of the gaseous bleach into the pulp can be avoided. At the top of the tower, an outlet pipe 31 is arranged to remove the gaseous bleach continuously or intermittently in order to prevent the bleach from becoming enriched with a gas which is unable to react with the pulp. If, for example, oxygen gas is used, although the same is true for other gaseous bleaches, difficulties may arise in preventing air from following the pulp. Enrichment with nitrogen gas can be avoided by passing it out through a pipe 31. In this case, a certain amount of gaseous bleach is lost.

Experiments and their results.

Oxygen bleaching is usually performed under the following conditions:

Temperature 80-120 ° C

2

Oxygen partial pressure 2-8 kg / cm Exposure time less than one hour.

The first bleaching experiments in the laboratory were performed with a pulp fiber content, i.e. consistency of 8-16%. Later, we presented values that showed that good bleaching results can be obtained in the laboratory even in the concentration range of 3-6%.

For equipment designers, the mentioned temperature range did not prove to be a particular problem. Likewise, no pressure or exposure time. However, a lower concentration range, 3-6%, would require too much stirring power to ensure adequate and even oxygen distribution during the reaction. The range of 8-16% was also found to be unsuitable for the same reasons. It was concluded that the ideal concentration range would be one in which the cellulose can be fluffed to such a porous state that gas can penetrate through the column of pulp in the reactor along its entire length, or at least that the interfibrous pores can trap oxygen consumed in the reaction. If this goal cannot be achieved, it is necessary to mix the pulp for at least part of the reaction time, requiring a complex and expensive reactor, with very high associated maintenance costs.

Laboratory experiments performed using fluffed cellulose pulp and various concentrations determined the relationship between pulp density and pulp pressure at each concentration value.

A layer of fluffed mass was introduced into a vertical cylinder whose inner walls were coated with Teflon to reduce wall friction to a minimum. A perforated plate was placed on top of the pulp layer 7,58522, and the volume of the layer was measured when the perforated plate was loaded with different weights. Two different cylinders with diameters of about 10 cm and about 3 cm were used. The pulp was mixed with 3.5% sodium hydroxide based on the weight of dry cellulose before the experiments.

Figure 4 shows a typical result obtained with 19% unbleached sulphate cellulose pulp (Pinus sylvestris). It was found that the 'density - pressure' relationship can be represented by the formula for each concentration value with a loading pressure greater than 2 n 1 PSIG (about 0.07 kp / cm).

B

ύ = ep where γ = mass density, g / cm 2

P = load pressure, PSIG

The values of the coefficient 6 and the exponent H varied depending on the mass concentration.

Using the dependence thus obtained and general physical mathematical calculations, the density of the fluffed mass was derived as a function of the layer depth, the results being shown in Fig. 5. As can be seen, the density increases considerably with increasing depth, i.e. distance from the surface. At a concentration of 25%, the density is about 6 m below the surface, almost twice as high as about 1 m below the surface, because the mass is compressed by its own weight. The dashed line depicts the boundary at which the fiber web begins to collapse and water drainage occurs, as has also been obtained in experiments.

Assuming a fiber density of 1.6 and a water density of 1.0, the relative gas volume in the pulp bed as a function of depth was further calculated, with the results shown in Figure 6.

From these graphs, it can be concluded that gas phase bleaching, which requires a considerable length of exposure time, must be performed using high concentrations to ensure adequate penetration of the gas into the entire pulp bed if mechanical mixing with the pulp by mechanical means is desired.

It was concluded that a suitable fiber content when treating cellulose made from said wood species in the reactor would be in the range of 25-30%. In addition, the results clearly showed that oxygen bleaching can be performed without mechanical mixing even at production rates as high as 800-1000 tons per day.

The results obtained from experiments with sulphate cellulose hall made from birch (Betula pubescens) were very similar to those presented above. In the case of other wood species, different mass densities can be observed. In such cases, adequate gas penetration requires a somewhat different concentration range.

Another problem that required attention was how to avoid excessive gas leakage (oxygen leakage) from the inlet and outlet end of the pressure reactor.

A screw feeder (Fig. 2) was designed for the feed end in such a way that it would compress the pulp into a gas-tight plug in the reactor feed line. The prototype was tested in a semi-factory facility. After a few changes, it proved possible to feed the high consistency mass into a pressure vessel of about 10 m 2, stop the screw feeder, leave everything in place and note the next day that the pressure in the tank was exactly the same as when stopping the feeder the day before.

An apparently effective way to prevent excessive gas losses at the outlet end is to use a dilution zone at the bottom of the reactor. Theoretically, at a temperature of 100 ° C and a pressure of 6 kp / cm 2 and a pulp removal content of 4%, the oxygen loss with the removal liquid is 0.4% by weight of cellulose and 0.25% at a pulp removal content of 6%, provided there is an equilibrium between the gas phase and the liquid phase.

Thus, it was decided to construct the bottom of the reactor as shown in Figure 12. The rotating plate feeder discharges the high consistency pulp into the dilution zone. With this arrangement, a very precise control of the discharge current from the high-consistency zone is achieved, as shown in Fig. 7. · By maintaining the adjusted level in the dilution zone, the discharge to the outlet line also becomes precisely controlled.

There are certain hazards in any activity that uses pure oxygen. Therefore, research on the safety aspects of oxygen bleaching was started at an early stage of the development work. Thus, extensive experiments have been performed, both in the laboratory and on a semi-technical scale, to determine the boundary conditions under which ignition and combustion of cellulose begin to occur in different combinations of pulp consistency and oxygen partial pressure. Somewhat complex instruments were used to measure and record rapid pressure and temperature changes. The following technique was used in all experiments: 9,58522

A layer of fluffed cellulosic pulp was placed in a pressure vessel equipped with an electrically heated filament to perform ignition. Oxygen was introduced into the vessel to the desired outlet pressure, and the filament was heated to ignite the contents. Vessels of various sizes were used from a 0.3 liter autoclave to a pressure vessel of about 10 m 2.

In summary, it proved very difficult to ignite cellulose at a concentration of 30% or less 2. .

even in pure oxygen and at a pressure of 10 kp / cm. However, combustion could be initiated by placing a small amount of pulp at a concentration of 38-40% around the filament. Figure 8 shows a typical pressure change as a function of time using said procedure in a 10 m tank.

In this experiment, 30 kg of BD cellulose pulp with a content of 30% was introduced into the tank, and in addition, 5.5 kg of pulp with a content of 38% was placed around the filament, the filament being located on the surface of the pulp layer. The initial pressure was 71 PSIG or 5 kp / cm. Ignition occurred at time 0 of the pattern. As can be seen, the pressure rose very slowly during the first 35 seconds. Since no attempt was made to prevent combustion, the puncture plate finally yielded 150 PSIG, or 10.5 kp / cm, after about 3 minutes.

Figure 9 shows the result of another experiment. In this experiment, 15 kg of BD cellulose pulp with a concentration of 39% y was ignited in the same tank as above. Also in this case, the pressure rise was relatively small during the first 30 seconds, namely from 2 71 PSIG to 85 PSIG, i.e. 6 kp / cm. When the pressure was 176 PSIG, i.e. 2 about 12 kp / cm in diameter, the exhaust valve with a diameter of about 10 cm was opened, whereby the pressure dropped to zero in 17 seconds and the fire went out.

In all experiments, it had been found that combustion occurred only on the surface of the pulp layer. Thus, it was concluded that it was possible to extinguish the ignited fire better with a jet of extinguishing agent than by opening the safety valve or breaking the puncture plate. Figure 10 illustrates the effect of a quench jet in a 10 m 2 tank. In this experiment, 20 kg of BD cellulose pulp with a concentration of 42% was ignited at an initial pressure of 71 PSIG. After 40 seconds, the jet valve was opened at a pressure of 114 PSIG and the pressure dropped to 85 PSIG

In 10-15 seconds, the fire goes out at the same time.

As a result of safety studies, the reactor was equipped with three independent safety systems. The first is affected by the pressure detector and / or temperature detectors in the reactor. Each of these can open the valve in the supply line of the water jet nozzles at the upper end of the reactor, open the discharge valve to quickly reduce the pressure, and close the valve in the oxygen supply line in the event of a fire. The second system is simply a conventional mechanical safety valve, and the third system consists of two blow-off plates. Each of these systems can extinguish the fire and lower the pressure and temperature before they are even close to dangerous levels.

Oxygen bleaching does not in itself reduce contamination in any significant amount, unless the solids dissolved in the oxygen step are recovered in some suitable manner. Oxygen bleaching can be performed in the presence of a certain amount of black liquor solids in the cellulosic pulp.

This fact leads to the idea of a real countercurrent recovery system in which the oxygen stage recovery liquid is used for washing in the last brown pulp scrubber.

In Figure 11, such a system is shown as three different alternatives. For simplicity, internal dilution circuits are not shown. However, the flow chart shows the recovery and return conditions that allow for a fully balanced fluid system, i.e., no fluid flow needs to be directed to the sewer or elsewhere. In Option I, the screening system is open. Any excess liquid from the scrubber downstream of the oxygen reactor is used as the scrubbing liquid in the last brown pulp scrubber. Screening (which does not involve branching) is performed after the oxygen step. In this way, the oxygen stage will be part of a brown pulp washing system, where the last scrubber is the scrubber downstream of the oxygen reactor.

In Option II, the screening system is closed, which means that the filtrate from the screened pulp thickener is used not only for internal dilution of the screening system but also for washing in a scrubber downstream of the oxygen reactor. Therefore, fresh water is used for washing in the thickener of the screened pulp, which then actually remains the last washing device in the brown pulp washing system, with the oxygen stage and the screening system being part of it.

Finally, in Option III, the screening residue is also stored in the system by purifying the residue and returning it to the first screens. Excess liquid left over from the residue treatment system is reused in the screening space.

11 58522

The relative amount of dissolved solids recovered from the oxygen step has been calculated for the different alternatives mentioned separately. Calculations were performed for washer displacement ratios of 0.70 and 0.85 to show the effect of this variable. In all cases, the calculations were based on a dilution factor of 3. The results were as follows:% solids recovered with a dilution factor of 3 Option I Option II Option ΙΠ Transfer ratio 0.70 64 83 88

Transmission ratio 0.85 77 90 95

Thus, it is possible to recover 80-90% of the dissolved solids in the oxygen phase to the evaporators without further dilution of the black liquor, i.e., contrary to current practice in most brown mass washing systems. This means that the B0D and COD load of the bleaching plant effluent is reduced, respectively. Likewise the color of the exhaust fluid. In addition, it means that sodium hydroxide in an amount corresponding to about 39-43 kg of glauber's salt per ton of cellulose is recovered from the oxygen bleaching step.

The invention is not limited to the described embodiments, but can be modified within the scope of the following claims.

Thus, the cellulosic pulp can be conveyed through the tower in the opposite direction, i.e. from the bottom up by means of suitable devices. The invention is also not limited to the described devices for introducing and removing pulp into the tower, but other suitable devices can be used to feed the pulp at a rate necessary to remove the pulp at specified intervals to ensure that the maximum column height remains within the scope of the invention.

Claims (13)

A process for bleaching a fibrous substance, such as cellulose pulp, in the gas phase, in particular oxygen gas, from inside a smooth or substantially smooth reaction tower having a gas space at the top of the tower into which the pulp is fed and allowed to fall in a finely divided space or swirl under a well-maintained gas space on the surface of a thick pulp column, characterized in that said unbroken pulp column has a content of 18-40% by weight, preferably 25-35 and most preferably about 30% and that the height of said unbroken thick pulp column is substantially greater than the diameter of the pulp column but the height of the pulp column in meters does not exceed 5/10 of the maintained concentration in weight percent in said continuous thick pulp column and that bleach gas is fed to the bottom of the pulp column and that gas which cannot react with the pulp is removed from the gas space above the pulp column. A method according to claim 1, characterized in that the unbroken thick pulp column extends into the dilution zone at the bottom of the tower. A method according to claim 1, characterized in that the unbroken thick pulp column extends to the bottom of the tower or to a bottom fitted to a special tower. Process according to one of the preceding claims, characterized in that the gaseous bleaching agent is an overpressure oxygen gas. Method according to one of the preceding claims, characterized in that the pulp is fed to the top of the tower by means of a screw which maintains the pulp plug against the overpressure prevailing in the tower. Method according to one of the preceding claims, characterized in that the pulp is comminuted when introduced into the tower or immediately before it. Method according to one of Claims 1 to 5, characterized in that the pulp is comminuted as it moves downwards in the tower to the surface of an unbroken thick pulp column. Apparatus for carrying out the method according to claim 1, comprising a smooth or substantially smooth vertical reaction tower (1) on the inside, an inlet line (2, 7, 8) connected to the upper end of the tower for pulp, at least one gas inlet pipe (29), pulp grinder (4), one or more water supply lines (22) for diluting the mass i3 5 8 5 2 2, if necessary agitators (23) and at least one outlet (24) at the bottom of the tower, characterized in that the height of the part of the tower enclosing the unbroken mass column the concentration is 18-40%, preferably about 30% t is substantially larger than the diameter of the tower, but that the height of the tower section enclosing said pulp column, in meters, does not exceed 0.5 times the pulp concentration, calculated as a percentage by weight, and that the gas inlet pipe (29) is arranged in the lower part of the tower and that at least one line (31) is arranged in the upper part of the tower to drain a gas which cannot react with the mass. Device according to Claim 8, characterized in that a dilution zone (17) is arranged on the bottom of the tower, the part of the tower in which the pulp column is located being located above the dilution zone (17). Device according to Claim 8 or 9, characterized in that a reversible loose base (19) is arranged above the base of the tower, on which the pulp column rests. Device according to one of Claims 8 to 10, characterized in that the shredding element (4) is arranged immediately after the opening of the pulp supply line (2) in the tower. Device according to one of Claims 8 to 11, characterized in that the gas supply pipes (29) are arranged in the lower part of the tower part which surrounds the unbroken thick pulp column. Device according to one of Claims 8 to 11, characterized in that the gas supply pipes (29) are arranged above and below the surface of the unbroken thick pulp column.