The present invention relates to a method and to an arrange- ment pertaining to the bleaching of pulp with pressurized peroxide, preferably chlorine-free bleaching of chemical pulp in association with its production, wherein a specific digester technique (steam/vapour phase digester) is used to achieve a relatively inexpensive and well-functioning pressurized peroxide stage.

It is known to delignify (no bleaching) cellulose pulp at an overpressure (superatmospheric pressure) . What is described is a so-called 0 ₂ stage, see for instance US-A 3,843,473. In the case of oxygen-gas delignification, the pulp moves upwards in the tower.

Chlorine-free bleaching with the aid of hydrogen peroxide is known from Swedish Patent 8902058-0 (EKA Nobel AB) , in which the so-called Lignox process is described.

Our Swedish Patent Application 9301960-2 describes the advantageous effects that are achieved when the peroxide bleaching stage is carried out at elevated temperatures and elevated pressures, these advantages including a considerably shorter residence time, among others. Despite this shortening of the residence time (in comparison with processes carried out at atmospheric pressure) , peroxide bleaching requires a relatively long pulp residence time in the vessel during the bleaching process.

However, the higher the pressure the greater the extent to which the requisite residence time can be shortened. In present times, static mixers are used in the pressurized stages to mix-in steam with the intention of raising the temperature, which results in an undesirable pressure drop. Furthermore, it is normally necessary to supply steam at an

intermediate pressure, normally a pressure of 10-13 bars, (which is relatively expensive) in order to achieve the temperature desired, i.e. a temperature above 90°C and preferably above 100°C.

It has surprisingly been found that higher pressures can be achieved by means of a novel inventive method and an inven¬ tive arrangement for carrying out the method.

One object of the inventive method is to provide optimal operating conditions from an energy aspect and cost aspect, among other things.

This object is achieved in accordance with the invention preferably by using a standing reactor, preferably a tall and narrow reactor (a so-called tower) into which the pulp is introduced at its upper end. In this regard, a Kvaerner's vapour phase digester or some similar vessel having a corresponding function and essentially corresponding dimen- sions, preferably exceeding 3 m ² , can be used in the bleach¬ ing process. By proceeding in this way, the diameter of the reactor vessel will be implicitly large at higher production levels.

A further object of the invention is to obtain a so-called plug flow in the reactor during the process. In order to achieve this the level of pulp at the top of the reactor should be as uniform as possible. By plug flow is meant a flow in which each part of the flow moves as a solid body under translatory motion, i.e. in the case of a tall and narrow vessel the flow properties will vary along the vessel. This can be achieved in accordance with the inventive method by using in the arrangement a pulp distributing or spreading device which functions to distribute the pulp over essen- tially the whole of the cross-sectional area of the contain¬ er, and by introducing the pulp at an appropriate rate.

In accordance with the invention, this is achieved preferably with the aid of a spreader provided at the top of the reactor vessel, preferably a pulp spreader or distributor of the kind described in our own Patent Application 9303019-5. The spreader functions to spread the pulp delivered to the vessel in a manner to form a generally uniform layer over the surface in the reactor, and it is possible to maintain a desired type of flow in the reactor by feeding-in the pulp generally uniformly. The arrangement includes a pulp-spread- ing impeller which includes pulp-spreading blades or vanes and which is rotatably mounted in the vessel, impeller drive means, and an inlet for delivering pulp to the impeller. The arrangement may optionally also include a further inlet for the optional introduction of chemicals.

Another object of the invention is to reduce the pressure requirement on the chemical pumps to the lowest possible level, therewith affording advantages with respect to both power and maintenance.

This object is achieved in accordance with the invention by delivering the necessary bleaching chemicals, with the exception of the peroxide, for instance to the upper end of a stand pipe arranged upstream of the reactor in which the bleaching process takes place, prior to having reached the system pressure, and therewith admixing the chemicals with the pulp to be bleached with the aid of an MC pump, for example. If necessary, further devices may be provided for mixing the supplied chemicals with the pulp. The peroxide is preferably delivered to the bottom end of the same stand pipe, prior to the pulp passing an MC pump. This positioning of the supply is advantageous from many aspects.

The further mixing equipment can be placed upstream of the reactor, in the conduit that leads to the top of the reactor vessel or alternatively in a pre-reactor, and the admixture of the chemicals can be effected at the top of the reactor

with the aid of the aforedescribed pulp-spreading arrange¬ ment.

A further object of the invention is to enable the desired temperature to be reached with the aid of low pressure steam, i.e. steam at a pressure lower than 6 bars ^<e) , preferably lower than 5 bars ^(e) , normally 3-5 bars ^(e) .

This is achieved in accordance with the present invention by supplying steam at the upper end of the reactor until a suitable temperature and preferably until a suitable system pressure is/are reached. The pressure can be adjusted in relation to the temperature by also delivering compressed air at the same location. In addition, the pulp will exert its own pressure on that part of the pulp located further down in the reactor, and consequently a gradual increase in pressure will be achieved as the pulp passes through the reactor from its upper end to its bottom end, which can be advantageous in certain cases.

It has also surprisingly been found that the steam delivered when practicing this special technique in accordance with the invention can be so-called low pressure steam as opposed to the technique used at present in which steam at intermediate pressure is used for the purpose of heating the pulp to a desired temperature (preferably above 100°C) prior to the bleaching process. Intermediate pressure steam is expensive whereas low pressure steam is much cheaper, and consequently the use of low pressure steam constitutes an important advantage. The intermediate pressure steam earlier used has also been used to raise the system pressure in addition to heating the pulp, although as distinct from the present invention, the delivery of the bleaching charges has earlier taken place within that part of the bleaching arrangement in which a desired bleaching pressure has been maintained.

When the pulp has passed down through the reactor vessel, it

is discharged from the bottom of the vessel by the system pressure and with the aid of preferably a conventional bottom scraper.

Final degassing of the pulp is preferably carried out in a downstream stand pipe provided with a flue stack.

Thus, the inventive method and the inventive arrangement enable the use of mixing devices which generate unnecessary drops in pressure to be avoided. Limitations to reactor pressure caused by said devices can thus also be avoided.

The inventive method and inventive arrangement also enable intermediate pressure steam to be replaced with low pressure steam. Because when practicing the inventive method pulp is introduced into the reactor vessel from above, the pressure of the vessel contents is increased to a significant extent such that low pressure steam will suffice in achieving desired process conditions. The inventive method is thus able to considerably reduce operation costs.

When practicing the inventive method and using the inventive arrangement, there is also obtained a highest pressure at the end of the bleaching process. Furthermore, when practicing the inventive method, it is not necessary to supply gas in order to maintain the pressure, although oxygen gas or some other gas, for instance ozone, can be supplied together with the steam to the gas phase at the top of the reactor, if considered desirable from a process aspect.

Because the inventive method and the inventive arrangement for carrying out the method simplify the admixture of chemicals with the pulp, less demanding mixer equipment is required, which in turn lowers the specific energy input required to effect the mixing process.

The invention will now be described with reference to the

accompanying drawings, in which

Fig. 1 illustrates a first arrangement for carrying out the inventive method; Fig. 2 illustrates a second arrangement for carrying out the inventive method; Fig. 3 is a partial cross-sectional view of the upper part of a paper pulp treatment container having mounted on the top thereof a first embodiment of a pulp-spreading device in accordance with the invention; Fig. 4 is a partial cross-sectional view of the upper part of a paper pulp treatment container having mounted on the top thereof a second embodiment of a pulp-spreading device in accordance with the invention; and Fig. 5 is a view of the inventive arrangement from above.

Like components in the two alternative embodiments of the invention have been identified in the two Figures with like reference numerals.

The arrangement shown in Fig. 1 includes a stand pipe 101 for feeding chemicals to the pulp. Mounted at the bottom end of the stand pipe is an MC pump 102 which, in addition to its standard function, also admixes the chemicals with the pulp. A peroxide inlet is provided at the bottom end of the stand pipe 101. The pulp is fed from the MC pump to the upper end of a reactor 103, where it is introduced into the reactor. Pulp feed is controlled by a valve 112. A connector or connectors is/are provided at the upper end of the reactor for the delivery of steam and optionally also oxygen gas. The delivery of steam to the reactor is controlled by a tempera¬ ture sensor unit TIC which is intended to sense the tempera- ture in the upper end of the reactor. The temperature sensor unit coacts with a valve 111 incorporated in the steam delivery conduit. A mixing device 104 is mounted at the upper

end of the reactor, this mixing device preferably being of the kind described in Swedish Patent Application 9303019-5. The normal level of pulp in operation is marked at the upper end of the reactor 103.

A pulp outlet is included at the lower end of the reactor 103, and a conventional bottom scraper is provided internally of the reactor. All of the pulp or part of the pulp may optionally be returned to the top of the reactor, through a conduit which extends from the reactor outlet to the upper part of said reactor (not shown) . The normal pulp level is shown at 105, 205 in the Figures.

Located downstream of the reactor 103 is a further stand pipe 107, which is provided with a flu stack or chimney 108 for degasification of the pulp. Incorporated in the conduit extending from the reactor to said stand pipe is a valve which coacts with a liquid indication device LIC, so as to be able to control the level of pulp in the reactor 103 so that pulp will not be conveyed from the reactor to the stand pipe 107 for degasification in unduly excessive quantities and thereby change the reaction conditions in the reactor in an undesirable manner.

The arrangement illustrated in Fig. 2 includes the components described above with reference to Fig. 1, and further includes a head box 109 provided with an inlet for unbleached pulp, and a pre-reactor 206 which is filled hydraulically. The purpose of the pre-reactor is partly to increase the effective time for the chemicals already supplied to the stand pipe 201 prior to adding optional substances in the reactor 203, and partly to enable the desired reaction conditions in the reactor 203 to be achieved by appropriate dimensioning of the pre-reactor 206. The hydraulically filled pre-reactor 206 can also act as a buffer in achieving a uniform pulp flow to the reactor. The pre-reactor 206 is thus able to assist in adjusting reaction conditions in the

reactor to an optimum.

The arrangement illustrated in Fig. 2 also includes the possibility of delivering steam to the head box 209, this supply being controlled through the medium of a temperature sensing device TIC mounted in the feed conduit extending from the stand pipe 201 to the pre-reactor 206, this temperature sensing device coacting with a valve 212 for delivering steam to the head box 209. A valve for delivering pulp from the stand pipe 201 to the pre-reactor 206 is provided downstream of an MC pump 202 in the same feed conduit.

The preferred device for spreading pulp at the upper end of the reactor in accordance with the aforegoing is a device primarily intended for storage towers, buffer tanks or reactors. The device is intended to be mounted centrally at the top of the container. The device is constructed to distribute or spread the pulp uniformly over the whole of the cross-sectional area of the container, which may be a tower, a tank or a reactor. This uniform spreading of the pulp over the cross-sectional area of the container results in a uniform outfeed speed (plug flow) , thereby eliminating the risk of channelization and enabling the reactor residence time to be used effectively.

Figs. 3 and 4 illustrate respectively two embodiments of the pulp-spreading device mounted at an upper part of a container 301, 401 for storing or for preparing paper pulp. Pulp is delivered to the container 301, 401, which in this case is a reactor, through a conduit 302, 402 which discharges through an upwardly directed inlet 303, 403 (spillway) at the top of the container. A spreader impeller 304, 404 is rotatably mounted on a drive shaft 305, 405, which in the case of the illustrated embodiment is journalled in the upper part of the respective container 301, 401 and driven by a motor 307 or by a frequency controlled motor 407, via a transmission 306 or a coupling 406. Frequency control of the

motor is significant to the mode of operation of the pulp- spreading device.

Also shown in Fig. 4 is a conduit 409 which discharges into a nozzle 410, which is mounted so that the nozzle will open into the conduit 402 and is preferably directed towards the inlet 403. In this case, the impeller 404 can be used to distribute a supplementary substance delivered to the pulp present in the container, such as a chemical substance or some liquid substance. The substance may be distributed either directly over the pulp in the container or together with pulp as the pulp is distributed in the container. Conceivable chemicals that can be charged through the conduit 409 are anti-foaming agents, such as EDTA (chelating agent) , bleaching agents such as peroxide (e.g. H ₂ 0 ₂ ) or pH adjusters such as NaOH or H ₂ S0 ₄ , etc. The nozzle, may also be rotatable or several pipes provided with nozzles or jets may be disposed along the conduit 402 to enable earlier mixing of the supplementary charge in the pulp. Another alternative is to connect the conduit 409 directly to the hub of the impeller from beneath, so that the supplementary charge will be delivered directly via openings (not shown) provided in the impeller.

Respective impellers 304 and 404 are described below in more detail with reference to Fig. 5. As will be seen, the impeller includes a hub 1 which is fixedly mounted on respective drive shafts 305 or 405, preferably by means of a wedge joint shown at 2. First spreader blades or vanes 10, which preferably have a surface 11 which is curved towards the direction of rotation, are mounted on the drive shaft at an angle α to the tangent of the outer surface of the hub, so that the pulp will be spread outwardly towards the periphery of the container. Second spreader blades or vanes 12, which also preferably have a surface 13 that curves towards the direction of rotation, are mounted on the drive shaft at preferably the same angle α. The angle α is

preferably chosen to achieve suitable spreading or distribu¬ tion of the pulp radially outwards in accordance with the configuration of the surfaces 11 and 13, which are preferably curved but which may alternatively have some other configura- tion, for instance a planar configuration. The first and the second blades 10 and 12 are mounted on the hub in pairs, such that the second blade 12, which - to avoid the build-up of fibres - may be shorter than the first blade 10, is prefera¬ bly placed before the first spreader blade 10 as seen in the direction of rotation. Although two pairs of hub-mounted spreader blades are shown in the Figure, it will be under¬ stood that the number of blades may be varied within the scope of the invention.

Each blade 10, 12 thus has a respective surface 11, 13 which faces towards the direction of rotation and which is config¬ ured and mounted relative to the hub in a manner to spread the pulp outwards towards the periphery of the container as respective impellers 304, 404 rotate. The respective surfaces 11, 13 have a radius of curvature which lies between 100-300 mm, and preferably between 150-200 mm.

So that pulp will also be spread or distributed towards the centre of the container 301/401, the pulp-spreading device includes a baffle 14 whose one end 15 may be connected to the outer end 16 of a respective first blade 10. Each baffle extends arcuately in the direction of rotation and has its centre of arc in the centre of the hub 1, such that the other end 17 of the baffle 14 will preferably extend beyond the outer end 18 of a respective second blade 12. Part of the pulp that is thrown outwardly by the second blade 12 will thus be deflected by the baffle 14 and therewith spread and distributed towards the centre of the container, so as to distribute pulp over the whole of the cross-sectional area of the container.

A generally radially extending reinforcement plate 19

connects the baffle 14 to the outer end 18 of the second blade 12. This reinforcement plate is thus placed generally centrally of the upper and lower edges of the blades on the side of the second blade 12 facing towards the direction of rotation, so as to provide more stability to the blades 10 and 12 and to the baffle 14 in operation.

The inlet 303/403 opens out immediately beneath the impeller 304/404, whereby the pulp exiting from the inlet is sliced and thrown-out and spread uniformly over the cross-sectional area of the container. The speed at which the impeller rotates and the vertical distance between impeller and the outlet orifice of the pulp tube are adapted to ensure uniform pulp distribution.

The component parts of the impeller are preferably made of metal and welded together. In the case of one preferred embodiment, the outer diameter of the impeller 304/404 is about 100 mm larger than the diameter of the outlet orifice of the pulp inlet 303/403.