US3660225A

US3660225A - Delignification and bleaching of cellulose pulp layers with oxygen gas

Info

Publication number: US3660225A
Application number: US840639A
Authority: US
Inventors: Abraham Jacob Verreyne; Leonard Austin Job; Paul Rerolle; Johan C F C Richter
Original assignee: Air Liquide SA; Kamyr AB
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Metso Fiber Karlstad AB
Priority date: 1968-07-11
Filing date: 1969-07-10
Publication date: 1972-05-02
Anticipated expiration: 1989-05-02
Also published as: DE1935067B2; ES369349A1; DE1935067A1; US3742735A; OA03094A; DE1935067C3; FI45473C; FI45473B; NO128336B; FR2012740A1

Abstract

A method of dividing a stream of pulp into a series of layers comprising discrete batches and progressively transferring the batches from layer to layer in controlled fashion so that the height of each layer does not exceed a maximum value at which the pulp at the bottom of a layer has a predetermined minimum gaseous content, the pulp being contacted with oxygen gas under pressure while progressively transferring pulp from one layer to the next in the series. The method may occur in an apparatus comprising a cylindrical pressure vessel including axially spaced floors which define chambers between them, the chambers being subdivided into a plurality of compartments by dividing walls disposed axially to the pressure vessel and transversely to the floors, each floor having an aperture and the compartments and floors being relatively movable to allow pulp in the compartments sequentially to be transferred from one chamber to the next as relative movement occurs.

Description

Verreyne et a1.

DELIGNIFICATION AND BLEACHING OF CELLULOSE PULP LAYERS WITH OXYGEN GAS lnventors: Abraham Jacob Verreyne, Petersfield; Leonard Austin Job, Bramley North, both of Republic of South Africa; Paul Rerolle, Nogent-sur Marne: .Iohan C. 1". C. Richter,

St. Jean Cap Ferrat. both of France Assignees: South African Pulp and Paper Industries Limited; LAlr Liquide, Societe Anonyse pour LEtude et LExploitation des Procedes Georges Claude and Aktiebolaget Kamyr Filed: July 10, 1969 Appl. No.: 840,639

Foreign Application Priority Data July 11, 1968 Sweden ..9540/68 July 15, 1968 Sweden... ..9689/68 [451 May 2, 1972 [56] References Cited UNITED STATES PATENTS 1,529,919 3/1925 Richter ..l62/65 X 2,431,478 11/1947 Hill l 162/1 7 3,298,900 l/l967 Laakso 162/17 3,492,199 l/197O Kindron et al 162/71 2,662,821 12/1953 Muench 162/237 Primary Examiner-S. Leon Bashore Assistant Examiner-Arthur L. Corbin An0rneyWaters, Roditi, Schwartz & Nissen [57] ABSTRACT A method of dividing a stream of pulp into a series of layers comprising discrete batches and progressively transferring the batches from layer to layer in controlled fashion so that the height of each layer does not exceed a maximum value at which the pulp at the bottom of a layer has a predetermined minimum gaseous content, the pulp being contacted with ox ygen gas under pressure while progressively transferring pulp from one layer to the next in the series. The method may occur in an apparatus comprising a cylindrical pressure vessel including axially spaced floors which define chambers between them, the chambers being subdivided into a plurality of compartments by dividing walls disposed axially to the pressure vessel and transversely to the floors, each floor having an aperture and the compartments and floors being relativelymovable to allow pulp in the compartments sequentially to be transferred from one chamber to the next as relative movement occurs.

11 Claims, 16 Drawing Figures PATENYEDHAY 2 1912 SHEET 2 a? 3 9 J 4 7 M w m X 0 m 1 a 6 4 jh enmnmr EA? w iwer 1 5 7 m .a 4 2 (W w o 3 A, i; 5 g t? M DELIGNIFICATION AND BLEACHING OF CELLULOSE PULP LAYERS WITH OXYGEN GAS This invention relates to the delignification and bleaching of cellulose pulps with oxygen gas under pressure.

For the purpose of this specification the term oxygen gas" includes any gas containing free oxygen, such as air.

it is known that a pulp can be delignified and bleached by subjecting it in an alkaline medium to the action of oxygen gas. French Pat. No. 1,387,853 discloses the treatment of a chemical pulp with oxygen gas under pressure in an alkaline medium in the presence of a catalyst or protector acting to preserve the physical and mechanical strength properties of the pulp. South African Pat. No. 67/3680 discloses a method of delignifying and bleaching an alkaline pulp by pretreating the pulp with an acidic medium and thereafter subjecting the pulp in an alkaline medium to the action of oxygen gas under pressure.

Delignification and bleaching agents other than oxygen gas are known and normally such other agents react with lignins in an aqueous medium. Generally the correct amount of these other agents can be dissolved in the aqueous phase of the wet pulp. Oxygen gas treatment must, however, be conducted under pressure and even then only small amounts of oxygen can be dissolved in water at such high oxygen partial pressures. As a result, it is necessary for the oxygen gas to be applied directly to the pulp mass in sufficiently large quantity if a satisfactory reaction is to be obtained.

it has been found that the amount of oxygen required for a satisfactory reaction can be accommodated by the pulp only if the bulk density of the pulp is sufficiently low and if substantially no free liquid drains from the pulp. The presence of drained liquid is detrimental in that it accumulates in certain areas where it occupies space that ought to be occupied by oxygen and excludes further oxygen from re-plenishing that which has been dissolved and then consumed. Drainage of liquid also results in unequal distribution of the alkali normally added and corresponding variation in the final bleaching of the pulp.

It has further been found that the design of conventional ulp bleaching towers are not satisfactory for the treatment of pulps with oxygen gas. Normally, such towers operate at relatively low consistencies with no free gas occluded in the pulp. Tests have shown that if such towers are operated at higher consistencies the pulp compresses under its own weight and liquid is inclined to collect at the bottom of such conventional towers so that the oxygen content of the pulp towards the bottom of such towers is too low for satisfactory reaction.

Although methods of repeated exposure of pulp to a gas are known such methods generally suffer the disadvantage of complex mechanisms with consequent excessive wastage of space between stages and difficulty of sealing against over pressure as well as high equipment costs.

Methods using a series of trays provided with rake arms causing transfer alternatively at the periphery and the center of the trays are known. These suffer the disadvantage that energy is used wastefully due to rollback of free flowing materials while sequential flow and control of residence time is poor as a consequence. The applicants have found by experiment that this type of apparatus is incapable of handling beds of pulp deeper than 0.4 meters in a manner capable of achieving good results by the oxygen bleaching process. As a result of this height limitation either the number of plough systems must be increased or the diameter of the pressure vessel must be increased. Both of these actions cause a high cost penalty.

It is accordingly an object of the present invention to provide improved treatment of cellulose pulp with oxygen gas under pressure with which the above disadvantages are at least minimized.

According to one aspect of the invention a method of treating a cellulose pulp with oxygen gas under pressure includes the steps of constraining the pulp for movement along a prescribed path, contacting the pulp with oxygen at a pressure in excess of atmospheric pressure during at least part of its movement along the path, dividing the pulp into a series of layers across the path, the height of each layer not exceeding a maximum value at which the pulp at the bottom of a layer has a predetermined minimum gaseous content; and progressively transferring pulp after a controlled residence period from one layer to the next in the series without increasing the height of any one layer beyond the maximum value.

Preferably the layers are located one above the other and the pulp is transferred from each layer to the next layer in the series under the influence of gravity.

The invention also includes the steps of introducing pulp at a high consistency of between 16 and 67 percent to the prescribed path and fluffing the pulp prior to dividing it into said series of layers.

The minimum amount of oxygen required at the bottom of a layer will depend on the number of layers in the series and the period of time the pulp is retained in each layer and may also depend on the temperature and pressure.

The pulp may be transferred continuously, but preferably the pulp is transferred in stepwise fashion from one layer to the next in the series and away from the last layer in the series.

Further according to the invention the layers are sub-divided in to a plurality of discrete subdivisions or batches. Preferably the batches are transferred from one layer to the next in a proper sequential manner without intermixing with other batches.

Pulp may be transferred away from the final layer in the series, re-united and discharged at a predetermined rate.

Preferably each quantity of pulp to be transferred through subsequent layers is determined towards the top of the path.

An advantage of such stepwise transfer of pulp is that the pulp may repeatedly be transferred through an atmosphere of oxygen gas, thereby to re-introduce further oxygen.

The pulp may preferably be contacted with oxygen at a partial pressure in excess of 3 bars absolute.

In a preferred arrangement, the pulp is contacted stepwise with oxygen substantially all the way along the said path. Preferably the pulp is continuously fed to and removed from the prescribed path.

The applicants have found that for satisfactory reaction between the pulp and the oxygen gas, the following are important,

a. The pulp should be at a consistency which is sufficiently high for the pulp to contain enough oxygen and so that there is no substantial drainage of liquid from the pulp;

b. The pulp is in a well divided form, such as in the form of noodles, or preferably in fiufied form since the liquid retention of flutfed pulp is better than that of noodle pulp and the bulk density of the former is lower than the latter;

c. The height of any pulp layer is not too great otherwise the pulp compacts under its own weight with a resultant expulsion of liquid and/or oxygen at the bottom of the layer.

The applicants tests have indicated the following:

a. That water begins to drain from softwood noodle pulp at a consistency of 15 percent under zero pressure and at a consistency of 18 percent under a pressure of a 2.40 meter high pulp layer.

b. The water begins to drain from hardwood noodle pulp at a consistency of 13 percent under zero pressure and at a consistency of 16 percent under the pressure of 2.4 meter high pulp layer.

c. That fluffed pulp is less inclined to drain water.

d. That for satisfactory reaction between pulp and oxygen gas, the pulp should contain at least 1 percent oxygen by weight on a dry pulp basis at the bottom of a layer of pulp. This condition is attained with different pulps with the following maximum layer heights under a selected typical reaction condition of l 70 p.s.i.g. and C and using pure oxygen:

Softwood noodle pulp at 20% consistency 3.00 meter. Softwood flufi'ed pulp at 18% consistency 3.00 meter. Hardwood noodle pulp at 18% consistency 3.00 meter. Hardwood fiulfed pulp at l8% consistency 3.00 meter.

The heights of these layers can be increased if the sidewalls exert any friction. However, the object is not to unnecessarily increase the friction.

In the light of the above results, the method of the present invention is preferably carried out with pulp at a consistency of not less than 18 percent in the case of hardwood noodle and fluffed pulps and of softwood fluffed pulp and with a pulp consistency of not less than 20 percent in the case of softwood noodle pulp. Fluffed pulp is to be preferred.

Furthermore, the heights of the various layers may be arranged so as not to exceed 3.00 meters and preferably not to exceed 1.20 meters, especially if lower operating pressures are used.

According to another aspect of the invention, pulp treating apparatus comprises a pressure vessel including a series of pulp trapping chambers so connected as to provide a pathway for the pulp to move gravitationally from chamber to chamber, at least one dividing wall projecting upwardly transverse to the floor of each chamber to divide the chamber into a plurality of compartments, means to permit relative movement of the floor and the dividing wall so as to effect to allow the wall to sweep over the floor about an axis, an inlet port for pulp to enter the chamber and an outlet port therefrom in the floor, the outlet port being characterized in a radial dimension substantially equal to the effective radial dimension of the chamber, and means to permit oxygen gas to contact the pulp during passage along the pathway.

Further according to the invention the chambers are defined in a cylindrical vessel which is sub-divided axially by spaced floors or decks extending substantially over the entire cross-section of the vessel.

Preferably, the decks are spaced so that the layers of pulp supported thereby do not exceed a maximum value at which the pulp at the bottom of a layer contains a predetermined minimum gaseous content as outlined above.

In order to more effectively introduce oxygen at each transfer level and in order to minimize any tendency for compaction, the chamber between the top two decks may be somewhat less in height than all subsequent chambers;

The chamber between the two top floors may thus be not more than 3.00 meters and preferably not more than 1.20 meters high, although for apparatus handling pulps well fluffed and not below 20 percent consistency this restriction need not apply.

Preferably the dividing wall extends over substantially the entire height of I the chamber. Thus the spaces above each deck are divided into a plurality of separate upright pulp holding compartments, each deck being arranged for pulp to be transferred sequentially from different compartments above it to the space below it. The pulp from a compartment above a deck may be transferred to a compartment below the deck and preferably, the pulp is transferred in a uniform manner without any hang-ups.

A plurality of dividing walls may be provided which radiate from a central axis to provide the plurality of pulp holding compartments.

In one arrangement, the dividing walls are, stationary and the floors of the chambers rotatable.

In another arrangement the dividing walls are rotated and the floors are stationary. Means for rotation may comprise a shaft extending axially in the pressure vessel, the floors or the dividing walls, as the case may be, being mounted on the shaft for rotation therewith.

In an alternative arrangement said means comprises a shaft extending axially in the pressure vessel and at least one scraper blade, or rake arm provided above each floor and fast with the shaft to rotate therewith.

Further according to the invention the lower regions of the dividing walls are so located relative to the chamber floor as to direct the pulp in a compartment towards the outlet port with minimal transfer of pulp between compartments in the same chamber.

The outlet port may comprise a substantially wedge shaped aperture in the chamber floor.

Preferably outlet ports of adjacent chambers are angularly off-set from one another with respect to the axis, so that substantially all the pulp passing into the compartment is retained for the controlled residence period.

Means may also be provided for fluffing pulp on introducing it to said pathway. Preferably a rotary feeding device is provided for distribution of pulp above the upper floor.

The pressure vessel may be provided with a pulp inlet towards its upper end and with a pulp outlet towards its lower end. The vessel may further be provided with one or more steam and oxygen gas inlets to maintain the temperature and pressure at a selected value such as C and p.s.i. g.

The pressure vessel towards its outlet may be provided with means to re-unite the pulp into a single column which may be washed, diluted, cooled and discharged from the vessel in known manner.

An advantage of the apparatus is that the shaft may enter the vessel at the bottom below the level of the diluted pulp in this way no glands operate in the gaseous phase thus simplifying the shaft sealing problem.

An added advantage is that the compartments are run substantially full which is an important factor in equipment operating under pressure.

For a clear understanding of the invention three preferred embodiments will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a vertical sectional view of one embodiment of a pulp treating apparatus according to the invention,

FIG. 2 is a cross-section on the line II-II in FIG. 1,

FIG. 3 is a cross-section on the line III-III in FIG. 1;

FIG. 4 is a cross-section on the line IVIV in FIG. 1;

FIG. 5 is a cross-section on the line V-V in FIG. 1;

FIG. 6 is a cross-section on the line VI-VI in FIG. 1;

FIG. 7 is a vertical sectional view of a second embodiment of a pulp treating apparatus according to the invention;

FIG. 8 is a cross-section on the line VIII-VIII in FIG. 7;

FIG. 9 is a cross-section on the line IX-IX in FIG. 7;

FIG. 10 is a crossasection on the line X-X in FIG. 7;

FIG. 11 is a cross-section on the line XIXI in FIG. 7;

FIG. 12 is a vertical sectional view with parts broken away of a third embodiment of a pulp treating apparatus according to the invention;

FIG. 13 is a cross-section on the line XIII-XIII in FIG. 12;

FIG. 14 is a cross-section on the line XIV-XIV in FIG. 12;

FIG. 15 is a cross-section on the line XV-XV in FIG. 12; and

FIG. 16 is a cross-section on the line XVI-XVI in FIG. 12.

Referring to FIGS. 1 to 6 reactor 1 comprises a cylindrical pressure vessel which is provided with a plurality of vertically spaced

pulp support decks

2a, 2b, 2c and 2d in the form of spaced discs, mounted on shaft 3 which is located co-axially within reaction vessel 1 and which is adapted to be rotatably driven by a motor (not shown). The number of decks may be increased to any convenient number and are not limited to 4 as in this example.

A plurality of vertically extending, radially disposed dividing walls 5 are located above each of the

decks

2a, 2b, 2c and 2d to divide the chamber above each deck into a plurality of upright compartments 6 disposed radially about shaft 3. The dividing walls 5 at the difierent levels are vertically aligned to provide corresponding compartments 6 at the different levels which are vertically aligned. The dividing walls 5 above the deck 2b are about 1.20 meters high and the dividing walls 5 above the

decks

2c and 2d are about 1.30 meters high. The height of the chamber above deck 2a is more than 1.20 meters in order to be able to provide the pulp layer of 1.20 meters in the chamber above deck 2b.

As can be seen from FIGS. 2 to 6 of the drawings, each deck is provided with an aperture 7 which may for instance conform to the cross-sectional configuration of the compartments 6, the apertures of the various decks being angularly off-set preferably by an angle greater than a but usually less than 2a where (1 corresponds to the angle subtended by one compartment 6 at the axis. The angle is so adjusted that all the pulp passing into the compartment is retained for the controlled residence period.

At its upper end, reaction vessel 1 is provided with a pulp inlet 8 and a rotary feeding device 9 which is mounted on shaft 3 and which is adapted to direct incoming pulp into the various compartments 6 above upper deck 2a. Device 9d is provided to fluff incoming pulp. At its lower end, reaction vessel 1 is provided with dilution nozzles 14 and with a scraper and extractor 10 for discharging treated pulp through outlet 1 1. The reaction vessel 1 is further provided at its upper end with oxygen and steam inlets 12 and at its lower end with oxygen inlets 13.

In operation, heated pulp plus chemicals and oxygen plus steam are introduced into reaction vessel 1 at its upper end through inlets 8 and 12 respectively. The oxygen is admixed with the pulp by the fluffing device and then directed into compartments 6 above upper deck 2a by feeding device 9. The pulp may be premixed with oxygen before entering reaction vessel 1, if necessary. By rotatably driving shaft 3, the

decks

2a, 2b, 2c and 2d are rotated so that the aperture 7 in each deck passes successively underneath the compartments 6 above such deck. It will be appreciated that as each deck rotates, a batch of pulp equal in quantity to the layer supported by deck 2b drops out of each successive compartment 6 above the deck into the corresponding compartment below the deck. Once the aperture 7 in a deck has passed a compartment 6 above the deck, the pulp in that compartment 6 above the deck remains substantially undisturbed until the aperture 7 reaches that compartment again during the next revolution of the deck. In this manner pulp is transferred compartment by compartment in stepwise fashion from the upper end of the pressure vessel 1 towards the bottom of the vessel from where treated pulp is discharged through outlet 11. It will be appreciated that the pulp contained in reaction vessel 1 is divided into a plurality of batches, on each of the

decks

2a, 2b, 2c and 2a by the dividing walls.

The speed of rotation of shaft 3 and therefore, of the decks, is dictated by the total height of the superimposed layers of pulp in reaction vessel 1 and the required reaction time. It will be appreciated that in the above example during each revolution less an angle equal to the angle of off-set of the decks, the pulp moves down about 1.20 meters, i.e., the height of the pulp layer above deck 2b. Where, for example, the total height of pulp in the reaction vessel is 15 meters; and the required reaction time is minutes, and the pulp is required to move downwardly at a speed of 0.5 meters per minute, the shaft speed would be 0.5/1.2 0.41 r.p.m.

It will be appreciated that the reaction time can be changed by changing the speed of rotation of the shaft and thus of the decks. In that event, it may also be necessary to change the rate of pulp introduction into and extraction from the reaction vessel for satisfactory continuous operation.

Referring to FIGS. 7 to 11, 21 designates an elongated upright essentially circular cylindrical vessel widened at its lower end at 22. The vessel is pressure-resistant in order to withstand an inner pressure of the order of 150 p.s.i.g. A continuous feed for cellulose pulp is provided of which merely supply tube 23 connected to the side of the vessel top is shown. The vessel 21 is further provided at its lower end with means for continuous discharge of treated pulp which comprise an outlet conduit 24 opening into the center of the vessel base and having associated therewith a blow valve 25, and a rotary scraper 26 for forwarding pulp towards that outlet conduit, the scraper operating close to the vessel base and over its entire cross-section. Pulp discharge is facilitated by its dilution with water or filtrate supplied through a conduit 27 and spread by means of nozzles 28 located in the vessel, well immediately above the base thereof. The scraper 26 is attached to a shaft 29 which is driven by a motor 30 coupled to gear box 31. The shaft, which passes through the vessel base extends along the entire length of the vessel 21 and has its upper end joumalled in a bearing 32 located inside the top of the vessel.

Over the greater part of its length the shaft 29 fonns a tube 33. Between the upper end of said tube and a cylindrical wall 33 attached thereto there is formed an annular tank 34 to which washing liquid preferably heated water, is supplied through a conduit 35 in a quantity controlled by the valve 36. Through

conduits

37 and 38 in the tube 33 and through cavities in the arms of the scraper 26 the washing liquid is supplied to spray tubes attached to the said arms which have nozzles 39 through which the washing liquid is spread between a set of hollow annular screen bodies 40 and 41 having cylindrical screen faces. By means of hollow radial arms 42 and outlets 43 the cavities of the said screen bodies communicate with an exterior conduit 44 for discharging the liquid displaced by the washing liquid which is screened off the pulp.

Connected to the upper end of the container is a conduit 45 for the supply of oxygen and a conduit 46 with valve 69 for the supply of hot water, e.g., of a temperature of 130 C, in controlled quantities. The conduit 46 is connected to a conduit 47 extending through the tube 33 and leading to one or more radially extending spreader tubes 48 having orifices distributed so that the hot water is spread over the entire crosssection of the container as evenly as possible. To the top of the container there is also connected a circulation conduit 49 with a low pressure pump 50 and a control valve 51. Through the last-mentioned conduit oxygen may be pumped into the conduit 52 and out through the orifices of the spreader tubes 48 together with the hot water.

inserted at different levels in the upper part of the container are

decks

55, 56, 57, 58 which are essentially horizontal but may be somewhat inclined towards the center. The decks are attached to the shell of the container and extend close to but are free from the rotary tube 33, where they are bordered by an upwardly directed cylindrical collar 59. Sector-shaped

apertures

65, 66, 67, 68 in the floors serve as passages for pulp.

Close to the decks there are pulp feeding means which are shaped as rake or scraper arms 70 attached to the tube 33 and extending radially therefrom. The scraper arms may be somewhat curved and/or turned or may be shaped as shovels and are arranged to move at different levels in the pulp layers collected upon the decks so that in addition to feeding pulp through the sector-shaped apertures they also serve to effect a certain stirring of the pulp. The apertures of the decks are angularly displaced relatively to each other so that the pulp dropping therethrough is caught by the deck located directly below.

Before the pulp to be bleached with oxygen is introduced into the above described reaction vessel, it is subjected to a pre-treatment consisting in impregnation thereof with NaOH and preferably a catalyst, preferably MgCO by immersion into a mixture thereof. Then the pulp is concentrated to a consistency of 12-30 percent but preferably below 20 percent and most preferably above 16 percent. It is important that chemicals are uniformly mixed into the pulp. The temperature of the pulp during the impregnation is preferably maintained at about C. By means of a high density pump the pulp is introduced into the vessel 21 and the pressure is maintained at about 150 p.s.i.g. and is additionally heated to approximately C, by supplying fresh steam. While being introduced into the vessel 21 the pulp is also loosened or broken up so that the pulp forms fiber bundles the greater part of which have a minimum cross-section size lying below 5 mm, preferably below 2 mm. The upper part of the container is filled with oxygen gas at the above pressure and said gas penetrates into the whirling or falling loose pulp before these are collected upon the uppermost deck 55. Continued access of oxygen to the interior of the pulp layer formed upon the deck 55 is facilitated by the agitation by means of the scraper arms 70. This feeds the pulp gradually towards the aperture 65, from which the pulp drops freely to the next deck 56. During said free gravity fall the pulp is spread again so that the oxygen has access to the spaces between the individual particles of the pulp and can penetrate into the pores thereof. Moreover, the pulp is subjected to a re-arrangement which is favorable for an even treatment of the same. In this manner the pulp moves in a series of drop steps through an upper treatment zone including the decks.

Below said zone the pulp is united into a coherent column which fills up the central and lower part of the vessel. The level 71 is maintained constant by controlling the feed and discharge of pulp into and out of the vessel.

The spaces between the particles of the pulp column are, at the lower part thereof, completely filled with the washing liquid supplied through the conduit 47 and more or less incompletely filled at the upper part thereof. By means of the pump 50 oxygen is forced out through the orifices of the rotary spreader arms 48 and therefore, in the treatment zone above said arm oxygen bubbles up through the pulp column. The oxygen not absorbed reaches the gas space and is recirculated.

In the next lower zone the reaction continues between the finally supplied oxygen and the pulp diluted with washing water. While using up essentially all oxygen in the pulp the bleaching reaction is finished in this zone.

Finally washing of the pulp takes place in the lowest zone of the vessel. Washing liquid supplied through the nozzles 39 displaces the treating liquid containing the reaction products. Such liquid, which still is of a temperature of about 130 C, is displaced out through the screens 40 and 41 and the conduit 44, to a pressure releasing tank 90. From the top outlet 91 thereof steam and remaining oxygen, if any, are let out and after condensing the steam, the oxygen may be returned to the vessel 21. Liquid leaving through the bottom outlet 92 is evaporated and transferred to a chemical recovery plant, preferably together with spent liquor obtained from the digestion of the pulp. The washed pulp is cooled and diluted with water supplied through the nozzles 28 and the pulp is discharged at the bottom of the container at a temperature below boiling temperature and its pressure is released to atmospheric pressure while passing the valve 25.

By using the above described apparatus bleaching of pulp by oxygen can take place at a high consistency and at a moderate pressure in excess of atmospheric pressure with great efficiency.

In the final embodiment shown in FIGS. 12 to 16 the apparatus is identical to that illustrated in FIGS. 7 to 11 except that the means for feeding pulp through the apertures 65 to 68 of the decks 55 to 58 is different.

In this arrangement feeding means are designed as radially extending plane

vertical vanes

101, 102, 103 and 104 having such a great height that they cover almost the entire space between the decks. The vanes are attached to the rotary shaft or tube 33 and extend to the vicinity of the cylindrical shell 21 of the vessel. A set of vanes is arranged above each deck and in each set the vanes are equal, in shape and height, but are angularly disposed as is shown in FIGS. 13 to 16. Above each deck the space is divided by said vanes into a number of sector-shaped compartments 105, in which the pulp is received. Stirring of the pulp is thus avoided when the shaft turns although the entire pulp layer resting upon a deck is moved by the vanes in a horizontal circular path.

As each compartment 105 between the vanes 101 of the upper set reaches the aperture 65 of the deck 55 the pulp batch in the compartment is stepwise emptied therethrough, the pulp dropping upon the next lower deck 56 and being collected in the compartments 105 between the vanes 102 arranged above said deck 55. The pulp drops by gravity in a stepwise cascade so that the pulp is spread or loosened and oxygen in the container has access and can penetrate evenly into the smaller spaces and pores of the pulp before the same forms a horizontal layer again.

The apertures of the decks are sector-shaped and preferably are congruent to the sectors of the above mentioned compartments. Furthermore, as mentioned the sector-shaped apertures of adjacent decks are angularly displaced relatively to each other, so that the pulp drops in a number of steps equal to the number of floors. The angular displacement is so chosen with regard to the direction of rotation of the shaft 33 that the compartments first pass over the aperture of the deck below for emptying the compartments and then pass immediately below the aperture of the deck above the compartments for re-filling thereof. Therefore, the pulp will rest upon each deck for a retention period corresponding to a rotary movement of the shaft of at least about 270.

We claim:

1. A continuous method of treating a cellulose pulp with oxygen gas under pressure comprising the steps of continuously introducing pulp at a consistency of l6 to 67 percent into the upper region of a vertically elongated pressure vessel; heating the pulp; distributing the heated pulp in well divided form in a series of separate layers individually supported within the pressure vessel at different levels along the length of the pressure vessel; progressively transferring pulp downwardly from one layer to the next in the series after a residence period in each layer; contacting the pulp in each layer with oxygen gas at a partial pressure in excess of 3 bars absolute; restricting the height of at least certain of the layers to a maximum value at which compaction of pulp under its own weight at the bottom of the layer is limited to a maximum value at which the pulp at the bottom of the layer has at least a predetermined minimum oxygen gas content; and continuously withdrawing treated pulp from the lower region of the pressure vessel.

2. A method as claimed in claim 1, wherein the pulp is fluffed prior to being distributed in layers.

3. A method as claimed in claim 1, wherein the pulp is transferred from one layer to the next in the series and away from the last layer in the series under the influence of gravity.

4. A method as claimed in claim 1, wherein pulp is transferred sequentially in stepwise fashion from one layer to the next in the series and away from the last layer in the series.

5. A method as claimed in claim 1, wherein the pulp in each layer is subdivided into a plurality of discrete batches which are transferred sequentially in stepwise fashion from one layer to the next in the series and away from the last layer in the series substantially without intermixing with other batches.

6. A method as claimed in claim 5, wherein the support for each layer of pulp is removed sequentially from underneath successive batches of pulp in the layer to permit the sequential transfer of successive batches of pulp from one layer to the next under the influence of gravity, the transfer of pulp from one layer to the next being controlled for a batch of pulp transferred from one layer to the next to occupy free space in said next layer which has previously been vacated by a batch of pulp transferred away from said next layer.

7. A method as claimed in claim 1, wherein the pulp in each layer is collected in a plurality of separate compartments as discrete batches which are transferred sequentially in stepwise fashion from one layer to the next in the series and away from the last layer in the series substantially without intermixing with other batches, new batches of pulp being collected sequentially in successive compartments in the first layer in the series to replace batches which are transferred from the first to the second layer in the series.

8. A method as claimed in claim 1, wherein the height of each layer with the possible exception of the first layer in the series does not exceed 3.00 meters.

53. A method as claimed in claim 7, wherein the height of each layer with the possible exception of the first layer in the series does not exceed 1.20 meters.

10. A continuous method of treating a cellulose pulp with oxygen gas under pressure comprising the steps of continuously introducing pulp at a consistency of 16 to 67 percent into the upper region of a vertically elongated pressure vessel; breaking up the pulp into well divided form; heating the pulp; distributing the heated and well divided pulp in a series of separate layers which are individually supported within the pressure vessel at different levels along the length of the pressure vessel, each layer of pulp comprising a plurality of discrete batches sequentially removing the support for each layer from underneath successive batches of pulp in the layer to permit the sequential transfer in stepwise fashion of successive batches of pulp from one layer to the next in the series and away from the last layer in the series under the influence of gravity substantially without intermixing with other batches; controlling the transfer of pulp from one layer to the next to permit a batch of pulp transferred from one layer to the next to occupy free space in the next layer which has previously been vacated by a batch of pulp transferred from said next layer; restricting the height of each layer of pulp with the possible exception of the first layer in the series not to exceed 3.00 meters; contacting the pulp in each layer with oxygen gas at a partial pressure in excess of 3 bars absolute; re-uniting in the lower region of the pressure vessel batches of pulp which have been transferred away from the last layer in the series; and continuously withdrawing treated pulp from the lower region of the pressure vessel.

11. A continuous method of treating cellulose pulp with oxygen gas under pressure comprising the steps of continuously introducing pulp at a consistency of 16 to 67 percent into the upper region of a vertically elongated pressure vessel; breaking up the pulp into well divided form; heating the pulp; distributing the heated and well divided pulp in a series of separate layers individually supported within the pressure vessel at different levels between the top and the bottom of the vessel; progressively transferring pulp downwardly in stepwise fashion under the action of gravity from one layer to the next in series; restricting the height of each layer of pulp with the possible exception of the first layer in the series not to exceed 3.00 meters; contacting the pulp in each layer with oxygen gas at a partial pressure in excess of 3 bars absolute; and continuously withdrawing treated pulp from the lower region of the pressure vessel.

Claims

2. A method as claimed in claim 1, wherein the pulp is fluffed prior to being distributed in layers.
3. A method as claimed in claim 1, wherein the pulp is transferred from one layer to the next in the series and away from the last layer in the series under the influence of gravity.
4. A method as claimed in claim 1, wherein pulp is transferred sequentially in stepwise fashion from one layer to the next in the series and away from the last layer in the series.
5. A method as claimed in claim 1, wherein the pulp in each layer is subdivided into a plurality of discrete batches which are transferred sequentially in stepwise fashion from one layer to the next in the series and away from the last layer in the series substantially without intermixing with other batches.
6. A method as claimed in claim 5, wherein the support for each layer of pulp is removed sequentially from underneath successive batches of pulp in the layer to permit the sequential transfer of successive batches of pulp from one layer to the next under the influence of gravity, the transfer of pulp from one layer to the next being controlled for a batch of pulp transferred from one layer to the next to occupy free space in said next layer which has previously been vacated by a batch of pulp transferred away from said next layer.
7. A method as claimed in claim 1, wherein the pulp in each layer is collected in a plurality of separate compartments as discrete batches which are transferred sequentially in stepwise fashion from one layer to the next in the series and away from the last layer in the series substantially without intermixing with other batches, new batches of pulp being collected sequentially in successive compartments in the first layer in the series to replace batches which are transferred from the first to the second layer in the series.
8. A method as claimed in claim 1, wherein the height of each layer with the possible exception of the first layer in the series does not exceed 3.00 meters.
9. A method as claimed in claim 7, wherein the height of each layer with the possible exception of the first layer in the series does not exceed 1.20 meters.
10. A continuous method of treating a cellulose pulp with oxygen gas under pressure comprising the steps of continuously introducing pulp at a consistency of 16 to 67 percent into the upper region of a vertically elongated pressure vessel; breaking up the pulp into well divided form; heating the pulp; distributing the heated and well divided pulp in a series of separate layers which are individually supported within the presSure vessel at different levels along the length of the pressure vessel, each layer of pulp comprising a plurality of discrete batches sequentially removing the support for each layer from underneath successive batches of pulp in the layer to permit the sequential transfer in stepwise fashion of successive batches of pulp from one layer to the next in the series and away from the last layer in the series under the influence of gravity substantially without intermixing with other batches; controlling the transfer of pulp from one layer to the next to permit a batch of pulp transferred from one layer to the next to occupy free space in the next layer which has previously been vacated by a batch of pulp transferred from said next layer; restricting the height of each layer of pulp with the possible exception of the first layer in the series not to exceed 3.00 meters; contacting the pulp in each layer with oxygen gas at a partial pressure in excess of 3 bars absolute; re-uniting in the lower region of the pressure vessel batches of pulp which have been transferred away from the last layer in the series; and continuously withdrawing treated pulp from the lower region of the pressure vessel.
11. A continuous method of treating cellulose pulp with oxygen gas under pressure comprising the steps of continuously introducing pulp at a consistency of 16 to 67 percent into the upper region of a vertically elongated pressure vessel; breaking up the pulp into well divided form; heating the pulp; distributing the heated and well divided pulp in a series of separate layers individually supported within the pressure vessel at different levels between the top and the bottom of the vessel; progressively transferring pulp downwardly in stepwise fashion under the action of gravity from one layer to the next in series; restricting the height of each layer of pulp with the possible exception of the first layer in the series not to exceed 3.00 meters; contacting the pulp in each layer with oxygen gas at a partial pressure in excess of 3 bars absolute; and continuously withdrawing treated pulp from the lower region of the pressure vessel.