AU595842B2

AU595842B2 - Pulp bleaching

Info

Publication number: AU595842B2
Application number: AU64671/86A
Authority: AU
Inventors: Jack A. Davidson; Derek Hornsey; Robert Gum Hong Lee; Arthur S. Perkins
Original assignee: Canadian Liquid Air Ltd
Current assignee: Canadian Liquid Air Ltd
Priority date: 1985-11-15
Filing date: 1986-11-03
Publication date: 1990-04-12
Anticipated expiration: 2006-11-03
Also published as: AU6467186A; NZ218294A; EP0226495B1; PT83745B; PT83745A; BR8605636A; EP0226495A1; DE3672580D1

Description

Melbourne, Australia *i i

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: 6'tfC 1i Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: 5 958A 4 2 4 I c.-ct for t I g- 1 C I C C C Ct

ICC

APPLICANT'S REF.: 2882X/225/07 CANADIAN LIQUID AIR Ltd AIR LIQUIDE CANADA Ltee Name(s) of Applicant(s): Address(es) of Applicant(s): Actual Inventor(s): C t Address for Service is: S t 1155, rue Sherbrooke Ouest Montreal H3A 1H8 Canada Robert Gum Hong Lee Derck Hornsey Arthur S Perkins Jack A Davidson iY PHILLIPS, ORMONDE AND FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia, 3000 Complete Specification for the invention entitled: "PULP BLEACHING" The following statement is a full description of this invention, including the best method of performing it known to applicant(s): P19/3/84

A

E L BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the dispersing and dissolving of a gas or gases in a pulp slurry.

More particularly, it is directed to a process in which oxygen is dispersed in cellulose pulp as a delignification or bleaching agent, to economize on the amount of more expensive chemicals used-for these purposes.

2. Description of the Prior Art Considerable work has been done on the use of oxygen along with other chemicals as evident from the report of a Symposium of the Technical Association of the Pulp and Paper Industry Inc. (TAPPI) held in November 1984 and devoted exclusively to the emerging era of chemical pulp delignification and 15 bleaching with oxygen, ozone and hydrogen peroxide.

The Symposium Chairman emphasized that the chemistry of this new technology is rather complex and little understood and represents most challenging research areas from a purely scientific point of view. The current conventional concepts of lignin removal do not hold anymore. While commercialization is proceeding, the basic chemistry of these processes is not widely known.

o° A problem of using oxygen is in introducing it into the pulp slurry to provide good dispersion for dissolution leading to delignification or bleaching. One form of mixing is with complex, expensive, high speed mixers, as shown in patents to Weyerhaeuser, namely, 4,295,925, 4,295,926, 4,295,927, 4,298,426 and 4,198,427.

Another type of mixing is with a rotatable orificed disk in the Li f 4 t

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sc C 1, C C: t (-r 3 flow path of the slurry, as shown in U.S. Patent 4,427,489, Jacobsen (1984).

A still further type of mixing is described in the advertisement entitled "EO Mixing Breakthrough", appearing in the TAPPI Journal for August 1984, at page 6, by Komax Sysems Inc.

Here a static mixer is used to mix oxygen into the pulp. Bubbles and pulp are mixed by eight internal eccentrically arranged baffles, which are said to.fluidize, backmix, and blend the bubbles and pulp so as to achieve a mixing greater than that of mechanical mixers, assuring that the bubbles areabsorbed by the fiber.

The mixing apparatus shown in these patents is relatively complex and expensive and has moving parts which can go out of order or static parts in the flow path, which can lead to 15 blockage.

It is, therefore, an aim of the present invention to provide apparatus for effectively dispersing and mixing oxygen into a pulp slurry which is simplified and economical compared with existing apparatus.

20 SUMMARY OF THE INVENTION The applicants have found that effective m ng of oxygen with an aqueous cellulose pulp slurry ing a consistency from about 8 to about 16%, pref ly 10 to 12%, can be effected by defining a relati y unobstructed flow path for the slurry past a zon oxygen introduction and beyond and moving the sl y through the flow path at a velocity at which it is olly fluidized and completely turbulent to have substanally "Newtonian" properties, where it acts substantially like -3a- According to the present invention, there is provided a method of reacting oxygen with a cellulose pulp slurry having a consistency from 8% to 16% to effect delignification or bleaching, comprising, dosing the pulp slurry with oxygen incrementally by, feeding a stream of the cellulose pulp slurry under pressure from a supply through a reaction path made up of a plurality of stages, in each of which the stream is first confined to a cross-section at which it has a velocity such as to provide turbulent liquid flow and is then expanded to a cross-section at which it has a velocity such as to provide plug flow, it t t f in each stage, injecting oxygen into the liquid flow under tit$ pressure in the form of a mass of minute bubbles to disperse tt them throughout whereby the oxygen remains in intimate tit.

contact with the pulp in the plug flow, and recovering treated slurry from the final stage.

According to the present invention there is also provided a method of mixing oxygen into a cellulose pulp slurry, having a consistency from 8% to 16%, to delignify or Ot 4 bleach the pulp, comprising, 0$ 4 defining a substantially uninterrupted flow path confining a o stream of the slurry under pressure, o 6 feeding the stream, from a source of supply, through said flow path at a superficial effective velocity to produce 9: flow while injecting oxygen under pressure into said stream 0 0 in the form of a mass of minute bubbles whereby the oxygen is dispersed throughout the slurry, immediately conducting the stream and dispersed oxygen into the pulp and slurry to thicken to plug form and to give time for the oxygen to react with the pulp, and recovering the pulp so treated.

According to the present invention there is further provided a method of dispersing a reactive gas throughout a pulp slurry which, at rest, is a thick dispersion, comprising, IRAZ moving the pulp in a confined stream at a velocity such that it behaves like a turbulent liquid while injecting oxygen -3binto the stream in the form of a mass of minute bubbles whereby they are dispersed throughout the liquid slurry, and immediately releasing the stream to confinement at a reduced velocity such that the slurry and entrained gas bubbles assume plug form for a time effective for the gas to react with the pulp.

According to the present invention there is further provided a method of reacting oxygen with a cellulose pulp slurry having a consistency from 8% to 16% to effect delignification or bleaching, comprising, dosing the pulp slurry with oxygen incrementally by, 00ooo0 00 feeding a stream of the cellulose pulp slurry under pressure from a supply through a reaction path made up of at least 00 one stage, in which the stream is first confined to a cross-section at which it has a velocity such as to provide aooo o turbulent liquid flow and is then expanded to a cross-section at which it has a velocity such as to provide plug flow, in said stage, injecting oxygen into the liquid flow under O. 0 pressure in the form of a mass of minute bubbles to disperse a0090 0o0 them throughout whereby the oxygen remains in intimate 0 0 contact with the pulp in the plug flow, .IQ and recovering treated slurry from said stage.

0 6 The present invention also provides an apparatus for reacting oxygen with a cellulose pulp slurry, comprising: a series of connected units providing a reaction path, each unit being made up of a conduit of restricted cross-sectional area leading to a reactor vessel of several times the cross-sectional area of the conduit, and a final unit having an outlet, 7 means for injecting oxygen in a mass of small bubbles into the conduit of each said unit, means for feeding the slurry from a source of supply under pressure through said reaction path, wherein, in each unit, said restricted cross-sectional area of said conduit confines slurry flow therethrough such that said flow has a velocity so as to achieve turbulent liquid flow and the cross-sectional area of said reactor vessel is S such that slurry flow therethrough is expanded and has a fr Olo AB I -3cvelocity so as to achieve plug flow; and means for controlling the supply of oxygen to each unit whereby an increment of oxygen is dispersed throughout the slurry in the conduit and most of the oxygen is reacted with the slurry in the reactor vessel.

The present invention further provides an apparatus for introducing oxidising gas into a pulp slurry, comprising: means forming a passage of restricted cross-sectional area which leads to a vessel having an expanded cross-sectional area relative to the cross-sectional area of the passage, means for feeding the slurry through the passage under I pressure, a bubble-producing surface having a mass of gas outlets of microscopic size exposed to the slurry in said passage, means for supplying gas under pressure to said outlets Swhereby the gas bubbles are dispersed in the slurry, wherein said restricted cross-sectional area of said passage confines slurry flow therethrough such that said flow has a velocity so as to achieve turbulent liquid flow and the cross-sectional area of said vessel is such that slurry flow Srll therethrough is expanded and has a velocity so as to achieve plug flow.

13

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i: I I r: 4C6 l. *4 -4 1 water and which produces highly effective mixing. Preferably, the oxygen is introduced into the slurry through diffusing means having a stationary porous surface in contact with the fast moving slurry, in the oxygen introduction zone, in finely divided form, preferably in the form of a mass of discrete bubbles having a diameter not more than, for example, about 2 to about microns. This ensures that the bubbles are dispersed throughout the slurry without substantial coalescence which might be expected from the behavior.or larger bubbles entering the liquid.

Preferred oxygen diffusing means includes an element of sintered metal of the type normally used for filtering. A typical diffuser is a sintered stainless steel element as described in Bulletin M201 of Pall Trinity Micro Corporation, a subsidiary of Pall Corporation. Such an element is produced by the process of U.S. Patent 2,554,343, the disclosure of which is hereby incorporated by reference. The alloyed metal powder of selected particle size is passed through a contrl atmosphere sintering furnace to produce sheets or shapes of the porous media. The powdered metal particles fuse at their points of contact, resulting in a bond with a homogeneous crystalline structure. No binders or other extraneous material are introduced, thus maintaining the basic properties of the metals or alloys used. Preferably, the element has a porous wall of thickness within the range from about 1/32nd of an inch to about J of an inch. The pores should have a diameter less than about 10 microns, preferably within the range from about 2 to about 10 microns and the porosity should be substantially uniform throughout the area of the porous wall, preferably within the range from about 40 to about It 4 5 Porous metals with up to 50% of their volume, made of interconnecting voids or pores, are available in a wide range of materials including nickel, Monel (trade mark), Inconel (trade mark) and m&iiy 300 and 400 Series Stainless Alloys. A widely used type is 316L carbon maximum) Stainless Steel. Varying pore sizes are available.

The diffusing device operates to produce a mass of very small bubbles in accordance with the principles expounded in U.S.

Patent 3,545,731, McManus (1970) with the exception that special conditions, discovered by the applicants and described herein, have to be observed for proper mixing of the oxygen into a pulp slurry.

tc Preferably, the diffusing element includes a cylinder through which the-slurry passes, the walls of the cylinder defining the flow path. The diffusing element may form a part of the wall of the cylinder, may take the form of a porous pipe extending transaxiallly of the cylinder, or may take the form of a porous pipe extending axially of the cylinder. In all cases, the path of the slurry is relatively unobstructed. Where the cylinder has a porous wall, the slurry passes directly through the cylinder without any obstruction. In the'case of a pipe within the cylinder, it takes up a minor space so that the major volume of the space in the cylinder is taken up by the slurry moving through it in contact with the porous diffusing surface.

An alternative preferred form of diffusing apparatus includes a conduit having a cylindrical wall defining the outer limits of the passage and an enclosed vessel supported within the conduit and having a cylindrical wall defining the inner -6limits of the passage. A bubble producing surface having a-mass of gas outlets of microscopic size is provided both on the conduit wall and on the wall of the vessel. A gas manifold surrounds the outer side of the conduit wall and is connected to a szpply of gas under pressure for supplying gas. The enclosed Vessel is also connected to a gas supply. With this arrangement, an annular pas r;e is provided through which a stream of pulp slurry is passed at fluidizing velocity, while gas is diffused into the stream through themicroporous surfaces on both sides of the annular passage.

So that the nature of the flow of the slurry through the passages will be more fully appreciated, it is described in an I '*.article entitled "Medium Consistency Technology" appearing in the Journal of the Technical Association of the Pulp and Paper 15 Industry (TAPPI), Vol. 64, No. 6, June 198! by Gullichsen and Haerkoenen, the disclosure of which is hereby incorporated by reference. Before any movement of the slurry through a pipe can occur, the plug of dispersed fibers which is in contact with the pipe wall has to be distorted by a shear force. Motion begins r 20 when this yield stress has been surpassed. The suspension moves first as a sheared, viscoelastic body with direct wall contact.

When the speed is increased, fibers and loosely held flocs break loose from the plug and move into the annulus between the wall and plug, where they act l ke rollers. The decline of the friction curve indicates another change of flow mechanism. A continuous and pure liquid annulus has been formed between plug and wall.

The annulus flow is laminar. A the flow rate increases, a velocity will be reached beyond which the flow becomes turbulent.

I

Increased speed intensifies turbulence, and the fiber plug decreases in size. More fibers are broken loose from the plug.

The whole suspension becomes turbulent when the velocity is escalated beyond the point where the fiber suspension and waterflow curves, after crossing, become parallel. All velocity and consistency gradients have disappeared, and the suspension behaves as a turbulent liquid. The shear field imposed through the mass of fibers now exceeds the shear stress value needed to disrupt the fiber network cpletely.

The applicants have also found that when the liquid slurry in which oxygen has been dispersed, as described, flows directly to a reactor in which it collects and moves at a much slower speed, as a plug, good reaction between the oxygen and the pulp is achieved.

The invention lends itself particularly to the incremental dosage of the pulp slurry with oxygen. This is done by feeding of a stream of cellulose pulp slurry under pressure, from a supply, through a reaction path made up of a plurality of successive stages, in each of which the stream is first confined to a cross-section at which it has a velocity such as to fluidize 1 ,,it to liquid flow and is then expanded to a cross-section at which it has a velocity such as to provide plug flow. In each stage, ~oxygen is injected into the liquid flow under pressure, in the form of a mass of minute bubbles to disperse them throughout, whereby most of the oxygen is reacted with the pulp in the plug flow in that stage. The treated slurry is then recovered from the final stage.

The invention also contemplates a preferred further

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mixing of the slurry leaving the'oxygen diffuser by a passage through a-pulp fluidizing mixer. This can be located adjacent to the diffuser or positioned some distance downstream. This mixer provides the effective further mixing and dispersing any agglomeration of oxygen so the oxygen remains in finely divided form and is more intimately mixed with the slurry. A preferred static mixer is made up of a pipe tapering down to a choke, followed by a pipe that flares out to the diameter of the pipe in which the pulp slurry is flowing. The velocity through the choke will be about the same as that in the diffuser.

The invention also contemplates means for coordinia-ing the flow of oxygen to the flow of pulp slurry so as to maintain the proper oxygen dosage. This may be accomplished by an apparatus which includes a flow measuring device, for example an orifice plate within the path of the slurry, in advance of the diffuser, the orifice plate being connected to a differential pressure cell which, in turn, is connected to a controller.

Th controller sends a signal to an oxygen flow control Svalve controlling the oxygen supply to the diffuser. There is an orifice plate or other device in the oxygen supply line that measures the oxygen flow. The control mechanism may include a computer that maintains a constant, predetermined and set ratio of oxygen to pulp so as to increase and decrease the dosage to keep pace with differing flow rates of the pulp slurry.

Injecting oxygen into cellulose pulp slurry, according to the invention, may be employed at various stages of the delignification and bleaching processes. The aim is to decrease the Kappa number of the pulp. For delignification, the Kappa number u y e I.I- j II u y ii Iiii H' I iLimaff Le may be reduced from 50 to 60 to within the range of 20 to 30. For bleaching, the Kappa number may be reduced from the range of to 10 within the range of 2.3 to Usually the velocity of the pulp passing through the oxygen introduction zone is from 1 to 50 meters a second, a preferred range being 5 to 40 meters a second. The stationary Sdiffuser surface may have an area from about 0.01 to 0.001 square feet per standard cubic foot per minute of oxygen gas or 30 to 300 normal cubic meters of oxygen per square meter of porous surface flow rate.

The dosage rate may range from 5 to 50 Ibs., preferably to 15 Ibs. per ton of pulp on an air dried (AD) basis for bleaching and 20 to 100 Ibs. per ton for delignification.

The oxygen employed may be molecular oxygen, as commercially available, containing 90% or more and preferably 98% or more of oxygen. Or, the oxygen can be in the form of a gas containing more than 50% oxygen. The starting pulp slurry may contain about 10 to about 20% of air by volume of the pulp dispersed which dilutes added oxygen. For a pulp containing little or no air, the concentration of the added oxygen containing gas may be at the lower end of the oxygen content range and for a pulp containing a lot of air, the added gas may contain oxygen toward the upper end of the oxygen range.

In order to control the oxygen dosage to meet requirements, the oxygen content of off-gas leaving the vent of the final treatment vessel is measured. From this it can be determined whether good mixing is being achieved. Adjustments can be made accordingly and the incremental dosage can be manipulated in order to achieve the best results.

In a multi-stage process, as described, large total amounts of oxygen can thus be added incrementally to a pulp slurry, by adding small amounts at each stage according to the formula: Volume of gaseous oxygen 0.1 Volume (gaseous oxygen pulp) TI 2 t f \rT O 2544j -9a-

I

10 order to achieve the best results. J In a multi-stage process, as descr large total amounts of oxygen can thus be add ncrementally to a pulp slurry,by adding small ounts at each stage according to the formula: Volume of oxygen gas Volume (oxygen gas.+ pulp) This formula is calculated at the operating pressure of that particular stage.

The dosage may be proportioned evenly to each stage or may be varied to suit varying conditions such as changes in the nature of the pulp or otherwise.

The invention is specially flexible in terms of the equipment and method so that it can be used in economically retrofitting a paper mill. The diffusion elements are relatively uncomplicated and inexpensive and can be readily installed in the system.

Each mill has a certain rating as to throughput. The present invention lends itself to adjustment to accommodate this rating. Changes in the flow rates of the pulp slurry and consequently of the oxygen could be dictated by changes in the pulp in which the slurry is introduced, the flow rate may have to be ireduced because of breakdown when the stock tanks become filled.

In a preferred apparatus, the pulp slurry flows through an initial oxygen diffuser and a plurality of reactors each preceded by an oxygen diffuser. Any or all may be passed into a reaction tower to allow further extraction with caustic soda.

i applicant(s): P19/3/84 -in*r 1 BRIEF -DESCRIPTION OF THE DRAWINGS Having thus generally described the invention, it will be referred to in more detail by reference to the accompanying drawings, which illustrate preferred embodiments, and in which: FIG. 1 is a diagrammatic view of a typical plant for carrying out the process of the invention; FIG. 2 is an enlarged side elevation partly in section illustrating in more detail one form of pulp fluidizing diffuser, as shown in Fig. 1, in series, with a pulp fluidizing mixer; FIGS. 3, 4, 5, 6 and 7 are views of alternative forms :tt:: of pulp fluidizing diffusers.

t DESCRIPTION OF THE PREFERRED EMBODIMENTS More specific reference will now be made to the drawings.

C 15 The typical plant illusi'ated will be understood from the following description of the path of a pulp slurry flowing through it.

By way of example, the particular stage illustrated, in the bleaching process, is the first alkaline extraction stage.

Fluidizino Oxygen Diffuser 20 An aqueous slurry of cellulose pulp, containing caustic soda, is pumped at a high superficial velocity from the pipe from a source of supply (not shown), through the pump A, to a line 17 entering a pulp fluidizing oxygen diffuser B within which oxygen is diffused and dispersed in fine bubbles into the slurry stream. The slurry stream is pumped from a source of supply through the line 17 by the pump A at a velocity such that the stream acts like water and there is high turbulence throughout.

The oxygen is diffused into the stream through the diffuser B in t-i I I_

I

12 fine bubbles, as will be described later in more detail, and provides rapid mixing of the oxygen throughout the pulp stream.

The rapidly moving stream in the line 17 downstream from the oxygen diffuser, now containing dispersed oxygen, is passed into the bottom of a first vertical reaction chamber C, in which it moves at a much lower velocity as a plug. At the lower velocity, the slurry loses its fluidity, trapping the dispersed oxygen within the plug, which allows the oxygen and the caustic soda to react with the pulp during the time the oxygen containing slurry remains in the chamber C.

The slurry is then passed rapidly, at turbulent velocity, through a pipe 19 into and through a second oxygen diffuser B and tl 1 a into the bottom of a second reaction chamber C in which it I t Ill' resides for a period to allow further reaction of the oxygen and LIlt 15 bleaching chemicals with the cellulose.

From the column C 1 slurry is passed through a pipe 21 and a third oxygen diffuser B into a third reaction chamber C 2 S ,The pulp then passes through a pressure control valve 23 and a pipe 24 into the top of a reaction tower D. The pulp remains in the tower D to provide further time for the bleaching chemical to react with the cellulose.

Alternatively, the pulp may proceed in the opposite direction, i.e. downward through the chambers, to the bottom or the top of the reaction tower, or some or all the reaction vessels may be horizontal.

The pulp slurry from the tower 0 is then recovered from an outlet pipe 25 and washed to remove bleaching chemicals and impurities or depending on the stage of the bleaching process

N

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I

1_3 the treated slurry may be passed to further treatment.

A vent 29 for off gases enables the monitoring of the excess oxygen, towards adjustment of the process for greater efficiency.

I

4 C 44 6 4CI 4 0446I 644 6644B o *4~ 4i 6066 Preferably, as shown in Fig. 2,there is a static pulp fluidizing mixer, or choke E, in series with the diffuser B, between it and the first reaction column through which the stream of oxygen containing slurry is passed. The choke has a restricted throat which narrows the stream and increases its velocity.

This ensures a better dispersion of the oxygen gas bubbles throughout the pulp slurry.

Diffuser The pulp fluidizing oxygen diffuser B, as shown-enlarged in Fig. 3, is made up of a cylinder 31 providing a passage 32 for 15 the pulp slurry. The cylinder has flanges 33, 34 on its ends for connecting into a pipeline. Extending diametrically across the otherwise uninterrupted passage 32 is a porous metal tube 35 which enters an opening in the side of the cylinder 31 through a nipple 31a and is secured and sealed in place by a SWAGELOK (trade mark) 20 type tube fitting 6. On the opposite side of the cylinder 31 is an opening closed by a cap 31b in which the lower end of the diffuser is held. The wall of the tube 35 is preferably made of sintered metal, as described.

The pulp is continuously passed at high velocity through the passage 32 while oxygen is supplied to the porous tube 35 and diffuses through its wall into the slurry.

An alternative form of diffuser is shown in Fig. 4. In this case, the wall of the cylinder 41 has a central cylindrical i 1 6 6 0 O 0 4 4 Itt^

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*c"I 07 14 porous section 45. A manifold 47 encloses this section from the outside and there is an oxygen connection 48 with the manifold.

The passage for the pulp slurry is thus completely uninterrupted.

Still another form of diffuser is shown in Fia. 5. Here a donut shaped device 55 is placed against the cylinder 51 and surrounds the passage 52. The pipe 55 is provided with suitable small orifices 56 from which the oxygen is diffused. The passage 52 is completely uninterrupted.

Still another form of diffuser is shown in Fig. 6. Here a porous tube 65 extends axially and centrally along the passage 62 and is held in it by suitable supports or spiders 69 which are connected to the inside of the wall of the cylinder 61. An oxygen It pipe 68 leads from outside the cylinder to the end of the diffusion tube Fig. 7 shows a still further preferred form of diffuser.

This diffuser is mounted between mounting flanges 100 and 101, from which extend inwardly short cylindrical pipe sections 102 and 103 respectively, merging into diagonal frusto-conical flanges 104 and 105. A jacket J, made up of an inner cylindrical porous wall 111, extends between the ends of the flanges-102 and 105, an outer imperforate cylindrical wall 107, and imperforate annular ends walls 108 and 109. The microporous inner wall 111 is preferably made of sintered metal, as described earlier in this application.

The pipe sections 102 and 103, the flanges 104 and 105, and the porous wall 111 provide an enclosure.

Axially mounted within this enclosure is a cylinder K having a microporous (preferably sintered metal) wall 113 capped H L 1 i 1 k OV AB l at its ends with impervious frusto-conical caps 115 and 117. The cylinder K is suitably supported by a spider arrangement S to retain its position within the conduit. The wall 113 of the cylinder K and the wall 111 of the jacket J form between them an annular passage P. An oxidant gas supply pipe 119 leads from a source o- oxidant gas under pressure to within the jacket J. An oxidant gas supply pipe 121 leads from a source of oxidant gas under pressure to within the cylinder K.

The diffuser is connected, similarly, to the connection of the earlier dsecribed embodiments, by the flange 100 to a pipe for incoming pulp slurry and by the flange 101 to a pipe .4 carrying outgoing pulp slurry.

In operation, the pulp slurry is fed into the lefthand end of the device, at fluidizing velocity, and rushes through the passage P between the microporous walls 111 and 113 and exits through the right-hand end. Oxidizing gas under presit t S, sure is fed through the pipes 119 and 121 and is diffused, in fine bubbles, through the opposed porous walls 111 and 113, into the fluidized pulp slurry rushing through the passage P in a manner already described in conjunction with other embodiments.

The diffusion of the gas into the slurry is most efficient because of the large diffusing area at both boundaries of the annular fluidized pulp stream.

Pulp Fluidizing Mixer The static pulp fluidizing mixer E, shown in Fig. 2, connected in series with the oxygen diffuser B, comprises an integral elongated tube of circular cross-section extending between connecting flanges 81 and 82. A first frusto-conical l r frust, o-c nical__- AB i 16part 83 tapers inwardly from the flange 81 to merge in a throat 84 with the small end of a longer outwardly tapered frusto-conical part 85 which terminates at the flange 82. The mixer E is connecte6 to a transition piece F which has an upstream flange connected to the flange 34 with a gasket intervening. The transition piece F has a downstream flange 91 connected to the flange 81 of the mixer. The pulp slurry is momentarily released to a greater cross-section and then confined substantially to the cross-section of the diffuser pipe.

The size (inside diameter) of the cylinder 31 to which the pulp slurry is pumped into contact with the oxygen will vary i t ,1 depending on the capacity of the pulp mill. Typical diameters t i t -range from 2 to 12 inches and usualiy from 3 to 6 inches.

i t Reaction Chamber Three reaction chambers C, C 1 and C 2 have been shown.

More reaction chambers or less can be employed depending on the t I t l material being processed, the amount of oxygen being added and the bleaching effect desired. The virtue of several diffusers each followed by a reaction chamber is that this allows large additions of oxygen to be added incrementally by adding small amounts at each diffuser so that: Volume of oxygen gas 0.1 Volume (oxygen gas pulp) This is calculated at the pressure of that particular stage.

Slurry Velocity The velocity at which the pulp slurry is pumped through the diffuser cylinder 31 is a high velocity effective to cause the stream to behave like water and making it easier to disperse small -i ic~ 17 bubbles of oxygen which are mixed throughout.

The pressure of the pulp slurry may be from 15 to 150 psig and the oxygen pressure from 20 to 200 psig.

The residence time of the slurry within the diffuser cylinder 31 will usually range from about 0.001 to about 0.120 seconds.

The residence time in each of the reactors C, C I and C 2 will usually range from about 1 minute to about 5 minutes, and the residence time in the tower D will range from about 30 to about minutes.

The pressure within each reactor will be from 15 to 150 psig.

4 4- High Speed Pump The invention does not preclude the use, the combination with the applicant's diffusers, for introducing the oxygen into the pulp, of high speed mixers as, for example, shown in U.S.

Patent 4,295,926, Weyerhaeuser or a Kamyr MC (trade mark) pump.

4 44 Nature of Pulp The nature of'the pulp treated can be softwood or hardwood or a mixture having a consistency within the range from 5 to il 16%. Pulp at various stages of the delignification-bleaching process, may be processed by the invention. For example, the starting pulp in the caustic extraction stage of a bleaching process may have a Kappa number within the range from 5.0 to 10.0 and the bleached pulp removed from the vessel D may have, after washing, a Kappa number of 2.5 to Nature of the Diffusion As a result of the high velocity of the pulp slurry 4 t- IS passing through the passage 32 in contact with the diffusion elements 35, 45, 56 or 65 the slurry takes on the characteristics of water. The slurry passes in close contact with the diffusing element and the oxygen emerges from its pores in very small diameter bubbles. These bubbles are well dispersed because of the turbulence of the slurry and present a large surface area as compared with their volume for mass transfer across the gas/ liquid interface. At high oxygen concentration gradients and large surface/volume ratio oxygen mass transfer is greatly enhanced by this fluidizing dispersal process.

Pre- and Post-Treatment of Slurry The slurry, prior to being pumped into the first diffuser B will have undergone normal treatments in a delignification-bleach'ing process. For example, the slurry will contain low concentrations of other bleaching chemicals. Moreover, I, the slurry will be provided at a temperature within the range -from about 40°C to about SThe slurry leaving the bleaching tower D will be washed to remove bleaching chemicals and other impurities.

I 20 Typical Plant Design A typical bleaching plant could have a capacity of processing 1,000 tons of pulp per day, of consistency 10% at Ibs. per ton of oxygen, or 10,000 Ibs. The flow rate could be 1,670 U.S. gallons per minute, with the velocity of the slurry through the diffuser 40 meters a second and the oxygen flow rate 6.9 Ibs. per minute. The diffusing area could be about 0.028 square feet.

Claims

1. A method of reacting oxygen with a cellulose pulp slurry having a consistency from 8% to 160 to effect delignification or bleaching, comprising, dosing the pulp slurry with oxygen incrementally by, feeding a stream of the cellulose pulp slurry under pressure from a supply through a reaction path made up of a plurality of stages, in each of which the stream is first con- fined to a cross-section at which it has a velocity such as to tO provide turbulent liquid flow and is then expanded to a cross- section at which it has a velocity such as to provide plug flow, in each stage, injecting oxygen into the liquid flow under pressure in the form of a mass of minute bubbles to dis- perse them throughout whereby the oxygen remains in intimate contact with the pulp in the plug flow, and recovering treated slurry from the final stage.

2. A method, as defined in claim 1, in which the oxyge s injected in each stage in an amount to satisfy the f ula: Volume of oxygen gas 0 1 Volume (oxygen gas to calculated at the operating pre re of that stage.

3. A method s defined in claim 1 or 2, in which the oxygen is' jected in the form of a mass of bubbles having an S ial diameter of less than about 10 microns. 4, F A ''II I. 2. A method according to claim 1, in which the oxygen is injected in each stage in an amount to satisfy the formula: Volume of gaseous oxygen 0.1 Volume (gaseous oxygen pulp) calculated at the operating pressure of that stage. 3. A method according to claim 1 or claim 2, in which the oxygen is injected in the form of a mass of bubbles having an initial diameter of less than about 10 microns.

4. A method according to claim 1 or claim 2, in which the confined slurry stream is at a pressure within the range from 15 to 150 psig. A method according to claim 1 or claim 2, in which the oxygen is injected at a pressure from 20 to 200 psig.

6. A method according to claim 2, in which the total rtl I oxygen dosage rate is from 5 to 50 Ibs. per ton on a dry basis. 0 t

7. A method according to claim 1 or claim 2, in which the oxygen is introduced through a porous surface having an area such that the rate of oxygen injection is within the range from 30 to 300 normal cubic meters per minute 0 per square meter of porous surface.

8. A method according to claim 1, in which the pulp It k 254 2544j -19a- -2z- 4. A method, as defined in claim 1 or 2, in which/he confined slurry stream is at a pressure within the r ;ge from to 150 psig. c A method, as defined in claim 1 r 2, in which the oxygen is injected at a pressure from 0 to 200 psig. 6. A method, as defined in claim 2, in which the total oxygen dosage rate is from 5 to 50 lbs. per ton on a dry basis. 7. A method,/ s defined in claim 1 or 2, in which the oxygen is introduced through a porous surface having an area iO 1to such that the rate of oxygen injection is within the range o, from 3 to 300 normal cubic meters per minute per square meter at of orous surface. 8. A method, as defined in claim 1, in which the pulp slurry velocity in said liquid flow is from one-half to fifty meters a second. a 9. A method of mixing oxygen into a cellulose pulp slurry, having a consistency from 8% to 16%, to delignify or bleach the pulp, cumprising, defining a substantially uninterrupted flow path con- fining a stream of the slurry under pressure, feeding the stream, from a source of supply, through said flow path at a superficial effective velocity to produce -tiui flow while injecting oxygen under pressure into said stream in the form of a mass of minute bubbles whereby the r4 oxygen is dispersed throughout the slurry, T o C07). -9a- I

21- immediately conducting the stream and dispersed oxyaen into a confined reaction zone of greater cross-section to allow the pulp and slurry to thicken to plug form and to give time for the oxygen to react with the pulp, and recovering the pulp so treated. A method of dispersing a reactive gas throughout a pulp slurry which at restia thick dispersion, comprising, moving the pulp in a confined stream at a velocity such that it behaves like a turbulent liquid while injecting oxygen into the stream in form of a mass of minute bubbles whereby they are dispersed throughout the liquid slurry, and immediately releasing the stream to confinement at a reduced velocity such that the slurry and entrained gas bubbles assume plug form for a time effective for the gas to react with the pulp. 11. A method of reacting oxygen with a cellulose pulp slurry having a consistency from 8% to 16% to effect deligni- fication or bleaching, comprising, dosing the pulp slurry with oxygen incrementally by, feeding a stream of the cellulose pulp slurry under pressure from a supply through a reaction path made up of at least one stage, in which the stream is first confined to a cross-section at which it has a velocity such as to provide turbulent liquid flow and is then expanded to a cross-section at which it has a velocity such as to provide plug flow, in said stage, injecting oxygen into the liquid i-ow g i under pressure in the form of a mass of minute bubbles to disperse them throughout whereby the oxygen remains in intimate contact with the pulp in the plug flow, and recovering treated slurry from said stage. 12. A method according to claim 11, in which the oxygen is injected in said stage in an amount to satisfy the formula: Volume of gaseous oxygen 0.1 Volume (gaseous oxygen pulp) calculated at the operating pressure of said stage. 13. A method according to claim 11, in which the confined slurry stream is at a pressure within the range from 50 to 150 psig, the oxygen is injected at a pressure from 20 to 200 psig. 14. A method according to claim 13, in which the oxygen is injected in said stage in an amount to satisfy the formula: Volume of gaseous oxygen 0.1 Volume (gaseous oxygen pulp) calculated at the operating pressure of said stage. An apparatus for reacting oxygen with a cellule pulp, comprising, a series of connected units providing-a reaction path, each unit being made up of a restricted conduit leading to a reactor vessel of several times the cross-sectional area "of the conduit and a final unit having an outlet ,and, neach stage, means for injecting oxygen in a mass small bubbles into the conduit, 7f a source of supply of slurry, 2544j JW -22- h; -22a- An apparatus for reacting oxygen with a cellulose pulp slurry, comprising: a series of connected units providing a reaction path, each unit being made up of a conduit of restricted cross-sectional area leading to a reactor vessel of several times the cross-sectional area of the conduit, and a final unit having an outlet, means for injecting oxygen in a mass of small bubbles into the conduit of each said unit, means for feeding the slurry from a source of supply under pressure through said reaction path, wherein, in each unit, said restricted cross-sectional area tit of said conduit confines slurry flow therethrough such that said flow has a velocity so as to achieve turbulent liquid flow and the cross-sectional area of said reactor vessel is 4441 such that slurry flow therethrough is expanded and has a 1411 velocity so as to achieve plug flow; and means for controlling the supply of oxygen to each unit whereby an increment of oxygen is dispersed throughout the o" slurry in the conduit and most of the oxygen is reacted with 4: the slurry in the reactor vessel. S16. An apparatus for introducing oxidising gas into a pulp o slurry, comprising: means forming a passage of restricted cross-sectional area which leads to a vessel having an expanded cross-sectional S area relative to the cross-sectional area of the passage, means for feeding the slurry through the passage under pressure, a bubble-producing surface having a mass of gas outlets of microscopic size exposed to the slurry in said passage, means for supplying gas under pressure to said outlets whereby the gas bubbles are dispersed in the slurry, wherein said restricted cross-sectional area of said passage confines slurry flow therethrough such that said flow has a velocity so as to achieve turbulent liquid flow and the cross-sectional area of said vessel is such that slurry flow therethrough is expanded and has a velocity so as to achieve plug flow. ~plug AB Ci S. 25- means for feeding che slurry from said supply so rce under pressure through said reaction path, whereby the lurry has a velocity in each conduit such that it is liquid as it passes through the conduit, and such that it is 'n plug form as it passes through the adjoining reactor v sel, means for controlling the suppl of oxygen to each stage whereby an increment of oxygen i dispersed throughout the liquid slurry and most of the o ygen is reacted with the pulp in plug form in its passag through the reactor vessel. 4/ Ot. o 16. An apparatus for/ ntroducing oxidizing gas into a pulp slurry, comprising, "means form/ng a restricted passage for the slurry, 4 means fr feeding the slurry through the passage at a pressure such hat it passes therethrough as a stream at fluidizing elocity, a bubble-producing surface having a mass of gas out- let of microscopic size exposed to the stream in said passage, Smeans for supplying gas under pressure to said outlets whereby the gas bubbles are dispersed in said fluidized pulp. 17. An apparatus a inP n claim 16, in which the fpassage is provided by a conduit and the bubble-producing surface is on a pipe extending transversely of the conduit in the path of the slurry semi. 18. An apparatus, at d-fin in a m. 16, in which the passage is provided by a conduit, the bubble-producing surface TR"'/ is on a part of the surface of the conduit, and a manifold 'T O ;-U-OCP~ J L b~ r d( t i- r i 1 -1 "11^~LI~ -24- surrounds said part of the surface for receiving said gas under pressure. 19. An apparatus according to claim 16, in which the passage is provided by a conduit and the bubble-producing surface is on a pipe extending longitudinally through the conduit, said apparatus further including means for supporting the pipe and conduit within the path of said slurry. An apparatus according to claim 16, in which the passage is provided by a conduit having a porous cylindrical wall and a centrally disposed closed vessel having a porous cylindrical wall concentric with and spaced from the wall of the conduit, the bubble-producing surface is provided both on the surface of the conduit and the surface of the vessel, the porous cylindrical wall of the conduit being surrounded by a manifold for receiving and supplying gas through the porous wall of the conduit, and means for connecting said manifold and said closed vessel with a source of oxidizing gas under pressure. 21. An apparatus according to claim 16, substantially as hereinbefore described with reference to the accompanying drawings.

22. A method according to claim 1, substantially as hereinbefore described with reference to the accompanying drawings. DATED: 31 JANUARY, 1990 PHILLIPS ORMONDE FITZPATRICK Attorneys For: CANADIAN LIQUID AIR LTD AIR LIQUIDE CANAALTEE I 4 I CIt (i L i