EP0226495B1 - Process and apparatus for bleaching paper pulp - Google Patents

Process and apparatus for bleaching paper pulp Download PDF

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Publication number
EP0226495B1
EP0226495B1 EP86402533A EP86402533A EP0226495B1 EP 0226495 B1 EP0226495 B1 EP 0226495B1 EP 86402533 A EP86402533 A EP 86402533A EP 86402533 A EP86402533 A EP 86402533A EP 0226495 B1 EP0226495 B1 EP 0226495B1
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EP
European Patent Office
Prior art keywords
suspension
oxygen
passage
pulp
diffuser
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP86402533A
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German (de)
French (fr)
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EP0226495A1 (en
Inventor
Arthur S. Perkins
Jack A. Davidson
Robert Gum Hong Lee
Derek Horney
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Air Liquide Canada Inc
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Air Liquide Canada Inc
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Filing date
Publication date
Priority claimed from CA000507000A external-priority patent/CA1300320C/en
Application filed by Air Liquide Canada Inc filed Critical Air Liquide Canada Inc
Priority to AT86402533T priority Critical patent/ATE54476T1/en
Publication of EP0226495A1 publication Critical patent/EP0226495A1/en
Application granted granted Critical
Publication of EP0226495B1 publication Critical patent/EP0226495B1/en
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    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
    • D21C9/10—Bleaching ; Apparatus therefor
    • D21C9/147—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F23/00—Mixing according to the phases to be mixed, e.g. dispersing or emulsifying
    • B01F23/20—Mixing gases with liquids
    • B01F23/23—Mixing gases with liquids by introducing gases into liquid media, e.g. for producing aerated liquids
    • B01F23/232—Mixing gases with liquids by introducing gases into liquid media, e.g. for producing aerated liquids using flow-mixing means for introducing the gases, e.g. baffles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F23/00—Mixing according to the phases to be mixed, e.g. dispersing or emulsifying
    • B01F23/20—Mixing gases with liquids
    • B01F23/23—Mixing gases with liquids by introducing gases into liquid media, e.g. for producing aerated liquids
    • B01F23/237—Mixing gases with liquids by introducing gases into liquid media, e.g. for producing aerated liquids characterised by the physical or chemical properties of gases or vapours introduced in the liquid media
    • B01F23/2376—Mixing gases with liquids by introducing gases into liquid media, e.g. for producing aerated liquids characterised by the physical or chemical properties of gases or vapours introduced in the liquid media characterised by the gas being introduced
    • B01F23/23761—Aerating, i.e. introducing oxygen containing gas in liquids
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F23/00—Mixing according to the phases to be mixed, e.g. dispersing or emulsifying
    • B01F23/20—Mixing gases with liquids
    • B01F23/23—Mixing gases with liquids by introducing gases into liquid media, e.g. for producing aerated liquids
    • B01F23/237—Mixing gases with liquids by introducing gases into liquid media, e.g. for producing aerated liquids characterised by the physical or chemical properties of gases or vapours introduced in the liquid media
    • B01F23/2376—Mixing gases with liquids by introducing gases into liquid media, e.g. for producing aerated liquids characterised by the physical or chemical properties of gases or vapours introduced in the liquid media characterised by the gas being introduced
    • B01F23/23761—Aerating, i.e. introducing oxygen containing gas in liquids
    • B01F23/237612—Oxygen
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
    • B01F25/30—Injector mixers
    • B01F25/31—Injector mixers in conduits or tubes through which the main component flows
    • B01F25/313—Injector mixers in conduits or tubes through which the main component flows wherein additional components are introduced in the centre of the conduit
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
    • B01F25/30—Injector mixers
    • B01F25/31—Injector mixers in conduits or tubes through which the main component flows
    • B01F25/313—Injector mixers in conduits or tubes through which the main component flows wherein additional components are introduced in the centre of the conduit
    • B01F25/3133—Injector mixers in conduits or tubes through which the main component flows wherein additional components are introduced in the centre of the conduit characterised by the specific design of the injector
    • B01F25/31331—Perforated, multi-opening, with a plurality of holes
    • B01F25/313311—Porous injectors
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
    • B01F25/30—Injector mixers
    • B01F25/31—Injector mixers in conduits or tubes through which the main component flows
    • B01F25/314—Injector mixers in conduits or tubes through which the main component flows wherein additional components are introduced at the circumference of the conduit
    • B01F25/3142—Injector mixers in conduits or tubes through which the main component flows wherein additional components are introduced at the circumference of the conduit the conduit having a plurality of openings in the axial direction or in the circumferential direction
    • B01F25/31421—Injector mixers in conduits or tubes through which the main component flows wherein additional components are introduced at the circumference of the conduit the conduit having a plurality of openings in the axial direction or in the circumferential direction the conduit being porous
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
    • B01F25/40—Static mixers
    • B01F25/42—Static mixers in which the mixing is affected by moving the components jointly in changing directions, e.g. in tubes provided with baffles or obstructions
    • B01F25/43—Mixing tubes, e.g. wherein the material is moved in a radial or partly reversed direction
    • B01F25/433—Mixing tubes wherein the shape of the tube influences the mixing, e.g. mixing tubes with varying cross-section or provided with inwardly extending profiles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
    • B01F25/40—Static mixers
    • B01F25/42—Static mixers in which the mixing is affected by moving the components jointly in changing directions, e.g. in tubes provided with baffles or obstructions
    • B01F25/43—Mixing tubes, e.g. wherein the material is moved in a radial or partly reversed direction
    • B01F25/433—Mixing tubes wherein the shape of the tube influences the mixing, e.g. mixing tubes with varying cross-section or provided with inwardly extending profiles
    • B01F25/4335—Mixers with a converging-diverging cross-section
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
    • B01F25/40—Static mixers
    • B01F25/42—Static mixers in which the mixing is affected by moving the components jointly in changing directions, e.g. in tubes provided with baffles or obstructions
    • B01F25/43—Mixing tubes, e.g. wherein the material is moved in a radial or partly reversed direction
    • B01F25/433—Mixing tubes wherein the shape of the tube influences the mixing, e.g. mixing tubes with varying cross-section or provided with inwardly extending profiles
    • B01F25/4337—Mixers with a diverging-converging cross-section
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F33/00—Other mixers; Mixing plants; Combinations of mixers
    • B01F33/80—Mixing plants; Combinations of mixers
    • B01F33/834—Mixing in several steps, e.g. successive steps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F35/00—Accessories for mixers; Auxiliary operations or auxiliary devices; Parts or details of general application
    • B01F35/71—Feed mechanisms
    • B01F35/715—Feeding the components in several steps, e.g. successive steps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
    • B01F2025/91—Direction of flow or arrangement of feed and discharge openings
    • B01F2025/919—Direction of flow or arrangement of feed and discharge openings characterised by the disposition of the feed and discharge openings
    • B01F2025/9191—Direction of flow or arrangement of feed and discharge openings characterised by the disposition of the feed and discharge openings characterised by the arrangement of the feed openings for one or more flows, e.g. for the mainflow and the flow of an additional component
    • B01F2025/91911—Direction of flow or arrangement of feed and discharge openings characterised by the disposition of the feed and discharge openings characterised by the arrangement of the feed openings for one or more flows, e.g. for the mainflow and the flow of an additional component with feed openings in the center of the main flow
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
    • B01F2025/91—Direction of flow or arrangement of feed and discharge openings
    • B01F2025/919—Direction of flow or arrangement of feed and discharge openings characterised by the disposition of the feed and discharge openings
    • B01F2025/9191—Direction of flow or arrangement of feed and discharge openings characterised by the disposition of the feed and discharge openings characterised by the arrangement of the feed openings for one or more flows, e.g. for the mainflow and the flow of an additional component
    • B01F2025/91912—Direction of flow or arrangement of feed and discharge openings characterised by the disposition of the feed and discharge openings characterised by the arrangement of the feed openings for one or more flows, e.g. for the mainflow and the flow of an additional component with feed openings at the circumference of the main flow
    • B01F2025/919121—Direction of flow or arrangement of feed and discharge openings characterised by the disposition of the feed and discharge openings characterised by the arrangement of the feed openings for one or more flows, e.g. for the mainflow and the flow of an additional component with feed openings at the circumference of the main flow with feed openings around the complete circumference of the main flow, e.g. being a perforated or porous part
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F25/00—Flow mixers; Mixers for falling materials, e.g. solid particles
    • B01F2025/91—Direction of flow or arrangement of feed and discharge openings
    • B01F2025/919—Direction of flow or arrangement of feed and discharge openings characterised by the disposition of the feed and discharge openings
    • B01F2025/9191—Direction of flow or arrangement of feed and discharge openings characterised by the disposition of the feed and discharge openings characterised by the arrangement of the feed openings for one or more flows, e.g. for the mainflow and the flow of an additional component
    • B01F2025/919125—Direction of flow or arrangement of feed and discharge openings characterised by the disposition of the feed and discharge openings characterised by the arrangement of the feed openings for one or more flows, e.g. for the mainflow and the flow of an additional component with feed openings in the center and at the circumference of the main flow

Definitions

  • the invention relates to the dispersion and dissolution of one or more gases in a pulp suspension.
  • It relates more particularly to a method according to the preamble of claim 1, as well as an apparatus for reacting oxygen in an oxygen-containing gas with a cellulose paste.
  • the mixing apparatus described in these patents is relatively complicated and expensive, and has moving parts which can cause breakdowns or static parts located in the flow path, which can lead to blockage.
  • GB-A-2 006 852 describes the addition of gaseous oxygen in any form in a dough (slurry). Despite the suggestion to use turbulence, not all of the oxygen can react to delignify the pulp.
  • US-A-3,545,731 describes an apparatus which can only be used with a liquid but cannot be used with a pulp such as a paper pulp.
  • An object of the present invention is therefore to provide an apparatus capable of efficiently dispersing and kneading oxygen in a pulp suspension, simplified and economical compared to the already existing apparatus.
  • the method according to the invention conforms to the characteristic of claim 1.
  • aqueous cellulose pulp suspension having a consistency of about 8 to about 16%, preferably 10 to 12%, by defining for the pulp a relatively unobstructed flow path passing through and past an oxygen introduction zone and circulating the suspension over the flow path at a rate at which it is fully fluidized and completely turbulent , so as to present substantially "Newtonian" properties, conditions in which it behaves substantially like water, and which produces a very effective mixing.
  • the oxygen is introduced into the suspension through diffusion means which have a fixed porous surface in contact with the rapidly moving suspension, in the oxygen introduction zone, in a finely divided form, of preferably in the form of a mass of fine bubbles having a diameter at most equal to about 2 to about 10 ⁇ m, for example. This causes the bubbles to disperse throughout the suspension without noticeable coalescence as it would occur with larger bubbles.
  • the apparatus according to the invention conforms to the characteristic of claim 11.
  • Preferred means for diffusing oxygen includes a sintered metal element of the type normally used for filtration.
  • a typical diffuser consists of a sintered stainless steel element produced by the process of US patent ⁇ A ⁇ 2,554,343. According to this process, the alloyed metal powder is passed through a particle size chosen in a sintering controlled atmosphere to produce sheets or shaped bodies made of porous media. The powdery metal particles melt at their contact points, resulting in a bonded mass with a homogeneous crystal structure. No binders or other foreign materials are introduced, which retains the basic properties of the metals or alloys used.
  • the element preferably has a thickness in the range of from about 0.8 mm to about 6.5 mm.
  • the pores must have a diameter less than about 10 ⁇ m, preferably in the range of from about 2 to about 10 ⁇ m, and the porosity should be substantially uniform over the entire surface of the porous wall and preferably in the range of about 40 to about 50%.
  • a widely used type is 316 L stainless steel (0.03% carbon maximum). Variable pore sizes are suitable.
  • the action of the diffusing device is to produce a mass of very fine bubbles in accordance with the principle set out in US patent ⁇ A ⁇ 3,545,731, with the exception that, to properly mix oxygen in a pulp suspension, special conditions must be observed which have been discovered and which are described in the present invention.
  • the diffuser element preferably includes a cylinder through which the suspension passes, and the walls of the cylinder define the flow path.
  • the diffuser element can form part of the cylinder wall, it can take the form of a porous tube placed transversely to the axis of the cylinder, or it can take the form of a porous tube along the axis of the cylinder .
  • the suspension path is relatively unobstructed.
  • the suspension passes directly through the cylinder without any obstruction.
  • this tube occupies a small space, so that most of the interior space of the cylinder is occupied by the suspension which passes through it while remaining in contact with the surface of the porous diffuser .
  • the diffusing apparatus comprises a duct having a cylindrical wall which defines the external limits of the passage and a closed capacity maintained inside the duct, and having a cylindrical wall which defines the internal limits of the passage .
  • a bubble producing surface having a large number of gas outlets, of microscopic dimensions is arranged on the wall of the duct as well as on the wall of the container.
  • a gas manifold surrounds the outer face of the wall of the duct and is connected to a source of pressurized gas which supplies the gas.
  • the closed capacity is also connected to a gas source.
  • the decrease in the friction curve indicates another change in the flow mechanism.
  • a continuous and purely liquid annular body was formed between the stopper and the wall. The flow in this ring is laminar.
  • a speed is reached beyond which the flow becomes turbulent.
  • the increase in speed intensifies the turbulence and the fiber plug decreases in size.
  • New fibers detach from the plug.
  • the whole suspension becomes turbulent when the speed is increased beyond the point where the flow curves of the fiber suspension and the water, after crossing, become parallel. All speed and consistency gradients have disappeared and the suspension behaves like a turbulent liquid.
  • the shear field imposed in the entire mass of fibers now exceeds the value of shear stress which is necessary to completely disintegrate the fibrous network.
  • the invention lends itself particularly well to a dosage of oxygen in stages in the suspension of pâté. This is obtained by introducing a stream of a suspension of cellulose pulp under pressure, supplied by a feed, into a reaction system composed of a plurality of successive stages, in each of which the stream first flows in a restricted section in which it acquires an appropriate speed to fluidize it and give it a liquid flow and, then, it circulates in an enlarged section in which it acquires a speed such that a plug flow occurs.
  • oxygen is injected into the flow of liquid under pressure, in the form of a large quantity of small bubbles to disperse these bubbles throughout the mass of the liquid, and as a result, most of oxygen reacts with the pulp in the plug flow at this point.
  • the treated suspension is then recovered at the exit from the final stage.
  • Another subject of the invention is another preferred method of kneading the suspension which leaves the oxygen diffuser by passing it through a dough fluidization mixer.
  • This mixer can be placed adjacent to the diffuser or placed some distance downstream. This mixer effectively ensures a new mixing and a new dispersion of the possible agglomerations of oxygen, so that the oxygen remains in a finely divided form and is mixed more intimately with the suspension.
  • a preferred static mixer consists of a tube which narrows to form a constriction, followed by a tube which widens to reach the diameter of the tube in which the dough suspension circulates. The speed through the throttle will be roughly the same as that seen in the diffuser.
  • the invention also relates to means for coordinating the flow of oxygen with the flow of the pulp suspension so as to maintain the appropriate dosage of oxygen.
  • a device for measuring the flow rate for example an orifice plate inserted in the path of the suspension, upstream of the diffuser, the orifice plate being connected to a differential pressure cell which, at in turn, is connected to a controller.
  • the controller sends a signal to an oxygen flow control valve, which regulates the supply of oxygen to the diffuser.
  • an orifice plate or other device in the oxygen supply line to measure the flow of oxygen.
  • the adjustment mechanism may include a computer which maintains a constant, predetermined and fixed ratio between oxygen and the dough, so as to increase or reduce the dosage to compensate for variations in the flow rate of the dough suspension.
  • the injection of oxygen into a suspension of cellulose pulp according to the invention can be used at different stages of the delignification and bleaching processes.
  • the aim is to decrease the Kappa index of the dough.
  • the Kappa index can be reduced from the range of 50 to 60 to the interval from 20 to 30.
  • the Kappa index can be reduced from the interval from 5 to 10 to the range 2.3 to 3.5.
  • the speed of the dough passing through the oxygen introduction zone is 0.5 to 50 meters per second, a preferred interval being 5 to 40 meters per second.
  • the surface of the fixed diffuser can have an area of approximately 0.0033 to 0.00033 dm 2 per unit of oxygen gas flow of 1 dm 3 per minute, or a flow of 30 to 300 normal cubic meters of oxygen per meter square of porous surface.
  • the dosage rate can vary from 2.5 to 25 kg, preferably from 5 to 7.5 kg per tonne of dough, on the basis of dry air (AD) for bleaching and from 10 to 50 kg per tonne for delignification.
  • AD dry air
  • the oxygen used can be molecular oxygen, as found commercially, which contains 90% or more and preferably 98% or more oxygen.
  • the oxygen can be in the form of a gas containing more than 50% oxygen.
  • the starting paste suspension can contain about 10 to about 20% air by volume of dispersed paste, which dilutes the added oxygen.
  • concentration of the gas containing added oxygen can be at the lower end of the oxygen content range and, for a dough containing a lot of air, the added gas may contain oxygen near the upper limit of the oxygen content range.
  • the oxygen content of the residual gas leaving the vent of the final treatment tank is measured. Based on this content, it can be determined whether a good mixing has been obtained. Adjustments can be made accordingly and the dosage can be changed in stages to obtain the best results.
  • the dosage can be uniformly proportioned at each stage or it can be modified to adapt to variations in conditions such as variations in the nature of the dough, or otherwise.
  • the invention is particularly flexible with regard to the equipment and the process, so that it can be used to economically equip a stationery after the fact.
  • the diffuser elements are relatively uncomplicated and inexpensive and can be easily installed in the installation.
  • Each factory has a certain nominal production capacity.
  • the present invention lends itself to being adjusted to suit this nominal capacity.
  • the variations in the flow rates of the pulp suspension and, therefore, of the oxygen, are related to the variations in the pulp demand, so the flow rate can be reduced due to a shutdown when the dough tanks are full .
  • the pulp suspension passes through a first oxygen diffuser and a plurality of reactors, each of which is preceded by an oxygen diffuser.
  • a fraction or all of the suspension can be introduced into a reaction tower to allow additional extraction with caustic soda.
  • Figs. 3,4, 5, and 7 are views showing variants of dough fluidizing diffusers.
  • the typical installation shown will be understood on reading the description of the path of the pulp suspension which circulates in this installation.
  • the particular stage represented in the bleaching process is the first stage of alkaline extraction.
  • An aqueous suspension of cellulose pulp containing caustic soda is pumped at a high surface speed, which arrives through the tube 15 from a supply source (not shown), through the pump A, in a line 17 which enters an oxygen diffuser B pulp fluidizer in which oxygen is diffused and dispersed in fine bubbles in the suspension stream.
  • the suspension current is pumped from a power source, via line 17, by pump A, at a speed such that the current behaves like water and that there is a strong turbulence in any mass. .
  • the oxygen is diffused in the stream through a diffuser B, in fine bubbles, as described below, in more detail, and it provides rapid mixing of the oxygen throughout the dough stream.
  • the stream of rapidly moving pulp which circulates in line 17 downstream of the oxygen diffuser, and which now contains dispersed oxygen, is introduced into the bottom of a first vertical reaction chamber C, in which it circulates at a much lower speed in the form of a plug. At this lower speed, the suspension loses its fluidity, trapping the oxygen dispersed in the stopper, which allows the oxygen and the caustic soda to react with the paste during the time when the suspension containing oxygen stay in room C.
  • the suspension is then sent rapidly, in turbulent flow, through a tube 19, into a second oxygen diffuser B, which it passes through and into the bottom of a second reaction chamber C, in which it remains for a certain period. to allow oxygen and bleaching chemicals to continue to react with cellulose.
  • the suspension is sent, by a tube 21 and through a third oxygen diffuser B 2 , to a third reaction chamber C 2 .
  • the dough then passes through a pressure adjustment valve 23 and, in a tube 24, to enter the upper end of a reaction tower D.
  • the dough stays in the tower D to give more time to the product bleaching chemical to react with cellulose.
  • the dough can flow in the opposite direction, that is to say from top to bottom through the chambers to reach the base or at the top of the reaction tower, or some or all of the reaction can be horizontal.
  • the pulp suspension leaving tower D is then collected at the outlet of an outlet tube 25 and it is washed to remove bleaching chemicals and impurities, or alternatively, depending on the stage of the bleaching process, the suspension processed can be sent to another processing.
  • a vent 29 for the gaseous effluents makes it possible to monitor the excess oxygen, with a view to regulating the process so as to ensure greater efficiency.
  • FIG. 2 there is a static mixer fluidizing dough or throttle E, in series with the diffuser B, between this diffuser and the first reaction column through which the suspension stream containing oxygen is passed.
  • the constriction has a constricted neck which narrows the current and increases its speed. This ensures better dispersion of the oxygen gas bubbles throughout the mass of pulp suspension.
  • the paste fluidiser oxygen diffuser B is composed, as shown on a larger scale in FIG. 3, a cylinder 31 which forms a passage 32 for the suspension of dough.
  • the cylinder has flanges 33, 34 on its ends, for connection to a pipe.
  • a porous metal tube 35 is placed diametrically across the passage 32 which, with this exception, is free; this tube enters an opening in the side of the cylinder 31, through a connector 31a which is fixed and sealed in place by a tube connector 6 of the "SWAGELOK" type (registered trademark).
  • the wall of the tube 35 is preferably made of sintered metal.
  • the paste is continuously introduced at high speed through the passage 32 while oxygen is supplied to the porous tube 35 and it diffuses into the paste through the wall of this tube.
  • FIG. 4 A variant of the diffuser is shown in FIG. 4.
  • the wall of the cylinder 41 has a porous cylindrical central section 45.
  • a manifold 47 encloses this section from the outside and an oxygen connection 48 is provided leading to the manifold. The passage of the paste suspension is thus entirely free.
  • FIG. 5 Another form of diffuser is shown in FIG. 5.
  • a device 55 in the form of an annular crown is placed against the cylinder 51 and surrounds the passage 52.
  • the tube 55 is provided with suitable small orifices 56 through which the oxygen is diffused.
  • the passage 52 is entirely free.
  • FIG. 6 Another form of diffuser is shown in FIG. 6.
  • a porous tube 65 extends axially and in a central position along the passage 62 and is held in this passage by suitable supports or cross-pieces 69 which are connected inside the wall of the cylinder 61.
  • a tube oxygen 68 is placed from the outside of the cylinder at the end of the diffusion tube 65.
  • FIG. 7 shows yet another preferred form of diffuser.
  • This diffuser is mounted between mounting flanges 100 and 101, from which start short sections of cylindrical tubes 102 and 103 respectively, which are connected to oblique tapered flanges 104 and 105.
  • a double wall J composed of a wall cylindrical inner wall 111, is placed between the ends of the flanges 104 and 105, a cylindrical outer wall 107 without perforation and the annular walls of perforated ends 108 and 109.
  • the microporous inner wall 111 is preferably made of sintered metal, as before described.
  • the tube sections 102 and 103, the flanges 104 and 105 and the porous wall 111 form an enclosure.
  • a cylinder K having a microporous wall 113 (preferably made of sintered metal), capped at its ends with impermeable frustoconical caps 115 and 117.
  • the cylinder K is suitably supported by a cross device S not shown, to maintain its position in the duct.
  • the wall 113 of the cylinder K and the wall 111 of the double wall J form between them an annular passage P.
  • a pipe 119 for supplying oxidizing gas connects a source of oxidizing gas under pressure to the interior space of the double wall J
  • An oxidizing gas supply tube 121 connects a source of oxidizing gas under pressure to the interior volume of the cylinder K.
  • the diffuser is connected, in the same way as in the connection of the other embodiments described above, by the flange 100, to an inlet tube for the paste suspension and, by the flange 101, to a tube which transports the outgoing dough suspension.
  • the paste suspension is introduced into the left end of the device, at a fluidization speed, and flows at high speed through the passage P between the microporous walls 111 and 113, and it exits through the far right.
  • An oxidizing gas under pressure is introduced through the tubes 119 and 121 and diffused in fine bubbles through the opposite porous walls 111 and 113, in the suspension of fluidized paste which circulates rapidly in the passge C, as previously described.
  • the diffusion of the gas in the suspension is extremely efficient due to the large diffusion surface formed at the two limits of the fluidized pulp stream.
  • the static dough fluidizer mixer E shown in FIG. 2, connected in series with the oxygen diffuser B, comprises an elongated tube in one piece of circular cross section, between the flanges 81 and 82.
  • a first frustoconical part 83 narrows from the flange 81 to be connected, at a neck 84, at the small end of a longer frusto-conical part 85, which flares, and which ends at the flange 82.
  • the mixer E is connected to a connection piece F which has an upstream flange 90 connected to the flange 34 with the interposition of a seal.
  • the transition piece F has a downstream flange 91 assembled to the flange 81 of the mixer.
  • the paste suspension momentarily circulates in a wider section then in a restricted section substantially to the section of the diffuser tube.
  • the size (the inside diameter) of the cylinder 31 into which the pulp suspension is pumped and brought into contact with oxygen varies according to the capacity of the pulp mill. Typical diameters range from about 50 to about 300 mm and usually from about 75 to about 150 mm.
  • reaction chambers C, C, and C 2 Three reaction chambers C, C, and C 2 are shown .
  • a greater or lesser number of reaction chambers can be used, depending on the material to be treated, the amount of oxygen added and the desired bleaching effect.
  • the effects of several diffusers, each of which is equipped with a reaction chamber, consist in adding in stages large additions of oxygen, by adding small amounts to each diffuser, so as to obtain: This expression is calculated at the pressure of the particular stage considered.
  • the speed at which the slurry suspension is discharged through the diffuser cylinder 31 is a high velocity which makes the current behave like water and makes it easier to disperse small bubbles of oxygen which are mixed throughout the mass of the current.
  • the pressure of the pulp suspension may be about 0.1 to 1 megapascals (MPa), and the oxygen pressure of about 0.14 to 1.4 MPa.
  • the residence time of the paste in the cylinder 31 of the diffuser is usually in the range of about 0.001 to about 0.120 seconds.
  • the residence time in each of reactors C, C 1 and C 2 is usually in the range of about 1 minute to about 5 minutes and the residence time in tower D is in the range of about 30 to about 90 minutes.
  • the pressure in each reactor is approximately 0.1 to 1 MPa.
  • the invention does not exclude the use or the combination with the diffusers for the introduction of oxygen into the dough, of high speed mixers such as, for example, those described in US patent brevet A ⁇ 4,295,926, in the name of WEYERHAEUSER, or of a "KAMYR MC" registered trademark pump).
  • the nature of the treated pulp can be wood softwoods, hardwoods or a mixture having a consistency in the range of 5 to 16%.
  • Pasta can be treated according to the invention at different stages of the delignification-bleaching process.
  • the starting dough in the caustic extraction stage of a bleaching operation may have a Kappa number in the range of 5.0 to 10.0 and the bleached dough exiting from tank D may have, after washing, an index of 2.5 to 3.5.
  • the suspension takes on the characteristics of water.
  • the suspension passes in close contact with the diffusing element and the oxygen leaves through the pores of this element in the form of bubbles of very small diameter.
  • These bubbles are well dispersed due to the turbulence of the dough and they have a large specific surface, compared to their volume, to ensure mass transfer through the gas / liquid interface.
  • the mass transfer of oxygen is considerably enhanced by this dispersing process with fluidization.
  • the suspension can be subjected to a pre and post-treatment before being pushed back into the first diffuser B, the suspension has undergone normal treatments in a delignification-bleaching operation.
  • the suspension contains small concentrations of other bleaching chemicals.
  • the suspension is provided at a temperature in the range of about 40 ° C to about 90 ° C.
  • the suspension leaving the bleaching tower D will be washed to remove bleaching chemicals and other impurities.
  • a typical bleaching facility could have a processing capacity of 1000 tonnes of pulp per day, 10% consistency with an oxygen quantity of 5 kg per ton, or 5000 kg of oxygen. Under these conditions, this flow could be 6.321 liters per minute, with a speed of circulation of the suspension through the diffuser of 40 meters per second, and an oxygen flow of 3.13 kg per minute. The diffusion surface would be around 0.26 square decimetres.

Description

L'invention concerne la dispersion et la dissolution d'un gaz ou de plusieurs gaz dans une suspension de pâte.The invention relates to the dispersion and dissolution of one or more gases in a pulp suspension.

Elle concerne plus particulièrement un procédé selon le préambule de la revendication 1, ainsi qu'un appareil par faire re'agir de l'oxygène an um gaz contenant de l'oxygène avec une pâte cellulosique.It relates more particularly to a method according to the preamble of claim 1, as well as an apparatus for reacting oxygen in an oxygen-containing gas with a cellulose paste.

On a déjà effectué un travail considérable sur l'utilisation de l'oxygène conjointement avec d'autres produits chimiques, ainsi que ceci ressort du compte rendu d'un Symposium de l'Association Technique de l'Industrie de la Pâte et du Papier ("Technical Association of the Pulp and Paper Industry Inc.") (TAPPI) qui s'est tenu en Novembre 1984 et a été exclusivement consacré à la délignification et au blanchiment de la pâte chimique par l'oxygène, l'ozone et le peroxyde d'hydrogène.Considerable work has already been done on the use of oxygen in conjunction with other chemicals, as is evident from the proceedings of a Symposium of the Technical Association of the Pulp and Paper Industry ( "Technical Association of the Pulp and Paper Industry Inc.") (TAPPI) which was held in November 1984 and was exclusively devoted to the delignification and bleaching of the chemical pulp with oxygen, ozone and peroxide hydrogen.

Le président du Symposium a souligné que la chimie de cette nouvelle technologie était relativement complexe et mal connue et qu'elle représentait des domaines de recherche extrêmement intéressants du point de vue scientifique. Les principes classiques actuels d'élimination de la lignine ne sont plusvalides. Bien que la commercialisation soit en cours, la chimie de base de ces procédés n'est pas largement connue.The president of the Symposium pointed out that the chemistry of this new technology is relatively complex and poorly understood and that it represents fields of research that are extremely interesting from a scientific point of view. The current conventional principles of lignin elimination are no longer valid. Although marketing is underway, the basic chemistry of these processes is not widely known.

On recherche dans l'utilisation de l'oxygène, un moyen d'introduction de ce gaz dans la suspension de pâte pour obtenir une bonne dispersion en vue d'une dissolution conduisant à la délignification ou au blanchiment. Selon un dispositif de mélange, on utilise des malaxeurs à grande vitesse, compliqués et coûteux, tels que ceux décrits dans les brevets US―A―4295925, US―A―4 295 926, US―A―4 295 927, US-A-4 298 426 et US―A―4 198 427. Dans un autre dispositif, on utilise un disque tournant à orifices intercalé dans le trajet de circulation de la suspension, comme décrit dans le brevet US―A― 4 427 489 au nom de Jacobsen (1984).One seeks in the use of oxygen, a means of introducing this gas into the pulp suspension to obtain a good dispersion with a view to dissolution leading to delignification or bleaching. According to a mixing device, high-speed, complicated and expensive mixers are used, such as those described in patents US ― A ― 4,295,925, US ― A ― 4,295,926, US ― A ― 4,295,927, US-A -4,298,426 and US ― A ― 4,198,427. In another device, a rotating disc with orifices inserted in the circulation path of the suspension is used, as described in US patent ― A― 4,427,489 in the name of Jacobsen (1984).

Un autre dispositif de mélange est décrit dans "EO Mixing Breakthrough", paru dans la revue "TAPPI Journal" Août 1984, page 6, au nom de Komax Systems Inc. Ici, on utilise un mélangeur statique pour mélanger l'oxygène dans la pâte. Les bulles et la pâte sont mélangées par huit chicanes internes disposées excentriquement, dont on dit qu'elles fluidisent, rebrassent et mélangent les bulles et la pâte de façon à réaliser un mélange supérieur à celui des mélangeurs mécaniques, en faisant en sorte que les bulles soient dispersées dans les fibres.Another mixing device is described in "EO Mixing Breakthrough", published in the review "TAPPI Journal" August 1984, page 6, on behalf of Komax Systems Inc. Here, a static mixer is used to mix the oxygen in the dough . The bubbles and the dough are mixed by eight internal baffles arranged eccentrically, which are said to fluidize, re-mix and mix the bubbles and the dough so as to produce a mixture which is superior to that of mechanical mixers, ensuring that the bubbles are dispersed in the fibers.

L'appareil mélangeur décrit dans ces brevets est relativement compliqué et coûteux, et comporte des parties mobiles qui peuvent provoquer des pannes ou des parties statiques situées sur le trajet de l'écoulement, qui peuvent conduire à un bouchage.The mixing apparatus described in these patents is relatively complicated and expensive, and has moving parts which can cause breakdowns or static parts located in the flow path, which can lead to blockage.

GB-A-2 006 852 décrit l'addition d'oxygène gazeux sous une forme quelconque dans une pâte (slurry). Malgré la suggestion d'utiliser une turbulence, on ne peut faire réagir tout l'oxygène pour délignifier la pâte à papier.GB-A-2 006 852 describes the addition of gaseous oxygen in any form in a dough (slurry). Despite the suggestion to use turbulence, not all of the oxygen can react to delignify the pulp.

US-A-3 545 731 décrit un appareil qui ne peut être utilisé qu'avec un liquide mais ne peut être utilisé avec une pâte telle qu'une pâte à papier.US-A-3,545,731 describes an apparatus which can only be used with a liquid but cannot be used with a pulp such as a paper pulp.

B. S. Kirk et al dans l'article intitulé "Low consistency oxygen delignification in a pipe line reac- tor" décrit un système semblable à celui décrit dans GB-A-2 006 852 et ayant les mêmes inconvénients.B. S. Kirk et al in the article entitled "Low consistency oxygen delignification in a pipe line reactor" describes a system similar to that described in GB-A-2 006 852 and having the same drawbacks.

Un objet de la présente invention est donc de réaliser un appareil capable de disperser et de malaxer efficacement l'oxygène dans une suspension de pâte, simplifié et économique comparativement à l'appareillage déjà existant.An object of the present invention is therefore to provide an apparatus capable of efficiently dispersing and kneading oxygen in a pulp suspension, simplified and economical compared to the already existing apparatus.

Le procédé selon l'invention est conforme à la caractéristique de la revendication 1.The method according to the invention conforms to the characteristic of claim 1.

Il a été constaté que l'on peut réaliser un malaxage efficace de l'oxygène avec une suspension de pâte cellulosique aqueuse ayant une consistance d'environ 8 à environ 16%, de préférence de 10 à 12%, en définissant pour la pâte un trajet d'écoulement relativement non obstrué passant au droit d'une zone d'introduction de l'oxygène et au-delà, et en faisant circuler la suspension sur le trajet d'écoulement à une vitesse à laquelle elle est entièrement fluidisée et complètement turbulente, de façon à présenter des propriétés sensiblement "Newtoniennes", conditions dans lesquelles elle se comporte sensiblement comme de l'eau, et qui produit un malaxage très efficace. De préférence, l'oxygène est introduit dans la suspension à travers des moyens de diffusion qui possèdent une surface poreuse fixe en contact avec la suspension en mouvement rapide, dans la zone d'introduction de l'oxygène, sous une forme finement divisée, de préférence sous la forme d'une masse de fines bulles ayant un diamètre au plus égal à environ 2 à environ 10 pm, par exemple. Ceci provoque la dispersion des bulles dans toute la suspension sans coalescence notable telle qu'elle se produirait avec des bulles plus grosses.It has been found that effective mixing of oxygen can be carried out with an aqueous cellulose pulp suspension having a consistency of about 8 to about 16%, preferably 10 to 12%, by defining for the pulp a relatively unobstructed flow path passing through and past an oxygen introduction zone and circulating the suspension over the flow path at a rate at which it is fully fluidized and completely turbulent , so as to present substantially "Newtonian" properties, conditions in which it behaves substantially like water, and which produces a very effective mixing. Preferably, the oxygen is introduced into the suspension through diffusion means which have a fixed porous surface in contact with the rapidly moving suspension, in the oxygen introduction zone, in a finely divided form, of preferably in the form of a mass of fine bubbles having a diameter at most equal to about 2 to about 10 µm, for example. This causes the bubbles to disperse throughout the suspension without noticeable coalescence as it would occur with larger bubbles.

L'appareil selon l'invention est conforme à la caractéristique de la revendication 11.The apparatus according to the invention conforms to the characteristic of claim 11.

Des moyens préférés de diffusion de l'oxygène comprennent un élément en métal fritté du type normalement utilisé pour la filtration. Un diffuseur type est constitué par un élément en acier inoxydable fritté produit par le procédé du brevet US―A―2 554 343. Selon ce procédé, on fait passer la poudre métallique alliée d'une dimension de particules choisies dans un four de frittage à atmosphère contrôlée pour produire des feuilles ou corps de forme faits de milieux poreux. Les particules métalliques pulvérulentes fondent à leurs points de contact, avec pour résultat une masse liée possédant une structure cristalline homogène. On n'introduit ni liant ni autres matières étrangères, ce qui conserve les propriétés de base des métaux ou alliages utilisés. L'élément a de préférence une épaisseur comprise dans l'intervalle allant d'environ 0,8 mm à environ 6,5 mm. Les pores doivent avoir un diamètre inférieur à environ 10 pm, de préférence dans l'intervalle allant d'environ 2 à environ 10 pm, et la porosité doit être sensiblement uniforme sur toute la surface de la paroi poreuse et de préférence dans l'intervalle d'environ 40 à environ 50%.Preferred means for diffusing oxygen includes a sintered metal element of the type normally used for filtration. A typical diffuser consists of a sintered stainless steel element produced by the process of US patent ― A ― 2,554,343. According to this process, the alloyed metal powder is passed through a particle size chosen in a sintering controlled atmosphere to produce sheets or shaped bodies made of porous media. The powdery metal particles melt at their contact points, resulting in a bonded mass with a homogeneous crystal structure. No binders or other foreign materials are introduced, which retains the basic properties of the metals or alloys used. The element preferably has a thickness in the range of from about 0.8 mm to about 6.5 mm. The pores must have a diameter less than about 10 µm, preferably in the range of from about 2 to about 10 µm, and the porosity should be substantially uniform over the entire surface of the porous wall and preferably in the range of about 40 to about 50%.

Les métaux poreux dans lesquels jusqu'à 50% du volume est fait de vides ou pores intercommu- nicants faisant partie d'une large gamme de matériaux comprenant le nickel, le "Monel" (marque déposée), "Inconel" (marque déposée) et un grand nombre d'alliages inoxydables des séries 300 et 400. Un type largement utilisé est l'acier inoxydable 316 L (0,03% de carbone au maximum). Des dimensions de pores variables conviennent.Porous metals in which up to 50% of the volume is made of intercommunicating voids or pores from a wide range of materials including nickel, "Monel" (registered trademark), "Inconel" (registered trademark) and a large number of stainless alloys of the 300 and 400 series. A widely used type is 316 L stainless steel (0.03% carbon maximum). Variable pore sizes are suitable.

Le dispositif diffuseur a pour action de produire une masse de bulles très fines conformément au principe exposé dans le brevet US―A―3 545 731, à l'exception du fait que, pour mélanger correctement de l'oxygène dans une suspension de pâte, on doit observer des conditions spéciales, qui ont été découvertes et qui sont décrites dans la présente invention.The action of the diffusing device is to produce a mass of very fine bubbles in accordance with the principle set out in US patent ― A ― 3,545,731, with the exception that, to properly mix oxygen in a pulp suspension, special conditions must be observed which have been discovered and which are described in the present invention.

L'élément diffuseur comprend de préférence un cylindre à travers lequel passe la suspension, et les parois du cylindre définissent le trajet d'écoulement. L'élément diffuseur peut former une partie de la paroi du cylindre, il peut prendre la forme d'un tube poreux placé transversalement à l'axe du cylindre, ou il peut prendre la forme d'un tube poreux selon l'axe du cylindre. Dans tous les cas, le trajet de la suspension est relativement non obstrué. Dans le cas où le cylindre a une paroi poreuse, la suspension passe directement à travers le cylindre sans aucune obstruction. Dans le cas d'un tube placé dans le cylindre, ce tube occupe un petit espace, de sorte que la majeure partie de l'espace intérieur du cylindre est occupée par la suspension qui le traverse en restant en contact avec la surface du diffuseur poreux.The diffuser element preferably includes a cylinder through which the suspension passes, and the walls of the cylinder define the flow path. The diffuser element can form part of the cylinder wall, it can take the form of a porous tube placed transversely to the axis of the cylinder, or it can take the form of a porous tube along the axis of the cylinder . In all cases, the suspension path is relatively unobstructed. In the case where the cylinder has a porous wall, the suspension passes directly through the cylinder without any obstruction. In the case of a tube placed in the cylinder, this tube occupies a small space, so that most of the interior space of the cylinder is occupied by the suspension which passes through it while remaining in contact with the surface of the porous diffuser .

Selon une variante préférée de réalisation, l'appareil diffuseur comprend un conduit présentant une paroi cylindrique qui définit les limites extérieures du passage et une capacité fermée maintenue à l'intérieur de la conduite, et possédant une paroi cylindrique qui définit les limites intérieures du passage. Une surface de production de bulles ayant un grand nombre de sorties de gaz, de dimensions microscopiques est disposée sur la paroi du conduit ainsi que sur la paroi de la capacité. Un collecteur de gaz entoure la face externe de la paroi du conuit et est relié à une source de gaz sous pression qui fournit le gaz. La capacité fermée est elle aussi reliée à une source de gaz. Avec cet agencement, il se forme un passage annulaire à travers lequel on fait circuler un courant de suspension de pâte à la vitesse de fluidisation, en même temps qu'on introduit le gaz dans le courant par diffusion à travers les surfaces micro-poreuses situées des deux côtés du passage annulaire.According to a preferred embodiment, the diffusing apparatus comprises a duct having a cylindrical wall which defines the external limits of the passage and a closed capacity maintained inside the duct, and having a cylindrical wall which defines the internal limits of the passage . A bubble producing surface having a large number of gas outlets, of microscopic dimensions is arranged on the wall of the duct as well as on the wall of the container. A gas manifold surrounds the outer face of the wall of the duct and is connected to a source of pressurized gas which supplies the gas. The closed capacity is also connected to a gas source. With this arrangement, an annular passage is formed through which a stream of pulp suspension is circulated at the fluidization speed, at the same time as the gas is introduced into the stream by diffusion through the microporous surfaces located on both sides of the annular passage.

La nature de l'écoulement de la suspension dans les passages, est décrité dans un article intitulé "Medium Consistency Technology" "Journal of the Technical Association of the Pulp and Paper Industry", volume 64, no. 6, Juin 1981, de Gullichsen et Haerkoenen. Avant qu'il ne puisse se produire un mouvement de la suspension dans un tube, on doit désagréger par une force de cisaillement le bouchon de fibres dispersées qui est en contact avec la paroi du tube. La mise en mouvement commence lorsque cette contrainte limite a été dépasée. La suspension se déplace tout d'abord sous la forme d'un corps cisaillé, visco-élastique, en contact direct avec la paroi. Lorsqu'on augmente la vitesse, des fibres et des flocons retenus de façon lâche se détachent du bouchon et pénètrent dans l'espace annulaire compris entre la paroi et le bouchon. La décroissance de la courbe de friction indique une autre modification du mécanisme d'écoulement. Il s'est formé un corps annulaire continu et purement liquide entre le bouchon et la paroi. L'écoulement dans cet anneau est laminaire. Lorsque le débit s'accroît, on atteint une vitesse au-delà de laquelle l'écoulement devient turbulent. L'accroissement de la vitesse intensifie la turbulence et le bouchon de fibres diminue de dimension. De nouvelles fibres se détachent du bouchon. L'ensemble de la suspension devient turbulent lorsque la vitesse est augmentée au-delà du point où les courbes d'écoulement de la suspension de fibres et de l'eau, après s'être croisées, deviennent parallèles. Tous les gradients de vitesse et de consistance ont disparu et la suspension se comporte comme un liquide turbulent. Le champ de cisaillement imposé dans toute la masse de fibres excède maintenant la valeur de contrainte de cisaillement qui est nécessaire pour désagréger totalement le réseau fibreux.The nature of the suspension flow in the passages, is described in an article entitled "Medium Consistency Technology" "Journal of the Technical Association of the Pulp and Paper Industry", volume 64, no. 6, June 1981, by Gullichsen and Haerkoenen. Before movement of the suspension in a tube can occur, the plug of dispersed fibers which is in contact with the wall of the tube must be disintegrated by shearing force. The setting in motion begins when this limiting constraint has been exceeded. The suspension first moves in the form of a sheared, visco-elastic body, in direct contact with the wall. As the speed is increased, loose fibers and flakes detach from the plug and enter the annular space between the wall and the plug. The decrease in the friction curve indicates another change in the flow mechanism. A continuous and purely liquid annular body was formed between the stopper and the wall. The flow in this ring is laminar. When the flow increases, a speed is reached beyond which the flow becomes turbulent. The increase in speed intensifies the turbulence and the fiber plug decreases in size. New fibers detach from the plug. The whole suspension becomes turbulent when the speed is increased beyond the point where the flow curves of the fiber suspension and the water, after crossing, become parallel. All speed and consistency gradients have disappeared and the suspension behaves like a turbulent liquid. The shear field imposed in the entire mass of fibers now exceeds the value of shear stress which is necessary to completely disintegrate the fibrous network.

Il a également été constaté que, lorsque la suspension liquide dans laquelle l'oxygène a été dispersé, parvient directement à un réacteur dans lequel elle se rassemble et se déplace à une vitesse beaucoup plus réduite, à la façon d'un bouchon, on obtient une bonne réaction entre l'oxygène et la pâte.It has also been found that, when the liquid suspension in which the oxygen has been dispersed, reaches directly a reactor in which it collects and moves at a much slower speed, in the manner of a plug, one obtains a good reaction between oxygen and the dough.

L'invention se prête particulièrement bien à un dosage d'oxygène par paliers dans la suspension de pâté. Ceci est obtenu en introduisant un courant d'une suspension de pâte cellulosique sous pression, fournie par une alimentation, dans un système réactionnel composé d'une pluralité d'étages successifs, dans chacun desquels le courant circule d'abord dans une section restreinte dans laquelle il acquiert une vitesse appropriée pour le fluidiser et lui conférer un écoulement liquide et, ensuite, il circule dans une section élargie dans laquelle il acquiert une vitesse telle qu'il se produit un écoulement en bouchon. Dans chaque étage, de l'oxygène est injecté dans l'écoulement de liquide sous pression, sous la forme d'une grande quantité de petites bulles pour disperser ces bulles dans toute la masse du liquide, et il en résulte que la majeure partie de l'oxygène entre en réaction avec la pâte dans l'écoulement en bouchon à ce stade. La suspension traitée est ensuite récupérée à la sortie de l'étage final.The invention lends itself particularly well to a dosage of oxygen in stages in the suspension of pâté. This is obtained by introducing a stream of a suspension of cellulose pulp under pressure, supplied by a feed, into a reaction system composed of a plurality of successive stages, in each of which the stream first flows in a restricted section in which it acquires an appropriate speed to fluidize it and give it a liquid flow and, then, it circulates in an enlarged section in which it acquires a speed such that a plug flow occurs. In each stage, oxygen is injected into the flow of liquid under pressure, in the form of a large quantity of small bubbles to disperse these bubbles throughout the mass of the liquid, and as a result, most of oxygen reacts with the pulp in the plug flow at this point. The treated suspension is then recovered at the exit from the final stage.

L'invention a également pour objet un autre mode préféré de malaxage de la suspension qui sort du diffuseur d'oxygène en la faisant passer dans un mélangeur de fluidisation de la pâte. Ce mélangeur peut être placé adjacent au diffuseur ou placé à une certaine distance en aval. Ce mélangeur assure efficacement un nouveau malaxage et une nouvelle dispersion des éventuelles agglomérations d'oxygène, de sorte que l'oxygène reste sous une forme finement divisée et est mélangé plus intimement avec la suspension. Un mélangeur statique préféré est composé d'un tube qui se rétrécit pour former un étranglement, suivi d'un tube qui s'évase pour atteindre le diamètre du tube dans lequel la suspension de pâte circule. La vitesse à travers l'étranglement sera à peu près la même que celle qu'on observe dans le diffuseur.Another subject of the invention is another preferred method of kneading the suspension which leaves the oxygen diffuser by passing it through a dough fluidization mixer. This mixer can be placed adjacent to the diffuser or placed some distance downstream. This mixer effectively ensures a new mixing and a new dispersion of the possible agglomerations of oxygen, so that the oxygen remains in a finely divided form and is mixed more intimately with the suspension. A preferred static mixer consists of a tube which narrows to form a constriction, followed by a tube which widens to reach the diameter of the tube in which the dough suspension circulates. The speed through the throttle will be roughly the same as that seen in the diffuser.

L'invention a également pour object des moyens pour coordonner l'écoulement de l'oxygène avec l'écoulement de la suspension de pâte de manière à conserver le dosage approprié d'oxygène. Ceci peut être obtenu par un appareil qui comprend un dispositif de mesure du débit, par exemple une plaque à orifice intercalée sur le trajet de la suspension, en amont du diffuseur, la plaque à orifice étant reliée à une cellule à pression différentielle qui, à son tour, est reliée à un contrôleur.The invention also relates to means for coordinating the flow of oxygen with the flow of the pulp suspension so as to maintain the appropriate dosage of oxygen. This can be obtained by an apparatus which comprises a device for measuring the flow rate, for example an orifice plate inserted in the path of the suspension, upstream of the diffuser, the orifice plate being connected to a differential pressure cell which, at in turn, is connected to a controller.

Le contrôleur envoie un signal à une valve de réglage du débit d'oxygène, qui règle la fourniture d'oxygène au diffuseur. Il y a une plaque à orifice ou un autre dispositif dans la conduite d'alimentation en oxygène, pour mesurer le débit d'oxygène. Le mécanisme de réglage peut comprendre un ordinateur qui maintient un rapport constant, prédéterminé et fixé, entre l'oxygène et la pâte, de façon à faire croître ou réduire le dosage pour compenser les variations de débit de la suspension de pâte.The controller sends a signal to an oxygen flow control valve, which regulates the supply of oxygen to the diffuser. There is an orifice plate or other device in the oxygen supply line, to measure the flow of oxygen. The adjustment mechanism may include a computer which maintains a constant, predetermined and fixed ratio between oxygen and the dough, so as to increase or reduce the dosage to compensate for variations in the flow rate of the dough suspension.

L'injection d'oxygène dans une suspension de pâte cellulosique selon l'inventon peut être utilisée à différents stades des processus de délignification et de blanchiment. Le but visé consiste à diminuer l'indice Kappa de la pâte. Pour la délignification, l'indice Kappa peut être ramené de l'intervalle de 50 à 60 à l'intervalle de 20 à 30. Pour le blanchiment, l'indice Kappa peut être ramené de l'intervalle de 5 à 10 à l'intervalle de 2,3 à 3,5.The injection of oxygen into a suspension of cellulose pulp according to the invention can be used at different stages of the delignification and bleaching processes. The aim is to decrease the Kappa index of the dough. For delignification, the Kappa index can be reduced from the range of 50 to 60 to the interval from 20 to 30. For laundering, the Kappa index can be reduced from the interval from 5 to 10 to the range 2.3 to 3.5.

Habituellement, la vitesse de la pâte qui traverse la zone d'introduction de l'oxygène est de 0,5 à 50 mètres par seconde, un intervalle préféré étant de 5 à 40 mètres par seconde. La surface du diffuseur fixe peut avoir une aire d'environ 0,0033 à 0,00033 dm2 par unité de débit de gaz oxygène de 1 dm3 par minute, ou un débit de 30 à 300 mètres cube normaux d'oxygène par mètre carré de surace poreuse.Usually, the speed of the dough passing through the oxygen introduction zone is 0.5 to 50 meters per second, a preferred interval being 5 to 40 meters per second. The surface of the fixed diffuser can have an area of approximately 0.0033 to 0.00033 dm 2 per unit of oxygen gas flow of 1 dm 3 per minute, or a flow of 30 to 300 normal cubic meters of oxygen per meter square of porous surface.

Le taux de dosage peut varier de 2,5 à 25 kg, de préférence de 5 à 7,5 kg par tonne de pâte, sur la base d'air sec (AD) pour le blanchiment et de 10 à 50 kg par tonne pour la délignification.The dosage rate can vary from 2.5 to 25 kg, preferably from 5 to 7.5 kg per tonne of dough, on the basis of dry air (AD) for bleaching and from 10 to 50 kg per tonne for delignification.

L'oxygène utilisé peut être de l'oxygène moléculaire, tel qu'on le trouve dans le commerce, qui contient 90% ou plus et de préférence 98% ou plus d'oxygène. Ou encore, l'oxygène peut être sous la forme d'un gaz contenant plus de 50% d'oxygène. La suspension de pâte de départ peut contenir environ 10 à environ 20% d'air en volume de pâte dispersée, qui dilue l'oxygène ajouté. Pour une pâte contenant peu ou pas d'air, la concentration du gaz contenant de l'oxygène ajouté peut être à l'extrémité inférieure de l'intervalle de teneur en oxygène et, pour une pâte contenant beaucoup d'air, le gaz ajouté peut contenir de l'oxygène à une valeur proche de la limite supérieure de l'intervalle de teneur en oxygène.The oxygen used can be molecular oxygen, as found commercially, which contains 90% or more and preferably 98% or more oxygen. Alternatively, the oxygen can be in the form of a gas containing more than 50% oxygen. The starting paste suspension can contain about 10 to about 20% air by volume of dispersed paste, which dilutes the added oxygen. For a dough with little or no air, the concentration of the gas containing added oxygen can be at the lower end of the oxygen content range and, for a dough containing a lot of air, the added gas may contain oxygen near the upper limit of the oxygen content range.

Pour régler le dosage de l'oxygène de façon à satisfaire les conditions, on mesure la teneur en oxygène du gaz résiduel qui sort de l'évent de la cuve de traitement final. En se basant sur cette teneur, on peut déterminer si l'on a obtenu un bon malaxage. On peut exécuter des réglages en conséquence et modifier le dosage par paliers de façon à obtenir les meilleurs résultats.To adjust the oxygen dosage to meet the conditions, the oxygen content of the residual gas leaving the vent of the final treatment tank is measured. Based on this content, it can be determined whether a good mixing has been obtained. Adjustments can be made accordingly and the dosage can be changed in stages to obtain the best results.

Dans un processus à étages multiples tel que celui décrit, on peut ainsi ajouter de grandes quantités totales d'oxygène, par paliers, à une suspension de pâte, en ajoutant de petites quantités à chaque stade conformément à la formule suivante:

Figure imgb0001
Cette formule est calculée à la pression de travail du stade particulier considéré.In a multi-stage process such as that described, it is thus possible to add large total quantities of oxygen, in stages, to a pulp suspension, by adding small quantities at each stage in accordance with the following formula:
Figure imgb0001
 This formula is calculated at the working pressure of the particular stage considered.

Le dosage peut être proportionné uniformément à chaque stade ou il peut être modifié de façon à s'adapter aux variations des conditions telles que les variations de la nature de la pâte, ou autrement.The dosage can be uniformly proportioned at each stage or it can be modified to adapt to variations in conditions such as variations in the nature of the dough, or otherwise.

L'invention est particulièrement souple en ce qui concerne l'équipement et le procédé, de sorte qu'elle peut être utilisée pour équiper économiquement après coup une papeterie. Les éléments diffuseurs sont relativement peu compliqués et peu coûteux et peuvent être installés facilement dans l'installation.The invention is particularly flexible with regard to the equipment and the process, so that it can be used to economically equip a stationery after the fact. The diffuser elements are relatively uncomplicated and inexpensive and can be easily installed in the installation.

Chaque usine a une certaine capacité nominale de production. La présente invention se prête à être ajustée pour s'adapter à cette capacité nominale. Les variations des débits de la suspension de pâte et, par conséquent, de l'oxygène, sont en relation avec les variations de la demande en pâte, ainsi le débit peut être réduit en raison d'un arrêt lorsque les réservoirs de pâte sont pleins.Each factory has a certain nominal production capacity. The present invention lends itself to being adjusted to suit this nominal capacity. The variations in the flow rates of the pulp suspension and, therefore, of the oxygen, are related to the variations in the pulp demand, so the flow rate can be reduced due to a shutdown when the dough tanks are full .

Dans un appareil préféré, la suspension de pâte traverse un premier diffuseur d'oxygène et une pluralité de réacteurs dont chacun est précédé d'un diffuseur d'oxygène. Une fraction ou la totalité de la suspension peut être introduite dans une tour de réaction pour permettre une extraction complémentaire par la soude caustique.In a preferred device, the pulp suspension passes through a first oxygen diffuser and a plurality of reactors, each of which is preceded by an oxygen diffuser. A fraction or all of the suspension can be introduced into a reaction tower to allow additional extraction with caustic soda.

L'invention ayant été décrite de manière générale, on se reporte de façon plus détaillée aux dessins annexés, qui illustrent des formes préférées de réalisation et sur lesquels:

  • La Fig. 1 est une vue schématique d'une installation type pour la mise en oeuvre du procédé de l'invention;
  • La Fig. 2 est une vue en élévation de côté agrandie, en partie en coupe, qui illustre de façon plus détaillée une forme de diffuseur fluidiseur de pâte tel que celui représenté sur la Fig. 1, en série avec un mélangeur fluidiseur de pâte;
The invention having been generally described rale, reference is made in more detail to the appended drawings, which illustrate preferred embodiments and in which:
  • Fig. 1 is a schematic view of a typical installation for implementing the method of the invention;
  • Fig. 2 is an enlarged side elevation view, partly in section, which illustrates in more detail a form of dough fluidizer diffuser such as that shown in FIG. 1, in series with a dough fluidizer mixer;

Les Fig. 3,4, 5, et 7 sont des vues représentant des variantes de diffuseurs fluidiseurs de pâte.Figs. 3,4, 5, and 7 are views showing variants of dough fluidizing diffusers.

L'installation type représentée sera comprise à la lecture de la description du trajet de la suspension de pâte qui circule dans cette installation. A titre d'exemple le stade particulier représenté, dans le processus de blanchiment, est le premier étage d'extraction alcaline.The typical installation shown will be understood on reading the description of the path of the pulp suspension which circulates in this installation. By way of example, the particular stage represented in the bleaching process is the first stage of alkaline extraction.

Une suspension aqueuse de pâte cellulosique contenant de la soude caustique est pompée à une grande vitesse superficielle, qui arrive par le tube 15 en provenance d'une source d'alimentation (non représentée), à travers la pompe A, dans une conduite 17 qui pénètre dans un diffuseur d'oxygène B fluidiseur de pâte dans lequel de l'oxygène est diffusé et dispersé en bulles fines dans le courant de suspension. Le courant de suspension est pompé depuis une source d'alimentation, par la conduite 17, par la pompe A, à une vitesse telle que le courant se comporte comme de l'eau et qu'il se manifeste une forte turbulence dans toute se masse. L'oxygène est diffusé dans le courant à travers un diffuseur B, en fines bulles, ainsi que décrit ci-après, de manière plus détaillée, et il procure un rapide malaxage de l'oxygène dans tout le courant de pâte.An aqueous suspension of cellulose pulp containing caustic soda is pumped at a high surface speed, which arrives through the tube 15 from a supply source (not shown), through the pump A, in a line 17 which enters an oxygen diffuser B pulp fluidizer in which oxygen is diffused and dispersed in fine bubbles in the suspension stream. The suspension current is pumped from a power source, via line 17, by pump A, at a speed such that the current behaves like water and that there is a strong turbulence in any mass. . The oxygen is diffused in the stream through a diffuser B, in fine bubbles, as described below, in more detail, and it provides rapid mixing of the oxygen throughout the dough stream.

Le courant de pâte en mouvement rapide qui circule dans la conduite 17 en aval du diffuseur d'oxygène, et qui contient maintenant de l'oxygène dispersé, est introduit dans le fond d'une première chambre de réaction verticale C, dans laquelle il circule à une vitesse beaucoup plus faible sous la forme d'un bouchon. A cette vitesse plus faible, la suspension perd sa fluidité, en emprisonnant l'oxygène dispersé dans le bouchon, ce qui permet à l'oxygène et à la soude caustique de réagir avec la pâte pendant le temps où la suspension contenant de l'oxygène reste dans la chambre C.The stream of rapidly moving pulp which circulates in line 17 downstream of the oxygen diffuser, and which now contains dispersed oxygen, is introduced into the bottom of a first vertical reaction chamber C, in which it circulates at a much lower speed in the form of a plug. At this lower speed, the suspension loses its fluidity, trapping the oxygen dispersed in the stopper, which allows the oxygen and the caustic soda to react with the paste during the time when the suspension containing oxygen stay in room C.

La suspension est ensuite envoyée rapidement, en écoulement turbulent, par un tube 19, dans un deuxième diffuseur d'oxygène B, qu'elle traverse et dans le fond d'une deuxième chambre de réaction C, dans laquelle elle séjourne pendant une certaine période pour permetttre à l'oxygène et aux produits chimiques de blanchiment de continuer à réagir avec la cellulose.The suspension is then sent rapidly, in turbulent flow, through a tube 19, into a second oxygen diffuser B, which it passes through and into the bottom of a second reaction chamber C, in which it remains for a certain period. to allow oxygen and bleaching chemicals to continue to react with cellulose.

De la colonne Ci, la suspension est envoyée, par un tube 21 et à travers un troisième diffuseur d'oxygène B2, dans une troisième chambre de réaction C2.From column Ci, the suspension is sent, by a tube 21 and through a third oxygen diffuser B 2 , to a third reaction chamber C 2 .

La pâte passe ensuite à travers une valve 23 de réglage de la pression et, dans un tube 24, pour pénétrer dans l'extrémité supérieure d'une tour de réaction D. La pâte séjourne dans la tour D pour donner plus de temps au produit chimique de blanchiment pour réagir avec la cellulose.The dough then passes through a pressure adjustment valve 23 and, in a tube 24, to enter the upper end of a reaction tower D. The dough stays in the tower D to give more time to the product bleaching chemical to react with cellulose.

En variante, la pâte peut circuler dans le sens inverse, c'est-à-dire de haut en bas à travers les chambres pour aboutir à la base ou au sommet de la tour de réaction, ou encore certaines ou la totalité des cuves de réaction peuvent être horizontales.Alternatively, the dough can flow in the opposite direction, that is to say from top to bottom through the chambers to reach the base or at the top of the reaction tower, or some or all of the reaction can be horizontal.

La suspension de pâte sortant de la tour D est ensuite récupérée à la sortie d'un tube de sortie 25 et elle est lavée pour éliminer les produits chimiques de blanchiment et les impuretés, ou encore, selon le stade du processus de blanchiment, la suspension traitée peut être envoyée à un autre traitement.The pulp suspension leaving tower D is then collected at the outlet of an outlet tube 25 and it is washed to remove bleaching chemicals and impurities, or alternatively, depending on the stage of the bleaching process, the suspension processed can be sent to another processing.

Un évent 29 pour les effluents gazeux permet de surveiller l'excès d'oxygène, en vue de régler le processus de manière à assurer un plus grand rendement.A vent 29 for the gaseous effluents makes it possible to monitor the excess oxygen, with a view to regulating the process so as to ensure greater efficiency.

De préférence, comme représenté sur la Fig. 2, il y a un mélangeur statique fluidiseur de pâte ou étranglement E, en série avec le diffuseur B, entre ce diffuseur et la première colonne de réaction à travers laquelle on fait passer le courant de suspension contenant de l'oxygène. L'étranglement présente un col étranglé qui rétrécit le courant et accroît sa vitesse. Ceci assure une meilleure dispersion des bulles d'oxygène gazeux dans toute la masse de suspension de pâte.Preferably, as shown in FIG. 2, there is a static mixer fluidizing dough or throttle E, in series with the diffuser B, between this diffuser and the first reaction column through which the suspension stream containing oxygen is passed. The constriction has a constricted neck which narrows the current and increases its speed. This ensures better dispersion of the oxygen gas bubbles throughout the mass of pulp suspension.

Le diffuseur d'oxygène fluidiseur de pâte B est composé, comme on l'a représenté à plus grande échelle sur la Fig. 3, d'un cylindre 31 qui forme un passage 32 pour la suspension de pâte. Le cylindre présente des brides 33, 34 sur ses extrémités, pour le raccordement à une canalisation. Un tube de métal poreux 35 est placé diamétralement en travers du passage 32 qui, à cette exception près, est libre; ce tube pénètre dans une ouverture ménagée dans le côté du cylindre 31, à travers une raccord 31a qui est fixé et scellé en place par un raccord de tube 6 du type "SWAGELOK" (marque déposée). Sur le côté opposé du cylindre 31, se trouve une ouverture fermée par un bouchon 31b dans lequel est insérée l'extrémité inférieure du diffuseur. La paroi du tube 35 est de préférence faite de métal fritté.The paste fluidiser oxygen diffuser B is composed, as shown on a larger scale in FIG. 3, a cylinder 31 which forms a passage 32 for the suspension of dough. The cylinder has flanges 33, 34 on its ends, for connection to a pipe. A porous metal tube 35 is placed diametrically across the passage 32 which, with this exception, is free; this tube enters an opening in the side of the cylinder 31, through a connector 31a which is fixed and sealed in place by a tube connector 6 of the "SWAGELOK" type (registered trademark). On the opposite side of the cylinder 31, there is an opening closed by a plug 31b into which the lower end of the diffuser is inserted. The wall of the tube 35 is preferably made of sintered metal.

La pâte est continuellement introduite à grande vitesse à travers le passage 32 tandis que l'oxygène est fourni au tube poreux 35 et qu'il diffuse dans la pâte à travers la paroi de ce tube.The paste is continuously introduced at high speed through the passage 32 while oxygen is supplied to the porous tube 35 and it diffuses into the paste through the wall of this tube.

Une variante de diffuseur est représentée sur la Fig. 4. Dans ce cas, la paroi du cylindre 41 possède une section centrale cylindrique poreuse 45. Un collecteur 47 enferme cette section de l'extérieur et il est prévu un raccord d'oxygène 48 aboutissant au collecteur. Le passage de la suspension de pâte est ainsi entièrement libre.A variant of the diffuser is shown in FIG. 4. In this case, the wall of the cylinder 41 has a porous cylindrical central section 45. A manifold 47 encloses this section from the outside and an oxygen connection 48 is provided leading to the manifold. The passage of the paste suspension is thus entirely free.

Une autre forme de diffuseur est représentée sur la Fig. 5. Ici, un dispositif 55 en forme de couronne annulaire est placée contre le cylindre 51 et entoure le passage 52. Le tube 55 est muni de petits orifices appropriés 56 à travers lesquels l'oxygène est diffusé. Le passage 52 est entièrement libre.Another form of diffuser is shown in FIG. 5. Here, a device 55 in the form of an annular crown is placed against the cylinder 51 and surrounds the passage 52. The tube 55 is provided with suitable small orifices 56 through which the oxygen is diffused. The passage 52 is entirely free.

Une autre forme de diffuseur est représentée sur la Fig. 6. Ici, un tube poreux 65 s'étend axialement et en position centrale le long du passage 62 et est maintenu dans ce passage par des supports ou croisillons appropriés 69 qui sont reliés à l'intérieur de la paroi du cylindre 61. Un tube à oxygène 68 est placé de l'extérieur du cylindre à l'extrémité du tube de diffusion 65.Another form of diffuser is shown in FIG. 6. Here, a porous tube 65 extends axially and in a central position along the passage 62 and is held in this passage by suitable supports or cross-pieces 69 which are connected inside the wall of the cylinder 61. A tube oxygen 68 is placed from the outside of the cylinder at the end of the diffusion tube 65.

La figure 7 montre encore une autre forme préférée de diffuseur. Ce diffuseur est monté entre des brides de montage 100 et 101, d'où partent de courtes sections de tubes cylindriques 102 et 103 respectivement, qui se raccordent à des brides tronconiques obliques 104 et 105. Une double paroi J, composée d'une paroi intérieure cylindrique poreuse 111, est placée entre les extrémités des brides 104 et 105, une paroi extérieure cylindrique 107 non perforée et les parois d'extrémités annulaires perforées 108 et 109. La paroi intérieure microporeuse 111 est de préférence faite de métal fritté, comme précédemment décrit.Figure 7 shows yet another preferred form of diffuser. This diffuser is mounted between mounting flanges 100 and 101, from which start short sections of cylindrical tubes 102 and 103 respectively, which are connected to oblique tapered flanges 104 and 105. A double wall J, composed of a wall cylindrical inner wall 111, is placed between the ends of the flanges 104 and 105, a cylindrical outer wall 107 without perforation and the annular walls of perforated ends 108 and 109. The microporous inner wall 111 is preferably made of sintered metal, as before described.

Les sections de tubes 102 et 103, les brides 104 et 105 et la paroi poreuse 111 forment une enceinte.The tube sections 102 and 103, the flanges 104 and 105 and the porous wall 111 form an enclosure.

Dans cette enceinte, est monté axialement un cylindre K possédant une paroi microporeuse 113 (de préférence en métal fritté), coiffé à ses extrémités de chapeaux tronconiques imperméables 115 et 117. Le cylindre K est convenablement supporté par un dispositif de croisillons S non représenté, pour conserver sa position dans le conduit. La paroi 113 du cylindre K et la paroi 111 de la double paroi J forment entre elles un passage annulaire P. Un tuyau 119 d'alimentation de gaz oxydant relie une source de gaz oxydant sous pression à l'espace intérieur de la double paroi J. Un tube 121 d'alimentation en gaz oxydant relie une source de gaz oxydant sous pression au volume intérieur du cylindre K.In this enclosure, there is axially mounted a cylinder K having a microporous wall 113 (preferably made of sintered metal), capped at its ends with impermeable frustoconical caps 115 and 117. The cylinder K is suitably supported by a cross device S not shown, to maintain its position in the duct. The wall 113 of the cylinder K and the wall 111 of the double wall J form between them an annular passage P. A pipe 119 for supplying oxidizing gas connects a source of oxidizing gas under pressure to the interior space of the double wall J An oxidizing gas supply tube 121 connects a source of oxidizing gas under pressure to the interior volume of the cylinder K.

Le diffuseur est raccordé, de la même façon que dans le raccordement des autres formes de réalisation décrites plus haut, par la bride 100, à un tube d'arrivée de la suspension de pâte et, par la bride 101, à un tube qui transporte la suspension de pâte sortante.The diffuser is connected, in the same way as in the connection of the other embodiments described above, by the flange 100, to an inlet tube for the paste suspension and, by the flange 101, to a tube which transports the outgoing dough suspension.

En fonctionnement, la suspension de pâte est introduite dans l'extrémité de gauche du dispositif, à une vitesse de fluidisation, et s'écoule à grande vitesse à travers le passage P compris entre les parois microporeuses 111 et 113, et elle sort à travers l'extrémité de droite. Un gaz oxydant sous pression est introduit à travers les tubes 119 et 121 et diffusé en bulles fines à travers les parois poreuses opposées 111 et 113, dans la suspension de pâte fluidisée qui circule rapidement dans le passge C, comme précédemment décrit. La diffusion du gaz dans la suspension est extrêmement efficace en raison de la grande surface de diffusion formée aux deux limites du courant de pâte fluidisée.In operation, the paste suspension is introduced into the left end of the device, at a fluidization speed, and flows at high speed through the passage P between the microporous walls 111 and 113, and it exits through the far right. An oxidizing gas under pressure is introduced through the tubes 119 and 121 and diffused in fine bubbles through the opposite porous walls 111 and 113, in the suspension of fluidized paste which circulates rapidly in the passge C, as previously described. The diffusion of the gas in the suspension is extremely efficient due to the large diffusion surface formed at the two limits of the fluidized pulp stream.

Le mélangeur statique fluidiseur de pâte E, représenté sur la Fig. 2, raccordé en série avec le diffuseur d'oxygène B, comprend un tube allongé d'une seule pièce de section circulaire, entre les brides 81 et 82. Une première partie tronconique 83 se rétrécit à partir de la bride 81 pour se raccorder, au niveau d'un col 84, à la petite extrémité d'une partie tronconique 85 plus longue, qui s'évase, et qui se termine au niveau de la bride 82. Le mélangeur E est raccordé à une pièce de raccordement F qui possède une bride amont 90 reliée à la bride 34 avec interposition d'un joint. La pièce de transition F possède une bride aval 91 assemblée à la bride 81 du mélangeur. La suspension de pâte circule momentanément dans une section plus large puis dans une section restreinte sensiblement à la section du tube diffuseur.The static dough fluidizer mixer E, shown in FIG. 2, connected in series with the oxygen diffuser B, comprises an elongated tube in one piece of circular cross section, between the flanges 81 and 82. A first frustoconical part 83 narrows from the flange 81 to be connected, at a neck 84, at the small end of a longer frusto-conical part 85, which flares, and which ends at the flange 82. The mixer E is connected to a connection piece F which has an upstream flange 90 connected to the flange 34 with the interposition of a seal. The transition piece F has a downstream flange 91 assembled to the flange 81 of the mixer. The paste suspension momentarily circulates in a wider section then in a restricted section substantially to the section of the diffuser tube.

La dimension (le diamètre intérieur) du cylindre 31 dans lequel la suspension de pâte est pompée et mise en contact avec l'oxygène varie en fonction de la capacité de l'usine à pâte. Les diamètres types vont d'environ 50 à environ 300 mm et, habituellement, d'environ 75 à environ 150 mm.The size (the inside diameter) of the cylinder 31 into which the pulp suspension is pumped and brought into contact with oxygen varies according to the capacity of the pulp mill. Typical diameters range from about 50 to about 300 mm and usually from about 75 to about 150 mm.

On a représenté trois chambre de réaction C, C, et C2. On peut utiliser un nombre de chambres de réaction plus grand ou plus petit, selon la matière à traiter, la quantité d'oxygène ajouté et l'effet de blanchiment désiré. Les effets de plusieurs diffuseurs dont chacun est équipé d'une chambre de réaction consistent en l'ajout par paliers de grandes additions d'oxygène, en ajoutant de petites quantités dans chaque diffuseur, de manière à obtenir:

Figure imgb0002
Cette expression est calculée à la pression du stade particulier considéré.Three reaction chambers C, C, and C 2 are shown . A greater or lesser number of reaction chambers can be used, depending on the material to be treated, the amount of oxygen added and the desired bleaching effect. The effects of several diffusers, each of which is equipped with a reaction chamber, consist in adding in stages large additions of oxygen, by adding small amounts to each diffuser, so as to obtain:
Figure imgb0002
 This expression is calculated at the pressure of the particular stage considered.

La vitesse à laquelle la suspension de pâte est refoulée à travers le cylindre diffuseur 31 est une vitessé élevée qui fait que le courant se comporte comme de l'eau et rend plus facile la dispersion de petites bulles d'oxygène qui sont mélangées dans toute la masse du courant.The speed at which the slurry suspension is discharged through the diffuser cylinder 31 is a high velocity which makes the current behave like water and makes it easier to disperse small bubbles of oxygen which are mixed throughout the mass of the current.

La pression de la suspension de pâte peut être d'environ 0,1 à 1 mégapascals (MPa), et la pression de l'oxygène d'environ 0,14 à 1,4 MPa.The pressure of the pulp suspension may be about 0.1 to 1 megapascals (MPa), and the oxygen pressure of about 0.14 to 1.4 MPa.

Le temps de séjour de la pâte dans le cylindre 31 du diffuseur est habituellement dans l'intervalle d'environ 0,001 à environ 0,120 seconde.The residence time of the paste in the cylinder 31 of the diffuser is usually in the range of about 0.001 to about 0.120 seconds.

Le temps de séjour dans chacun des réacteurs C, C1 et C2 est habituellement dans l'intervalle d'environ 1 minute à environ 5 minutes et le temps de séjour dans la tour D est dans l'intervalle d'environ 30 à environ 90 minutes.The residence time in each of reactors C, C 1 and C 2 is usually in the range of about 1 minute to about 5 minutes and the residence time in tower D is in the range of about 30 to about 90 minutes.

La pression dans chaque réacteur est d'environ 0,1 à 1 MPa.The pressure in each reactor is approximately 0.1 to 1 MPa.

L'invention n'exclut pas l'utilisation ni la combinaison avec les diffuseurs pour l'introduction de l'oxygène dans la pâte, de mélangeurs à grande vitesse tels que, par exemple, ceux qui sont décrits dans le brevet US―A―4 295 926, au nom de WEYERHAEUSER, ou d'une pompe "KAMYR MC" marque déposée).The invention does not exclude the use or the combination with the diffusers for the introduction of oxygen into the dough, of high speed mixers such as, for example, those described in US patent brevet A― 4,295,926, in the name of WEYERHAEUSER, or of a "KAMYR MC" registered trademark pump).

La nature de la pâte traitée peut être du bois de résineux, du bois de feuillus ou un mélange ayant une consistance dans l'intervalle de 5 à 16%. On peut traiter selon l'invention des pâtes à différents stades du processus de délignification-blanchiment. Par exemple, la pâte de départ dans le stade d'extraction caustique d'une opération de blanchiment peut avoir un indice Kappa dans l'intervalle de 5,0 à 10,0 et la pâte blanchie sortant de la cuve D peut avoir, après lavage, un indice de 2,5 à 3,5.The nature of the treated pulp can be wood softwoods, hardwoods or a mixture having a consistency in the range of 5 to 16%. Pasta can be treated according to the invention at different stages of the delignification-bleaching process. For example, the starting dough in the caustic extraction stage of a bleaching operation may have a Kappa number in the range of 5.0 to 10.0 and the bleached dough exiting from tank D may have, after washing, an index of 2.5 to 3.5.

Par suite de la grande vitesse de la suspension de pâte qui passe dans le passage 32 en contact avec les éléments de diffusion 35, 45, 56 ou 65, la suspension prend les caractéristiques de l'eau. La suspension passe en contact étroit avec l'élément diffuseur et l'oxygène sort par les pores de cet élément sous la forme de bulles de très petit diamètre. Ces bulles sont bien dispersées en raison de la turbulence de la pâte et elles présentent une grande surface spécifique, comparativement à leur volume, pour assurer le transfert de masse à travers l'interface gaz/ liquide. A des gradients élevés de concentration de l'oxygène et avec un grand rapport surface/ volume, le transfert de masse d'oxygène est considérablement renforcé par ce procédé de dispersion avec fluidisation.As a result of the high speed of the pulp suspension which passes through the passage 32 in contact with the diffusion elements 35, 45, 56 or 65, the suspension takes on the characteristics of water. The suspension passes in close contact with the diffusing element and the oxygen leaves through the pores of this element in the form of bubbles of very small diameter. These bubbles are well dispersed due to the turbulence of the dough and they have a large specific surface, compared to their volume, to ensure mass transfer through the gas / liquid interface. At high oxygen concentration gradients and with a large surface / volume ratio, the mass transfer of oxygen is considerably enhanced by this dispersing process with fluidization.

La suspension peut être soumise à un pré et post-traitment avant d'être refoulée dans le premier diffuseur B, la suspension a subi des traitements normaux dans une opération de délignification-blanchiment. Par exemple, la suspension contient de petites concentrations d'autres produits chimiques blanchissants. En outre, la suspension est fournie à une température dans l'intervalle d'environ 40°C à environ 90°C.The suspension can be subjected to a pre and post-treatment before being pushed back into the first diffuser B, the suspension has undergone normal treatments in a delignification-bleaching operation. For example, the suspension contains small concentrations of other bleaching chemicals. In addition, the suspension is provided at a temperature in the range of about 40 ° C to about 90 ° C.

La suspension sortant de la tour de blanchiment D sera lavée pour l'élimination des produits chimiques de blanchiment et des autres impuretés.The suspension leaving the bleaching tower D will be washed to remove bleaching chemicals and other impurities.

Une installation de blanchiment type pourrait avoir une capacité de traitement de 1000 tonnes de pâte par jour, d'une consistance de 10% avec une quantité d'oxygène de 5 kg par tonne, ou de 5000 kg d'oxygène. Dans ces conditions, ce débit pourrait être de 6.321 litres par minute, avec une vitesse de circulation de la suspension à travers le diffuseur de 40 mètres par seconde, et un débit d'oxygène de 3,13 kg par minute. La surface de diffusion serait d'environ 0,26 décimètres carrés.A typical bleaching facility could have a processing capacity of 1000 tonnes of pulp per day, 10% consistency with an oxygen quantity of 5 kg per ton, or 5000 kg of oxygen. Under these conditions, this flow could be 6.321 liters per minute, with a speed of circulation of the suspension through the diffuser of 40 meters per second, and an oxygen flow of 3.13 kg per minute. The diffusion surface would be around 0.26 square decimetres.

Claims (15)

1. A multistage process for dispersing a gas containing oxygen within a suspension of cellulose pulp, dissolving it therein and reacting it with the latter for the purpose of delignifying or bleaching the said cellulose pulp whilst causing a flow of the pulp suspension to pass through a feed duct (15) coming from a source and injecting oxygen into it characterized in that, in each stage, it consists in:
effecting the feed within an injection and mixing duct forming a passage of restricted cross-section (17, 19, 21) for the pulp and continuing the feed within a reaction tank (C, Cl), diffusing a gas containing oxygen into the passage via a diffuser (B, Bi, B2) establishing an interface with the pulp to form microdimensional gas bubbles therein and causing the suspension stream resulting therefrom and containing the bubbles to pass into the reaction tank;
keeping unobstructed at least the greater part of the space comprised within the passage, to ensure a free linear flow of the pulp suspension flow through the said passage;
causing the pulp suspension to pass through the said passage at a speed appropriate to give rise to a turbulent fluid flow, imparting to it substantially "Newtonian" properties which cause it to behave substantially like water and the gas bubbles formed in the suspension to be exposed to mixing in the said suspension without appreciable coalescence;
causing the suspension containing the bubbles to flow into the reaction tank at a speed such as to establish a plug flow; and,
during the final stage (C2), recovering the processed suspension at the outlet of the reaction tank.
2. A process according to claim 1, characterized in that in each stage, the oxygen is injected in an appropriate quantity in accordance with the formula:
Figure imgb0005
calculated at the working pressure of this stage.
3. A process according to one of claims 1 or 2, characterized in that the oxygen is injected in the form of a large quantity of bubbles having an initial diameter of less than about 10 um.
4. A process according to claim 1 or 2, characterized in that the current of suspension flowing through the passage of restricted cross-section is at a pressure contained within the range extending from approximately 0.1 to 1 MPa.
5. A process according to claim 1 or 2, characterized in that the oxygen is injected at a pressure approximately 0.14 to 1.4 MPa.
6. A process according to claim 1 or 2, characterized in that the amount of the total oxygen dosage is from 2.5 to 25 Kg per tonne of dry pulp.
7. A process according to claim 1 or 2, characterized in that the oxygen is fed in through a microporous surface having an area such that the oxygen injection flow lies within the range from 30 to 300 normal cubic meters per minute and per square metre of microporous surface.
8. A process according to claim 1, characterized in that the speed of the pulp suspension within the said passge is from 0.5 to 50 metres per second.
9. A process according to claim 1, characterized in that the diffuser has a microporous surface.
10. A process according to claim 1, characterized in that the cellulose pulp has a consistency of 8% to 16%.
11. Apparatus for reacting oxygen or a gas containing oxygen with a cellulose pulp, characterized in that it comprises:
a series of connected units forming a reaction path,
each unit comprising a restricted passage (17, 19, 21) leading to a reaction tank (C, C1) having a cross-section equal to several times the cross-section of the passage and a final unit having an outlet (C2), each restricted passage being unobstructed to ensure a free linear flow of the cellulose pulp,
and, in each stage a diffuser (B, B1, B2) which allows oxygen to be injected into the passage in the form of a large quantity of small microdimensional bubbles,
a suspension feed source,
means (15) serving to cause the said suspension originating from the said feed source under pressure into the said reaction circuit (17), in such a way that the suspension has a speed in each passage such that it is liquid whilst passing through this passage and has "Newtonian" properties which have the effect that it behaves like water and that the bubbles formed in the suspension are exposed to mixing in the said suspension without appreciable coalescence and such that it is in the form of a plug upon passing into the adjacent reaction tank,
means allowing the infeed of oxygen to be regulated in each stage in such a way that a dose of oxygen is dispersed throughout the liquid suspension and that the major part of the oxygen reacts with the pulp in plug form during the passage of this pulp into the reaction tank.
12. Apparatus according to claim 11, characterized in that the passage is formed by a conduit (32) and the diffuser is situated on a pipe which extends transversely to the duct (32) in the path of the flow of suspension.
13. Apparatus according to claim 11, characterized in that the passage is formed by a conduit (42), the diffuser is situated on a part of the surface of the conduit and a collector (47) surrounds the said part of the surface to receive the said gas under pressure.
14. Apparatus according to claim 11, characterized in that the passage is formed by a conduit (62) and the diffuser is situated on a pipe (65) which extends longitudinally in the conduit (62) and means (69) are provided to support the pipe and the conduit in the path of the said flow.
15. Apparatus according to claim 11, characterized in that the passage is formed by a conduit having a cylindrical wall (107) and an enclosed capacity (K) situated at its centre and having a cylindrical wall concentric with the wall of the conduit and spaced apart from this wall,
the bubble generating surface is provided at the same time on the surface of the conduit (111) and on the surface of the said capacity (113),
the porous portion of the cylinder (111) being surrounded by a collector (J) intended to receive and feed the gas through the porous wall of the duct,
and means serving to connect the collector and the cylinder to a source of oxidising gas under pressure (119).
EP86402533A 1985-11-15 1986-11-14 Process and apparatus for bleaching paper pulp Expired - Lifetime EP0226495B1 (en)

Priority Applications (1)

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AT86402533T ATE54476T1 (en) 1985-11-15 1986-11-14 METHOD AND APPARATUS FOR BLEACHING PAPER PULP.

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US79828685A 1985-11-15 1985-11-15
US798286 1985-11-15
CA507000 1986-04-18
CA000507000A CA1300320C (en) 1985-11-15 1986-04-18 Pulp bleaching

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EP0226495A1 (en) 1987-06-24
NZ218294A (en) 1990-05-28
DE3672580D1 (en) 1990-08-16
PT83745B (en) 1988-08-17
PT83745A (en) 1986-12-01
AU595842B2 (en) 1990-04-12
AU6467186A (en) 1987-05-21

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