PULP AND BLEACHING REAGENT MIXER AND METHOD

Abstract: A contacting apparatus (50) for providing intimate contact between pulp and a gaseous bleaching reagent has an elongate substantially cylindrical chamber (60) having an inside diameter proportional to a mixing capacity and an inlet (52) for pulp and for the carrier gas and the gaseous bleaching reagent; a rotatable shaft (54) extending through the chamber along an axial centerline of the chamber; provisions, mounted on the rotatable shaft, for radially tossing the pulp within successive transverse planes to maintain a substantially uniform distribution of pulp across the radial extent of the cylindrical chamber (60) with a minimum of axial mixing; and further provisions for regulating gas flow through the inlet (52) to transport the pulp through the mixing apparatus, the flow being proportioned to provide a predetermined pulp residence time within the chamber as determined by dimensions of the chamber, rotation speed of the shaft, concentration of the bleaching reagent, pulp consistency, and pulp production requirements.

Description: PULP AND BLEACHING REAGENT MIXER AND METHOD BACKGROUND OF THE INVENTION This invention relates generally to pulp manufacturing processes and equipment and more particularly to contactors for promoting intimate contact between high consistency pulp and a gaseous bleaching reagent. Laboratory studies have shown that the optimum pulp consistency for reacting with gaseous bleaching reagents is in the range of 40 - 45%. As discussed in U.S. Patent No. 3,814,664, 3,964,962 and U.S. Application S/N 07/651,586, both of which are incorporated herein by reference, ozone bleaching is best performed in the gas phase, with the pulp at high consistency and well fluffed and the ozone supplied as a dilute mixture at about 2 - 6% concentration in a carrier gas of oxygen. Fluffed pulp at 40-45% consistency contains a considerable fraction of individual fibers, but the majority of the fiber is present in agglomerated particles called floes, each containing a substantial number of individual fibers, and measuring about 1/4 to 1/2 inch in diameter, or about 2 - 4 times the fiber length. These particles are porous and permeable to gaseous bleaching reagents. NOTE; Although flocculation has been widely studied in dilute water suspensions of pulp fibers, it has only recently been recognized that the same phenomenon occurs in gaseous suspensions of moist fibers - for example, at the discharge of a refiner or fluffer used to prepare pulp for gaseous bleaching. (Garner & Kerekes, TAPPI, 63 (6): 103(1980). When fluffed pulp is blown into a conventional bleaching apparatus, the floes settle into a bed with a bulk density of about 3.0 pounds (dry fiber weight) per cubic foot. At this density, the fluff consists of about 5% fiber and 95% gas (or void) volume, as the water is mostly contained within the fibers. The volume of gas required for bleaching is, however, much larger than the void volume in the fluffed pulp. For example, at a typical dosage of 1 % ozone, (basis dry pulp weight), an ozone concentration of 6% in oxygen carrier gas, and a fluffed pulp bulk density, as above, of 3.0; the process requires that 6.5 cubic feet of gas be contacted with each cubic foot of fluffed pulp, at typical reaction conditions of 1 atmosphere pressure and 120 degrees Fahrenheit. This means that, with the pulp in bed or mat form, 6.5 volumes of bleaching gas must pass uniformly through the one volume of fluffed pulp. Clearly the density and hence the permeability of the fluffed pulp must be extremely uniform, in order to avoid non-uniform results. The fluffed pulp mass is easily compressed by the action of bulk solids handling equipment to form wads and clumps having much higher density and much lower gas permeability. Bleaching gas flows much more slowly through such wads and clumps and much more rapidly through the wad-to-wad contact areas. The result is overbleached contact areas and underbleached wad cores. Thus it has been found that bleaching systems which employ conventional bulk materials handling equipment to move the fluffed pulp through a bleaching retention chamber while bleaching it with ozone gas cannot successfully produce uniformly bleached pulp fluff. One system which conveys fluffed pulp through several chambers using a pan conveyor, and which makes multiple transfers of the pulp when discharging from one bleaching chamber to another, does not produce uniformly bleached pulp. Another uses scraper arms to push a large mass of fluffed pulp through apertures or slits in order to drop it from one treating chamber to another and also produces non-uniform bleaching results. In general, systems which employ mechanical conveying devices cannot produce uniform bleaching results using ozone gas on high consistency pulp, due to the variable permeability produced by the action of the scraper arms. The concern for uniformity of contact between the fluffed pulp and the bleaching reagent gas, in the case of ozone bleaching, is fostered by the rapid reduction in the concentration of ozone gas in contact with the fluffed pulp. This reduction is attributable to the extremely fast reaction rate of ozone with wood pulp. Since the reaction rate is concentration dependent, this characteristic increases the non-uniform bleaching results attendant upon the variable permeability of the pulp. Conventional pulp mixer designs such as peg rolls have two additional disadvantages when used for ozone bleaching. First, the mixer blades produce rotary motion but also move the pulp forward and backward. This is desirable for mixing when the reaction rate is slow enough that only a small portion of the reaction is completed within the time that the material is retained in the mixer itself, and the mixed pulp and bleach chemical are passed into a second chamber where the retention time is sufficient for completion of the reaction. In ozone bleaching, however, the delignification (or bleaching) reaction proceeds so quickly that it will be about 90 - 95% completed within the transit chamber between the fluffer and the second chamber, even though the transit chamber is usually fairly small as will be illustrated below. Thus, should some of the pulp be retained in transit for a few seconds longer than the rest of the material, it will become overbleached and suffer degradation of the cellulose fibers. Likewise, should some of the pulp proceed forward to the discharge faster than the rest, it will be underbleached and will not reach the desired brightness. These considerations require that the mixing blades be designed in such a way that the "plug flow" of the wood pulp not be disturbed by the action of the rotor, and that the axial transport of the pulp be achieved without "backmixing" or "forwardmixing" (dispersion) within the mixing chamber. The second disadvantage is that the rotary motion of the mixer causes centrifugal forces to pack the pulp against the wall of the mixer, leaving a clear path for the reagent gas to pass through it without contacting the pulp to any appreciable degree. The foregoing illustrates limitations known to exist in present apparatus for providing contact of a gaseous reagent with pulp. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. SUMMARY OF THE INVENTION In one aspect of the present invention, this is accomplished by employing a contactor for providing intimate contact between pulp and a gaseous bleaching reagent including an elongate substantially cylindrical chamber having an inlet and outlet for pulp and for the gaseous bleaching reagent; a rotatably driven shaft extending along an axial centerline of the chamber; means, mounted on the rotatable shaft, for radially tossing the pulp to maintain a substantially uniform distribution of pulp within successive transverse planes of the cylindrical chamber; and means for regulating flow of gaseous bleaching reagent for transporting the pulp through the contacting chamber to the outlet. The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a general overall fragmentary schematic view of a bleaching system to illustrate the contactor of the present invention in its operating environment; Fig. 2 is a side elevation sectional view of an embodiment of the contactor to show key operating features thereof; Fig. 3 is an end sectional view looking upstream from line 3-3 of Fig. 2; Fig. 4 is a view, as in Fig. 3, to represent the contactor operating with a charge of pulp; Fig. 5 is a fragmentary side elevation sectional view illustrating an alternative contactor blade/arm assembly configuration; Fig. 6 is a view, as in Fig. 5, showing a third alternative design of the contactor blade/arm assembly; and Fig. 7 is a graphic illustration of the relationship between gas flow velocity and pulp transport velocity through the contactor of the present invention. DETAILED DESCRIPTION Fig. 1 shows a pulp bleaching system for bleaching high consistency pulp with a gaseous reagent. The pulp is fluffed in the presence of the reagent/carrier gas mixture in fluffer 10 and fed to inlet 52 of contactor 50. After passing through contactor 50, the pulp and gas mixture pass through discharge outlet 58 and into finishing reactor vessel 100. The relative locations of the system components in this figure are chosen for ease of illustration purposes and are not intended to limit the locations of such components in practice. As seen in Figs. 2 and 3, the contactor 50 consists of a substantially cylindrical chamber 60 having a rotatable shaft 54 extending along the axial centerline of the chamber 60. A number of arms 55 radiate outwardly from the shaft 54 and carry scoop-shaped blades 56 at their outboard ends. Blades 56 are shaped with leading edges closely following the wall of chamber 60 to avoid compacting the pulp while lifting it free of the wall and trailing edges inclined inwardly to toss the fluffed pulp radially inward within a transverse plane and toward shaft 54. The arms 55 and blades 56 are made with no bias so they do not drive the pulp forward or backward but only inwardly from the wall. The "no bias" design significantly reduces dispersion of the pulp within the contactor chamber and improves uniformity of contact time, while the "radial only" tossing behavior of the blades effectively counteracts the centrifugal forces due to rotary motion and results in a fairly uniform dispersion of pulp, across the radial extent of chamber 60. The pulp is transported from inlet 52 to outlet 58 solely by the gas flow through the chamber. The blades 56 are mounted on two arms 55 in a symmetric manner to avoid any axially forward or backward unbalanced influences on the pulp suspension. This also offers increased strength and rigidity by providing support at both axial extremes of each blade. The blades resist axial misalignment due to the improved strength and rigidity. This also permits use of thinner and lighter arms which interfere less with travel of the fluffed pulp through the contactor. The face of the blade 56 which contacts the pulp is either curved or straight. In either case the angle of the face of the blade at its leading edge is very important. On a curved blade, the minimum practical angle for structural strength is about 10 degrees from the tangent to the circle defining the locus of the tip of the blade during operation of the contactor. The blade can then curve inward so that the angle of its heel reaches an angle of about 35 degrees from the same tangent line. A straight blade requires a contacting face which is set at between about 15 degrees and 25 degrees from the tangent. Arms 55 and blades 56 are positioned on the shaft 54 and are spaced apart generally only enough so that they do not overlap adjacent arms and blades. Occasionally, a spacing equal to 1/4 to 1/2 the axial extent of a single blade is allowed as required for contactor performance. In smaller diameter machines it is important that only one arm 55 and blade 56 be positioned at each axial location on the shaft 54, as multiple blades running in one plane impart too much rotary motion and not enough lifting motion to the fluffed pulp as explained below. The blades 56 and arms 55 are arranged at different angular positions on the shaft 54 to balance the rotor against vibration. Fig. 4 is a representation of the action of the scoop-shaped blades 56 on the fluffed high consistency pulp in the contactor 50. Although the pulp drops to the bottom of chamber 60, blades 56 continuously scoop it up and toss it upward and inward, within a transverse plane, toward the shaft so that the particles 75 follow a transverse flight path across the chamber. During this flight, the pulp particles 75 are carried axially forward in chamber 60 by the flow of reagent gas mixture until they again hit the chamber wall and are stopped by friction. Thus the pulp particles 75 travel axially through the chamber in a series of short gas borne leaps until they reach discharge outlet 58 and are deposited in finishing reactor 100 where the low-rate portion of the bleaching reaction is carried to its practical limit. The transport mechanism described has been confirmed, in tests using a contactor chamber having transparent walls, by stopping the pulp feed and shaft rotation simultaneously during normal running. The pulp suspension stopped rotating and fell to the bottom of the chamber, filling it to a level between one third and one half full. (The bulk density of the pulp at rest is about 3.0 pounds of dry fiber per cubic foot while the bulk density in motion during normal contactor operation is about 1.2 pounds of dry fiber per cubic foot.) Although the gas flow was maintained, the pulp did not move forward again until the rotor was restarted. When the gas flow and pulp feed were stopped with the rotor still running, the rotating pulp suspension continued to fill the chamber but did not travel forward or backward. This demonstrated the "no bias" operation of the blades 56 and arms 55 and confirmed the achievement of controllable gas transport through contactor 50. Fig. 5 shows a contactor 50 with blades 56 each symmetrically mounted on a single arm 55. This is an alternative embodiment of the blade/arm assembly which provides pulp tossing which is virtually bias free in net effect. As a whole, the pulp receives no transport bias from the blades; however there is a slight degree of local forward and backward dispersion due to pulp particles falling off the axial ends of the blades. For this reason, the two arm design of Fig.2 is preferred. The alternative embodiment shown in Fig. 6 is the same as the embodiment already discussed with respect to Fig. 5 except that blades 56 are attached, at one end extremity, to arms 55 to produce a slight transport bias in the direction of excess projection of the blades. This is also thought to be attributable to some axially forward spillage of pulp from the free ends of the offset mounted blades 56. It is most useful in setting the limits for the gas flow requirements for the desired pulp transport rate in chambers of different sizes. The offset of blades 56 is shown as downstream, but it can be upstream asell, which would create a slight upstream bias. For a given tonnage rate of production, with equipment of a given size, and a reagent gas mixture of a given concentration, an optimum residence time or dwell time for the pulp in the contactor is determined. By varying the offset of blades 56, the degree of bias is varied and, as a consequence of that bias variation, so is the gas flow required for such pulp dwell time. The relationship between the gas flow velocity and the pulp transport velocity is schematically illustrated in Fig. 7 for a 15 inch diameter contactor at a production rate of 30 TPD (tons per day). In general, the velocity of pulp transport through the contactor, as seen in Fig.7, is approximately 1/10 of the velocity of the reagent gas mixture. This is due to the interruptions in the flight of the pulp particles each time they hit the chamber wall. Of course, without this velocity difference the bleaching reaction would be much less uniform and much less efficient. As described, the pulp transport is by gas borne jumps accomplished during the transverse flights of the fluffed pulp particles caused by the blades 56. Since forward axial movement of pulp only occurs during those flights, it is clear that, for a given gas flow velocity, the rate of pulp transport is directly proportional to the rotor speed or to the amount of time the pulp particles are in flight due to the action of the blades. The following typifies a bleaching reaction on wood pulp using ozone as the reagent gas in an oxygen carrier: Pulp is fed at 500 tons per day (694 pounds per minute dry fiber) to fluffer 10. Gaseous reagent consisting, for example, of ozone at a concentration of 1 3/4% in oxygen carrier gas is injected through valve 102 and pipe 120 into the fluffer where it is mixed with the pulp. The pulp and the gas pass through inlet 52 into contactor 50 and together pass through outlet 58 into finishing reactor 100. For a contactor of 3 1/2 foot diameter and 11 foot length, a rotor speed of 60 to 90 revolutions per minute is about optimum. At 120 degrees Fahrenheit, the gas flow is about 6000 cubic feet per minute, resulting in a gas velocity through the contactor of about 10 feet per second. The resulting pulp velocity is about 0.9 feet per second, creating a pulp retention time of 12 seconds. This invention eliminates many of the difficulties encountered in bleaching high consistency pulp using ozone in an oxygen carrier gas. In particular, the non-uniform brightness caused by overbleaching and underbleaching is greatly reduced by the intimate contact and plug flow of the fluffed pulp provided by the gas transport of the pulp through the contactor. The non-bias design of the blades and the arms assures that only by intimate contact with the transporting reagent gas mixture can the pulp travel through the contactor. Provision of proportioning valves for balancing volumetric flows of bleaching gas and carrier gas to yield a required quantity of gaseous bleaching reagent, determined by pulp transport and pulp bleaching requirements, allows a high degree of adjustability of the reaction within the contactor chamber.

Claims: What is claimed is: 1. A contactor for providing contact between pulp and a gaseous bleaching reagent, comprising: an elongate substantially cylindrical chamber having an inlet and outlet for said pulp and said gaseous bleaching reagent; a rotatably driven shaft extending along an axial centerline of said chamber; means, mounted on said rotatably driven shaft, for radially tossing said pulp to mechanically suspend said pulp substantially uniformly within successive transverse planes of said cylindrical chamber without providing any pulp transport bias; and means for regulating flow of gaseous bleaching reagent for transporting said pulp through said contacting chamber, to said outlet. 2. The contactor of claim 1, wherein the means for radially tossing said pulp comprises a plurality of blades, each said blade mounted on one or more arms attached to said rotatable shaft, said blades having leading edges oriented axially parallel with a wall of said chamber and providing radial lifting forces to toss pulp toward the shaft. 3. The contactor of claim 1, wherein the means for regulating flow of gaseous bleaching reagent comprises a proportioning valve means for controlling a volume flow rate of said gaseous bleaching reagent. 4. The contactor of claim 3, wherein the means for regulating flow of gaseous bleaching reagent further comprises mixing means for combining a required amount of a bleaching gas with a quantity of carrier gas to provide a required volume flow rate capacity of gaseous bleaching reagent at a desired concentration. 5. In a pulp bleaching apparatus of the type having a pulp fluffer, a supply of bleaching gas and carrier gas, a contactor chamber, and a bed-type finishing reactor, the improvement, in combination with said contactor chamber, comprising: a rotatably driven shaft extending along an axial centerline of said contactor chamber and having means, mounted thereon, for radially tossing said pulp within successive transverse planes of said cylindrical chamber without providing any pulp transport bias; and means for transporting pulp through said contacting chamber, to said outlet by regulating a flow of gaseous bleaching reagent. 6. The improvement of claim 5, wherein the means for radially tossing said pulp comprises a plurality of blades, each said blade mounted on one or more arms attached to said rotatable shaft, said blades having leading edges oriented axially parallel with a wall of said chamber and providing radial lifting forces to toss pulp toward the shaft with a minimum of axial mixing and without any axial transport of said pulp. 7. A contactor for providing uniform contact between wood pulp and a gaseous bleaching reagent, comprising: an elongate substantially cylindrical chamber having inner and outer surfaces; means for mechanically agitating said pulp for forming a uniform suspension of said pulp across a radial extent of said chamber without providing any pulp transport bias; and means for controlling transport speed of pulp through said chamber by regulating flow of gaseous bleaching reagent through said chamber. 8. The contactor of claim 7, wherein the means for mechanically agitating said pulp comprises a rotatably driven shaft extending along an axial centerline of said chamber, said shaft having a arms mounted thereon, and one or more blades, each said blade attached to at least one of said radially extending arms and having an axially oriented leading edge parallel to said chamber inner surface. 9. The contactor of claim 7, wherein the means for controlling transport speed of pulp through said chamber comprises mixing means, for combining a required amount of a bleaching gas with a quantity of carrier gas to provide a required volume flow rate capacity of gaseous bleaching reagent at a desired concentration, and a proportioning valve means for regulating volume flow rate of said gaseous bleaching reagent. 10. A method for improving uniformity of contact between wood pulp and a gaseous bleaching reagent, in a pulp contactor chamber of a pulp bleaching apparatus, comprising: providing a rotatably driven shaft extending along a central axis of said chamber; mounting one or more radially extending arms on said shaft and attaching a blade to each one or more arms, said blade having a leading edge axially aligned and parallel with a wall of said chamber to provide radial lifting forces to the pulp and to toss said pulp toward the shaft without imparting any axial transport bias thereto; and introducing a flow of gaseous bleaching reagent into an inlet of the contactor chamber and regulating said flow, in proportion to a given shaft speed, to transport said pulp at a speed which yields a required residence time in said contactor.

Inventors: CARLSMITH L ALLAN, LUTHI OSCAR, MURPHY PATRICK D, CARLSMITH, L., ALLAN, LUTHI, OSCAR, MURPHY, PATRICK, D.
Applicants: INGERSOLL RAND CO, INGERSOLL-RAND COMPANY
Application year: 1993
Main classification: D21C9
IPC codes: B01F7, B01J19, D21C9