AU646267B2 - Method and apparatus for treating fibrous materials with a gaseous reagent - Google Patents

Description

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AUSTRALIA

Patents Act 4 3 *K7 COMPLETE SPECIFICATION

(ORIGINAL)

Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Int. Class f 4 I L I Name of Applicant: Ingersoll-Rand Company Actual Inventor(s): Lawrence A. Carlsmith Address for Service: PHILLIPS ORMOND!- FITZPATRICK Patent and Trade Mark Attorneys 357 Collins Street Melbourne 3000 AUSTRALIA Invention Title: METHOD AND APPARATUS FOR TREATING FIBROUS MATERIALS WITH A GASEOUS REAGENT Our Ref 221928 POF Code: 1428/1428 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1- 6006 L Docket No. 1197-IR-PA METHOD AND APPARATUS FOR TREATING FIBROUS MATERIALS WITH A GASEOUS REAGENT BACKGROUND OF THE INVENTION This invention relates generally to pulp processing for papermaking and more particularly to pulp bleaching using a gaseous bleaching reagent.

Bleaching of wood pulp using gaseous reagents such as chlorine dioxide or ozone involves a large volume percentage of carrier gas along with the reactive gas. This results from the methods of preparation of such gaseous bleaching reagents.

Ozone is generated by passing air, or oxygen, through an electrical field of sufficient intensity to generate a corona discharge which converts a portion of the oxygen into ozone. For example, passing oxygen through the corona, in some ozone generators, will result in conversion of as much as S 15 six percent of the oxygen into ozone. The remaining oxygen passes into the bleaching process with the ozone as a "carrier gas". When reacted with the pulp, the ozone fraction will be substantially Docket No. 1197-IR-PA consumed and the oxygen must be removed from the pulp as it is not reactive under the bleaching conditions used and its recovery is important to the economics of ozone generation.

One currently available ozone bleaching apparatus treats pulp at a high consistency of 20 to percent. The pulp is fluffed by a mechanical fluffer in the presence of the reacting gas, is transported by the gas through a conduit; and is dropped onto a porous bed of fluffed pulp which continuously moves downward through a cylindrical reaction tower toward an expanded section which acts as a gas separation chamber. The ozone/carrier gas mixture is initially mixed with the pulp in the fluffer to form a gas suspended mixture for transport and initiation of reaction, which then passes through a conduit to the top of the tower.

The ozone and carrier gas flows through the porous bed at a substantially higher velocity than that at which the bed moves down through the reaction tower. The carrier gas leaves the pulp bed peripherally into the gas separation chamber from which it is recycled to the ozone generator.

Bleaching in this type reactor frequently results in a mottled appearance due to the variable 2 Docket No. 1197-IR-PA permeability of the fluffed pulp agglomerations.

This variability is also attributed to non-uniform compaction of the porous pulp bed which results from localized impact of the falling pulp. Greater local depth of the pulp bed also increases local compaction. These compacting factors are further aggravated by the pressure drop of the ozone and carrier gas through the pulp bed. Since the pressure drop is proportional to the compacted density of the pulp bed, the aggravating character of the gas flow is clear. In addition, the exothermic bleaching reaction produces higher temperatures in the zones of lower density and thus higher bleaching rates in those zones. Since the ozone/pulp reaction is very fast, and since the rate decreases rapidly with concentration decrease, the non-uniform bleaching effects of prior art reactors have been further aggravated because a large fraction of the ozone is consumed before it has an opportunity to reach the fibers in more densely packed zones of the pulp bed.

One important characteristic of the porous bed type reactor is that it has proved highly efficient in removing the ozone from its oxygen carrier gas.

Measurements of the gas leaving the porous bed into Docket No. 1197-IR-PA the gas separation chamber have shown a very low residual ozone content, for example 30 parts per million (when the feed gas contained 4% ozone in oxygen at the top of the reactor). This high utilization of the ozone gas is important to the economics of ozone bleaching as ozone is a relatively expensive chemical. It is believed that this high efficiency in ozone utilization is the result of the fact that the bulk density of the pulp bed is substantially increased near the gas separation zone by the combined effects of weight of the pulp above and pressure drop of the gas flowing through the entire height of the bed. For example, in a reactor operating with eight feet of pulp bed above the separation zone, and a gas velocity in the upper portion of the porous bed reactor of about ft./minute, the bulk density of the pulp at the gas separation zone will have risen to about ilbs./cu.ft. and the porosity will have been reduced to about 36% of the bulk volume.

In this zone of reduced porosity, the ozone gas is forced into intimate contact with the pulp fibers, and the last traces of ozone are consumed in bleaching rather than passing through the porous bed and being destroyed in the recycling system used to recover the oxygen carrier gas.

The foregoing illustrates limitations known to exist in present gaseous reagent pulp bleaching reactors. 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.

OF THE INVENTION According to a first aspect of this invention there is provided an apparatus for treating fibrous material with a gaseous reagent including: means for fluffing the fibrous material in the presence of the gaseous reagent to form a gas .suspension of the fibrous material; means for receiving and agitating said gas suspension of fibrous material to mechanically maintain the fibrous material in suspension until the gaseous reagent has substantially reacted with said fibrous material; and means for receiving said gas suspension of fibrous material wherein the reaction is completed and the remaining reaction gases are removed from the fibrous material.

2 According to a second aspect of this invention there is provided an apparatus for treating a fluffed fibrous :material with a gaseous reagent including: means for introducing a gas suspension of fluffed o: :fibrous material into a mixing chamber; means within said mixing chamber for mechanically agitating to maintain the fibrous material in suspension in the gaseous reagent for a dwell time sufficient that a majority of the gaseous reagent has reacted with the fibrous material; and means for dispensing the mixture of fibrous material and gaseous reagent into a reaction and degasification chamber to form a porous bed in which the reaction can continue until the gaseous reagent is substantially completely consumed.

SAccording to a third aspect of this invention there

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5a is provided in an apparatus, for bleaching wood pulp with a gaseous reagent, of the type having a mechanical pulp fluffer which fluffs the pulp in the presence of the gaseous reagent to form a gas suspension of pulp and a porous bed reaction and degasification chamber, an improvement including: a mechanical mixing chamber having a rotor with a plurality of mixing paddles, and means for conveying the pulp and gaseous reagent, as a gas suspension of pulp, through the mixing chamber at a rate such that a substantial majority of said gaseous reagent is consumed in the mixer, said mixing chamber being positioned between said mechanical pulp fluffer and said porous bed reaction and degasification chamber.

According to a fourth aspect of this invention there is provided a method for treating fibrous material with a gaseous reagent including the steps of: fluffing the fibrous material in the presence of the 20 gaseous reagent to form a gas suspension of the fibrous S: 20 material; SI. agitating to mechanically maintain the fibrous material in suspension in the gaseous reagent until said gaseous reagent has substantially reacted with said fibrous material; and dispensing the mixture into a porous bed reaction and degasification chamber to complete the reaction and remove the remaining reaction gases from the fibrous material.

ee o The foregoing and other aspects will become apparent from the following non-limiting detailed description of the invention when considered in conjunction with the accompanying drawings.

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Docket No. 1197-IR-PA The foregoing-ad uLther aspec tItbecome apparent from the follow'n -detailed description of the ion when considered in conj n with the accompanying drawing -DREF--DESCRIPTIONO-F THE-DRAWING-FI-GUR Fig. 1 is a schematic partially sectional view of an apparatus which embodies the features of the present invention; Fig. 2 is a fragmentary partially sectional view of the apparatus as seen from line 2-2 of Fig. 1; Fig. 3 is a graphic presentation of the reaction rate of ozone with wood pulp in a porous bed reactor; and Fig. 4 is a fragmentary partially sectional view of the apparatus of another embodiment of the present invention in which the reagent gas is introduced into the feed hopper of the mixing chamber rather than into the fluffer.

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Docket No. 1197-IR-PA DETAILED DESCRIPTION Fig. 1 shows the preferred embodiment of a gas/solid reactor system 20 of the present invention. It consists principally of a fluffer/blower 60, which forms the gas suspended mixtuo'e of the fibrous solid as previously described, a mixing chamber 40, and a porous bed reactor 30. The fluffer/blower 60 and porous bed reactor 30 are well-known in the art, but are described here in order to illustrate the function of the present invention within the system.

Fluffer/blower 60 receives high consistency pulp 5 (20% to 50% consistency) from a dewatering press, not shown, through conduit 66. It also receives a gas mixture 71 of feed gas 70 and excess blowing gas 76 through gas loop 50. Feed gas 70 is composed of a gaseous reagent and, depending upon the reaction desired, the gaseous reagent may be mixed with any appropriate substantially 7 non-reactive carrier gas. For purposes of this description, a wood pulp bleaching operation using ozone as the gaseous reagent and oxygen as the carrier gas will be used. Feed gas mixture 70 is injected through valve 77 into gas loop 50 where it 7 f Docket No. 1197-IR-PA 4 mixes with excess blowing gas 76 and forms gas mixture 71. It enters fluffer/blower 60 where it is i mixed with pulp 5 to form a gas suspended pulp fluff mixture and provides the mass flow necessary to transport gas/fluff mixture 54 which is discharged through conduit 51 into fluff discharge cyclone 52.

From there pulp 5 falls into mixing chamber feed hopper 46 and is fed into mixing chamber 40 by auger 43.

Within mixing chamber 40, the gas suspended pulp fluff mixture is mechanically agitated by the intense action of paddle blades 44 on rotor shaft 42 which is driven by motor 41. The velocities of the fluffed pulp and the contacting gas 69, which is composed of the remaining gaseous reagent in combination with the non-reactive carrier gas plus

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the developing gases resulting from the progressive bleaching reaction, through the mixing chamber are not the same. Usually the gas is traveling faster.

The intense mechanical action of the paddle blades maintains the pulp fluff in suspension in the contacting gas 69 and, thus, ensures intimate contact between the contacting gas 69 and the fluffed pulp so that no portion of the fluffed pulp leaves the mixing chamber without contacting the S8 Docket No. 1197-IR-PA gaseous reagent. This ensures unifo. bleaching during the high rate portion of the bleaching 1 reaction. The mixture is discharjed from the mixing chamber 40 through discharge neck 45 into the porous j 05 bed reactor 30. The action of the paddle blades 44 at the entrance to discharge neck 45 prevents plugging of the neck by the pulp exiting the mixing i chamber ii The fluffed pulp, having formed a porous bed, moves slowly downward through the porous bed reactor and becomes more compacted toward the bottom as Spreviously described.

i The contacting gas 69 passes down through the permeable pulp bed 7, since it has a much higher velocity than that of the pulp through the reactor until it reaches gas exit 37 where it escapes peripherally to the annular gas exit chamber 32.

i The contacting gas 69, by this time stripped of all but a trace of ozone and predominantly composed of oxygen plus gases produced as a by-product of the bleaching reaction, is vented through nozzle 75 and recycled to the ozone generator system. Bleached pulp 10 at the bottom of porous bed reactor 30 is diluted with recycled filtrate through dilution Docket No. 1197-IR-PA nozzle 34 and is discharged through pulp discharge 36 as dilute bleached pulp Fig. 2 shows a partially sectioned schematic L view from line 2-2 of Fig. 1 of fluffer/blower and its associated equipment. Pulp 5 from the dewatering press enters compression screw feeder 62 through conduit 66 and from there is fed into fluffer/blower 60. The compression screw feeder compacts the pulp into a substantially impermeable plug 99 which prevents backward flow of the mixed gas from reactor system 20. Mixed gas 71 is fed into fluffer/blower 60 through gas loop 50. It is mixed with the fluffed pulp which it conveys as a gas suspension of fluffed pulp by entrainment into pulp and gas discharge conduit 51.

Referring to Figs. 1 and 2, the novel features of the invention can best be understood. The functions of the fluffer/blower 60 and its related equipment, as well as the porous bed reactor 30, are well-known in the art. Despite the shortcomings of the prior art porous bed reactor, already described, its high efficiency of ozone removal makes it a i desirable element, in combination with the present invention, as explained below. The bleaching Docket No. 1197-IR-PA reaction of ozone with pulp is very fast. Fig. 3 shows a graphic representation of the concentration of ozone (in percent) versus time (in seconds) in contact with fluffed pulp under conditions such as those previously described for a porous bed reactor. The concentration of ozone in oxygen coming out of an ozone generator is shown as being approximately It can be seen, that for a porous bed reactor, approximately 60% of the ozone is consumed in less than one second of contact with the pulp, while at six seconds, over 90% of the ozone is consumed. For this reason, thorough mixing of the ozone with the pulp fibers is imperative in order to avoid wide variations in bleached brightness of the pulp. Thus, mechanical mixing chamber 40 is provided between fluffer/blower 60 and porous bed reactor The fluffed pulp/gas mixture 54 is injected through fluff discharge cyclone 52 and mixing chamber hopper 46 into auger 43 which conveys it to the mixing paddles 44 mounted on rotor shaft 42.

The feed rate is maintained so that the bulk density of the intensely agitated pulp in the substantially horizontal mixing chamber is the maximum possible which will not result in plugging of the chamber.

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Docket No. 1197-IR-PA This ensures that all of the fluffed pulp passing through chamber 40 is exposed, to the maximum possible extent, to the gaseous reagent in the chamber. Paddles 44 provide intensive mixing action between the pulp fluff and the gas to maintain the gas suspension of the pulp fluff which was formed in the fluffer/blower 60 during the dwell time in the mixing chamber 40. The mixing paddles closest to discharge neck 45 prevent plugging of the neck by the pulp exiting the mixing chamber 40 and dropping 7 into reactor 30 on top of pulp bed 7. The contacting gas 69 which, after six seconds mixing, contains less than half its initial ozone, continues downward through the porous pulp bed until it reaches gas exit 37 at which time its concentration is typically less than 30 parts per million ozone.

The mixing chamber provides a pulp fluff dwell time in the range of approximately 10 seconds while maintaining the pulp in suspension in the contacting gas 69 through intimate intensive mechanical mixing of the gas and pulp fluff. This ensures the maximum practical reaction extent prior to discharge of the mixture into the porous bed reactor. Since the ozone concentration entering the porous bed reactor Docket No. 1197-IR-PA is substantially less than half its initial concentration, the reaction rate is considerably slower, and non-uniformity of bleaching due to variable porosity of the pulp bed is further mitigated.

Fig. 4 shows an alternative embodiment of the present invention which is similar to that of Fig.

1, except that, here, fluffer/blower 60 has been replaced by fluffer 90 which, without any blowing action, deposits fluffed pulp fibers directly into mixing chamber hopper 46. Gas loop 50 has also been eliminated in this embodiment, since the reagent gas mixture is introduced in the mixing chamber hopper rather than in the fluffer.

This invention, in its preferred embodiment, provides the advantage of attaining uniform bleaching reaction results in the example considered without wasting expensive ozone or other gaseous reagent. By providing the mechanical mixing chamber prior to the porous bed reactor chamber, the desired gas/pulp contact time is achieved while intimate mixing is maintained. Thus, the invention provides uniform substantially complete reaction under commercially practical conditions over a wide range of capacities.

Docket No. 1197-IR-PA The ozone/wood pulp reaction is used as an example to illustrate the features of this invention. Any gas/solid reaction in which the reaction kinetics are similar to those of ozone/wood pulp and in which the solid has fluffy fibrous characteristics similar to wood pulp is an appropriate application for this invention.

Claims (4)

12. The apparatus of claim 10,Awherein said 1 means for conveying comprises a bias on the mixing 2 paddles. 19

13. The bleaching apparatus according to any one of claims 30 to 12, wherein said means for conveying provides a dwell time for the gas suspension of pulp in the mixing chamber of 10 seconds.

14. A method for treating fibrous material with a gaseous reagent including the steps of: fluffing the fibrous material in the presence of the gaseous reagent to form a gas suspension of the fibrous material; agitating to mechanically maintain the fibrous material in suspension in the gaseous reagent until said gaseous reagent has substantially reacted with said fibrous material; and dispensing the mixture into a porous bd reaction and degasification chamber to complete the reaction and remove the remaining reaction gases from the fibrous material. An apparatus for treating fibrous material with a gaseous reagent substantially as herein described with respect to any one of the embodiments illustrated in the accompanying drawings.

16. A method for treating fibrous material with a gaseous reagent substantially as herein described with respect to any one of the embodiments illustrated in the 25 accompanying drawings. I DATED 3 December 1993 PHILLIPS ORMONDE FITZPATRICK Attorneys for: INGERSOLL-RAND COMPANY 6136e Docket No. 1197-IR-PA ABSTRACT OF THE DISCLOSURE A method and apparatus for treating fluffed high consistency fibrous materials with gaseous reagents provides a mechanical mixing chamber with a dwell time sufficient to allow reaction of a substantial majority of the gaseous reagent prior to dispensing the mixture into a porous bed reaction and degasification chamber. The gas flows through the porous bed of fibrous material in the reaction chamber until virtually all the active reagent has been consumed. This results in uniformly treated fibrous materials and efficient reagent utilization. I II