Classifications

• • 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
  • D21C9/153—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications with ozone

Definitions

• the invention relates to a method for bleaching medium consistency pulp with an ozo- ne-containing gas.
• the invention relates to the proper utilization of the very fast reaction of ozone, by providing efficient but pulp-preserving mixing immediately on introducing a substantial amount of ozone into the pulp.
• ozone bleaching of medium consistency (MC) pulp can be described as ozone generation followed by compression before introducing the ozone containing gas into the the MC pulp flow.
• the gas-liquid-fiber suspension is vigourously treated in one or several high shear mixers before the suspension is lead to a bleach tower.
• the ozone may be introduced at several points along the pulp stream. Vent gases must be treated because of excess ozone carried over.
• the principle described may be a result of the application of oxygen bleaching methods. Oxygen, however, operates at a much slower rate, and the temperatures used are significantly higher than those employed in ozone bleaching.
• Typical and frequent problems arise from the difficulty to keep the suspension uniform. Segregation into two-phase flow easily occurs, and the ozonisation rate drops signifi- cantly (to 1 or even 0.1 % of its optimum rate) This is a dominant problem, which may be reduced by using a higher quality ozone, resulting in less gas void and consequently less need for vigorous mixing.
• a typical solution in the present state of the art is the use of more than one mixer. This does not, however, eliminate the problem, and by applying more shear forces to the pulp, the strength properties of the resulting product are severe- ly affected.
• a basic problem with such mixers is the short residence time, and if mixing time is increased, undesired backmixing may occur. After leaving the mixers, the gas-pulp suspension soon segregates into two-phase flow having a relatively small gas-liquid interface per unit volume. The chemical consequences of this are low capacity and a non-uniform bleaching result. Obvious evidence of this phenomenon is the significant ozone surplus often remaining after the bleaching stage, representing both a hazard and an economical loss.
• a pulp bleaching method comprising introduction of high pressure ozone in a carrier gas into a pulp stream with vigorous mixing and subsequent removal of carrier gas is disclosed in, e.g. EP-A 511 433.
• the major issue of this document is the removal of gas from the pulp after injection into the mixer; the reaction is said to take place essentially within ten seconds in a vertical reaction vessel situated immediately following the flui- dizing mixer. Gas at about 10-13 bar containing about 3-10 % ozone by weight (6.8 vol %) is used.
• the gas-pulp mixture is carried in a horizontal path following the vertical reaction step to effect separation of the large amount of carrier gas involved.
• Austrian patent application 2203/92 describes a method wherein medium consistency pulp is treated with an ozone-containing gas comprising more than 120 g O 3 / normal m gas (5.6 vol %) whereby the gas is introduced as fine bubbles with a low differential pressure (preferably less than 1 bar). It is considered that using gas with a high ozone content, a sufficient amount of ozone is can be suspended into the gas to achieve the desired bleaching. Further, AT 2203/92 discloses the use of mixers with or without fluidisation effects, and of an ozone reaction stage subsequent to the mixing stage, as well as additional ozone addition stages with degassing stages in between.
• the highly concentrated ozone is introduced in static mixers at several points, possibly removing the inert carrier gas (normally oxygen) between stages, and the final reaction between ozone and fiber takes place in a bleach reactor, typically of the traditional upflow tower type.
• inert carrier gas normally oxygen
• a common feature of several other publications disclosing ozone bleach processes for medium consistency pulp is the use of fluidizing mixers in connection with the injection of ozone-carrying gas, and the use of subsequent, relatively extended reaction stages and gas separation.
• MC ozonisation can be described as ozone molecules in a gas phase that must be transported to the vicinity of the fiber and react with the fiber or other substrates.
• the ozone must diffuse through the gas-liquid interface, through the liquid to the fiber.
• the applied mixing affects the size and the relative velocity of the gas bubbles, and also the amount of fiber-liquid interface.
• the rate limiting step completely dominating the interaction of ozone with the fiber material is the transport of ozone through the gas- liquid interface.
• the gas-liquid transfer rate in a given volume is heavily dependant on the bubble size, i.e. gas-liquid surface area m gas/m suspension, and on the partial pressure of ozone.
• Other rate limiting steps like diffusion in the fiber material itself, are determined by the nature and the consistency of the pulp, which is dominantly affected by the temperature.
• reaction rate of ozone Due to its dependency on mass transfer, the reaction rate of ozone is, theoretically and empirically, first order.
• RTD residence time distribution
• the high gas void i.e. the low concentration of ozone generated by most present ozone generators, limits the possibilities to improve the situation.
• Reduced gas void in subsequent generations of ozone generators will reduce the need for mixing and reduce ener- gy requirements as well as the size of the equipment. Higher ozone concentrations will also increase the ozonisation rate. Disclosure of the invention
• high-concentration, high pressure ozone is introduced into the pulp line, whereby conditions approaching plug flow are achieved, a high concentration of ozone is reached with a mass transfer area in the sus- pension which is sufficient for effective delignification.
• the ozone is introduced using effective injection nozzles providing for the efficient dispersion necessary for obtaining a uniform distribution as well as sufficient mass transfer area to overcome the rate-delimiting mass transfer treshold present in methods according to the prior art.
• the need for fiber- destroying high shear fluidizing mixers is removed.
• a dynamic low to medium intensity mixer is provided in the pulp stream immediately downstream of the ozone injection site.
• Such a mixer delivers to the pulp stream amounts of energy which are well below fluidization energies, and does not mechanically affect the fiber.
• ozone with a concentration of up to 18-20 % by volume may be generated. References to concentrations as high as 300 g O 3 /Nm 3 have been made in prior art publications (e.g. EP-A-426 652, priority 30.10.1989), but such concentrations have not been technically feasible until recently.
• the reaction between ozone and fiber is allowed to take place at such a rate that the subsequent use of an upflow bleach tower is not necessary.
• the gas pressure is obtained by using precompressed oxygen, optionally mixed with other gases or liquids (e.g. argon) to maintain a suitable conductivity for ozone generation.
• Oxygen is the most common carrier gas used for ozone.
• Highly concentrated ozone is usually considered an explosion hazard.
• the concentration of ozone in the gas introduced to the pulp stream is sufficient for achieving bleaching without any fiber-destroying mechanical impact.
• the initial distribution of highly concentrated ozone into the pulp is of importance, for the selectivity, as the carbohydrate component itself may be attacked by ozone if ex- posed for an extended time.
• the absence of backmixing, as may occur in high shear mixers, and the presence of plug flow conditions counteract this phenomenon.
• Figure 1 shows a comparison between the changes in reaction rates against time in a prior art ozone pulp bleaching process using a medium consistency mixer, and a process according to the present invention.
• Example 1 Ozone-carrying gas having a pressure of about 15 bar and an ozone concentration 14 % by volume is introduced into a medium consistency pulp line carrying 1000 tons/day via a collar of radially arranged nozzles.
• the nozzles are arranged to direct the gas radially into the pulp flow, essentially in a direction perpendicular to the pulp flow.
• a number of nozzles sufficient for distributing the gas evenly must be used. On this production scale, 186 nozzles with an inlet diameter of maximum 1 mm may be used.
• a sufficient mean residence time (10-40 seconds) must be allowed before any other disturbing action to the pulp.
• Example 2 A medium intensity (low-shear) mixer is adapted into the pulp stream of the previous example, preferably immediately following the gas injection site.
• the mixer turbine is preferably a double or multiple screw with blade angles and rotation rate balanced to maintain the plug flow residence time distribution (RTD) and giving good radial mixing efficiency.
• the center blade has a steeper angle than the outer screw blade.
• porous metal injector devices for introduction of ozone can be arranged peripherical- ly or on the turbine.
• Figure 1 shows a comparison between a system employing a traditional medium consistency mixer with a vei y high capacity for a short interval dropping rapidly to zero, compared to a system according to the invention with a moderately high capacity kept constant for a long period.
• the dotted line represents state-of-the-art traditional medium consistency mixer technology.
• the first, steep section shows the effect of the mixer with high reaction and uniform distribution.
• the low rate section shows the effect of the corruption of the gas-suspension interface.
• the reaction takes place with a nonuniform distribution and the pulp is mechanically stressed by high shear mixing.
• the solid line represents a system according to the invention. Throughout the process, a moderately fast reaction is taking place in a mildly stressed pulp and with a uniform distribution of ozone.
• Table 1 shows a comparison in numbers between a typical conventional MC bleaching system, a state-of-the-art system and a system according to the present invention.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

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Citations (5)

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• 1998
  • 1998-08-24 FI FI981808A patent/FI981808A/fi unknown
• 1999
  • 1999-08-24 ID IDW20010438A patent/ID28478A/id unknown
  • 1999-08-24 AU AU53760/99A patent/AU753807B2/en not_active Ceased
  • 1999-08-24 AT AT99939477T patent/ATE386836T1/de active
  • 1999-08-24 PT PT99939477T patent/PT1115946E/pt unknown
  • 1999-08-24 DE DE69938189T patent/DE69938189T2/de not_active Expired - Lifetime
  • 1999-08-24 CA CA002341490A patent/CA2341490C/en not_active Expired - Fee Related
  • 1999-08-24 BR BRPI9913123-4A patent/BR9913123B1/pt not_active IP Right Cessation
  • 1999-08-24 JP JP2000566505A patent/JP4888749B2/ja not_active Expired - Fee Related
  • 1999-08-24 EP EP99939477A patent/EP1115946B1/en not_active Expired - Lifetime
  • 1999-08-24 WO PCT/FI1999/000696 patent/WO2000011262A1/en active IP Right Grant
  • 1999-08-24 ES ES99939477T patent/ES2303383T3/es not_active Expired - Lifetime

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