WO2011149797A1 - Déshydratation du papier - Google Patents

Déshydratation du papier Download PDF

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Publication number
WO2011149797A1
WO2011149797A1 PCT/US2011/037485 US2011037485W WO2011149797A1 WO 2011149797 A1 WO2011149797 A1 WO 2011149797A1 US 2011037485 W US2011037485 W US 2011037485W WO 2011149797 A1 WO2011149797 A1 WO 2011149797A1
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WIPO (PCT)
Prior art keywords
press
paper
roll
dewatering
papermachine
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PCT/US2011/037485
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English (en)
Inventor
Jason Max Stotter
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Eltron Research & Development Inc.
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Publication of WO2011149797A1 publication Critical patent/WO2011149797A1/fr

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Classifications

    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
    • D21F3/00—Press section of machines for making continuous webs of paper
    • D21F3/02—Wet presses
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/46—Treatment of water, waste water, or sewage by electrochemical methods
    • C02F1/469—Treatment of water, waste water, or sewage by electrochemical methods by electrochemical separation, e.g. by electro-osmosis, electrodialysis, electrophoresis
    • C02F1/4698—Treatment of water, waste water, or sewage by electrochemical methods by electrochemical separation, e.g. by electro-osmosis, electrodialysis, electrophoresis electro-osmosis

Definitions

  • This invention relates to a method and apparatus for efficient dehydration of pulp and paper (slurries, web, etc.) by application of electro-osmosis.
  • the method and apparatus of this invention can for example be employed in conventional pulp and paper drying methods and papermachine systems to achieve energy and cost saving.
  • Paper production is energy intensive. In 2002, the paper industry accounted for 15% of industrial energy use in the United States. Dewatering accounts for a significant portion of energy costs for paper production, ca. 4-5 million BTU per ton of pulp. Replacing or supplementing heat-assisted mechanical dewatering with more energy efficient dewatering methods can be an important step in making paper production more cost competitive.
  • Paper is produced by chemically and mechanically digesting wood to produce a cellulose and lignin fiber slurry, called pulp. Additives that change the properties of the paper product are often added to the pulp. Non-fibrous additives used, including sizing agents, alum, resins, filllers, dyes/pigmnets, brighteners etc., depend at least in part on the type of paper being made. See: Smook (2002) Chapter 15 pp. 218-225. Depending on the type of pulp, solids content is ca. 5%. To make paper, pulp is applied to a screen and water is removed by vacuum applied to one side to increase the solids content and form a continuous, wet sheet of paper, or paper web.
  • the paper web is a type of paper fiber slurry.
  • the paper web is thereafter mechanically dried, either with a series of roll presses or with a shoe press, to 40-45% solids content.
  • the paper making processes described to this point are collectively known as the wet end.
  • the paper web is then transferred to the dry end, where the web is further dried to the final solids content of ca. 95%. This drying is often achieved by passing the web through hot rollers which are gas fired in modern plants.
  • Smook 2002 provides an overview of wet end (Chapter 16, pages 227-260) and dry end (Chapter 17, pages 263- 282) operations for paper manufacture. This reference is also incorporated by reference for its descriptions of paper manufacture, its descriptions of of the wet end and dry end processes of paper making.
  • Mechanical dewatering can achieve a slurry composition of ca. 30% solids for slurries consisting of fine, compressible particles, such as paper slurries. Mechanical dewatering is not further effective for slurries with particles smaller than 10 ⁇ because the paths between particles are so small as to present a significant resistance to liquid flow.
  • Heat-assisted mechanical dewatering utilizes heat to help drive the interparticle water from mechanically dewatered slurries. The energy required to generate the heat needed to assist mechanical dewatering is significant.
  • 290 is a review article reporting electrically enhanced dewatering of kaolinite (clay) suspensions, with the goal of estimating the feasibility of up-scaling the process.
  • the reference also refers to dewatering soils, clays, and wastewater treatment sludges.
  • Vijh, A. K. (1999) Electro- Osmotic Dewatering of Clays, Soils and Suspensions in Modern Aspects of Electrochemistry Chapter 4, p: 301 -332 is a technical review of the energetics of dewatering.
  • the reference provides analysis of dewatering of soils and clays.
  • Pulsed electric field (PEF) enhanced expression and juice quality of white grapes Separation and purification technology 52 520-526 reports the application of pulsed electric fields (PEF) to the pressing of grapes to increase juice expression.
  • PEF pulsed electric fields
  • PEF of moderate intensity (500-1 OOOV/cm) and short duration is reported to allow permeabilization of cell membranes and to facilitate expression of juice and its components.
  • WO/2009/016935 reports a sludge dewatering method which makes it possible to dewater to a water content of 60% or below through electro-osmotic dehydration.
  • the sludge is conditioned by the addition of an iron inorganic coagulant and then dewatered in an electro-osmotic dehydration apparatus.
  • the iron inorganic coagulant is preferably ferric sulfate and the amount added is preferably 5 to 20wt% in terms of Fe based on the SS of the sludge.
  • WO 2008/029961 reports an electro- osmotic dehydrator using three phase alternating current.
  • U.S. patent 6,871 ,744 relates to an apparatus for electrodewatering a liquid/solid mixture.
  • the patent reports an apparatus having a conveyor belt for receiving an electric charge and an element above the conveyor belt for receiving an opposite charge.
  • the conveyor belt and element are adapted for applying a compressive force to the mixture.
  • the conveyor is reported to include a plurality of discrete electrically conductive segments permeable to the liquid.
  • U.S. patent 7,578,918 relates to a process an apparatus for treating sludge comprising electrodes.
  • U.S. patents 1 ,229,203; 3,773,640; 3,980,547; 4,003,81 1 ; 4,003,819; 4,048,038; 4,107,026; 4,1 10,189; 4,208,259; 4,208,260; 4,246,039; 4,312,729; 4,367,132; 4,639,300; 4,755,305; 5,019,230; 5,160,593; 5,259,940; 5,344,533; 5,362,371 ; 5,403,455; and 6,089,788 relate variously to electrokinetic separation of solid particles from liquids; electroflocculation, electro-osmotic processes, dewatering with an electrical augmented vacuum filter; removal of liquid from sludge and vertical drains for soil stabilization.
  • U.S. 5,891 ,342 reports a dewatering process for application to dewatering of sludge where the sludge is subjected to compressive mechanical forces and electro- osmosis using a belt comprising electrically conductive material.
  • the belt is reported to comprise a plurality of spiral yarns. It is reported that the potential difference applied is preferably no greater than 30 V and the electrical current is preferably no greater than 120A.
  • U.S. patent 3,705,847 relates to a method for forming a uniform continuous web of paper in which an aqueous dispersion of paper making fibers are flowed between spaced-apart electrically charged endless belts forming a deposition zone. Guide blocks are provided to maintain a "proper" electrode spacing. An electrostatic field is reported to be maintained between conducting surfaces at a predetermined potential to deposit paper-making fibers by electrophoresis to form a fiber mat. The fiber mat is reported to be removed from the conducting surface upon which it was formed by bringing the formed mat into contact with another charged surface having potential sufficient to attract the paper mat. Thereafter the fiber mat is reported to be dried and pressed.
  • U.S. published application 2009/01 14359 relates to a method in the wet end of a web forming process.
  • the publication reports that "at least one electrode pair is placed in the wet end," and that the electrode pair is supplied with a current/voltage from a power source, so that an electric field is set up between the electrode pair (102, 103).
  • the electric field is said to cause "the material particles in the pulp suspension in the wet end to be electrically charged in a desired manner and to move in a desired manner in the pulp suspension in the wet end.”
  • the current/voltage of the power source is said to be controlled “by means of a measuring and control unit so as to cause the material particles in the pulp suspension to be electrically charged and to move in a desired manner, thus allowing the retention, formation and orientation of the material particles in the pulp suspension to be substantially improved.”
  • the method is directed to the improvement of "retention” to reduce the "considerable amount of fibers and additives” that are "removed together with the water from the fiber suspension.”
  • JP2004300591 relates to a papermachine which is reported to " improve the efficiency of dehydration and the quality of the made paper.”
  • the papermachine is described as having a "section for feeding a pulp suspension between upper and lower wire cloths ...to form the wet paper ...simultaneously pressing the wet paper through the wire cloths to remove the water or squeeze the wet paper.”
  • the wire cloths are said to comprise a high resistant material or an insulating material and not having water absorbability.”
  • a DC voltage is said to be "applied to the wet paper nipped between the wire cloths at a plurality of points.”
  • JP 2002138383 relates to a dehydrating apparatus for a papermachine, reported to be capable of further increasing efficiency of water removal of wet paper.
  • the apparatus is described as pressing wet paper against a suction roller through a water absorption material.
  • the water absorption material is described as composed of an electroconductive material and is connected through a guide roller to an anode of a DC power supply source.
  • a cathode of the DC power supply source is said to be connected to the suction roller.
  • the invention provides effective and efficient electroosmotic processes for dewatering wet biomass or textile product, particularly pulp and paper slurries.
  • the methods and papermachines adapted for use in such methods are generally useful for dewatering/drying of any paper or textile product formed in a continuous web.
  • the methods and apparatus of this invention are more specifically useful for production of paper products, including among others, market pulp, paperboard, liner, tissue, and towel.
  • the invention relates to a dewatering process for paper or paper pulp (paper) that uses application of electrical energy to assist mechanical dewatering in which pressure is applied so that the dewatering achieved during pressing is enhanced.
  • Application of the process of this invention can result in energy cost significantly lower than removing the same amount of additional water with heat.
  • the papermachine apparatus employed most generally comprises at least one mechanical press that is electrified.
  • the term press is used generically herein to refer to a mechanical press of any type that is employed in the press section of a papermachine to apply pressure to wet web, mat or textile.
  • a press generally comprises two surfaces between which the wet web (e.g., paper slurry mat) passes.
  • One or more surfaces of a press can be a roll.
  • one roll or surface of the press is electrified against the opposing roll or surface of the press.
  • Current preferably direct current, passes from one surface to the opposing surface through the wet web or mat.
  • An appropriate power supply is employed to electrify the press surfaces (e.g., rolls of a roll press).
  • the press that is electrified is a press in the wet end of a conventional papermachine. More specifically, the press that is electrified is a roll press or shoe press in the wet end of a conventional papermachine. In another embodiment, the press that is electrified is a dedicated press that is added to a conventional papermachine. More specifically, the dedicated press that is electrified is a roll press or shoe press added into the press configuration of the wet end of a conventional papermachine. In a specific embodiment, the pressure applied at the electrified press is selectively controllable. The pressure applied at a press is selected or adjusted to achieve a desired level of dewatering. In an embodiment, the pressure applied at a press is 200 pli or more.
  • the pressure applied is 200 pli to 1000 pli.
  • the potential that is applied across the press is selectively controllable. Application of the potential across the press enhances dewatering compared to that obtained by application of pressure alone. More specifically, application of the potential across the press provides at least a 5% enhancement of dewatering. In related embodiments, application of the potential across the press provided at least a 10% enhancement of dewatering.
  • the resistance of the web or mat e.g., paper slurry mat
  • the potential is preferably generated at or overlapping the point or region of closest approach between the surfaces of the press.
  • the potential gradient formed drives water from one side of the web or mat passing through the press to the other.
  • Mechanical force is applied in a way that forces the two surfaces together at the point or region of contact with the web or mat moving between them.
  • the potential can be applied at one or more points where mechanical force is applied.
  • the invention provides an improved method for dewatering of a paper slurry mat in the wet end of a paper-making process in a papermachine.
  • the papermachine is adapted as is known in the art for the type of paper product being made.
  • the method of the invention is applied to making paperboard, liner, tissue or towel.
  • a method of the invention comprises the steps of providing an electric field across at least one press in a papermachine by applying a voltage across the components of the at least one press.
  • the voltage applied is selectively variable.
  • the electric field is applied after the formation of a paper slurry mat.
  • the electric field is applied in the press section of the papermachine.
  • the paper slurry mat is formed in the papermachine for passage though the wet and dry ends of the papermachine.
  • the voltage applied at the press results in an electric field strength greater than 200 V/cm between the surfaces of the press components at the point of closest distance between the surfaces.
  • the electric field strength obtained by application of voltage is between 400 V/cm and 30,000 V/cm. In other embodiments, the electric field strength obtained by application of voltage is between 400 V/cm and 15,000 V/cm.
  • one or more presses of the papermachine can be electrified, wherein at least one of the electrified presses is in the wet end after formation of the paper slurry mat.
  • two or more presses are electrified.
  • two or more presses in the wet end are electrified.
  • two or more presses in the press section of the wet end are electrified.
  • the press across which the voltage is applied is the first press after formation of the paper slurry mat.
  • the press across which the voltage is applied is the last press in the wet end.
  • the presses across which the voltage is applied are the first press after formation of the paper slurry mat, and the last press in the wet end section of the papermachine.
  • the press across which the voltage is applied is a press added to a conventional papermachine in the wet end.
  • the press across which the voltage is applied is a press located in the press section of the papermachine at a point where the paper slurry mat passing through the roll press has a solids content of 25% or more.
  • the press across which the voltage is applied is a press located in the press section of the papermachine at a point where the paper slurry mat passing through the roll press has a solids content of 40% or more.
  • the press may be a roll press or a shoe press.
  • the pressure applied at the electrified press will depend upon the type of press that is employed. In specific embodiments, the pressure applied at an electrified press is 100 pli or higher. In related embodiments, the pressure applied at an electrified press is 200 pli or higher. In related embodiments, the pressure applied at an electrified press is 400 pli or higher. In other related embodiments, the pressure applied ranges from 200 pli to 1000 pli and subranges thereof.
  • Papermachines adapted for use in this invention are preferably operated at conventional speed for the paper making process for which the papermachine is employed.
  • the speed at which the paper slurry mat passes through the electrified press is 200 feet/minute or higher.
  • the speed at which the paper slurry mat passes through the press is 500 feet/minute or higher.
  • the invention also relates to certain dewatering apparatus and devices for implementation of the methods herein.
  • the invention also relates to methods for adapting existing papermachines for electroosmotic dewatering by electrifying existing roll presses therein and/or by adding additional electrified roll presses therein.
  • the invention additionally relates to methods for making paper of various kinds employing such adapted equipment.
  • FIG. 1 is a schematic drawing of an exemplary first dewatering device employing rollers particularly for application to pulp and paper dewatering applications.
  • FIG. 2 is a schematic drawing illustrating a conventional Fourdrinier machine which can be implemented for electroosmotic dewatering by generation of electric fields at one or more locations in the machine or by addition of electrified nips, for example, prior to the steam roller section.
  • FIG. 3 is a schematic drawing illustrating a paper machine conventionally employed for manufacture of tissue which has a Yankee cylinder (7).
  • This machine can be implemented for electroosmotic dewatering by generation of electric fields at one or more locations in the machine or by addition of one or more electrified roll press nips, for example, before, at or after the Yankee cylinder.
  • the invention relates in part to methods for increasing the solid content of pulp slurries and particularly wet paper web.
  • the term slurry is used generally herein to apply to a combination of liquid and solid which is largely insoluble in the liquid.
  • the term is not intended to refer to mixtures having only certain ranges of solids content.
  • the term applies to liquid solid mixtures wherein the solids content ranges from 1 % to 90%, or 10% to 80% or 5% to 50% among others.
  • the particles in the slurry can be suspended in the liquid.
  • the liquid is water or an aqueous solution.
  • paper slurry and the term pulp slurry may be used interchangeably herein.
  • paper web refers to the web formed typically at the beginning of a papermachine (which term is used herein to refer to the combined presses, rollers, felts and other elements of equipment configurations employed to make various dried paper products and includes both the wet end and the dry end).
  • Solids content can be determined by any known method and for example can be determined as noted below.
  • Paper manufacturing terms employed herein are intended to take their broadest art recognized meaning unless otherwise indicated.
  • Papermachines include a wet end and a dry end, as is known in the art. A portion of the wet end is designated the press section, again as is known in the art.
  • the method of this invention can be applied or adapted to any papermachine configuration including those having roll presses and those having shoe presses.
  • the method of the invention comprises a step of applying a mechanical force to the slurry (after fiber mat is formed) while applying a controllable electric field to the slurry.
  • the controllable electric field is applied to electrodes, comprising one or more anodes and one or more cathodes in fluid contact with the slurry.
  • one or more of the electrodes are liquid permeable.
  • one or more filter media (or porous separation material) are inserted between the slurry and the one or more liquid permeable electrodes.
  • the filter medium is filter paper.
  • the filter medium is absorbent.
  • the filter medium is felt of the type commonly used on roller presses in a papermaking machine.
  • the filter medium is a porous ceramic material.
  • the filter medium is a textile.
  • the electric field is controlled to achieve desired effects or avoid undesired effects.
  • the electric filed can be controlled, for example, to maintain or achieve a desired temperature of the slurry.
  • the dewatering papermachine device comprises at least one roll press that is electrified.
  • a roll press can comprise a roll and an opposing surface or a pair of rolls each such pair being spaced apart and capable of being pressed together with mechanical force, through which the slurry passes and is mechanically dewatered.
  • Application of a voltage to the at least one roll press such that current passes through the slurry passing there though facilitates dewatering.
  • one or both rolls in each pair is covered with a porous, absorbent material, such as felt, that absorbs water from the slurry when in contact therewith.
  • the dewatering papermachine device comprises one or more roll presses, each of which are electrified with opposite and equal voltages to pass a current through the slurry and any absorbent material, by electrifying the metal rolls or opposing surfaces, by electrifying the absorbent material by means of a conductor intertwined in the absorbent material, or by covering the absorbent material with a porous conductor, or a combination of these approaches.
  • a dewatering papermachine apparatus comprises: one or more roll presses for receiving a slurry there between wherein at least one roll press comprises a roll and an opposing surface such that on rotation of the roll a mechanical force is applied between the roll and the surface and to the slurry and a pair of spaced apart electrodes in fluid contact with the slurry received between the roll and surface for application of an electric field to or across the slurry.
  • the roll and the opposing surface form the pair of electrodes.
  • a dewatering papermachine apparatus comprises: one or more pairs of spaced apart rolls for receiving a slurry there between such that on rotation of the rolls a mechanical force is applied to the slurry and a pair of spaced apart electrodes in fluid contact with the slurry received between the rolls for application of an electric field to or across the slurry.
  • the rolls Preferably the rolls form the pair of electrodes.
  • One or more electrified roll presses can be combined with one or more non-electrified roll presses.
  • One or more actuators can be provided for a roll or pair of rolls to facilitate application of a mechanical force through the rolls to the slurry there between.
  • the mechanical force applied by the electrified roll press is selectively adjustable.
  • the mechanical force and the electric field applied at the electrified are both independently selectively adjustable.
  • the electrodes are formed on the rolls (or the roll and surface) to provide for continuous flow operation.
  • one or both of the rolls and opposing surface are electrically isolated from the balance of the dewatering device, except where intentionally contacted by electrical brushes, by means of electrically isolated bushings, bearings, or mounts.
  • one or both of the rolls and opposing surface are electrically isolated from the balance of the device, except where intentionally contacted by electrical brushes, by being mounted to a portion of the device which is itself electrically isolated from the balance of the device.
  • each roll is sleeved or opposing surface is covered in an electrically insulating material which is sleeved in a conductive material, providing electrical insulation of the electrically conductive contact surface of the roll press from the balance of the device, except where contacted by electrical brushes.
  • the electric field is applied at one roll press in the papermachine at the wet end after paper web formation. In an embodiment, the electric field is applied at two roll presses in the wet end after paper web formation.
  • the roll which is electrified in the papermachine of the invention is not a suction roll.
  • the electric field is applied after formation of the paper web.
  • the electric field is applied at the first roll press in the wet section after paper web formation in a conventional papermachine. In an embodiment, the electric field is applied at any roll press in the wet end other than the last roll press before the dry end.
  • the voltage applied across the electrified roll press is such that
  • the improved papermachine of this invention does not contain an endless belt.
  • a vacuum is not applied to the slurry or a surface of the slurry at the same location at which an electric field is applied.
  • Tissue is made with few additives so that it is expected that effects on zeta potential will be minimal in application of electroosmotic dewatering to tissue paper manufacture.
  • Tissue is a growing market segment.
  • Tissue products are made on different paper machines than the other products because of the delicate nature of the product and the desire to dewater without applying pressure that would compress the fibers. This is important to maintaining absorbency. (See exemplary tissue specific machine in Figure 3).
  • a caliper assumed for tissue is 0.1 mm.
  • EOD effective 40 V to 300 V, 0.04 A to 3 A
  • EOD effective in paper 500 V to 1 ,320 V, 0.45 A to 1 .2 A
  • EOD effective in paper and felt 1 ,320 V to 9,000 V, 1 .2 A to 8.2 A
  • Market pulp is formed into high basis weight sheets that are then dried, lapped into 4'x4' sheets which are baled up and sold to paper makers without their own pulp mills. This application usually involves no additives that might interfere with the application of our technology. Market pulp is pressed with high pressure, achieving up to 50% solids at the end of the press section. An exception is fluff pulp which is pressed with less pressure to prevent damage of the fibers. Most market pulp machines support the web with felt in the press section, so no ranges are given for "no felt” applications.
  • Basis weight 200 to 500 lbs per 1000 square feet
  • EOD not effective 0 V to 600 V 2.
  • EOD effective in paper 600 V to 1 ,800 V (0.4 A to 1 .2 A)
  • Liner is the part of corrugated boxes on the visible surface (not the fluted medium inside corrugated). It is made from unbleached pulp. It is not washed as well as tissue and market pulp, so it will be more conductive, decreasing energy efficiency of electroosmotic dewatering. Solids content after a traditional roll press section is 45%, and after a shoe press is 50%. Most liner machines support the web with felt in the press section. Caliper is assumed to be 0.02 cm, based on measurements of some liner samples with a micrometer. Numbers are calculated assuming implementation in a roll press. Note that for the "with felt" case, the felt is estimated to be effectively dewatered at lower potentials than the paper.
  • Basis weight 20 to 90 lbs per 1000 square feet (most of market is 42 lbs per 1000 square feet)
  • EOD effective 80 V to 600 V, 3.2 A to 24 A
  • Electroosmotic dewatering is less preferred for application to making fine paper and newsprint due to likelihood of additives, including particles, dyes and retention aides, that can affect zeta potential.
  • Paper fibers are negatively charged, so there should be some electroosmotic effect when wet paper fibers are placed in a voltage field.
  • a hydraulic press in which a hydraulic press was used to squeeze paper fiber mat with two porous electrodes, across which a voltage was applied very poor results were achieved.
  • the rate of water leaving the paper was too slow to suggest success at the high speed of a paper machine. This is likely because the void spaces between the paper fibers are small and tortuous when the paper is compressed.
  • the path traveling through this mat of paper fibers which several millimeters in thickness, is believed to have provided too much resistance to flow.
  • using a paper handsheet roll press the fiber mat is significantly thinner, presenting less resistance to flow, even under pressure.
  • the voltage field is also increased for the same applied voltage because the two electrodes, e.g. the rolls, are much closer together than in a piston press.
  • the paper and its felts are so thin that it does not take a large voltage magnitude to achieve a significant voltage field (V/cm).
  • Voltage fields previously reported for successfully dewatering materials other than paper slurry have been around 100 V/cm.
  • application of voltages across paper slurry higher than 100V/cm is more successful. Effective increases in electroosmotic dewatering combined with mechanical pressing have been observed at 8,000 V/cm.
  • the slurry in electroosmotic dewatering, can be thought of as a collection of nearly touching, charged particles.
  • the void space between the particles can be thought of as a network of passages with charged walls filled with an electrolyte solution.
  • the ions in the electrolyte that are opposite in charge to the charges on the particles will be more concentrated near the particle walls in order to balance the charge.
  • these concentrated ions When an electric field is applied across the slurry, these concentrated ions will migrate towards the oppositely polarized electrode, of the two electrodes used to apply the voltage field. The migration of these ions induces a net flow of the electrolyte solution towards the oppositely charged electrode.
  • the surfaces of the fibers have a net negative charge. Therefore, cations in the electrolyte will preferentially concentrate at the walls of the fibers.
  • the bulk electrolyte will move towards the negatively charged electrode.
  • the rate at which the bulk electrolyte moves towards the cathode is slower than the theoretically predicted rate due to the resistance to flow by in the confined pore space.
  • the initial dewatering rate with EOD applied to paper slurry was estimated based on the zeta potential of pure cellulose fibers in neutral solution to be 1 .5 m/secA//cm.[Lindstrom, T. "Electrokinetics of the Paper Industry.” In Paper Chemistry, Roberts, J. C, Ed.; Chapman and Hall: New York, 1996; pp 25-43.]
  • the linear velocity of water in the pulp, in the absence of resistance to flow, was estimated by multiplying this initial dewatering rate by the voltage field strength, 4000 V/cm to be 6x103 ⁇ /sec 6x10-3 m/sec.
  • the estimated electroosmotic effort, in the absence of resistance to flow within the paper, can contribute about 27 mL/sec of the required about 58 mL/sec flux of water from the paper passing through the nip. This represents a significant amount (near 50%) of the required dewatering.
  • the estimated values that were used in these calculations represent conservative values from ranges of values that might be observed in practice. Therefore, the amount of dewatering provided by application of EOD in the absence of resistance to flow within the paper in actual practice may be even higher than estimated. In any case, these estimates indicate that the electroosmotic dewatering rate predicted in the absence of resistance to flow can contribute significantly to the dewatering of paper at industry relevant press pressures and speeds. This first approximation model does not however predict the effect of resistance to flow on the actual observed flux.
  • the current models of dewatering paper describe the process as movement of water through a network of fibers, first with water moving against the resistance of other water in the pores, and as the network passageways decrease with increasing solids content as dewatering proceeds, with water being pressed from pores within the fibers.
  • Models generally start from the Kozeny-Carman equation, governing movement of liquid through porous networks formed by impermeable particles. These models are somewhat simplistic because it is known that paper is formed by porous, compressible particles. While these models recognize that resistance to flow increases with decreasing water content, the effects of different types of pulp, different pressures, and different press impulses at different starting water contents or, more importantly, at different water contents during the pressing have not been reported..
  • One empirical model (Kerekes, R. J.
  • the first is to sleeve the rolls (or cover any opposing surface) with a non-conductive material, and sleeve or cover this material with a conductive material.
  • the surface of the rolls (or opposing surface) that contact the paper are then electrically isolated from the balance of the apparatus.
  • Another approach is to electrically isolate the bushings or bearings on which the roll(s) or opposing surface are mounted from the frame of the apparatus.
  • Another approach is to electrically isolate the armature or frame element to which each of the roll(s) or opposing surface is mounted from the balance of the apparatus. Electrical brushes can then be used to make electrical contact between the rolls and the power supply in all of these cases.
  • Another approach is to use one of these three methods to isolate one of the pair of rolls or roll and surface, electrically ground the frame of the apparatus, and use an electrical brush to polarize the isolated roll or surface relative to earth ground.
  • Electrical isolation of the synchronization portion of the drive for the rolls may be achieved by use of electrically insulating gears with conductive drive chain, metal chain with metal gears that are insulated from the roll axles by use of insulated sleeving, or pulleys and a belt where the belt is electrically insulated, the pulleys are electrically insulating, or the pulleys are electrically isolated from the roll axles by use of insulated sleeving.
  • a force applied between the rolls can bend the roll(s) sufficiently that physical contact is made between the electrified surfaces at the ends where there is no paper separating the surfaces. This would result in an electrical short, eliminating the electrical dewatering force.
  • a roll or rolls can be crowned to account for bending of the rolls due to force, preventing physical contact anywhere in the roll press.
  • the ends of the rolls may be alternatively or in addition coated with an insulating polymer or other material that overlaps with the paper contact area. This area of paper contact with the non- conductive material coated segment of the rolls and opposing surface will not be effectively dewatered by the electrical dewatering force.
  • the coating applied must be electrically conducting in the portions of the rolls that come in contact with the paper that is to be dewatered.
  • the roll press surfaces in contact with the paper mat can be coated with a conducting polymer.
  • a variety of such conductive polymers are known in the art.
  • Electrified roll or other contact surfaces in the roll press are preferably constructed from a non- corroding material of sufficient hardness and thickness to minimize or prevent deformation when mechanical pressure is applied to the paper or pulp and have sufficient conductivity to minimize electrical resistance to the dewatering circuit.
  • Preferred materials include platinum, niobium, and conductive polymers.
  • the bottom roll (with respect to the ground in a typical horizontal apparatus implementation) should be polarized negative relative to the upper roller. This causes the water that is driven by the electrical force in a downward direction (by gravity).
  • a safety control circuit is preferably included in the system to prevent loss of electrical current through the apparatus and prevent electrical hazards to operators.
  • This circuit could be in the form of a ground fault circuit interrupt placed between the roll press and the DC power supply.
  • An electrified roll press of this invention may be in the form of an extended-nip press or shoe press type, where one roll contacts an extended nip in a solid shoe that does not rotate.
  • the belt that passes over the shoe to carry the paper through the nip which is typically made of a non-conductive polymer material, must be made from a conductive material for implementation of EOD in a shoe type press.
  • a shoe press would replace either the nip 5,6 or 7,8 in Figure 2.
  • U.S. patents 6,514,385, 6,527,916, 7,008,506 and 7,922,869 illustrate a press section configuration including exemplary shoe presses which can serve as the electrified press of this invention.
  • the roll or rolls of an electrified roll press may be undriven or driven.
  • One or both rolls may be driven.
  • Dryer paper, which has more tensile strength, or paper supported on felts, may not require driven rolls. If both rolls are driven, they must be synchronized so the roll surfaces at the nip are traveling with the same linear velocity to prevent tearing of the paper.
  • any roll driving mechanism known in the art can be employed, driving of the electrified rolls must not create and electrically conducting path from one electrified roll to the other. The conditions and mechanisms for driving rolls will be known to one of ordinary skill in the art.
  • Electrical current passed through the paper can be direct current, pulsed direct current or alternating current.
  • the current is preferably direct, not alternating.
  • the current employed can be alternating. See energy saving analysis below for discussion of alternating current application.
  • the roll(s) must apply mechanical force to the paper in the location of the nip, with the force preferably being applied along the line between the rotational axis of the roll(s).
  • rolls should be crowned. Crowning means the rolls are slightly thicker in the middle than at the ends, allowing for a uniform force to be applied to the paper across the length of the roll despite deformation of the roll due to the applied force. Crowned rolls are common employed in the press sections of paper machines.
  • Press surfaces in contact with paper sluury mat must have electrically conductive surfaces. Rolls or other such surfaces in presses can be manufactured from appropriately conductive materials or electrically conductive roll covers can be applied to the surface of non-conductive rolls. If precautions are not taken, the surface of the anode roll will corrode with the application of the current applied for dewatering.
  • Anode rolls are preferably either prepared from or coated with a material that can resist the corrosion.
  • a material that can resist the corrosion For example, when wet paper towel was placed in the nip of the apparatus shown in Figure 1 , with rolls stopped and with dewatering pressure applied, and 50 V was applied for 10 min, considerable etching of the anode roll was observed at the point of contact with the paper. Also at the point of contact of the paper with the roll, the paper was stained with a red-brown stain, presumably iron and other metal oxides from corrosion of the stainless steel roll cover. This corrosion would lead with extended use to destruction of the anode roll and staining of the paper product being dewatered.
  • the experiment was repeated with a piece of platinum foil placed between the anode roll surface and the paper at the point of contact. After a 10 min application of 50 V, there was no discoloration on the paper and no discoloration or etching visible on the anode roll or the platinum foil.
  • Other approaches for protecting the anode roll from corrosion include a conductive polymer roll cover, a conductive ceramic roll cover, or a coating of another noble metal other than platinum.
  • the experiment was repeated with a piece of palladium foil, resulting in discoloration of the palladium at the point of contact, but no visible evidence of corrosion products left in the paper.
  • Another preferred roll cover material is niobium.
  • niobium forms a conductive, stable, and protective oxide in the presence of water giving niobium electrode surfaces noble-metal-like properties.
  • niobium forms a conductive, stable, and protective oxide in the presence of water giving niobium electrode surfaces noble-metal-like properties.
  • the conductive roll cover should be less resistive than wet felt, in the direction of the current flow through the paper.
  • the cover should be less resistive than conductive polymers used in static dissipation rubber materials, which have a resistance on the order of 10 6 ohms in this direction. If the roll cover is too resistive, the current needed to achieve the electroosmotic dewatering potential across the paper or paper and felt will consume power through the generation of heat that exceeds the value of the steam savings in the steam drying section.
  • the maximum applied voltage is 30,000 V/cm, which is the breakdown field strength for air (arc discharge), or approximately 6,000 V for a 2 mm thick felt. For application to a medium weight paper, with thickness of approximately 0.01 cm, the estimated maximum applied potential is 300 V.
  • the applied voltage for the best effects was 50 V for 675 g/m 2 basis weight of 100% softwood pulp hand sheets.
  • the spacing between the rolls was measured to be approximately 0.128 mm. This represents a field strength of approximately 3900 V/cm.
  • the paper speed through the press was approximately 90 ft min.
  • the pressure applied between the rolls was in the range of 300 to 500 pounds per linear inch (PLI). Under these conditions, solids content achieved was 35% with mechanical pressure only and 47% with mechanical pressure and electrical force.
  • the sheet was carried through the nip on a sheet of polymer film with a smaller than 4" diameter hole cut in it.
  • the film provided electrical insulation between the rolls until the sheet was passing through the nip. No felts were used.
  • the thickness of this film which defined the distance between rolls for most experimental results reported here, was 0.0125 cm.
  • Handsheets in these experiments had basis weight of 109 g/m 2 and were made from a 50/50 mix of bleached, kraft hardwood and softwood fibers.
  • Figure 4A and 4B show experimental results for one set of conditions.
  • the first plot shows the % solids increasing for sheets that were passed through the nip 15 times. Two sets of points are shown, one for 5 V (control) applied across the paper and one at 50 V. Application of 5V is expected to too low a potential to generate EOD, but is used to measure the resistance of the sheet for other calculations.
  • the plot shows the solids content for the 50 V sheet increasing faster than the 5 V sheet in passes 6 and higher.
  • the second plot shows the difference between the % solids for the two sheets at each pass. There is a clear improvement in % solids with 50 V applied.
  • the enhanced dewatering effectiveness decreases with increased roll speed because the residence time of the paper in the roll contact area decreases with increasing speed. As shown in Table 1 , the difference between an average of three sheets passed with 50 V and the average of three sheets passed at 5 V (all at 410 PLI) decreases with increasing speed. A nearly linear relationship between press speed and maximum % solids is observed.
  • the pressure applied to the rolls can vary for different paper products.
  • a paper machine generally has several roll presses through which the paper passes, with each roll press applying increasing mechanical pressure on the paper or pulp. It was expected that implementation of EOD would be most effective when the electrical force was applied to the last, highest pressure roll in a paper machine. However, as shown in Table 3, at the maximum pressure tested (640 pli) the application of the voltage actually decreased the effectiveness of the mechanical dewatering.
  • the data in Table 3 represent averages of three sheets passed under each set of conditions, all at a speed of 150 ft/min and a potential field strength of 4000 V/cm.
  • FIG. 1 illustrates application of an exemplary device of the invention to dewatering of paper web.
  • rollers 30a and 30b with conductive surfaces 31a and 31 b are spaced apart to receive a paper web (34) which is to be dewatered.
  • the spacing between the rollers is selectably adjustable such that a desired amount of pressure is applied to the paper web.
  • the rollers rotate in opposition to one another, as indicated by arrows and the paper web passes between the rollers.
  • an optional support material 35 (such as felt) can be employed to support the paper web as it passes through the rollers. Electrical power is applied between the rollers by electrical brushes 36.
  • the device of FIG. 1 was used to dewater test paper sheets formed from 100% softwood pulp and water. Control mechanical dewatering (rollers with no application of an electric field to the rollers) resulted in paper sheet with 35% solids content. In contrast, electromechanical dewatering with voltage applied to the rollers provided paper sheet with 47% solids content. In this experiment, the rollers were run at 93 ft/min using no felt support.
  • FIG. 1 illustrates schematically how brushes can be used to make contact with the conductive surface of the rolls. Nips and rolls as described hereafter can be electrified, for example, as illustrated in FIG. 1 .
  • FIG. 2 exemplifies application of the methods and apparatus of the invention for papers made with a traditional Fourdrinier machine, including market pulp and liner. Two embodiments are illustrated.
  • the machine illustrated is a conventional configuration adapted for implementation of electroosmotic dewatering.
  • the illustrated machine has a headbox (1 ), felted press nip (5) with press felt (4) , press roll (6), unfelted press nip (7), press roll (8), dewatering section hood (9) and a plurality of steam dryer rolls (11 ) and dryer felt (10, two illustrated).
  • Wet paper web (2) is conventionally formed on forming fabric (3), passed sequentially between felted press nip (5) and press roll (6) and unfelted press nip (7) and press roll (8) and thereafter proceed into the dewatering section hood between dryer felts (10) and steam dryer rolls (11 ) to result in finished paper 12.
  • felted press nip (5) and/or unfelted press nip (7) are electrified as described above. These implementations can be readily implemented in an existing paper machine press section having felted and unfelted nips. It is noted that one or more felted or unfelted nips in a given machine can be electrified.
  • electroosmotic dewatering can be implemented in such a process by adding one or more additional press nips, for example, one or more unfelted press nip and roller positions in the paper pathway prior to the dewatering hood and steam rolls.
  • the term electrified is used to indicate that an electric field as described above is generated at the places indicated by application of a current to the element. It is further noted that appropriate insulation as discussed above must be provided to prevent shorting.
  • FIG. 3 illustrates implementation of electroosmostic dewatering for tissue manufacture application.
  • tissue manufacture the paper machine has no press section and no traditional steam dryer section. Instead, paper is pressed to the surface of a Yankee cylinder (7), a single steam filled roll.
  • an exemplary machine for tissue manufacture has a headbox (1 ), pressure roll (4), nip (5), Yankee cylinder (7), at least partially surrounded by hood (6), doctor blade (8) and nip (9).
  • Wet paper web (2) is conventionally formed on forming fabric (3), and passes to the Yankee cylinder (7) via nip (5). The web travels partway around the roll, which is covered by a hood (6) to remove wet air.
  • nip (5) and/or nip (9) are electrified as described above. If the electrified nip is implemented at nip 5, the cathode will be roll (4) and the anode is the surface of the Yankee cylinder (7). If nip (9) is electrified, one of the two rolls at (9) will be the anode (preferably the top roll as illustrated) and the other roll (the bottom roll as illustrated) will be the cathode.
  • additional electrified nip can be added which are not directly in contact with the Yankee cylinder, for example, such additional nips can be insert prior to nip 5 or after nip 9. Note that in a given implementation, one or more of such nips can be electrified as described above.
  • the rollers When paper web is pressed by rollers in a typical papermachine, the rollers are usually covered with a layer of a compressible, absorbent material called a felt. As the paper web passes out of a pair of rollers, the felt decompresses and absorbs water from the web.
  • the presence of felt on electrified rollers may be advantageous to the application of electro-osmotic dewatering of paper web for two reasons. First, if the properties of the felt allow for removal of water from the felt by application of electro-osmotic force, and if the dewatering rate for felt is sufficiently fast, dewatering the compressed felt between the rollers may result in more effective dewatering of the contacted paper web.
  • the dewatered felt will absorb more water from the paper web when decompressing after leaving the rollers.
  • the properties of the felt may be more important to dewatering than that of the paper. This indicates that the dewatering process may also be improved by adjusting the properties of the felt, which may be done without altering the properties of the paper web or resulting paper.
  • the device may operate with felt covering only the roller polarized as the cathode, with the electrical force driving water out of the felt during compression, allowing it to absorb more water from the slurry.
  • the device may operate with felt covering both anode and cathode rollers, with water driven out of the cathode felt during contact, allowing it to absorb more water from the slurry.
  • the felt may be electrified as an electrode by intertwining a conductive material in it or by covering it with a compressible, porous, conductive material.
  • the felt may cover electrified metal rollers which act as anode and cathode.
  • BTU/L British thermal units expended to produce one ton of finished paper
  • the energy cost per liter of water, BTU/L is calculated in the tables below for two different sets of conditions using the following steps: (1 ) determining the power expended in a pass to dewater a handsheet, (2) using the sheet size and speed to calculate kW-h and converting to BTU, and (3) calculating the amount of water removed during the pass in L.
  • 60C is used in this calculation because the wet paper web is at about this temperature in the press section of a paper machine.
  • Sheets are 5.5 g of 16% solids 50/50 hardwood softwood that are air dried after forming to 50% solids
  • alternating current can be applied by maintaining a DC potential in the same direction and range of magnitudes that have been discussed above and/or tested herein and then impose a small AC signal on top of DC potential.
  • Table 4 summarizes measurements made using two different power supplies.
  • the current passed was at the lower end of the range of the current measurement instrument in one power supply.
  • This instrument reported a potential between 0 and 10 V for currents between 0 and 1 1 A.
  • a current of 6 mA is approximately 0.05% of the full scale and appears to be the noise level of the current monitor output. The reported current therefore may be erroneously low.
  • the smallest step in current reporting by this power supply that appeared to be the noise level was 39 mA, for the run at 198 mA. This value was taken as an upper bound to estimate the highest Power and Energy Cost, which are the higher values in the ranges for these runs. For these runs, the entry in the Current column has the upper estimate current value in parenthesis.
  • Sheets are 5.5 g of 16% solids 50/50 hardwood softwood that are air dried after forming to 30% solids
  • the applied potential was set at 600 V, but was effectively limited by the current limit on the power supply. Increasing this limit allowed higher potentials to be reached on successive runs. Results are provided in Table 5.
  • an apparatus consists of two press rolls that are installed in the mechanical press section of a papermaking machine, a power supply and a control/safety box.
  • the power supply can apply a sufficient electrical potential to effect electro-osmotic dewatering of paper or paper and felt between the rolls and sufficient electrical current to maintain this potential between the surfaces of the two rolls, which are electrically isolated from the balance of the press.
  • the control/safety box monitors the electrical lines between it and the roll surfaces for ground faults. In the case of a ground fault, the safety/control box will open a relay between the power supply and the modified nip and will also remotely disable the power supply. Both measures have the effect of stopping flow of electricity to the modified press.
  • the press is loaded at a certain applied pressure to press the rolls together, and this pressure is comparable to the pressure that would be applied for the type of paper passing through the press.
  • the rotation of the rolls is controlled and synchronized by a drive system that does not prevent the roll surfaces from remaining electrically isolated from the balance of the press.
  • the press roll rotation may be controlled by the passage of the felt.
  • the dewatering effect of the mechanical pressing is enhanced by the electro-osmotic driving of water out of the paper, or paper and felt, onto one of the press rolls, resulting in a higher consistency paper product leaving the mechanical press.
  • the paper web typically experiences higher pressure in later presses. As water is removed from the paper web, its tensile strength increases. It may be transferred to other felts or pass unsupported through later presses. Following the press section is a steam drying section where the paper web passes, supported by felts, over steam-filled rolls. In this section, the paper web is dried to its final water content by evaporation.
  • the preferred modification in this type of paper machine is to electrify the press rolls that form the last press nip before the paper leaves the press section and enters the steam drying section. This will provide the greatest benefit of steam drying energy savings by enhancing the consistency achieved by the mechanical press section.
  • some or all of the roll presses in the press section are replaced by a single shoe press.
  • a shoe press the paper web passes between the surface of a roll and a continuous belt which passes over a stationary shoe, which applies pressure to the belt, paper, and felt, as they pass through the nip between the roll and shoe surfaces. After passing through the shoe press, the paper enters a steam drying section.
  • the preferred modification is to electrify the shoe press.
  • the paper web is transferred from the former to the surface of a single, steam-filled roll called a Yankee cylinder, sometimes with felt supporting the paper web between the former and the Yankee cylinder.
  • the paper web is often pressed to the Yankee surface by a smaller press roll.
  • the preferred modification is to electrify the small press roll surface and the surface of the Yankee cylinder, if the paper solids content is high enough when making first contact with the Yankee cylinder for electroosmotic dewatering to be applied (approximately 40% solids or higher).
  • the tissue paper may be too high in solids content after passing over the Yankee cylinder to be effective later in the tissue making process.
  • any recitation herein of a phrase “comprising one or more claim element” e.g., “comprising A and B
  • the phrase is intended to encompass the narrower, for example, “consisting essentially of A and B” and “consisting of A and B.”
  • the broader word “comprising” is intended to provide specific support in each use herein for either “consisting essentially of” or “consisting of.”
  • the invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.

Abstract

La présente invention concerne des procédés électro-osmotiques efficaces pour déshydrater une biomasse humide ou un produit textile, en particulier pour déshydrater la pâte à papier et les suspensions de pâte à papier. Lors du procédé de déshydratation, l'application d'une énergie électrique induit et améliore la déshydratation mécanique pendant laquelle est appliquée une pression. Plus particulièrement, un champ électrique est créé au niveau d'une ou de plusieurs presses dans une machine à papier. Les procédés et appareils de la présente invention sont plus particulièrement utiles dans la production de produits papetiers, y compris entre autres, de la pâte commerciale, du carton, du papier à couvrir, du papier mouchoir, et de l'essuie-tout.
PCT/US2011/037485 2010-05-22 2011-05-22 Déshydratation du papier WO2011149797A1 (fr)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111732318A (zh) * 2020-07-24 2020-10-02 山东创新华一环境工程有限公司 固液分离挤压机
CN113430863A (zh) * 2021-07-21 2021-09-24 杭州众材科技股份有限公司 一种避免纸张水性脱酸后发生褶皱变形的干燥方法

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1881985A (en) * 1931-04-06 1932-10-11 Scott Paper Co Method and means for manufacture of paper
US5192413A (en) * 1987-04-13 1993-03-09 Fuji Electric Co., Ltd. Electroosmotic dewaterer
JP2002138383A (ja) * 2000-10-27 2002-05-14 Shoji Mizumura 抄紙機用脱水装置
US20040238140A1 (en) * 2001-08-21 2004-12-02 Sakari Laitinen-Vellonen Method in a paper or pulp process to control the chemical state of the pulp and circulation water system
US20090114359A1 (en) * 2005-10-31 2009-05-07 Savcor Process Oy Method For Use In The Wet End Of A Paper Machine, Cardboard Machine Or An Equivalent Web Forming Machine

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1881985A (en) * 1931-04-06 1932-10-11 Scott Paper Co Method and means for manufacture of paper
US5192413A (en) * 1987-04-13 1993-03-09 Fuji Electric Co., Ltd. Electroosmotic dewaterer
JP2002138383A (ja) * 2000-10-27 2002-05-14 Shoji Mizumura 抄紙機用脱水装置
US20040238140A1 (en) * 2001-08-21 2004-12-02 Sakari Laitinen-Vellonen Method in a paper or pulp process to control the chemical state of the pulp and circulation water system
US20090114359A1 (en) * 2005-10-31 2009-05-07 Savcor Process Oy Method For Use In The Wet End Of A Paper Machine, Cardboard Machine Or An Equivalent Web Forming Machine

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111732318A (zh) * 2020-07-24 2020-10-02 山东创新华一环境工程有限公司 固液分离挤压机
CN113430863A (zh) * 2021-07-21 2021-09-24 杭州众材科技股份有限公司 一种避免纸张水性脱酸后发生褶皱变形的干燥方法

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