EP0755469A1

EP0755469A1 - Use of ultrasonics in connection with paper making

Info

Publication number: EP0755469A1
Application number: EP95916891A
Authority: EP
Inventors: Bo Nilsson
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-04-14
Filing date: 1995-04-12
Publication date: 1997-01-29
Also published as: WO1995028521A1; JPH09512063A; SE9401272L; FI964078A0; CA2187734A1; SE9401272D0; CN1149898A; FI964078A; AU2377895A

Abstract

Method for preventing fibre flocculations to occur and to disperse already formed fibre flocculations, better to distribute additive chemicals, to increase fibrillation, to improve reaction and yield from two component chemicals, to prevent the forming of microbiological and other types of build-ups onto machine parts and other parts of the white water circulations, to prevent the filling of wires and felts, to prevent flocculation in coating equipments, to reduce content of air in the stock, and to reduce the foaming. This is achieved by that ultrasonic energy with a frequency of 15 - 60 kHz is added to the stock, machine parts, washing trays or coaters via externally mounted ultrasonic transducers or internally applied submersible ultrasonic units. The ultrasonic energy is added preferably by means of externally onto the head box mounted ultrasonic transducers or internally applied submersible ultrasonic units. The level of ultrasonic energy is controlled by in-line installed formation or flocculation size measurement systems to arrive at desired flocculation. The measured flocculation is used as a criterium for set point optimisation of weight per unit area for the paper or the board produced, and for maximum production per unit of time.

Description

USE OF ULTRASONICS IN CONNECTION WITH PAPER MAKING

This patent application deals with the application of ultrasonic energy to the papermaking process in order to achieve significant advantages. The ultrasonic energy is hereby added to the part of the process generally referred to as the short white water system, i.e. headbox or headboxes, the white water which is separated in the wire part and which is used for dilution of the stock to a desired stock consistency and to white water trays and tanks within this white water system. In addition ultrasonic energy is added to wash systems for wires and felts in order to maintain them completely clean and to prevent the blocking of them, which leads to constant and high dewatering capacities for those parts of the papermaking process. In summary this means, improved paper quality, increased yield for paper constituents, reduced steam consumption and increased production capacity per production unit.

The addition of ultrasonic energy to the white water system is done in order to prevent the forming of fibre flockulation in the paper, and to prevent microbiological growth, which leads to the forming of lumps of slime which in combination with lumps of fibres will cause breaks and machine stops for wash-ups and thereby loss of production time. It will in some cases, when a microbiological contamination has been found to come from a certain raw material which is added to the white water system be necessary to add ultrasonic energy also to that raw material in order to prevent the development of such a source of contamina¬ tion.

The utilization of ultrasonic energy in the pulp and paper industry is known earlier, but has been limited to the production of pulp and within the stock preparation system and it has been added far ahead of the actual paper machine.

Patent GB A 1 577 664 describes a method whereby one uses ultrasonic energy to remove a very troublesome type of resin from a pulp manufactured from specific wood species, a resin which caused big problems in the form of percipitations of resin of different paper machine parts. The patent thus addresses a completely different problem than those addressed in this patent, and the addition of ultrasonic energy is made far ahead of the paper machine.

Derwent's Abstract No. 82-97003E/45, SU 896130 describes a method whereby ultrasonic energy is added to high consistency screens, used for the screening of pulp particularily to prevent the forming of fibre knots, i.e. far ahead of the paper machine.

Derwent's Abstract No. 82-85514E/40, Sϋ 887610 describes a method whereby ultrasonic energy has been used in the stock preparation in order to improve the coloring of the stock, which also is done far ahead of the paper machine. The patent describes a batch processing method, which has not been foreseen to be used in a continuous process.

Derwent's Abstract No. 81-17053/D, SU 746012 is also dealing with a batch process in the stock preparation, where ultrasonic energy has been used in order to be able to add a paraffin-wax dis¬ persion and copolymers in order to manufacture a paper with improved mechanical strength and with water repelling properties.

In addition, it is known from Tappi Journal Vol. 77, No. 3, that ultrasonic energy can be used to improve the deinking of recycled fibre stock made from paper which has been laser printed, which means, that the toner has been fixed onto the sheet using high temperature and is very difficult to remove.

The use of ultrasonic energy in above mentioned processes aims at solving completely different problems that does this patent, and involve different processes e.g. manufacturing of pulp from wood and secondary fibres, and stock preparation far ahead of the actual papermaking process. The utilization of ultrasonic energy to solve the problems covered by this patent application cannot be anticipated by the abovce mentioned publications and is also not obvious to the expert in these fields.

IMPACT ON ECONOMY

The uneven distribution of fibres in the paper sheet caused by fiber flocculation is considerable. The statistical fibre weight variation when tested with paper rondelles with an area of 23.5 square millimeters was 7-10% whereas it was 0.5 - 0.8% when the traditional rondelles of 20 000 square millimeters were used. Large profits can thus be achieved, if flocculation of fibres can be eliminated or reduced, which in turn makes it possible to arrive at the same strength properties with less weight per unit area. For the Swedish paper market the total profit potential is SEK 350 625 000 per percent reduction of the weight per unit area.

The reduction of the number of breaks and stops for cleaning due to the elimination of slime and fibre agglomorations gives a direct profit for the Swedish paper production of SEK 730 000 000 per year in addition to environmental profits, since no pulluting chemicals are used for the slime control.

The impact on profits from improved dewatering in the wire- and drying sections is also considerable. Continuous cleaning of all pressfelts should result in a 0.4 - 0.5% higher dry solids content after the press part. A 0.5% higher dry solids content corresponds to a possible production increase of 2.21% correspon¬ ding to a profit potential for the Swedish paper industry of SEK 620 000 000 per year.

DESCRIPTION OF INSTALLATION

Fig. 1 is a schematic, partly sectioned profile of a head box with an air cushion and equipped with ultrasonic transducers 1, suitable to utilize for this invention. Figs. 2-5 are schematic, partly sectioned profiles of different hydraulic head boxes equipped with ultrasonic transducers 1, suitable to utilize for this invention.

Fig. 6 is a two layer hydraulic head box equipped with ultrasonic transducers 1, suitable to utilize for this invention.

Fig. 7 is a head box where the perforated roll has been replaced by submersible units equipped with ultrasonic transducers 1, suitable to utilize for this invention.

Figs. 8 and 9 are examples of washing systems for wires and felts, equipped with ultrasonic transducers 1, suitable for the utilization of this invention.

Above mentioned arrangements are examples only of how the invention can be applied, but other solutions are possible which are intended to be within the scope of this invention. PREVENTION OF FLOCCULATION

In the papermaking process one tries by means of various mechanical devices to prevent the forming of floes within the fibre suspension which flows onto the fourdrinier wire from the head box, so that one gets as even fibre and filler distribution in the finished paper product as possible. The floes are built up by fibres and fillers but are initially made up by fibres. Higher pulp consistency, higher degree of refining and longer fibres will increase the flocculation, which in turn forces the paper manufacturers to work with very low consistencies during the production of e.g. sack paper. The size and form of the floes can easily be determined by observing the paper sheet by means of translucent light. The floes are then represented by the darker parts of the paper with lighter parts between them. The paper is thus built-up by darker parts with a higher weight per unit area and lighter parts with lower weight per unit area. The mechanical strength properties of the paper are increased by higher and decreased by lower weight per unit area. This means, that the strength properties of the paper are controlled by the lighter parts between the darker parts of the paper. Filler and fibre retention, particularily the retention of fines, is higher in the floes, whereas they in the lighter parts tend to follow the white water which is drained from the paper machine wet end, i.e. the wire and press sections.

A reduction of the flocculation, referred to as improvement of the formation by the paper maker, thus leads to either that one can manufacture a paper with improved strength properties with the same weight of fibres per unit area, or manufacture a paper with the same strength properties from less fibres per unit area. An improvement of the formation thus gives the possibility of better fibre yield and in turn to an improved raw material economy. The floes also contain more water and will reduce the dewatering on the wire as well as in the press section which in turn leads to an increased tendency of crushing in the press section which is caused by the fast water flow caused by that more water must be separated from the sheet will tear fibres away from it, and when the hydraulic pressure is large enough cause a break. In order to reduce the number of breaks, one then will have to work with less pressure in the presses, which reduces the dry solids content from the press section for the paper entering into the dryer section. This in turn leads to a higher steam consumption per ton, which also makes a lower production speed necessary unless the dryer section capacity is increased. In summary: improved formation leads to a better raw material and energy management, and offers a possibility to increase produc¬ tion per unit time which in turn reduces the specific capital costs. The largest cost reductions will be achieved when one can take total advantage of the possibility to produce paper with reduced weight per unit area, but still maintain the same strength properties of the produced paper.

An improvement of the formation is thus of vital interest technically as well as economically. Different designs of head boxes have been developed in order to achieve that goal, generally by building in rotating perforated rolls and/or repeated expansions and reductions of the through flow area which causes stock flow turbulence, which at least in theory should prevent the forming of new floes. One could say that the situation has been improved but that the problem remains unsolved.

The addition of ultrasonic energy in the range of 15 kHz - 60 kHz to arrive at the advantages mentioned above solves or significal- ly reduces these problems. The ultrasonic energy is in this case added to the stock in the head box either by gluing the ultraso¬ nic transducers which are mounted on aluminium bars onto the head box for air cushion and open head boxes preferably underneath the head box, and all around the head box for hydraulic head boxes (figs. 1-6) . One could also mount the transducers within a number of submersible units installed inside the head box (fig. 7) or by means of a combination of the different ways of installation. The ultrasonic transducers are installed such that the ultrasonic energy is added to the stock within the entire length, width and height of the head box using several parallel combinations of transducers. In this way one assertains that sufficient amount of ultrasonic energy to solve the problem is added to the stock.

The stock is pumped into the head box via a manifold and a diffusor bank. When it enters into the head box after the diffusor bank, a very rapid reduction of flow velocity occurs which assertains that the floes are dispersed. The reforming of the floes occurs very rapidly and it is desirable that one assertains that the reforming of the floes is prevented. The present technology has not made it possible to arrive at these results in spite of the many different designs that have been tested and been in operation during decades.

The dispersing effect of the ultrasonic energy is caused by the well known cavitation which is used for ultrasonic cleaning of different components, e.g. anilox rolls or gravured rolls for flexo- and rotogravure printing and where it is also possible to remove even dried waterbased ink from the gravured cells. When an ultrasonic sound wave traverses a material, the molecules of the material are induced to vibrate longitudinal in the direction of the wave propagation. This results in pressure variations which in a liquid leads to cavitation or the formation of small bubbles about 0.1 - 0.2 mm in diameter. When these small bubbles are irradiated with ultrasound, they will continuously absorb energy from the compression and expansion cycles of the ultraso¬ nic sound waves and grow to their critical size which is dependent upon wave length after which they start to grow in size very rapidly and will finally implode which they do in less than a microsecond. The imploding bubbles will transmit energy to the nearby medium which in a fibre suspension means that they disperse existing floes as well as prevent new floes to occur and in addition increase the fibrillation. That the fibrillation occurs has been verified by measurements of freeness values, where the difference of freeness values before and after the addition of ultrasonic energy has been verified. The dispersing effect has also been documented in the Swedish patent application 9002006-6 (468754) where the dispersing effect of the ultrasonic energy has resulted in print quality improvements and to ink savings.

The addition of ultrasonic energy will prevent the regeneration of floes and disperse already formed floes. If the ultrasonic energy is added over a range of wavelengths with wavelength sweeps over that range, it will since the length of one wave¬ length is directly dependent upon frequency, cause, that wavelengths of different frequencies are located on different distances from the transducer and is transmitted through the medium in multiples of these distances. In that way the ultraso¬ nic cavitation will be distributed in the entire fibre suspension and prevent regeneration or forming of floes. It will furthermore contribute to degassing and reduced foaming.

Other chemicals are also used in the production of paper, e.g. retention aids, which are polymers, often two component types which are used for improving the retention of fibres and fillers in the web in the wire section and thus reduce the content of these paper constituents in the white water and to increase them in the web leaving the wire section entering into the press section. The ultrasonic energy disperses each component of these chemicals which increases the reactivity and improves the yield. They will in addition distribute them more evenly which leads to improved retention for the same amount of retention aid or the same retention with less retention aids.

Fibre agglomerations on machine parts which after some time will come off and eventually cause breaks, are other areas wehre the dispersing effect of ultrasonic energy can be used. They are generally formed at locations where there are eddy currents e.g. around transducers, inspection doors, deckel boards and the like. An installation of ultrasonic transducers near these locations will prevent the forming of such fibre agglomorations.

Other parts of the paper machine can also take advantage of the dispersing and cleaning effect of the ultrasonic energy. Examples of this are felts and wires where one, if they are passed through a tray with an ultrasonic cleaning bath, sometimes also contai¬ ning chemicals to prevent microbiological growth, percipitation of pitch and other organic and inorganic substances which can reduce dewatering effect and useful life. Examples of application as per figs. 8 and 9.

One has earlier been of the opinion that the presence of cellulose fibres in the stock, the large stock flow and the large volume in the head box has made it impossible to apply ultrasonic energy to the papermaking process since it will be rapidly absorbed and therefore cannot have an impact within the entire stock volume. It has been demonstrated that this is not the case and the application of ultrasonic energy to the papermaking process thus represents a completely new thinking and it is therefore a technical novelty. The ultrasonic energy is very rapidly distributed in a fibre suspension. It is true that the cellulose fibres absorb part of the ultrasonic energy due to an increase of fibrillation and a decrease or prevention of flocculation, but this is a necessary and desired function and will not prevent the distribution of ultrasonic energy throughout the entire volume in the head box. The reason for this is that the lumen is completely filled with water and that the cell walls also absorb a large volume of water. In such a way the entire volume of cellulose suspension acts as a liquid and the ultraso¬ nic energy can, if sufficient ultrasonic energy is added, be distributed throughout the entire stock volume in the head box. Increased degree of refining, increased pulp consistency and increased speed of stock flow will increase the specific energy requirements.

The utilisation of systems for installation in the paper machine wet end for inspection and measurements of formation and fibre weight, can be used for the optimisation and control of stock consitency, ultrasonic power and other production parameters, e.g. sufficient paper grammage to meet paper property specifica¬ tions. The specific utilisation of raw materials such as paper making pulp, chemicals and energy can be optimised in a way that until now has not been possible, and with revolutionary possibi¬ lities for improvements of quality and profitability.

Microbiological growth in the stock and water circulations has traditionally been a big problem in the paper making process. Aerobic as well as anaerobic slime forming microorganisms form slime build-up onto machine components and walls of white water and stock tanks. The presence of anaerobic growth is explained by the fact, that the aerobic growth forms anaerobic conditions under itself due to the fact that the aerobic organisms will consume all available oxygen. The anaerobic growth of, partieula¬ rily sulphate reducing bacteria such as Desulfovibrio Desulfuric- nas, will increase metal corrosion, and will directly influence brightness, partieularily when reducing beaching with zinc or sodium hydrosuphite is used.

The microbiological growth causes slime build-up onto machine parts, which build-up, sooner or later will come off as lumps, which are very difficult to dissolve. They are e.g. passing through fan pumps or other pumps and through diffusor banks and perforated rolls and causes slime spots and production breaks. The lumps consists many times as well of fungi and bacteria as of organic and inorganic substances. The presence of eddy currents in water and stock systems will favour the growth and forming of fibre lumps, which will agravate the situation.

The control of these problems have traditionally been carried out by means of biologiclly active substances, biocides and biostats, often combined with a dispersant. Initially organic mercury compounds or chlorinated phenols were used which cused much environmental harm to the external environment and this was the reason for demands of non-toxic control. It is certainly difficult or impossible to come up with a chemical both, biologically active, which at the same time must not influence on biological life.

Ultrasonic energy will solve this problem. An addition of ultrasonic energy to the part where microbiological growth occurs will effectively prevent agglomerations there. The ultrasonic energy can be added externally as is done onto the head box for components and tanks made of metal, partieularily stainless steel. In tanks made of concrete, tiled or plastic coated, it will have to be added by means of submersible units, in which case the distance to the wall and the energy level will have to be matched so that the cavitation is powerful enough to solve the problem, but not so powerful that it will peel of the tile or coating.

The addition of ultrasonic energy to the paper making process is not limited to the examples mentioned above, but similar problems can be solved with other installations e.g. in decculators to accomplish improvements in deaeration, or to prevent the forming of flocculations in different coating stations or laminating machines where water based products are applied. The expert in the field will find, that this technology can be applied in many different parts of the paper machine or coater, in- or off-line, but those applications are intended to be within the scope of this invention.

Claims

Patent Claims

1. Method for preventing fibre flocculations to occur and to disperse already formed fibre flocculations, increase fibrilla¬ tion, improve retantion and yield from two component chemicals, to prevent the forming of microbiological and other types of build-up onto machine parts and other parts of the white water circulations, prevent the filing of wires and felts, prevent flocculation in coating stations or machines, reduce content of air in the stock, and reduce foaming characterized by that ultrasonic energy with a frequency of 15 - 60 kHz is added to the stock, machine parts, washing trays and coaters via externally mounted ultrasonic transducers to internally applied submersible ultrasonic units.

2. Method according to claim 1 characterized by that the ultrasonic energy is added by means of externally onto the head box mounted ultrasonic transducers or internally applied submersible ultrasonic units as per fig. 2 or 3, and that the ultrasonic energy is used to improve formation and retention.

3. Method according to claim 1 and 2 characterized by that the level of ultrasonic energy is controlledby in-line installed formation or flocculation size measurement systems to arrive at desired flocculation, and that this measured flocculation is used as a criterium for set point optimisation of weight per unit area for the paper or the board produced, and for maximum production per unit of time.

4. Method according to claim 1 characterized by that the ultrasonic energy is added to machine parts or tanks in the white water circulation to prevent the build-up of microbiologic, organic or inorganic nature.

5. Method according to claim 1 characterized by that the ultrasonic energy is added to special washing trays to prevent filing of felts and wires of microbiologic, organic or inorganic nature.

6. Method according to claim 1 characterized by that the ultrasonic energy is added to the coating substance in coating or lamination stations to prevent the forming of different types of flocculations in the coating substance circulation or form build-ups onto machine parts in the station.

7. Method according to claim 1 characterized by that the ultrasonic energy is added to the stock in order to reduce foaming and to improve deaeration.

8. Method according to claims 1 - 7 characterized by that the added ultrasonic energy sweeps over a frequency range within 15 kHz - 60 kHz.

9. Method according to claims 1 - 7 characterized by that the added ultrasonic energy has one or several predetermined frequencies between 15 kHz - 60 kHz.