Patent Application
20210189650
This application claims priority to European patent application 19218416.6, filed Dec. 20, 2019, the entirety of which is incorporated by reference.
The present invention relates to a method of and an arrangement for adding at least one additional stock component to an approach flow system of a fiber web machine headbox. More specifically the present invention concerns mixing of at least one additional stock component to a fibrous stock flowing in the approach flow system of a fiber web machine headbox.
A paper machine or, in broader terms, a fiber web machine is preceded by an approach flow system in which the fibrous suspension, called stock, used for making a fibrous web at the fiber web machine is prepared. At the approach flow system various stock components, i.e. virgin and/or recycled pulp and/or broke are mixed together with white water recovered from the fiber web machine, fillers and other desired additives are mixed with the stock and the stock or one or more of its components are, if considered necessary, deaerated. Further, at the approach flow system the stock is introduced by means of a headbox feed pump, normally a specifically designed centrifugal pump, to a headbox screen where the stock is finalized for the web production at a headbox. The approach flow system is also provided with a mixer or mixers for adding retention chemical(s), as well as possibly other chemicals or additives, to the stock. Depending on the type of the chemical, such may be added to the stock as early as in the mixing chest or as late as just upstream of the headbox, i.e. between the headbox or machine screen and the headbox.
Wet end chemicals have been used for a long time in paper making for example for retaining fine solids like for instance fillers and other additives used in the production of paper, in the fibrous web while water, so called white water, is drained from the fibrous web at the forming and drying sections of the fiber web machine. The wet end chemicals such as retention chemicals bind the solids to one another and to fibers of the stock soon after they are introduced into the mixture of fibers and the solids. The introduction of wet end or retention chemical(s) may take place before or after the headbox screen. However, usually the introduction is performed in such a stage that the agglomerations formed by the wet end or retention chemical(s) are formed only after the headbox screen, as, on the one hand, the screen could easily sort out such agglomerations from the stock the web is later on made, or, on the other hand, the screen by creating turbulence could break the agglomerations and thereby reduce the retention of solids in the web. Therefore, quite often the wet end or retention chemical(s) are added to the outlet duct of the headbox screen where there is such weak turbulence left that is not capable of breaking the agglomerations to be formed but is still capable of aiding in the mixing of the wet end or retention chemical(s).
As long as paper has been made with paper machines the strive has been towards higher quality end products. A factor having an essential role in paper quality is the use of various chemicals and/or additives. However, for a number of reasons there is a constant aim to reduce the use of chemicals and additives. Firstly, the chemicals are more costly than cellulose fibers, whereby the cost-aspects push towards reduction in the chemical usage. Secondly, naturally, when chemicals are mixed with paper making stock a part of the chemicals end up into circulation waters, and irrespective of the efficiency of water-recirculation a small part of the chemicals always enter the waste water treatment and possibly into lakes, rivers or sea. Thereby, both for economical and environmental reasons the use of chemicals and additives should be made as efficient as possible. In other words, a minimal amount of chemicals should be used and the chemical should be mixed as evenly as possible with the stock flow.
A starting point for the present invention is a chemical mixer of Valmet Corporation, called OptiMixer™ and discussed in US Patent Application Publication 2002/0121350 (350 application). The 350 application discloses a static mixer, which comprises a mixer flow pipe that has a diameter equal with that of the stock flow pipe taking the paper making stock towards the paper machine headbox. The mixer flow pipe is fastened to the stock flow pipe by means of flanges and the inner circumference of the mixer flow pipe is provided with a number of circumferentially arranged form parts. The purpose of the form parts is to generate turbulence for mixing chemicals to the stock. The 350 application teaches several ways of adding the chemical to the stock. In a first embodiment the chemical is introduced via the form parts to the stock, in a second embodiment the chemical is introduced to the stock immediately upstream of the form parts, in a third embodiment the chemical is introduced immediately after the form parts. And, in a fourth embodiment the chemical is introduced, in circumferential direction, between the form parts to the stock.
In all the above discussed embodiments the principle of mixing the chemical is the same. However, in recently performed experiments it has been learned that the use of chemicals may be made more effective if a few facts are taken into account. Firstly, the stock flow coming from the headbox screen or the machine screen or any other device along a stock flow pipe to the static mixer has an irregular consistency profile, like in any flow pipe. In other words, the parts of the stock flowing nearest to the flow pipe surface have the lowest consistency, and the parts flowing at the center of the flow pipe have the highest consistency. Secondly, now that a chemical is introduced to the stock either directly through the wall of the mixer flow pipe or via the form parts, the chemical gets into contact with such parts of the stock that have the lowest consistency. The 350 application also discusses embodiments, where the chemical is introduced to the stock flow at a distance from the wall whereby the stock consistency is somewhat higher than that at the pipe surface, but still not the same as in the middle of the flow. And the embodiment has such a clear downside that the nozzle extending from the pipe wall inside the pipe gathers impurities that make the feed of the chemical more difficult and deteriorates the quality of the end product.
What the above consistency-related problems mean in practice is that the concentration of the active chemical (not yet reacted or fastened to solid matter) is at its highest there where the solids consistency of the stock is at its lowest. Accordingly, the chemical performs its action efficiently with the solids in the low consistency stock, and, as a result, the concentration of the chemical decreases and less chemical is left for the solids in the higher consistency stock. Thereby the solids in the higher consistency stock have less chemical to react with or to be fastened to. Thereby the final result of such mixing is uneven treatment of the solids with chemicals, and in order to make sure that in all parts of the stock there is sufficient amount of chemical, the chemical dosage has to be increased beyond optimal. Additionally, there is a risk that, for instance, the filler retention is not even throughout the stock whereby the cross-machine profile of the filler in the end product is not even, and the quality of the end product is not as good as it could optimally be.
The present invention may be embodied to solve one or more one of the above discussed problems.
The present invention may be embodied to use optimal amount of chemical(s) and/or additives and thereby reduce the chemical/additive costs involved in the production of a web-like article.
The present invention may be embodied to reduce the amount of chemicals/additives loading the waste water treatment of the paper mill.
The present invention may be embodied to improve the construction of the headbox feed pipe such that minimal amount of consistency differences end up in the headbox.
The present invention may be embodied to improve the construction of the headbox feed pipe such that the positions of various devices provided in connection with the headbox feed pipe are carefully determined.
The invention may be embodied to provide chemical/additive dosing method for new functional fiber based products having more demanding requirements for chemical/additive dosing than conventional paper and board making.
At least part of the objects is achieved by TÄHÄN VAATIMUS 1 and TÄHÄN VAATIMUS 8.
The present invention may be embodied as arranging in a headbox feed pipe, a stock flow equalizer that mixes the stock flow efficiently upstream of the chemical introduction such that any consistency differences are minimized or totally removed. The stock flow equalizer forms a strong turbulent zone of chaos-like turbulence that continues up to chemical introduction. The chemical(s) is/are introduced to the equalized, but still turbulent, stock flow by means of an injection mixing station that is known to be able to spread chemical(s) evenly to the entire cross-section of a stock flow pipe. A good example of such an injection-type chemical mixer or injection mixing station, is TrumpJet™ mixer of Wetend Technologies Ltd that has gained wide acceptance in the field of mixing retention and other chemicals to fibrous suspensions. The above mentioned TrumpJet™ mixer and its use are discussed in, for instance, European Patent B1-1219344. The TrumpJet™ mixer is used as a mixing station formed either of a single injection mixing unit or of a number of injection mixers or injection mixing units arranged on the circumference of the stock flow pipe in which the medium to which the chemical is supposed to be mixed flows.
The present invention may include adding at least one additional stock component to a stock flow in an approach flow system of a fiber web machine headbox, wherein a fibrous stock is introduced by means of a headbox feed pump via a headbox screen, a headbox feed pipe and a chemical mixer to the headbox of a fiber web machine, in which method the stock flow is treated by equalizing consistency differences prevailing in the stock flow downstream of the headbox screen and mixing the at least one additional stock component with the stock.
The present invention may include an arrangement for adding at least one additional stock component to a stock flow in an approach flow system of a fiber web machine headbox, the approach flow system comprising a headbox feed pump, a headbox screen a chemical mixer and a headbox feed pipe; a stock flow equalizer is arranged in the headbox feed pipe at a distance from the chemical mixer arranged between the headbox screen and the headbox.
In the following the prior art and the present invention are discussed in more detail with reference to the accompanying drawings, in which
As discussed already above, feeding one or more chemicals to the stock flow that has not yet been completely mixed so that the consistency variations in the stock flow would have been equalized brings about problems that may be seen in both the quality of the end product and in the unnecessarily high dosage of chemicals, resulting in increased chemical costs.
The form parts 58 function (see
As to the stock flow equalizer 50 it should be understood that the number of form parts 58 thereof may vary from 3-15, the form parts may be arranged on the same circumference of the inside surface 56 of the stock flow equalizer 50, or they may be divided in several groups on several circumferences, or they may be freely arranged in arbitrary positions on the inside surface. Naturally, their axial distance from one another should not be too long, which would reduce the efficiency of the stock flow equalization function. A length of the stock flow equalizer, i.e. from the leading edge of the upstream form part to the trailing tip of the downstream form part may be in a range of 0.5 to 3×D, where D is the diameter of the headbox feed pipe. The radial height of the form parts 58 is may be in a range of 0.1 to 0.5×D, where D is the diameter of the headbox feed pipe. A width of the form part may be in a range of 0.1 to 1×length of the form part. Also the leading surface 60 of the form part is not necessarily planar, but it may be curved, i.e. convex or concave, in either radial or axial cross sections thereof, or in both.
As already discussed above in connection with
The chemical mixer 80 may be an injection mixing station, which may be formed of a single injection mixer or a set of injection mixers. In narrow pipes a single injection mixer may suffice, but with larger pipes an injection mixing station with several injection mixers on the periphery of the headbox feed pipe are needed. In operation, at least one additional stock component and an injection liquid is brought to the injection mixer, each along its own feed channel. The additional stock component and the injection liquid are introduced with one another via a common nozzle to the equalized stock flow such that the additional stock component is mixed with the injection liquid and the injection liquid, due to its high speed and injection pressure, ensures that the mixture of a relatively small amount of the additional stock component and the injection liquid penetrates deep enough in the equalized stock flow in the headbox feed pipe. The injection liquid may be the same stock to which the additional stock component is to be mixed. Such an injection liquid may be taken upstream of the mixer to be injected by means of the mixer together with the additional stock component to the stock flow in the headbox feed pipe. It was already earlier mentioned that the injection liquid may be taken from the headbox feed pipe either upstream of the stock flow equalizer, between the equalizer and the injection mixing station or downstream of the injection mixing station. Other options for the injection liquid are fresh pulp, recycled pulp, white water, fresh water, etc. just to name a few options without any intention of limiting the applicable liquids to the listed alternatives only.
It should also be understood that the present invention covers mixing either a single additional stock component or two or more additional stock components to the stock upstream and/or downstream of the stock flow equalizer. In other words, it is possible that in cases where more than one additional stock component is used (in addition to the one mixed downstream of the stock flow equalizer), one additional stock component is mixed to the stock between the headbox screen and the stock flow equalizer and/or one additional stock component is mixed to the stock upstream of the headbox screen. Naturally, it is preferable in these two latter alternatives that the additional stock component is such a slowly reacting one that it functions or forms agglomerations only after the stock flow equalizer.
And finally, it should also be understood that at least one additional stock component is at least one of retention chemical, sizing agent, like alkenyl succinic anhydride (ASA) or alkyl ketene dimer (AKD), starch, filler, paper dye or pigment, micro- or nano-fibrillated cellulose (MFC, NFC) or other natural/bio-based fiber or synthetic fiber, just to name a few options. These additional stock components may be mixed with the stock either together with another additional stock component(s) or independently thereof. If introduced together with some other additional stock component these may be either premixed with the additional stock component(s) prior to being injected to the stock or just injected together with the additional stock component(s) to the stock.
It should be also understood that the above presented dimensions in different embodiments in relation to the headbox feed pipe diameter D may vary significantly depending on the actual application. In a small size approach flow systems of a fiber web machine headbox, the diameter D may be 50 to 100 mm. In large fiber web machine, the diameter D may be approximately 1200 mm.
Although the invention has been described with reference to specific illustrated embodiments, it is emphasized that it also covers equivalents to the disclosed features, as well as changes and variants obvious to a man skilled in the art, and the scope of the invention is only limited by the appended claims.
While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.
| Number | Date | Country | Kind |
|---|---|---|---|
| 19218416.6 | Dec 2019 | EP | regional |
