WO2002025012A1

WO2002025012A1 - Method and apparatus for feeding chemicals into a liquid flow

Info

Publication number: WO2002025012A1
Application number: PCT/FI2001/000825
Authority: WO
Inventors: Jouni Matula
Original assignee: Wetend Technologies Oy
Priority date: 2000-09-22
Filing date: 2001-09-21
Publication date: 2002-03-28
Also published as: FI20002087A0; FI20002087A; AU2001287783A1; FI111284B

Abstract

The present invention relates to a method and apparatus for feeding chemical into a liquid flow. The invention is particularly well applicable in the feeding of liquid chemical smoothly into a liquid flow. The method and the apparatus of the invention are advantageously applied in the feeding of retention aid into the fiber suspension flowing into the head box of a paper machine.

Description

Method and apparatus for feeding chemicals into a liquid flow

The present invention relates to a method and apparatus for introducing chemical into a liquid flow. The method and apparatus of the invention are particularly well suitable in adding liquid chemical smoothly into a liquid flow. The method and apparatus of the invention are advantageously applied in feeding retention aid into the fiber suspension to be fed into the head box of a paper machine.

Naturally, there are practically uncountable numbers of methods of introducing different chemicals into liquid flows. However, these methods may be classified into a few main types, as may be seen from the following. Firstly, it is possible just to let the liquid to be added flow freely into the other liquid without using any control or mixing means. This kind of addition methods cannot be used where the mixing ratio or the homogeneity of the mixing is important; neither in situations where the price of the chemical to be added is significant. The next applicable method is to add the chemical at a precise ratio into the liquid flow, which secures a correct and economical dosage. It should be noted, however, that also in this case the chemical dose is usually a little more that the optimal as mixing is know to be inadequate. The mixing can, however, be improved by feeding the chemical for example via a perforated wall of the flow duct; thereby the chemical to be mixed has at least been distributed evenly around the liquid flow. The last method discussed here is supplying the chemical accurately dosed either to the liquid flow upstream of the mixer or via the mixer itself to the liquid. In this case the design of the mixer totally determines how efficiently the chemical is mixed into the liquid flow.

Paper manufacture is a special area, exacting in its own way, in the mixing of chemicals. When using paper chemicals, it should be remembered that precise and homogenous mixing of chemicals is of vital importance in the short circulation of a paper machine. Uniform mixing directly yields improved and homogenous paper quality. At the same time the operation of the process becomes free from disturbances and problems. Poor mixing, on the other hand, requires an overdose of chemical, which may increase production costs remarkably. It is quite obvious that with poor mixing the paper quality and the operation of the process are not good. On the one hand, present mixing techniques use clean water fractions both as dilution water, and as so-called "whip water" to intensify the mixing. On the other hand, the tendency is to close the water circulations of a paper mill, which means that introduction of clean water to the system should be reduced and internally purified fractions, or untreated flows, such as filtrates obtained directly from a process, should be used instead. Present chemical mixing systems do not allow, or allow only to a very limited extent, the use of internal water fractions of the system.

An essential mixing process in manufacturing paper is the mixing of a retention chemical or chemicals into the fiber suspension flow to be supplied into the head box of a paper machine. Retention chemicals are used in the paper manufacture particularly to improve the retention of fines at the wire section of the paper machine. A chemical with long molecule chains is used as a retention aid, the molecule chains binding the solids particles of the suspension together and thus preventing the fines from being drained with water through the wire in the web formations stage. At least in theory, the retention aid should be mixed into the pulp as uniformly as possible in order to achieve the maximum effect of the chemical and in order to avoid variations in the paper properties caused by retention fluctuations. However, mixing subjects the liquid to a turbulent flow the shear forces of which break/may break the long molecule chains which in turn impairs the effect of the retention aid. However, there are different retention aids available. For example polyacrylic amides are sensitive to the influence of a turbulent flow; its broken molecule chains do not, as far as known, regain their original length after the turbulence has died. But there are also retention aids (for example polyethylene imines) the molecule chains of which are quickly restored essentially to their original dimensions after the turbulence has died out. In the short circulation of a paper machine, the point at which the retention chemical is fed in, depends greatly on the retention aid used, on the flow space from the introduction point to the head box lip, and on the pulp used. The retention aids sensitive to shear forces are usually fed in immediately after the apparatus, which may be a pump, a screen, or a vortex cleaner, preceding the head box and bringing about the shear forces in the flow, either at one point or, for example, into the accept pipe of each pressure screen. Also several retention aids of different types may be used at the same time and they may be fed into the fiber suspension at different stages. The portion of the retention aids resistant to shear forces may be supplied already to the high-consistency pulp or prior to the head box feed pump, and the retention aids sensitive to shear forces are usually not fed until to the fiber suspension feed duct in the vicinity of the head box screen prior to the head box.

For the sake of clarity, the following explains which and what kind of apparatus are used in the short circulation of a paper machine. Prior art paper machine approach systems, a good idea of which is given for example by U.S. patent no. 4,219,340, are almost always composed of the following components: a mixing tank, a feed pump, a vortex cleaning plant, a gas separation tank, a head box feed pump, a head box screen, a paper machine head box, and white water collection vats. The components mentioned are placed in connection with the paper machine and arranged to operate as follows. The fiber material and fillers which are used in the paper manufacture and are diluted with water from the paper machine, mostly the so-called white water obtained from the wire section, are dosed into the mixing tank, which is often called a wire pit and is usually located on the ground level of the mill. The feed pump also located on the ground level of the mill is used to pump the fiber suspension to the vortex cleaning plant usually located on the machine level of the mill, i.e. the level on which the paper machine is located, or, as in the patent mentioned, above the machine level. The fiber suspension accepted by the vortex cleaner continues its travel by means of the pressure generated by the feed pump to the gas separation tank located on a level above the machine level. From the gas separation tank the fiber suspension from which gas has been removed as completely as possible, flows to the head box feed pump on the ground level of the mill which pumps the fiber suspension to the head box screen (not illustrated in the U.S. patent) located also on the ground level of the mill from which the fiber flows to the head box of the paper machine on the machine level.

As was stated above, the tendency has been to mix the retention chemical into the fiber suspension early to give the chemical adequate time to be mixed with the pulp uniformly but, on the other hand, it has also been known that some retention chemicals do not tolerate strong turbulence and therefore they have been mixed as late as possible, however, paying attention to the requirement of uniform mixing. Generally, the feed duct from the head box screen to the head box has been considered a good space for the mixing because of the turbulence prevailing there. In other words the tendency has been to feed the retention chemical to the beginning end of the feed duct in question, i.e. the so- called approach duct, in other words for example to the discharge conduit of the head box screen whereby the retention chemical is no subjected to the turbulence in the head box screen but, however, is influenced by the approach duct turbulence long enough to give grounds to believe that the mixing is adequately uniform.

This theory is supported also by patent publications relating to the mixing of retention chemicals. FI 86754 discusses a process according to which the retention chemicals are added in several portion, as several components. One of the components is added during the production of fiber slurry and one or two components are reported to be added immediately before the head box. The publication in question, however, discloses in example 4 on page 6 that "aluminium sulphate is added after the screens and the deaeration means immediately prior to the head box" and also on page 9 that "dilution water is added immediately before the head box... a little after that... the aluminium salt is introduced" (the retention aid). The distance from the screens and the deaeration means to the head box is usually dozens of meters. The dilution, on the other hand, is in the short circulation of a paper machine performed already before the deaeration.

Thus it is clear that, according to the publication, the retention chemical should be mixed in the short circulation of a paper machine either after the screens or the deaeration means, i.e. in good time before the head box or even before the vortex cleaning, degassing or the head box feed pump, in any case dozens of meters before the head box.

Another FI publication, no. 75200, mentions that retention chemicals are supplied to the short fiber fraction chest. It is said that fillers, which someone might argue to include retention chemicals, can be dosed straight to the suction side of the mixing pump. In both cases the addition of the retention chemical takes place at a very early stage prior to the head box.

U.S. patent 6,086,718 relates to the feed pipe line of a head box beginning from the machine chest. According to the publication, the retention chemical is fed to the first pulp flow coming from the pulp chest at a point before the paper machine head box. It should be noted that the retention chemical is mixed into the first pulp flow which is after that mixed with a second pulp flow guided either to the head box feed pump or directly to the head box. In other words, according to the invention the retention chemical is supplied to the first pulp flow already before the first pulp flow is added to the second pulp flow to be supplied to the head box.

However, none of the prior art publications disclose the structure of the mixing apparatus itself but in most cases it is believed that the turbulence of the pulp flow takes care of the mixing efficiently enough. It is possible that this happens if the mixing time is adequate. In fact, it has been found out commonly in the industry that if the time reserved for the mixing is not at least of the order of 5 - 6 seconds, the system is unstable. This means that the chemical has not had enough time to be mixed properly, which results in uneven wire retention. The mixing time of 5 - 6 seconds means in practice that the feed point of the chemical must be located at least 15 meters from the head box as the guide line flow speed in the head box feed line is 3 m/s.

However, Wetend Technologies Oy has recently introduced into the market a TRUMPJET™ mixer, which has been designed for mixing retention chemicals and is disclosed for example in FI patent application no. 980437. Mill-scale tests performed with the apparatus have proved that the apparatus is capable of mixing the retention chemical into the pulp in an essentially smoother way that prior art apparatus; we could even speak of optimal mixing.

When studying how small retention chemical dosages really would be adequate, as optimal mixing as such reduces the amount of retention chemical required, it was surprisingly found out that it is characteristic of retention chemicals that the durability of the bondings they form and the floes developed with these bondings are very much bound to time. In other words, the curve in Fig. 2 illustrating schematically the efficiency of retention chemicals as a function of time can be considered valid for all retention chemicals used. The efficiency can be measured for example as the strength of the bondings or the number of bondings in a certain volume, or in some other way. The best efficiency of the retention chemical illustrated schematically in the example of the figure was reached in about two seconds after the chemical was mixed into the fiber suspension. After this, the efficiency of the chemical begins to decrease so that after about 5 - 6 second from the mixing only approx. 50 percent of the bondings still remain. This simple example reveals that too much retention chemical is dosed today to the fiber suspension as the retention mixing point is located in most cases somewhere close to the head box screen in the paper machine approach system. In practice this means that the chemical is mixed into the fiber suspension at a point 20 - 30 meters before the head box, which means that, when the speed of the pulp flow is 3 m/s, it takes 6 - 10 second before the chemical bondings and floes reach the head box. In other words, in order to achieve an adequate floe lever on the paper machine wire the retention chemical dose must be double or ever triple compared with a system employing mixing at the optimal time.

Thus, in order to intensify the use of chemical it is essentially important to place the chemical mixer at a suitable distance from the head box which in practice means the same as the paper machine wire as the travel of floes from the head box onto the wire takes only fractions of a second. The optimal way is to find out for each retention chemical its characteristic curve according to figure 2 and to determine according to that how far from the head box the chemical mixer should be installed.

The characteristic features of the method and the apparatus of the invention for feeding chemical into a fiber suspension, which method and apparatus solve for example problems discussed above, are disclosed in the appended patent claims.

The method of controlling the operation of the approach system of a paper machine or a corresponding apparatus according to the invention is explained more in detail with reference to the accompanying drawing figures of which

Fig. 1 illustrates mainly the prior art solution disclosed in U.S. patent no. 4,219,340; and

Fig. 2 illustrates schematically the typical efficiency of each retention chemical as a function of time.

The prior art paper machine approach system illustrated in Fig. 1 comprises a mixing tankwire pit 10, a feed pump 12, a vortex cleaning plant 14, a gas separation tank 16, a head box feed pump 18, a head box screen 20, a paper machine head box 22 and white water collection vats (not illustrated). These components have been provided in connection with a paper machine 24 and arranged to work as follows. Fiber material used for paper production is dosed into the mixing tank 10, which may also be the wire pit into which white water is collected, and located usually on the bottom level of the mill. The fiber material may be composed of virgin pulp, secondary pulp or broke, and fillers, which, in order to produce so-called paper pulp, are diluted with white water obtained from the paper machine, mostly from its wire section. The feed pump 12 also disposed on the bottom level of the mill is used to pump the paper pulp from the mixing tank 10 to the vortex cleaning plant 14 located usually on the machine level K of the mill which is the level on which the paper machine 24 is located. The paper pulp accepted by the vortex cleaning plant 14 continues its travel by means of the pressure generated by the feed pump 12 to the gas separation tank 16 located on a level T above the machine level. From the gas separation tank 16 the essentially gas-free paper pulp, from which gas has been removed as completely as possible, flows to the head box feed pump 18 on the bottom level of the mill which pumps the paper pulp to the head box screen 20 also located on the bottom level of the machine; from the screen 20 the accepted pulp flows to the machine level K to the head box 22 of the paper machine 24. In prior art systems, as was already stated above, the retention chemical is mixed either to the high-consistency pulp, i.e. prior to the mixing tank 10, or before the head box feed pump 18, depending on the type of the chemical, or the portion of the retention chemical sensitive to shear forces not until to the fiber suspension feed duct in the vicinity of the head box screen 20 prior to the head box 22.

Figure 2 illustrates schematically the efficiency of the retention aid as a function of time. The efficiency of the retention aid can be measured with for example the number of bondings or floes in a volume unit. Correspondingly, the time is measured from the mixing moment of the retention chemical. In the situation illustrated by the figure, the retention chemical begins to form bondings and floes immediately after the mixing and the number of bondings and floes is at its maximum after a time t1 from the mixing moment which in practice in most cases is about two seconds; at that point the efficiency is E1 , or 100 % expressed as percentage. After the maximum point, the number of floes reduces as illustrated in Fig. 2, which is due to the properties of the chemical itself and to the turbulence, which disrupts the floes. Thus, the form of the descending curve, i.e. the disrupting speed of the floes, depends both on the chemical and the intensity of the turbulence. The figure further illustrates a time T2, which depicts prior art, i.e. a situation, in which the retention chemical is mixed to the fiber suspension already in connection with the head box screen, in other words before the screen or immediately after it. It can be seen that the retention aid efficiency E2 corresponding to the time t2 is less than half of the maximum efficiency E1 , or at least substantially poorer than the maximum efficiency E1. In order to reach an adequately good retention and floe level in a prior art process, more than twice more retention chemical should be fed in the situation of Fig. 2 so as to reach an effect of the retention chemical use after time t2 that would correspond to the efficiency E1.

Thus when we aim at as efficient use of the retention chemical as possible, the peak of the curve should be arranged in the head box, i.e. the mixing point of the chemical should be located at a distance corresponding the delay t1 before the head box. In practice, the location of the peak of the curve may be in the flange between the approach duct and the head box, in the head box itself, or at the beginning of the wire section as measured in time the difference between these is very little. In practice the optimal location of the retention chemical mixer can be determined separately for each chemical used by drafting a curve according to Fig. 2 for each chemical and by calculating after that, based on the flow speed in the head box feed duct and the time indicated by the peak point of the curve according to Fig. 2, the distance of the mixer from the head box. In practice, the curve determined according to Fig. 2 for many retention chemicals in the market gives a "delay time" of the peak point of about 0.5 - 2.0 seconds. The delay time with some retention aids, however, may be up to 2.5 seconds or even three seconds. Correspondingly, the guideline value for the fiber suspension flow speed in the head box approach duct is in most cases 2.5 - 3.5 m/s. These numerical values indicate that the distance of the chemical mixer from the head box is at its most about 1.2 -10.5 m, although for most retention chemicals the optimal distances are less than 9 meters, preferably less than 8 meters. In prior art approach systems the distance mentioned has been almost without an exception of the order of 20 meters or more, because the head box approach duct has been considered to be a very good mixing space because of its turbulent state.

If the efficiency curve typical to the retention aid is not available the location of the chemical mixer may be determined based on the following information obtained experimentally. One possibility is to use the diameter of the approach duct in determining the location. A location very close to the optimal may be found by providing the mixer at a distance of 2 - 5 x D from the head box, where D is the diameter of the approach duct. Another dimensioning possibility is to arrange the mixer at a distance of less than 10 meters, preferably less than 8 meters from the head box.

A retention chemical composed of several components may be supplied, and should be supplied, at different points to the head box approach duct. It has been found out, for example, that it is best for the end result, i.e. the paper formation itself, to feed retention chemicals at two stages. The retention aids, for example polymer-based retention aids, which are used to form large floes, are supplied either prior to or after the head box screen. Then its clear that the large floes they form will star to disrupt both due to the turbulence prevailing in the head box feed duct and due to the long delay time. However, after this, another retention aid, for example a mineral-based retention aid, is supplied as optimally as possible, i.e. close to the head box so that the floes which it builds and which actually form the wire retention, remain whole until the wire. The method and the apparatus according to the present invention are particularly suitable for feeding in the latter retention aid. As may be understood from the above, a way has been developed both to reduce the costs of paper production and to reduce the use of chemicals at the paper machine. It should be noted from the above that the readings given in connection with the exemplary Figure 2 are only trend setting and exemplary and that the values in question may well deviate even to a remarkable extent from the ones given.

Claims

We claim:

1. A method of feeding chemical into a liquid flow, according to which method retention chemical is supplied via a special chemical mixer into the fiber suspension to be fed into the head box (22) of a paper machine or a corresponding web forming apparatus, characterized in that the mixer mentioned is located in the approach duct between a head box screen (20) and the head box (22) at distance of less than 10 meters from the head box (22).

2. A method according to claim 1 , characterized in that the mixer is located at a distance of less than 8 meters from the head box (22).

3. A method of feeding chemical into a liquid flow, according to which method retention chemical is supplied via a special chemical mixer into the fiber suspension to be fed into the head box (22) of a paper machine or a corresponding web forming apparatus, characterized in that a typical efficiency curve is determined for the retention chemical, a delay time corresponding to the peak point of the curve is determined from the curve, the chemical mixer is placed in the approach duct of the head box (22) at a distance corresponding to the delay time from the head box (22), and the retention chemical is fed from the mixer into the fiber suspension.

4. A method according to claim 1 or 2, characterized in that the mixer is arranged in the approach duct substantially at a distance from the head box

(22) corresponding to the calculated delay time giving the best efficiency for each retention chemical and the retention chemical is fed through the mixer mentioned to the fiber suspension.

5. A method according to claim 3 or 4, characterized in that the delay time mentioned is of the order of 0.5 - 3 seconds, preferably 0.5 - 2.5, most preferably 0.5 - 2.0.

6. A method according to claim 1 or 2, characterized in that the mixer is disposed at a distance of 2 - 5 x D from the head box (22), where D is the diameter of the approach duct.

7. An apparatus for feeding chemical into a liquid flow, the apparatus including as components of the approach system of a paper machine or a corresponding web forming device, a head box screen (20) and a head box (22) and a so- called approach duct connecting these two and a retention chemical mixer, characterized in that the distance of the retention chemical mixer mentioned from the head box (22) is less than 10 meters.

8. An apparatus as claimed in claim 7, characterized in that the distance of the retention chemical mixer mentioned from the head box (22) is less than 8 meters.

9. An apparatus according to claim 7, characterized in that the distance mentioned corresponds to a delay or flow time of about 0.5 - 3.0, preferably 0.5 - 2.5, most preferably 0.5 - 2.0 seconds in the approach duct of the head box (22).

10. An apparatus according to claim 7, characterized in that the distance mentioned corresponds to the optimal delay time determined for each retention chemical.