FI111284B - Method and apparatus for introducing a chemical into a liquid stream - Google Patents

Description

. 111284

Method and apparatus for introducing a chemical into a liquid stream

The present invention relates to a method and apparatus for feeding a chemical into a liquid stream. Particularly well, the method and apparatus of the invention are suitable for continuously adding a liquid chemical to a liquid stream. Preferably, the method and apparatus of the invention are used to deliver a retention agent to a fiber suspension going to the headbox of a papermaking machine.

There are, of course, virtually innumerable ways of feeding various chemicals into fluid streams. However, these ways can be divided into a few main types, as shown below. First, it is quite possible simply to allow the fluid to be added to flow freely among the second fluid without any specific control or mixing means. This type of addition cannot be used in situations where the mixing ratio or uniformity of mixing would matter. And even in a situation where the price of the chemical to be added matters. Another useful way is to feed the chemical in an accurate proportion to the fluid flow, which will make dispensing correct and economical. Again, however, it should be borne in mind that the chemical is generally dosed somewhat more than the optimum amount, since the mixing is known to be deficient. However, the mixing can be improved by feeding the chemical through, for example, the perforated wall of the flow passage, whereby at least the chemical to be mixed has been applied throughout the liquid stream. As a last resort, it is possible to deal with a situation where the chemical is metered into a liquid stream either upstream of the mixer or through the mixer itself into the liquid. In this case, it is completely dependent on the design of the mixer how efficiently the mixing agent is mixed with the liquid stream.

The following describes, by way of example, one of many possible alternative blending situations for papermaking. However, from the outset, it should be noted that very similar problems, which are described in greater detail below, apply to almost all blending situations, so our invention to solve these problems is not limited to use specifically in papermaking. Paper making is one of the most demanding specialties in the mixing of chemicals. When using paper chemicals, it is good to keep in mind that mixing them accurately and evenly is a vital step in a short machine cycle. Smooth blend marks-35 see directly for better paper quality and consistency. At the same time, process operation 2 111284 is made trouble-free and trouble-free. Poor mixing, in turn, requires an overdose of the chemical, which can significantly increase production costs. It goes without saying that with poor blending, the quality of the paper and the operation of the process are not good. Current mixing technology, on the other hand, uses pure water fractions for both dilution water and so-called dilution. As "whisker water" to enhance mixing efficiency. On the other hand, the paper mill's water circulations are being blocked, whereby the purging of the water into the system should be reduced and instead use of internally purified and / or untreated process streams such as filtrates. Current chemical mixing systems do not allow, or only permit to a small extent, the use of in-process water fractions.

One essential blending process associated with papermaking is the blending of the retention chemical or chemicals into the fiber suspension stream to the headbox of the papermaking machine. Retention chemicals are used in papermaking, in particular to improve fine retention on the wire section of a papermaking machine. The retention agent is a chemical whose long molecular chains bind together a mass of solid particles and thus prevent the fines from being transported through the wire during the forming step. The retention agent should, at least in theory, be blended with the pulp as uniformly as possible to achieve maximum chemical performance and to avoid variations in paper properties due to retention variations. However, mixing, on the other hand, means that the liquid is subjected to a turbulent flow where shear forces break / can break long molecular chains, which naturally reduces the effectiveness of the retention agent. However, there are various retention agents. Sensitive to the effects of turbulent flow are e.g. broken polyacrylamides: 25 molecular chains are known not to recover after turbulence suppression.

But there are also retention agents (e.g., polyethyleneimines) whose molecular chains recover rapidly after turbulence is substantially reduced to their original size.

In a short run of a papermaking machine, the retention chemicals feed point is highly dependent on the retention agent used, the flow space at the feed point to the headbox lip, and. of the pulp used. Shear-sensitive retention agents are usually fed immediately downstream of the headbox shear-applying device, which may be a pump, a strainer or a vortex cleaner, either in one location or, for example, in an accept tube for each pressure strainer. Several different types of retention agents can also be used simultaneously and fed into the fiber suspension stepwise. . The shear-resistant portion of the retention aids may be fed to the bulk mass prior to or up to the headbox feed pump, and a portion of the retention agents susceptible to shear forces will generally only be introduced into the fiber suspension inlet tube near the headbox screen.

5 For the sake of clarity, the following describes what and what types of equipment are involved in the short cycle of a papermaking machine. Prior art paper machine approach systems which are well understood by, e.g. U.S. Patent No. 4,219,340 consists almost exclusively of the following components. Mixing tank, feed pump, rotary purification plant, gas separation tank, headbox feed pump, headbox strainer, ίο paper machine headbox and sewage collection basins. Said components are located in connection with the paper machine and arranged to operate as follows. A mixing tank, also often referred to as a wire pit, which is usually located at the bottom of the mill, is dosed with papermaking fiber and fillers, which are diluted using a paper machine, mainly from its wire section. white water.

Likewise, with a feed pump located at the bottom of the mill, the fiber suspension is pumped from the mixing tank, usually at the mill level, to the plane where the paper machine sits or, as in the aforementioned patent, to a vortex cleaning plant. The fiber suspension, accepted by the vortex purification plant, continues to be pressurized by said feed pump into a gas separation tank positioned at a level above machine plane 20. From the gas separation tank, the deaerated fiber suspension is flushed to a mill bottom-level headbox feed pump, which also pumps the fiber suspension to a bottom-level headbox screen (not disclosed in the aforementioned U.S. Patent) from which the fiber suspension flows to the machine bed headbox.

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It has already been stated that retention chemicals have been attempted to be mixed with the fiber suspension and at a sufficiently early stage to allow the chemical to mix evenly with the pulp, but it has also been recognized that some retention chemicals do not withstand turbulence and are blended as late as possible. taking into account the requirement of uniformity of mixing. However, in general, the feed tube from the headbox screen to the headbox has been considered a good mixing space due to the turbulence there. In other words, an attempt has been made to feed the retention chemical to the feed tube in question. to the upstream end of the approach tube, e.g., the headbox screen outlet, whereby the retention chemical is not exposed to turbulence in the head-35 duct strainer, but is exposed long enough to be believed to be sufficiently uniform.

Recently, a TRUMPJET ™ blender designed by Andritz-Ahlstrom Oy for the retention chemical mix has been launched. Fl patent application 980437. In practical tests of this device, it has been found that the device is capable of mixing the retention chemical with the pulp substantially more uniformly than prior art devices, one could even speak of optimum mixing.

Further investigating how low doses of the retention chemical could actually cope, since the optimum mixing in itself reduces the amount of retention chemical required, it was surprisingly found that the retention chemicals and the flocs they develop are very durable. In other words, as a function of time, the retention chemicals as depicted in Figure 2 as a function of time can be considered suitable for all retention chemicals in use. The power can be measured, for example, in the strength of the dressings or in the number of dressings per unit volume or the like. The best effect of the retantiochemical shown in the schematic example shown in the figure was achieved about two seconds after the chemical was mixed with the fiber suspension. Thereafter, the potency of the ke-20 chemical begins to diminish, so that after about 5 to 6 seconds, approximately 50% of the dressings remain after mixing. From this simple example, it can be inferred that the retention chemical is still dosed too much in the current fiber suspension, since the retention chemical mixing point is usually located somewhere in the headbox screen in the paper machine approach system. In practice, this means that the chemical is mixed 20 to 30 meters before the headbox with the fiber suspension, where, at a mass flow rate of 3m / s, it takes 6 to 10 seconds before the chemical bonds and flocks reach the headbox. In other words, in order to achieve a sufficiently high level of flocculation on a papermaking machine wire, the retention chemical must be dosed in two or even three times the optimal mixing time.

Thus, in order to enhance the use of the chemical, it would be imperative to place the chemical mixing device at a suitable distance from the headbox, which in practice means the same as a papermaking machine wire, since transferring flocs from the headbox to the wire takes Aino-35 seconds. The optimum method is to determine the characteristic curve of Fig. 2 for each retention chemical and determine how far from the headbox the chemical mixer should be installed.

The method and apparatus of the invention for supplying a chemical to a fiber-5 pension, which is solved e.g. the foregoing problems, the features of which are set forth in the appended claims.

In the following, the method of controlling the operation of a papermaking machine or similar approach system according to the invention will be explained in more detail with reference to the accompanying drawings, of which Figure 1 illustrates the prior art solution of US Patent 4,219,340;

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The prior art paper machine approach system shown in Figure 1 includes a mixing tank / wire well 10, a feed pump 12, a vortex cleaning plant 14, a gas separation tank 16, a head box feed pump 18, a head box sieve 20, a paper machine headbox 22 and wire runners. Said components are disposed on paper machine 24 and arranged to operate as follows. The mixing vessel 10, which may also be a wire well for collecting the wastewater and which is usually located at the bottom of the mill, is dosed with papermaking fiber, which may consist of fresh pulp, secondary pulp or wreck and diluents used tap water, so-called. 25 to form a pulp. Likewise, with the mill bottom feed pump 12, said pulp is pumped from the mixing tank 10 to a vortex cleaning plant 14, usually at mill level K, the plane where the paper machine 24 sits. from the gas separator tank 16 to a substantially non-degassed pulp, thus degassing as accurately as possible, is fed to a bottom-level headbox feed pump 18 which also pumps the pulp to a bottom-level headbox screen 20 from which the accepted pulp flows to the machine platform K 22. In the prior art systems, as noted above, the retention chemical was either mixed with the viscous mass, depending on the type of chemical 111284. prior to mixing tank 10 or prior to headbox feed pump 18, or to a portion of retention agents exposed to retention agents only in the fiber suspension suspension in the vicinity of headbox screen 20 before headbox 22.

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Figure 2 is a schematic representation of the retention agent's efficacy over time. The effectiveness of the retention agent can be measured, for example, by the number of dressings or flocs per unit volume. Similarly, time is measured from the time the retention chemical is mixed. In the situation shown in the figure, the retention chemical begins to form bonds and flocculants immediately after mixing and after t1, which in practice corresponds in most cases to about two seconds, at the time of mixing the maximum amount of flocks and dressings is E1 or 100%. After the maximum point, the number of flocks decreases as shown in Figure 2, due to both the properties of the retention chemical itself and the turbulence that ruptures the flocks. In other words, the shape of the slope of the curve, i.e. the rate at which the flocs break, depends on both the chemical and the intensity of the turbulence. The figure further shows the time t2 corresponding to the prior art solution, i.e. the situation where the retention chemical is already mixed with the fiber suspension in connection with the headbox screen. That is, either before or immediately after the screen. It is noted that the retention agent te-20 has an E2 efficiency of less than half the maximum efficiency E1, or at least substantially less than the maximum efficiency E1. In order to achieve sufficiently good retention or flock level in the prior art processes, in the situation of Figure 2, more than twice as much retention chemical should be applied so that the practical effect of the retention chemical after t2 is equivalent to E1.

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Thus, in order to maximize the use of a retention chemical, the peak of the curve should aim at the headbox, i.e., the chemical mixer should be positioned at a delay t1 before the headbox. In practice, it makes no difference whether the tip of the curve is placed in the flange between the approach tube and the headbox, in the headbox itself or at the beginning of the wire section 30, since the time difference between them is very small. In practice, the location of the retention chemistry mixer can be determined to be optimal for each chemical used by plotting the curve of Figure 2 for each chemical and then calculating the headbox feed tube flow rate and the time given by the peak of the curve 2 from the headbox. In practice, the curve shown in Figure 2 for a number of retention chemistry-35 cabbages on the market gives a peak 'residence time' of about * 7111284 0.5 to 2.0 seconds. However, with some retention agents the delay time can reach up to 2.5 seconds or even up to three seconds. Similarly, a value of 2.5 to 3.5 m / s is often used as a guide for the fiber suspension flow rate in the headbox approach tube. From these values, the distance of the chemical mixer from the headbox to the widest 5 at any distance of about 1.2 to 10.5 m is obtained, although optimum distances for most retention agents are less than 9, preferably less than 8 meters. In prior art approach systems, this distance has been almost invariably of the order of 20 meters or more, precisely because of its turbulent space, the headbox approach tube has been regarded as an excellent mixing space.

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If a retention agent specific performance curve is not available, it is also possible to determine the placement of the chemical mixer based on the following empirical information. One possibility is to use the approach tube diameter to assist in sizing. By placing the mixer at a distance of 2 - 5 * D from the headbox, where 15 D is the diameter of the approach tube, it is very close to the optimum position of the mixer. Another possible dimension is to place the mixer at a distance of less than 10 meters, preferably less than 8 meters, from the headbox.

A multi-component retention chemical can and must be fed to different locations in the headbox approach piping. For example, it has been found to be best to feed the retention chemicals in two steps for the final result, i.e. the actual paper formation. Retention agents, for example polymer-based retention agents, which form large flocs, are fed either before or after the headbox strainer. It is clear, then, that the flocs they form begin to disintegrate due to both turbulence in the headstock feed tube and the long delay. Thereafter, however, a second retention agent, for example a mineral based retention agent, is fed as optimally as possible, i.e. near the headbox, so that the flocs formed by them to form the actual wire retention remain intact up to the wire. Especially in the supply of the latter retention agent, it is possible to utilize the method and apparatus according to our invention.

As noted above, there has once again been a way to both reduce paper production costs and reduce the use of chemicals in the paper machine. From the foregoing, it should be noted that the exemplary

Claims (10)

5 Patent Claims A method for feeding a chemical into a liquid stream, wherein the retention chemical is fed through a special chemical mixer to a fiber suspension to be fed to the headbox (22) of a papermaking or similar forming machine, characterized by positioning said agitator less than 10 meters from the headbox (22). 20) and the approach tube between the headbox (22). A method according to claim 1, characterized in that the mixer is located less than 8 meters from the headbox (22). 15 3. A method for feeding a chemical into a liquid stream, wherein the retention chemical is fed through a special chemical mixer to a fiber suspension fed to a paper machine or similar web forming headbox (22), characterized by: from the headbox (22) to its approach tube and feeding the retention chemical from said mixer to the fiber suspension. Method according to claim 1 or 2, characterized in that the chemical mixer is located substantially from the headbox (22) in the head tube (22) of the retention chemical, which is characterized by the best efficiency of each retention chemical, and feeds the retention chemical into the fiber suspension. A method according to claim 3 or 4, characterized in that said delay time is in the order of 0.5 to 3 seconds, preferably 0.5 to 2.5, most preferably 0.5 to 2.0. A method according to claim 1 or 2, characterized in that the mixer is located at a distance of 2 to 5 * D, where D is the diameter of the approach tube, from the headbox (22). 35 9 111284 Apparatus for supplying a chemical to a liquid stream comprising, as part of the approach system of a papermaking or similar web-forming device, a headbox screen (20) and a headbox (22) and a so-called connecting device. an approach tube and a retention chemical mixer, characterized in that said retention chemical mixer is less than 10 meters from the headbox (22). A method according to claim 7, characterized in that said retention chemical mixer is less than 8 meters from the headbox (22). Apparatus according to claim 7, characterized in that said distance corresponds to a delay or flow time of about 0.5 to 3.0, preferably 0.5 to 2.5, most preferably 0.5 to 2.0 seconds in the approach tube of the headbox (22). Apparatus according to claim 7, characterized in that said distance 15 corresponds substantially to the optimum residence time determined for each retention chemical. 111284