The invention relates to a process for blending liquid flows, particularly in the approach system to a paper machine, as well as a device for implementing the process.
In paper-making, a pulp suspension is distributed evenly over a wire and the greater part of the water is removed from the pulp in the first part. Before the suspension is fed onto the wire, impurities, in particular, are removed. The pulp suspension also contains gas, particularly air, as free air in the form of bubbles and as dissolved air. This air, especially in the form of bubbles, causes problems in the paper production process, particularly if present in larger quantities. As a result there may be problems with foam, instabilities in the process, pulsations in the approach system to the paper machine, reduced dewatering performance and, as a further consequence, small holes may appear in the paper web.
A process to achieve maximum possible degassing is described, for example, in U.S. Pat. No. 4,219,340. This evacuation is, however, very complex and in many cases, there is no need for complete evacuation.
In the approach system to the paper machine, different pulp components (long fibres, short fibres, broke, etc.) are currently fed into a tank and blended. Then various chemicals are added (e.g. wet strength agent, dye, filler, etc.). As an alternative, the individual components and also additives can be fed into a mixing pipe.
The problem with these set-ups is that the substances are not mixed adequately and also contain a large proportion of gas, both in the individual flows of the pulp components and in the white water. EP 0 543 866 B1 shows a plant, for example, in which several pumps are used to remove the gas from the pulp that has been blended beforehand and from the white water coming from the paper machine. The plant is not capable, however, of mixing pulp components and additives.
Although the sensors for measuring quantities and consistency are located in the de-aerated pulp, there is no device here to mix the dilution water homogenously into the pulp.
The present invention is intended to prevent these disadvantages and is thus characterised by the individual liquid flows being merged, blended with one another, and degassed, all at the same time. Since the mixture is degassed at the same time, a constant status is achieved after blending, which means it is possible to do without the large mixing tanks needed hitherto. In addition, it is possible to obtain exact measurements of the pulp data, particularly the consistency.
It is a particular advantage if individual liquid components are blended with one another, during which process additives can also be mixed into the pulp as this produces a homogenous pulp suspension from which also a homogenous paper web can be produced.
It has proved favourable to blend dilution water, e.g. white water from a paper machine, into the individual liquid flows, where the entire white water can also be mixed into the suspension. When the pulp components are blended with the white water, the white water can then also be de-aerated together with the suspension. Thus, a level of de-aerating can be achieved in many cases that renders complex vacuum de-aerating unnecessary.
An advantageous configuration of the invention is characterised by the blended and degassed suspension being fed to a storage tank, e.g. machine chest, standpipe. With this storage tank it is possible to obtain a yet more uniform suspension and particularly, to eliminate any pulsations, however it is important to have a small volume so that any grade or colour change can be carried out promptly. As the suspension has been well blended beforehand, there is no longer any need for the mixing chest required previously.
Particularly low volumes and thus, particularly favourable grade changes, are obtained if the blended and degassed suspension is fed directly to a pump.
A favourable further development of the invention is characterised by at least one characteristic value of the suspension being measured after blending and degassing, where the consistency of the pulp suspension can be measured and, advantageously, the dilution water is added according to the consistency of the blended and degassed pulp suspension. Since the dilution water is mixed in well, it is also possible to obtain high accuracy. Other pulp data, however, such as ash content, brightness, or freeness, can also be measured online with particular accuracy.
The invention also relates to a device for blending liquid flows, particularly in the approach system to a paper machine. According to the invention, this is characterised by a degassing device, particularly a rotor with degassing holes, being provided in a mixing pipe. In this way, the pulp suspension can be blended particularly well, and degassed at the same time.
An advantageous further development of the invention is characterised by several pipes for liquid flows, particularly pulp components, leading into the mixing pipe, into which a dilution water pipe can also discharge. As a result, the consistency of the pulp suspension especially can be regulated particularly well to the desired value. Blending and homogenising is much more intensive here compared to a mixing chest.
A particularly favourable embodiment of the invention is characterised by the white water pipe of a paper machine discharging into the mixing pipe. Thus, the entire white water can also be degassed together with the liquids added, particularly pulp components.
A favourable variant of the invention is characterised by the mixing pipe being connected to a storage tank after the degassing device, where this storage tank can be designed as a standpipe. Here, the standpipe together with the white water tank can form a communicating vessel, which makes the system self-regulating.
An alternative advantageous embodiment of the invention is characterised by the mixing pipe being connected to a pump after the degassing device. This results in particularly low storage volumes and thus, a particularly favourable means of changing the grade or colour of paper produced.
An advantageous embodiment of the invention is characterised by a measuring device for at least one of the suspension's characteristic values being provided after the degassing device, where the measuring device can be a consistency meter and where it is an advantage if this consistency meter is connected to a valve in the dilution water pipe via a controller. In this way, the consistency of the pulp suspension can be set particularly accurately in the feed to the paper machine. In addition, other measuring devices, e.g. for brightness, ash content or freeness, can be used and will provide particularly accurate measuring values, especially on account of the virtually gas-free and homogenous suspension.
The invention will now be described in examples and referring to the drawings, where
According to the state of the art, the approach system to a paper machine shown in
The invention avoids the need for large degassing tanks, which leads in turn to considerable savings in investment. Thus, a “short flow” concept can be implemented by simple and low-cost means.
Number | Date | Country | Kind |
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A 830/2004 | May 2004 | AT | national |