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. The 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. The 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.