Patent Description:
In a modern pulp mill, it is increasingly important that the water consumption of the mill is as low as possible and at the same time the mill produces as little effluent as possible. Such operation is beneficial for environmental reasons and often required by laws or guidelines as well. Accordingly, there is a need for control of flows of water, filtrate and effluent in a mill.

During normal operation a pulp mill is well balanced also with respect to water and effluent flows, as excess water from a certain production phase can be used elsewhere in the mill. For example, excess water produced in pulp drying can be recycled back to the fiber line. However, situations arise in which the fiber line cannot receive all or some of the excess water from pulp drying and in such a case the excess water has typically been diverted to effluent thus increasing water consumption and effluent handling need.

Similar situations have been previously addressed in publication <CIT> in pulp washing by storing the extra filtrate in a pulp tower. However, the solution of said publication changes the consistency in the tower by mixing the filtrate and the stored pulp and accordingly is not suitable for most situation, e.g. in pulp drying.

It is the object of the present invention to provide a method for control of flows that decreases water usage and amount of effluent thus mitigating the problems of prior art.

According to a first example aspect of the present invention, there is provided a method for control of flows of in a pulp mill, comprising.

The method may further comprise when the fiberline is again able to receive the white water from the first water tower, evacuating the white water from the second pulp tower and directing it to the first pulp tower.

The method may further comprise circulating a part of the white water being directed from the second pulp tower to the first pulp tower past a measurement element for measuring the consistency of the white water from the second pulp tower.

The method may further comprise determining that the white water has been substantially emptied from the second pulp tower when the consistency measured with the measurement element is above a predetermined threshold.

The predetermined threshold may be about <NUM>,<NUM>%.

The consistency of the pulp in the second pulp tower may be about <NUM>,<NUM> to <NUM>%.

The method may further comprise directing pulp from the second pulp tower to pulp drying.

According to a second example aspect of the present invention, there is provided a system for control of flows in a pulp mill, comprising.

<FIG> shows a schematic block view of a system according to an example embodiment of the invention. <FIG> shows a part of the elements of a pulp mill pulp and flow structures. It is to be noted that <FIG> is a schematic representation of elements for explaining embodiments of the present invention and accordingly only a part of the elements of the pulp mill are depicted, i.e. it is clear to a skilled person that the majority of the elements and structures of the pulp mill are not shown which does not in any way define the presence or absence of the same. Hereto it is further noted that the actuators, such as piping, pumps and valves, of the system used in carrying out the method according to embodiments of the invention have not been represented and a skilled person appreciates that they comprise actuators and further elements of conventional construction.

The system <NUM> according to an embodiment of the invention comprises a first pulp tower <NUM>. In an embodiment the first pulp tower <NUM> comprises a bleached pulp tower configured to receive and store bleached pulp arriving from the fiberline A prior to the pulp being dried at B. The system <NUM> further comprises a second pulp tower <NUM>. In an embodiment, the second pulp tower <NUM> comprises a transition pulp tower configured to receive and store pulp from the fiberline at A in transition situations in which the pulp type is changed, for example from hardwood to softwood or vice versa. In a further embodiment, the second pulp tower <NUM> comprises a pulp tower with similar functionality and purpose as the first pulp tower <NUM>, for example in a mill having two fiberlines or a fiberline with two pulp towers even if producing single pulp quality. In this case. in an embodiment, during regular operation of the mill, the pulp from the second pulp tower <NUM> is dosed in small amounts into production together with the pulp from the first pulp tower <NUM>.

The system <NUM> further comprises a first water tower <NUM>, i.e. a white water tower configured to receive white water from pulp drying at C. In regular operation of the mill, the water tower <NUM> is configured to forward the white water received from pulp drying back to the fiberline at D. In an embodiment, the water tower <NUM> is further configured to forward white water into the first pulp tower for dilution. <FIG> shows the regular operating situation of the system <NUM>. The pulp is delivered from the fiberline at A to the first pulp tower <NUM> in which it is stored at a consistency in the range of <NUM>,<NUM>-<NUM>%, for example at a consistency of about <NUM>%. In an embodiment, the volume of the first pulp tower <NUM> is for example between about <NUM> and <NUM> cubic meters. From the first pulp tower <NUM> the pulp is then forwarded to pulp drying at B. Pulp drying produces white water which is delivered to the water tower <NUM>, stored therein and forwarded back to the fiberline at D. In an embodiment, white water is further forwarded into the first pulp tower for dilution. In the regular operating situation, the fiberline is able to receive substantially all white water from the water tower <NUM>.

<FIG> shows a further schematic block view of a system according to an example embodiment of the invention. <FIG> shows the elements of the system <NUM> as described hereinbefore with respect to <FIG>. <FIG> shows an operating situation in which the fiberline is not able to receive white water from the water tower <NUM> for some reason, for example because the fiberline is undergoing a service break. In such an operating situation the water tower <NUM> would start to overfill relatively quickly and accordingly, according to the present invention, the white water from the water tower <NUM> is forwarded into the second pulp tower <NUM> to be stored therein.

The white water is pumped into the second pulp tower <NUM> from the lower part thereof. As the consistency of the pulp in the second pulp tower <NUM> is from about <NUM>,<NUM>% to <NUM>%, the white water and the pulp substantially remain in separate layers with the white water layer below and a pulp cake above it. It is to be noted that remaining substantially in separate layers does not mean that in an embodiment, there could not exist an intermediate layer in which the pulp and the white water start, at least given time, to mix to a certain extent. Since the excess white water from the water tower <NUM> can be diverted to the second pulp tower <NUM>, the pulp drying can remain operational even in the situation in which the fiberline can not receive the excess water without causing a large amount of effluent.

<FIG> shows a further schematic block view of a system according to an example embodiment of the invention. <FIG> shows the elements of the system <NUM> as described hereinbefore with respect to <FIG>. <FIG> shows an operating situation in which the fiberline is again able to receive white water from the water tower <NUM> after the operating situation of <FIG> in which white water has been forwarded to the second pulp tower <NUM>. In such an operating situation, the white water temporarily stored at the second pulp tower <NUM> is removed therefrom.

in order to empty the white water from the second pulp tower <NUM>, the white water is evacuated from the bottom part thereof and forwarded to the first pulp tower <NUM> and used to dilute the pulp therein. Simultaneously, a part of the white water being evacuated from the second pulp tower is circulated past a measurement element <NUM> in order to measure the consistency of the flow leaving the second pulp tower <NUM>. When the consistency measured by the measurement element <NUM> raises above a certain predefined threshold, it is determined that the white water stored in the second pulp tower <NUM> has been successfully emptied therefrom and substantially only the pulp layer remains and/or the intermediate layer in which the white water and pulp have mixed or started to mix to a certain extent is being evacuated. In an embodiment, the value of the predefined threshold is chosen in accordance with the situation, for example by an operator. in an example embodiment, the predefined threshold is about <NUM>,<NUM>% to <NUM>%, in an embodiment about <NUM>,<NUM>%. In a further embodiment, the water stored in the second pulp tower <NUM> is forwarded substantially in its entirety to the fiberline, for example in an operating situation in which pulp drying is not operational and cannot supply water to the fiberline. In a further embodiment, the water from the second pulp tower <NUM> is forwarded to the water tower, for example in a situation in which the water tower <NUM> needs to be filled up.

in the operating situation of <FIG>, the water tower <NUM> receives white water from pulp drying at C and forwards white water back to the fiberline at D and in an embodiment to the first pulp tower <NUM> for dilution. Furthermore, pulp from the first pulp tower <NUM> is forwarded to pulp drying at B and pulp arrives at the first pulp tower <NUM> from the fiberline at A. In an embodiment, the pulp from the fiberline could also be forwarded to the second pulp tower <NUM> while the pulp from the first pulp tower is forwarded to pulp drying. In a situation in which white water is evacuated form the second pulp tower <NUM>, the amount of white water forwarded to the fiberline increases by the amount of white water taken from the second pulp tower <NUM> in order to keep the system balanced and to prevent the need to forward white water into effluent. Furthermore, in a further embodiment, the water emptied from the second pulp tower <NUM> is in some situations forwarded directly, i.e. without being first forwarded to the fiberline or the water tower <NUM>, to pulp drying in order to be used for control of consistency.

<FIG> shows a further schematic block view of a system according to an example embodiment of the invention. <FIG> shows the elements of the system <NUM> as described hereinbefore with respect to <FIG>. <FIG> shows an operating situation after the operation situation hereinbefore described with reference to <FIG> in which the white water has been emptied from the second pulp tower <NUM>. The system <NUM> operates in the regular operating situation described hereinbefore with reference to <FIG> and pulp from the second pulp tower <NUM> is also forwarded to pulp drying mixed with the pulp from the first pulp tower <NUM>. ln a further embodiment, only pulp from the second pulp tower <NUM> is forwarded to pulp drying. Furthermore, the water tower <NUM> receives white water from pulp drying at C and forwards white water back to the fiberline at D.

<FIG> shows a flow chart of a method according to an example embodiment of the invention. In an embodiment, the method according to an example embodiment of the invention is caused to be carried out by a control system. In an embodiment, the control system is a standalone control system configured to control the method. In a further embodiment, the control system is integrated into a mill-wide control system.

At step <NUM> the system is in a regular operating situation and it is checked whether the fiberline is receiving the excess water of pulp drying, i.e. from the water tower <NUM>. If the fiberline is not able to receive the excess water from pulp drying, the excess water from white water tower <NUM> is directed to the bottom of the second pulp tower as hereinbefore described with reference to <FIG> at step <NUM>. As soon as the fiberline is again able to receive the excess water from pulp drying, the excess water is returned to the fiberline at step <NUM> and the water is evacuated from the second pulp tower by directing it to the first pulp tower at step <NUM> as described hereinbefore with respect to <FIG>. Simultaneously the consistency of the water being evacuated from the second pulp tower <NUM> is monitored with a measurement element 40and it is checked whether the consistency is above a predefined threshold at step <NUM>. If the consistency is above the predefined threshold, normal operation of the system as described hereinbefore with reference to <FIG> and <FIG> is resumed at step <NUM>.

Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein is reduction of pulp mill water usage. Another technical effect of one or more of the example embodiments disclosed herein reduction of effluent and need for effluent purification capacity. Another technical effect of one or more of the example embodiments disclosed herein is ensuring uninterrupted operation of pulp drying in irregular operating situations without compromising the water balance. A still further technical effect of one or more of the example embodiments disclosed herein is a more environmentally friendly operation. A still further technical effect of one or more of the example embodiments disclosed herein is energy saving.

Claim 1:
A method for control of flows of in a pulp mill, comprising
storing pulp from fiberline in a first pulp tower (<NUM>);
directing the pulp from the first pulp tower (<NUM>) to pulp drying;
storing white water from the pulp drying into a first water tower (<NUM>) and directing the water therefrom back to the fiberline; and
when the fiberline is not able to receive the white water from the first water tower (<NUM>), directing the white water to second pulp tower (<NUM>) comprising pulp with such consistency that the white water and the pulp remain substantially separated in different layers in the second pulp tower (<NUM>).