TECHNICAL FIELD

The invention relates to a cellulose producing system comprising a cooking device, an oxygen delignification arrangement, and a post-delignification washing arrangement comprising a first wash press device and a subsequent second wash press device. The invention also relates to a method for controlling such a system.

BACKGROUND

In systems for producing cellulose where an oxygen delignification arrangement is used to delignify the cellulose pulp, there is a need to wash the cellulose pulp after delignification, i.e. prior to a subsequent bleaching step. This is advantageously performed using two consecutive wash press devices with an intermediate storage tower. The twin-roll press is a well-known type of such a wash press device, which is provided with two counter-rotating rolls with perforated outer surfaces. A web of pulp is formed on the respective rolls and is transported in the direction of rotation in a vat partially surrounding the rolls, to the press nip between the rolls. The liquid removed from the pulp, i.e. the filtrate, passes through the perforated roll surface in a radial inwards direction and is led to the ends of the press roll, where it is output. Washing liquid or other treatment liquid may be supplied to the pulp web through inlets in the vat. The twin-roll press uses displacement washing, where dirty liquid (liquor) in the pulp is replaced by cleaner wash liquid added to the vat, and pressing, and where the dirty liquid is pressed (squeezed) out from the pulp, in particular at the press nip.

The residence time in such twin-roll presses is quite short, which means that the use of such presses in this application (post-delignification washing) becomes sensitive for disturbances in the process. For instance, if the digester/cooking device is stopped, but the other parts of the mill is still in operation, production before and after the intermediate storage power differs, which means that more filtrate/dirty liquor may be produced in the second wash press than what is required as washing liquid in the first wash press.

Today, this problem is solved by the use of a liquor buffer tank after the second wash press in which excess liquor is stored during disturbances or upset conditions. During normal steady state operation, the liquor buffer tank is not of any use, however. The liquor buffer tank thus adds cost and space to the mill without any advantages during normal operation. Thus, there is a need for a more cost- effective solution to the above-mentioned problem.

SUMMARY An object of the invention is to solve or improve on at least some of the problems mentioned above in the background section.

These and other objects are achieved by the present invention by means of a method and a system according to the independent claims.

According to a first aspect of the invention, there is provided a system for producing cellulose pulp comprising a cooking device configured to digest wood chips to produce cellulose pulp, a screening arrangement configured to screen the cellulose pulp, a washing arrangement configured to wash the cellulose pulp after screening, an oxygen delignification arrangement configured to delignify the cellulose pulp after washing, a wash liquor buffer tank connected to said cooking device and to said washing arrangement to receive filtrate from the washing arrangement for use as washing liquid in said cooking device, and a post delignification washing arrangement connected to the delignification arrangement to wash the cellulose pulp after delignification. The post delignification washing arrangement comprises a first wash press device and a subsequent second wash press device, with an optional intermediate storage tower. The first wash press device is connected to the second wash press device to receive a filtrate flow therefrom for use as washing liquid. The second wash press device is connected to the wash liquor buffer tank to allow a filtrate flow from the second wash press to the wash liquor buffer tank. The second wash press device is connected to the wash liquor via a conduit and a pumping means. The system may further comprise a control system configured to:

Determine a required wash liquid flow for the first wash press device;

Determine a filtrate flow obtained from the second wash press device, and

If said required wash liquid flow is lower than said filtrate flow, controlling the pumping means to pump excess filtrate flow to said wash liquor buffer tank.

According to a second aspect of the invention, there is provided a method for controlling a system according to the first aspect of the invention or embodiments thereof. The method comprises:

Determining a required wash liquid flow for the first wash press device;

Determining a filtrate flow obtained from the second wash press device, and

If said required wash liquid flow is lower than said filtrate flow, transporting excess filtrate flow to said wash liquor buffer tank.

It is understood that the term connected to should be interpreted in the sense that two devices or arrangements are in fluid communication/contact directly or indirectly with each other, for instance via a conduit with or without pumping means and/or via one or more intermediate devices which may comprise a storage tower. It is further understood that the cooking device, also referred to herein as the digester, is normally preceded by or comprises an impregnation device. It is further understood that the system (and corresponding method) described herein is a continuous system for producing cellulose pulp comprising a cooking device of the continuous type.

The invention is based on the insight that disturbed/upset conditions make up a quite small portion of the overall operating time of a pulping mill, which means that the volume of the excess filtrate from the second wash press of the post delignification over time is quite small. This had led the inventors to the insight that, although the excess filtrate from the second wash press of the post delignification washing arrangement is relatively clean, it is overall substantially more economical to use this excess filtrate as washing liquid further upstream in the process rather than investing in a separate buffer tank at the second wash press for storing the excess filtrate. In particular, the inventors have realized that it is advantageous to use the excess liquor as washing liquid for the digester (which is normally recycled from the brown stock washing, and thus much less clean) by pumping it back to the already present wash liquor tank of the digester. The invention is furthermore based on the insight that such a connection is advantageously combined with a control of the pumping means based on a required wash liquid flow for the first wash press device and the actual filtrate flow obtained from the second wash press device.

In embodiments of the system according to the first aspect of the invention, the system further comprises a source of wash liquid connected to the first wash press device, wherein the control system is further configured to control the source of wash liquid to provide wash liquid to the first wash press if the required wash liquid flow is higher than said filtrate flow. In other words, if the upset conditions result in a greater need for wash liquid to the first wash press device than what the second wash press can produce as filtrate, the filtrate is supplemented with fresh wash liquid, for instance from a tank or an external source.

In a corresponding embodiment of the method according to the second aspect of the invention, the method further comprises, if the required wash liquid flow is higher than the filtrate flow, supplementing the filtrate flow with wash liquid from a tank and/or an external source of wash liquid.

In embodiments of the system according to the first aspect of the invention, the system further comprises an evaporation arrangement, also referred to as the recovery system, having at least one evaporation liquor tank and at least one evaporator connected thereto, wherein at least one of the evaporation liquor tank(s) is connected to the wash liquor buffer tank, for instance via a conduit with pumping means. At least one of the evaporation liquor tank(s) may also be connected to the cooking device to receive weak black liquor therefrom. This is normally the case. At least one of the evaporation liquor tank(s) and may also be connected to an impregnation device preceding the cooking device to receive spent liquor therefrom.

In an embodiment comprising such an evaporation arrangement, the control system may be further configured to:

Determine a liquid level in the wash liquor buffer tank using an associated level sensor;

Determine if the liquid level is above a predetermined maximum level, and if so, control said evaporation arrangement to increase production and control the pumping means to increase a flow from said wash liquor buffer tank to said at least one of the evaporation liquor tank(s), and

Determine if said liquid level is below a predetermined minimum level, and if so, control said evaporation arrangement to reduce production and control the pumping means to decrease a flow from said wash liquor buffer tank to said at least one of the evaporation liquor tank(s).

Such a control system is based on an overall control strategy for the system, where production is maximized by minimizing disturbances in the system as a whole by means of i) handling excess filtrate from the post delignification washing by pumping it at an appropriate flow rate to the wash liquor buffer tank of the digester, and ii) controlling the evaporation arrangement to maintain the liquid level in the wash liquor buffer tank between the predetermined minimum and maximum levels. The predetermined minimum level corresponds to a wash liquid level needed to maintain operation of the cooking device for a certain (predetermined) time providing margin if disturbances would occur in downstream brown stock washing or further down in the process which (would reduce inflow of washing liquid to the buffer tank). The predetermined maximum level corresponds to a wash liquid level a predetermined level below the maximum capacity of the buffer tank providing margin for a ramping up time for production in the evaporation arrangement. This control strategy provides many advantages in terms of operation of the system. For instance, the evaporation arrangement can be kept working for a certain time even if the cooking device is stopped.

In a corresponding embodiment of the method according to the second aspect of the invention, the method comprises method steps corresponding to those described above with respect to the control system.

In an alternative embodiment comprising an evaporation arrangement, the control system may be further configured to: Determine liquid levels in the wash liquor buffer tank and in each of the at least one evaporation liquor tank using level sensors arranged in or at respective tanks;

Controlling the cooking device and each evaporator of said evaporation arrangement to maximize production in said cooking device while maintaining each determined liquid level within a range defined by corresponding maximum and minimum liquid levels for each tank.

Such a control system is based on an alternative overall control strategy for the system, where production is maximized and disturbances in the system as a whole are minimized by means of i) handling excess filtrate from the post delignification washing by pumping it at an appropriate flow rate to the wash liquor buffer tank, and ii) controlling the cooking device and each evaporator is to maintain the liquid level in the wash liquor buffer tank and the evaporation tanks between the respective minimum and maximum levels. The maximum and minimum liquid levels for each tank may be predetermined. Alternatively, the maximum and minimum liquid levels for each tank may be determined by the control system using a learning algorithm comprising adapting said levels to maximize production in said cooking device. Learning algorithms suitable for this purpose are well known in the art and will not be described in further detail here. This control strategy is advantageous since it makes effective use of all the tanks in the system, rather than controlling each part of the system as an independent sub-system as in most systems today.

In a corresponding alternative embodiment of the method according to the second aspect of the invention, the method comprises method steps corresponding to those described above with respect to the control system.

The features of the embodiments described above are combinable in any practically realizable way to form embodiments having combinations of these features. Further, all features and advantages of embodiments described above with reference to the first aspect of the invention may be applied in corresponding embodiments second aspect of the invention and vice versa. BRIEF DESCRIPTION OF THE DRAWINGS

Above discussed and other aspects of the present invention will now be described in more detail using the appended drawings, which show presently preferred embodiments of the invention, wherein: fig. 1 shows a prior art system for producing cellulose pulp; fig. 2 shows an embodiment of a system for producing cellulose pulp according to the first aspect of the invention; fig. 3 shows a flowchart illustrating an embodiment of the method according to the second aspect of the invention; fig. 4 shows a flowchart illustrating additional method steps of another embodiment of the method according to the second aspect of the invention, and fig. 5 shows a flowchart illustrating additional method steps of yet another embodiment of the method according to the second aspect of the invention.

DETAILED DESCRIPTION

Fig. 1 shows a schematic illustration of a prior art system for producing cellulose pulp. The system comprises a cooking device (also referred to as a digester) 2 configured to digest wood chips to produce cellulose pulp. The cooking device is fed from an impregnation device 9 which impregnates the chips with white/black liquor. The cooking device 2 is connected to a screening arrangement 3 via storage tower 19 in which the pulp is stored at medium consistency (about 12%). The screening arrangement is shown for illustrative purposes as a single screening device. The screening arrangement is connected to a (brown stock) washing arrangement 4 illustrated as a single wash press device (shown as a twin roll press). The washing arrangement is connected to an oxygen delignification arrangement 5 configured to delignify the cellulose pulp after washing. It is understood that the screening arrangement 3, the washing arrangement 4 and the delignification arrangement each normally comprises at least two stages. For instance, the screening arrangement may comprise at least one coarse screen device and at least one fine screen device. A wash liquor buffer tank 6 is arranged adjacent the cooking device and is f I uidica lly connected thereto as well as to the filtrate tank 4' of the washing arrangement 4 to receive filtrate from the washing arrangement for use as washing liquid in the washing section (bottom part) of the cooking device. A post-delignification washing arrangement 7 is connected to the delignification arrangement to wash the cellulose pulp after delignification, the washing arrangement comprising a first wash press device 7a and a second wash press device 7b with an intermediate storage tower 7c where washed pulp at medium consistency is stored. The first wash press device 7a is also connected to the second wash press device 7b via its filtrate tank 7b' to receive a filtrate flow therefrom for use as washing liquid. The filtrate tank 7b' is also connected to a liquor buffer tank 8 having a substantially greater volume than the filtrate tank 7b' in order to handle excess filtrate flow from the second wash press device which may be provided during disturbances in the system. The system further comprises an evaporation arrangement 10, also referred to as a recovery system. In the figure, the evaporation arrangement is illustrated as having two evaporation liquor tanks 10a, 10b and one evaporator 10c connected thereto, but may comprise further tanks and evaporators. The first evaporation liquor tank 10a is connected to the cooking device to receive weak black liquor therefrom, and is also connected to the wash liquor buffer tank 6 such that also excess wash liquor from the brown stock washing 4 will be sent to evaporation.

Fig. 2 shows an embodiment of a system for producing cellulose pulp according to the first aspect of the invention. The illustrated cooking device 2, screening arrangement 3, brown stock washing arrangement 4, delignification arrangement 5, wash liquor buffer tank 6, post delignification washing arrangement 7 (including 7a, 7b, 7c), impregnator 9 and evaporation arrangement 10 correspond to the devices/arrangements in fig. 1. The main difference from the prior art system in fig. 1 is that the liquor buffer tank 8 is omitted. Instead, the filtrate outlet of the second wash press device 7b is connected (via its filtrate tank 7b') to the wash liquor buffer tank 6 of the cooking device 2 via a conduit 11 and pumping means 12 to allow a filtrate flow from the second wash press to the wash liquor buffer tank 6. A further difference is that the first wash press 7a is provided with a connection to a source of fresh wash liquid, here illustrated as wash liquid conduit 14. Furthermore, the embodiment in fig. 2 is shown illustrated with an electronic control system 13 arranged in electronic communication with pumping means 12, 16 and the evaporator 10c. The control system 13 is configured to determine a required wash liquid flow for the first wash press device 7a, determine a filtrate flow obtained from the second wash press device 7b, and if the required wash liquid flow is lower than the filtrate flow, control the pumping means 12 to pump excess filtrate flow to the wash liquor buffer tank 6. The required wash liquor flow is determined using the cellulose pulp mass production inputted to the first wash press device 7a and the desired dilution factor. The control system may also be configured to control the source of fresh wash liquid 14 to provide wash liquid to said first wash press if the required wash liquid flow is higher than the filtrate flow. In embodiments, black liquor extracted from the (bottom) of the impregnator is also extracted and sent to the evaporation arrangement 7 (not illustrated in the figure).

The system is optionally provided with a level sensor 17 (shown with dotted lines) at/in the liquor buffer tank 6. In such an embodiment, the control system may be further configured to determine a liquid level in said wash liquor buffer tank using the level sensor 17, determine if the liquid level is above a predetermined maximum level, and if so, control the evaporator 10c to increase production and control additional pumping means 16 to increase a flow from the wash liquor buffer tank to the evaporation liquor tank 10a, and determine if the liquid level is below a predetermined minimum level, and if so, control the evaporator 10c to reduce production and control the additional pumping means 16 to decrease a flow from the wash liquor buffer tank to the evaporation liquor tank 10a.

The system is optionally provided with a level sensors 18a, 18b (shown with dotted lines) at/in the evaporation liquor tanks 10a, 10b. In such an embodiment, the control system may be further configured to determine liquid levels in the wash liquor buffer tank and in each evaporation liquor tank 10a, 10b using the level sensors 17, 18a, 18b, and to control the cooking device 2 and each evaporator 10c with the target to maximize production in the cooking device while maintaining each determined liquid level within a range defined by corresponding maximum and minimum liquid levels for each tank.

Although not illustrated in fig. 2, it is understood that also the level sensors 18, 18a, 18b are in electronic communication with the control system 13.

The devices and arrangements in the above described system are all well known in the art and do not need to be described in further detail here. For example, the impregnator may be an atmospheric vessel combining the function of a chip bin, flash tank and an impregnation vessel. Further, the cooking device may be of a known type having a plurality of cooking zones and a washing zone at its bottom. Further, it is understood that one or more pump device of known type is/are present to transport the pulp between the shown devices/arrangements. The system may also comprise further devices such as further screening devices for instance for screening the extracted liquor prior to evaporation.

Fig. 3 shows a flowchart illustrating an embodiment of the method according to the second aspect of the invention. The method comprises determining 101 a required wash liquid flow for the first wash press device, determining 102 a filtrate flow obtained from the second wash press device, and if said required wash liquid flow is lower than said filtrate flow, transporting 103 excess filtrate flow to said wash liquor buffer tank, and ff said required wash liquid flow is higher than said filtrate flow, supplementing 104 the filtrate flow with wash liquid from a tank and/or an external source of wash liquid. The method is performed continuously (is looped) during operation of the system such that excess filtrate flow, or lack of filtrate flow, is handled at all times. The method steps 101-104 can also be understood from the above described operation of the control system of the system in fig. 2.

Fig. 4 shows a flowchart illustrating additional method steps of another embodiment of the method according to the second aspect of the invention. The method steps shown in fig. 4 concern handling the liquid level in the liquor buffer tank 6 and are performed in addition to the method steps shown in fig. 3. The method comprises determining 105 a liquid level in the wash liquor buffer tank, determining 106 if said liquid level is above a predetermined maximum level, and if so, increasing 107 production in said evaporation arrangement and increasing a flow from said wash liquor buffer tank to said at least one of the evaporation liquor tank(s), and determining 108 if said liquid level is below a predetermined minimum level, and if so, decreasing 109 production in said evaporation arrangement and decreasing a flow from said wash liquor buffer tank to said at least one of the evaporation liquor tank(s). The method steps 105-109 can also be understood from the above described operation of the control system of the system in fig. 2.

Fig. 5 shows a flowchart illustrating additional method steps of yet another embodiment of the method according to the second aspect of the invention. These method steps concern handling the liquid level in the liquor buffer tank 6 and are performed in addition to the method steps shown in fig. 3 as an alternative embodiment to the embodiment in fig. 4. The method comprises determining 110 liquid levels in said wash liquor buffer tank and in each of said at least one evaporation liquor tank, determining 111 maximum and minimum levels for each tank, which in this embodiment are predetermined and may thus be read from a table. In other embodiments, the maximum and minimum liquid levels for each tank may be determined using a learning algorithm comprising adapting said levels to maximize production in said cooking device. The method further comprises controlling 112 said cooking device and each evaporator of said evaporation arrangement to maximize production in said cooking device while maintaining each determined liquid level within a range defined by corresponding maximum and minimum liquid levels for each tank.

The description above and the appended drawings are to be considered as non-limiting examples of the invention. The person skilled in the art realizes that several changes and modifications may be made within the scope of the invention. In particular, as explained above, the devices and arrangements shown above may be of other known types. The invention however applies in particular to systems having a post-delignification washing arrangement comprising wash press devices.