WO2024132474A1

WO2024132474A1 - A paper machine

Info

Publication number: WO2024132474A1
Application number: PCT/EP2023/084123
Authority: WO
Inventors: Scott WIGHTWICK
Original assignee: Valmet Ab
Priority date: 2022-12-21
Filing date: 2023-12-04
Publication date: 2024-06-27

Abstract

A paper machine (1; 101; 201; 301) and a method for controlling the paper machine. Said paper machine comprises at least two dryness sensors (9a-p) which are positioned at separate positions of a machine direction, (MD), of the paper machine, said dryness sensors being configured for measuring a dryness profile over a cross direction, (CD), of the paper web (7). Said paper machine further comprises at least two drying devices (13a-e) positioned at separate positions of the machine direction, (MD), of the paper machine and configured for drying said paper web in said paper machine and a control unit (11) which is connected to the dryness sensors and to the drying devices, wherein said control unit is configured to control said drying devices such that a drying of the paper web performed by said drying devices is adjusted in dependence of said measured dryness profile of the paper web from at least two different positions along the machine direction, (MD).

Description

A PAPER MACHINE

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a paper machine and to a method for controlling a paper machine.

BACKGROUND

Paper machines typically operate by injecting a stock into a forming gap in a forming section to create a paper web that moves through various machine sections to be formed, dewatered, dried, and optionally structured to form a finished paper. The paper web is transported on a series of wires or fabrics, e.g. felts, that are arranged in endless loops and that serve to form and protect the paper web and optionally also to create a structure in the paper web.

Since the stock typically has a water content of around 97 % or greater, removal of water is a main objective when producing paper and this takes place by a combination of pressing, sucking and heating. In some sections of the paper machine, water or air is transported through the wire or felt in order to suck water from the paper web or to allow a flow of hot air to reach the paper web.

A uniform moisture content or dryness profile of the paper over a cross direction, CD, of the paper web is advantageous. A variation in CD dryness profile can lead to paper breaks, Yankee coating issues, poor creping, reduced crepe blade life, variation in paper quality, poor quality reels, increased energy usage and machine clothing issues. Diagnosing the source of the moisture variation is often difficult.

In for example a Through Air Drying, TAD, paper machine an Infra-Red scanner is located after the TAD dryers. This is a traversing scanner/sensor which can measure a CD moisture profile of the paper web after it has been dried. Hereby a variation in CD moisture profile can be detected but from this information it is not possible to identify the source of the moisture issue in the machine direction, MD. Hereby, in today's solutions steam is simply added for compensation according to the measured CD moisture profile of the paper web. This may not be the most efficient or cost efficient solution.

SUMMARY

An object of the invention is to provide an improved paper machine and method for controlling a paper machine.

A further object is to provide a paper machine and a method for controlling a paper machine which provides a more uniform dryness profile over a cross direction of the paper web.

A further object is to improve process efficiency in a paper machine.

This is achieved by a paper machine and a method for controlling a paper machine and a computer program product according to the independent claims.

According to one aspect of the invention a paper machine is provided comprising different machine sections configured for at least forming, dewatering and drying a paper web being formed and transferred along a machine direction, MD, of the paper machine, wherein said paper machine further comprises:

- at least two dryness sensors which are positioned at separate positions of the machine direction, MD, of the paper machine, said dryness sensors being configured for measuring a dryness profile over a cross direction, CD, of the paper web;

- at least two drying devices positioned at separate positions of the machine direction, MD, of the paper machine and configured for drying said paper web in said paper machine; and

- a control unit which is connected to the dryness sensors and to the drying devices, wherein said control unit is configured to control said drying devices such that a drying of the paper web performed by said drying devices is adjusted in dependence of said measured dryness profile of the paper web from at least two different positions along the machine direction, MD. According to another aspect of the invention a method for controlling a paper machine is provided, said paper machine comprising different machine sections configured for at least forming, dewatering and drying a paper web being formed and transferred along a machine direction, MD, of the paper machine, said method comprising the steps of: measuring a dryness profile over a cross direction, CD, of the paper web in at least two different positions along a machine direction, MD, of the paper machine, wherein said measuring is performed by at least two dryness sensors which are positioned at separate positions of the machine direction, MD, of the paper machine;

- controlling at least two drying devices which are positioned at separate positions in the machine direction, MD, of the paper machine, such that a drying of the paper web performed by said drying devices is adjusted in dependence of said measured dryness profile of the paper web from at least two different positions along the machine direction, MD, said controlling being performed by a control unit which is connected to the dryness sensors and the drying devices.

Hereby the dryness profile over a cross direction of the paper web is known for at least two different positions of the machine direction, MD. Hereby a source of a moisture problem can be more easily identified, and a production process can be better optimized, and energy can be saved. A balance of vacuum dewatering and for example Through Air Drying, TAD, can be optimized if the source of a moisture problem is known whereby energy costs can be saved. Gas for TAD drying may be cheaper than electricity for vacuum drying. With a better diagnose of from where a dryness profile problem is originating the problem can be solved more quickly and downtime can be minimized whereby efficiency is improved and costs are saved. Furthermore, with a better CD moisture uniformity quality of the paper web is improved. By knowing more details about moisture profile changes throughout the machine direction the process can also be adapted faster for different process changes, such as different pulp types, chemistry changes, or other changes in the preparation of the stock for example fibre blending, refining changes, etc. Furthermore, by measuring the dryness profile at different machine direction positions, the performance of machine clothing can be better monitored, controlled and optimized and the performance of clothing cleaning devices (e.g. showers) can be monitored and optimized. In one embodiment of the invention said paper machine comprises at least three dryness sensors which are positioned at separate positions of the machine direction, MD, of the paper machine, wherein at least two of said at least three dryness sensors are positioned in different machine sections of the paper machine. Hereby dryness profile information is achieved from different machine sections whereby drying control can be improved.

In one embodiment of the invention said paper machine is a paper machine for producing structured tissue and comprises at least one structured fabric for transporting and forming the paper web through at least one of the machine sections.

In one embodiment of the invention said control unit further is configured to control said drying devices such that a drying of the paper web is adjusted also in dependence of cost for running the different types of drying devices. Hereby costs can be saved while quality of paper web is maintained.

In one embodiment of the invention said drying devices comprise at least two of: a vacuum dewatering box, a suction roll, a steam profiler, a press roll system, an Infrared dryer and a TAD, Through Air Drying, drying system, whereby said control unit is configured for controlling a drying effect of the drying devices and/or a drying effect profile over the cross direction of the paper web, by controlling a vacuum level in the vacuum dewatering boxes and/or suction rolls and/or varying a load in the press roll systems and/or varying a temperature in TAD drying systems and/or Infrared dryers and/or controlling profiling actuators in steam profilers.

In one embodiment of the invention said dryness sensors are configured for measuring dryness at, at least five positions over a cross direction of the paper web, i.e. the step of measuring a dryness profile over a cross direction, CD, of the paper web comprises measuring dryness at, at least five positions over a cross direction of the paper web. Hereby a dryness profile over a cross direction, CD, of the paper web is achieved.

In one embodiment of the invention at least one of said dryness sensors is a microwave based sensor and/or an IR scanner.

In one embodiment of the invention at least one of said dryness sensors is a microwave based sensor which is built into a ceramic surface of a vacuum dewatering or transfer box. In one embodiment of the invention said paper machine is a Through Air Drying, TAD, paper machine comprising a drying section comprising a Yankee dryer, an intermediate drying and dewatering section comprising a TAD drying system, a dewatering section and a forming section and wherein the at least two dryness sensors comprise at least one dryness sensor positioned in the intermediate drying and dewatering section and at least one dryness sensor positioned in the dewatering section.

In one embodiment of the invention one of the dryness sensors is positioned between the TAD drying system and the Yankee dryer and at least one of the dryness sensors is positioned in a vacuum dewatering box or a vacuum transfer box.

In one embodiment of the invention said step of controlling comprises determining which ones of the drying devices should be used and by which drying effect and possibly by which drying effect profile over the cross direction of the paper web, based on said measured dryness profile of the paper web from at least two different positions along the machine direction, MD.

In one embodiment of the invention said step of measuring a dryness profile over a cross direction of the paper web comprises measuring in at least three different positions along a machine direction, MD, of the paper machine, and in at least two different machine sections of the paper machine, wherein said measuring is performed by at least three dryness sensors which are positioned at separate positions of the machine direction, MD, of the paper machine.

In one embodiment of the invention said step of controlling at least two drying devices further comprises controlling said drying devices such that a drying of the paper web is adjusted also in dependence of cost for running the different types of drying devices.

In one embodiment of the invention said step of controlling at least two drying devices comprises controlling a drying effect of the drying devices and/or a drying effect profile over the cross direction of the paper web, by controlling a vacuum level in a drying device being a vacuum dewatering box and/or a suction roll and/or varying a load in a drying device being a press roll system and/or varying a temperature in a drying device being a TAD drying system and/or an Infrared dryer and/or controlling profiling actuators in a drying device being a steam profiler.

In one embodiment of the invention said method further comprises a step of controlling a process for maintenance of fabrics in the paper machine in dependence of said measured dryness profile of the paper web from at least two different positions along the machine direction. Hereby a quality or condition of the fabrics can be optimized whereby also a quality of the paper web will be optimized.

In one embodiment of the invention said method further comprises a step of adjusting tension of fabrics in the paper machine in dependence of said measured dryness profile of the paper web from at least two different positions along the machine direction. Hereby tension of fabrics can be optimized whereby the paper making process can be improved.

In one embodiment of the invention said step of controlling at least two drying devices comprises identifying which part of the paper machine is causing a non-uniform dryness profile of the paper web by analysing said measured dryness profile of the paper web from at least two different positions along the machine direction.

In one embodiment of the invention the method further comprises the step of optimizing one or more of the following paper machine parameters: forming geometry, headbox position, headbox slice opening, transfer points between inner and outer fabrics and stock/water temperature in dependence of said measured dryness profile of the paper web from at least two different positions along the machine direction. Hereby the paper making process in the paper machine can be further optimized.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1-4 are schematic drawings of paper machines according to different embodiments of the invention.

Figure 5 is a flow chart of a method for controlling a paper machine according to one embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS

Figures 1-4 show four different specific examples of paper machines 1; 101; 201; 301 comprising dryness sensors 9a-p according to the invention. These paper machines 1; 101; 201; 301 are all used in processes for producing structured tissue, i.e. they all comprise a machine section 3b; 103b; 203b; 303b comprising a fabric 21 which is a structured fabric. These are however, four specific examples with specific positions given for the dryness sensors 9a-p. The examples are further described in detail below. The invention is however not limited to these specific examples and these specific number of and positions of dryness sensors 9a-p and dryness devices 13a-e. First a more general description of the invention is given with reference to all the Figures 1-4.

A paper machine 1; 101; 201; 301 is provided comprising different machine sections 3a-d: 103a-d; 203a-e; 303a-d configured for at least forming, dewatering and drying a paper web 7 being formed and transferred on fabrics 21 along a machine direction, MD, of the paper machine 1; 101; 201; 301. The different machine sections may for example comprise a drying section 3a; 103a; 203a; 303a, a forming and dewatering section 3c, 3d; 103d; 203d; 303d, an intermediate drying and dewatering section 3b, a press and dewatering section 103c; 203e; 303c, a belt section 103b; 203c and/or a structuring section 3b; 103b; 203b; 303b. The drying section 3a; 103a; 203a; 303a may comprise a Yankee dryer 41.

According to the invention the paper machine 1; 101; 201; 301 further comprises at least two dryness sensors 9a-p which are positioned at separate positions of the machine direction, MD, of the paper machine. The dryness sensors 9a-p are configured for measuring a dryness profile over a cross direction, CD, of the paper web 7. The paper machine 1; 101; 201; 301 comprises furthermore at least two drying devices 13a-e which are positioned at separate positions of the machine direction, MD, of the paper machine and which are configured for drying said paper web 7 in said paper machine. The paper machine 1; 101; 201; 301 comprises furthermore a control unit 11 which is in communication with the dryness sensors 9a-p and with the drying devices 13a-e, i.e. the control unit 11 is connected to the dryness sensors 9a-p and the drying devices 13a-e, wherein the connection can be a wired connection or a wireless connection. Hereby the control unit 11 is in communication contact with the dryness sensors 9a-p and with the drying devices 13a-e. The control unit 11 is configured to control said drying devices 13a-e such that a drying of the paper web 7 performed by said drying devices 13a-e is adjusted in dependence of said measured dryness profile of the paper web 7 from at least two different positions along the machine direction, MD. Hereby the drying of the paper web 7 can be better controlled and balanced between the various drying devices. Different drying devices can be specifically adjusted in accordance with different measured dryness profiles of the paper web whereby a more uniform dryness profile of the paper web is achieved. Thanks to the different dryness sensors 9a-p being separated along the machine direction of the paper machine it can be identified better where a possible problem is located, and the effects of a process change can be better identified. Hereby the drying devices 13a-e throughout the machine direction of the paper machine can be controlled with more precision and a more efficient process is achieved. Furthermore, other process parameters such as forming geometry, headbox position, transfer points between different fabrics, fabric tensions, stock and water temperature can all be optimized more efficiently to maximize drying and process efficiency.

In some embodiments of the invention said paper machine 1; 101; 201; 301 comprises at least three dryness sensors 9a-p which are positioned at separate positions of the machine direction, MD, of the paper machine, wherein at least two of said at least three dryness sensors 9a-p are positioned in different machine sections 3a-d: 103a-d; 203a-e; 303a-d of the paper machine.

In some embodiments of the invention said paper machine 1; 101; 201; 301 is a paper machine for producing structured tissue. Hereby the paper machine comprises at least one fabric 21 which is a structured fabric for transporting and forming the paper web 7 through at least one of the machine sections 3b; 103b; 203b; 303b, which may be called a structuring section.

The dryness sensors 9a-p are configured for measuring a moisture profile, also called a dryness profile, over a cross direction of the paper web 7. The dryness sensors 9a-p may measure dryness at, at least five positions over a cross direction of the paper web 7. The dryness sensors 9a-p may be for example a microwave based sensor and/or an IR scanner. In some embodiments of the invention at least one of the dryness sensors is a microwave based sensor which is built into a ceramic surface of a vacuum dewatering and/or transfer box. Hereby a dryness profile is achieved over a cross direction of the paper web 7 for a number of different positions throughout the machine direction where the dryness sensors 9a-p are positioned. Hereby, it will be possible to better identify where a possible problem in the paper machine is located. It may also be possible to identify how different changes to process parameters will affect the dryness profile of the paper web 7 and where in the paper machine it is most efficient to compensate for such changes.

Some of the dryness sensors 9a-p are configured for measuring the dryness of the paper web 7 through the fabric while others measure from the paper side. The sensor type would be selected based on the position in the machine (ability to be able to mount the senor(s), impact/interference with the process, operating conditions (local environmental conditions such as temperature)) and which sensor would be expected to give the most accurate reading for that location given the expected moisture range in that location.

The drying devices 13a-e may comprise at least two of: a vacuum dewatering box 13a, a suction roll 13d, a steam profiler 13b, a press roll system 13e, an Infrared dryer and a TAD, Through Air Drying, drying system 13c. The control unit 11 is configured for controlling a drying effect of the drying devices and/or a drying effect profile over the cross direction of the paper web 7, by controlling a vacuum level in the vacuum dewatering boxes 13a and/or suction rolls 13d and/or varying a load in the press roll systems 13e and/or varying a temperature in TAD drying systems 13c, Infrared dryers and/or steam profilers 13b.

A vacuum dewatering box 13a is a vacuum box where vacuum is pulling water from the paper web 7 through the fabric and into the dewatering box. By sucking water from the paper web it is increasing the dryness of the paper. Normally the control of a vacuum box is the vacuum level within the vacuum box. Each vacuum box has a device which measures the amount of vacuum within the vacuum box. The Operator normally sets how much vacuum he/she wants in each vacuum box and this vacuum level can be adjusted by a manual or automatic valve to regulate the vacuum level. With this invention the vacuum level of a dewatering box can be adjusted with more precision in order to provide a uniform dryness profile of the paper web 7. A vacuum dewatering box 13a can also be called a molding box if it also shapes the paper web. A vacuum dewatering box 13a can also be called a fabric dewatering box or a uhle box if it used against a fabric that is not carrying the paper web 7 and the action of the vacuum is to clean and remove water from the fabric. A steam box 13b is also called a steam profiler. The purpose of the steam profiler is to increase the dryness of the paper and even out moisture variations across the width of the paper (cross direction on the machine). The steam profiler comprises multiple independently controlled profiling zones across the width of the machine. An individual profiling zone is typically 100mm wide but can be narrower or wider. Depending upon the width of a machine a steam profiler can have 50 or more individual profiling zones. By applying steam to a particular area of the wet paper the steam increases the temperature of the water in the paper in that specific area reducing the viscosity of the water in that area. Water with a lower viscosity can be more easily removed from the paper web by applying vacuum in a dewatering box increasing the dryness of the paper in that area. Therefore, by measuring the dryness profile of the paper web 7 across its width it can be seen which areas are wetter and dryer and the individual profiling zones of the steam profiler can be controlled accordingly. For example, if an area of the paper web is wetter than surrounding areas, the corresponding steam profiler zone can be controlled to admit more steam in this wetter area to decrease the viscosity of the water in this area and thus more water will be removed from the paper web in this area when it passes over the next vacuum dewatering box 13a. Conversely in dryer areas the corresponding steam profiling zones will close more to reduce the amount of steam in that area.

A TAD, Through Air Drying, drying system 13c comprises one or more TAD rolls 31 and corresponding TAD hoods 32. Hot air is blown from the TAD hoods 32 through the paper web 7, supported by fabric 21, into the TAD rolls 31. In another TAD configuration hot air can be blown from inside the TAD rolls 31 through the paper web 7, through the fabric 21 supporting the web into the TAD hoods 32. The temperature of the TAD drying systems can be varied.

A suction roll 13d is a special roll used to dewater the paper web 7. The outer shell of the roll is normally some type of metal (commonly bronze) which has thousands of small holes drilled into it to create a perforated roll surface. Inside the roll is a stationary component in the shape of a long cylindrical tube. Part of this tube is open and vacuum is applied to the inside of the tube. The tube can be rotated inside the roll so that the opening (vacuum zone) in the tube is lined up with where the felt and paper web is contacting the outer shell of the suction roll. As the roll shell rotates the felt and paper web pass over the stationary vacuum zone and vacuum pulls water from the paper web and felt through the holes in the suction roll shell thus dewatering the paper web increasing its dryness. The vacuum level inside the suction roll is measured and can be varied/controlled by the Operator.

A press roll system 13e may be used in different parts of a paper system and may be used for different purposes whereof some of them are related to drying. A load can be varied in the press roll systems for adjusting a drying effect. All paper machine concepts use a Transfer or Pressure roll to transfer the paper web from the last fabric (could be a structured fabric, felt or belt) to the Yankee 41. The transfer mechanism is facilitated by pressure (or load) forcing the sheet on the fabric to be pressed against the surface of the Yankee. Not much (if any) drying takes place in the step. In an eTAD paper machine a Visconip press roll system is used. The purpose of the Visconip press roll is to remove water from the paper web/sheet and to transfer the sheet to a Backing roll. The hydraulic press unit inside the press roll conforms to the shape of the Backing roll, and produces a uniform nip load over a very wide range of linear loads. The press unit inside the roll consists of three hydraulic fluid chambers, which are positioned parallel to each other in the cross-machine direction, in the press unit. The chambers apply pressure to the belt (Visconip roll cover), which in turn presses the sheet. By controlling the pressure in the chambers, the linear pressing load can be adjusted. Each chamber can be adjusted individually, which makes it possible to optimize the pressing load curve to either maximise water removal or to provide a balance between water removal and increasing bulk/caliper. A Symbelt press roll is used in NTT and QRT paper machines. This is similar to a Visconip press roll in eTAD. The purpose of the Symbelt press roll is to remove water from the paper web and to transfer the paper web from one fabric to the next. In the case of the NTT and QRT tissue machine concepts the transfer is from a felt to a belt. The internals of the Symbelt roll are different compared to the Visconip roll. The Symbelt has a single hydraulically loaded press shoe extending across the width of the machine which is loaded against the inside of the Symbelt cover. The Symbelt roll is loaded against a solid Counter roll with the felt, belt and paper web sandwiched between the Symbelt and Counter rolls. As the paper passes through this nip point the load applied by the Symbelt squeezes water out of the paper web increasing the dryness of the paper while at the same time transferring the paper web from the felt to the belt. The amount of load applied by the Symbelt press can be adjusted by the Operator which can vary the dryness of the paper.

Infrared dryer is another possible drying device. The principle of an infrared dryer is similar to a steam profiler except that it is using infrared heat lamps to heat different zones of the paper instead of steam. Infrared dryers use electricity and can be costly to run (depending on the local cost of electricity) and the cost of steam is usually cheaper, hence steam boxes at more typically used instead of infrared dryers.

Hereby, according to the invention the drying devices 13a-13e can be controlled in dependence of a measured dryness profile of the paper web 7 as measured from different positions along the machine direction of the paper machine and hereby the paper making process can be better optimized. A dryness uniformity of the paper web can be more easily achieved which will provide better quality of the paper and both process efficiency and process cost can be better optimized by using the drying devices in a better way.

The control unit 11 may further be configured to control said drying devices 13a-e such that a drying of the paper web 7 is adjusted also in dependence of cost for running the different types of drying devices. Some of the drying device, such as vacuum dewatering boxes, suction rolls, press roll systems and Infrared Dryers, use only electrical energy for the drying, while other drying devices, such as TAD drying systems and steam boxes, also use gas and/or steam for the drying, whereby the cost of electrical energy and the cost of gas and/or steam can be used as factors in the control unit when controlling the drying devices to dry the paper web. Hereby the drying of the paper web can be controlled to be as cost efficient as possible.

In one embodiment of the invention said paper machine 1 is a Through Air Drying, TAD, paper machine 1 comprising a drying section 3a comprising a Yankee dryer 41, an intermediate drying and dewatering section 3b comprising a TAD drying system 13c, a dewatering section 3c and a forming section 3d. In this embodiment there are at least two dryness sensors 9a-p and at least one dryness sensor 9a, 9f, 9g, 9h is positioned in the intermediate and dewatering section 3b and at least one dryness sensor 9b, c, d, e, e' is positioned in the dewatering section 3c. In one embodiment one of the dryness sensors 9a is positioned between the TAD drying system 13c and the Yankee dryer 41 and at least one of the dryness sensors 9b-9g is positioned in one of the drying devices 13a-c being a vacuum dewatering box 13a-c.

In Figures 1-4 four different specific examples of paper machines 1; 101; 202; 301 in which this invention can be used are schematically illustrated. Each example is briefly described below.

Figure 1 shows schematically a Through Air Drying, TAD, paper machine 1 which comprises a drying section 3a comprising a Yankee dryer 41, an intermediate drying and dewatering section 3b comprising a TAD drying system 13c, a dewatering section 3c and a forming section 3d. The TAD drying system 13c comprises in this example two TAD rolls 31 and corresponding TAD hoods 32. In this example as much as eight dryness sensors 9a-h are illustrated. They are all positioned at separate positions throughout the machine direction of the TAD paper machine 1. This is only to illustrate different suitable positions for dryness sensors 9a-h. The number of dryness sensors 9a-h can vary. According to one embodiment of the invention at least two dryness sensors 9a-h are provided and according to another embodiment at least three dryness sensors 9a-h are provided and according to another embodiment at least four dryness sensors 9a-h are provided. For example one, two, three, four or even more dryness sensors 9a, 9f, 9g, 9h can be positioned in the intermediate drying and dewatering section 3b. Four suitable positions are illustrated in Figure 1. One dryness sensor 9a is positioned between the TAD rolls 31 and the Yankee dryer 41. Another dryness sensor 9g is positioned in, or in close connection with, a vacuum transfer box 14a provided for transferring the paper web 7 from the dewatering section 3c to the intermediate drying and dewatering section 3b. Vacuum dewatering boxes are often positioned on the opposite side of the fabric 21 compared to where the paper web 7 is transferred on the fabric 21 and as discussed above a dryness sensor 9a-p can be positioned in a ceramic surface of a vacuum dewatering box and hereby measure a dryness profile of the paper web 7 through the fabric 21. Another dryness sensor 9f is shown positioned on the paper web 7 side between the TAD rolls 31 and the dewatering section 3c. Still another dryness sensor 9h is shown positioned between the two TAD rolls 31. Four dryness sensors 9b,c,d,e are also illustrated to be positioned in the dewatering section 3c. The number of dryness sensors may however vary as discussed above. Three of these dryness sensors 9c,d,e are provided in, or in close connection with, one vacuum dewatering box 13a each which vacuum dewatering boxes 13a are positioned at separate positions along the fabric 21 in the dewatering section 3c. One dryness sensor 9b is positioned in connection with a vacuum transfer box 14b provided for transferring the paper web 7 from the forming section 3d to the dewatering section 3c. All these dryness sensors 9b,c,d,e are shown positioned on the side of the fabric 21 opposite of the paper web 7. However, one or more dryness sensors may also be provided on the paper web side 7. This is illustrated by one additional dryness sensor 9e'. The dryness sensors 9a-h can as discussed above be for example microwave based sensors and/or IR scanners. By measuring dryness profile of the paper web 7 at many different positions throughout the machine direction of the paper machine 1 the different drying devices 13a, 13b, 13c can be controlled accordingly, i.e. drying is adjusted at different positions of the paper machine in accordance with measured dryness profiles of the paper web for different positions. Vacuum levels in vacuum dewatering boxes 13a can be adjusted and temperature of steam from steam boxes 13b can be adjusted and temperature in the TAD drying system 13c can be adjusted. By independently control of different drying devices 13a-c a more efficient process control is achieved and a better quality of paper is achieved because a more uniform dryness profile over a cross section of the paper web 7 can be achieved. Furthermore, by optimizing process control costs are saved. The control unit 11 is connected to the dryness sensors 9a-9h and to the drying devices 13a-13c which is schematically illustrated by a dotted arrow to the control unit 11. The connection can be by wire or a wireless connection and hereby the control unit 11 is in communication with the dryness sensors 9a-9h and the drying device 13a-13c.

Figure 2 shows schematically a NTT, New Tissue technology, paper machine 101 which comprises a drying section 103a comprising a Yankee dryer 41, a belt section 103b, a press and dewatering section 103c and a forming section 103d. In this example two dryness sensors 9i, 9j are illustrated, where one dryness sensor 9i is positioned in the press and dewatering section on the paper web side 7 of the fabric 21 between a suction roll 13d and a press roll system 13e. The suction roll 13d and the press roll system 13e are examples of drying devices in the NTT paper machine 101. A steam box 13b is furthermore provided in the press and dewatering section 103c in connection with the suction roll 13d but on the opposite side of the fabric 21 and paper web 7 compared to the position of the suction roll 13d. The other dryness sensor 9j is positioned in the belt section 103b on the paper web side of the fabric 21. The control unit 11 is in communication with the dryness sensors 9i, 9j and with the drying devices 13b, 13d, 13e which is schematically illustrated by dotted lines.

Figure 3 shows schematically another type of paper machine 201 which comprises a drying section 203a comprising a Yankee dryer 41, a structuring section 203b, a belt section 203c, a press and dewatering section 203d and a forming section 203e. In this example three dryness sensors 9k, 91, 9m are illustrated, whereof one dryness sensor 9k is positioned in the belt section 203c on the paper web side 7, one dryness sensor 91 is positioned in, or in close connection with a vacuum dewatering box 13a which is provided in the structuring section 203b and one dryness sensor 9m is positioned in the structuring section 203b between the dewatering box 13a and the drying section 203a. In this paper machine 201 further drying devices are provided in the form of a vacuum dewatering box 13a in the press and dewatering section 203d, a press roll system 13e positioned at the transfer of the paper web 7 from the press and dewatering section 203d to the belt section 203c and a suction roll 13d and an opposing steam box 13b in the press and dewatering section 203d. The control unit 11 is in communication with the dryness sensors 9k-m and with the drying devices 13a, 13b, 13d, 13e which is schematically illustrated by dotted lines.

Figure 4 shows schematically another type of paper machine 301 which comprises a drying section 303a comprising a Yankee dryer 41, a structuring section 303b, a press and dewatering section 303c and a forming section 303d. Drying devices 13a, 13b, 13d, 13e provided in this paper machine 301 are a vacuum dewatering box 13a provided in the structuring section 303b, a steam box 13b and a suction roll 13d provided in the press and dewatering section 303c in connection with each other on opposite sides of the fabric 21 and paper web 7 and a press roll system 13e provided at a position where the paper web 7 is transferred from the press and dewatering section 303c to the structuring section 303b. Three dryness sensors 9n, 9o, 9p are shown in this example, whereof one dryness sensor 9n is positioned in connection with the press roll system 13e, one dryness sensor 9o is positioned in or in connection with the vacuum dewatering box 13a in the structuring section 303b and one dryness sensor 9p is positioned in the structuring section 303b between the vacuum dewatering box 13a and the Yankee dryer 41. The control unit 11 is in communication with the dryness sensors 9n-p and with the drying devices 13a, 13b, 13d, 13e which is schematically illustrated by dotted lines. In Figure 5 a flow chart of a method for controlling a paper machine 1; 101; 201; 301 according to one embodiment of the invention is shown. The paper machine comprises as described above different machine sections 3a-d: 103a-d; 203a-e; 303a-d configured for at least forming, dewatering and drying a paper web 7 being formed and transferred along a machine direction, MD, of the paper machine 1; 101; 201; 301. The method steps are described below with reference to the flow chart:

SI: Measuring a dryness profile over a cross direction, CD, of the paper web 7 in at least two different positions along a machine direction, MD, of the paper machine, wherein said measuring is performed by at least two dryness sensors 9a-p which are positioned at separate positions of the machine direction, MD, of the paper machine. In some embodiments said step of measuring a dryness profile over a cross direction of the paper web 7 comprises measuring in at least three different positions along a machine direction, MD, of the paper machine, and in at least two different machine sections 3a-d: 103a-d; 203a-e; 303a-d of the paper machine, wherein said measuring is performed by at least three dryness sensors 9a-p which are positioned at separate positions of the machine direction, MD, of the paper machine 1; 101; 201; 301. The step of measuring a dryness profile over a cross direction, CD, of the paper web 7 may comprise to measure dryness at, at least five positions over a cross direction of the paper web 7.

S2: Controlling at least two drying devices 13a-e which are positioned at separate positions in the machine direction, MD, of the paper machine, such that a drying of the paper web 7 performed by said drying devices 13a-e is adjusted in dependence of said measured dryness profile of the paper web 7 from at least two different positions along the machine direction, MD, said controlling being performed by a control unit 11 which is in communication contact with the dryness sensors 9a-p and the drying devices 13a-e. In some embodiments said step of controlling at least two drying devices 13a-e further comprises controlling said drying devices 13a-e such that a drying of the paper web 7 is adjusted also in dependence of cost for running the different types of drying devices. In some embodiments said step of controlling at least two drying devices 13a-e comprises controlling a drying effect of the drying devices and/or a drying effect profile over the cross direction of the paper web 7, by controlling a vacuum level in a drying device being a vacuum dewatering box 13a and/or a suction roll 13d and/or varying a load in a drying device being a press roll system 13e and/or varying a temperature in a drying device being a TAD drying system 13c, Infrared dryer and/or controlling the profiling actuators in steam profiler 13b. Some of the drying devices 13a-13e can also be controlled differently over a cross direction of the paper web 7. A drying effect can be provided only to a part of a cross direction of the paper web or differently for different parts. The drying effect over a cross direction of the paper web can hereby be controlled in dependence of measured dryness profiles of the paper web.

Said step of controlling S2 may comprise to determine which ones of the drying devices 13a- e should be used and by which drying effect and possibly by which drying effect profile over the cross direction of the paper web (7), based on said measured dryness profile of the paper web (7) from at least two different positions along the machine direction, MD.

Said step of controlling S2 at least two drying devices 13a-e may further comprise to identify which part of the paper machine is causing a non-uniform dryness profile of the paper web 7 by analyzing said measured dryness profile of the paper web 7 from at least two different positions along the machine direction.

The method according to the invention may comprise one or more of the additional method steps as described below:

S3: Controlling a process for maintenance of fabrics 21 in the paper machine in dependence of said measured dryness profile of the paper web 7 from at least two different positions along the machine direction. Maintenance of fabrics (also called belts and felts) can be cleaning and/or changing of fabrics. Cleaning can be optimized for example by adjusting shower pressures of cleaning devices. An optimum time for changing of fabrics can be determined by analysing said measured dryness profile of the paper web 7 from at least two different positions along the machine direction.

S4: Adjusting tension of fabrics 21 in dependence of said measured dryness profile of the paper web 7 from at least two different positions along the machine direction.

S5: Optimizing one or more of the following paper machine parameters: forming geometry, headbox position, headbox slice opening, transfer points between inner and outer fabrics and stock/water temperature in dependence of said measured dryness profile of the paper web 7 from at least two different positions along the machine direction. The method according to the invention, i.e. the steps of measuring a dryness profile and controlling dryness devices 13a-e can be performed continuously. This will enable a greater control when there is a significant change of the process, such as for example a significant increase of machine speed, a furnish ratio change, a pulp change, a grade change, a stock preparation change (for example refining load change), any paper making trial (for example new papermaking clothing style, chemical trials, new product grade development) or a change of wet end chemistry.

According to the invention a computer program product is also provided comprising instructions which, when executed in a processor 11a in the control unit 11 provided in the paper machine 1; 101; 201; 301 as described above, cause the control system 11 to perform the method as described above according to the invention. The control unit 11 comprises further a memory for storing the computer program instructions.

Claims

1. A paper machine (1; 101; 201; 301) comprising different machine sections (3a-d: 103a-d; 203a-e; 303a-d) configured for at least forming, dewatering and drying a paper web (7) being formed and transferred along a machine direction, MD, of the paper machine (1; 101; 201; 301), wherein said paper machine further comprises:

- at least two dryness sensors (9a-p) which are positioned at separate positions of the machine direction, MD, of the paper machine, said dryness sensors being configured for measuring a dryness profile over a cross direction, CD, of the paper web (7);

- at least two drying devices (13a-e) positioned at separate positions of the machine direction, MD, of the paper machine and configured for drying said paper web in said paper machine; and

- a control unit (11) which is connected to the dryness sensors (9a-p) and to the drying devices (13a-e), wherein said control unit (11) is configured to control said drying devices (13a-e) such that a drying of the paper web (7) performed by said drying devices (13a-e) is adjusted in dependence of said measured dryness profile of the paper web (7) from at least two different positions along the machine direction, MD.

2. Paper machine according to claim 1, wherein said paper machine (1; 101; 201; 301) comprises at least three dryness sensors (9a-p) which are positioned at separate positions of the machine direction, MD, of the paper machine, wherein at least two of said at least three dryness sensors (9a-p) are positioned in different machine sections (3a-d: 103a-d; 203a-e; 303a-d) of the paper machine.

3. Paper machine according to claim 1 or 2, wherein said paper machine (1; 101; 201; 301) is a paper machine for producing structured tissue and comprises at least one structured fabric for transporting and forming the paper web through at least one of the machine sections (3b; 103b; 203b; 303b).

4. Paper machine according to any one of the preceding claims, wherein said control unit (11) further is configured to control said drying devices (13a-e) such that a drying of the paper web (7) is adjusted also in dependence of cost for running the different types of drying devices.

5. Paper machine according to any one of the preceding claims, wherein said drying devices (13a-e) comprise at least two of: a vacuum dewatering box (13a), a suction roll (13d), a steam profiler (13b), a press roll system (13e), an Infrared dryer and a TAD, Through Air Drying, drying system (13c), whereby said control unit (11) is configured for controlling a drying effect of the drying devices and/or a drying effect profile over the cross direction of the paper web, by controlling a vacuum level in the vacuum dewatering boxes (13a) and/or suction rolls (13d) and/or varying a load in the press roll systems (13e) and/or varying a temperature in TAD drying systems (13c) and/or Infrared dryers and/or controlling profiling actuators in steam profilers (13b).

6. Paper machine according to any one of the preceding claims, wherein said dryness sensors (9a-p) are configured for measuring dryness at, at least five positions over a cross direction of the paper web (7).

7. Paper machine according to any one of the preceding claims, wherein at least one of said dryness sensors (9a-p) is a microwave based sensor and/or an IR scanner.

8. Paper machine according to any one of the preceding claims, wherein at least one of said dryness sensors (9a-p) is a microwave based sensor which is built into a ceramic surface of a vacuum dewatering or transfer box (13a).

9. Paper machine according to any one of the preceding claims, wherein said paper machine (1) is a Through Air Drying, TAD, paper machine (1) comprising a drying section (3a) comprising a Yankee dryer (41), an intermediate drying and dewatering section (3b) comprising a TAD drying system (13c), a dewatering section (3c) and a forming section (3d) and wherein the at least two dryness sensors (9a-p) comprise at least one dryness sensor (9a) positioned in the intermediate drying and dewatering section (3b) and at least one dryness sensor (9b, c, d) positioned in the dewatering section (3c).

10. Paper machine according to claim 9, wherein one of the dryness sensors (9a) is positioned between the TAD drying system (13c) and the Yankee dryer (41) and at least one of the dryness sensors (9b-9g) is positioned in a vacuum dewatering box (13a) or a vacuum transfer box (14a, 14b).

11. A method for controlling a paper machine (1; 101; 201; 301) comprising different machine sections (3a-d: 103a-d; 203a-e; 303a-d) configured for at least forming, dewatering and drying a paper web (7) being formed and transferred along a machine direction, MD, of the paper machine (1; 101; 201; 301), said method comprising the steps of: measuring (SI) a dryness profile over a cross direction, CD, of the paper web (7) in at least two different positions along a machine direction, MD, of the paper machine, wherein said measuring is performed by at least two dryness sensors (9a-p) which are positioned at separate positions of the machine direction, MD, of the paper machine;

- controlling (S2) at least two drying devices (13a-e) which are positioned at separate positions in the machine direction, MD, of the paper machine, such that a drying of the paper web (7) performed by said drying devices (13a-e) is adjusted in dependence of said measured dryness profile of the paper web (7) from at least two different positions along the machine direction, MD, said controlling being performed by a control unit (11) which is connected to the dryness sensors (9a-p) and the drying devices (13a-e).

12. Method according to claim 11, wherein said step of controlling (S2) comprises determining which ones of the drying devices (13a-e) which should be used and by which drying effect and possibly by which drying effect profile over the cross direction of the paper web (7), based on said measured dryness profile of the paper web (7) from at least two different positions along the machine direction, MD.

13. Method according to claim 11 or 12, wherein said step of measuring (SI) a dryness profile over a cross direction of the paper web (7) comprises measuring in at least three different positions along a machine direction, MD, of the paper machine, and in at least two different machine sections (3a-d: 103a-d; 203a-e; 303a-d) of the paper machine, wherein said measuring is performed by at least three dryness sensors (9a- p) which are positioned at separate positions of the machine direction, MD, of the paper machine (1; 101; 201; 301).

14. Method according to any one of the claims 11-13, wherein said paper machine (1; 101; 201; 301) is a paper machine for producing structured tissue and comprises at least one structured fabric for transporting and forming the paper web through at least one of the machine sections (3b; 103b; 203b; 303b).

15. Method according to any one of the claims 11-14, wherein said step of controlling (S2) at least two drying devices (13a-e) further comprises controlling said drying devices (13a-e) such that a drying of the paper web (7) is adjusted also in dependence of cost for running the different types of drying devices.

16. Method according to any one of the claims 11-15, wherein said step of controlling (S2) at least two drying devices (13a-e) comprises controlling a drying effect of the drying devices and/or a drying effect profile over the cross direction of the paper web (7), by controlling a vacuum level in a drying device being a vacuum dewatering box and/or a suction roll and/or varying a load in a drying device being a press roll system and/or varying a temperature in a drying device being a TAD drying system and/or an Infrared dryer and/or controlling profiling actuators in a drying device being a steam profiler.

17. Method according to any one of the claims 11-16, wherein the step of measuring (SI) a dryness profile over a cross direction, CD, of the paper web (7) comprises measuring dryness at, at least five positions over a cross direction of the paper web (7).

18. Method according to any one of the claims 11-17, wherein said paper machine is a Through Air Drying, TAD, paper machine (1) and wherein the step of measuring a dryness profile over a cross direction, CD, of the paper web (7), comprises measuring a dryness profile of the paper web (7) when it is passing between a TAD drying system (13c) provided in the TAD paper machine (1) and a Yankee dryer (41) provided in the TAD paper machine (1) and measuring a dryness profile of the paper web (7) by a dryness sensor (9b-9e) positioned in a drying device (13a) being a vacuum dewatering box (13a) and/or vacuum transfer box (14b) positioned in a machine section (3c) of the TAD paper machine (1) being a forming and dewatering section (3c).

19. Method according to any one of the claims 11-18, wherein said method further comprises a step of controlling (S3) a process for maintenance of fabrics (21) in the paper machine in dependence of said measured dryness profile of the paper web (7) from at least two different positions along the machine direction.

20. Method according to any one of the claims 11-19, wherein said method further comprises a step of adjusting (S4) tension of fabrics (21) in the paper machine in dependence of said measured dryness profile of the paper web (7) from at least two different positions along the machine direction.

21. Method according to any one of the claims 11-20, wherein said step of controlling (S2) at least two drying devices (13a-e) comprises identifying which part of the paper machine is causing a non-uniform dryness profile of the paper web (7) by analysing said measured dryness profile of the paper web (7) from at least two different positions along the machine direction.

22. Method according to any one of the claims 11-21, further comprising the step of optimizing (S5) one or more of the following paper machine parameters: forming geometry, headbox position, headbox slice opening, transfer points between inner and outer fabrics and stock/water temperature in dependence of said measured dryness profile of the paper web (7) from at least two different positions along the machine direction.

23. A computer program product comprising instructions which, when executed in a processor in a control unit (11) provided in a paper machine (1; 101; 201; 301) according to any one of the claims 1-10, cause the control system (11) to perform the method according to any one of the claims 11-22.