FI89284C - Impulse drying method and apparatus for removing water from a paper or cardboard web - Google Patents

Description

39284

Impulse drying method and apparatus for removing water from a paper or board web Impulstorkningsförfarande och -ordordör för att 5 avlägsna vatten frän en pappers- eller kartonbana

The invention relates to an impulse drying method for removing water from a paperboard or paper web, which method comprises a pressing step or steps in which the web to be pressed is guided against the surface of a cylinder or roll heated to a temperature above 100 ° C so that the aqueous layer against the surface evaporates and the steam thus generated blows water from the web fiber into the pressing fabric in contact with the opposite surface of the web, and wherein the method presses the web against the surface of the hot cylinder or roll in one or more pressing steps, at least one of which is a long nip pressing step.

The invention further relates to an impulse drying device for carrying out the method of the invention for removing water from a paper or board web, which device comprises a hot cylinder or roll whose surface is heated by heating devices inside and / or outside the cylinder, which device comprises a press shoe device with a long nip press shoe, which forms a long compression zone with said hot cylinder and through which a web to be pressed against the surface of the hot cylinder and a water-receiving compression fabric on the opposite side of the web and a compression conveyor belt on its opposite side are arranged.

It is known from the paper web that the removed water is obtained by pressing so that the dry matter content of the web is also 40-45%. The rest of the water has had to be removed by evaporation, which consumes substantially more energy than the dewatering obtained by pressing per unit mass.

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It has already been known to increase the efficiency of dewatering from a paper web by raising the temperature of the web going to the press and the water in it, e.g. with steam boxes. Thus, the viscosity of the water has been reduced and the elastic properties of the web have been changed, whereby the removal of water-5 in normal roll nips or so-called in long nipples becomes more effective. It has not always been possible to achieve a sufficient increase in dry matter by these means, but a significant part of the so-called even free water, which is not a hydrogen bond attached to the fibrous material, has had to be removed from the web by evaporation.

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In the paper machine, the so-called closed press sections in which one or generally more press nips are formed in connection with the central roll. An example of such a previously known press section is the press section marketed by the applicant under the trademark "Sym-Press II", the smooth-surfaced center roll of which is a roll with a larger diameter than other press rolls, usually made of stone, usually granite. As an inhomogeneous natural material with low tensile strength, granite is quite problematic in mechanical engineering. If a granite roll were to be heated, its temperature-dependent deformations are nonlinear and difficult to predict. Granite as a press roll material has relatively good web release properties, which is at least partly due to its popularity. However, there is room for improvement in release properties, especially for unbleached paper grades.

25 As the production speeds of paper machines increase, dewatering by nip pressing has become a bottleneck limiting the increase in speed. This is because the press-nips formed by the pair of rollers are short in area, so at high speeds the residence time of the web in these press-nips remains short. However, especially due to the flow resistance of the fibrous structure of the web, water needs a certain amount of time to leave the web on the hollow surface of the roll or the press fabric.

If an attempt is made in nip presses to increase the dewatering efficiency by increasing the nip pressure, a certain line pressure is reached at the limit where increasing the nip pressure no longer helps, as the web structure can no longer withstand the compression.

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Pulse drying and hot pressing methods are also known, for which reference is made, by way of example, to U.S. Patent No. 4,324,613, according to which a paper web is pressed in a roll nip in which another roll or cylinder is heated to a temperature above 100 ° C by surface heating.

In said nip, the surface water of the paper web is made to evaporate and the pressurized steam blows water pressed into the intermediate spaces of the paper fiber into the press felt. The dry matter content achieved by this known hot pressing method is quite good, but the problem is the short nip time of the high speed machine 10, because the pressing time of the roll nip is only about 1 ... 3 ms, so that evaporation does not start properly and water is not blown out of the fiber. the temperature of the hot cylinder or roll is not very high. The high temperature of the roll causes problems, especially with respect to the compression fabric and the durability of the rolls.

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With regard to the applicant's recent inventions related to the web compression treatment of the present invention, reference is made as examples to FI applications Nos. 871870, 870309, 874136, 881711, 882312 and 891380.

It is already known to use an ultrasonic field to enhance the separation of water from materials. Improving the dewatering of the paper web using ultrasound is already known in some special applications, e.g. the applicant's FI patent 62,692 and Oy Tampella Ab's FI publication 77,286, which, however, do not disclose 25 hot pressing methods.

For example, in the drying of peat, laboratory-scale experiments have caused the final dry matter to rise significantly by applying an ultrasonic field to the compressed peat mass.

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The object of the present invention is to develop previously known impulse drying methods and devices, because impulse drying offers quite high potential possibilities for raising the dry matter, so that the energy costs and equipment costs of evaporative dewatering can be substantially reduced.

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It is a further object of the present invention to enhance known impulse drying methods and apparatus so that a reasonable surface temperature of the hot cylinder can be used without problems with the durability of the compression fabrics and the hot cylinder or rolls.

It is a non-essential additional object of the invention to provide a method and apparatus by which the transverse moisture profile of the web can be controlled by rapid and precise control devices in impulse drying without always having to use expensive mechanical-hydraulic compression shoe deflection and compression pressure control devices.

In order to achieve the above and later objects, the method of the invention is mainly characterized in that in said long nip pressing step an ultrasonic field is applied to the pulse-dried web, the oscillation frequency spectrum and intensity being selected that the water is removed from the web during the pulse drying step or steps before the water has time to bind, and that the dry matter content of the web entering the combined pulse drying and ultrasonic treatment step is in the range KAb -25-75% and that after said combined treatment step the substance content KA ^, is in the range ΚΑΜ 50 ... 80%.

The device according to the invention, in turn, is mainly characterized in that an ultrasonic transmitter or transmitters are arranged in said long nip press shoe and the device comprises electrical power supply and control devices 30 for supplying electrical power to said ultrasonic transmitters. that the device uses a plurality of ultrasonic transmitters in the transverse direction of the web, to which adjustable electrical power 35 can be applied to control the transverse moisture profile of the web.

In the present invention, an ultrasonic field is used in pulse drying in a long nip region to enhance the water expansion from the fibers. The ultrasound shakes off the water adhering to the fibers, which remains free inside the fiber 5 of the web. In the invention, the ultrasound is applied specifically to the web under pulse drying. In this case, the ultrasonically removed water can be removed so quickly that it does not have time to re-bind. By means of impulse drying mechanisms, water can be transferred from inside the web to a felt or similar water-receiving tissue.

It has been found that the dewatering enhancing effect is maximal in the narrow frequency range. This "removal frequency" is e.g. compression pressure function. The frequency range of the ultrasound used in the invention is preferably between 1 and 100 kHz.

If the ultrasonic treatment were carried out on the web in an uncompressed state, the ultrasonic energy of the treatment leading to the same result would be considerably higher, since then the ultrasound has to remove water from the fibers 20 as well as the web.

To implement the pulse drying method of the invention, a shoe nip device is used in which ultrasonic transmitters are placed in a press shoe forming a long nip with a hot cylinder or roll. Although there are several interfaces between the pu-25 crossing shoe and the web, which to some extent reduce the efficiency of the ultrasonic transmitters, sufficient ultrasonic energy can be transferred to the web to achieve the objectives of the invention. Without the simultaneous effect of compression pressure and steam-wrinkling, the efficiency of the ultrasonic treatment would be substantially worse. Ultrasonic transmitters can also be arranged in the same shoe roll induction heating devices, e.g. with devices disclosed in the applicant's FI patent applications 881711 and 882312.

When the pulse drying in a long shoe nip and the simultaneous ultrasonic treatment are combined according to the invention, surprising interactions are obtained. These effects are mainly due to the fact that ultrasonic treatment can simultaneously both lower the viscosity of the water and cause the web fibers to vibrate so that the water separates from the surfaces of the web fibers. In this case, the water vapor used in the im-5 pulse drying is even better and immediately able to blow water away from the fibers into the water-receiving and displaceable felt or similar fabric in contact with the web.

In the invention, a roll nip can be used as the preceding or following pressing component of the combined ultrasonic hot pressing shoe, in which hot pressing is started or in which the hot pressing step is terminated. In the latter case, the water removed by the ultrasonic field and having a reduced viscosity in it is efficiently transferred to the press fabric in the latter sharp and high-pressure roll nip, since it has not yet had time to bind back to the fiber.

In the following, the invention will be described in detail with reference to some application examples of the invention shown in the figures of the accompanying drawing, to the details of which the invention is not limited.

Figure 1 shows a schematic side view of an embodiment of the invention in which an ultrasonic hot pressing shoe and before and after roll nips are connected in connection with the same hot cylinder.

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Fig. 2 shows the different ultrasonic treatment compression and dewatering steps and the compression pressures prevailing therein.

Figure 3 shows a machine direction vertical cross-section of a combined ultrasonic and hot pressing shoe according to the invention in connection with a hot cylinder.

The impulse drying device shown in Fig. 1 comprises a relatively large diameter 35 hot cylinder 10 with a drive 10a having a smooth or porous outer surface 10 ', which is metal or induction heating when using e.g. electrically conductive assemblies.

The temperature T0 of the surface 10 'of the cylinder 10 is preferably arranged to be T?> 5 100 ° C when the surface 10' encounters a web W to be pulsed-dried on the surface of the press felt 12, the dry matter content of which is marked KA 1. Depending on the position of the device according to the invention in the process, KAta varies between KA ,,, - 25 ... 75%.

The device further comprises a press roll 21 placed before the pressing shoe device 30, having a smooth or patterned jacket surface 21 'and provided with a drive 21a. The press roll 21 is placed inside the loop of the conveyor belt 25. The roll 21 forms a nip N with the hot cylinder 10. The web W is introduced into the nip N under the support of the press felt 12, so that the web W comes immediately against the heated smooth surface 10 'of the cylinder 10. Correspondingly, the press felt 12 is separated after the second roll nip Nj from the web W following the surface 10 'of the cylinder 10, from which it is separated as a free pull Wp.

The surface 10 'of the cylinder 10 is heated by a medium introduced inside it, such as steam and / or an induction heating device 11, which acts on the coating of the electrically conductive roll 10 through the air gap V. Figure 1 schematically shows the steam supply devices in block 16 and joint 17.

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The press shoe device 30 of the impulse drying device according to Figs. 1 and 3 comprises a long nip press shoe 31 with a sliding surface 32 opposite the impermeable conveyor belt 25, which may have hydrodynamic and / or hydrostatic pressure chambers (not shown). The press shoe device 30 30 comprises a frame beam 30a extending over the entire width of the paper web W. A cylinder block 36 is arranged on the frame beam 30a, to the pressure space 33 of which the pressure of the pressure medium or pressures can be led through the channel 36 from the pressure source. The cylinder block 32 has a sealed piston 35 which loads the shoe 31 and its lubricated sliding surface.

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A splash water collecting trough 26 is arranged around the loop of the conveyor belt 25. The second press roll 27 is provided with a smooth surface 27 'and a drive 27a and has a lubricant collecting trough 24 behind it, from which the lubricant on the inner surface of the belt 25 is fed by a recirculation device.

According to the present invention, the pressing shoe 31 is further arranged to act as an ultrasonic treatment shoe, by means of which the ultrasonic field is applied to the hot-pressed web W. Figures 1 and 3 show ultrasonic sensors 40 arranged in series with the sliding surface 32 of the shoe 31. The ultrasonic sensors 40 are supplied with electrical power via an electric line 41, the frequency spectrum f of which is mainly preferably in the range f to 1-100 kHz. Block 42 shown in Figure 3 schematically illustrates electrical power supply and control devices.

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The ultrasonic boost of the pulse drying can advantageously be combined with the control of the transverse moisture profile of the web W by using several (N) parallel ultrasonic sensor arrays 40, ... 40n in the transverse direction, to which adjustable electrical power P 1 can be conducted from block 42 via different lines 41, ... 40N. ..PN. In this way, the transverse moisture profile of the web W can be efficiently controlled quickly and accurately by electrical control devices, so that it is not necessary to provide expensive mechanical deflection control and profile control devices in connection with the pressing shoe 31.

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In Fig. 2, the vertical axis shows both the pressure distribution P of the pressing shoe 31 and the ultrasonic power Pu applied to the web, which is illustrated in Fig. 2 by a curve US drawn in broken lines.

The ultrasonic field US provided by the ultrasonic sensors 40 according to the invention acts as follows. As the water in the web to be pressed against the hot cylinder 10 evaporates, a vapor layer is formed in connection with the surface 10 'of the cylinder 10' in step C of Fig. 2 and possibly also in steps D and E, blowing water towards the hood 12 through the web W. The ultrasonic-35 field US, which acts simultaneously, reduces both the viscosity of the water and causes the web W fibers to vibrate so that the water separates from the i, QS2R4 fibers to more efficient and the water enters the compression fabric 12 in step C and possible intense nip step E before it can bind back. fiber mesh.

The second press roll 27 together with the hot cylinder 10 forms a second roll nip N2, after which the web W follows the surface 10 'of the cylinder 10, from which it is separated by a draw Wp by a guide roll 13 and transferred to a support roll 15 guided drying fabric 15. The fabric 15 takes the web W to a drying section (not shown) where the dewatering is continued by evaporation.

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The dry matter content of the web W after the impulse dryer is denoted by KA 2. In general, this dry matter content KA ,,, - 50 ... 80%.

According to Fig. 2, the paper web W is pressed against the hot (T0> 100 ° C) surface 10 'of the cylinder 10 by means of a belt 25 and a press felt 12 by means of a relatively low pressure p, long nip press shoe 31, and the surface of the paper web W against the surface 10' is heated above 100 °. C temperature. Said surface temperature 10 'when it comes into contact with the web W is generally in the range Tc - 150 ... 500T. The corresponding temperature To1 when the web W separates from the surface 10 'is generally in the range T01 to 100 ... 300 ° C. The pressure level of the long nip press shoe 31 is e.g. p, = 0.1 ... 5 MPa. Said temperatures T0, T01 can be kept at a reasonable level due to the ultrasonic field US according to the invention.

The long nip shoe 31 is hydrostatic, hydrodynamic or a combination thereof. According to Fig. 2, after the long nip pressing step C, the pressure on the paper web W decreases to a level d determined by the tension of the belt 25 in the zone D, and the water evaporation of the paper web W is enhanced by the pressure drop p, -> p „. Pressure po - T / R, where the tension of the T - belt 25 30 and the radius of the R - cylinder 10. After zone D, an intensive pressing step takes place in the nip N2, in which the paper web W is pressed with a high pressure between the cylinder 10 or the corresponding roll and the pressing roll 27. This step is shown in Figure 2 by E, and the maximum compression pressure is then preferably p__, »7 MPa. In the pressing step E, water vapor is blown through the paper web W, causing it to dissipate the water released by the ultrasonic field in the interstices of the fibers, and thus to have an enhanced compression result and an even higher dry matter content KA ,,,,.

The so-called conveyor belt 25 can also be used. an elastic belt by means of which the zones A and E of the roll roll nips N, and N3 and at the same time their pressing time can be extended and the pressing pulse can be increased. If necessary, a separate elastic band can also be used, which is passed between the conveyor belt 25 and the felt 12. Since water cannot be pressed from the press felt 12 onto the hollow surfaces of the rollers 21,27, if necessary, it is possible to make the water-receiving hollow surface on the belt 25, to which the broken line 25 'of the outer surface of the belt 25 in Fig. 1 refers.

In step A shown in Fig. 2, in which the peak compression pressure Pml is used in the nip N, it is a question of the first step. Step B is a pressure lowering step, step C is an ultrasonically enhanced second compression step according to the invention. Stage D is a depressurization and vapor generation stage, and stage E (peak pressure p ^ j) is an intensive ultrasonic field-enhanced compression and blow-through stage in which the water removed by the ultrasonic field US has not yet had time to bind back to the web 20 W fiber. In the middle row (L) below the zone markings A-E, Fig. 2 shows examples of preferred lengths (mm) of said zones and corresponding residence times (ms) in the lower rows (t) when using a machine speed v to 20 m / s.

The following claims, within the scope of the inventive idea defined by which the various details of the invention may vary and differ from those set forth above by way of example only.

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Claims (8)

1. Impulse drying method for dewatering a paperboard or paperboard web (W), in which the method using a pressing stage or spoon, wherein the web (W) to be pressed is guided to the surface (10 ') by a cylinder (10) or roller heated to a temperature above 100 ° C so that the water layer against said surface (10 ') of the web (W) is extended and with the mud thus formed, water is blown from the web (W ) fiber mills in a press fabric (12) in conjunction with the opposite surface of the web (W) and in which method the web (W) is pressed against the surface (10 ') of the hot cylinder (10) or roller in one or more the pressing stage (A, C, E), at least one of which is a longitudinal pressing stage (C), characterized in that in said longitudinal pressing stage (C) an ultrasonic field (US) is directed towards the web (W) which is subjected to pulse drying and the vibrational frequency spectrum and the intensity of the ultrasonic field (US) are selected so that the ultrasonic field (US) is essentially bed reduces the viscosity of the water to be blown away from the web (W) by means of the meadow and shakes off water from the fiber plant, and water is removed from the web (W) during the impulse drying stage or stages (C, E) before the water is allowed to bond and that the dry matter content of the web (W) arriving at the combined impulse drying and ultrasonic processing stage is in the range of KA 2, - 25 ... 75%, and that after said combined stage, the dry matter content of the web KA - 50 ... 80%. 25 Method according to claim 1, characterized in that the frequency (f) or the frequency spectrum and / or the intensity (P, ... PN) of said ultrasonic treatment is controlled in the transverse direction of the path (W) and that with this control the path of the path is at least partially controlled. (W) transverse moisture profile. Method according to claim 1 or 2, characterized in that the frequency spectrum of the ultrasound used in the method is mainly in the range 1 kHz ... 100 1Hz. 35 4. A method according to any of claims 1-3, characterized in that in the method the ultrasonic field (US) is directed towards the web (W) by means of a longitudinal nip shoe (31) via a press slip belt (25). and a press tissue (26) directed to its sliding surface (32). 5. A method according to any one of claims 1-4, characterized in that an intensive roller cutting press (A, E) is used before and / or after the method in connection with the hot cylinder (10). combined elongated squeezing and ultrasonic treatment stage (C). 10 A pulse drying device intended to carry out the method according to any of the patent rows 1-5 for dewatering a paper or cardboard web (W), the device comprising a hot cylinder (10) or roller, the surface (10 ') of which is heated by heating means (11, 16, 17) located inside and / or outside the cylinder, which means comprises a press shoe device (30), wherein there is a long press shoe (31) forming a long press zone (C) with said hot cylinder (10). ) and through which long press zone (C) the web (W) to be pressed is directed toward the surface (10 ') of the hot cylinder (10) and the opposite side of the web (W) receiving the press tissue (12) and a on the opposite side of this pressurized sliding belt (25), the inner surface of which is arranged to run over the lubricated sliding surface (32) of longitudinal pressing cone (31), characterized in that one or more ultrasonic transmitters have been placed in said longitudinal pressing shoe (31). (40) and that the device comprises electric power supply and control means (42), by which electrical power is supplied in said ultrasonic transmitter (40), from which one can direct an ultrasonic field (US) of sufficient intensity in view of the longitudinal squeeze stage (C). the efficiency of the dewatering of the path (W), and that in the device several (Nst) parallel ultrasonic transmitters (40, ... 40N) are used in the transverse direction of the path (W) in which one can conduct an adjustable electrical power (P1). ..PN) for checking the transverse moisture profile of the web (W). 7. A pulse drying device according to claim 5 or 6, characterized in that electric induction heating devices are provided in the combined ultrasonic pressing shoe (31) with which the opposite hot cylinder (10) or roller surface (10 ') is heated. An impulse drying device according to any of claims 5-7, characterized in that the device includes one or more roll nips (N 1, N 2) arranged in connection with the hot cylinder (10) or roll, which / which are placed before ultrasonic press cones (31) and / or after this. 10