WO2008040852A1

WO2008040852A1 - Method and arrangement for controlling the drying of a coated or surface-sized fibrous web

Info

Publication number: WO2008040852A1
Application number: PCT/FI2007/050539
Authority: WO
Inventors: Jouni Haavisto; Olli Niemi; Aleksi Sipilä
Original assignee: Metso Paper, Inc.
Priority date: 2006-10-03
Filing date: 2007-10-03
Publication date: 2008-04-10
Also published as: DE112007002214B4; FI20065630A0; AT509529B1; FI120656B; AT509529A1; DE112007002214T5

Abstract

The invention relates to method for drying a coated or surface-sized fibrous web (W), in which method the fibrous web is coated or surface-sized and after the coating or surface-sizing the fibrous web is dried in a drying line (1) comprising two or more drying units (6). In the method, the final moisture content of the web (W) being dried is measured after (16) the drying line (1), the temperature (T1) of the web (W) being dried is measured in connection (8) with each drying unit (6) respectively and the drying capacity of each drying unit (6) is adjusted on the basis of the difference between the measured temperature (T1) and the set value (T0) of the maximum temperature set for the drying unit in question to control the temperature of the web (W). The invention also relates to an arrangement for implementing the method.

Description

Method and arrangement for controlling the drying of a coated or surface- sized fibrous web

The invention relates to a method for controlling the drying of a fibrous web, in which method the fibrous web is coated or surface-sized and then dried in two or more drying units. The invention also relates to an arrangement for implementing the method.

Especially when drying heat-sensitive products, it is important to control the temperature of the product being dried during drying. A heat-sensitive product thus mainly refers to a paper- or board-based product which reacts readily to heat. Heat-sensitive fibrous products, especially thermal papers, are used, for example, in self-adhesive labels, receipts, tickets, name tags and similar objects. The product consists, for example, of one or usually more different layers of a paper or board web, in one or more of which layers are arranged components reacting readily to temperature. When the temperature rises to a sufficiently high level, these components bring about chemical reactions and as a result, on the surface of the product are formed optically detectable marks or changes in their colour or darkness. One way of arranging the layers containing these components is to apply a coating layer or coating layers with a suitable composition on the surface of the fibrous web. Similarly, adhesive layers or the like may be applied on the surface of the web. This humid layer diluted with a suitable volatile liquid, for example water, must be dried before rolling the fibrous web with a reel or the like.

In the manufacture of a product, and now especially during the drying stage, also the temperature of the web, in addition to the desired final moisture content, must be adjusted and controlled throughout the drying stage. Especially if a particularly heat-sensitive product is being dried, very definite maximum values may well be set for web temperature. In respect of heat- sensitive products, the maximum temperature is typically within the range from 45 to 80°C, more typically 50 - 65°C. i The temperature adjustment of the web has previously been carried out by adjusting the running speed of the entire machine. The temperature of the web was measured after drying, and if the temperature of the web was found to rise excessively, the dwell time of the web in each individual drying unit was shortened by increasing the running speed of the machine and thus reducing heat transfer to the web. Correspondingly, the temperature of the fibrous web was increased by slowing down the running speed of the machine. This adjustment method is, however, difficult to run and its adjustment principle is inadequate. Changing the running speed due to drying requires adjustment measures in different parts of the machine and running the machine on this adjustment principle requires constant attention. Furthermore, since the adjustment measures required can only be carried out at a limited speed, there is a continuous risk of exceeding the temperature limits and thus of the sensitive product being damaged and rendered useless. A common problem is also the distribution of the heat load between the drying units when several drying units are used. Solutions based on calculatory modelling have also been proposed, where a drying process consisting of several units is modelled and simulated by using the respective process parameters. Based on these modellings, the drying capacities of each drying unit are then adjusted. This type of solution is, however, costly and cumbersome and it is also susceptible to failure. In addition, adjusting it to possible modifications made in the installation would be laborious and it is, therefore, a rather inflexible solution.

The aim of the present invention is to solve some of the above-mentioned problems and to provide a simple, reliable and economically applicable method by means of which web drying can be performed both at an automatically maximized drying capacity and in a manner protecting the product from damage more reliably than before.

The method according to the invention is characterised by what is stated in the characterising part of the first claim. The characteristics of the arrange- merit according to the invention, on the other hand, appear rrom tne cnarac- terising part of claim 6. Further preferred embodiments of the invention are disclosed in the dependent claims.

According to the invention, both the final moisture content of the fibrous web after the drying line and its temperature in connection with each drying unit are measured by means of web temperature sensors, and the capacity of individual drying units, respectively, is adjusted on the basis of both the measurement results and the set values in such a way that the maximum temperatures, hereinafter limit values, set for the web are not exceeded. The final moisture content of the web preferably acts simultaneously as a general main control variable in the process. The capacity of individual driers can now be arranged to be automatically adjustable according to the desired final moisture content of the web and at the same time by observing the limit values set for the temperature in connection with each drying unit.

In the arrangement according to the invention, a moisture controller provided for calculation and adjustment calculates, on the basis of measurement results and set web temperature limit values, the changes in set values that are distributed to the drying units in proportion to the measured web temperatures. The temperature of the web being dried can thus now be set per drying unit by adjusting the drying capacity of each dryer as necessary on the basis of the measured values. By means of the invention is provided a simple and cost-effective, but at the same also a reliable method both for controlling the temperatures and for making drying more effective.

The invention is described in greater detail in the following by means of embodiments, referring to the accompanying drawing, in which

Figure 1 shows highly diagrammatically an arrangement implementing the method for controlling the drying of a fibrous web. Figure 1 shows highly diagrammatically an arrangement according to the invention for drying a fibrous web W in a drying line 1, which comprises several drying units 6 arranged in succession. The web W is coated in a coating device 4, where the suitable components for obtaining the properties typical of a heat-sensitive product are brought on the bottom layer of the web in the form of a coating. To support the web and to control the web tension, it is possible to arrange support points in the form of guide rolls or the like (not shown) between or in conjunction with the drying units.

After, or in addition to, the drying arrangement according to the invention may also be used other drying stages or devices. In addition may be mentioned that as concerns the backside of the web, it is preferable in connection with the invention to arrange separate equipment or a separate unit for it. In it is mainly controlled the curvature of the web by adding a liquid consisting mainly of water and suitable additives on the backside. After this "straightening unit" may still be provided additional drying for the web, for example, by means of conventional cylinder drying. The said additional equipment or units are not shown in Figure 1.

The manner of applying the coating or surface-sizing is not significant from the point of view of the invention and it is not, therefore, described in greater detail herein. As examples could be mentioned, for example, curtain coating and film transfer coating. It should also be noted that the components or compositions bringing about the above-mentioned characteristics of a heat-sensitive product may be provided in the fibrous web already at an earlier stage, for example, at the stage of forming the fibrous web itself. Or at least some of these components may be provided in the web already at its formation stage.

In connection with each drying unit 6, preferably immediately after it or inside it, is arranged web temperature measurement 8. In principle, measure- ment at one point suffices, but measurement may also be arranged to take place over the whole width of the web in the CD-direction to improve the reliability of the measurement results. In the latter case, it is possible to arrange for adjustment of the transverse temperature profile of the web. The temperature measurement 8 itself can be carried out, for example, as IR- measurement. Dryer unit specific web temperature measurement is most preferably carried out either immediately after the dryer 6 or inside the dryer itself. In the latter case it is also possible to take into account the possibly slightly higher temperature of the web inside the dryer itself than that of a web travelling and at the same time cooling somewhat between the dryers. Web temperature measurement 12 may also be arranged before the first dryer, especially for adjusting the first dryer. To facilitate adjustment, it is also possible to arrange the web to travel from one dryer to another while thermically insulated to minimise temperature variations or, on the other hand, also to arrange cooling means for controlling and lowering the temperature of the web between dryers.

In addition, means 16 are provided for determining or measuring the final moisture content of the fibrous web. To determine the moisture content of the web, means 16b can be arranged also both before the drying line 1 and in connection with the drying units. By means of them, both the initial moisture content of the web and the decrease in moisture content in the drying line can be determined. This information can be utilized both in a derivative and a real time manner for adjusting the drying procedure.

Information is transmitted from the temperature measuring devices 8 to the moisture controller 10 acting as the actual adjustment device. On the basis of the measured temperature values and the final moisture value of the web obtained from the frame 16, as well as the entered dryer-specific maximum temperatures of the web, the controller calculates the capacity requirement for the drying line and the dryer-specific capacities and gives the correspond- ing commands to the dryers. Both the limit values for the web temperature and the target value for the final moisture content of the web can be entered in the moisture controller at this control centre 20. The controller solution may be implemented, for example, by means of unit controllers, that is, as a combination of several single unit controllers or as multivariate control.

When drying a heat-sensitive fibrous web, the dryers typically used are air dryers. For the sake of clarity, only the term dryer is used hereafter. The solution relating to the invention is not, however, in any way limited to using an air dryer, but also other known devices suitable for drying a fibrous web from drying cylinders to radiation dryers can conceivably be used. In this connection should, however, be mentioned that the method according to the invention is particularly well-suited for adjusting and controlling the drying of a curtain-coated heat-sensitive fibrous web. The method according to the invention also provides highly preferably means of process control for the drying phase taking place within the solidification point range, which is significant to the mottling-phenomenon. In this case, both the maximum temperature of the web and the maximum speed of drying are significant, which may be increased to a value exceeding 60kg/m2h within the solidification point range. Using the invention for the aforementioned applications is thus disclosed herein as a preferable additional embodiment of the invention.

In an air dryer, where hot dry air, superheated steam or a similar gaseous fluid is directed at the web by blowing, the control parameters used may be either the outlet velocity, temperature or both of the fluid. The absolute humidity of the fluid may also be adjusted to adjust the drying capacity. In the following fluid refers to air only.

According to the invention, the adjustment principle is arranged in such a way that the capacity of a single dryer 6 is adjusted in accordance to how far the web temperature Ti,i measured immediately after the dryer 6 at the measuring point 8 is from the dryer-specific set value T₁^ of the maximum temperature. When the measured value Ti approaches the set value T₀, the drying capacity of the said dryer is gradually reduced. This adjustment is carried out for each dryer. On the whole, the adjustment aims at a desired final moisture content of the web, however, ensuring at the same time that the temperature of the web remains below the set values To set during the drying phase. This ensures that the components contained by the product will not be able to cause premature dyeing or darkening of the product. Typically, the set value for the maximum temperature T₀ of the fibrous web W is arranged for each respective drying unit 6 to be within a range from +45 to +80⁰C, preferably from +50 to +65°C, most preferably from +50 to +55°C

In the following, adjustment is considered from the point of view of individual dryers. The first dryer 6 may in principle mostly be run at almost full capacity. As concerns this dryer, the temperature of the drying gas used, that is, mainly air, may be arranged so as to always be even as high as 200⁰C. Basically, the temperature of the drying air in the dryers is lowered 6 towards the end of the drying line. The temperature used at the last dryer is typically already below +100⁰C, for example +80⁰C. The same also applies to drying air flow rates, which are typically of the order of 40-65 m/s at the first dryer and 15-30 m/s at the last. The final moisture content of the web is, on the other hand, typically within the range from 2 to 12%, most typically from 3 to 8%.

On arrival at the first dryer, the temperature of the web is typically still low and the set value of the web temperature following a dryer is not easily exceeded. On the other hand, on arrival at the second dryer, following the first dryer the temperature is already clearly closer to the maximum temperature allowed, that is, the set value T₂,o of the maximum temperature of the web at the second dryer. Therefore, at the second dryer, the thermal power of the steam is limited to prevent excessive temperature increase of the web. In the units following the first dryer, the temperature of the steam is typically set within the range from 130 to 150⁰C. The method according to the invention itself does not, however, limit these temperature ranges in any way, but on the contrary, gives the opportunity to use higher drying capacities than before, and thus wider adjustment ranges for operating temperatures.

When calculating the changes in drying capacities for different drying units, in a preferred embodiment of the invention the adjustment is arranged with an aim to divide the capacities to the dryers in an equalising manner, taking into account the difference between the measured temperature of the web and the drying unit specific set temperature. When the aim is to increase the drying capacity of the entire drying line, that is, the aim is to increase the amount of heat transferred to the web to improve the efficiency of drying, the aim is to transfer capacity to the dryer after which the web temperature Ti is furthest from the set value T₀ of the maximum temperature prescribed to the said dryer. Similarly, the capacity of a dryer coming closest to its set value is either aimed to be maintained constant or even lowered somewhat.

Similarly, when the aim is to lower the drying capacity of the drying line, dryer-specific capacities are reduced by lowering most the capacity of the dryer after which the measured web temperature is closest to the set temperature prescribed for the said dryer. Similarly, the capacity of the dryer furthest from its set value is preferably lowered least or not at all.

Obviously, in all of the above cases it is presumed that the measured temperature Ti is always lower than the set temperature T₀ set for each dryer, respectively. If the measured temperature exceeds the set temperature, the capacity of the dryer in question is lowered immediately. The maximum value of the adjustment range of the measured temperature of the web and thus the temperature adjusted by the capacity of the dryers used as an adjust- ment parameter is thus the above set value, which the web temperature should not exceed.

If then, at any dryer, the measured temperature T₁ reaches or even exceeds the set value T₀ for maximum temperature, the aim is basically always to lower the capacity of such dryer immediately, and also regardless of what the web temperature in connection with other dryers is.

In the method according to the invention, measures taken in connection with the adjustment of one dryer obviously affect the operation of the other dryers following it through changes in web temperature. To improve and expedite adjustment, these changes can be taken into account. For example, if the capacity is increased at the first dryers, the increase in web temperature resulting from this will at the same time affect the capacity requirement of the dryers to follow as the web temperature rises. Thus, in this example the capacity requirement of the subsequent dryers decreases. The limits of the drying capacity of the drying unit, that is, the minimum and maximum drying capacity value of each drying unit may be considered in the same way. If, for example, the capacity of the previous dryer is already being used to the full, the aim will be to provide the additional capacity required by adjusting the following dryer or by increasing the capacity of the previous dryer, obviously taking into account the web temperature range set for each dryer, respectively.

The general principle mentioned previously in the case of both decreasing and increasing the capacity of the drying line works in all cases in principle, but for fine adjustment is preferably carried out, for example, a continuous iterative adjustment process with the aim of finding a suitable balance between all drying units. This type of adjustment process is now easy to implement by means of the arrangement according to the invention. A substantial difference compared with the earlier method is, therefore, the possibility of automating the entire drying line adjustment process in a simple and reliable manner. This adjustment was previously mostly done manually by adjusting the running speed of the whole machine on the basis of the final moisture content and temperature of the web. Now the entire adjustment can be carried out regardless of the running speed of the machine and without interfering with it in any way. At the same time, this has an extremely advantageous effect on the use of the machine itself and its reliability. The control arrangement according to the invention is in turn simple and reliable as concerns both its operating principle and implementation and it thus promotes for its part the improvement of the operational reliability of the drying process.

Furthermore, since adjustability in general is markedly improved, it is also possible to connect the drying stage provided with the method according to the invention to a coating line acting as an online device. The example shown in Figure 1 relates to an off line product, but the method can also be utilised as an online application. Due to their adjustment principles, earlier methods were only applicable to off line machines.

In one embodiment of the invention are in addition provided dryer-specific cooling devices for decreasing the temperature of the web if it is observed to rise to an excessively high level above the set values. For cooling may be used, for example, cold air blowing, water blowing or directing some other cold fluid on the web or bringing a cold heat transfer surface in contact with the web. Cooling may be carried out either once or also in several stages during drying according to the invention. The means for cooling the web may, if necessary, be arranged in connection with each drying device, possibly on both sides of the device. In yet another embodiment of the invention, a suitable film or similar layer is applied on the surface of the fibrous web, by means of which either the transfer of heat to the web may be adjusted as desired or by means of which, for example, heat generated by friction or transferred to the web may be reduced. The film thus acts as a kind of insulation or friction-reducing surface and for its part prevents or slows down excessive rising of the temperature in the type of fibrous product where exceeding the set maximum temperature leads to the properties of the product changing in a specific way. This type of film or other layer may be applied on the surface of the product in connection with the application of the actual coating or surface-sizing or after it, either before the drying stage or partly also during it. This type of protective film may also be applied on the surface of the product after drying, on the finished product, to prevent unintentional exceeding of the above-mentioned maximum temperature.

Claims

1. A method for drying a coated or surface-sized fibrous web (W), in which method the fibrous web is coated or surface-sized and after the coating or surface-sizing the fibrous web is dried in a drying line (1) comprising two or more drying units (6), characterised in that, in the method, the final moisture content of the web (W) being dried is measured after (16) the drying line (1), that the temperature (Ti) of the web (W) is measured in connection (8) with each drying unit (6), and that the drying capacity of each drying unit (6) is adjusted on the basis of the difference between the measured final moisture content of the web (W), the measured temperature (Ti) and the set value (T₀) of the maximum temperature set for the drying unit in question to control the temperature of the web (W).

2. A method as claimed in claim 1, characterised in that the method is used for drying heat-sensitive fibrous material (W).

3. A method as claimed in claim 1 or 2, characterised in that when increasing the drying capacity of the drying line (1), the aim is to increase especially the capacity of the drying unit (6) in connection with which the measured web temperature (Ti) is furthest from the set value (T₀) of the maximum temperature of the web set for it.

4. A method as claimed in any of the claims 1 to 3, characterised in that when lowering the capacity of the drying line (1), the aim is to lower especially the capacity of the drying unit (6) in connection with which the measured web temperature (Ti) is closest to the set value (To) of the maximum temperature of the web set for it.

5. A method as claimed in any of the claims 1 to 4, characterised in that the set value (T₀) of the maximum temperature of the fibrous web (W) is arranged for each drying unit (6) within the range from +45 - +8U°L, preferably within the range from +50 - +65°C, more preferably within the range from +50 - +55°C.

6. A method as claimed in any of the claims 1 to 5, characterised in that the temperature of the web (W) is lowered at one or two stages during the said drying.

7. A method as claimed in any of the claims 1 to 6, characterised in that an air dryer is used as the drying device in one or more drying units (6).

8. A method as claimed in any of the claims 1 to 7, characterised in that the adjustment is carried out by means of unit controllers and/or as multi- variable control.

9. An arrangement for drying a heat-sensitive fibrous web, in which arrangement the fibrous web is arranged to be coated in a coating machine (4) and after coating to be dried in a drying line (1) comprising two or more drying units (6), characterised in that in the arrangement, after the drying line (1) are arranged means (16) for determining the final moisture content of the web (W), that in connection with each drying unit (6) are arranged means (8) for measuring the temperature (Ti) of the web (W) being dried, and that the drying capacity of each drying unit (6) is arranged to be adjusted on the basis of the difference between the measured temperature (Ti) and the set value (T₀) of the maximum temperature set for the drying unit in question to control the temperature of the web (W).

10. An arrangement as claimed in claim 9, characterised in that in one or more drying units (6), an air dryer is provided as the drying device.

11. An arrangement as claimed in claims 9 to 10, characterised in tnat as adjusters are arranged unit controllers and/or a multivariable controller.