Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2001-01-05
2001-11-06
Walberg, Teresa (Department: 3742)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S445000, C034S446000, C034S557000
Reexamination Certificate
active
06311410
ABSTRACT:
The object of the present invention is a method and device specified in the independent claims presented below for drying a coated web.
Traditionally, various types of airborne web-dryers, air dryers, infrared radiators and cylinder groups have been used for drying coated paper and board. The drying of a coated web is characterised by the fact that the initial drying stage must be carried out without contact with the web, that is, traditionally by means of air drying or infrared drying because the wet coating will not withstand mechanical contact.
The method of drying a pigment coating has a significant effect on the qualities of the finished paper. Both evaporation and the absorption of water into the base paper at the initial drying stage cause rearrangement of the binders in the coating. The time lag between the application of the coating and the start of drying, and the evaporation rate during drying proper, influence the binder distribution of the coating in the direction of its thickness and also the density of the surface.
The initial drying stage can be divided into the heating stage and the drying stage proper, that is, the evaporation stage. After the evaporation stage, the absorption of water into the base paper has practically ceased and the coating has solidified to such an extent that the drying no longer affects the quality of the coating in the same way. After this stage the coating can be allowed to come into contact with the rolls and cylinders.
In order to dry a coated web, it must be subjected to a certain amount of energy during drying. By far the greatest part of the energy is obviously consumed by evaporation, but in some cases the amount of energy used for heating the web may also rise to a high level. In air dryers, the energy is transmitted to the web by convection. In an infrared dryer, the energy is transmitted to the web through electromagnetic radiation.
It has been found that if coated paper is only dried by means of conventional air dryers, that is, by blowing dry hot air towards the coated web, the coating often becomes blotchy and its surface strength properties will be poor. It has been assumed that this is due to the fact that air drying heats the coating slowly. Attempts have been made to eliminate the problem by heating the coated web by means of infrared dryers before air drying proper. The infrared dryers heat the web more evenly than conventional air dryers. The infrared dryers can also raise the temperature of the coated web to the desired, sufficiently high level, usually to a temperature of about 70° C., considerably more rapidly than air dryers.
Infrared heaters are, however, expensive devices and their operating costs are much higher than those of conventional air dryers, that is, airborne web-dryers.
It has previously also been suggested, in the American patent publication U.S. Pat. No. 5,536,535, that the coated web be dried by means of superheated steam with a wet bulb temperature of more than 85° C. By means of superheated steam, the temperature of the web can be raised rapidly to the required drying temperature, which is, however, high when drying by means of superheated steam, that is, almost 100° C. In the solution disclosed in the US publication, the web thus has to be dried at a considerably higher temperature than when drying with hot air.
It has also been found that interrupting the conventional drying of a coated web at the initial drying stage by free drawing of the web between individual air dryers may impair the quality of the paper. This is assumed to be due to the fact that the drying of the coating layer slows down during a free draw, which means that during a free draw, more water is able to pass from the coating layer to the base web than at the air dryer. The water thus transferred causes the base web to swell and causes fibre based roughness in a wood-containing base web and changes resembling such roughness in a wood-free base web. These should be eliminated.
The aim of the present invention is, therefore, to achieve an improved method and device for drying a coated web compared with those described above.
The aim is particularly to achieve a method and device by means of which the web to be dried can be heated to the required drying temperature more economically than before.
The aim is also to achieve a method and device by means of which the cooling of the web between air dryers is prevented.
In order to achieve the aims presented above, the method and device relating to the invention are characterised by what is specified in the characterising parts of the independent claims presented below.
A typical method relating to the invention concerns a drying method in which the coated web is first taken through a heating unit, in which the temperature of the coated web is raised to drying temperature or close to drying temperature, and in which the coated web thus heated is then taken through at least one air dryer, in which drying air jets are directed at the coated web in order to dry the coated web.
In this solution relating to the invention, the heating of the web in the heating unit is carried out by means of humid, hot air. The heating typically takes place in an air dryer by blowing hot air jets towards the coated web, the humidity a
1
of the air jets being substantially higher than the humidity a
2
of the air jets of the dryer section proper that follow. In this way the, temperature of the coated web can be increased safely and rapidly to the required drying temperature, typically to a temperature of approximately 60-80° C.
The high temperature of the coating reduces migration of the binder. The most important reason for this is that the coating can solidify or lock more quickly and in wetter form than when the coating is heated more slowly by means of a conventional air dryer. Rapid heating gives a more porous coating which absorbs printing inks more evenly.
The steam in the hot air jets relating to the invention condenses on the surface of the coated web, forming a thin film of water and emitting heat evenly to the coated web. This means that no areas that heat up and dry faster or more slowly will be formed in the web. As long as the temperature of the coated web is lower than the dew point of the drying air, water will condense on the surface of the web and release an amount of energy corresponding to evaporation enthalpy. In consequence of this, the solidification of the coating can be achieved rapidly by means of humid, hot blast air. For example, by means of blast air with a wet bulb temperature of 70° C., film formation of latex can be achieved rapidly. Evaporation which starts at this stage will thus no longer cause binder migration. Neither does the surface-direction structure of the coating vary to any significant degree at this stage.
Since the rate of condensation on the coated web is the faster, the cooler the area on the surface of the web, the release of energy due to evaporation, as described above, causes the surface-direction temperature differences to level out in this type of dryer, which means that the surface-direction film formation of latex is also even, and the printing properties of the surface become uniform.
The hot air jets with high humidity can, in the case of one-sided coating, only be directed towards the coated side of the web.
The heating of the web according to the invention can in practice be simply arranged to take place e.g. in the first part of an air dryer divided into successive segments, preferably in its first segment, in which substantially more humid hot air is blown towards the web than in the other segments.
The humidity of the heating air jets can preferably be maintained within the humidity range of >0.1 kg
water
/kg
dry air
, typically within the range of 0.2-0.5 kg
water
/kg
dry air
.
The humidity of the drying air jets, on the other hand, is usually below 0.2 kg
water
/kg
dry air
, typically about 0.1 kg
water
/kg
dry air
, however, lower than that of the heating air jets.
According to a preferred embodiment of the invention, the initial drying o
Heikkila Pertti
Juppi Kari
Rajala Pasi
Dahbour Fadi H.
Metso Paper Inc.
Nixon & Vanderhye P.C.
Walberg Teresa
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