GB2418929A - Producing paper product - Google Patents

Abstract

A method of producing a tissue or towel provides continuous support through a dryer section of a papermaking machine to a paper web 100 by a unitary supporting fabric 4 as the web and fabric pass over, in sequence, at least one through air dryer 5 and at least one dryer cylinder 29. A dryer section of a papermaking machine provides a paper web support means comprising a unitary supporting fabric 4 adapted to provide continuous support to a paper web 100 as it passes over, in sequence, at least one through air dryer 5 and at least one dryer cylinder 29.

Description

24 1 8929

METHOD AND APPARATUS FOR PRODUCING

A THROUGH AIR DRIED TISSUE OR TOWEL PRODUCT

FIELD OF THE INVENTION

This invention relates to a method and apparatus for producing a through air dried tissue, towel or similar paper product in the dryer section of a papermaking machine.

BACKGROUND OF THE INVENTION

In the manufacture of tissue or towel, an aqueous slurry of water and cellulosic fibres and other papermaking constituents is deposited from a headbox onto a moving forming fabric, or between two moving forming fabrics on a two-fabric papermaking machine. The web is initially formed and partially drained in the forming section, and in a conventional wet press process (CWP), carried by a press felt and pressed onto a Yankee cylinder where it is dried and then removed from the cylinder by a creping doctor and wound onto a reel.

However, if the finished tissue and towel sheet is required to have enhanced properties, such as bulk, absorbency and hand feel, improved properties can be obtained by the use of a through air drying (TAD) process which is characterized by the absence of wet pressing and the use of a TAD dryer, either alone or in combination with a Yankee cylinder and hood, to dry the web.

, . . . In a TAD process, the web is transferred from a forming fabric to a TAD fabric by a transfer means, generally a pick up roll or shoe. Alternatively, the web may be transferred from a forming fabric to an intermediate transfer fabric and then to the TAD fabric. A vacuum box or moulding box downstream of the pick-up applies vacuum to the back of the TAD fabric, opposite the sheet side, so as to mould the web to the surface topography of the TAD fabric. The moulding creates a more threedimensional web, creating regions of low fibre density adjacent to the TAD fabric openings or pores, thus increasing the bulk and absorbency of the finished web. The TAD fabric may be constructed so as to have a particular surface design topography which will impart to the web conveyed thereon a desired shape or pattern.

The web is then dried on one or more TAD dryers, where heated air is directed through the paper web thus providing for a high rate of evaporation. In comparison to conventional impingement dryers, such as Yankee hoods, the TAD dryer yields very high drying rates. Typical drying rates for impingement dryers are in the range of from about 80-120 kg/h/m2 as compared to about 160 to 240 kg/h/m2 for TAD dryers.

In a variation of the conventional TAD process, the web is partly dried on one or more TAD dryers and then carried by the TAD fabric through a press nip and transferred onto a Yankee cylinder which dries the web to completion.

Typically, an impingement hood or Yankee hood is used to enhance evaporation rates and remove water vapour. The web is then creped from the Yankee, the creping process imparting stretch to the web, before going to the reel. In another variation of the conventional TAD processes, the partially dried web is transferred from the TAD fabric, following the one or more TAD dryers, to one or more transfer fabrics which subsequently carry the web through the press nip onto the Yankee.

In another variant of the TAD process, sometimes referred to as the uncreped TAD process, such as described by Rugowski et al in US 5,591,309, and by Klerelid et al. in US 6,743,334 and US 2004/0181966, the web is dried to completion on one or more TADS without the benefit of a Yankee cylinder and is then transferred to the reel without being creped. In the absence of creping, tissue and towel webs generally tend to be quite stiff and unsuitable for certain consumer uses. However, in the uncreped TAD process, the stretch capabilities and softness of the web can be enhanced without undue loss of bulk, by employing a speed differential between the moving forming fabric and a slower-moving immediately following transfer fabric. This is known in the art as a 'rush transfert', and is described for example in US 5,667,636 to Engel et al., and US 5, 399,412 to Sudall. The use of rush transfer is well known in combination with other steps to achieve softness in a tissue product.

A disadvantage of the conventional TAD process arises from the use of a press roll to transfer the web to the Yankee cylinder. In order to obtain good adhesion of the web to the Yankee cylinder, high press loads must be used. The pressing action causes the densification of the web in the areas adjacent to the fabric 'knuckles" which are raised areas (yarn cross-overs on woven fabrics, or "land'' areas on resin-patterned belts) of the papermaking fabric adjacent to and in contact with the paper sheet, as distinguished from the recessed areas of the fabric. The web densification at these locations results in a loss of bulk and absorbency in the web.

It has been found from practice that the TAD fabric must have a sufficiently high surface contact area (generally 20% and more) and a correspondingly high number of contact points to cause the web to adhere to the Yankee. This generally limits the range of designs which can be used for the TAD fabric as a fairly fine,' fabric (i. e. relating to the yarn density, or high number of threads per unit area of fabric) is required to produce the necessary adhesion. It is also known in the art to "surface" the fabric - a process which involves grinding the paperside surface of the fabric to increase the amount of contact area between the fabric and web, and subsequently web and cylinder, to obtain the necessary adhesion. These factors generally lead to greater compaction of the web, resulting in its densification, and loss of bulk and other properties.

The use of the pressure roll to transfer the web to the Yankee precludes the use of seamed TAD fabrics as, in general, most pin or coil seams would leave a very noticeable mark on the paper web and could also lead to unwanted vibration of the press during passages by the seam through the press nip, due to variations in fabric calliper and density at the seam. AS almost all TAD fabrics are supplied endless (i.e. without a seam, as in a tube sock), the rolls and framing of the paper machine must be cantilevered to allow for fabric installation; this increases the weight and complexity of the papermaking machine as well as its supporting foundations.

Additional disadvantages arise from the use of one or more transfer fabrics after the TAD fabric. As noted previously, the TAD transfer fabric is required to have a relatively fine weave, to ensure that good contact is made by the web with the Yankee. When the web is transferred from the TAD to the transfer fabric, it is highly unlikely that any registration will occur between the weave patterns of the two fabrics and the pattern imparted to the web. Thus, a reduction in sheet bulk results from the mix-registration of the patterns of the transfer and TAD fabrics.

In practice, it has been found that adequate adhesion between the web and Yankee cylinder is achieved by a combination of fabric design (the fabric having sufficiently high contact area and contact density with the sheet), coupled with the use of adhesives, which are applied to the Yankee cylinder. However, the use of adhesives results in the accumulation of glue on the Yankee as well as on the machinery. Chemical release agents may also be required to ensure the subsequent separation of the web from the Yankee.

In the uncreped TAD process, described earlier, the web is dried entirely on TADs. In the TAD, the hot air flows through the web and the openings in the TAD fabric. As the s areas of the fabric occupied by the fabric yarns obstruct airflow, the regions of the web above and in direct contact with yarn knuckles (or land', areas) of the TAD fabric will tend to remain wetter than areas over the openings. Thus, in order to achieve a desired average finished sheet solids content, the areas of the web which are adjacent the open areas of the TAD fabric will be significantly overdried to compensate for the wetter areas of the web adjacent the air flow obstructed areas of the fabric. From an energy consumption perspective, this is a disadvantage as the amount of heat required to evaporate water increases with increasing sheet solids. In the conventional TAD process, this problem is offset somewhat as the wet areas of the sheet which were adjacent to the fabric knuckles are pressed onto the Yankee surface and dried preferentially. The amount of drying energy required by the Yankee hood is generally low due to the high dryness of the sheet adjacent the open areas of the fabric.

One difficulty common to all tissue making processes is ensuring that the desired sheet properties, softness for example, are achieved while maintaining adequate tensile strength of the sheet during the manufacturing process.

For example, in the transfer of the web from the Yankee to the reel, the unsupported distance the web travels is relatively long and the web must be strong enough to withstand the aerodynamic forces it encounters in the long open draw. However, if continuous support is provided for the web up to the reel, the requirement for additional tensile strength to avoid tearing of the sheet in unsupported sections is removed, so that the tensile strength of the sheet can be somewhat reduced.

Rugowski in US 5,591,309 discloses a TAD machine which does not employ a Yankee dryer and which has sheet transfer without open draws from the TAD fabric by means of dry end transfer fabrics carrying the web to the nip between the reel and reel drum. He further teaches a transfer from TAD fabric to the reel drum, employing vacuum assist in the reel drum to aid the transfer and, in another variant, the use of intermediate vacuum rolls between the TAD fabric and reel drum. Linden in US 6,669,818 discloses several methods of providing improved support for the tissue web, including the use of foils and or belts located between the creping doctor of a Yankee and the reel to provide support for the web. He further discloses the continuous support of the web on TAD machines without Yankees through the TAD section to the reel.

A problem common to all TAD tissue manufacture is the very high drying energy cost associated with the TAD process.

This is largely due to the relatively low solids content of the web at the start of the drying phase. Typically, the solids content of TAD tissue prior to the drying phase is in the 20 to 25% range, as compared to 35 to 45% for wet pressed tissue; this results in drying loads which are 3 to 4 times higher for TAD than wet pressed tissue. The use of TAD alone as a means to achieve final web solids content is extremely energy intensive because the areas of the sheet adjacent to the open areas of the fabric will be over-dried in order to compensate for the wetter areas shielded from the drying air flow by the fabric yarns or elands areas.

Several other factors contribute to the high energy cost of TAD. Compared to the impingement drying hoods (Yankee hoods) used on CWP machines, whether or not in conjunction with TAD, the TAD itself uses a relatively low rate of drying air recirculation i.e. the humidity in the exhaust air stream is low, from 0.15 to 0.25 kg water/ kg dry air for TAD as compared to 0.35 to 0.45 kg water/kg dry air for a Yankee hood. AS a consequence, a considerable amount of energy is spent heating up the fresh air for circulation through the TAD. This is in part due to the need to limit the amount of fibre dust in the recirculation air stream. From a drying capacity perspective, increasing the humidity increases the web temperature in the dryer; it is well known that the web temperature in the dryer can be approximated by the adiabatic saturation temperature of the supply air. This in turn reduces the temperature difference between the drying air and the web, thus the driving force for heat transfer and consequently the drying rate. This same effect holds true for impingement dryers; but as the supply air temperature for TADS tends to be lower (175 - 220 C) as compared to impingement dryers (315 - 510 C) the effect of increased humidity in the dryer on the drying rate is more pronounced in the TAD.

The TAD process is the most energy intensive of the methods used to dry tissue and towel, followed by the Yankee hood and Yankee cylinder. Typical specific energy consumption (SEC) for TAD dryers ranges from 3750 kj/kg to 4500 kj/kg. For Yankee hoods, the SEC will be in the range of 3250 kJ/kg to 4200 kJ/kg. Cylinder drying is the least S energy intensive drying technique with an SEC of approximately 2550 kJ/kg. From a drying economy standpoint therefore, it is preferable to maximize cylinder and impingement drying.

SUMMARY OF THE INVENTION

It has been found that the problems described above can be substantially reduced or eliminated by means of a novel TAD fabric arrangement in the dryer section of a papermaking machine. According to the invention, a single fabric is used to provide continuous support for the web as it is conveyed through the TAD and dryer section, the fabric partially wrapping one or more TAD dryers and one or more drying cylinders, the arrangement maintaining the advantageous drying features of the TAD dryer and Yankee, or other type of drying cylinder and hood.

Thus, the present invention seeks to provide a method of producing a tissue or towel by providing continuous support through a dryer section of a papermaking machine to a paper web by a unitary supporting fabric as the web and fabric pass over, in sequence, (i) at least one through air dryer; (ii) at least one dryer cylinder.

The present invention further seeks to provide a dryer section of a papermaking machine wherein a paper web support means comprises a unitary supporting fabric adapted to provide continuous support to a paper web as it passes over, in sequence, (i) at least one through air dryer; and (ii) at least one dryer cylinder.

A particular advantage of the use of a single TAD fabric to carry and support the web through the drying section in its entirety is that the moulded topography of the web and the desired sheet bulk can be substantially maintained throughout the drying process. Eliminating the sheet transfer to the Yankee by the pressure roll allows the design of the fabric to be optimized towards the end product properties rather than having to compromise fabric design to meet the requirements for transfer to the Yankee through a press roll nip. Thus the range of fabric designs suitable for the process can be greatly expanded to include a broad range of contact areas and fabric topography. An example of fabrics suitable for the process is described in PCT application WO 0306732 to Johnson et al. and GB patent application 03/18220 to Stone et al. Further, the absence of a pressure roll in the dryer section of this invention now allows for the use of seamed TAD fabrics, the seams of which are designed so as to have a low profile to reduce the possibility of any excessive sheet marking. Further, by employing a seamed TAD fabric, it is no longer necessary to provide a cantilevered dryer section as the fabric can be installed on the papermaking machine in the exact same manner as any other seamed papermaking fabric.

Elimination of the sheet transfer by pressure roll from the TAD fabric to the Yankee also improves operating efficiency as the success of the transfer is not always guaranteed owing to the large number of influencing variables, such as variation in TAD fabric contact area, insufficient contact area and variations in the Yankee adhesive coating system.

A further benefit provided by the fabric arrangement of the present invention is that the complexity of the tissue machine is reduced. By eliminating the pressure roll, the design of the Yankee cylinder is simplified in that it only needs to be designed for the requirements of a steam pressure vessel, and not a combined nip load and pressure IS load as in the conventional case when a pressure roll is employed. Thus, a further benefit of the present invention is that the dryer section can now accept conventional steam heated cylinders whose quantity and size can be selected to best meet the demands of the drying load.

This in turn also simplifies the shipment and installation of these dryer cylinders, because the problems associated with the transport of the large diameter Yankee cylinders is now avoided. Maintenance of the cylinder dryers can be simplified because the absence of a creping doctor, the need for regrinding of the Yankee cylinder can be eliminated or reduced, downtime for changes of the doctor blade is eliminated, and machine downtime for general maintenance is reduced.

To overcome the sheet characteristics of stiffness and loss of stretch and softness which results from the elimination of the creping step, it is possible to employ a rush transfer such as is described above between the forming section and the TAD fabric or, alternatively, between the forming section and one (or more) intermediate transfer fabrics and the TAD fabric.

In one embodiment of the invention, the TAD fabric provides continuous support from the drying section to the reel drum or reel, eliminating the open draw and reducing sheet transfer problems. This also allows the sheet properties to be optimized for end product requirements rather than for manufacturing or processing strength requirements. It may however be advantageous from a wear resistance perspective to use a separate transfer fabric to pick up the web from the TAD fabric in a closed transfer to convey the web to the reel drum or reel.

Alternatively, two fabrics can be used to sandwich the web in the transfer to the reel drum or reel.

A limitation of this invention arises from the use of the TAD fabric to convey the paper web over the Yankee (or other type of) drying cylinder. This is because the TAD fabric will now be directly exposed to the impinging jets of high temperature air (in the range of about 300 C or more) from the Yankee (impingement) hood as it conveys the web over the drying cylinder(s). In general, the air temperature at the Yankee hood in the dryer section of this invention will be below that which is usually required in a COP (conventional wet pressed tissue) process because the drying load on the drying cylinder and hood will be less than those typically found in conventional wet press tissue, because the sheet will have a higher solids content.

Also, the majority of the drying following the TAD section in the apparatus of this invention will be done by the cylinder, the areas of the web adjacent the open areas of the fabric having been preferentially dried on the preceding TAD section. Further, the relatively short dwell time under the hood means that fabric temperatures will not rise too high. If high temperature hood operation is required, it will be preferable to employ materials having high heat and hydrolysis resistance, such as PPS (polyphenylene sulphide) or PEEK (polyetherether ketone) in the TAD fabric so as to increase its service life. To limit any possible hydrolytic damage caused by the impingement of high temperature air on the TAD fabric when it is stationary, a system of bypass ducts and dampers can be used on the Yankee hood.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying drawings, in which: Figure l is a highly simplified schematic illustration of

a conventional prior art TAD dryer section layout;

Figure 2 is a prior art variation of the arrangement shown in Figure l;

Figure 3 is a second prior art variation of the

arrangement shown in Figure l; Figure 4 is a schematic illustration of a dryer section according to a first embodiment of the present invention; Figure 5 is a schematic illustration of a dryer section according to a second embodiment of the invention; Figures 6 to 11 are schematic illustrations of various alternate arrangements for transfer of the dry web from the last cylinder dryer to the reel.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 shows a conventional, prior art TAD dryer section layout. The paper web 100 is transferred from a forming fabric or transfer fabric 1 to the TAD fabric 4 by means of a pick-up shoe 2. Alternatively a pick-up roll (not shown) could be used in place of pick-up shoe 2. The TAD fabric 4 carries the web 100 over a vacuum moulding box 3, over at least one through air dryer 5, the air being supplied by the hood 6 to the dryer 5. After the web 100 is dried partially, it is carried by the TAD fabric 4 over turning roll 7, to a pressure nip between press roll 8 and Yankee dryer cylinder 9. The web 100 is pressed onto the surface of cylinder 9, and dried to completion by means of the cylinder 9 and heat from hood 10. Thereafter it is removed from the cylinder 9, for example by creping doctor 11, to travel by an open draw to reel drum 12 and reel 13.

In this arrangement, the paper web 100 is carried on the outside of the TAD fabric 4 while passing around each dryer 5. After separation of the web 100, TAD fabric 4 follows a suitable return path, shown in Figure 1 as passing over turning roll 14, guide roll 15, and through cleaning showers and a Uhle box at region 17 and thence over take-up roll 16.

Figure 2 shows a variant of the conventional TAD layout in which after the web 100 is transferred to the TAD fabric 4, it is carried over two turning rolls 7 to be carried over each dryer 5 sandwiched between the TAD fabric 4 and the dryer 5, to be dried by air flowing from each dryer 5 to a hood 26. Thereafter the web 100 is carried through the nip at press roll 8, and transferred to the Yankee cylinder 18, dried through the combined effect of the cylinder 18 and hood 19, before passing to reel 13 in the same manner as shown in Figure 1. For simplicity, the guide roll, take-up and showers are not shown.

Figure 3 shows a further prior art variant of the TAD process, in which creping doctor 11, Yankee dryer cylinder 9 and hood 10 (all shown in Figure 1) are omitted, and instead the web 100 is dried to completion on the dryers 5, before being conveyed to the reel section by means of vacuum assisted transfer roll 22, a pair of transfer fabrics 20, and nip pressure roll 21.

Figure 4 is a schematic illustration of a first embodiment of the present invention. As in Figure 1, the web 100 is transferred to the TAD fabric 4 from a forming fabric or transfer fabric 1 by pick-up means 2 and is then moulded to the TAD fabric 4. The web 100 is carried by, and on the outside of, the TAD fabric 4 through at least one through air dryer 5. After passing optional turning roll 7, the TAD fabric 4 carries the web 100 onto the Yankee cylinder 29. As the web passes around the cylinder 29, it is held in contact with the cylinder 29 by the TAD fabric 4, i.e. between the cylinder 29 and the TAD fabric 4, while being dried to completion simultaneously by the cylinder 29 and S the hood 30 in the conventional manner. At or shortly after the outlet of the Yankee hood 30, the TAD fabric 4 and web 100 separate from the dryer 29, passing over roll 23, which may employ a vacuum section to assist in the web separation. Thereafter, the TAD fabric 4 returns to complete its loop while the web 100 is carried in a suitable manner to the reel drum 12 and reel 13. In the embodiment shown in Figure 4, an open draw is provided between roll 23 and reel drum 12. If desired, calender means can be provided at this open draw section, or such IS means can be provided off-line.

Referring to Figure 5, the single Yankee dryer cylinder 29 (Figure 4) can be replaced by one or more cylinders of the same size or smaller, shown here as three cylinders 39 and hoods 40. As discussed above, the smaller cylinders are significantly lighter, less expensive and easier to transport and install than the conventional large Yankee cylinder 9.

Referring to Figures 6 to 11, various means are shown by which the web 100 can be carried to the reel 13, after leaving the TAD fabric 4, to meet the requirements for particular end uses of the web 100. In each of Figures 6 to 11, the TAD fabric 4 and the web 100 are shown as they leave the (or the last) dryer cylinder 49 and hood 50.

Further, in each of the means discussed below, an open draw is avoided, i. e. the web 100 is provided with maximum support at all times after leaving the TAD fabric 4.

Referring to Figure 6, roll 25 acts to remove the web 100 from the cylinder 49. Located between that point and the point at which the web 100 leaves the TAD fabric 4 to pass over the reel drum 12 to the reel 13, sheet stabilizer box 24 provides support by vacuum means.

Referring to Figure 7, reel drum 12 can be located inside the run of the TAD fabric 4. The transfer of the web 100 (not shown) to the reel 13 takes place at nip 31 by means of pressurized air assist from inside reel drum 12.

Referring to Figures 8 and 9, one or more carrier or transfer fabrics 27 can be used to transfer the web 100 (not shown) from the TAD fabric 4 and the reel 13. In the embodiment shown in Figure 8, the web 100 is transferred directly from TAD fabric 4 to a single carrier fabric 27 which provides support up to the nip between pressure roll 21, within the loop of carrier fabric 27, and reel 13. In the embodiment shown in Figure 9, the web 100 is transferred from the TAD fabric 4 to carrier fabric 27b.

Thereafter the web 100 is further stabilized by being carried between carrier fabrics 27b and 27a, to pass through the nip provided by pressure roll 21, within the loop of carrier fabric 27b, and reel 13.

Referring to Figure 10, in a further embodiment of the invention, the web 100 (not shown) is transferred from the TAD fabric 4 to the reel drum 12 by what is known as a "kiss" transfer, and thence to the reel 13.

Referring to Figure 11, in a further embodiment of the invention, the web 100 is transferred directly to the reel 13 at a nip 28 formed between the TAD fabric 4 itself and the reel 13.

Claims (14)

1. A method of producing a tissue or towel by providing continuous support through a dryer section of a papermaking machine to a paper web by a unitary supporting fabric as the web and fabric pass over, in sequence, (i) at least one through air dryer; and (ii) at least one dryer cylinder.

2. A method of producing a tissue or towel as claimed in Claim 1, comprising the further step of providing continuous support to the paper web, after leaving the last of the at least one dryer cylinder, to a reeling location.

3. A method of producing a tissue or towel as claimed in Claim 2 wherein the further continuous support is provided by the unitary supporting fabric.

4. A method of producing a tissue or towel as claimed in Claim 2 wherein the further continuous support is provided by a closed transfer to at least one transfer fabric for carrying the paper web to the reeling location.

5. A method of producing a tissue or towel as claimed in Claim 1 including the further step of providing a rush transfer from a forming fabric to carry the paper web to the unitary supporting fabric.

6. A method of producing a tissue or towel as claimed in Claim 1 including the further step of providing a rush transfer from a forming fabric to carry the paper web to at least one transfer fabric and thereafter to the unitary supporting fabric.

7. A method of producing a tissue or towel as claimed in Claim 1 wherein the unitary supporting fabric is seamed by a method selected from pin seaming or coil seaming.

8. A dryer section of a papermaking machine wherein a paper web support means comprises a unitary supporting fabric adapted to provide continuous support to a paper web as it passes over, in sequence, (i) at least one through air dryer; and (ii) at least one dryer cylinder.

9. A dryer section of a papermaking machine as claimed in Claim 8, wherein further continuous support is provided to the paper web by a secondary support means between the last of the at least one dryer cylinder and a reeling location.

10. A dryer section of a papermaking machine as claimed in Claim 9 wherein the secondary support means is the unitary supporting fabric.

11. A dryer section of a papermaking machine as claimed in Claim 9 wherein the secondary support means is at least one transfer fabric.

12. A dryer section of a papermaking machine as claimed in Claim 8 wherein a rush transfer means is provided from a forming fabric to the unitary supporting fabric.

13. A dryer section of a papermaking machine as claimed in Claim 8, wherein a rush transfer means is provided from a forming fabric to at least one transfer fabric adapted to cooperate with the unitary supporting fabric.

14. A dryer section of a papermaking machine as claimed in Claim 8 wherein the unitary supporting fabric is seamed by a method selected from pin seaming or coil seaming.