Method for studying water removal and air penetration during Through Air Drying of tissue in laboratory scale

Award Winner: 2022 BioResources Early Career Investigator Award

Sjöstrand, B., Danielsson, M., and Lestelius, M. (2023). “Method for studying water removal and air penetration during Through Air Drying of tissue in laboratory scale,” BioResources 18(2), 3073-3088.

Abstract

Energy use, together with consumption of raw materials, machine clothing, and wet end chemicals, are some of the most critical aspects in successful tissue making today. This work was aimed at developing a laboratory-scale method of estimating dewatering mechanisms, vacuum efficiency, and energy use of Through Air Drying (TAD) of tissue. When compared to pilot data, the results of the new laboratory method for investigating dewatering during TAD were in the same magnitude, around 24 to 26% dryness after vacuum dewatering, and 27 to 29% dryness after TAD molding. Sheet properties, such as caliper and surface profile, were evaluated and compared to commercial tissue sheets. The results indicate that it will be possible to precisely measure accurate dryness development and penetrated air volume for tissue sheet forming and TAD molding at a laboratory scale. This can contribute to the efforts of implementing a circular forest-based bioeconomy by increasing the fundamental understanding of dewatering of tissue paper materials, which is facilitated by improvements in energy use. The new method developed in this work will make it easier to assess ideas that are difficult to bring to pilot scale or full scale before learning more of the dewatering capabilities. The authors are convinced that improved knowledge of tissue dewatering mechanisms, forming, and material transport during and after TAD dewatering can increase the efficiency of the industrial manufacturing processes.

Download PDF

Full Article

Method for Studying Water Removal and Air Penetration during Through Air Drying of Tissue in Laboratory Scale

Björn Sjöstrand,^a,* Mikael Danielsson,^b and Magnus Lestelius^a

Award Winner: 2022 BioResources Early Career Investigator Award

DOI: 10.15376/biores.18.2.3073-3088

Keywords: Through air drying; TAD; Dewatering; Tissue; Papermaking

Contact information: a: Department of Engineering and Chemical Sciences, Karlstad University, Sweden; b: Albany International AB, Halmstad, Sweden; *Corresponding author: bjorn.sjostrand@kau.se

INTRODUCTION

Energy use, together with consumption of raw materials, machine clothing, and wet end chemicals are some of the most critical aspects in successful tissue making today. While about half of the energy can be produced in-house by the mills using biomass residues (IEA 2017), this is almost never the case for tissue production. There is potential for making the processes more energy efficient (Håkansson 2010; Lahtinen and Karvinen 2010; Holmberg et al. 2013; Sjöstrand 2020).

The forming section in a tissue machine is typically much shorter than paper machines manufacturing other paper grades and is not aligned horizontally. The press section can be a single press nip against a Yankee cylinder. The sheet is pressed and adhered onto the Yankee surface, dried with steam inside the cylinder and in a covered gas-fired hood, and later creped off with a doctor blade (Klerelid 2000; Tysén 2018). This is called Dry Creped Tissue (DCT). Another concept of tissue manufacturing is Through Air Drying (TAD). In a TAD machine (Fig. 1), the press is replaced by a TAD cylinder, or in some machine configurations two or more cylinders, with a structured molding TAD fabric, where the sheet is transferred with vacuum boxes and much of the water is removed with hot air blown through sheet and fabric. The structured fabric provides the tissue sheet with its structure, bulk, and allows for efficient drying due to its open nature. There is also a speed difference between the forming section and TAD section, which has a big impact on sheet properties compared to DCT. The TAD method is less energy efficient but gives higher performing products when compared with DCT (Tysén 2018). Producing tissue differs from other grades relating to the basis weight of the network being much lower; thus, the efforts in dewatering are considerably lower compared to production of, for example, paperboard grades (Klerelid 2000; Tysén 2018). Nevertheless, energy efficiency becomes increasingly important for tissue producers, especially for the more energy demanding TAD method.

Tissue papers are mainly produced from fiber blends of softwood and hardwood mixtures (de Assis et al. 2018). Fiber type can affect drainage and dewatering, mainly by their varying fines content (de Assis et al. 2018). The fines have a negative effect on drainage by clogging drainage channels (de Assis et al. 2018; Hubbe et al. 2020). Conformability of individual fibers can also affect dewatering by sealing of forming fabric surface when using flexible fibers (Sjöstrand et al. 2019; Hubbe et al. 2020).

Fig. 1. Schematic drawing of a simplified TAD tissue machine, redrawn from Tysén (2018)

A long tradition of studying conventional vacuum dewatering can be found in literature. A compilation of these literature is shown in Table 1.

Table 1. Notable Publications Regarding Different Aspects of Vacuum Dewatering

The overarching conclusion from earlier work is that the dewatering behavior of the vacuum suction boxes is consistent with the overall dewatering behavior of the papermaking process, with easy dewatering in the early parts and increasingly harder to remove water, diminishing returns (Attwood 1962; Neun 1994, 1996; Räisänen et al. 1996; Ramaswamy 2003; Pujara et al. 2008). The location of water within the sheet’s structure is the reason why it becomes increasingly difficult to dewater. Water is removed in the following order: from in between fibers, from the lumen, and lastly, from the fiber walls (Paulapuro 2000; Stenström and Nilsson 2015). Other aspects, such as sheet basis weight, forming fabric, pulp beating, vacuum level, and dwell time, are shown to affect vacuum dewatering. The dewatering mechanisms are shown to be drainage, before the vacuum dewatering (Britt and Unbehend 1985), compression dewatering (Åslund and Vomhoff 2008a, 2008b), air displacement dewatering (Räisänen et al. 1996; Åslund and Vomhoff 2008b; Nilsson 2014a, 2014b), and external rewetting (Åslund et al. 2008b; Sjöstrand et al. 2015).

Work addressing laboratory TAD dewatering has been published by Tysén et al. (2015, 2018). They described the influence of grammage and pulp type on water removal with TAD in laboratory scale (Tysén et al. 2015) and assisting the TAD dewatering with infrared radiation (Tysén et al. 2018). The dewatering mechanism was suggested to be evaporation due to air flow. The increase in temperature by infrared radiation also increased drying capacity of the TAD. The dwell times were relatively long compared to industrial scale tissue production, which makes it hard to connect the experimental results with pilot and full-scale manufacturing.

This work was aimed at developing a laboratory scale method for measuring dewatering and air volume through the sheet under commercial conditions, and estimating vacuum efficiency and energy use, of TAD of tissue. The dewatering mechanism for water removal with laboratory TAD will be investigated.

EXPERIMENTAL

Fibers and Fabrics

Bleached chemical kraft hardwood pulp fibers from Eucalyptus were supplied by Valmet AB (Karlstad, Sweden), with a dryness of 96%. The hardwood pulp was characterized with a Valmet Fiber Image Analyzer (Valmet FS5, Valmet AB, Karlstad, Sweden). The fiber length was approximately 0.8 mm and the fiber width was 13 μm based on a single test in the FS5. The drainage resistance was measured with the Shopper-Riegler method, ISO 5267-1 (1999) to a value of 15.5 °SR.

Bleached chemical kraft softwood pulp fibers mixed from Norway spruce and Scots pine were supplied by StoraEnso AB (Skoghall, Sweden) with a dryness of 4%. The softwood pulp was characterized with an L&W Fiber Tester (ABB AB / Lorentzen and Wettre, Kista, Sweden). The fiber length was 2.2 mm, and the fiber width was 31 μm based on a single test in the Fiber Tester. For both FS5 and Fiber Tester the single values of fiber length were based on many images of individual fibers in the pulp samples. The drainage resistance was measured with the Shopper-Riegler method, ISO 5267-1 (1999) to a value of 23 °SR.

Two fabrics provided by Albany International Inc. (Rochester, NH, USA) were used in the experiments. One tissue grade triple layer forming fabric simulating the inner position in the TAD machine configuration, fabric number 2 in Fig. 1, and a TAD fabric, corresponding to fabric number 3 in Fig. 1 (molding fabric). The TAD fabric is a relatively coarse single layer fabric mainly used for towel tissue applications. Basic data for the two fabrics are summarized in Table 2.

Table 2. Yarn Set Up in Machine and Cross Machine Direction and Basic Data for Caliper and Permeability

Methods

The fibers were mixed to a 75/25 weight ratio with 75% softwood and 25% hardwood. The fiber mixture was then diluted to 0.2% in tap water with no additional chemicals added, later referred to as the stock suspension. The stock solution was assumed to have a density of 1 g/cm³; this assumption was valid because the fibers are a small part of the total weight and also completely saturated with the water. To form a tissue paper sheet of 20 g/m², 266 g stock suspension was measured on a scale and added to a modified sheet former together with an additional 3.5 L of tap water. The sheet former had a diameter of 18.4 cm and the sheet was formed directly on the commercial forming fabric. The forming fabric with the formed tissue sheet was removed from the sheet former and transferred to the vacuum dewatering equipment. This apparatus (Fig. 2) was described in detail by Granevald et al. (2004), and in this experiment a plate with a single 5-mm opening was used. The tissue paper, still on the forming fabric, was then subjected, triplet samples for each setting, to -30 kPa pressure drop during a number of different dwell times: 0, 1, 2.5, 5, 10, and 20 ms. The dwell times were designed to capture the dewatering during a typical suction box with the total dwell time of 20 ms. Before transfer to the vacuum dewatering equipment, and after each vacuum dewatering run, each tissue sheet was scraped off the forming fabric, and the dryness was measured according to ISO 638-1 (2022). Because the edges of the sheets are not completely dewatered, the middle part of the sheets with a diameter of 10 cm was used for dryness measurements. All dryness measurements were made at full rewetting, as rewetting was shown by Sjöstrand et al. (2015) to be complete in considerably faster than a second. For this reason, all dryness values are considered to represent dryness after rewetting.

A number of sheets dewatered 20 ms were manually transferred to the TAD fabric and brought back to the vacuum suction box, with no speed differential between fabrics. This time for another set of vacuum dewatering representing the transfer and TAD molding with -65 kPa and the following dwell times: 2.5, 5, 10, and 20 ms. After TAD dewatering, the whole sheets were measured for dryness according to ISO 638-1 (2022).

Fig. 2. Schematic drawing of the custom-built laboratory scale vacuum suction box. The pressure drop is varied with the vacuum pump and measured with the pressure transducer. The vacuum dwell time is varied with varying speeds of the moving plate, dwell time can be as small as 0.5 ms.

All sheets were supported by the forming fabric during vacuum dewatering and supported by the TAD fabric during TAD molding. This indicates that the sheets were in tension in a similar way as the sheets in the forming section of a pilot machine.

The pressure drops in the vacuum dewatering machine, both for forming fabric dewatering and TAD dewatering, were recorded during vacuum exposure for each sample. A typical recording is shown in Fig. 3. The vacuum level dropped during one test in accordance with Fig. 3, both for forming fabric and TAD dewatering. It was, however, always reset to original value before next sample. The logged pressure drops of the vacuum dewatering test were used to calculate the air volume that passed through the sheet using Eq. 1 (Granevald et al. 2004; Sjöstrand et al. 2016). The air volume for TAD sheets consists of the forming fabric dewatering air volume for 20 ms, added with the respective TAD dewatering air volume, because these sheets effectively are dewatered twice. Equation 1 is as follows,

(1)

where V_air is the penetrated air volume (dm³), V_tank is the volume of the tank (dm³), P_atm is the atmospheric pressure (Pa), and ΔP is the change in pressure during the test (Pa), as explained by Granevald et al. (2004).

Fig. 3. Typical appearance of change in vacuum level during testing, this sample was dewatered on the forming fabric in -30 kPa and 20 ms

Sheets from both forming fabric dewatering and TAD dewatering were freely dried at room temperature, without prior pressing to conserve the structure, and analyzed once dry. Single ply thickness according to ISO 12625-3 (2014) was measured on laboratory forming fabric sheets, laboratory TAD sheets and commercial TAD sheets used for reference, thickness was measured on conditioned sheets according to ISO 187 (2022). The sheets’ appearance was qualitatively captured with scanning electron microscopy (SEM) with a Hitachi SU-3500 (Tokyo, Japan) instrument, and also with X-ray microtomography (XRM) with a Bruker Skyscan 1172 (Karlsruhe, Germany). The SEM is an environmental SEM with normal conditions and no metal plating of the material. The XRM was used with a resolution of 0.95 μm/pixel, source voltage 32 kV, and current 192 μA. The global structural caliper of the SEM sheets was also estimated by measuring in the SEM images, this value was used as an indication on the achieved structure from the TAD fabric.

RESULTS AND DISCUSSION

Sheet formation and coverage are shown in Fig. 4. The image clearly indicates that the laboratory method was sufficiently proficient at forming tissue grade sheets. The mix of large softwood fibers and smaller hardwood fibers are visible in the image. The sheet is observed to be porous and would not impede water or air flowing to a large extent.

Fig. 4. XRM images of a laboratory made TAD tissue sheet

The development of solids content (%) for the tissue sheet in vacuum dewatering is shown in Fig. 5. The typical shape of the diminishing returns of the dewatering curve is consistent with both previous data from the same laboratory equipment (Sjöstrand et al. 2016, 2017, 2019; Rahman et al. 2018; Sjöstrand and Brolinson 2022), and with laboratory and pilot data from other publications of vacuum dewatering (Attwood 1962; Neun 1994, 1996; Räisänen et al. 1996; Ramaswamy 2003; Pujara et al. 2008). The shape of the dewatering curve shows the well-known dewatering mechanism for vacuum dewatering where water between the fibers is removed first, to an initial high dewatering rate, and as the water becomes increasingly less accessible the dewatering rate levels out, and gradually goes to zero. Comparable data from a pilot scale tissue machine, indicated by red markers in Fig. 5, confirm that the solids content after laboratory scale vacuum dewatering matches well with the pilot scale measurements. Note that the pilot results varied somewhat with regard to basis weight, fiber mixture, vacuum level, and dwell time.

Fig. 5. Solids content development during vacuum dewatering on a conventional forming fabric. Note that the red ‘x’ markers indicate corresponding values from pilot scale testing.

After vacuum dewatering on a forming fabric, the sheet was transferred to the TAD fabric and subjected to another vacuum pulse. The development of solids content (%) is shown in Fig. 6.

Fig. 6. Solids content development during vacuum dewatering on a TAD fabric. Note that the red ‘x’ markers indicate corresponding values from pilot scale testing.

The shape of the dewatering curve differed somewhat from the vacuum dewatering on a forming fabric in Fig. 5. The dewatering rate was more constant due to a more linear solids content versus dwell time graph. This indicates that the dewatering mechanism for laboratory, and possibly pilot and production scale, TAD molding is air displacement dewatering (Räisänen et al. 1996; Åslund and Vomhoff 2008b; Nilsson 2014a, 2014b). Air displacement dewatering is similar to drying with air of room temperature (20 °C) and is an energy-demanding process.

Figures 7 and 8 show the calculated penetrated air volume (using Eq. 1) for vacuum dewatering on a forming fabric and the TAD molding, respectively. The numbers are not compared with pilot or production scale tissue manufacturing but clearly show that TAD molding requires high pump capacity, based on the higher penetrated air volume, and is therefore concluded to require more energy. This is in line with the suggested dewatering mechanism. To further develop the laboratory method, tailored pilot trials could be performed to connect the penetrated air volume values to actual energy use. The presumed higher pump energy demand for TAD molding, relative DCT, leads to reasonable questioning regarding economic efficiency, mainly because pressing is not used in TAD.

Fig. 7. Penetrated air volume during vacuum dewatering on a conventional forming fabric

An important aspect of TAD tissue manufacturing is the sheet properties. In this work the sheets were evaluated according to appearance from SEM images, mainly comparison between laboratory sheets dewatered on forming fabric, laboratory TAD sheets, and commercial TAD sheets. A rough global caliper estimation was made on the SEM images to show that the laboratory TAD sheets were similar to commercial ones.

Fig. 8. Penetrated air volume during vacuum dewatering on a TAD fabric. Note that the air volume is representing both forming fabric vacuum dewatering and TAD dewatering.

Figure 9 shows the laboratory sheet before TAD (A) and after TAD molding (B), where a difference in global sheet caliper can be observed.

Fig. 9. SEM images of A) laboratory 20 g/m² sheet formed on a standard forming fabric, B) laboratory 20 g/m² sheet formed on a TAD fabric, and C) commercial 20 g/m² sheet formed on a TAD fabric

The tissue paper sheet before TAD (A) was measured as 0.28 mm, the laboratory TAD (B) was 0.46 mm, and the commercial TAD (C) was 0.55 mm. The qualitative appearance of the laboratory TAD sheet (B) was more similar to the commercial TAD (C) than the sheet formed on a forming fabric (A). When comparing the laboratory TAD (B) to the other two sheets (A and C) in Fig. 9, it was observed that the laboratory method of TAD molding produced sheets with similar properties of commercial TAD manufacturing processes.

The single-ply sheet caliper was also measured quantitatively with a thickness measurement (Fig. 10), which showed that the single ply thickness for the TAD sheets, both laboratory and commercial was slightly higher that the laboratory forming fabric sheets. This indicates that the structure generated by TAD molding was successfully achieved throughout the sheets that were produced. The single ply caliper was naturally lower than the global caliper estimation in Fig. 9 because the measurement relates to structural buckling of the sheet, as indicated in Fig. 9.

Fig. 10. Single-ply thickness measurements for laboratory forming fabric sheets, laboratory TAD sheets and commercial TAD sheets, with a 95% confidence interval shown with error bars, based on 10 measurements over five single ply sheets

CONCLUSIONS

This work describes a successful laboratory scale method of measuring dryness development and penetrated air volume of vacuum dewatering and through-air drying (TAD) molding of tissue paper. The method has commercial conditions of machine speed, vacuum level, and provides TAD sheets with similar properties as full scale production.
The dewatering mechanism of TAD molding is more closely related to drying than vacuum dewatering, and is proposed to be more energy demanding, based on measured penetrated air volume during laboratory testing. The dewatering mechanism is suggested to be air displacement dewatering.
This new laboratory method will allow investigation of multiple parameters affecting TAD dewatering.
The authors plan to validate the laboratory scale method further with support of more pilot scale and full-scale measurements, mainly to connect the penetrated air volume to actual energy consumption of the manufacturing processes.

ACKNOWLEDGMENTS

The authors are grateful for the financial support of the Knowledge foundation, Grant No. 2022-0024, as well as generous in-kind contributions from Albany International Inc., Karlstad University and Valmet AB. Sara Christensson and Cecilia Westling at Stora Enso AB are gratefully acknowledged for providing the beaten pulp fibers. Anders Ottosson, Bruno Tremblay, Karl-Johan Tolfsson and Viktor Bergström at Valmet AB are acknowledged for help with ideas, consultations, and providing pilot scale data. Darrin Curley and John LaFond at Albany International Inc. are acknowledged for help with discussions regarding experiments and results. Carl-Anton Karlsson at Karlstad University is acknowledged for help with laboratory equipment. The authors BS and ML are a part of Pro2BE at Karlstad University, the research environment for Processes and products for a circular forest-based bioeconomy.

REFERENCES CITED

Åslund, P., and Vomhoff, H. (2008a). “Method for studying the deformation of a fibre web,” Nordic Pulp & Paper Research Journal 23(4), 398-402. DOI: 10.3183/npprj-2008-23-04-p398-402

Åslund, P., and Vomhoff, H. (2008b). “Dewatering mechanisms and their influence on suction box dewatering processes – A literature review,” Nordic Pulp & Paper Research Journal 23(4), 389-397. DOI: 10.3183/npprj-2008-23-04-p389-397

Åslund, P., and Vomhoff, H. (2008c). “Evaluation of membrane-assisted dewatering on a pilot,” Nordic Pulp & Paper Research Journal 23(2), 152-155. DOI: 10.3183/npprj-2008-23-02-p152-155

Åslund, P., and Vomhoff, H. (2008d). “Web deformation during membrane-assisted dewatering,” Nordic Pulp & Paper Research Journal 23(4), 415-419. DOI: 10.3183/npprj-2008-23-04-p415-419

Åslund, P., Vomhoff, H., and Waljanson, A. (2008a). “The deformation of chemical and mechanical pulp webs during suction box dewatering,” Nordic Pulp & Paper Research Journal 23(4), 403-408. DOI: 10.3183/npprj-2008-23-04-p403-408

Åslund, P., Vomhoff, H., and Waljanson, A. (2008b). “External rewetting after suction box dewatering,” Nordic Pulp & Paper Research Journal 23(4), 409-414. DOI: 10.3183/npprj-2008-23-04-p409-414

ASTM D737-96. (2018). “Standard test method for air permeability of textile fabrics,” ASTM International, West Conshohocken, PA, USA.

Attwood, B. W. (1962). “A study of vacuum box operation,” Paper Technology 3(5), 144-153.

Baldwin, L. (1997). “High vacuum dewatering,” Paper Technology 38(4), 23-28.

Britt, K. W., and Unbehend, J. E. (1980). “Water removal during sheet formation,” TAPPI Journal 63(4), 67-70.

Britt, K. W., and Unbehend, J. E. (1985). “Water removal during paper formation,” TAPPI Journal 68(4), 104-107.

de Assis, T., Reisinger, L. W., Pal, L., Pawlak, J., Jameel, H., and Gonzalez, R. W. (2018). “Understanding the effect of machine technology and cellulosic fibers on tissue properties – A review,” BioResources 13(2), 4593-4629. DOI: 10.15376/biores.13.2.Deassis

Granevald, R., Nilsson, L. S., and Stenström, S. (2004). “Impact of different forming fabric parameters on sheet solids content during vacuum dewatering,” Nordic Pulp & Paper Research Journal 19(4), 428-433. DOI: 10.3183/npprj-2004-19-04-p428-433

Holmberg, K., Siilasto, R., Laitinen, T., Andersson, P., and Jäsberg, A. (2013). “Global energy consumption due to friction in paper machines,” Tribology International 62, 58-77. DOI: 10.1016/j.triboint.2013.02.003

Hubbe, M. A., Sjöstrand, B., Nilsson, N., Koponen, A., and McDonald, J. D. (2020). “Rate-limiting mechanisms of water removal during the formation, vacuum dewatering, and wet-pressing of paper webs: A review,” BioResources 15(4), 9672-9755. DOI: 10.15376/biores.15.4.Hubbe

Håkansson, C. (2010). “Energy savings by process optimization. Reducing vacuum demand in the paper machine,” in: Proceedings of TAPPI PaperCon 2010, Atlanta, GA, USA, pp. 1164-1191.

International Energy Agency (IEA) (2017). Tracking Clean Energy Progress 2017, Informing Energy Sector Transformations, International Energy Agency, Paris, France.

ISO 187 (2022). “Paper, board and pulps – Standard atmosphere for conditioning and testing and procedure for monitoring the atmosphere and conditioning of samples,” International Organization for Standardization, Geneva, Switzerland.

ISO 638-1 (2022). “Paper, board, pulps and cellulosic nanomaterials – Determination of dry matter content by oven-drying method – Part 1: Materials in solid form,” International Organization for Standardization, Geneva, Switzerland.

ISO 5267-1 (1999). “Pulps – Determination of drainability – Part 1: Schopper-Riegler method,” International Standardization of Organization, Geneva, Switzerland.

ISO 12625-3 (2014). “Tissue paper and tissue products – Part 3: Determination of thickness, bulking thickness and apparent bulk density and bulk,” International Organization for Standardization, Geneva, Switzerland.

Klerelid, I. (2000). “Drying of tissue,” in: Papermaking Part 2, Drying, M. Karlsson (ed.), Fapet Oy, Jyväskylä, Finland, pp. 147-185.

Kullander, J., Nilsson, L., and Barbier, C. (2012). “Evaluation of furnishes for tissue manufacturing; suction box dewatering and paper testing,” Nordic Pulp & Paper Research Journal 27(1), 143-150. DOI: 10.3183/npprj-2012-27-01-p143-150

Lahtinen, J., and Karvinen, J. (2010). “Energy savings in paper machine vacuum system. How to utilize modern process and variable speed drive technology,” Proceedings of TAPPI PaperCon 2010, Atlanta, GA, USA, pp. 1129-1163.

Neun, J. A. (1994). “Performance of high vacuum dewatering elements in the forming section,” TAPPI Journal 77(9), 133-138.

Neun, J. A. (1996). “High-vacuum dewatering of newsprint,” TAPPI Journal 79(9), 153-157.

Nilsson, L. (2014a). “Stepwise development of a mathematical model for air flow in vacuum dewatering of paper,” Drying Technology 32(13), 1587-1597. DOI: 10.1080/07373937.2014.909844

Nilsson, L. (2014b). “Air flow and compression work in vacuum dewatering of paper,” Drying Technology 32(1), 39-46. DOI: 10.1080/07373937.2013.809732

Paulapuro, H. (2000). “Wet pressing,” in: Papermaking Part 1: Stock Preparation and Wet End, H. Paulapuro and J. Gullichsen (eds.), Fapet Oy, Jyväskylä, Finland, pp. 284-340.

Pujara, J., Siddiqui, M. a., Liu, Z., Bjegovic, P., Takagaki, S. S., Li, P. Y., and Ramaswamy, S. (2008). “Method to characterize the air flow and water removal characteristics during vacuum dewatering. Part II—Analysis and characterization,” Drying Technology 26(3), 341-348. DOI: 10.1080/07373930801898125

Rahman, H., Engstrand, P., Sandström, P., and Sjöstrand, B. (2018). “Dewatering properties of low grammage handsheets of softwood kraft pulps modified to minimize the need for refining,” Nordic Pulp & Paper Research Journal 33(3), 397-403. DOI: 10.1515/npprj-2018-3037

Räisänen, K. O., Karrila, S., and Maijala, A. (1996). “Vacuum dewatering optimization with different furnishes,” Paperi ja Puu 78(8), 461-467.

Ramaswamy, S. (2003). “Vacuum dewatering during paper manufacturing,” Drying Technology 21(4), 685-717. DOI: 10.1081/DRT-120019058

Rezk, K., Nilsson, L., Forsberg, J., and Berghel, J. (2013). “Modelling of water removal during a paper vacuum dewatering process using a level-set method,” Chemical Engineering Science 101, 543-553. DOI: 10.1016/j.ces.2013.07.005

Rezk, K., Nilsson, L., Forsberg, J., and Berghel, J. (2015). “Simulation of water removal in paper based on a 2D level-set model coupled with volume forces representing fluid resistance in 3d fiber distribution,” Drying Technology 33(5), 605-615. DOI: 10.1080/07373937.2014.967401

Shands, J., and Hardwick, C. (2000). “Dewatering on a high vacuum flat box,” in: Proceedings of Paptac, 86^th Annual Meeting, Montreal, QC, Canada, pp. 113-117.

Sjöstrand, B. (2020). Vacuum Dewatering of Cellulosic Materials, Doctoral Thesis, Karlstad University, Karlstad, Sweden.

Sjöstrand, B., Barbier, C., and Nilsson, L. (2015). “Rewetting after high vacuum suction boxes in a pilot paper machine,” Nordic Pulp & Paper Research Journal 30(4), 667-672. DOI: 10.3183/npprj-2015-30-04-p667-672

Sjöstrand, B., Barbier, C., and Nilsson, L. (2016). “Influence on sheet dewatering by structural differences in forming fabrics,” in: Proceedings of TAPPI PaperCon 2016, Cincinatti, OH, USA, pp. 767-776.

Sjöstrand, B., Barbier, C., and Nilsson, L. (2017). “Modeling the influence of forming fabric structure on vacuum box dewatering,” TAPPI Journal 16(8), 477-483.

Sjöstrand, B., Barbier, C., Ullsten, H., and Nilsson, L. (2019). “Dewatering of softwood kraft pulp with additives of microfibrillated cellulose and dialcohol cellulose,” BioResources 14(3), 6370-6383. DOI: 10.15376/biores.14.3.6370-6383

Sjöstrand, B., and Brolinson, A. (2022). “Addition of polyvinylamine in chemi-thermomechanical pulp and kraft pulp and the effects on dewatering, strength, and air permeance,” BioResources 17(3), 4098-4115. DOI: 10.15376/biores.17.3.4098-4115

Stenström, S., and Nilsson, L. (2015). “Predicting water removal during vacuum dewatering from fundamental fibre property data,” Nordic Pulp & Paper Research Journal 30(2), 265-271. DOI: 10.3183/npprj-2015-30-02-p265-271

Tysén, A. (2018). Through Air Drying: Thermographic Studies of Drying Rates, Drying Non-uniformity and Infrared Assisted Drying, Doctoral Thesis, Karlstad University, Karlstad, Sweden.

Tysén, A., Vomhoff, H., and Nilsson, L. (2015). “The influence of grammage and pulp type on through air drying,” Nordic Pulp & Paper Research Journal 30(4), 651-659. DOI: 10.3183/npprj-2015-30-04-p651-659

Tysén, A., Vomhoff, H., and Nilsson, L. (2018). “Through air drying assisted by infrared radiation: The influence of radiator power on drying rates and temperature,” Nordic Pulp & Paper Research Journal 33(4), 581-591. DOI: 10.1515/npprj-2018-2002

Article submitted: January 20, 2023; Peer review completed: February 25, 2023; Revised version received: March 3, 2023; Accepted: March 4, 2023; Published: March 7, 2023.

DOI: 10.15376/biores.18.2.3073-3088