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The influence of grammage and pulp type on through air drying
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences. (Pro2BE)ORCID iD: 0000-0002-5864-4576
Innventia AB.
Innventia AB.
2015 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 30, no 4, p. 651-659Article in journal (Refereed) Published
Abstract [en]

The influence of grammage and pulp type on through air drying was studied. The temperature of a sample was measured during the drying process and the observed temperature changes were used to evaluate the drying process. Laboratory sheets with grammages 15- 60 g/m², from two softwood and two hardwood bleached chemical pulps were used. All samples were analysed with respect to formation, flow resistance, modified permeability, mean drying time, non-uniformity of drying time, and area- and mass-specific drying rate. The pulps had different modified permeabilities but showed similar behaviour when analysed as a function of grammage. A constant value was found for higher grammages, while an increase in modified permeability was found at decreasingly low grammages. Almost all pulp and grammage combinations had similar area-specific drying rates, but the mass-specific drying rates decreased with grammage. However, the samples with lower grammages had mass specific drying rates independent of modified permeability, where samples with increasing grammage showed an increasing dependency. This implies that the drying efficiency at low grammages was not controlled by the volume flow of the drying air. A good correlation was found between energy needed to evaporate water and energy supplied by the drying air as estimated from the surface temperature and air flow measurement. The surface temperature can therefore be used to quantify the drying process.

Place, publisher, year, edition, pages
2015. Vol. 30, no 4, p. 651-659
Keyword [en]
Drying, Moisture, Non-uniformity, Pulp, TAD, Thermography, Tissue
National Category
Paper, Pulp and Fiber Technology
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kau:diva-39096DOI: 10.3183/NPPRJ-2015-30-04-p651-659ISI: 000366040300013OAI: oai:DiVA.org:kau-39096DiVA, id: diva2:896304
Funder
Knowledge Foundation
Available from: 2016-01-20 Created: 2016-01-20 Last updated: 2018-04-18Bibliographically approved
In thesis
1. Through air drying: Thermographic studies of drying rates, drying non-uniformity and infrared assisted drying
Open this publication in new window or tab >>Through air drying: Thermographic studies of drying rates, drying non-uniformity and infrared assisted drying
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The removal of water is an integral part of papermaking. The drying process is responsible for a great share of the energy used in the paper machine. Premium grade tissue products are dried by through air drying. Large volumes of natural gas are burned to heat the air drawn through the paper web to achieve the drying. The low grammages for which this technique is used are believed to have material properties differing from the bulk properties achieved at higher grammages. If through air drying could be performed more efficiently, premium products could be produced with less environmental impact and at a lower cost.

The objective of this thesis was to investigate the non-uniformity and the rate of through air drying. The aspects considered were grammage, pulp type, formation, web-fabric interaction and infrared radiation. A method was developed, where the time-dependent change in surface temperature of a drying sample was recorded. The experimental equipment allowed paper samples to be dried by drawing air through them, with the option of additional energy from infrared radiation. An infrared camera captured the spatial variations in surface temperature during drying.

Abstract [en]

The objective of this thesis was to investigate parameters concerning the drying rate and the non-uniformity in the through air drying process. Parameters considered were grammage, pulp type, formation, web-fabric interaction and infrared radiation. A piece of equipment was therefore developed which allowed the paper samples to be dried by air being drawn through them, with the option to supply additional drying energy through infrared radiation. The time-dependent local surface temperature of a drying sample was recorded using an infrared camera. In addition, the air flow through the samples was measured.

Samples with grammages ranging from 15 to 60 g/m² were made on a laboratory sheet former from a range of different commercial chemical pulps. The pulps comprised both hardwoods and softwoods. Samples with both good and bad formation were made.

The measurements showed that the air flow through the sample varied with grammage and pulp type. The air permeability, i.e. the specific air flow, was constant at higher grammages, as could be expected. In contrast to that, at lower grammages, the air permeability was higher, and also a function of grammage. The permeability was also highly influenced by the fibre morphology, with softwood samples being much more permeable.

The non-uniformity in drying increased with bad formation and was influenced by the web-fabric interaction. Local drying time maps quantified the spatial non-uniformity of drying. Formation had little or no impact on the total drying time, but the non-uniformity of drying increased with worse formation. For commercial through air drying fabrics, the effect of web-fabric interaction was observed for hardwood samples, where the web areas in contact with the knuckles of the fabric weave had longer drying times. However, virtually no non-uniformity could be measured for the softwood samples.

The average drying rate was mainly influenced by pulp type, and was increased by the addition of infrared radiation. Interestingly, even though the permeability differed significantly between pulps, the drying rate was independent of the varying permeability at lower grammages. Thus, a higher air flow did not increase the drying rate. Adding additional drying energy by using an infrared radiator allowed for an increase in the drying rate. However, as the radiator power was increased, the corresponding increase in the drying rate was less than proportional.

Energy utilisation was evaluated for both the case with only air flow, and the combination of air flow and infrared radiation. It was found that the energy supplied by air correlated well with the theoretical energy required to remove the water in the fibre wall. When the infrared radiation was added, the efficiency in using the supplied energy appeared to diminish as the radiator power increased.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2018. p. 56
Series
Karlstad University Studies, ISSN 1403-8099 ; 2018:19
Keyword
Through air drying, TAD, Drying, Dewatering, Water removal, Infrared drying, Non-uniformity, Variability, Formation, Thermography, Tissue, Fabric, Wire, Pulp, Grammage
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-67061 (URN)978-91-7063-852-7 (ISBN)978-91-7063-947-0 (ISBN)
Public defence
2018-05-22, 9C 204, Rejmersalen, 10:15 (English)
Opponent
Supervisors
Available from: 2018-04-27 Created: 2018-04-18 Last updated: 2018-04-27Bibliographically approved

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