Characterisation and Removal of Gas in Papermaking

Topi Helle

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Forest Products Technology for public examination and debate in Auditorium Puu II at Helsinki University of Technology (Espoo, Finland) on the 18^th of January, 2008, at 12 noon.

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Abstract

The objective of this dissertation was to characterise gas in papermaking and to study the effects of centrifugal force on flotation fibre recovery and the removal of gases from fibre suspensions. In order to reach these goals, laboratory and pilot paper machine trials with dispersed and precipitated gas bubbles were carried out. Laboratory and mill trials subjecting the fibre suspension to centrifugal force were also made.

Entrained gas bubbles had mostly adverse effects on papermaking, which was to be expected from previous studies reported in the literature. Even quite low entrained gas contents were found to affect papermaking. Precipitated gas bubbles appeared to have largely the same effect on papermaking as dispersed gas bubbles. However, differences were found in their effects on tear and wet tensile strength.

In this study, centrifugal force was found to remove gas bubbles effectively, so it can be used to remove gas from a papermaking fibre suspension. Centrifugal force did not remove dissolved gas effectively, but with correct process design sufficient removal of entrained air should be enough. In addition, it is possible to remove dissolved gas with centrifugal force with the help of turbulence or vacuum. According to the results in this study, centrifugal force cannot be used to improve flotation fibre recovery.

This thesis consists of an overview and of the following 5 publications:

1. Topi-Matti Helle, 2000, Effect of centrifugal force on flotation of air bubbles onto fibres and fillers, Paperi ja Puu, volume 82, number 4, pages 251-254. © 2000 Paperi ja Puu Oy. By permission.
2. Topi-Matti Helle, Paul Meinander, and Hannu Paulapuro, 1998, Removal of entrained air from white water by application of centrifugal force, Paperi ja Puu, volume 80, number 5, pages 379-382. © 1998 Paperi ja Puu Oy. By permission.
3. Topi-Matti K. Helle, Paul O. Meinander, Risto J. Nykänen, Kimmo S. Molander, and Hannu V. Paulapuro, 1999, Air removal mill trials using pomp deaerator, Tappi Journal, volume 82, number 6, pages 146-149. © 1999 by authors.
4. Topi-Matti Helle, 2000, Qualitative and quantitative effects of gas content on papermaking, Paperi ja Puu, volume 82, number 7, pages 457-463. © 2000 Paperi ja Puu Oy. By permission.
5. Topi-Matti Helle and Hannu Paulapuro, 2004, Effect of precipitated gas bubbles in papermaking, Appita Journal, volume 57, number 6, pages 444-447. © 2004 Appita. By permission.

Keywords: papermaking, gas, air, degassing, deaeration, flotation

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© 2008 Helsinki University of Technology